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Antioxidants, nature and chemistry
Submitted by Dr. Tamer Fouad, M.D.
Antioxidants are substances that
protect other chemicals of the body from damaging oxidation
Ascorbic acid (vitamin C)
Source and Nature:
Ascorbic acid (vitamin C) is a
water-soluble, antioxidant present in citrus fruits, potatoes, tomatoes and
green leafy vegetables.
Humans are unable to synthesize
l-ascorbic acid from d-glucose due to absence of the enzyme L-gulacolactone
Hence, humans must therefore obtain ascorbic acid from dietary sources.
Mechanism of Action:
The chemopreventive action of vitamin C
is attributed to two of its functions. It
is a water-soluble chain breaking antioxidant (Ishwarial
et at 1991). As an antioxidant, it scavenges free
radicals and reactive oxygen molecules, which are produced during metabolic
pathways of detoxification. It also prevents formation of carcinogens from
precursor compounds (Block and Menkes,
1988). The structure of
ascorbic acid is reminiscent of glucose, from which it is derived in the
majority of mammals.
property is its ability to act as a reducing agent (electron donor). Ascorbic
acid is a reducing agent with a hydrogen potential of +O.08V, making it capable
of reducing such compounds as a molecular oxygen, nitrate and cytochromes a and
c. Donation of one electron by ascorbate gives the semi-dehydroascorbate radical
(DHA). Ascorbate reacts rapidly with O2?⁻and
even more rapidly with ?OH to
give DHA. DHA, itself can act as a source of vitamin C.
Ascorbic acid + 2O2?⁻
H2O2 + DHA
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It has also been
shown that ascorbate is more potent than a-tocopherol
in inhibiting the oxidation of LDL in a cell free system (Jialal
et at 1990). Co-incubation of LDL with ascorbate during similar oxidative
condition inhibited LDL oxidation and resulted in preservation of the endogenous
antioxidant in the LDL particle (Ishwarial
et at, 1991). The concentration of ascorbate used to inhibit LDL oxidation
is well within the normal plasma range (23-85 pm).
Vitamin C also
contributes to the regeneration of membrane bound oxidized vitamin E. It will
react with the a
-tocopheroxyl radical, resulting in the generation of tocopherol in this process
itself being oxidized to dehydroascorbic acid (Ward
& Peters 1995). Vitamin C supplementation in animals leads to increased
plasma and tissue levels of vitamin E.
In vitro studies suggest that the
antioxidant properties of ascorbic acid may not increase linearly as ascorbic
acid concentrations rise (Frei
et al. 1989). Moreover, ascorbic acid alone can act as a
"pro-oxidant" or reducing agent to react with copper or iron salts.
Ferric iron (Fe3+) formed by the reaction, Fe2+ + H2O2
HO + ?OH + Fe3+, is converted by ascorbic acid to ferrous (Fe2+)
ion. Ferrous iron is therefore recycled to promote the conversion of more H2O2
to ?OH (Halliway et al. 1992).