Action of Ascorbic Acid on Enzymes

The formation of serotonin by carcinoid tissue and in smaller amounts by certain normal tissues has been demonstrated to occur from L-trypto-phan by 5-hydroxylation followed by decarboiq lation. The enzyme for 

Hydroxylation of Adrenal Steroids Studies of the biosynthesis of corticosteroids by adrenal glands, either perfused, minced, or in isolated mitochondria do not reveal any increased reaction with ascorbic acid, although the high ascorbic acid content of the adrenals has prompted many such experiments. Under certain conditions such an effect can be observed on the hydroxylation reactions which require molecular oxygen. An example of such a reaction is the 

11-P-hydroxylase of mitochondria which converts DOGA to corticosterone, and which requires TPNH as a reducing cosubstrate. DPNH and ascorbic acid are ineffective in this reaction, but together with the microsomal preparation containing the ascorbic acid-dependent DPNH-oxidase described below, they will replace TPNH. 

Staudinger s interpretation of this involves the central problem of such hydroxylation reactions the generation of free radicals from O2 as the possible hydroxylating agents (K4). The hypothesis is that the reaction of TPNH with O2 in Ae mitochondrial 11-P-hydroxylase generates such free radicals, and so does the oxidation of ascorbic acid by the DPNH-oxidase-cytochrome be system (Eq. 15). The monodehydroascorbic acid formed in the latter system is already well documented. 

Ascorbic Acid-dependent DPNH-Oxidase Microsomes of adrenal medullas contain an enzyme which oxidizes DPNH but not TPNH, and like the similar enzyme from plant tissues (K2, N2) utilizes oxidized ascorbic acid but not dehydroascorbic acid as the acceptor. Complexed Fe + in ferricyanide or cytochrome c can also serve 

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