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Journal of Cell Science
RESEARCH ARTICLE
Light-harvesting chlorophyll pigments enable mammalian
mitochondria to capture photonic energy and produce ATP
Chen Xu, Junhua Zhang, Doina M. Mihai and Ilyas Washington*
ABSTRACT
Sunlight is the most abundant energy source on this planet. However,
the ability to convert sunlight into biological energy in the form of
adenosine-59-triphosphate (ATP) is thought to be limited to
chlorophyll-containing chloroplasts in photosynthetic organisms.
Here we show that mammalian mitochondria can also capture light
and synthesize ATP when mixed with a light-capturing metabolite of
chlorophyll. The same metabolite fed to the worm Caenorhabditis
elegans leads to increase in ATP synthesis upon light exposure,
along with an increase in life span. We further demonstrate the same
potential to convert light into energy exists in mammals, as
chlorophyll metabolites accumulate in mice, rats and swine when
fed a chlorophyll-rich diet. Results suggest chlorophyll type
molecules modulate mitochondrial ATP by catalyzing the reduction
of coenzyme Q, a slow step in mitochondrial ATP synthesis. We
propose that through consumption of plant chlorophyll pigments,
animals, too, are able to derive energy directly from sunlight.
KEY WORDS: ATP, Light, Mitochondria
INTRODUCTION
Determining how organisms obtain energy from the environment
is fundamental to our understanding of life. In nearly all
organisms, energy is stored and transported as adenosine-59-
triphosphate (ATP). In animals, the vast majority of ATP is
synthesized in the mitochondria through respiration, a catabolic
process. However, plants have co-evolved endosymbiotically
to produce chloroplasts, which synthesize light-absorbing
chlorophyll molecules that can capture light to use as energy
for ATP synthesis. Many animals consume this light-absorbing
chlorophyll through their diet. Inside the body, chlorophyll is
converted into a variety of metabolites (Ferruzzi and Blakeslee,
2007; Ma and Dolphin, 1999) that retain the ability to absorb light
in the visible spectrum at wavelengths that can penetrate into
animal tissues. We sought to elucidate the consequences of light
absorption by these potential dietary metabolites. We show that
dietary metabolites of chlorophyll can enter the circulation, are
present in tissues, and can be enriched in the mitochondria. When
incubated with a light-capturing metabolite of chlorophyll,
isolated mammalian mitochondria and animal-derived tissues,
have higher concentrations of ATP when exposed to light,
compared with animal tissues not mixed with the metabolite.
We demonstrate that the same metabolite increases ATP
concentrations, and extends the median life span of
Caenorhabditis elegans, upon light exposure; supporting the
hypothesis that photonic energy capture through dietary-derived
metabolites may be an important means of energy regulation in
animals. The presented data are consistent with the hypothesis
that metabolites of dietary chlorophyll modulate mitochondrial
ATP stores by catalyzing the reduction of coenzyme Q. These
findings have implications for our understanding of aging, normal
cell function and life on earth.
RESULTS
Light-driven ATP synthesis in isolated mammalian mitochondria
To demonstrate that dietary chlorophyll metabolites can modulate
ATP levels, we examined the effects of the chlorophyll
metabolite pyropheophorbide-a (P-a) on ATP synthesis in
isolated mouse liver mitochondria in the presence of red light
(l
max
5670 nm), which chlorin-type molecules such as P-a
strongly absorb (Aronoff, 1950), and to which biological tissues
are relatively transparent. We used P-a because it is an early
metabolite of chlorophyll, however, most known metabolites of
chlorophyll can be synthesized from P-a by reactions that
normally take place in animal cells. Control samples of
mitochondria without P-a, and/or kept in the dark were also
assayed. In the presence of P-a, mitochondria exposed to red light
produce more ATP than mitochondria without P-a (Fig. 1A)
or mitochondria kept in the dark (supplementary material
Fig. S1A–D). Mitochondrial membrane potential (Fig. 1B) and
oxygen consumption (Fig. 1C) increased upon increased light
exposure in P-a-treated mitochondria. Light or P-a alone had no
effect on any of the above measures of mitochondrial activity
(supplementary material Fig. S1E–G). With too much added P-a,
ATP concentrations and the rate of oxygen consumption started
to return to the levels in mitochondria not incubated with P-a
(supplementary material Fig. S1G). Addition of the electron
transport inhibitor, sodium azide, reduced the light- and P-a-fueled
oxygen consumption by 57% (supplementary material Fig. S1H–I),
consistent with oxygen consumption occurring through the electron
transport system. Observations were consistent with enhanced ATP
production driven by oxidative phosphorylation.
To determine whether P-a associates with mitochondria, we
measured P-a fluorescence at 675 nm in the presence of increasing
amounts of heart mitochondrial fragments obtained from sheep
(Fig. 2A,B). After increasing the concentration of mitochondria, P-
a fluorescence increased abruptly, by fivefold, and quickly reached
a plateau (Fig. 2B). The abrupt change in fluorescence reflects a
change in the environment of P-a, consistent with its change from
an aqueous environment to one in which it is presumably
associated with a protein. This threshold-sensitive behavior is
consistent with zero-order ultrasensitivity, or positively
cooperative binding, as described by Goldbeter and Koshland,
and suggests a coordinated interaction between the metabolite and
Columbia University Medical Center, Ophthalmology, New York, NY 10032, USA.
*Author for correspondence (iw2101@columbia.edu)
Received 30 April 2013; Accepted 15 October 2013
ß2014. Published by The Company of Biologists Ltd | Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
388
Journal of Cell Science
mitochondrial fragments (Goldbeter and Koshland, 1981). In
contrast, this threshold sensitivity was not observed when
increasing amounts of bovine serum albumin (BSA) were added
to a solution of P-a; instead, fluorescence steadily increased
(supplementary material Fig. S1J).
Catabolic reduction of coenzyme Q
10
(CoQ
10
) is a rate limiting
step in respiration (Crane, 2001). The majority of CoQ
10
molecules exist in two alternate states of oxidation: ubiquinone,
the oxidized form, and ubiquinol, the reduced form. To show
that the P-a metabolite could catalyze the photoreduction of
mitochondrial CoQ
10
, we measured the oxidation state of CoQ
10
in the above sheep heart mitochondrial fragments in response to
exposure to red light. We exposed the mitochondria to light for
10 minutes and measured the percentage of reduced and oxidized
CoQ
10
by high performance liquid chromatography (HPLC) (Qu
et al., 2013). In the freshly isolated mitochondria fragments,
nearly all the CoQ
10
was oxidized in the form of ubiquinone.
However, when we incubated the mitochondria with P-a and
exposed the suspension to light, 46% of CoQ
10
was reduced
(Table 1, entry 1). In comparison, as a positive control, we
energized the mitochondria with glutamate/malate and kept the
suspension in the dark, yielding a 75% reduction of CoQ
10
within
10 minutes (entry 2). In the absence of light, no reduction
occurred (entry 3). Upon denaturing the mitochondrial proteins
with heat, no reduction occurred (entry 4). Likewise, there was a
lack of CoQ
10
reduction with CoQ
10
, P-a and light in the absence
of mitochondria (entry 5). These observations are consistent with
the fluorescence data in Fig. 2A,B, showing that mitochondrial
proteins sequester and organize P-a. In the absence of added P-a,
a 2–14% reduction was observed, depending on the mitochondrial
preparation used (entry 6). We attribute this ‘background
reduction’ to the actions of endogenous chlorophyll metabolites,
Fig. 1. Chlorophyll metabolite P-a allows isolated mouse liver mitochondria to capture light to make ATP. (A) ATP synthesis in mouse liver mitochondria
incubated with P-a (treated) and exposed to light compared to controls (no P-a). Light exposure started at time zero and ADP was added at 30 seconds. Aliquots
were obtained at times shown and relative ATP levels measured using the firefly luciferase assay. Means and standard deviations are shown for each time point.
The experiment was run in triplicate with the same batch of mitochondria. *P,0.05 for treated versus control samples. (B) Mitochondrial membrane potential
(Dym) under different treatments as measured by safranin fluorescence. Lower fluorescence equals higher membrane potential. Mitochondria, with or without
P-a, were exposed to light for 2 minutes or kept in the dark. Safranin was added at time zero and safranin fluorescence was continuously measured while
samples remained under the light. The experiment was run in triplicate with the same batch of mitochondria. Curves shown are the average traces for triplicate
runs. (C) Representative oxygraph trace (black line) for mitochondria treated with 4 mM P-a. The light was turned on or off at the times indicated by the arrows.
Steeper slope denotes faster oxygen consumption. Dotted lines show slopes when the light was off. When the light was turned on the slope of the black line
increased by twofold. That is, oxygen consumption increased when the light was turned on. When the light was turned off, oxygen consumption returned to
baseline levels (i.e. the two gray lines have the same slope).
Fig. 2. Cooperative binding of P-a to mitochondrial fragments.
(A) Fluorescence spectra of P-a before and after addition of sheep heart
mitochondrial fragments. Upon addition of mitochondrial fragments, the
fluorescence intensity of P-a increased and shifted to a longer wavelength,
and the shape of the curve (ratio of the shoulder to main peak) changed.
(B) Ultrasensitive steady state response of the P-a–mitochondrial interaction.
We measured fluorescence intensity for a 1 mM P-a solution while increasing
the concentration of mitochondrial fragments. A Hill coefficient of 36, with a
95% confidence interval from 7 to 65, was obtained by fitting the data to the
Hill equation [y5ax
b
/(c
b
+x
b
)+offset]. Fit (R
2
): 0.96.
Table 1. Photoreduction of CoQ
10
is an early event in light-
stimulated ATP synthesis
Condition number Reaction conditions
Percent reduction in
coenzyme Q
1 P-a, light, Mito 4662
2 Glutamate/malate, Mito 7562
3 P-a, dark, Mito 261
4 P-a, light, denatured Mito 561
5 P-a, light, coenzyme Q None
6 Light, Mito 2–14
Mito, mitochondria.
Sheep heart mitochondria were used under the listed conditions. All
reactions were for 10 minutes under an anaerobic atmosphere, employing
the same amount of mitochondria. Longer reaction times did not increase the
percentage of reduced CoQ
10
(data not shown). Ubiquinone and ubiquinol
were quantified by HPLC. Entries 1–5 are results for two experiments each.
Entry 6 is a range of values observed for two different mitochondrial
preparations.
RESEARCH ARTICLE Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
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Journal of Cell Science
which we were able to detect by fluorescence spectroscopy (see
Distribution of light-absorbing dietary chlorophyll, below).
Light-driven ATP synthesis in rodent tissue homogenates
To determine whether chlorophyll metabolites and light could
influence ATP production in whole tissues, we treated mouse
brain homogenates with P-a and exposed them to 670-nm light.
The treated brain homogenates synthesized ATP at a 35% faster
rate than a control homogenate that was not incubated with P-a
[relative ATP synthesis rates (means with standard error and
95% confidence intervals (CI) were: treated, 171.768.1 (CI:
154.6–188.7); control, 111.369.1 (CI: 92.5–130.0); Fig. 3A]. No
linear correlation between the increase in ATP concentrations
and the amount of added P-a was observed. Increasing
concentrations of P-a elicited the same increase in ATP
(supplementary material Fig. S2A,B).
To demonstrate that photon absorption by P-a was necessary to
enhance ATP production, we exposed the P-a-treated brain
homogenates to greenish (500 nm) and red (630, 670 and
690 nm) light, all with the same total energy. Wavelengths of
light that were more strongly absorbed by P-a produced the
largest increase in ATP. For example, the ATP concentration
increased by ,16-fold during exposure to 670 nm light; relative
to the same sample kept in the dark, it increased by two-to-
fivefold during exposure to 500, 630 and 690-nm light of equal
energy (Fig. 3B).
In addition to brain homogenates, P-a also enhanced ATP
production in adipose, lens and heart homogenates (supplementary
material Fig. S2C–E). Quantification of ATP by both the luciferase
assay and high-performance liquid chromatography (HPLC) gave
similar results (supplementary material Fig. S2E–F).
Distribution of light-absorbing dietary chlorophyll
Chlorophylls and its metabolites, both chlorins, have signature
absorption and admission spectra (Aronoff, 1950). Namely they
absorb strongly (e<50,000 M
21
cm
21
)at,665–670 nm and
demonstrate intense fluorescence emissions at ,675 nm, which
differentiate chlorins from endogenous molecules in mammals
(Aronoff, 1950). To examine whether dietary chlorophyll and/or
its metabolites were present in animal tissue after oral
consumption, we fed mice a chlorophyll-rich diet. Brain
(Fig. 4A) and fat (Fig. 4C) extracts from these mice exhibited
red fluorescence at 675 nm when excited with a 410-nm
light [brain: treated, 15.466.7 (n56); control: 4.262.6 (n56;
means 6s.d.); P,0.01]. The excitation spectrum of this 675-nm
peak (Fig. 4B) was similar to that of known chlorophyll
metabolites with an intact chlorin ring: with maxima at 408,
504, 535, 562 and 607 nm. This red fluorescence diminished, as
measured by the area under the 675 nm peak, when animals were
Fig. 3. Chlorophyll metabolite P-a allows mouse brain tissue
homogenates to capture light to make ATP. (A) ATP synthesis in mouse
brain homogenate with light exposure. Homogenates were incubated with
ADP 6P-a and exposed to light starting at time zero. Aliquots were
withdrawn at the times shown. Relative ATP in the aliquots was measured
using the firefly luciferase assay. The experiment was run in triplicate with the
same batch of homogenized brains. Means and standard errors are shown
for each time point. For the control, the standard errors are smaller than the
line markings and thus cannot be seen. *P,0.05 for treated versus untreated
samples. (B) Overlay of the absorption spectrum of P-a (dotted line) and the
wavelengths tested for ATP production in samples treated with P-a and
exposed to light for 20 minutes. Peak ATP production correlated with peak
P-a absorption. Experiments were done in triplicate. Means and standard
errors were calculated, however, standard errors are smaller than the
markings and thus cannot be seen.
Fig. 4. Dietary chlorophyll results in chlorophyll-
metabolite-like fluorescence in tissues.
(A) Representative fluorescence spectra of brain
extracts following excitation at 410 nm. Relative peak
areas for a total of six control animals fed a
chlorophyll-poor diet and six treated animals fed a
chlorophyll-rich diet. (B) Representative excitation
spectrum (emission at 675 nm) of a brain extract from
mice fed a chlorophyll-rich diet. (C) Representative
fluorescence spectra of abdominal fat extracts from
mice fed chlorophyll-poor and rich diets. (D) A
675610-nm fluorescence image of skinned mice
raised on chlorophyll-rich and -poor diets.
RESEARCH ARTICLE Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
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Journal of Cell Science
given a chlorophyll-free diet for 2 weeks. Red fluorescence could
also be seen using fluorescence imaging; fluorescence was
stronger in the bodies and brains of animals fed chlorophyll than
in animals given a chlorophyll-poor diet [Fig. 4D; mean gray
value in the boxed areas with standard deviation and minimum
and maximum gray value shown in brackets were: treated brain,
118 (97–138); control brain, 82 (60–100); treated back fat pad,
116 (97–132) and control back fat pad, 35 (25–46)]. The red
fluorescence was enriched in the gut and intestines, consistent
with dietary chlorophyll being the source of the fluorescence.
To determine whether the red fluorescence was localized to
mitochondria, we measured the relative 675-nm fluorescence in
whole liver homogenates and mitochondria isolated from these
homogenates. As measured by fluorescence intensity, isolated
mitochondria contained 2.3-fold as much of the 675-nm
fluorescent metabolite(s) per milligram of protein as did the
whole liver homogenate. This observation suggests that P-a was
concentrated in the mitochondria, consistent with data
summarized in Fig. 2A,B, and literature reports (MacDonald
et al., 1999; Tang et al., 2006).
Fat and plasma extracts from rats fed chlorophyll-rich diets
were further analyzed by HPLC to elucidate the source of the
red 675-nm fluorescence. Fig. 5A shows a representative
chromatogram with compounds in the eluting solvent that
displayed 675-nm fluorescence when excited with 410-nm light.
Rat fat extracts and plasma extracts both contained similar
chlorophyll-derived metabolites (similar chromatograms not
illustrated). Two groups of compounds eluting at 23–
30 minutes and 40–46 minutes were detected. Compounds
eluting between 23 and 30 minutes had similar retention times
to those of the chlorophyll metabolites without the phytyl tail,
with at least one carboxylate group, such as P-a. The absorption
spectra (the locations of the absorbance maxima and the Soret-to-
Q
y
-band ratios) of this group of compounds were consistent with
demetalated chlorophylls (Rabinowitch, 1944), as shown in
Fig. 5B. In addition, the spectra of this group of peaks were
indicative of coordination to a metal ion. A representative
spectrum of such a presumably metalated metabolite is shown in
Fig. 5C, showing a red shifted Soret band, a blue shifted Q
y
-band
and a Soret-to-Q
y
-band ratio of ,1. The compounds eluting
between 40 and 46 minutes had similar retention times to that of
the demetalated chlorophyll-a standard (pheophytin-a). In
addition, these compounds partitioned with hexanes (polarity
index50.1) when mixed with hexanes and acetonitrile (polarity
index55.8). This latter characteristic is consistent with a lack of a
carboxylic acid group, or an esterified P-a, such as pheophytin-a.
Similar HPLC chromatograms from fat extracts of swine fed
chlorophyll rich diets (Mihai et al., 2013) were recorded
(supplementary material Fig. S2G), suggesting that uptake and
distribution of chlorophyll metabolites were not unique to mice
and rats.
We quantified total blood pigments from rats that absorbed at
665 nm. Using an extinction coefficient of 52,000 at 665 nm
(Lichtenthaler, 1987), which is typical of chlorophyll-a-derived
pheophytins, we estimated a plasma concentration of 0.05 mMin
two rats fed a chlorophyll-rich diet. The 665-nm peak was absent
in animals fed a chlorophyll-poor diet. The amount of measured
total metabolite was five- and two-times higher than that reported
for the fat soluble vitamins K (Tovar et al., 2006) and D (Halloran
and DeLuca, 1979), respectively, in the rat.
Light-driven ATP synthesis in C. elegans
Next, we used C. elegans to evaluate the effects of light-
stimulated ATP production in a complex organism. As C. elegans
age, there is a drop in cellular ATP (Braeckman et al., 1999;
Braeckman et al., 2002). We hypothesized that the worm would
live longer if it could offset this decline in ATP by harvesting
light energy for ATP synthesis. As our model system, we used
firefly luciferase-expressing C. elegans, which upon incubation
with luciferin emit a luminescence that is proportional to their
ATP pools (Lagido et al., 2009; Lagido et al., 2008; Lagido et al.,
2001). Upon incubation with P-a, worms incorporated the
metabolite, as measured by fluorescence spectroscopy
(supplementary material Fig. S3A). To determine whether there
Fig. 5. Light-absorbing metabolites of chlorophyll
are present in adipose tissue. (A) HPLC
chromatogram of an adipose extract. 2.5 grams of
abdominal adipose tissue from a rat fed a chlorophyll-
rich diet was extracted with acetone and the acetone
concentrate subjected to HPLC. In the chromatogram,
only compounds that displayed 675-nm fluorescence,
characteristic of chlorophyll and its metabolites
possessing a chlorin ring, are shown. Five major peaks
are observed along with several minor peaks. For
peaks with letters, the corresponding absorption
spectra are shown below. (B–D) Absorption spectra of
labeled peaks in A (b–d, respectively). Numbers above
peaks are peak maxima in nm. Numbers in the center
are the ratios of the Soret band, around 400 nm, to the
Q
y
band at around 655 nm. All spectra are consistent
with those of metabolites of chlorophyll. Spectrum C
has been assigned to a metalated porphyrin.
RESEARCH ARTICLE Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
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Journal of Cell Science
were changes in ATP stores in response to light, we plated two
groups of worms into 96-well plates containing luciferin
substrate. We measured worm luminescence at time zero. We
then exposed one group to 660-nm light and kept the other in the
dark and periodically measured luminescence in both groups
of worms (summarized in Fig. 6A,B). To determine whether
ATP increased in light-exposed animals, we subtracted the
luminescence signal of the worms kept in the dark from that of
the worms exposed to light (Fig. 6C). Worms that were given P-a
had a statistically significant increase in ATP when exposed to
light, whereas control worms showed no increase. The metabolite
alone had no effect on ATP levels when the worms were kept in
the dark (i.e. luminescence intensity remained constant
throughout the experiment). The elevated luminescence signal
persisted for 1 hour after the light was turned off, at which time
measurement ceased. However, the luminescence intensity did
not further increase during the time the light was off. It was
unclear whether this persistent signal reflected the kinetics of the
luciferase–luciferin reaction, luciferase expression, or actual ATP
pools. Thus ATP was quantified by additional methods.
As an alternative means of determining whether light
stimulated ATP synthesis, we plated luciferase-expressing
worms into a 96-well plate without the luciferin substrate, and
exposed them to light. ATP status was determined at time zero,
immediately before light exposure, and at 15-minute intervals for
a total of 45 minutes by adding the luciferin substrate to a group
of worms and measuring luminescence (Fig. 6D,E). We found an
increase in ATP when 5-day-old and 10-day-old adult worms
were fed the metabolite and exposed to light.
We further confirmed the in vivo increase in ATP using two
additional ex vivo methods. After light treatment, we lysed
the worms, extracted their ATP and quantified ATP in the
homogenate using either the firefly luciferase assay or HPLC
(supplementary material Fig. S3B,C). Both methods were
consistent with the in vivo ATP measurements.
In addition to an increase in ATP, worms treated with P-a
exhibited a 13% increase in respiration when exposed to light, as
measured by oxygen consumption. However, light had no effect on
the respiration rates in untreated worms (supplementary material
Fig. S3D). This observation is consistent with an increase in ATP
Fig. 6. P-a treatment enables worms to capture light to generate ATP. Black lines show results from worms incubated with P-a at the indicated
concentrations; gray lines show results from worms not incubated with P-a. (A) In vivo, real-time ATP levels in 1-day-old worms were tracked during exposure to
light. Luciferase-expressing worms were incubated with luciferin and exposed to light at time zero. Luminescence was measured at the times shown. Data
represent triplicate experiments of 12 separate sets of worms plated in 12 wells of a 96-well pate. Means and standard deviations are shown for each of the three
separate runs. (B) In vivo, real-time ATP levels in worms kept in the dark. The same experiment as in A in the same 96-well plate, but the worms were kept in the
dark. (C) Percentage ATP increase for worms in A relative to worms in B. (D) In vivo, real-time ATP monitoring. Groups of worms were incubated with or without
P-a; light exposure began at time zero and in vivo ATP levels were determined at the times shown in each group of worms by measuring worm luminescence
after the addition of luciferin. Each time point represents a different group of worms exposed to light for the times shown. Each experiment was performed in
triplicate sets of 12; averages and standard deviations are shown. P-values of Student’s t-tests are also shown, representing the significance compared with the
controls at the same light exposure. (E) The same experiment as described in D, but using 10-day-old worms.
RESEARCH ARTICLE Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
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Journal of Cell Science
through oxidative phosphorylation, in accordance with the
mitochondrial data. Despite the increase in ATP, the levels of
reactive oxygen species (ROS) were equivalent in treated and
untreated worms during 5 hous of light exposure, as measured using
29,79-dichlorofluorescin diacetate (supplementary material Fig. S3E).
In fact, although the difference was not statistically significant,
treated worms exhibited, on average, lower levels of ROS.
Light harvesting to extend life span
We next tested whether photonic energy absorption by P-a could
prolong life. Life span measurements were taken in liquid
cultures according to the method of Gandhi et al. and Mitchell
et al. (Gandhi et al., 1980; Mitchell et al., 1979). Adult worms
were incubated with P-a for 24 hours. Beginning at day 5 of
adulthood, we exposed the worms to red light in a daily
5 hours:19 hours light:dark cycle. Control worms were not given
P-a or exposed to light, but otherwise were kept under identical
conditions. Counts were made at 2- to 3-day intervals and deaths
were assumed to have occurred at the midpoint of the interval. To
obtain the half-life, we plotted the fraction alive at each count
verses time and fitted the data to a two-parameter logistic
function, known to accurately fit survival of 95% of the
population (Vanfleteren et al., 1998). The group treated with
P-a and light had a 17% longer median life span than the group
that was not treated with P-a, but exposed to light (Fig. 7A,B).
P-a treatment alone, in the absence of light, had no effect on life
span (supplementary material Fig. S4B). Light treatment alone
decreased life span by 10% (supplementary material Fig. S4B), in
accordance with reports that nematodes survive better in
complete darkness (Thomas, 1965). This decrease in median
life span brought on by light was reversed when the worms were
treated with P-a. The increased median life span with light
and P-a was reproducible with different batches of worms
(supplementary material Fig. S4B–E). Increasing the amount of
P-a past a certain threshold, however, lead to a gradual decrease
in lifespan approaching that of animals not treated with P-a
(supplementary material Fig. S4B,C).
We also examined life span longitudinally. We placed 6-day-
old adult P-a- and non-P-a-treated worms into a 96-well plate,
exposed them to red light for 5 hours per day and compared the
percentage dead and alive after 15 days. Result: 47% of the P-a-
treated worms were alive (175 alive; 200 dead) after 15 days,
versus 41% of the control worms (111 alive; 163 dead), consistent
with the cross-sectional experiments above.
DISCUSSION
Photoreduction of coenzyme Q
Upon incubation of: (1) isolated mouse mitochondria; (2) mouse
brain, heart and lens homogenates; (3) homogenized duck fat; and
(4) live C. elegans, with a representative metabolite of chlorophyll,
light exposure was able to increased ATP concentrations. These
observations in a variety of animal tissues perhaps demonstrate the
generality of this phenomenon. To synthesize ATP, mitochondrial
NADH reductase (complex I) and succinate reductase (complex II)
extract electrons from NADH and succinate, respectively. These
electrons are used to reduce mitochondrial CoQ
10
,resultingin
ubiquinol (the reduced form of CoQ
10
). Ubiquinol shuttles the
electrons to cytochrome creductase (complex III), which uses the
electrons to reduce cytochrome c, which shuttles the electrons to
cytochrome coxidase (complex IV), which ultimately donates the
electrons to molecular oxygen. As a result of this electron flow,
protons are pumped from the mitochondrial matrix into the inner
membrane space, generating a trans-membrane potential used to
drive the enzyme ATP-synthase.
The ‘pool equation’ of Kro¨ ger and Klingenberg describes the
total rate of electron transfer: V
obs
5V
ox
V
red
/(V
ox
+V
red
), where
V
red
is ubiquinone reduction and V
ox
is ubiquinol oxidation
(Kro¨ ger and Klingenberg, 1973). Based on this equation, the
major roles of complexes I and II can be considered to maintain
the mitochondrial ubiquinol pool, and to reduce ubiquinone,
which should result in increased ATP synthesis. We reasoned the
reduction of CoQ
10
could be a potential step in the respiratory
pathway in which chlorophyll metabolites could influence ATP
levels, as it is known that chlorophyll-type molecules can
photoreduce quinones (Chesnokov et al., 2002; Okayama et al.,
1967). Indeed, a primary step during photosynthesis is the
reduction of the quinone, plastoquinone, by a photochemically
excited chlorophyll a (Witt et al., 1963). We hypothesized that if
the reduction of mitochondrial ubiquinone could be catalyzed by
a photoactivated chlorophyll metabolite, such as P-a, then ATP
synthesis would be driven by light in mitochondria with these
dietary metabolites. In the proposed mechanism, electrons would
be transferred by a metabolite of chlorophyll to CoQ
10
, from a
chemical oxidant present in the mitochondrial milieu. Many
molecules, such as dienols, sulfhydryl compounds, ferrous
compounds, NADH, NADPH and ascorbic acid, could all
potentially act as electron donors. Throughout mammalian
Fig. 7. P-a and light increase C. elegans median life span. (A) Median life
spans of worms treated with P-a and exposed to light versus those exposed
to light but not treated with P-a. Numbers in parentheses are 95% confidence
intervals (CI). (B) Life span plots of the values used for A. P-value is from an
f-test. Experiments were run in triplicate. The L4 molt was used as time zero
for life span analysis. Worms were grown in liquid culture at 500 worms/ml.
For counting, aliquots were withdrawn and placed in a 96-well plate to give
,10 worms per well; the worms were scored dead or alive on the basis
of their movement, determined with the aid of a light microscope. A total of
60–100 worms, representing 1–2% of the total population, were withdrawn
and counted at each time point for each flask.
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Journal of Cell Science
evolution, photons of red light from sunlight have been present
deep inside almost every tissue in the body. Photosensitized
electron transfer from excited chlorophyll-type molecules is
widely hypothesized to be a primitive form of light-to-energy
conversion that evolved into photosynthesis (Krasnovsky, 1976).
Thus it is tempting to speculate that mammals possess conserved
mechanisms to harness photonic energy.
Photoexcitation of chlorophyll and derivatives produces the
excited singlet state (*1). Oxidative quenching of this singlet state
by ubiquinone is possible. Electron transfer could take place
through proteins or in solution. Escape from the charge transfer
complex and protonation would yield ubisemiquinone, which
accounts for 2–3% of the total ubiquinone content of
mitochondria (De Jong and Albracht, 1994). Ubisemiquinone
can be reduced to ubiquinol by repeating the above sequence or
by disproportionation to give one molecule of ubiquinol and one
molecule of ubiquinone. Back-electron transfer, from the
photoreduced metabolite to the oxidized quinone, could be
inhibited by disproportionation or by organizing the chlorophyll
derivative and ubiquinol through protein binding. In line with the
CoQ
10
photoreduction hypothesis, we observed mitochondrial
CoQ
10
was reduced when isolated mitochondria were exposed to
light and P-a (Table 1). Also consistent with light and/or P-a
acting upstream of complexes I and II, in isolated mitochondria
we observed an increase in ATP in the absence of added electron
transport substrates, such as glutamate and malate (Fig. 1A;
supplementary material Fig. S1A–C). However, further evidence
is needed to confirm this mechanistic hypothesis.
The effect of light in vivo
Intense red light between 600 and 700 nm has been reported to
modulate biological processes (Hashmi et al., 2010; Passarella
et al., 1984; Wong-Riley et al., 2005), and has been investigated
as a clinical intervention to treat a variety of conditions (Hashmi
et al., 2010). Exposure to red light is thought to stimulate cellular
energy metabolism and/or energy production by, as yet, poorly
defined mechanisms (Hashmi et al., 2010). In the presence of P-a,
we observed changes in energetics in animal-derived tissues
initiated with light of intensity and wavelengths (<670 nm at
<0.860.2 W/m
2
) that can be found in vivo when outdoors on a
clear day. On a clear day the amount of light illuminating your
brain would allow you to comfortably read a printed book
(Benaron et al., 1997). In humans, the temporal bone of the skull
and the scalp attenuate only 50% of light at a wavelength of
,670 nm (Eichler et al., 1977; Wan et al., 1981). In small
animals, light can readily reach the entire brain under normal
illumination (Berry and Harman, 1956; Massopust and Daigle,
1961; Menaker et al., 1970; Vanbrunt et al., 1964). Sun or
room light over the range of 600–700 nm can penetrate an
approximately 4-cm-thick abdominal wall with only three-to-five
orders of magnitude attenuation (Bearden et al., 2001; Wan et al.,
1981). Photons between 630 and 800 nm can penetrate 25 cm
through tissue and muscle of the calf (Chance et al., 1988).
Adipose tissue is bathed in wavelengths of light that would excite
chlorophyll metabolites (Bachem and Reed, 1931; Barun et al.,
2007; Zourabian et al., 2000). Thus, identification of pathways,
which might have developed to take advantage of this photonic
energy, may have far-reaching implications.
Dietary chlorophyll in animals
A potential pathway for photonic energy capture is absorption by
dietary-derived plant pigments. Little is known about the
pharmacokinetics and pharmacodynamics of dietary chlorophyll
or its chlorin-type metabolites in human tissues. Here, we
observed the accumulation of chlorin-type molecules in mice,
rats and swine administered a diet rich in plant chlorophylls
(Figs 4, 5; supplementary material Fig. S2G). Data suggests that
sequestration from the diets of chlorophyll-derived molecules,
which are capable of absorbing ambient photonic energy, might
be a general phenomenon.
To date, the reported chlorophyll metabolites isolated from
animals have been demetalated (Egner et al., 2000; Fernandes
et al., 2007; Scheie and Flaoyen, 2003). The acidic environment
of the stomach is thought to bring about loss of magnesium from
the chlorophyll (Ferruzzi and Blakeslee, 2007; Ma and Dolphin,
1999). Our absorbance data from extracted pigments from rat fat
is consistent with the presence of chlorophyll metabolites bonded
to a metal (Fig. 5). If true, the presence of a metal derivative in
fat tissue suggests that the pigment was actively re-metalated to
take part in coordination chemistry. The identification of several
metabolites in the fat and plasma of rats and swine fed a
chlorophyll-rich diet that are similar to ones found in plants is
significant. However, the structures of the metabolites remain to
be elucidated. Chlorin-type molecules are similar in structure
and photophysical properties and thus can carry out similar
photochemistry (Gradyushko et al., 1970). Our data demonstrate
that dietary metabolites of chlorophyll can be distributed
throughout the body where photon absorption may lead to an
increase in ATP as demonstrated for the chlorin P-a. Indeed, P-a
could have been transformed into other metabolites, as most
known metabolites of chlorophyll can be formed from P-a by
reactions that normally take place in animal cells.
There relationship between the increase in ATP and the amount
of added P-a was not linear (supplementary material Fig. S2A,B).
ATP stimulation by light in the presence of P-a better fitted a
binary on/off, rather than a graded response to P-a. Increasing
concentrations of P-a elicited the same increase in ATP, after
light exposure. However, with too much added P-a, ATP levels
began to fall. This on/off response was also consistent with the
observed cooperative binding mode of P-a with mitochondria
fragments, suggesting that the threshold response may be
regulated by mitochondrial binding of P-a. If chlorophyll
metabolites are found to be involved in energy homeostasis,
a better understanding of their pharmacodynamics and
pharmacokinetics will be needed.
ATP stores and life span
Light of 670 nm wavelength that penetrates the human body,
yields ,43 kcal/mol (1.18610
222
kcal/photon). Given estimated
concentrations of chlorophyll derivatives in the body (Egner
et al., 2000; Fernandes et al., 2007; Scheie and Flaoyen, 2003)
and the photon flux at 670 nm (Bachem and Reed, 1931;
Barun et al., 2007; Bearden et al., 2001; Benaron et al., 1997;
Chance et al., 1988; Eichler et al., 1977; Menaker et al., 1970;
Vanbrunt et al., 1964; Wan et al., 1981; Zourabian et al., 2000),
each chlorophyll metabolite would be expected to absorb only
a few photons per second. As such, one might anticipate
negligible amounts of additional energy. Organization of
chlorophyll metabolites into supramolecular structures, similar
to chlorophyll antenna systems in photosynthetic organisms,
would increase the effective cross-sectional area of photon
absorption and, thus, photon catch. Indeed, our observed
positively cooperative binding with mitochondrial fragments is
evidence for such organization. Even so, to approach the rate of
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ATP synthesis powered by NADH or FADH
2
, sufficient P-a
pigment would have to be added to turn animals green.
Nevertheless, in model systems, we measure an increase in
ATP upon light absorption and changes in fundamental biology
(extention in life span). Regardless of the mechanism by which
ATP is increased or the measured amount of the increase, perhaps
the larger question is: how much of an increase in ATP is enough
to make a biological difference?
In animals, treatment with P-a and light both increased ATP and
median life span, suggesting that light in the presence of these light
absorbing dietary metabolites can significantly affect fundamental
biological processes. We previously observed that chlorophyll
metabolites enabled photonic energy capture to enhance vision
using a mouse model (Isayama et al., 2006; Washington et al.,
2004; Washington et al., 2007). Because ATP can regulate a broad
range of biological processes, we suspect that ATP modulation also
played a role in vision enhancement. The increase in life span may
seem contradictory, given that there are studies suggesting that
limiting metabolism and ATP synthesis increases the life span of C.
elegans. It has been proposed that the life span of this worm might
be determined by the metabolic status during development (Dillin
et al., 2002) and that there might be a coupling of a slow early
metabolism and longevity (Lee et al., 2003). Other observations
have led to the hypothesis that increased life span may be achieved
by decreasing total energy expenditure across the worm’s entire
life span (Van Raamsdonk et al., 2010). However, most studies
decrease ATP synthesis from hatching through genetic
engineering. By contrast, here, we were able to increase ATP
during adulthood at a time when ATP stores reportedly begin to
decline. For example, by day 4 of adulthood, the level of ATP and
oxygen consumption can drop by as much as 50% compared to day
zero (Braeckman et al., 1999; Braeckman et al., 2002). This
difference in timing might account for why we observed an
increase in life span in response to an increase in ATP. We note that
besides caloric restriction, there are only a few interventions that
are known (Petrascheck et al., 2007) to increase life span when
given to an adult animal.
Alternative mechanisms of life-span extension cannot be ruled
out. For example, an increase in reactive oxygen species (ROS) is
thought to increase life span in C. elegans (Heidler et al., 2010;
Schulz et al., 2007). Upon photon absorption, metabolites of
chlorophyll can transfer energy to oxygen, resulting in the
generation of singlet oxygen, a ROS. Thus life-span extension
seen here might be a result of an increase in ROS due to the
generation of singlet oxygen. However, our published data with
blood plasma (Qu et al., 2013) and data here from C. elegans do
not show an increase in ROS. As ubiquinol is a potent lipid
antioxidant (Frei et al., 1990) any ROS increase might be offset
by an increase in ubiquinol, generated from the photoreduction of
coenzyme Q. Indeed, by producing ubiquinol, P-a might have
increased life span by an alternative method by protecting against
long-term oxidative damage, which is also a mechanism that has
been shown to increase C. elegans life span (Ishii et al., 2004).
Further research will be needed to distinguish between the above
possible mechanisms.
Conclusion
Both increased sun exposure (Dhar and Lambert, 2013; John
et al., 2004; Kent et al., 2013a; Kent et al., 2013b; Levandovski
et al., 2013) and the consumption of green vegetables
(Block et al., 1992; Ferruzzi and Blakeslee, 2007; van’t Veer
et al., 2000) are correlated with better overall health outcomes in
a variety of diseases of aging. These benefits are commonly
attributed to an increase in vitamin D from sunlight exposure and
consumption of antioxidants from green vegetables. Our work
suggests these explanations might be incomplete. Sunlight is
the most abundant energy source on this planet. Throughout
mammalian evolution, the internal organs of most animals,
including humans, have been bathed in photonic energy from the
sun. Do animals have metabolic pathways that enable them to
take greater advantage of this abundant energy source? The
demonstration that: (1) light-sensitive chlorophyll-type molecules
are sequestered into animal tissues; (2) in the presence of the
chlorophyll metabolite P-a, there is an increase in ATP in isolated
animal mitochondria, tissue homogenates and in C. elegans, upon
exposure to light of wavelengths absorbed by P-a; and (3) in the
presence of P-a, light alters fundamental biology resulting in up
to a 17% extension of life span in C. elegans suggests that,
similarly to plants and photosynthetic organisms, animals also
possess metabolic pathways to derive energy directly from
sunlight. Additional studies should confirm these conclusions.
MATERIALS AND METHODS
General procedures
Two light sources were used for all experiments, either a 300 W halogen
lamp equipped with a variable transformer and band pass interference
filters [500, 632, 670, 690 nm with full-width half maximum (FWHM) of
10 nm] or a 1.70 W, 660 nm, LED light bulb. Luminous power density
was set to 0.860.2 W/m
2
as measured by a LI-250A light meter (LI-COR
Biosciences, Lincoln, NE). The intensity of red light used was 30–60
times less than the level of red light that we measured on a clear March
afternoon in New York City and is less than the level that several organs
are exposed to in vivo. Pyropheophorbide-a (P-a, 95% purity) was
obtained from Frontier Scientific, Logan, UT. For all experiments, prior
to exposing samples to light, we minimized light exposure by preparing
samples/experiments with laboratory lights turned off, using a minimum
amount of indirect sunlight that shone through laboratory windows
(.0.001 W/m
2
).
Animal protocols were approved by the Institutional Animal Care and
Use Committee of Columbia University. Mice (ICR, Charles River,
Wilmington, MA) weighing 22–28 g and rats (Fisher 344, Harlan Teklad,
Indianapolis, IN), weighing 300 g were used. Swine, fed a chlorophyll-
rich diet have been described previously (Mihai et al., 2013).
Continuous ATP monitoring in isolated mouse liver mitochondria
Mice were fed a chlorophyll-poor, purified rodent diet supplied by Harlan
(Indianapolis, IN) for a minimum of 2 weeks. We isolated mouse
liver mitochondria by differential centrifugation according to existing
procedures (Frezza et al., 2007) and used only preparations with a
minimum respiratory control ratio above 4.0 [state III/II, using glutamate
(5 mM final) and malate (2.5 mM final) as measured with an
oxygen electrode from Qubit Systems Inc., Kingston, ON, Canada].
Mitochondria at a final concentration of <1 mg protein/ml as determined
by the Coomassie Plus (Bradford) protein assay (Thermo Fisher
Scientific, Rockford, IL) in buffer A (0.250 M mannitol, 0.02 M
HEPES, 0.01 M KCl, 0.003 M KH
2
PO
4
, 0.0015 M MgAc
2
?H
2
O,
0.001 M EGTA, 1 mg/ml fatty acid–poor BSA, pH 7.4) were
incubated with P-a for 30 minutes at 0˚
C. ADP was added (0.5 mM
final concentration) and then 250 ml aliquots of this suspension were
placed in nine wells of a 96-well plate for exposure to light at room
temperature. At various times, 20 ml aliquots were withdrawn, added
to 150 ml lysis buffer (10 mM Tris, pH 7.5; 100 mM NaCl; 1 mM
EDTA and 1% Triton X-100), and ATP levels were determined
with a commercial kit (Invitrogen, Carlsbad, CA) according to the
manufacturer’s instructions. Controls were treated in the same way,
except they were: (1) incubated at 0˚
C without P-a (shown), (2) not
exposed to light and (3) were incubated without P-a and not exposed
to light.
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Membrane potential measurement
Mitochondrial membrane potential was monitored in buffer A as
described by Feldkamp et al. (Feldkamp et al., 2005). Measurements
were made in a 3 ml cuvette placed inside a fluorescence spectrometer
(Fluorormax-4, HORIBA Jobin Yvon, Horiba Scientific, Kyoto, Japan)
with a final reaction volume of 1 ml. For light exposure, we used a fiber
optic light guide to capture and direct light from a 660 nm LED light bulb
into the spectrometer. The end of the fiber optic cable was positioned
1 cm above the reaction mixture. Prior to these experiments, light power
was measured 1 cm from the end of the fiber optic cable.
Oxygen consumption measurement
Mitochondrial oxygen consumption was measured using an oxygen
electrode cuvette (OX1LP-1 ml; Qubit Systems Inc., Kingston, ON,
Canada) according to the manufacturer’s instructions. Reactions were
run with mitochondria at a concentration of <1 mg protein/m’ in
buffer A. For light exposure, a 660-nm LED was directed at the plastic
[poly-(methyl methacrylate)] chamber.
For inhibition of respiration, sodium azide was added at a final
concentration of 0.005 M from a stock solution in water. Sodium azide
inhibits cytochrome oxidase (complex IV): oxygen consumption during
state 3 respiration is progressively inhibited by increasing concentrations
of azide (Bogucka and Wojtczak, 1966).
Analysis of zero-order ultrasensitivity
Mitochondria from sheep hearts were prepared as previously described
(Smith, 1967) on two separate occasions from 2 and 1 sheep heart(s) using
‘Procedure 1’. We used mitochondrial fragments to allow P-a direct access
to the respiratory chain, to minimize potential complications due to
variable rates of P-a import. Mitochondrial isolation started within 1 hour
of the death of the animal and the hearts were transported to the laboratory
in a bath of 0.25 M sucrose, 0.1 M tris(hydroxymethyl)aminomethane
(Tris) at pH 7.5, which was surrounded by ice. Mitochondria were isolated
and stored in 250 ml aliquots at a concentration of ,60 mg of protein/ml in
300 mM trehalose, 10 mM HEPES–KOH pH 7.7, 10 mM KCl, 1 mM
EGTA, 1 mM EDTA and 0.1% BSA at 280˚
C (Yamaguchi et al., 2007)
until use. The thawed mitochondria exhibited a respiratory control ratio of
,1, indicating mitochondrial fragmentation.
Analysis of coenzyme Q redox status
We used sheep heart mitochondria because they contain relatively
large amounts of CoQ
10
, which expedited analysis. For evaluation of
CoQ
10
redox ratios, frozen mitochondria were thawed at 37˚
C and diluted
with 500 ml buffer A to create a mitochondrial stock solution, which was
kept on ice until use. For reactions, 50 ml of this stock suspension was
added to 500 ml of buffer A, containing 0.5 mg/ml antimycin A from a
25 mg/ml stock solution in ethanol. Antimycin A binds to the Q
i
site of
cytochrome creductase (complex III), thereby inhibiting the upstream
oxidation of any produced ubiquinol. Pa was added (25 mM final
concentration) from a 1.3 mg/ml stock solution in DMSO. The
suspension was added to a test tube, mixtures purged with argon and
the reactions initiated by placing the tube between two LED light
bulbs (previously described). We irradiated the samples for 10 minutes
at room temperature. For negative controls, we repeated the above
sequence changing the following: (1) in the absence of light; (2) in the
absence of added P-a; (3) with heat denatured mitochondria; and (4) in
the absence of added mitochondria but with added coenzyme Q. For a
positive control we added 10 ml of a stock solution of 0.25 M glutamate/
0.125 M malate in Tris buffer at pH 7. For mitochondrial denaturing,
200 ml of stock mitochondrial suspension was purged with argon and
placed in a bath at 70˚
C for 5 minutes. For control reactions without
added mitochondria, a coenzyme Q stock solution in buffer A was
prepared by adding ALL-QTM (DSM Nutritinal products, Switzerland),
a water-soluble coenzyme Q solution containing 10% coenzyme Q,
modified food starch, sucrose and medium chain triglycerides, to
buffer A. For these reactions 50 ml of the water-soluble Coenzyme Q
stock or the denatured suspension was used as above in place of
the mitochondrial stock solution. All reactions were adjusted to give
the same amount of coenzyme Q in the reaction mixture as measured
by HPLC.
To quantify relative ubiquinone and ubiquinol concentrations, a 50 ml
aliquot was taken from the reaction mixture and was added to 200 mlof
0.4 M perchloric acid and 100 ml isopropyl ether containing 1 mg of
butylated hydroxytoluene/ml as an antioxidant. The solution was
vortexed for 1 minute, centrifuged for 2 minutes at 15,000 r.p.m. and
the organic phase analyzed by HPLC. HPLC conditions have been
reported previously (Qu et al., 2009; Qu et al., 2011). Briefly, we used an
isocratic elutent consisting of 1% sodium acetate 3% glacial acetic acid,
5% butanol in methanol at 0.6 ml/minute. The HPLC column was
5062.1 mm, C-18, 2.6 u, 100 A
˚(Phenomenex, Torrance, CA). A PDA
detector set at 290 nm for ubiquinol and 275 nm for ubiquinone was
used. We determined relative ubiquinol and ubiquinone concentrations
by their online absorption spectra using extinction coefficients of
14,200 M
21
cm
21
at 275 nm in ethanol for ubiquinone and
4640 M
21
cm
–1
at 290 nm in ethanol for ubiquinol (Lester et al., 1959).
Analysis of ATP synthesis in mouse brain homogenates
To produce homogenates of mouse brain, the frontal lobe was
homogenized using two strokes of a Potter S homogenizer (Sartorius
AG, Goettingen, Germany) at 4˚
C (20 mg of brain to 1 ml buffer A). The
homogenate (80 ml) was added to buffer A (920 ml) and treated as
described above for liver samples. Reactions were run in triplicate and
data obtained between 5 and 50 minutes after lysis. ATP production
showed a linear increase during this time, which was fitted to a line, the
slope of which is reported as the relative ATP synthesis rate.
Analysis of ATP synthesis in mouse lens and heart homogenates
Lenses from mice were homogenized (KONTESHDUALLHtissue
grinder with glass pestle) in ATP assay buffer (0.15 mM sucrose,
0.5 mM EDTA, 5 mM magnesium chloride, 7.5 mM sodium phosphate,
2 mM HEPES) at 50 ml buffer per lens. We added 1 ml of P-a stock
(1 mM) and 1 ml of ADP stock (10 mM) to 100 ml lens homogenate. The
mixture was exposed to red light (671 nm at 0.8 W/m
2
) or kept in dark
for 20 minutes. ATP concentrations were determined using a luciferase-
based ATP quantification kit according to the manufacture’s instructions
(Life Technologies, Grand Island, NY).
Heart tissue (20 mg) was homogenized as above in 1 ml ATP assay
buffer. 10 ml of P-a (1 mM) and 10 ml of ADP (10 mM) and 940 mlof
ATP assay buffer were added into 40 ml tissue homogenate. The mixture
was exposed to red light and ATP was determined as described above
using a luciferase based ATP kit.
Analysis of ATP concentrations in duck adipose
We removed visceral fat from a duck (Anas platyrhynchos domestica)
less then 30 minutes after death by decapitation and homogenized the fat
at 4˚
C (without buffer) in a loose-fitting Potter-Elvehjem homogenizer.
We then added P-a (70 ml of a 3.3 mg/ml stock solution) and ADP
(800 ml of a 10 mg/ml stock solution). The homogenate was divided into
two groups: one group was kept in the dark, while the other was exposed
to red light (671 nm at 0.8 W/m
2
); both dishes were kept at 37˚
C. 200-ml
aliquots were taken from each dish and ATP was measured using
the luciferase assay or by HPLC, as described in the literature (Ally and
Park, 1992).
Analysis of the effect of light wavelength
The entire brain of a mouse was homogenized with a Dounce
homogenizer (20 mg of brain to 1 ml buffer C: 0.15 mM sucrose,
0.5 mM EDTA, 5 mM MgCl
2
, 7.5 mM Na
2
HPO
4
, 2 mM HEPES) at 4˚
C.
We took a 40-ml aliquot of the homogenate and added it to 940 ml buffer
C. We added 10 ml P-a (from a 1 mM stock in DMSO) and placed the
sample on ice for 1 hour. We then added 10 ml ADP (from a 10 mM
stock). Five 100-ml portions of the suspension were added to each well of
a 96-well plate and exposed to light for 40 minutes. Then, 20-ml aliquots
of the mixture were lysed with 200-ml lysis buffer for 1 hour on ice,
and ATP levels were determined as above using a luciferase-based
ATP kit.
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Analysis of red fluorescence in tissue extracts
The chlorophyll-rich diet (Harlan Teklad, Indianapolis, IN) contained
15% by weight spirulina [a food supplement produced from
cyanobacteria (Ciferri, 1983)], which is equivalent to ,0.15% by
weight chlorophyll-a. The control diet was a purified diet devoid of
dietary chlorophylls (Harlan Teklad). The swine chlorophyll-rich diet has
been described previously (Mihai et al., 2013).
For fluorescence spectroscopy, five pigs each were given these
respective diets ad libitum for 2 weeks. Whole brain or 2–7 grams of
abdominal fat was homogenized with a hand-held homogenizer (Omni
Micro Homogenizer (mH), Omni International, Kennesaw, GA), HPLC
grade acetone (40 ml) was added and the sample was vortexed for
1 minute. Insoluble material was precipitated by centrifugation and the
acetone evaporated with a rotary evaporator. The samples were
resuspended in 3 ml chloroform and measured directly.
For HPLC and UV spectroscopy, we extracted 2.5 grams of fat, as
described above, from rats or swine that had been given a chlorophyll-
rich diet, to give a clear oil. We then added 10 ml of absolute ethanol,
cooled the sample to 220˚
C for 30 minutes, pelleted the insoluble
material by centrifugation, separated and evaporated the ethanol with a
rotary evaporator and re-suspended the sample in 500 ml of absolute
ethanol. For plasma, we added 4 ml of plasma to 1 ml of saturated NaCl
and 10 ml ethyl acetate, vortexed the sample for 1 minute and separated
the layers by centrifugation. We removed the ethyl acetate layer,
evaporated the ethyl acetate and re-suspended the resulting film in 300 ml
of absolute ethanol. The samples were then used for HPLC and UV
spectroscopy. A Waters (Milford, MA) HPLC system with a 600 pump, a
2475 fluorescent detector, a 2998 photodiode array (PDA) detector and a
C18, 2.6 u, 100 A
˚,15062.10 mm column (Phenomenex, Torrance, CA)
was used for HPLC. Excitation was set to 410 nm and emission set to
675 nm. Absorbance between 275 and 700 was recorded. We used a
mobile phase of acetonitrile containing 10% isopropyl alcohol and 0.1%
formic acid (solvent A) and water containing 0.1% formic acid (solvent B).
Compounds were eluted at a flow rate of 0.3 ml/minute with a 50:50
mixture of A:B for 5 minutes, which was changed linearly to 100:0, A:B
over 15 minutes. At 35 minutes, the flow was increased to 0.5 ml/minute.
In vivo
imaging
Animals were imaged with a Maestro
TM
In-Vivo Imaging System (CRi,
Hopkinton, MA), as described by Bouchard et al.; the animals were
skinned to reduce interference from skin autofluoresence (Bouchard
et al., 2007).
General
C. elegans
maintenance
Worms were a gift from Dr Cristina Lagido (Department of Molecular
and Cell Biology, University of Aberdeen Institute of Medical Sciences,
Foresterhill, Aberdeen, UK) (Lagido et al., 2009; Lagido et al., 2001).
Nematode husbandry has been described previously (Wood, 1988).
Briefly, animals were maintained on nematode growth medium (NGM)
agar (Nunc) using E. coli strain OP50 as a food source. To obtain
synchronous populations, we expanded a mixed population on egg yolk
plates (Krause, 1995). Worm eggs were isolated from the population by
treatment with 1% NaOCl/0.5 M NaOH solution (Emmons et al., 1979)
and transferred to a liquid culture with E. coli strain OP50, carbenicillin
(50 mg/ml) and amphotericin B (0.1 mg/ml; complete medium).
Real-time ATP monitoring in
C. elegans
We administered the P-a chlorophyll metabolite by adding it to the
culture medium for a minimum of 24 hours. To confirm P-a uptake, we
washed away the culture medium containing P-a, suspended the worms in
fresh medium and determined the fluorescence spectra in the worms.
Treated worms had signature chlorophyll-derived fluorescence, whereas
control worms that were not given P-a exhibited no such fluorescence,
confirming metabolite uptake.
Method A
Worms were grown in liquid culture at a density of 10,000 worms/ml.
Twenty-four hours before the experiment, the culture was split into
control and treatment groups and varying amounts of a P-a stock solution
in DMSO were added to the treated groups. Control worms were given
DMSO vehicle. Worms were washed with M9 buffer (IPM Scientific,
Eldersburg, MD) to remove food and unabsorbed P-a and resuspended at
3000 worms/ml. 50 ml of worm suspension from each of these groups
were plated into a well of a 96-well plate. Each experimental group
was plated into a minimum of 12 wells. To assay ATP stores by
luminescence, 100 ml of luminescence buffer containing D-luciferin was
added to each well, according to the literature (Lagido et al., 2009;
Lagido et al., 2001) and luminescence was recorded in a plate reader. The
luminescence buffer was a citric phosphate buffer at pH 6.5, 1% DMSO,
0.05% Triton X-100 and D-luciferin (100 mM). After initial ATP
measurements, half of the worms from each experimental group were
exposed to LED light centered at 660 nm at 162 W/m
2
; the other half
was kept in the dark by covering the plate with aluminium foil. ATP
(luminescence signal) was recorded periodically. The amount of ATP
synthesized was reported as the difference within an experimental group
between the luminescence signal of worms kept in the dark and the
worms exposed to light. All experimental procedures outside of red light
exposure were performed under dim light. The experiment was repeated
three times with different populations of worms.
Method B
Worms were plated as above, with each experimental group divided into
12 wells of a 96-well plate. Four identical 96-well plates were made, each
containing worms treated with varying concentrations of P-a and control
worms. At time zero, 100 ml of luminescence buffer was added to a plate
and in vivo ATP was assayed as luminescence. The remaining three
plates were exposed to light and ATP assays were performed every
15 minutes for 45 minutes by the addition of 100 ml of luminescence
buffer and the recording of luminescence.
In vitro
ATP monitoring in
C. elegans
One-day-old adult worms in liquid culture were incubated with P-a for
24 hours, washed with M9 buffer and re-suspended in M9 buffer at 50,000
worms/ml. The control group was incubated in DMSO vehicle without P-a.
100 ml of each worm suspension was placed into 18 centrifuge tubes. At
time zero, six tubes from each group were placed in liquid nitrogen and the
remaining tubes exposed to red light. Then, at 15 and 30 minutes, six tubes
from each group were placed into liquid nitrogen. To measure ATP, we
removed the centrifuge tubes from the liquid nitrogen and placed them in
boiling water for 15 minutes to lyse the worms (Artal-Sanz and
Tavernarakis, 2009). The resulting solution was cleared by centrifugation
for 5 minutes at 15,000 rpm and ATP in the lysate was measured using the
luciferase assay according to the manufacturer’s instructions or by HPCL
according to established protocols (Ally and Park, 1992).
Analysis of
C. elegans
oxygen consumption
Oxygen consumption was measured using a Clark-type oxygen electrode
(Qubit Systems Inc.), as described (Anderson and Dusenbery, 1977;
Zarse et al., 2007). One-day-old adult worms in liquid culture at a
density of ,10,000 worms/ml were incubated with P-a (25 mM) for
24 hours in complete medium. Animals were washed three times with
M9 buffer to remove bacteria and excess P-a and resuspended in M9
buffer at 10,000 worms/ml. One-ml aliquots of this suspension were
transferred into the respiration chamber and respiration was measured at
25˚
C for 10 minutes while being exposed to an LED light centered at
660 nm at 162 W/m
2
. The control group was treated in the same way but
not incubated with P-a.
Analysis of ROS formation in
C. elegans
ROS formation was quantified as described by Schulz et al. (Schulz et al.,
2007). Three-day-old worms were synchronized in liquid culture at a
density of 500 worms/ml in complete medium, then divided into control
and treatments groups. The treatment group was incubated for 24 hours
with 12 mM P-a and the control group in DMSO vehicle. Bacterial food
and P-a were removed by three repeated washes with M9 and the worms
resuspended to 500 worms/ml M9 buffer. 50 ml of the suspension from
RESEARCH ARTICLE Journal of Cell Science (2014) 127, 388–399 doi:10.1242/jcs.134262
397
Journal of Cell Science
each group was added to the wells of a 96-well plate with opaque walls
and a transparent bottom. A 100 mM29,79-dichlorofluorescin diacetate
(Sigma-Aldrich, St. Louis, MO) solution in M9 buffer was prepared from
a 100 mM 29,79-dichlorofluorescin diacetate stock solution in DMSO.
50 ml of this solution were pipetted into the suspensions, resulting in a
final concentration of 50 mM. Additional controls included worms
without 29,79-dichlorofluorescin diacetate and wells containing 29,79-
dichlorofluorescin diacetate without animals; these were prepared in
parallel. Five replicates were measured for each experimental and control
group. Immediately after addition of 29,79-dichlorofluorescin diacetate,
the fluorescence was measured in a SpectraMax M5 microplate reader
(Molecular Devices, LLC, Sunnyvale, CA) at excitation and emission
wavelengths of 502 and 523 nm. The plates were then exposed to red
LED light and fluorescence was re-measured at 2.5 and 5 hours under
conditions equivalent to those used previously.
Life span analysis
Population studies
Life span measurements were performed according to the method of
Gandhi et al. and Mitchell et al. (Gandhi et al., 1980; Mitchell et al.,
1979) with some modifications. Eggs were harvested and grown in
darkness in a liquid culture at room temperature. To prevent any progeny
developing, 5-fluoro-29-deoxyuridine (FUDR) (Sigma-Aldrich, 120 mM
final) was added at 35 hours after egg isolation, during the fourth larval
molt. At day 4 of adulthood, the culture was split into control and
experimental groups. The experimental group was treated with 12 mMP-
a from a stock solution in DMSO. The control group was given the
DMSO vehicle alone. The treated and control cultures were then split into
two or three. The final density of worms in all reaction flasks was 500
worms/ml; each flask contained 10 ml, therefore a total of 5000 worms.
The following day (day 5 of adulthood), worms were exposed to LED
light centered at 660 nm at 162W/m
2
for 5 hours. Light exposure was
repeated every day until the end of the experiment. For counting, aliquots
were withdrawn and placed in a 96-well plate to give ,10 worms per well;
the worms were scored dead or alive on the basis of their movement,
determined with the aid of a light microscope. A total of 60–100 worms
(representing 1–2% of the total population) were withdrawn and counted at
each time point for each flask. Counts were made at 2–3-day intervals and
deaths were assumed to have occurred at the midpoint of the interval. Any
larvae that hatched from eggs produced before the FUDR was added
remained small in the presence of FUDR and were not counted. We used
the L4 molt as time zero for life span analysis. To obtain the half-life, we
plotted the fraction alive at each count verses time and fitted the data to a
two-parameter logistic function using the software GraphPad Prism
(GraphPad Software, Inc., La Jolla, CA). The two-parameter model is
known to fit survival of 95% of the population fairly accurately
(Vanfleteren et al., 1998). Because changes in environment, such as
temperature, worm density and the amount of food, can influence life span,
control measurements were conducted at the same time under identical
conditions. The concentration of P-a dropped (,75%) throughout the life
span studies and it was not adjusted (supplementary material Fig. S4F).
Life span measurements in 96-well microtiter plates
Life span was measured as described in the literature (Solis and Petrascheck,
2011), except that P-a was added at day 4 and light treatment commenced at
day 5. Scoring (fraction alive) was done once on day 15.
Competing interests
The authors declare no competing interests.
Author contributions
C.X. conducted studies with worms, and ATP measurements in mitochondria and
tissue homogenates. J.Z. conducted ATP measurements in mitochondria and
tissue homogenates. D.M. conducted metabolite-binding distribution studies. I.W.
designed and supervised the study and wrote the manuscript.
Funding
This work was supported by the Department of the Navy, Office of Naval
Research [grant number N00014-08-1-0150 to I.W.]; the Nanoscale Science and
Engineering Initiative of the National Science Foundation [grant numbers CHE-
0117752, CHE-0641532 to I.W.]; and the New York State Office of Science,
Technology and Academic Research (NYSTAR).
Supplementary material
Supplementary material available online at
http://jcs.biologists.org/lookup/suppl/doi:10.1242/jcs.134262/-/DC1
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