Serine-borate complex as a transition-state inhibitor of gamma-glutamyl transpeptidase

gamma-Glutamyl transpeptidase, a membrane-bound enzyme, functions in the gamma-glutamyl cycle to catalyze utilization of glutathione. It has been postulated that the amino-acid-stimulated utilization of glutathione by gamma-glutamyl transpeptidase reflects an aspect of amino acid translocation. As one approach to the effective in vivo inhibition of this enzyme, the inhibition of the enzyme by L-serine in the presence of borate buffers [Revel, J.P. & Ball, E.G. (1959) J. Biol. Chem. 234, 577-582] was reinvestigated. Inhibition by L-serine, D-serine, and alpha-methyl-DL-serine in the presence of borate is competitive with respect to gamma-glutamyl substrate and such inhibition is parallel to the activity of transpeptidase toward L-gamma-glutamyl, D-gamma-glutamyl, and L-gamma-(alpha-methyl)glutamyl derivatives. L-Serine and borate effectively protect against inactivation of the enzyme by the gamma-glutamyl analogs, 6-diazo-5-oxonorleucine and azaserine, which bind to the gamma-glutamyl site of the enzyme. These studies, kinetic investigations, equilibrium dialysis experiments, and other data support the view that inhibition is produced by formation of serine-borate complex which binds at the gamma-glutamyl binding site of the light subunit of gamma-glutamyl transpeptidase. The data indicate that serine-borate complex is a transition state inhibitor of gamma-glutamyl transpeptidase.