Studies on the stability of the choline oxidase.

The choline oxidase was first described by Bernheim and Bernheim (1). Later it was shown (2) that choline at pH 6.7 was oxidized to betaine aldehyde which was then only slowly oxidized further, whereas at pH 7.8 the oxidation proceeded rapidly to betaine. This indicated that betaine aldehyde was oxidized by another enzyme system, and Klein and Handler (3) were able to show that its oxidation required DPN’ but that of choline did not. The choline oxidase is inhibited by fatty acids (4) and its activity is greatly depressed in fatty livers (5). Its importance in the development of fatty livers caused by choline deficiency is shown by the fact that guinea pigs which lack the enzyme cannot be made to develop fatty livers (6) and hamsters which have some enzyme but much less than rats never accumulate as much liver fat on a choline-deficient diet as rats do (7). The enzyme can thus be considered a disadvantage when minimal amounts of choline are present for fat transport and metabolism. On the other hand, the recent work of Dubnoff (8) and Muntz (9) shows that it plays an important r81e in transmethylation reactions, for apparently only betaine aldehyde or betaine can act as a methyl donor. Animals which lack the enzyme can therefore presumably obtain their methyl groups from methionine only. Because of the importance of the enzyme a further study of its properties seems in order. It has previously been shown (1) that it is cyanide-sensitive and that it is readily inhibited by copper and phenylhydraeine (10). Barron and Singer (11) place it in the group of sulfhydryl enzymes. It is very sensitive to diamidines (12). The following is a study of the stability of the enzyme and the effect of cystine and certain metal ions upon it.