Monodehydroascorbic acid reductase

As shown in Figure 13.3, oxidation of ascorbic acid, for example, by the reduction of superoxide to hydrogen peroxide or Fe + to Fe +, 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-dependent monodehydroascorbate reductases, which reduce the radical back to ascorbate. Ascorbate is thus an effective quencher of singlet oxygen and other radicals. 

Harwood Jr., H. J., Greene, Y. J., and Stacpoole, P. W., 1986, Inhibition of human leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by ascorbic acid An effect mediated by the free radical monodehydroascorbate, J. Biol. Chem. 261 7127-7135. 

Oxidoreductases (such as ascorbate oxidase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, superoxide dismutase and ascorbate cytochrome-fo reductase) that act in the metabolism of vitamin C in animals and plants can be considered to be antivitamins C. Other oxidoreductases, such as enzymes known trivially as polyphenoloxidases and some others, may indirectly cause loss of ascorbic acid. 

As shown in Figure 1, oxidation of ascorbic acid proceeds by a one-electron process, forming mono-dehydroascorbate, which disproportionates to ascorbate and dehydroascorbate. Most tissues also contain monodehydroascorbate reductase (EC 1.6.5.4), a flavoprotein that reduces the radical back to ascorbate. Dehydroascorbate is reduced to ascorbate by dehydroascorbate reductase (EC 1.8.5.1), a glutathione-dependent enzyme little is oxidized to diketogulonic acid in human beings. 

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