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

Abstract

.gamma.-Glutamyl transpeptidase, a membrane-bound enzyme, functions in the .gamma.-glutamyl cycle to catalyze utilization of glutathione. The amino-acid-stimulated utilization of glutathione by .gamma.-glutamyl transpeptidase may reflect an aspect of amino acid translocation. As one approach to the effective in vivo inhibition of this enzyme, the inhibition of the [rat kidney] enzyme by L-serine in the presence of borate buffers 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 a 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.