Human erythrocyte membrane thiol methyltransferase. S-methylation of captopril, N-acetylcysteine, and 7 alpha-thio-spirolactone.

Thiol methylation is an important pathway in the biotransformation of sulfhydryl drugs such as captopril. Human red blood cell (RBC) membranes contain a thiol methyltransferase (TMT) activity that catalyzes the S-methylation of 2-mercaptoethanol (2-ME). These experiments were performed to determine whether human RBC membranes contained enzymes that could catalyze the S-methylation of thiol drugs, and, if so, to determine whether those enzymes were similar to the RBC membrane TMT that catalyzes the S-methylation of 2-ME. Human RBC membranes were able to catalyze the methylation of captopril, N-acetylcysteine, and 7 alpha-thio-spirolactone, a sulfhydryl metabolite of spironolactone. Those activities and 2-ME TMT were similar with respect to subcellular distribution, inhibitor sensitivity, and thermal stability. When activities of the methyltransferase enzymes for 2-ME and for the three thiol drugs were measured in RBC membranes from 19 individual subjects, there were highly significant correlations among all four activities (r greater than 0.96 for all comparisons). These observations suggested either that a single enzyme in the human RBC membrane catalyzed the S-methylation of all of these compounds, or, less likely, that these were four separate activities regulated in parallel with similar properties. Experiments were then performed to identify the products of the enzyme reactions. The product of the reaction performed with 7 alpha-thio-spirolactone was the expected S-methyl derivative, 7 alpha-thiomethyl-spirolactone. However, it was found that lipophilic conjugates of the S-methyl derivatives of captopril and N-acetylcysteine were formed during the in vitro enzyme reaction.(ABSTRACT TRUNCATED AT 250 WORDS)