title: Inactivation of gamma-glutamylcysteine synthetase, but not of glutamine synthetase, by S-sulfocysteine and S-sulfohomocysteine.
abstract: 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.
mesh_terms: D000818:Animals; D001921:Brain / Q000201:enzymology; D003538:Cystamine / Q000494:pharmacology; D003545:Cysteine / Q000031:analogs & derivatives* / Q000494:pharmacology; D005974:Glutamate-Ammonia Ligase / Q000037:antagonists & inhibitors*; D005721:Glutamate-Cysteine Ligase / Q000037:antagonists & inhibitors*; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000494:pharmacology; D007668:Kidney / Q000201:enzymology; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D010453:Peptide Synthases / Q000037:antagonists & inhibitors*; D051381:Rats; D012756:Sheep; D013237:Stereoisomerism
chemical_list: D006710:Homocysteine; C054372:S-sulfohomocysteine; D008345:Manganese; C011119:S-sulphocysteine; D005974:Glutamate-Ammonia Ligase; D010453:Peptide Synthases; D005721:Glutamate-Cysteine Ligase; D008274:Magnesium; D003545:Cysteine; D003538:Cystamine