Pro and Antioxidant Roles of Ascorbate

Ascorbate can act as a radical-trapping antioxidant, reacting with superoxide and a proton to yield hydrogen peroxide, or with the hydroxy radical to yield water. In each case, the product is monodehydroascorbate.

The antioxidant activity of ascorbate is variable. From consideration of the chemistry involved, itwouldbe expectedthat, overall, 2 moles ofperoxyl radical would be trapped per mole of ascorbate, because of the reaction of 2 moles of monodehydroascorbate to regenerate ascorbate and yield dehydroascorbate (see Figure 13.3). However, as the concentration of ascorbate increases, so the molar ratio decreases, and it is only at very low concentrations of ascorbate that it tends toward the theoretical 2:1.

As well as its antioxidant role, ascorbate can be a source of hydroxyl and superoxide radicals. At high concentrations, it can reduce molecular oxygen to superoxide, being oxidized to monodehydroascorbate. At lower concentrations of ascorbate, both Fe3+ and Cu2+ ions are reduced by ascorbate, yielding monodehydroascorbate. Fe2+ and Cu+ are readily reoxidized by reaction with hydrogen peroxide to yield hydroxide ions and hydroxyl radicals. Cu+ also reacts with molecular oxygen to yield superoxide.

It seems likely that the prooxidant actions of ascorbate are of relatively little importance in vivo. Except in cases of iron overload, there are almost no transition metal ions in free solution. They are all bound to proteins, and because the renal transport system is readily saturated, plasma and tissue concentrations of ascorbate are unlikely to rise to a sufficient extent to lead to radical formation (Halliwell, 1996; Carr and Frei, 1999a). Reduction of the Vitamin E Radical by Ascorbate Asdiscussed in Section 4.3.1, one of the major roles of vitamin E is as a radical-trapping antioxidant in membranes and lipoproteins. a-Tocopherol reacts with lipid peroxides forming the a-tocopheroxyl radical, which reacts with ascorbate in the aqueous phase, regenerating a-tocopherol, and forming monodehydroascorbate. Vitamin C may have a vitamin E-sparing antioxidant action, coupling lipophilic and hydrophilic reactions.

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