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COENZYME Q10
Coenzyme Q10 belongs to a family of compounds known as
ubiquinones. All animals, including humans, can
synthesize ubiquinones, so coenzyme Q10 cannot be
considered a vitamin. The name ubiquinone refers to
the ubiquitous presence of these compounds in living
organisms and their chemical structure, which contains
a functional group known as a benzoquinone.
Ubiquinones are fat-soluble molecules with anywhere
from 1 to 12 isoprene (5-carbon) units. The ubiquinone
found in humans, ubidecaquinone or coenzyme Q10, has a
"tail" of 10 isoprene units (a total of 50
carbons) attached to its benzoquinone "head"
.
FUNCTION
Coenzyme Q is highly soluble in lipids (fats) and is
found in virtually all cell membranes, as well as
lipoproteins. The ability of the benzoquinone head
group of coenzyme Q to accept and donate electrons is
a critical feature in its physiological functions.
Coenzyme Q can exist in three oxidation states : 1)
the fully reduced ubiquinol form (CoQH2), 2) the
radical semiquinone intermediate (CoQH·), and 3) the
fully oxidized ubiquinone form (CoQ).
Mitochondrial ATP synthesis
The conversion of energy from carbohydrates and fats
to adenosine triposphate (ATP), the form of energy
used by cells, requires the presence of coenzyme Q in
the inner mitochondrial membrane. As part of the
mitochondrial electron transport chain, coenzyme Q
accepts electrons from reducing equivalents generated
during fatty acid and glucose metabolism and transfers
them to electron acceptors. At the same time, coenzyme
Q transfers protons outside the inner mitochondrial
membrane, creating a proton gradient across that
membrane. The energy released when the protons flow
back into the mitochondrial interior is used to form
ATP.
Lysosomal function
Lysosomes are organelles within cells that are
specialized for the digestion of cellular debris. The
digestive enzymes within lysosomes function optimally
at an acid pH, meaning they require a permanent supply
of protons. The lysosomal membranes that separate
those digestive enzymes from the rest of the cell
contain relatively high concentrations of coenzyme Q.
Recent research suggests that coenzyme Q plays an
important role in the transport of protons across
lysosomal membranes to maintain the optimal pH for
cellular recycling.
Antioxidant functions
In its reduced form, CoQH2 is an effective fat-soluble
antioxidant. The presence of a significant amount of
CoQH2 in cell membranes, along with enzymes that are
capable of reducing oxidized CoQ back to CoQH2,
supports the idea that CoQH2 is an important cellular
antioxidant. CoQH2 has been found to inhibit lipid
peroxidation when cell membranes and low-density
lipoproteins (LDL) are exposed to oxidizing conditions
outside the body (ex vivo). When LDL is oxidized ex
vivo, CoQH2 is the first antioxidant consumed.
Moreover, the formation of oxidized lipids and the
consumption of a-tocopherol (vitamin E) are suppressed
while CoQH2 is present. In isolated mitochondria,
coenzyme Q can protect membrane proteins and DNA from
oxidative damage that accompanies lipid peroxidation.
In addition to neutralizing free radicals directly,
CoQH2 is capable of regenerating a-tocopherol.
Nutrient Interactions
Vitamin E: Alpha-tocopherol and
coenzyme Q are the principal fat-soluble antioxidants
in membranes and lipoproteins. When alpha-tocopherol
(a-TOH) neutralizes a free radical, such as a lipid
hydroperoxyl radical (LOO·), it becomes oxidized
itself, forming the a-tocopheroxyl radical (a-TO·),
which can promote the oxidation of lipoproteins under
certain conditions in the test tube. When the reduced
form of coenzyme Q (CoQH2) reacts with a-TO·, a-TOH
is regenerated and the semiquinone radical (CoQH·) is
formed. It is possible for CoQH· to react with oxygen
(O2) to produce superoxide (O2·-), which is a much
less oxidizing radical than LOO·. However, CoQH· can
also reduce a-TO· back to a-TOH, resulting in the
formation of fully oxidized coenzyme Q (CoQ), which
does not react with O2 to form O2·
Vitamin B6: The first step in
coenzyme Q10 biosynthesis (the conversion of tyrosine
to 4-hydroxyphenylpyruvic acid) requires vitamin B6 in
the form of pyridoxal 5'-phospate. Thus, adequate
vitamin B6 is essential for coenzyme Q biosynthesis. A
pilot study in 29 patients and healthy volunteers
found significant positive correlations between blood
levels of coenzyme Q10 and measures of vitamin B6
nutritional status. However, further research is
required to determine the clinical significance of
this association.
SOURCES
Biosynthesis
Coenzyme Q10 is synthesized in most human tissues. The
biosynthesis of coenzyme Q10 involves three major
steps: 1) synthesis of the benzoquinone structure from
the amino acids, tyrosine or phenylalanine, 2)
synthesis of the isoprene side chain from
acetyl-coenzyme A (CoA) via the mevalonate pathway,
and 3) the joining or condensation of these two
structures. The enzyme hydroxymethylglutaryl (HMG)-CoA
reductase plays a critical role in the regulation of
coenzyme Q10 synthesis as well as the regulation of
cholesterol synthesis.
Food Sources
Based on food frequency studies the average dietary
intake of coenzyme Q10 in Denmark was estimated to be
3-5 mg/d. Most people probably have a dietary intake
of less than 10 mg/d of coenzyme Q10. Rich sources of
dietary coenzyme Q10 include mainly meat, poultry, and
fish. Other relatively rich sources include soybean
and canola oils, and nuts. Fruits, vegetables, eggs,
and dairy products are moderate sources of coenzyme
Q10. Approximately 14-32% of coenzyme Q10 was lost
during frying, but the coenzyme Q10 content of
vegetables and eggs did not change when boiled. Some
relatively rich dietary sources and their coenzyme Q10
content in milligrams (mg) are listed in the table
below.
Summary
Coenzyme Q10 is a fat-soluble compound
primarily synthesized by the body and also consumed in
the diet.
Coenzyme Q10 is required for mitochondrial ATP
synthesis and functions as an antioxidant in cell
membranes and lipoproteins.
Endogenous synthesis and dietary intake appear
to provide sufficient coenzyme Q10 to prevent
deficiency in healthy people.
Oral supplementation of coenzyme Q10 increases
plasma, lipoprotein, and blood vessel levels, but it
is unclear whether tissue coenzyme Q10 levels are
increased, especially in healthy individuals.
Coenzyme Q10 supplementation has resulted in
clinical and metabolic improvement in some patients
with hereditary mitochondrial disorders.
Although coenzyme Q10 supplementation may be a
useful adjunct to conventional medical therapy for
congestive heart failure, additional research is
needed.
Roles for coenzyme Q10 supplementation in
other cardiovascular diseases, neurodegenerative
diseases, cancer, and diabetes require further
research.
Coenzyme Q10 supplementation does not appear
to improve athletic performance.
Although coenzyme Q10 supplements are
relatively safe, they may decrease the anticoagulant
efficacy of warfarin (Coumadin).
More information
Presently, it is unclear whether individuals
taking cholesterol-lowering medications, known as
HMG-CoA reductase inhibitors (statins), would benefit
from coenzyme Q10 supplementation.
By The Linus Pauling Institute
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