HUMAN-ERYTHROCYTE MEMBRANE THIOL METHYLTRANSFERASE - S-METHYLATION OF CAPTOPRIL, N-ACETYLCYSTEINE, AND 7-ALPHA-THIO-SPIROLACTONE

  • 1 January 1984
    • journal article
    • research article
    • Vol. 12  (6) , 717-724
Abstract
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 catalyzed 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 3 thiol drugs were measured in RBC membranes from 19 individual subjects, there were highly significant correlations among all 4 activities (r > 0.96 for all comparisons). Apparently, a single enzyme in the human RBC membrane catalyzed the S-methylation of all of these compounds, or, less likely, these were 4 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.-thiospirolactone was the expected S-methyl derivative, 7.alpha.-thiomethylspirolactone. Lipophilic conjugates of the S-methyl derivatives of captopril and N-acetylcysteine were formed during the in vitro enzyme reaction. The lipophilic conjugate of captopril was identified as the ethyl ester of the S-methyl metabolite. It remains to be determined whether lipophilic conjugates of the S-methyl metabolites of these and other carboxylic acid sulfhydryl drugs may be formed in vivo.