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Clinical Chemistry 22: 417-421, 1976;
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Clinical Chemistry, Vol 22, 417-421, Copyright © 1976 by American Association for Clinical Chemistry

Gamma-glutamyltransferase: Substrate inhibition, kinetic mechanism, and assay conditions

JH Stromme and L Theodorsen

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.




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Copyright © 1976 by the American Association for Clinical Chemistry.