SUICIDE INACTIVATION OF THIOETHER S-METHYLTRANSFERASE BY ETHYL VINYL SULFIDE

Thioether S-methyltransferase is an important enzyme in the metabolism of sulfur and selenium-containing compounds in animals. Ethyl vinyl s ulfide was previously shown to be a substrate for this enzyme yielding methyl ethyl vinyl sulfonium ion (MEVS(+)) upon reaction with S-adeno sylmethionine. Since vinyl sulfonium ions are reactive toward nucleoph iles, the inactivation of thioether S-methyltransferase as a result of its methylation of ethyl vinyl sulfide was investigated, Ethyl vinyl sulfide was found to inactivate thioether S-methyltransferase in a tim e-dependent, pseudo-first-order process with k(inact) and K-I values o f 0.05 min(-1) and 0.275 mM, respectively, Calculation of the partitio n ratio revealed one inactivation event for every 100 turnovers, Dimet hyl sulfide, an alternate substrate for thioether S-methyltransferase which yields the nonreactive product trimethyl sulfonium ion, protecte d the enzyme from inactivation by ethyl vinyl sulfide. The inactivatio n is a result of covalent reaction of methyl ethyl vinyl sulfonium ion with the enzyme as shown by comigration of radioactivity with the enz yme during denaturing gel filtration of reaction mixtures containing t hioether S-methyltransferase, ethyl vinyl sulfide, and S-adenosyl[meth yl-H-3]methionine, Using this method the stoichiometry of inactivation was determined to be 1 mel of [H-3]-methyl group/mol of thioether S-m ethyltransferase inactivated, Both the alternate substrate, dimethyl s ulfide, and the competitive product inhibitor, S-adenosylhomocysteine, inhibited such covalent labeling of the enzyme by ethyl vinyl sulfide and S-adenosyl[methyl-H-3]methionine, Chemically synthesized MEVS(+) inactivated thioether S-methyltransferase, and [methyl-C-14]MEVS(+) co valently labeled the enzyme with C-14. These results reveal a previous ly unrecognized mechanism for biochemical activation of vinyl thioethe rs by methylation to form reactive vinyl sulfonium ions.