As shown in Figure 13.3, oxidation of ascorbic acid, for example, by the reduction of superoxide to hydrogen peroxide or Fe3+ to Fe2+, and similar reduction of other transition metal ions, proceeds by a one-electron process, forming the monodehydroascorbate radical. The radical rapidly disproportionates into ascorbate and dehydroascorbate. Most tissues also have both nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione-dependentmon-odehydroascorbate reductases, which reduce the radical back to ascorbate. Ascorbate is thus an effective quencher of singlet oxygen and other radicals.
Dehydroascorbate is unstable in solution, undergoinghydrolytic ring opening to yield diketogulonic acid. However, in vivo, it is normally reduced to
ascorbate by either NADPH or glutathione-dependent reductases. Dehydroascorbate may also be reduced by reaction with homocysteine, forming homocysteic acid; this may be an important source of homocysteic acid for the synthesis of phosphoadenosine phosphosulfate (PAPS) for sulfation reactions (McCully, 1971).
In plants, ascorbate oxidase reduces oxygen to water, in a series of four single-electron steps, forming monodehydroascorbate. This enzyme, and onward nonenzymic oxidation to diketogulonic acid, is responsible for the oxidative loss of much of the vitamin C in vegetables after harvesting. In animals, where the role of ascorbate seems to be mainly as a reducing agent, there is no specific ascorbate oxidase.
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