Ascorbic acid is a required cofactor for several important dioxygenases, functioning as an electron donor in each of these reactions (Table 28.12). These dioxygenases are involved in the hydroxylation of many important biomolecules during their synthetic sequence including collagen, peptide hormones, steroids, and norepinephrine. The resulting oxidized product, semidehydroascorbate (Fig. 28.50), can be reduced back to ascorbic acid through cytochrome-^ reductase (EC 22.214.171.124) or thioredoxin-disulfide reductase (EC 126.96.36.199). Alternatively, two molecules of semidehydroascor-bate can undergo a disproportionation reaction yielding one molecule of ascorbic acid and one molecule of dehy-droascorbate. The latter can also be converted to ascorbic acid by thioredoxin-disulfide reductase (EC 188.8.131.52) or reduction by glutathione that may be enzymatically or nonen-zymatically mediated.242
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