Thetin–homocysteine transmethylase. Some further characteristics of the enzyme from rat liver

Abstract

The ability of a number of methylsulfonium and ethylsulfonium salts related to dimethylthetin to act as substrates for partially purified preparations of thetin-homocysteine transmethylase from rat liver was examined. Dimethylthetin is the most effective methyl donor so far investigated. Trimethylsulfonium chloride acts as a substrate for the enzyme, and gives rise to a linear reaction rate and a linear enzyme activity/concen-tration relationship. The compound has therefore been considered as a suitable substrate for the manometric assay of the transmethylase. Homocysteine was the only effective methyl-acceptor among the thiols tested, although some evidence was obtained to suggest that L-cysteine might be acting as a substrate. None of the metal ions which were tested activated the enzyme; several heavy-metal ions and potassium ions were inhibitory. Several other commonly-used inhibitors had no effect on the enzyme. Possible mechanisms for the participation of the enzyme in the transmethylation reaction have been proposed.