Antioxidant Activity of Quercetin: A Mechanistic Review

Abstract

Flavones and flavonoids are known to have potent antioxidant activity due to intracellular free radical scavenging capacities. Flavonoids are found ubiquitously in plants as a member of polyphenolic compounds which share diverse chemical structure and properties. Quercetin is among the most efficient antioxidants of the flavonoids. The antioxidant property of quercetin has been highlighted in this review. These compounds have pivotal role in treatment of diabetes, cancers and some cardiovascular diseases.

Full text

Turkish Journal of Agriculture - Food Science and Technology, 4(12): 1134-1138, 2016
Turkish Journal of Agriculture - Food Science and Technology
www.agrifoodscience.com,
Turkish Science and Technology
Antioxidant Activity of Quercetin: A Mechanistic Review
Senay Ozgen1, Ozgur Kivilcim Kilinc1, Zeliha Selamoglu2*
1Department of Plant Productions and Technologies, Faculty of Agricultural Sciences and Technologies, Omer Halisdemir University,
51240 Niğde, Turkey
2Department of Biotechnology, Faculty of Arts and Science, Omer Halisdemir University, 51240 Niğde, Turkey
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 09 November 2016
Accepted 29 November 2016
Available online, ISSN: 2148-127X
Flavones and flavonoids are known to have potent antioxidant activity due to intracellular
free radical scavenging capacities. Flavonoids are found ubiquitously in plants as a
member of polyphenolic compounds which share diverse chemical structure and
properties. Quercetin is among the most efficient antioxidants of the flavonoids. The
antioxidant property of quercetin has been highlighted in this review. These compounds
have pivotal role in treatment of diabetes, cancers and some cardiovascular diseases.
Keywords:
Quercetin
Oxidative stress
Flavonoid
Antioxidant
Plant
Türk Tarım – Gıda Bilim ve Teknoloji Dergisi, 4(12): 1134-1138, 2016
Kuersetinin Antioksidan Aktivitesi: Mekanik Bir Derleme
M A K A L E B İ L G İ S İ
Ö Z E T
Geliş 09 Kasım 2016
Kabul 29 Kasım 2016
Çevrimiçi baskı, ISSN: 2148-127X
Flavonlar ve flavonoidlerin, hücre içi serbest radikal süpürme kapasitelerinden dolayı
güçlü antioksidan aktiviteye sahip oldukları bilinmektedir. Flavonoidler, polifenolik
bileşiklerin çeşitli kimyasal yapı ve özelliklerini paylaşan bir üyesi olarak bitkilerde
bulunmaktadır. Quercetin, flavonoidlerin en etkili antioksidanları arasında yer almaktadır.
Bu derlemede quercetin'in antioksidan özellikleri vurgulanmıştır. Bu bileşikler diyabet,
kanser türleri ve bazı kardiyovasküler hastalıkların tedavisinde önemli rol
oynamaktadırlar.
Anahtar Kelimeler:
Kuersetin
Oksidatif stres
Flavonoidler
Antioksidanlar
Bitki
* Corresponding Author:
E-mail: zselamoglu@ohu.edu.tr
* Sorumlu Yazar:
E-mail: zselamoglu@ohu.edu.tr

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Introduction
Flavonoids are found ubiquitously in plants as a
member of polyphenolic molecules that share diverse
chemical structure and properties. There are more than
4.000 various flavonoids have been characterized within
the main flavonoid group which involve flavonols,
flavones, flavanones, catechins, anthocyanidins,
isoflavones, dihydroflavonols and chalcones (Cook et al.,
1996). The food industry uses natural antioxidants to
protect nutrients and color in the food. Recently, the
numbers of the studies conducted for the use of
flavonoids in different areas of industry are increasing.
Similarly, possible use of these compounds is being
common due to their antioxidant properties in the area of
food, textile, leather, metallurgy, medicine and
agriculture. Thus, quercetin is a common source for food
and pharmaceutical industries. Quercetin (3, 5, 7, 3′, 4′-
pentahydroxyflavone) is classified as a flavonol which is
one of the five subclasses and major dietary flavonoids
distributed in both cultivated and wild plants. (Cook et al.,
1996; D'Andrea, 2015).
Many factors, for instance, development of
technologies, the greenhouse effect, environmental
pollution, smoking, radiation and many chemicals, cause
negative effects of oxidative stress in the human body, as
a result, stress-free radicals occur. It is proven that
oxidative stress and free radicals provoke aging and
disease in the body. Characterization of components
capable of natural flavonoid antioxidants and their
antioxidative effects have become increasing interest
(Selamoglu et al., 2016). Antioxidants are organic
compounds with non enzymatic low concentrations that
prohibit the free radical oxidation mechanism (Das,
1989). Flavones and flavonoids, particularly quercetin,
are known to reveal important cytotoxicity process
against cultured human cells via raising intracellular
reactive oxygen species amount (Yáñez et al., 2004). In
this review, the effect of quercetin for the role of free
radical scavenger will be presented.
Chemistry of Quercetin
A hydroxyl group in third carbon, a double bond
between second and third carbon, a carbonyl group in
fourth carbon and polyhydroxylated A and B aromatic
rings (Figure 1) have main role in antioxidant proprieties
of these compounds (Cook et al., 1996).
The first resonant structure where B ring has an ortho-
catechol group may enable the forming of intra- and inter-
molecular hydrogen bonds. Indeed, the flavones, include
an ortho-catechol group (myricetin, quercetin, and
luteolin) are more acidic than apigenin and kaempferol, in
which the B-ring only have a 4′ hydroxyl group. Luteolin
and quercetin have two hydroxyl groups in the B-ring, on
the contrary of myricetin. Thus, the more numbers of
connected hydroxyl groups in the B-ring defines the
acidity capacity.
The scavenging mechanism of the free radical by
quercetin has been discussed on the rational of different
AM1 and quantum mechanics calculation, the attained
boundary orbitals and total spin intensity (Vasilescu and
Girma, 2002). The analysis results of the spin density near
by the attained free radical 4′-quercetin implies that the
essential intensity of spin α(↑) is condensed in the B ring
on the O4′ oxygen. They noticed also a light
delocalization of spins α(↑) and β(↓) on the B ring and on
a part of the C ring. Authors concluded that quercetin
antioxidant capacity may be related to basically with the
B ring and a half of the C ring (Vasilescu and Girma,
2002).
The correlation of these flavonoids with Cu2+ ions was
examined for their obscure composition of chelation or
modification through oxidation, as well as in their
structural relation (Brown et al., 1998). It has been
implied that the ortho 3', 4'-dihydroxy substitution in the
B ring is critical for a Cu2+-chelate formation which
affects the antioxidant activity. Furthermore, it has been
shown that the existence of a 3-hydroxy group in the
flavonoid structure promotes the oxidation of quercetin
and kaempferol. On the other hand, luteolin and rutin are
absent in the 3-hydroxy group which they do not oxidize
as easily in the existence of Cu2+ ions (Brown et al.,
1998). In addition, it is observed that complexation of
magnesium (Mg+2) cation increases the free radical
scavenging capacity of quercetin which inhibits oxidant
damage and cell mortality via different pathways (Ghosh
et al., 2015).
Figure 1 The structure of quercetin (Modified from
Mitchell, 1965).
Sources of Quercetin
Quercetin is one of natural flavonoid group that is
most common as a secondary metabolite in plants.
Production of synthetic flavonoids has not been practiced
yet. Hence, plants are the only sources for quercetin
(Abdelmoaty et al., 2010). Major vegetables and fruits
that are commonly consumed comprise different classes
of flavonoids in varied amount. It has been found that
onion has the highest amount of quercetin (about 300
mg/kg) among tested nutrition (Beecher, 1999). Other
vegetables, such as broccoli and kale, included quercetin

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1136
and kaempferol, but at much lower content. However, tea
contains a high amount of catechins but low amounts of
flavonoid quercetin (Beecher, 1999). The colour of fruits
and vegetables indicates amount of flavonoids, as red
grapes, cherries and blueberries have a significant amount
of variant anthocyanins. Most of these fruits also contain
flavonoids, especially quercetin (Beecher, 1999).
Quercetin and Oxidative Stress
The antioxidant character of quercetin is associated to
chemical structure, especially the presence and location of
the hydroxyl (–OH) substitutions and the catechol-type B-
ring (Rice-Evans et al., 1996; Wang et al., 2006). The
structural properties of a potent antioxidant capacity is
due to the presence of (i) an ortho-dihydroxy or catechol
group in the B-ring, (ii) a 2,3-double bond, and (iii)
hydroxyl substitution at positions 3 and 5 (Bors et al.,
1990). Growing evidence has demonstrated that quercetin,
which is featured by a hydroxylation form of 3, 5, 7, 30,
and 40 and a catechol B-ring, contains all the structural
properties of an antioxidant agent (Silva et al., 2002;
Rietjens et al., 2005). Quercetin has anticarcinogenic and
anti-inflammatory properties with antioxidant and free
radical scavenging effects. However, quercetin may also
be diverted into reactive molecules (Metodiewa et al.,
1999; Boots et al., 2003). In vitro, the oxidative
degradation of quercetin has been showed to result in the
formation of a free radical orthosemiquinone
intermediate, which may afterward be changed to the
parent molecule or to an orthoquinone, nearby the
manufacturing of reactive oxygen species (Metodiewa et
al., 1999; Boots et al., 2003). In conclusion, the possible
pro-oxidant property of quercetin, especially at high dose
levels, must be emphasized (Rietjens et al., 2005). As a
result, the prooxidant effect may be accountable for the in
vitro mutagenic action of quercetin. Also, the researchers
reported that under aerobic conditions, quercetin was
demonstrated to produce dose-dependent DNA damage
and lipid peroxidation in isolated rat liver nuclei and
oxygen radicals produced by autooxidation of the
flavonol (Rahman et al., 1992; Sahu and Washington,
1991). Oxidative stress is related to reactive oxygen
species which is the main factor for viral hepatitis,
fibrosis, cirrhosis and liver cancer formation (Preedy et
al., 2014). Recent studies show that some flavonoids
prevent the occurrence of superoxide and hydroxyl
radicals which cause lipid peroxidation.
Some researchers suggested that quercetin has
antimicrobial, antiviral, antioxidative and anti-
inflammatory activities (Doğan et al., 2015; Gutteridge,
1995). Also, it is able to increase the cellular antioxidant
potential via the Nrf2 pathway (Gutteridge, 1995).
Another study by Doğan et al. (2015) has showed that
quercetin is able to prevent against the toxic action of
chemotherapeutic substances treated prior to pregnancy
(Doğan et al., 2015). Quercetin was applied at a dose of
10 mg/kg/day by oral gavage. After 48 h of the
experimental chemotherapy exposure, female rats were
transferred to cages including male rat for mating.
According to this study, women who have been exposed
to chemotherapy and may be pregnant should be treated
with antioxidant molecules, such as quercetin to decrease
the risk of injury to fetal brain tissues. In addition, the
data of this investigation assessed the hypothesis that
quercetin can prevent the toxic actions of
chemotherapeutic compounds treated prior to pregnancy
(Doğan et al., 2015).
Some experimental studies on animals are declared
that the antioxidant effects of quercetin decrease oxidative
injury to the tissues such as the brain, heart in ischemic
reperfusion damage and exposure to agents that induce
oxidative stress (Doğan et al., 2015; Bayne and Sohal,
2002). It is well established that natural antioxidants are
usually harmless to human body. They are molecules that
prevent early aging via blocking the catasthropic effects
of the free radicals, many diseases and chain reactions
(Elik et al., 2007).
Antioxidant enzyme activities are substantially
enhanced by quercetin treatment (Elik et al., 2007). The
study of Elik et al. (2007) proved that quercetin as a
flavonoid with antioxidant characteristics demonstrates
antidiabetic effects (Elik et al., 2007). This compound
also has protection against oxidant damage to the heart,
brain, liver, aorta and kidney for mid-term or long-term
diabetic rat (Elik et al., 2007). Thus, quercetin increases
the antioxidant defence capacity.
It has been shown that flavonoids could inhibit
enzymes like cyclooxygenases and protein kinases where
they are part of cell proliferation and apoptosis processes
(Abdelmoaty et al., 2010). In addition, doses of 15–50
mg/kg body mass quercetin was able to of normalize
blood glucose level, augmenting liver glycogen
ingredients and dramatically decreasing serum cholesterol
and low density lipoprotein (LDL) levels in alloxan–
diabetic rats (Abdelmoaty et al., 2010). Furthermore,
treatment of quercetin to isolated rat islets increased
insulin production by 44–70% with changing in calcium
flows and in cyclic nucleotide metabolism (Abdelmoaty
et al., 2010).
Consumption of flavonoids showed a reduction in
coronary heart disease (Hertog et al., 1994). The study
conducted in Japan showed that there was a decreased on
both total and LDL-cholesterol concentration when
plasma quercetin is increased (Hertog et al., 1994). In
another study in Finland showed that diet rich in apple
and onion increased quercetin level which was helpful to
reduce coronary mortality (Knekt et al., 1996).
Each alive organism is able to prevent negative effects
of free radicals with antioxidative protection system.
However, this system is not strong enough to prevent an
increase in the amount of radical. Consequently, oxidative
stress (cell damage) occurs in this situation. Low level of
stress induces a cell to activate extra defence system,
although, high-stress level causes the death of cells which
damages organisms (Çıkrıkçı, 2005). All these studies are
demonstrating that quercetin has an anti-oxidative effect
to inhibit oxidative stress.
A series of epidemiological studies proved that a lack
of association between flavonoids consumption up to 68

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mg total flavonoids/day (in large portion by high
quercetin level) and incidence of all variety of cancer
(Hertog et al., 1995; Lin et al., 2006). On the other hand,
there are studies indicated a negative correlation between
up to 40 mg/day flavonoid consumption (95% of it
quercetin) and cancer incidence (Knekt et al., 1997; Knekt
et al., 2002).
It is demonstrated that triptolide (TP)-induced
oxidative stress and a decrease of testosterone generation
could be prevented by quercetin (Hu et al., 2015).
Different concentrations of TP were applied to Leydig
cells to cause oxidative stress with high intracellular
reactive oxygen species resulted in reduction activities
and expressions of glutathione peroxidase and superoxide
dismutase. Results of this study imply that quercetin
could lower the TP-induced reproductive toxicity, which
support the usage of TP.
The inhibition effect of quercetin against oxidative
stress, which caused by sodium fluoride, was investigated
in rat’s liver (Nabavi et al., 2012). Five groups of rats
were treated with different diets; the first group served
standard diet, the second group was intoxicated with
sodium fluoride (600 ppm) via drinking water for a week.
The third, fourth and fifth groups were applied with
quercetin at a dose of 10 and 20 mg/kg and vitamin C (as
the positive control) at a dose of 10 mg/kg
intraperitoneally for 1week ahead of sodium fluoride
intoxication, seriatim. Activities of superoxide dismutase
and catalase, the amount of decreased glutathione and
lipid peroxidation end product were measured 1 week
later treatments of rat liver. According to the data of this
work that quercetin preserves rat liver from sodium
fluoride activated oxidative stress, most likely by its
antioxidant action (Nabavi et al, 2012). It is also
demonstrated that quercetin prevents perfluorooctanoic
acid-induced liver damage via mitigating oxidative stress
and inflammatory response in mice (Zou et al., 2015). As
pointed out, mitochondrial oxidative stress has a main
role in the pathology of myocardial infarction (Czepas
and Gwoździński, 2014). In this study, pretreatment of
quercetin reduced the activities of serum creatine kinase,
lactate dehydrogenase, heart mitochondrial lipid
peroxidation products and dramatically enhanced the
amounts of mitochondrial antioxidants. In addition,
quercetin also cured the activities of tricarboxylic acid
cycle and respiratory chain enzymes almost normal level
in myocardial infarcted rats. The action of quercetin on
cardiac mitochondrial oxidative stress could reduce
mitochondrial lipid peroxidation; enhance levels of
mitochondrial antioxidants and activities of mitochondrial
marker enzymes. As a result, heart mitochondria of rat are
protected to isoproterenol-stimulated oxidative stress in
vivo in myocardial infarction (Czepas and Gwoździński,
2014).
Quercetin is the most abundant polyphenolic in human
food and the absences of its toxicity-genotoxicity has
been proved. It has high importance due to chemo
preventive and anticancer values. In general, all these
values might point out the possible treatment of quercetin
as cardio protectant through anthracycline chemotherapy.
Moreover, all these favourable impacts to anthracycline-
induced complications of chemotherapy need to be more
studied and validated both in animal and clinical works.
The experiment was established to test the
hepatoprotective effect of quercetin compared to N-
acetylcysteine (NAC) against hepatic I/R injury in rats
and to determine iNOS, eNOS, and NOSTRIN protein
expressions, as a possible mechanism of its
hepatoprotective effect. As a result, quercetin application
improved eNOS protein expression with a simultaneous
decline in iNOS and NOSTRIN protein expressions. In
addition, pretreatment of quercetin decreased serum
aspartate aminotransferase, alanine aminotransferases and
hepatic myeloperoxidase activities and recover the extinct
content of decreased glutathione, malondialdehyde, and
nitric oxide levels (Abd-Elbaset et al., 2015).
Neurodegenerative disorders are formed with complex
processes, basically associated to advance brain injury
capturing cellular death. Biochemical reactivity related to
these processes in Alzheimer’s disease contains, among
others, metal-induced oxidative stress promoting to
neuronal cell demise. One of the active redox metals
causing oxidative stress is Cu (II) (Naday et al., 2015).
Lately, the effort is to produce bioactive hybrid
nanoparticles that have the capability to work as host-
carriers for potential antioxidants, for instance, the natural
flavonoid quercetin. In molecular technology silica
nanoparticles were assembled with synthetic protocols to
produce PEGylated and CTAB-modified materials.
Conclusion and Future Prospects
The current review deals with the basic biological
functions of quercetin, such as antioxidant, anti-
carcinogenic, anti-inflammatory, and cardio protective
properties. Furthermore, prevention of tumour
development and possibility of flavonoid-drug interaction
have been also discussed. We shared these biological
properties of quercetin with a common mechanism of
antioxidant action in this review. Critical evaluation of
biological effects of quercetin leads to raising a
conclusion that at estimated dietary intake levels would
not cause adverse health problems, even; daily
consumption would be extremely beneficial for human
daily activities.
Competing Interest
The authors declare that they have no conflict of
interest.
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