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Antioxidants, nature and chemistry

 Submitted by Dr. Tamer Fouad, M.D.

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 oxidase (Ensimnger et al.1995). 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.

One important 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?⁻ + 2H⁺ ? H2O2 + DHA

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 (40-60 mm) 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).

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