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molecules
Review
Coenzyme Q and Its Role in the Dietary Therapy
against Aging
Alfonso Varela-López 1, Francesca Giampieri 2, Maurizio Battino 2, 3, * and José L. Quiles 1, *
1Department of Physiology, Institute of Nutrition and Food Technology “José Mataix”,
Biomedical Research Center (CIBM), University of Granada, Avda. del Conocimiento s.n., Armilla,
Granada 18100, Spain; avarelalopez@gmail.com
2Dipartimento di Scienze Cliniche Specialistiche ed Odontostomatologiche (DISCO), Facoltà di Medicina,
Università Politecnica delle Marche, Ancona 60131, Italy; f.giampieri@univpm.it
3Centre for Nutrition & Health, Universidad Europea del Atlantico (UEA), Santander 39011, Spain
*Correspondence: m.a.battino@univpm.it (M.B.); jlquiles@ugr.es (J.L.Q.);
Tel.: +39-071-2204646 (M.B.); +34-958-240057 (ext. 20316) (J.L.Q.)
Academic Editor: Derek J. McPhee
Received: 9 February 2016 ; Accepted: 11 March 2016 ; Published: 18 March 2016
Abstract:
Coenzyme Q (CoQ) is a naturally occurring molecule located in the hydrophobic domain
of the phospholipid bilayer of all biological membranes. Shortly after being discovered, it was
recognized as an essential electron transport chain component in mitochondria where it is particularly
abundant. Since then, more additional roles in cell physiology have been reported, including
antioxidant, signaling, death prevention, and others. It is known that all cells are able to synthesize
functionally sufficient amounts of CoQ under normal physiological conditions. However, CoQ
is a molecule found in different dietary sources, which can be taken up and incorporated into
biological membranes. It is known that mitochondria have a close relationship with the aging process.
Additionally, delaying the aging process through diet has aroused the interest of scientists for many
years. These observations have stimulated investigation of the anti-aging potential of CoQ and its
possible use in dietary therapies to alleviate the effects of aging. In this context, the present review
focus on the current knowledge and evidence the roles of CoQ cells, its relationship with aging, and
possible implications of dietary CoQ in relation to aging, lifespan or age-related diseases.
Keywords: aging; antioxidants; diet; mitochondria; nutrition; oxidative stress; ubiquinone
1. Coenzyme Q: Structure, Localization and Forms
Coenzyme Q (CoQ) is a naturally occurring molecule formed from the conjugation of
a benzoquinone ring with a hydrophobic isoprenoid chain of varying chain length, depending on
the species [
1
–
3
]. It is mainly located in the hydrophobic domain of the phospholipid bilayer of
the inner membrane system of the mitochondria, but it is also present in all the other biological
membranes at significant levels [
4
–
9
], as well as in plasma lipoproteins [
10
]. Moreover, CoQ is
found in every plant and animal cell [
2
,
11
]. Due to its ubiquitous presence in nature and its quinone
structure, it is also called ubiquinone [
2
,
12
]. Ubiquinone is referred to as “coenzyme” because of
its unique ability to participate in chemical reactions but it remains at steady-state levels in the
cell [3,13]
. There are different ubiquinone molecules that are classified based on the length of their
isoprenoid side chain with a subscript indicating the number of carbons in the chain (CoQ
n
) [
1
,
3
].
CoQ
9
(2,3-dimethoxy-5-methyl-6-noneprenyl-1,4-benzoquinone) is the predominant form in rats
and mice, whereas in humans and other long-living mammals, the major homologue is CoQ
10
(2,3-dimethoxy-5-methyl-6-decaprenyl-1,4-benzoquinone) [
2
,
3
,
14
]. The importance of the length of
the polyisoprenyl chain is related to the stability of the molecule within the hydrophobic lipid bilayer.
Molecules 2016,21, 373; doi:10.3390/molecules21030373 www.mdpi.com/journal/molecules
Molecules 2016,21, 373 2 of 26
In addition, this feature seems to affect other properties, such as mobility, intermolecular interaction
with membrane proteins, and autoxidizability [3,15,16].
The benzoquinone ring of CoQ can assume three alternate redox states due to the existence of
different possible levels of protonation yielding three alternative CoQ forms (Figure 1): the fully
oxidized (CoQ) or ubiquinone, the fully reduced (CoQH
2
) or ubiquinol and the partially reduced
(CoQH) or ubisemiquinone [
17
,
18
]. Because of its extreme hydrophobicity, it is possible to find natural
CoQ in three physical states: dissolved in lipid bilayers, forming micellar aggregates, or bound to
proteins. In cells, CoQ is distributed between the two first states [
18
], whereas the importance of the
last one is only experimental [
17
]. All cells are able to synthesize functionally sufficient amounts of this
molecule under normal physiological conditions [
19
]. However, the content of CoQ as well as the ratios
between its forms are different depending on the analyzed species, tissue or even organelle. For instance,
murine kidney and heart show higher CoQ levels than brain or liver homogenates [
20
,
21
]. Likewise, it
has been reported that lysosomes and Golgi membranes contain relatively higher concentrations of CoQ
than mitochondrial membranes or microsomes [
22
,
23
]. In non-mitochondrial biological membranes,
CoQ continuously cycles between reduced and oxidized states thanks to different enzymes with CoQ
reductase activity [
24
]. These enzymes are NAD(P)H dehydrogenases which form part of the plasma
membrane redox system, an electron transport where CoQ acts as a mediator accepting electrons
from cytosolic NAD(P)H [
6
]. This system has been related to the maintenance of intracellular redox
homeostasis, membrane antioxidant protection, regulation of cell signaling and other functions [
6
] that
will be discussed below.
Molecules 2016, 21, 373 2 of 24
affect other properties, such as mobility, intermolecular interaction with membrane proteins, and
autoxidizability [3,15,16].
The benzoquinone ring of CoQ can assume three alternate redox states due to the existence of
different possible levels of protonation yielding three alternative CoQ forms (Figure 1): the fully
oxidized (CoQ) or ubiquinone, the fully reduced (CoQH2) or ubiquinol and the partially reduced
(CoQH) or ubisemiquinone [17,18]. Because of its extreme hydrophobicity, it is possible to find natural
CoQ in three physical states: dissolved in lipid bilayers, forming micellar aggregates, or bound to
proteins. In cells, CoQ is distributed between the two first states [18], whereas the importance of the
last one is only experimental [17]. All cells are able to synthesize functionally sufficient amounts of
this molecule under normal physiological conditions [19]. However, the content of CoQ as well as
the ratios between its forms are different depending on the analyzed species, tissue or even organelle.
For instance, murine kidney and heart show higher CoQ levels than brain or liver homogenates [20,21].
Likewise, it has been reported that lysosomes and Golgi membranes contain relatively higher
concentrations of CoQ than mitochondrial membranes or microsomes [22,23]. In non-mitochondrial
biological membranes, CoQ continuously cycles between reduced and oxidized states thanks to
different enzymes with CoQ reductase activity [24]. These enzymes are NAD(P)H dehydrogenases
which form part of the plasma membrane redox system, an electron transport where CoQ acts as a
mediator accepting electrons from cytosolic NAD(P)H [6]. This system has been related to the
maintenance of intracellular redox homeostasis, membrane antioxidant protection, regulation of cell
signaling and other functions [6] that will be discussed below.
Figure 1. Alternative redox forms of Coenzyme Q.
2. Physiological Roles of CoQ
2.1. Mitochondrial CoQ Is an Essential Factor for Cell Bioenergetics
CoQ was isolated and characterized in 1955 and only two years later it was shown to be an
essential mitochondrial electron transport chain (mtETC) component [25] (Figure 2). Thus, CoQ is an
essential factor for cell bioenergetics [6]. It acts as carrier of electrons from respiratory complexes I
and II to complex III [23,26–29]. Additionally, CoQ also accepts electrons from other donors including
dihydroorotate dehydrogenase, and acyl-CoA dehydrogenase. Since it has several electron donors
but only transfers them to complex III, it may be considered a crossroad in the mitochondrial electron
transport activity. Therefore, CoQ continuously cycles between the reduced and the oxidized forms in
mitochondria, but it is also bound to proteins and in a membrane free form. The sum of CoQ in any
of these possible states constitutes the mitochondrial CoQ pool [23]. In relation to this, Lopez-LLuch
et al. [6] have proposed that an equilibrated CoQ pool may perform a better electron flow adaptation
than a higher or lower CoQ pool by keeping a better mitochondrial homeostasis control. In addition,
it has been reported that protein-bound CoQ participates in and is needed for the maintenance of
complex III stability in yeast [30]. According to this finding, CoQ levels in mitochondria would not
only affect cell bioenergetics by its participation as coenzyme in mtETC, but also could affect protein
complex activity and structure [6]. Moreover, by means of plasma membrane redox system, CoQ is
involved in balancing the NAD+:NADH ratio in cells, which also regulates cell bioenergetics [31].
Figure 1. Alternative redox forms of Coenzyme Q.
2. Physiological Roles of CoQ
2.1. Mitochondrial CoQ Is an Essential Factor for Cell Bioenergetics
CoQ was isolated and characterized in 1955 and only two years later it was shown to be an essential
mitochondrial electron transport chain (mtETC) component [
25
] (Figure 2). Thus, CoQ is an essential
factor for cell bioenergetics [
6
]. It acts as carrier of electrons from respiratory complexes I and II
to complex III [
23
,
26
–
29
]. Additionally, CoQ also accepts electrons from other donors including
dihydroorotate dehydrogenase, and acyl-CoA dehydrogenase. Since it has several electron donors
but only transfers them to complex III, it may be considered a crossroad in the mitochondrial electron
transport activity. Therefore, CoQ continuously cycles between the reduced and the oxidized forms in
mitochondria, but it is also bound to proteins and in a membrane free form. The sum of CoQ in any of
these possible states constitutes the mitochondrial CoQ pool [
23
]. In relation to this,
Lopez-LLuch et al. [6]
have proposed that an equilibrated CoQ pool may perform a better electron flow adaptation than
a higher or lower CoQ pool by keeping a better mitochondrial homeostasis control. In addition, it has
been reported that protein-bound CoQ participates in and is needed for the maintenance of complex
III stability in yeast [
30
]. According to this finding, CoQ levels in mitochondria would not only affect
cell bioenergetics by its participation as coenzyme in mtETC, but also could affect protein complex
activity and structure [
6
]. Moreover, by means of plasma membrane redox system, CoQ is involved in
balancing the NAD+:NADH ratio in cells, which also regulates cell bioenergetics [31].
Molecules 2016,21, 373 3 of 26
Molecules 2016, 21, 373 3 of 24
Figure 2. Different activities exerted by coenzyme Q in cells: (a) coenzyme Q has a central role in
mitochondrial electron transport chain; (b) plasma membrane redox system components; (c) antioxidant
roles of coenzyme Q (this can occurs in all membranes where ubiquinol is present); (d) coenzyme Q
prevent permeability transition pore opening; (e) coenzyme Q participates in lysosomal pH maintenance;
(f) coenzyme Q stimulatory ( ) or inhibitory ( ) effects on gene expression. Abbreviations:
α-tp: α-tocopherol; α-tp
*−
: α-tocopheryl anion radical; CoQ: oxidized coenzyme Q; CoQ·H: partially
reduced coenzyme Q; CoQH
2
: fully reduced coenzyme Q; Cytb5R: NADH: cytochrome b5 reductase; e
−
:
electron; FAD
+
: reduced flavine adenine dinucelotide; FADH
2
: reduced flavine adenine dinucleotide;
H
2
O
2
: hydrogen peroxide; LOO
*
: lipid hydroperoxyl radical; LOOH: lipid hydroperoxyde; MAPKs:
mitogen-activated protein kinases; mPTP: mitochondrial permeability transition pore; NAD
+
: oxidized
nicotine adenine dinucleotide; NADH: reduced nicotine adenine dinucleotide; NQO1: NAD(P)H:Ubiquinone
oxidase 1; O
2·−
: superoxide anion; ROS: reactive oxygen species; SIRT1: sirtuin1.
2.2. CoQ Acts as Lipid-Soluble Antioxidant in All Biological Membranes
One of the main functions of CoQ in biological membranes is as an antioxidant [6] (Figure 2).
Furthermore, it constitutes the only lipid-soluble antioxidant endogenously synthesized that has
shown to efficiently prevent oxidation of proteins, lipids and DNA [32]. In this role, CoQ can act as
antioxidant by itself or as free radical quencher [11,16,33]. It also participates in the regeneration of
other antioxidants such as ascorbate [6,34,35] and α-tocopherol [33,36]. In particular, it has been
proposed that the antioxidant effect of quinones in mitochondrial membranes is mediated by
α-tocopherol recycling [6,37,38]. By itself, reduced CoQ
10
has also demonstrated to be an important
physiological lipid-soluble antioxidant [39]. In contrast to other antioxidants, ubiquinol can inhibit
both the initiation and propagation of lipid peroxidation by reacting with perferryl radical and
radicals generating ubisemiquinone and a non-radical lipid hydroperoxide [19,25]. Physiological
Figure 2.
Different activities exerted by coenzyme Q in cells: (
a
) coenzyme Q has a central
role in mitochondrial electron transport chain; (
b
) plasma membrane redox system components;
(
c
) antioxidant roles of coenzyme Q (this can occurs in all membranes where ubiquinol is present);
(
d
) coenzyme Q prevent permeability transition pore opening; (
e
) coenzyme Q participates in lysosomal
pH maintenance; (
f
) coenzyme Q stimulatory (
Molecules 2016, 21, 373 3 of 24
Figure 2. Different activities exerted by coenzyme Q in cells: (a) coenzyme Q has a central role in
mitochondrial electron transport chain; (b) plasma membrane redox system components; (c) antioxidant
roles of coenzyme Q (this can occurs in all membranes where ubiquinol is present); (d) coenzyme Q
prevent permeability transition pore opening; (e) coenzyme Q participates in lysosomal pH maintenance;
(f) coenzyme Q stimulatory ( ) or inhibitory ( ) effects on gene expression. Abbreviations:
α-tp: α-tocopherol; α-tp
*−
: α-tocopheryl anion radical; CoQ: oxidized coenzyme Q; CoQ·H: partially
reduced coenzyme Q; CoQH
2
: fully reduced coenzyme Q; Cytb5R: NADH: cytochrome b5 reductase; e
−
:
electron; FAD
+
: reduced flavine adenine dinucelotide; FADH
2
: reduced flavine adenine dinucleotide;
H
2
O
2
: hydrogen peroxide; LOO
*
: lipid hydroperoxyl radical; LOOH: lipid hydroperoxyde; MAPKs:
mitogen-activated protein kinases; mPTP: mitochondrial permeability transition pore; NAD
+
: oxidized
nicotine adenine dinucleotide; NADH: reduced nicotine adenine dinucleotide; NQO1: NAD(P)H:Ubiquinone
oxidase 1; O
2·−
: superoxide anion; ROS: reactive oxygen species; SIRT1: sirtuin1.
2.2. CoQ Acts as Lipid-Soluble Antioxidant in All Biological Membranes
One of the main functions of CoQ in biological membranes is as an antioxidant [6] (Figure 2).
Furthermore, it constitutes the only lipid-soluble antioxidant endogenously synthesized that has
shown to efficiently prevent oxidation of proteins, lipids and DNA [32]. In this role, CoQ can act as
antioxidant by itself or as free radical quencher [11,16,33]. It also participates in the regeneration of
other antioxidants such as ascorbate [6,34,35] and α-tocopherol [33,36]. In particular, it has been
proposed that the antioxidant effect of quinones in mitochondrial membranes is mediated by
α-tocopherol recycling [6,37,38]. By itself, reduced CoQ
10
has also demonstrated to be an important
physiological lipid-soluble antioxidant [39]. In contrast to other antioxidants, ubiquinol can inhibit
both the initiation and propagation of lipid peroxidation by reacting with perferryl radical and
radicals generating ubisemiquinone and a non-radical lipid hydroperoxide [19,25]. Physiological
) or inhibitory (
Molecules 2016, 21, 373 3 of 24
Figure 2. Different activities exerted by coenzyme Q in cells: (a) coenzyme Q has a central role in
mitochondrial electron transport chain; (b) plasma membrane redox system components; (c) antioxidant
roles of coenzyme Q (this can occurs in all membranes where ubiquinol is present); (d) coenzyme Q
prevent permeability transition pore opening; (e) coenzyme Q participates in lysosomal pH maintenance;
(f) coenzyme Q stimulatory ( ) or inhibitory ( ) effects on gene expression. Abbreviations:
α-tp: α-tocopherol; α-tp
*−
: α-tocopheryl anion radical; CoQ: oxidized coenzyme Q; CoQ·H: partially
reduced coenzyme Q; CoQH
2
: fully reduced coenzyme Q; Cytb5R: NADH: cytochrome b5 reductase; e
−
:
electron; FAD
+
: reduced flavine adenine dinucelotide; FADH
2
: reduced flavine adenine dinucleotide;
H
2
O
2
: hydrogen peroxide; LOO
*
: lipid hydroperoxyl radical; LOOH: lipid hydroperoxyde; MAPKs:
mitogen-activated protein kinases; mPTP: mitochondrial permeability transition pore; NAD
+
: oxidized
nicotine adenine dinucleotide; NADH: reduced nicotine adenine dinucleotide; NQO1: NAD(P)H:Ubiquinone
oxidase 1; O
2·−
: superoxide anion; ROS: reactive oxygen species; SIRT1: sirtuin1.
2.2. CoQ Acts as Lipid-Soluble Antioxidant in All Biological Membranes
One of the main functions of CoQ in biological membranes is as an antioxidant [6] (Figure 2).
Furthermore, it constitutes the only lipid-soluble antioxidant endogenously synthesized that has
shown to efficiently prevent oxidation of proteins, lipids and DNA [32]. In this role, CoQ can act as
antioxidant by itself or as free radical quencher [11,16,33]. It also participates in the regeneration of
other antioxidants such as ascorbate [6,34,35] and α-tocopherol [33,36]. In particular, it has been
proposed that the antioxidant effect of quinones in mitochondrial membranes is mediated by
α-tocopherol recycling [6,37,38]. By itself, reduced CoQ
10
has also demonstrated to be an important
physiological lipid-soluble antioxidant [39]. In contrast to other antioxidants, ubiquinol can inhibit
both the initiation and propagation of lipid peroxidation by reacting with perferryl radical and
radicals generating ubisemiquinone and a non-radical lipid hydroperoxide [19,25]. Physiological
) effects on gene expression.
Abbreviations:
α
-tp:
α
-tocopherol;
α
-tp*
´
:
α
-tocopheryl anion radical; CoQ: oxidized coenzyme
Q; CoQ
¨
H: partially reduced coenzyme Q; CoQH
2
: fully reduced coenzyme Q; Cytb5R: NADH:
cytochrome b5 reductase; e
´
: electron; FAD
+
: reduced flavine adenine dinucelotide; FADH
2
: reduced
flavine adenine dinucleotide; H
2
O
2
: hydrogen peroxide; LOO*: lipid hydroperoxyl radical; LOOH:
lipid hydroperoxyde; MAPKs: mitogen-activated protein kinases; mPTP: mitochondrial permeability
transition pore; NAD
+
: oxidized nicotine adenine dinucleotide; NADH: reduced nicotine adenine
dinucleotide; NQO1: NAD(P)H:Ubiquinone oxidase 1; O
2
¨ ´
: superoxide anion; ROS: reactive oxygen
species; SIRT1: sirtuin1.
2.2. CoQ Acts as Lipid-Soluble Antioxidant in All Biological Membranes
One of the main functions of CoQ in biological membranes is as an antioxidant [
6
] (Figure 2).
Furthermore, it constitutes the only lipid-soluble antioxidant endogenously synthesized that has shown
to efficiently prevent oxidation of proteins, lipids and DNA [
32
]. In this role, CoQ can act as antioxidant
by itself or as free radical quencher [
11
,
16
,
33
]. It also participates in the regeneration of other
antioxidants such as ascorbate [
6
,
34
,
35
] and
α
-tocopherol [
33
,
36
]. In particular, it has been proposed
that the antioxidant effect of quinones in mitochondrial membranes is mediated by
α
-tocopherol
recycling [
6
,
37
,
38
]. By itself, reduced CoQ
10
has also demonstrated to be an important physiological
lipid-soluble antioxidant [
39
]. In contrast to other antioxidants, ubiquinol can inhibit both the initiation
and propagation of lipid peroxidation by reacting with perferryl radical and radicals generating
ubisemiquinone and a non-radical lipid hydroperoxide [
19
,
25
]. Physiological ubisemiquinones having
long side chains do not react with oxygen, except in complex III under the very special conditions of
the CoQ-cycle [
18
]. On the other hand, short-chain ubiquinones have a pro-oxidant effect in complex I.
Molecules 2016,21, 373 4 of 26
Prevention of the lipid peroxidation propagation by the quenching of the initiating perferryl radicals
also protects proteins from oxidation. In relation to this, it is also important to point out that ubiquinol
effectively regenerates vitamin E from the
α
-tocopheroxyl radical which additionally contributes to
slow the propagation step of lipid peroxidation. Lastly, CoQ has also been shown to prevent DNA base
oxidation, which is particularly important in the case of mtDNA [
19
]. Furthermore, deficiency in some
nutrients considered as antioxidants like vitamin E and selenium could be compensated by the induction
of the CoQ-dependent antioxidant system [
6
]. This has been observed in hepatocytes where both, CoQ
levels and CoQ reductases, have been shown to be affected by this compensatory mechanism [
40
].
The latter reinforces the role of CoQ as antioxidant particularly under some physiological situations such
as nutritional deficiencies, which is particularly interesting because it is endogenously synthesized.
Explanations for the exceptionally high efficiency of CoQ as antioxidant include its intramembranous
localization, its general and abundant distribution and its effective reduction/reactivation by a number of
cellular systems [
19
,
25
]. Firstly, its localization is of central significance since
¨
OH and O
2
¨ ´
generated
in the membrane would otherwise rapidly react with neighboring lipid and protein molecules that
necessitate the availability of effective protective agents close to the site of radical production [
19
].
Secondly, several effective enzymatic systems catalyze CoQ reduction to achieve its active (reduced)
form in eukaryotic cells [
32
]. In mitochondria, the reduced antioxidant form of CoQ is regenerated
by the respiratory chain [
19
]. Besides the mitochondrial respiratory chain, other enzymes with such
function have been found. These include NADH-cytochrome b5 reductase that can reduce CoQ through
a one-electron reaction mechanism [
40
,
41
] and the soluble enzyme NAD(P)H-quinone oxidoreductase
1 (NQO1) that can reduce quinones by a two-electron reaction [
18
,
42
]. This enzyme is induced under
oxidative challenge and can maintain the electron flow through the plasma membrane redox system
to CoQ when this system is working mainly as antioxidant [
43
,
44
]. In addition, a distinct cytosolic
NADPH-CoQ reductase different from NQO1 has also been described [
38
], which seems to be a main
factor for non-mitochondrial CoQ reduction [
45
–
47
]. However, more studies are needed to clarify its
role and regulation under oxidative stress [6].
Lastly, it is important to note that ubiquinol is also present in lipoproteins, where it exerts its
antioxidant activity. In particular, it has been reported that ubiquinol is the most efficient antioxidant
in low-density lipoproteins (LDL) [
48
] which also contain
α
-tocopherol [
19
,
48
]. As a consequence of
LDL protection from oxidation, CoQ would also have anti-atherosclerotic properties.
2.3. CoQ Collaborates in Redox State Regulation by Plasma Membrane Redox System Activity
It has been proposed that plasma membrane redox system activity becomes essential in
the maintenance of bioenergetics in cells when activity in mitochondria decreases, as occurs in
aging [
6
,
49
] (Figure 2). Actually, the plasma membrane redox system is up-regulated in cells lacking
functional mitochondria [
31
,
50
,
51
]. In more detail, it has been reported that the amount of CoQ in
plasma membrane increases and CoQ-dependent reductases are induced after mtDNA removal that
induces the accumulation of cytosolic NADH. This mechanism allows cells to maintain the cytosolic
NAD
+
:NADH ratio in response to increase in cytosolic NADH [
31
,
51
–
53
] which is necessary for
glycolysis to occur correctly. CoQ is also able to act as an oxidant by autoxidation of ubisemiquinone
form [
11
,
16
,
33
]. Based on its apparently paradoxical property to potentially act both as a pro-oxidant
and an antioxidant, Sohal and Forster [
23
] suggest that CoQ may also be a modulator of the cellular
redox state under physiological and/or pathological conditions affecting the aging process in this way.
2.4. CoQ Participates in Cell Signaling by Plasma Membrane Redox System
The activity of plasma membrane redox system also plays an important role in growth and
development of organisms [
6
,
33
]. In this sense, CoQ-dependent NADH oxidases of plasma membrane
have been shown to be involved in regulation of cell growth and differentiation. In particular,
Gómez-Díaz et al. [
34
] found that the activity of CoQ reductases in plasma membrane is modulated
during erythrocyte differentiation. Likewise, the vitamin D
3
-induced differentiation of myeloid cells
Molecules 2016,21, 373 5 of 26
to monocytes is enhanced when this system is activated by ascorbate [53,54]. Similarly, in serum-free
cultures, addition of CoQ induces cell growth [
55
]. In support of the role of plasma membrane redox
system in cell signaling, it has been reported that extracellular signaling molecule growth factors,
insulin and pituitary extracts activated NADH oxidoreductases of plasma membrane [
56
]. At least
in the case of myeloid cell differentiation, it has been observed that modulation of the differentiation
program by plasma membrane redox system activation includes modulation of intracellular second
messengers and regulation of the activity of transcription factors [
54
,
57
]. Findings from studies on this
topic suggest different mechanisms by means of which CoQ might exert these effects on cell signaling,
which include the following.
1.
Reactive oxygen species (ROS): As previously stated, NAD(P)H dehydrogenases found in plasma
membrane, cytochrome b5-reductase and other NAD(P)H dehydrogenases found in plasma
membrane, reduce CoQ in a one-electron mechanism yielding semiquinone forms [
58
]. These CoQ
forms have pro-oxidant activity of generating O
2
¨ ´
or H
2
O
2
that would act as second messengers
on cell signaling mechanisms. Consequently, these would modulate different cell responses
affecting cell growth and differentiation processes [59].
2.
Tyrosine kinases: It has been suggested that the redox state of CoQ in plasma membrane could
control the activity of tyrosine kinases indirectly by generation of H
2
O
2
and further inactivation of
protein phosphatases, or directly by induction of redox-dependent conformational changes [
6
,
60
].
3.
Voltage-dependent anion channel (VDAC) proteins: As in mitochondria, plasma membrane also
contains proteins belonging to the VDAC protein family. One of the components of this family,
VDAC1, can function as NADH-ferricyanide reductase, an activity associated to the plasma
membrane redox system [
61
]. Taken into consideration that CoQ is involved in the regulation of
VDAC/permeability transition pore in mitochondria [
62
], López-Lluch et al. [
6
] have suggested
a putative relationship of the activity of CoQ in plasma membrane and cell signaling linked to
VDAC1 to be considered in future research.
4.
NAD+-dependent deacetylases: This group of proteins implicated in genetic expression, such as
sirtuins, could be affected and regulated in some manner by the activity of NADH-dependent
reductase in plasma membrane [
63
]. It has been proposed that the variations in activity of
CoQ-dependent NADH oxidoreductases in the different biological membranes could also regulate
sirtuins because of the effects on redox state [6].
5.
Mg
2+
-dependent neutral sphingomyelinase: It has been reported that CoQ-dependent plasma
membrane redox system is involved in inhibition of Mg
2+
-dependent neutral sphingomyelinase
after oxidative stress [
64
–
66
]. This is an integral plasma membrane protein involved in the release
of ceramide from plasma membrane sphingomyelin and participates in cell signaling, apoptosis,
and the modulation of cell responses [6,67].
2.5. CoQ Exerts Anti-Inflammatory Effects through Its Antioxidant Activity
CoQ exerts multiple anti-inflammatory effects by influencing the expression of Nuclear factor
κ
B (NF-
κ
B)1-dependent genes [
68
]. Likewise, H
2
O
2
has been identified as an activator of the
pro-inflammatory trancription factor NF-
κ
B [
69
]. In view of the antioxidant properties of the reduced
form of CoQ
10
and the effective enzymatic conversion of oxidized CoQ
10
into its reduced form,
CoQ
10
might mediate its observed anti-inflammatory effects via gene expression (Figure 2). For the
study of the expression of genes implicated in inflammatory processes, in vitro models based on cells
triggered with lipopolysaccharide (LPS) have aroused interest. This is because this bacterial product
induces downstream signaling cascades of the transcription factor NF-
κ
B, which in turn leads to the
induction of inflammatory genes. In this context, it has been reported that CoQ
10
down-regulates
LPS-inducible genes in the monocytic cell line THP-1, presumably due to its antioxidant impact on
gene expression [
70
]. Thus, as Genova and Lenaz [
18
] indicated, it is likely that all effects of CoQ at the
genetic level may be mediated by its antioxidant effect.
Molecules 2016,21, 373 6 of 26
2.6. Mitochondrial CoQ Prevents Events Leading to Programmed Cell Death
Different studies have confirmed the protective role of CoQ
10
against apoptosis by inducing the
inhibition of cell death independently from its free radical scavenging properties or antioxidant effects [62,71,72]
.
Presumably, this occurs by direct inhibition [
62
] of the opening of the permeability transition pores
a high-conductance protein channel located in the internal mitochondrial membrane [
73
] (Figure 2).
Permeability transition pore opening by its inducers has shown to lead to cell death [
74
]. Opening of
the mitochondrial membrane transition pore depolarizes the mitochondrion [
18
], but also allows
the translocation of molecules as large as 1500 Da in size [
62
] leading to the release of proteins
present in mitochondrial intermembrane space into the cytosol. These last constitute different
factors that trigger the process of programmed cell death like cytochrome c [
18
]. Thus, CoQ would
counteract other downstream apoptotic events such as ATP depletion, caspase-9 activation, and DNA
fragmentation [
75
,
76
]. Moreover, the increase of mitochondrial CoQ levels has been related to the lack
of induction of permeability transition pore opening in diabetic rats [77].
To elucidate the possible mechanism under this effect, it is important to point out that permeability
transition pore shows an ubiquinone binding site where different (natural and artificial) forms of
CoQ interact stabilizing the pore in the closed conformation [
75
,
76
]. In particular, quinones have
also been shown to exert a direct effect on permeability transition pores. These compounds are able
to modulate the permeability transition pore through a common binding site rather than through
oxidation-reduction reactions. Occupancy of this site can modulate the permeability transition pore
open-closed transitions, possibly through secondary changes of the Ca
2+
-binding affinity for the
pore [
18
]. Based on their relation with permeability transition pores, Walter et al. [
76
] distinguish
three functional classes of quinone analogs:
‚permeability transition pores inhibitors, like CoQ0,CoQ2, and decylubiquinone.
‚
permeability transition pores inducers, like idebenone 2,3-dimethoxy-5-methyl-6-(10-hydroxydecyl)-
1,4-benzoquinone).
‚
permeability transition pores-inactive quinones, which counteract the effects of both inhibitors
and inducers, such as CoQ1.
An indirect study [
78
] suggests that CoQ
10
may be a permeability transition pore inhibitor.
These authors exposed SHSY5Y neuroblastoma cells to neurotoxic
β
-amyloid peptides and oxygen-glucose
deprivation to investigate the neuroprotective effect of CoQ
10
. In these neuronal cells CoQ
10
increased
resistance against
β
-amyloid peptides-induced cell death that was related to the regulation of
permeability transition pore opening. In addition, a decrease of O
2
¨ ´
production was also noted [
78
].
Similar studies indicated a protective effect of CoQ
10
on permeability transition pore opening against
amitriptyline toxicity [
79
]. However, because ROS signaling could be implicated in permeability
transition pore opening, it is not yet fully clear whether these CoQ
10
effects on the transition pore
depend on a direct mechanism or are mediated by the antioxidant effect [18].
2.7. CoQ Present in Lysosomal Membrane Participates in pH Maintenance
The NADH-dependent CoQ reductase present in lysosomal membrane is implicated in H
+
translocation from cytosol to lysosomal lumen by means of an ATP-independent mechanism having
O
2
as the final electron acceptor [
80
] (Figure 2). As expected, this activity contributes to the maintenance
of acidic pH into lysosomes. However, the exact role of this enzyme on lysosomal activity is currently
under study [6,81].
3. Endogenous and Exogenous Sources of CoQ
3.1. CoQ Biosynthesis
CoQ endogenous bionsynthesis requires the synthesis of both a benzoquinone ring and
an isoprenoid side chain. The precursor of the benzoquinone ring is 4-hydroxybenzoate that is
Molecules 2016,21, 373 7 of 26
synthesized from tyrosine or, at least theoretically, from phenylalanine [
19
]. Meanwhile, the isoprenoid
side chain is synthesized by a series of reactions starting from acetyl-CoA and ending up with farnesyl
pyrophosphate (farnesyl-PP), which comprises the mevalonate pathway [
24
,
82
,
83
]. Consequently, some
reactions involved in CoQ bionsynthesis are shared with cholesterol and other lipid compounds [
82
].
Both the final product of this pathway, farnesyl-pyrophosphate (farnesyl-PP), and the intermediary
of this pathway, isopentenyl pyrophosphate (isopentenyl-PP) are utilized for the synthesis of the
isoprenoid side-chain of CoQ [
19
]. Then, the long isoprenoid side-chain of CoQ (which contains
6–10 isoprene units in different species) is synthesized by trans-prenyltransferase, which condenses
farnesyl-PP with several molecules of isopentenyl-PP, all in the trans configuration [84].
Then, the 4-hydroxybenzoate-polyprenyl transferase, which is encoded by the gene Coq-2 in
humans [
85
], acts by catalyzing the condensation of the isoprenoid side-chain with 4-hydroxybenzoate.
After condensation, the benzoquinone ring undergoes a sequence of modifications including
C-hydroxylations, decarboxylation, O-methylation and C-methylation to synthesize CoQ. This sequence
has been studied mainly in bacteria and yeast. Meanwhile, in mammals, various functions of
genes possibly implicated in these modifications have been established through the complementary
recognition in yeast [
19
]. At the moment, six genes encoding different enzymes that catalyze reactions
in this sequence have been identified in humans (termed from Coq-3 to Coq-8) [
11
,
12
]. However, full
details of the synthesis of CoQ in animal tissues have not yet been clarified [19,86].
Endogenous CoQ
10
, synthesized using the radiolabelled precursor
14
C-para-hydroxybenzoate
(pHB), was first detected in mitochondria and later incorporated into mitochondria-associated
membranes and endoplasmic reticulum, from where it is transported to other membranes in the
cell [87]. This suggests that CoQ biosynthesis mostly occurs in mitochondria but it is redistributed to
other organelles. Although this process is largely unknown, as mentioned, variations in the amounts
present in different cellular organelles have been observed, with mitochondria, lysosomes and Golgi
vesicles showing the highest concentrations [
11
]. Despite the fact that all cells are able to produce CoQ,
its distribution is not uniform among different organs, being present at the highest concentrations
in heart, kidney and liver [
88
]. Likewise, CoQ amounts can also vary among different tissues and
structures of the same organ as occurs in bovine brain where CoQ
10
levels range from 3 pg/g in the
white matter to 25 pg/g in the striatum [
89
]. Moreover, interindividual variations in total CoQ
10
have been described, as well as a significant difference in total CoQ
10
between healthy males and
females [
90
]. All these variations would indicate that CoQ
10
concentrations are tightly distributed
around a homeostatic set point in both organs and individuals. In any case, all cells should functionally
synthesize sufficient amounts of CoQ under normal physiological conditions. Thus, in contrast to
cholesterol, no redistribution via uptake from the circulation is required. As already mentioned, the
liver releases a certain amount of CoQ that associates with very low-density lipoproteins (VLDLs), but
this pool is not redistributed to other organs [19,91].
Under normal circumstances, it has been assumed that there is a lack of dependence on exogenous
sources of CoQ, since all tissues are able to synthesize CoQ. However, situations may arise in which
some cell synthetic capacity is insufficient to reach its CoQ
10
requirements. Metabolically active
cells presumably have the highest requirements for CoQ
10
which explains that CoQ
10
deficiency
susceptibility appears to be greatest in them [
3
,
12
]. For this reason, deepening on our understanding
about CoQ synthesis regulation, it is interesting to deal with different pathologies. Hydroxybenzoate is
generally present in excess, so that the rate of this reaction is determined by the availability of the
polyisoprenoid chain [
19
]. Farnesyl-PP produced by mevalonate pathway is precursor for cholesterol,
CoQ, dolichol and isoprenylated proteins [
82
]. Moreover, the intermediary isopentenyl-PP is utilized
for the synthesis of the isoprenoid side-chain of CoQ, but also for the synthesis of dolichol. It has
been suggested that synthesis of all end-products of the mevalonate pathway is co-regulated since the
initial sequence of reactions leading to such lipids is identical [
19
]. However, terminal regulation must
also occur [
19
], which would explain greatly varying synthesis rates and different amounts found for
these lipids [
92
]. The terminal points of regulation probably involve the branch-point enzymes that
Molecules 2016,21, 373 8 of 26
have farnesyl-PP as substrate which are considered to be rate-limiting for the terminal portion of the
biosynthetic sequences [
93
]. These are squalene synthase, trans-prenyltransferase, cis-prenyltransferase
and farnesyl- or geranylgeraniol-protein transferases for cholesterol, CoQ, dolichol and isoprenylated
proteins. Traditionally, a central role in the regulation of the mevalonate pathway has been attributed
to 3-hydroxy-3-metylglutaryl-CoA (HMG-CoA) reductase, but it seems that its primary regulatory role
falls to cholesterol biosynthesis [
19
]. Since squalene synthase Michaellis constant (KM) for farnesyl-PP
is high, it is expected that the total amount of farnesyl-PP in the cell exerts its major influence on
cholesterol synthesis. Therefore, when this substrate concentration decreases, the enzyme is not
saturated and the cholesterol synthesis rate is reduced [
94
]. In contrast, all of the other branch-point
enzymes, i.e., trans- and cis-prenyltransferases, farnesyl- and geranylgeraniol-protein transferases,
exhibit low KMs and remain saturated even when the farnesyl-PP pool is smaller.
As in other metabolic pathways, it seems that endogenous compounds also play regulatory roles
on CoQ biosynthesis [
19
]. Epoxidated derivatives of certain all-trans polyisoprenoids, solanesol,
tocotrienols, vitamin K
2
and CoQ up-regulate CoQ synthesis in HepG2 cells by themselves.
Additionally, it has been reported that several of these compounds inhibit the biosynthesis of cholesterol
in the same cultures [
95
]. However, none of the epoxidated poly-cis polyisoprenoids, which occur
naturally without an epoxy group in large numbers, exhibit any effect on products of the mevalonate
pathway in this same system [
19
]. Tocotrienols exert the most profound effects on this biosynthetic
pathway. It has been observed that dolichol synthesis is also up-regulated but the importance of this
phenomenon has not been clarified since the function for non-phosphorylated form in animals has
not been established despite its abundance. In general, improved CoQ biosynthesis is achieved by
regulating entire biosynthetic machinery, while decreasing cholesterol synthesis specifically involves
the inhibition of oxidosqualene cyclase. Therefore, it has been suggested that small amounts of mono-
and diepoxides polyisoprenoids, as those having many organisms, could be biological regulators of
the mevalonate pathway [
19
]. At translational level, some molecules have been shown to stimulate
CoQ
10
synthesis by Coq-7 expression regulation [
96
,
97
]. In particular, it has been found that CoQ
10
biosynthesis is dependent on NF-
κ
B, which binds specifically to two binding sites present in the
5’-flanking region of the Coq-7 gene, inducing both the Coq-7 expression and CoQ
10
biosynthesis [
98
].
3.2. Dietary CoQ Has Shown to Increase CoQ Levels in Different Body Compartments
CoQ is a molecule naturally found in different dietary sources, which can be taken up from
intestinal lumen in a similar way to other lipids [
83
]. According to results from different studies in
rats [
96
–
103
], it has been historically considered that approximately 6% of orally administered CoQ
permeates the gastrointestinal tract into the blood and is transferred to liver and spleen. Consequently,
uptake in the whole body ranged between 2% and 3% of the total dose. Moreover, CoQ
10
has been
found in plasma, largely in the reduced form [
100
].
In vitro
studies with CaCo-2 models of absorption
have suggested that CoQ
10
is reduced to ubiquinol either during or following absorption [
104
]. In turn,
CoQ uptake by other tissues such as heart, kidney, brain and skeletal muscle was considered low or
completely absent [100,102,103], unless endogenous levels fall below a critical threshold.
In humans, different supplementation studies have been carried out [
105
–
114
] varying in duration
(20 days, 3 or even 16 months) and dosages (100–2400 mg/day), as well as in experimental group’s
characteristics. Despite the differences, all showed an increase of CoQ
10
levels in blood. Further, such
increase seems to be a dose-dependent one according to a study where the effects of supplementation
with 90, 150, and 300 mg/day of the reduced form of CoQ
10
were compared [
108
]. In elderly women it
was reported that the change was inversely associated with the baseline concentration [
107
], which
suggests that CoQ uptake is also affected by levels present in the body. However, when CoQ
10
concentration was assessed in other samples (homogenized muscle and mitochondria), no increase
was noted [
105
]. Regarding how duration of dietary treatment affects CoQ body levels, it was observed
that plasma ubiquinol concentration nearly reached steady-state by two weeks after the start of
treatment, and plasma ubiquinol levels returned to those before administration six months after
Molecules 2016,21, 373 9 of 26
completion of treatment [
108
]. In another study [
109
], maximum plasma CoQ
10
concentration was
reached in two weeks of supplementation with all-trans form of CoQ
10
and then decreased to basal
level after withdrawal, but CoQ
10
levels increased by a third compared to the study by Hosoe et al.
using ubiquinol [108].
Results from more recent studies in mice and rats [
20
,
21
,
110
,
111
,
115
–
122
] firmly refute this
long-held notion that CoQ content of tissues other than plasma, liver or spleen cannot be significantly
augmented by dietary administration of CoQ
10
. It was first shown by Matthews et al. [
110
] that
CoQ
10
intake in diets for two months by 12- or 24-months-old rats increased CoQ content in brain
mitochondria. Subsequently, a series of studies conducted by Sohal et al. demonstrated that
CoQ
10
administration (from 0.7 to 370 mg/Kg per day) via food to mice [
20
,
21
,
116
,
120
,
121
] and
rats [
113
,
115
–
119
] caused an increase in amounts of CoQ
10
in plasma and in tissue homogenates
and mitochondria of brain, heart, kidney, skeletal muscle and liver. In all tissues, the amount of
CoQ augmentation was greater in mitochondria than in the homogenate, suggesting its preferential
sequestration in mitochondria [
6
]. In brain, the increase was of a lesser magnitude and occurred
primarily in mitochondria and not in the homogenate. With this background, it seems more clear
that different tissues tend to vary in their capacity for CoQ accretion, with liver and skeletal muscle
exhibiting the highest elevations, and brain showing the lowest [
23
]. The mentioned studies in animals
were performed with relatively high daily dietary amounts of CoQ suggesting that higher plasma
CoQ
10
concentrations are necessary to facilitate uptake by peripheral tissues [
83
,
116
], which could
explains differences with earlier studies. However, it has also been observed that feeding rats with very
low-dosages of CoQ
10
from weaning leads to increase of mitochondrial CoQ levels of heart and liver in
12- and 24-months-old rats. Moreover, it has been observed that this difference observed between rats
increases as the animals aged [
118
,
123
]. Thus, age at beginning and duration of the treatment must be
taken into account too. Importantly, dietary CoQ
10
also leads to an increase of the rat specific form
CoQ9in all tissues where it was measured [20,21,118].
Notwithstanding, as Sohal and Forster [
23
] pointed out, the uptake of CoQ and other lipid
substances is a complex process dependent upon a considerable number of different factors. In that
sense, it has been reported that intestinal absorption is threefold faster if CoQ
10
is administrated with
food intake in rats [
124
]. In mice, the increase of plasma levels per 100 mg values was remarkably
higher with the reduced form of CoQ
10
when compared to the results obtained with the oxidized
form CoQ
10
[
22
]. Additionally, other aspects of CoQ form also seem to be important.
Villalba et al. [83]
reviewed the efficacy of a variety of commercial formulations that have been developed to solubilize
CoQ
10
and promote its better absorption
in vivo
, and its use in the therapy of pathologies associated
with low CoQ levels with emphasis in the results of the clinical trials. They concluded that the relative
bioavailability of CoQ
10
is dependent on the type and amounts of oil in the formulations as well as
on its delivery system, the order of decreasing bioavailability being: nanoparticulated, solubilized,
oil-emulsioned and powder. Some additional dietary factors have been shown to influence CoQ
contents in the body and they could also influence the magnitude of dietary CoQ effect. In that
sense, it has been reported that monounsarturated fatty acids (MUFA)-rich dietary fats increased
CoQ mitochondrial contents, whereas these decreased in diets rich in polyunsarturated fatty acids
(PUFA) [
125
]. In the same sense, it has been observed that a diet rich in saturated fat for 50 days
decreased CoQ levels in plasma and liver mitochondria of rabbits, which was restored by oral
administration of soluble CoQ10 (25 mg/kg per day) over 30 days after such period [126].
Regarding CoQ supplements safety, different assessments in human and animals reviewed by
Hidaka et al. [
127
] concluded that the endogenous biosynthesis of CoQ
10
is not influenced by exogenous
inputs. Moreover, it does not accumulate into plasma or tissues when supplementation ends. In rats,
a 52-week chronic toxicity study has indicated that the acceptable daily intake is 12 mg/kg per day,
calculated from the no-observed-adverse-effect level of 1200 mg/kg per day. In humans, clinical trials
suggest an observed safety level for CoQ
10
was 1200 mg/day per person. In addition, doses up to
3000 mg/day of CoQ
10
did not cause serious adverse effects in humans. However, several moderate
Molecules 2016,21, 373 10 of 26
adverse effects as nausea and other adverse gastrointestinal effects have been reported, although
these effects were not causally related to the active ingredient because there was no dose-response
relationship [
127
]. At very high amounts (2.6 mg/g), prolonged intake of CoQ
10
has shown to
exacerbated cognitive and sensory impairments in old mice. In turn, intake at lower amounts had no
discernable negative impact on these functions [128].
4. Evidence for CoQ as Anti-Aging Compound
4.1. CoQ Levels Are Affected by Aging
It has been proposed that along the entire life of organisms, the synthesis of CoQ changes and
is significantly reduced from the initial phases of aging [
10
,
129
]. This statement seems to contradict
findings from C. elegans where older individuals showed increases in CoQ content [
130
]. However,
measures of CoQ amount in mammals, mostly rodents, have differed. In mice, age-related decreases
of CoQ levels have been observed in liver homogenates [
130
] as well as in mitochondrial CoQ levels of
skeletal muscle, but not of kidney, brain or heart [
120
]. Like in mice, liver CoQ levels also decrease
with age in pigs (Sus scrofa domestica) [
130
]. However, mitochondria of liver, heart and kidney from
19-month-old ad libitum fed rats showed lower CoQ
9
than those from 4-month-old animals, although
this CoQ depletion was not detectable in tissue homogenates [
131
]. Authors attributed this fact to the
preferential sequestration of CoQ in the mitochondrial fraction. Similarly, no changes in the levels of
CoQ in rat brain after an increase during the first months of life have been reported [
132
], but in this
organ no changes were observed when only mitochondria were evaluated [
133
]. On the other hand,
an age-associated decrease of CoQ concentration was detected in pancreas and adrenal gland, brain,
heart and lung after an early increase like in brain reaching highest values at 30 days of age [
134
].
In an old study, CoQ increased between 2 and 18 months and decreased significantly at 25 months in
the heart and kidney, and the gastrocnemius, oblique and deep aspect (red) of vastus lateralis muscles.
On the other hand, CoQ concentration of rat liver increased over the life span, while it remained
relatively constant in brain, lung, and the superficial aspect (white) of the vastus lateralis muscle [
135
].
These results suggest that decrease could appear relatively later in life explaining why some studies
do not find clear age-associated changes.
Results of the various studies described do not support the existence of a common trend for all
living organisms, although age-associated changes in CoQ content are most evident in mitochondria
as was suggested by Sohal and Forster [
23
]. In mammals, CoQ levels tend to decrease with aging, but
this depends on tissues and organisms and probably many other factors. This could explain, at least in
part, the differences found in aging processes among different organisms, and also why some tissues
are more susceptible to aging or aging-related diseases. Studies evaluating the association between
age and CoQ concentration in humans have also supplied contradictory results. In elderly women, no
significant correlations were found between CoQ
10
plasma levels and age [
107
]. In human pancreas
and adrenal gland, CoQ
10
levels were highest at one year of age, and then they decrease, whereas in
the brain, heart and lung, the corresponding peak value was at 20 years of age and was followed by
a continuous decrease upon further aging [
134
]. Age-associated decrease in brain has been confirmed
by later studies [
136
,
137
]. In fact, it has been suggested that the mevalonate pathway is affected in the
aged brain [137].
4.2. Several Age-Related Pathologies Are Associated with Low Levels of CoQ
Diseases such as cardiovascular diseases, neuropathies, inflammation, metabolic syndrome, arthritis,
carcinogenesis, diabetes or hypercholesterolemia worsen during aging and are considered as major
age-related diseases [
6
]. Different studies have reported a beneficial effect of CoQ on them [
138
–
142
],
almost all in animal models. This fact has been taken into account to suggest that CoQ becomes
an essential factor in the maintenance of the normal activity of cells in such conditions [
6
]. However,
Molecules 2016,21, 373 11 of 26
it is important to note that some interventions in humans in relation to Parkinson disease [
112
] and
Amyotrophic Lateral Sclerosis [113] have not offered clear benefits for CoQ on disease progression.
These and other neurodegenerative diseases have been widely studied in this respect. Case-control
studies have reported lower CoQ
10
levels in serum of Lewy’s body disease patients [
143
] and plasma
of subjects with amyotrophic lateral sclerosis [
144
] compared with healthy controls. Likewise, lower
levels of this molecule have been found in cerebrospinal fluid of Alzheimer
´
s disease patients [
145
].
In addition, a negative correlation has been reported between CoQ levels and the duration of illness
in Amyotrophic Lateral Sclerosis [
145
], Alzheimer’s [
146
] and Parkinson’s disease case studies [
147
].
These observations, together with the decrease of mitochondrial activity and the increase of free
radical levels found in neuroneurodegenerative diseases [
147
–
150
], highlight the importance that
CoQ has in these processes as suggested by Lopez-Lluch et al. [
6
]. Actually, it has been reported that
NQO1 expression increases during the initial steps of Alzheimer’s disease, indicating a higher lipid
peroxidation coupled to a higher necessity for CoQ-dependent antioxidant activity [6,151].
Some authors have also focused their studies on a possible relationship between CoQ amount
and some metabolic syndrome components as risk factors of cardiovascular disease. An important
component associated to aging is hypercholesterolemia since it affects a significant part of the aged
population. In elderly women, no differences were seen between hyperlipidemic and normolipidemic
subjects in relation to serum CoQ
10
levels. Similarly, no significant correlations were found with body
mass index, another risk factor for cardiovascular disease [
108
]. However, as indicated above, CoQ
is the main antioxidant in LDL [
48
]. In relation to this, it has been shown that total CoQ
10
: total
cholesterol ratio was reduced in diabetes mellitus patients, another component of metabolic syndrome,
compared with subjects with normal glucose tolerance and impaired fasting glucose [152].
The development of certain types of cancer also could be related with low CoQ levels.
Epidemiological studies in humans have shown negative associations between CoQ levels in blood
and breast cancer [
153
,
154
], myeloma [
153
], melanoma [
155
], Graves’ disease or follicular and papillary
thyroid carcinomas [
156
]. Some studies have also suggested that tumoral cells have lower CoQ
levels than normal cells. In women presenting carcinomas and non-malignant-breast cancer, CoQ
10
concentrations in breast tumor tissues significantly decreased when compared to the surrounding
normal tissues [
157
]. Similarly, melanoma cell lines also presented low concentrations of CoQ
10
[
155
].
In relation to these groups of diseases, it has been found that CoQ treatment decreases cell growth of
a prostate cancer line (PC3 line) but not of non cancer cells (PNT2) and was associated with a high
production of ROS [158], suggesting an interesting use for this molecule in cancer therapy.
4.3. Aging, Development and Lifespan Are Associated to Changes in CoQ Bionsynthesis
In mammals, there are at least 10 different proteins participating in the biosynthesis of CoQ that
are encoded by Coq genes. These have both catalytic and regulatory activities. The first group is
encoded by the genes Coq-1, Coq-2, Coq-3, and Coq-7, whereas it has been suggested that Coq-4,
Coq-8 and Coq-9 gene products have regulatory functions [
6
,
84
]. Different studies using animal model
mutants for these genes have been carried out to elucidate the role of endogenous CoQ synthesis on
aging. C. elegans models have been extensively used in this way. clk-1 (mammals Coq-7 orthologue
gene) mutant nematodes that produce a very low amount of CoQ [
159
] showed an increased lifespan
compared to wild-type animals [
160
,
161
]. However, analysis of mutants in Coq genes, other than clk-1,
provided different results. In most of cases, Coq genes knockout showed deleterious defects leading
to early developmental arrest [
162
–
164
]. This has been reported for Coq-1, Coq-3, Coq-3 and Coq-8.
Extension of longevity by silencing Coq genes [
165
] occurs upon moderate low levels of global CoQ
content (up to 50%) but not in the case of more severe CoQ depletions as observed in Coq-8 [
162
] and
Coq-3 [
164
] mutants. To explain this difference it has been proposed that moderated CoQ depletion is
associated to lower ROS production extending lifespan [
165
], whereas a higher CoQ depletion would
lead to developmental and reproductive inefficiency [162,164].
Molecules 2016,21, 373 12 of 26
According to findings from the studies with C. elegans, it has also been suggested that Coq-7 and
CoQ synthesis is also related to aging in humans [
166
] and other mammals. However, experimental
evidence suggests that important differences could exist between mammals and C. elegans in this sense.
In contrast to observations in nematodes, murine Coq-7 knockout embryos arrest development at
midgestation [
167
]. Similarly, it has been reported that some deletions in Coq-7 affect mitochondrial
integrity and neurogenesis [
168
], which is comparable to some effects of CoQ deficiency found in
humans [
169
,
170
]. However, heterozygous mice for this gene carrying a single functional copy show
a notable increase in lifespan [171].
Mutations in genes implicated in CoQ biosynthesis have been identified as a cause of different
pathologies associated with CoQ deficiencies in humans, most of which appear early in life [
172
–
176
].
The most severe human CoQ
10
deficiencies are due to autosomal recessive mutations and can be
classified as primary deficiencies when mutations affect CoQ
10
biosynthetic genes, or secondary if the
cause is related to other genetic defects [
177
]. Hereditary CoQ
10
deficiencies caused by these mutations
usually lead to cardiomyopathies and degenerative muscle and neuronal diseases [
169
,
173
,
178
–
180
].
The major phenotypes provoked by CoQ
10
deficiencies are encephalomyopathy, severe infantile
multisystemic disease, cerebellar ataxia, Leigh syndrome with growth retardation, ataxia and deafness,
and isolated myopathy [181].
5. Studies on Dietary Therapies with CoQ on Aging
According to previous observations, dietary supplementation with CoQ
10
could constitute
an anti-aging strategy. In humans, there is evidence, mainly indirect, that exogenous orally
administered CoQ
10
may be incorporated into mitochondria, at least in conditions of partial CoQ
tissue deficiency, where it may enhance electron transfer and ATP synthesis with improvement of
pathological situations such as cardiac failure [
182
,
183
], Parkinson’s disease [
115
,
184
–
186
], Alzheimer’s
disease [187–189] and Friedreich’s ataxia [190].
Results from animal studies are not clear about the dietary CoQ effects on longevity. In C. elegans,
it has been reported that dietary CoQ prolonged lifespan [
162
], but this was also noted with
a CoQ-deficient diet [
191
]. One possible explanation for poor diet effect could be in the adaptability of
these nematodes to stressful conditions. C. elegans life span is extended by the intake of antimycin A,
an mtETC inhibitor, whereas this molecule is toxic to most other aerobic species [
192
]. According to
this observation, CoQ deficiency might induce a hypometabolic or a dauer-like state, which would
facilitate survival under adverse conditions. However, a study focused on features of bacteria used to
feed worms has evidenced that this phenomenon may be more complex. In this referred study, a diet
based on respiratory incompetent E. coli, regardless if they were CoQ-less or CoQ-replete, produced
a robust life extension in wild-type C. elegans [
193
]. An explanation for these observations was that the
fermentation-based metabolism of the E. coli diet is an important parameter of C. elegans longevity [
193
].
As in invertebrates, simple dietary CoQ supplementation has shown no direct concluding results
on lifespan extension in rodents. Information from this model indicates that CoQ
10
supplementation
with daily dosages ranged from 10 to 370 mg/Kg has no effect on longevity [
117
,
119
,
129
,
194
].
Despite the absence of evidence supporting that dietary CoQ can increase lifespan in animals,
some interventions in the same way have been found to retard certain aging detrimental aspects
in different animal models for aging or age-related diseases. In SAMP mice, a mouse model
for accelerated senescence and severe senile amyloidosis, life-long supplementation with CoQH
2
substantially decreased the senescence grading scores at different ages, although it did not alter some
age-associated features of the model like the senile amyloid deposition rate. Again, this intervention
did not have an effect on the lifespan [
195
]. In older mice with clear cognitive and psychomotor
impairments, short-time (15 days) CoQ-supplementation improved spatial learning [
196
]. A cardinal
event of diabetes like diabetic neuropathy has also been reported to be positively modified upon
CoQ administration in diabetic rats [
140
]. In the hypercholestolemic ApoE knockout mouse, dietary
CoQ had an anti-atherogenic effect preventing the accumulation of lipid peroxides in aorta [
139
].
Molecules 2016,21, 373 13 of 26
In turn, in most of cases beneficial effects of CoQ over mitochondrial function and oxidative
stress have been demonstrated [
139
,
143
,
196
,
197
]. However, long-term CoQ
10
intake in healthy
mice fed a standard diet failed to modulate mitochondrial respiratory capacity in liver or levels
of oxidative stress in liver, kidney, skeletal muscle or brain [
23
,
119
]. According to different findings,
López-LLuch et al. [6]
suggested that supplementation with CoQ is not needed when the organism
is young and healthy because cell membranes seem to be nearly saturated at the functional level.
However, this supplementation becomes necessary when the organism shows deficiency, as in aging.
This could explain why CoQ effects are clearer in animal models of disease than on lifespan of healthy
animals if they are related to low CoQ levels.
On the other hand, the combination of dietary treatments using CoQ supplements in certain
nutritional conditions associated with higher oxidative stress levels and age-related detrimental effects
offers interesting expectations. From this standpoint, studies comparing CoQ effects between isocaloric
diets with different lipid profile are particularly interesting. The effects of long-term supplementation
with daily CoQ
10
at 0.7 mg/kg on rats fed on MUFA-rich diets have been compared with those found
in n-6 PUFA-rich diets [
118
,
123
,
198
–
201
]. One of the most interesting findings from such studies was
that dietary CoQ
10
produced significant increases of mean and maximum lifespan in rats fed a diet rich
in n-6 PUFA [
123
,
199
,
200
]. At the histopathological level, when sunflower oil was the main fat in the
diet, CoQ supplementation seemed to improve endocrine pancreas structure and in particular
β
-cell
mass resembling positive effects of virgin olive oil [
201
]. Similar effects were noted in rat alveolar
bone loss associated to aging [
202
]. Dietary CoQ treatments have also been shown to be effective
in counteracting many of the high-fat diet consequences in animals [
129
,
203
–
208
]. In other mouse
models, post-weaning dietary supplementation with CoQ
10
rescued many of the detrimental effects of
nutritional programming on cardiac aging by low birth-weight and catch-up growth [197].
The biochemical basis of potential beneficial effects of CoQ on lifespan or other aging detrimental
effects may include enhancement of the cellular antioxidant protection systems in cell membranes,
where CoQ sustains lipids in its reduced redox state preventing lipid peroxidation, particularly the
unstable PUFA [
6
,
209
]. In previous studies in rats, diets containing CoQ were associated with lower
lipid peroxidation markers [
118
,
199
], as well as with lower oxidative damage of other macromolecules
such as DNA or proteins. A higher antioxidant capacity [
199
,
200
] compared to that found in
animals maintained on the same diet without additional CoQ
10
has been reported [
118
,
122
,
198
,
202
].
In addition, a lower impairment in mitochondrial function was also observed in CoQ-fed animals [
118
].
All these findings would indicate that dietary CoQ
10
avoids, at least in part, oxidative stress
linked to aging under certain conditions. Furthermore, it has been shown that life-long dietary
supplementation with CoQ
10
attenuated a variety of changes in enzymatic activities associated
with aging in rats [
209
,
210
]. These include increases in the hepatic activities of Mg
2+
-dependent
sphingomielinase [
209
] and of cytosolic and membrane-bound NQO1 activities [
210
], as well
as decreases in cytosolic glutathione-S-transferase and microsomal Se-independent glutathione
peroxidase in liver plasma membrane [
209
]. Proteomic analysis in rats under similar conditions
has shown that serum albumin, which decreases with age in the rat, was significantly increased by
CoQ
10
supplementation. Additionally, it induced significant modifications of several proteins in
plasma. These modifications support the beneficial role of dietary CoQ
10
decreasing both oxidative
stress and cardiovascular risk, and modulating inflammation and osteogenesis during aging [211].
In humans, some studies have suggested similar effects for dietary CoQ in relation to oxidative
stress, at least in combination with certain dietary patterns. Short-term (4 weeks) dietary CoQ effects
on aging have been tested in combination with the Mediterranean diet. In this regard, elderly subjects
ingested a Western diet rich in saturated fatty acids (SFA), a Mediterranean diet (rich in MUFA), and
a Mediterranean diet supplemented with CoQ following a cross-over design [
212
–
216
]. CoQ
10
addition
to MUFA-rich diet reduced some postprandial oxidative stress marker levels when subjects took
a breakfast with a lipid profile similar to their experimental diets [
212
], which correlated with a lower
expression of antioxidant enzyme components [
212
,
215
]. Moreover, dietary CoQ has also been shown
Molecules 2016,21, 373 14 of 26
to improve DNA repair systems [
213
,
214
] and modulate inflammatory signaling cascade as well as to
reduce endoplasmic reticulum stress [214].
All these results suggest that although CoQ supplementation does not directly extend lifespan,
it may help to prevent life span shortening due to oxidative insults [
6
] as it has been suggested by
its effect in all aspect related to mitochondrial function, oxidative stress and antioxidant defenses
both in animals [
139
,
196
,
197
] and humans [
114
,
143
]. However, despite animal studies have shown
certain beneficial effects on the health of different disease models, there are clinical trials reported
no significant effects on the progression of some nervous central system disease [
113
,
114
]. As has
been stated, brain CoQ uptake is very low compared to other organs [
23
] and animal studies reported
beneficial effects on this organ [
195
,
196
] have used very high daily dosages in relation to body weight
compared with those used in clinical trials [
113
,
114
]. Future trials in humans focused on diseases
affecting tissues and organs that have shown low CoQ uptake capacity should be more careful at this
respect and to use higher CoQ dosages and/or chemical formulation with higher bioavailability such
as those nanoparticulated or solubilized [
83
]. In the same sense, possible differences in bioavailability
and efficacy between ubiquinol and ubiquinone should also be taken into account. On the other hand,
CoQ effectivity for different disease treatment could depend on many other conditions not considered
in clinical trials: among others, possible differences in etiology and pathophysiology among animal
models and human diseases.
Due to differences in dosage, duration, chemical formulation and subject age at the beginning of
the treatment, it is very difficult to establish “ideal conditions” for CoQ use as “anti-aging therapy”.
Notwithstanding, according some to animal studies [
118
,
122
,
198
,
202
], life-long dietary interventions
could result useful to prevent negative consequences of different health insults throughout life,
particularly in relation to lifestyle. However, ubiquinone dosages used (0.7 mg/kg) already seem very
high to take them. In this sense, supplementing with some foods, particularly those that are more
“pro-oxidant”, could result useful.
Acknowledgments:
F.G. was supported by a Fondazione Umberto Veronesi Fellowship. A.V.-L. is recipient of
a predoctoral FPU contract from the Spanish Ministry of Education, Culture and Sport and member of the research
group CTS-627 supported by Junta de Andalucia and University of Granada.
Author Contributions:
A.V.-L. and F.G. wrote the review. Figures were drawn by A.V.-L. M.B. and J.L.Q. proposed
the scheme, supervised and corrected the review.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
CoQ Coenzyme Q
CoQH partially reduced coenzyme Q
CoQH2fully reduced coenzyme Q
Cytb5R NADH: cytochrome b5 reductase
FAD+reduced flavine adenine dinucleotide
FADH2reduced flavine adenine dinucleotide
farnesyl-PP farnesyl-pyrophosphate(isopentenyl-PP)
HMG-CoA 3-hydroxy-3-metylglutaryl-CoA
LDL low-density lipoprotein
LOO lipid hydroperoxyl radical
LOOH lipid hydroperoxyde
LPS lipopolysaccharide
MAPKs mitogen-activated protein kinases
mPTP mitochondrial permeability transition pore
Molecules 2016,21, 373 15 of 26
mtDNA mitochondrial DNA
mtETC mitochondrial electron transport chain
MUFA monounsarturated fatty acid
NQO1 NAD(P)H-quinone oxidoreductase 1
pHB para-hydroxybenzoate
PUFA polyunsarturated fatty acids
ROS Reactive oxygen species
SFA saturated fatty acids
SIRT1 sirtuin1
VDAC Voltage-dependent anion channelNuclear factor κB (NF-κB)
VLDL very low-density lipoprotein
α-tp α-tocopherol
α-tp*´α-tocopheryl anion radical
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