Oxidation of choline-like substances by rat liver preparations; inhibitors of choline oxidase.

The work of Moyer and du Vigneaud (1) has demonstrated that, of a considerable number of compounds related to choline, choline itself, betaine, its sulfur analogue dimethylthetin, and the dimethylethyl analogue of choline, i.e. dimethylethyl-P-hydroxyethylammonium chloride, are the only ones which will permit rat growth on a methionine-free homocystinecontaining diet. Subsequent work by Dubnoff and Borsook (2, 3) and by Muntz (4) suggests that choline can furnish a labile methyl group for methionine synthesis only after it has been oxidized to betaine by means of the enzyme choline oxidase. In this paper are reported the studies of choline oxidase with various analogues a,nd homologues of choline as substrates in order to ascertain whether or not this high degree of compound specificity of dietary methyl donors could be related to the specificity of choline oxidase for substrate. For these studies, rat liver homogenates and preparations of rat liver mitochondria (5) were used as sources of the enzyme. It was observed that not only were choline and the dimethylethyl analogue of choline oxidized, but several choline-like compounds which are known (1) not to be methyl donors in vivo were also oxidized. Among these substances were diethylmethylcholine, p-methylcholine, and 01, a-dimethylcholine. It was also observed that several of these substances which mere not oxidized at all, or were oxidized only very slowly, had the ability to inhibit markedly the oxidation of choline. Compounds of this class were dimethylaminoethanol, a, a-dimethyltriethylcholine, and 2-amino-2-methylpropanol-1. The inhibitory activity exhibited by these compounds indicated that choline oxidase had a strong affinity for them. Since in these studies purified choline oxidase was not used, it could only be presumed that in the rat liver homogenat,es and rat liver mitochondrial preparations choline oxidase was the effective enzyme. However, those compounds which were oxidized by the rat liver homogenates and rat liver mitochondrial preparations were not oxidized at all by guinea pig liver homogenates, a preparation which has been demonstrated to contain no