A Novel, Species-specific Class of Uncompetitive Inhibitors of -Glutamyl Transpeptidase*

Expression of -glutamyl transpeptidase (GGT) in tumors contributes to resistance to radiation and chemotherapy. GGT is inhibited by glutamine analogues that compete with the substrate for the -glutamyl binding site. However, the glutamine analogues that have been evaluated in clinical trials are too toxic for use in humans. We have used high throughput screening to evaluate small molecules for their ability to inhibit GGT and have identified a novel class of inhibitors that are not glutamine analogues. These compounds are uncompetitive inhibitors, binding the -glutamyl enzyme complex. OU749, the lead compound, has an intrinsicKiof 17.6 M. It is a competitive inhibitor of the acceptor glycyl-glycine,which indicates thatOU749occupies the acceptor site while binding to the -glutamyl substrate complex. OU749 is more than 150-fold less toxic than the GGT inhibitor acivicin toward dividing cells. Inhibition of GGT by OU749 is species-specific, inhibiting GGT isolated from human kidney with 7–10-fold greater potency than GGT isolated from rat ormouse kidney.OU749doesnot inhibitGGT frompig cells. Human GGT expressed in mouse fibroblasts is inhibited by OU749 similarly toGGT fromhuman cells, which indicates that the species specificity is determined by differences in the primary structure of the protein rather than species-specific, post-translational modifications. These studies have identified a novel class of inhibitors of GGT, providing the basis for further development of a new group of therapeutics that inhibit GGT by a mechanism distinct from the toxic glutamine analogues.