Inhibition of γ-Glutamylcysteine Synthetase by l-Methionine-S-sulfoximine

Abstract

Abstract Methionine sulfoximine was found to be an effective inhibitor of γ-glutamylcysteine synthetase. The data indicate that methionine sulfoximine binds to the glutamate site of the enzyme and that, in the presence of ATP and metal ions (Mg2+ or Mn2+), it is converted to methionine sulfoximine phosphate. Of the four stereoisomers of methionine sulfoximine, only l-methionine-S-sulfoximine inhibits the enzyme; this is the same stereoisomer which inhibits glutamine synthetase and which causes convulsions in mice. Chemically synthesized l-methionine sulfoximine phosphate inhibits the enzyme in the presence of Mg2+ or Mn2+. These findings indicate that the mechanism of inhibition of γ-glutamylcysteine synthetase by methionine sulfoximine is similar to that by which this compound was previously shown to inhibit glutamine synthetase. However, methionine sulfoximine phosphate is less tightly bound to γ-glutamylcysteine synthetase than to glutamine synthetase; thus, under certain conditions methionine sulfoximine phosphate can be released from inhibited γ-glutamylcysteine synthetase with restoration of catalytic activity. The findings are in accord with the view that enzyme-bound γ-glutamyl phosphate is an intermediate in the reaction catalyzed by γ-glutamylcysteine synthetase, and that phosphorylation of methionine sulfoximine by the enzyme reflects the phosphorylation step of the normal catalytic reaction.