Gamma-glutamyltransferase: Substrate inhibition, kinetic mechanism, and assay conditions.
Gamma-glutamyltransferase activity in serum is shown to be competitively inhibited by the two substrates gamma-glutamyl-4-nitroanilide and glycylglycine. Awareness of this is of importance when one is choosing final reaction conditions for the assay of the enzyme. Gamma-glutamyltransferase probably acts by a "ping-pong bi-bi" kinetic mechanism, which fits with the double competitive substrate inhibition demonstrated. The product, 4-nitro-aniline, appears to be an uncompetitive dead-end inhibitor of both substrates. Various amino acids, particularly glycine and L-alanine, inhibit the enzyme. Their inhibition patterns are uncompetitive with glycylglycine and competitive with gamma-glutamyl-4-nitroanilide. On the basis of the present and other studies, the Scandinavian Society for Clinical Chemistry and Clinical Physiology is going to recommend for routine use a gamma-glutamyltransferase method in which the final concentrations of gamma-glutamyl-4-nitroanilide and glycylglycine are 4 and 75 mmol/liter, respectively.
D000596:Amino Acids / Q000494:pharmacology; D000813:Anilides / Q000378:metabolism / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D005246:Feedback; D006033:Glycylglycine / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D013329:Structure-Activity Relationship; D005723:gamma-Glutamyltransferase / Q000037:antagonists & inhibitors / Q000097:blood / Q000378:metabolism*
D000596:Amino Acids; D000813:Anilides; D006033:Glycylglycine; D005723:gamma-Glutamyltransferase