Stimulation of Enzyme Activity by Ascorbate In Vitro

Over the years, a number of enzymes have been assumed to be ascorbate-dependent, because their activity is stimulated in vitro by the addition of ascorbate to the incubation medium. In general, these reactions are not ascorbate-dependent ascorbate is one of a variety of reducing reagents that enhance the reaction. 

Ascorbate is also frequently added to the incubation medium to remove hydrogen peroxide formed during a variety of reactions. Again, a number of reducing agents have the same action, and in vivo this role would presumably be performed by catalase. 

In vitro, ascorbate and Fe + ions are frequently used as a source of superoxide for such enzymes as indoleamine dioxygenase. Although ascorbate does have prooxidant and superoxide generating activity (Section 13.3.7), there is no evidence that it is the physiological source of this radical for superoxideutilizing enzymes. 

Nonheme iron is absorbed as Fe +, and not as Fe + ascorbic acid in the intestinal lumen will both maintain iron in the reduced state and also chelate it, thus increasing absorption. A dose of 25 mg of vitamin C taken together with a semisynthetic meal increases the absorption of iron 65%, whereas a 1 -g dose gives a nine-fold increase (Hallberg, 1982). 

This is an effect of ascorbic acid present together with the test meal neither intravenous administration of vitamin C nor supplements several hours 

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Ascorbic acid has been shown to increase collagen synthesis by fibroblasts in vitro (99, 280) and to maintain collagen synthesis In nonmitotic fibroblEists for extended periods (99). Prolyl hydroxylase, the enzyme hydroxylating prolyl and lysyl residues of procollagen, requires ascorbate to function in vitro (181), and the addition of ascorbic acid to tissue cultures stimulates the prolyl hydroxylase activity of fibroblasts (174). 

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