Inactivation of Human Gamma-glutamylcysteine Synthetase by Cystamine. Demonstration and Quantification of Enzyme-ligand Complexes

Human erythrocyte gamma-glutamylcysteine synthetase is inactivated by the disulfide cystamine (2,2'-dithiobis-(ethylamine)) at pH 8.2 with a rate constant of 1020 min-1 mM-1. Magnesium ion and various combinations of substrates and products confer differing degrees of protection against cystamine inactivation, thus allowing the detection and quantification of certain enzyme-ligand interactions. By measuring inactivation rates as a function of ligand concentrations in incomplete reaction mixtures, we have obtained evidence for the following complexes: enzyme . Mg2+; enzyme . Mg2+ . MgATP2-; enzyme . Mg2+ . L-glutamate; enzyme . Mg2+ . MgATP2- . L-glutamate; enzyme . Mg2+ . L-gamma-glutamyl-L-alpha-aminobutyrate. The data also imply the existence of enzyme . (Mg2+)2 . MgATP2- . L-glutamate and several enzyme forms resulting from the weak binding to L-alpha-aminobutyrate. The methods used permit the calculation of cystamine inactivation rates for most of these enzyme forms and also give values for the equilibrium constants describing their formation.