Mouse liver thiol methyltransferase. Assay conditions, biochemical properties, and strain variation

Thiol methyltransferase (TMT) catalyzes the S-methylation of aliphatic sulfhydryl drugs and xenobiotic compounds. It would be useful if there were an experimental animal model in which the regulation and function of TMT could be studied. Therefore, TMT activity was measured in hepatic microsomes from A/J mice. Substrate kinetics for mouse liver microsomal TMT, like those for the enzyme in human red blood cell membranes and human kidney microsomes, were biphasic, with apparent "high" and "low" affinity forms of TMT with 2-mercaptoethanol as the methyl acceptor substrate. Apparent Michaelis (Km) constants of the high and low affinity activities for 2-mercaptoethanol were 30 microM and 12 mM, respectively. Apparent Km values of the high and low affinity activities for S-adenosyl-L-methionine, the methyl donor for the reaction, were 47 microM and 61 microM, respectively. The optimal pH for the high affinity activity was between 7.2 and 8.1, whereas the optimal pH for the low affinity activity was approximately 9.2. Differential centrifugation showed that more than 85% of both activities was associated with membrane fractions. SKF 525A, a potent inhibitor of TMT in human tissues, inhibited mouse liver high and low affinity TMT activities by 88% and 46%, respectively, at a concentration of 0.5 mM. TMT activities were then measured in hepatic microsomes from nine additional inbred strains of mice. High affinity TMT activities varied 1.7-fold, whereas low affinity activities varied 2.1-fold among these strains. Since the properties of TMT in mouse liver are similar to those of the enzyme in human tissue, the inbred mouse will be a useful experimental animal model in which to study the regulation and function of TMT.