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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
lactone oxidase. The term vitamin C refers to both ascorbic acid and dehydroascorbic acid (DHA), since both exhibit anti-scorbutic activity. Ascorbic acid, the functional and primary in vivo form of the vitamin, is the enolic form of an α-ketolactone (2,3-didehydr L -threo-hexano-1,4-lactone). The two enolic hydrogen atoms give the compound its acidic character and provide electrons for its function as a reductant and antioxidant. Its one-electron oxidation product, the ascorbyl radical, readily dismutates to ascorbate and DHA, the two-electron oxidation products. Both the ascorbyl radical and DHA are readily reduced back to ascorbic acid in vivo. However, DHA can be hydrolyzed irreversibly to 2,3-diketogulonic acid. The molecular structure of ascorbic acid contains an asymmetric carbon atom that allows two enantiomeric forms, of which the L form is naturally occurring (the D -form, isoascorbic or erythorbic acid, provides antioxidant but little or no anti-scorbutic activity), as shown in Figure 5-1.
The biological functions of ascorbic acid are based on its ability to provide reducing equivalents for a variety of biochemical reactions. Because of its reducing power, the vitamin can reduce most physiologically relevant reactive oxygen species (Buettner, 1993). As such, the vitamin functions primarily as a cofactor for reactions requiring a reduced iron or copper metalloenzyme and as a protective antioxidant that operates in the aqueous phase both intra- and extracellularly (Englard and Seifter, 1986; Halliwell and Whiteman, 1997; Tsao, 1997). Both the one- and the two-electron oxidation products of the vitamin are readily regenerated in vivo—chemically and enzymatically—by glutathione, nicotinamide adenine dinucleotide (NADH), and nicotinamide adenine dinucleotide phosphate (NAD-PH) dependent reductases (May et al., 1998; Park and Levine, 1996).
Vitamin C is known to be an electron donor for eight human enzymes. Three participate in collagen hydroxylation; two in carnitine biosynthesis; and three in hormone and amino acid biosynthesis. The three enzymes that participate in hormone and amino acid biosynthesis are dopamine-β-hydroxylase, necessary for the biosynthesis of the catecholamines norepinephrine and epinephrine; peptidyl-glycine monooxygenase, necessary for amidation of peptide hormones; and 4-hydroxyphenylpyruvatedioxygenase, involved in tyrosine metabolism. Ascorbate's action with these enzymes in-