Hepatic oxidation of choline and arsenocholine

Abstract

Arsenocholine, like choline, was oxidised by choline oxidase in rat liver to the corresponding aldehyde. This was followed probably by oxidation to arsenobetaine. The substitution of N by As reduced the affinity of the substrate for the enzyme. Secondary decomposition of arsenocholine took place in the presence of rat liver with liberation of a volatile reducing compound having a strong garlic odor (?trimethylarsine). In the presence of semicarbazide, fixation of betaine aldehyde and arsenobetaine aldehyde took place, kinetic studies showing an O2 uptake corresponding to the formation of these substances. Choline oxidase consisted partly of a typical dehydrogenase. This was shown by the fact that rat liver extracts, in the presence of choline or arsenocholine, rapidly reduced Na ferricyanide under anaerobic conditions. This system was cyanide-insensitive and was studied mano-metrically in a bicarbonate medium. Choline dehydrogenase, in the presence of choline or arsenocholine, rapidly reduced cytochrome C at room temp. The choline oxidase system probably consisted of choline dehydrogenase, cytochrome C and cytochrome (indophenol) oxidase. Choline oxidation was inhibited by ammonium ions, trimethyl ammonium ions, and betaine. The group NR3 (R = H or CH3) was evidently of primary importance for the access of the substrate to the enzyme.