Human hepatic microsomal thiol methyltransferase. Assay conditions, biochemical properties, and correlation studies.

Thiol methyltransferase (TMT) catalyzes the S-methylation of aliphatic sulfhydryl drugs and xenobiotic compounds. As a first step in the determination of whether genetic control of TMT activity in an easily accessible human cell, the red blood cell (RBC), might reflect the regulation of TMT in the human liver, we determined optimal conditions for the assay of human hepatic microsomal TMT activity. We then used those assay conditions to study the biochemical properties and regulation of TMT in the human liver for comparison with the properties of TMT in the human RBC. Substrate kinetic studies of hepatic microsomes performed with 2-mercaptoethanol (2-ME) revealed biphasic kinetics similar to those found in the human RBC, with apparent "high-" and "low"-affinity forms of TMT activity. The high-affinity form had an optimal pH of 7.2-7.6 and apparent KM values of 9.0 microM for 2-ME and 7.5 microM for S-adenosyl-L-methionine. The low-affinity form had an optimal pH of 8.8 and apparent KM values of 20 mM for 2-ME and 44 microM for S-adenosyl-L-methionine. Both forms were inhibited by compounds that also inhibited human RBC membrane TMT. The two kinetic forms of hepatic microsomal TMT activity were inactivated approximately 50% by heating for 15 min at 53 degrees C and, as was found with RBC TMT, had very similar thermal stability profiles.(ABSTRACT TRUNCATED AT 250 WORDS)