Suicide inactivation of thioether S-methyltransferase by ethyl sulfide

Thioether S-methyltransferase is an important enzyme in the metabolism of sulfur and selenium-containing compounds in animals. Ethyl vinyl sulfide was previously shown to be a substrate for this enzyme yielding methyl ethyl vinyl sulfonium ion (MEVS+) upon reaction with S-adenosylmethionine. Since vinyl sulfonium ions are reactive toward nucleophiles, 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 time-dependent, pseudo-first-order process with k(inact) and KI values of 0.05 min(-1) and 0.275 mM, respectively. Calculation of the partition ratio revealed one inactivation event for every 100 turnovers. Dimethyl sulfide, an alternate substrate for thioether S-methyltransferase which yields the nonreactive product trimethyl sulfonium ion, protected the enzyme from inactivation by ethyl vinyl sulfide. The inactivation is a result of covalent reaction of methyl ethyl vinyl sulfonium ion with the enzyme as shown by comigration of radioactivity with the enzyme during denaturing gel filtration of reaction mixtures containing thioether S-methyltransferase, ethyl vinyl sulfide, and S-adenosyl[methyl-3H]methionine. Using this method the stoichiometry of inactivation was determined to be 1 mol of [3H]-methyl group/mol of thioether S-methyltransferase inactivated. Both the alternate substrate, dimethyl sulfide, and the competitive product inhibitor, S-adenosylhomocysteine, inhibited such covalent labeling of the enzyme by ethyl vinyl sulfide and S-adenosyl[methyl-3H]methionine. Chemically synthesized MEVS+ inactivated thioether S-methyltransferase, and [methyl-14C]MEVS+ covalently labeled the enzyme with 14C. These results reveal a previously unrecognized mechanism for biochemical activation of vinyl thioethers by methylation to form reactive vinyl sulfonium ions.