Inactivation of gamma-glutamylcysteine synthetase, but not of glutamine synthetase, by S-sulfocysteine and S-sulfohomocysteine

The reactions catalyzed by gamma-glutamylcysteine synthetase and glutamine synthetase are thought to proceed via enzyme-bound gamma-glutamyl phosphate intermediates. We investigated the possibility that S-sulfocysteine and S-sulfohomocysteine might act as analogs of gamma-glutamyl phosphate or of the associated putative tetrahedral intermediates. The D- and L-enantiomers of S-sulfocysteine and S-sulfohomocysteine were found to rapidly inactivate rat kidney gamma-glutamylcysteine synthetase but to be reversible inhibitors of sheep brain glutamine synthetase. Inactivation of gamma-glutamylcysteine synthetase does not require ATP and is associated with noncovalent binding of close to 1 mol of inactivator/mol of enzyme. The findings indicate that the S-sulfo amino acids are transition-state analogs, and that binding of S-sulfo amino acid to the enzyme induces formation of a very stable enzyme-inactivator complex. The data suggest that stabilization of the enzyme-inactivator complex results from interactions involving the sulfenyl sulfur atom of the S-sulfo amino acid and the active site thiol group of the enzyme.