gamma-Glutamyltransferase in human serum: an analysis of kinetic models

The purpose of the study was to elucidate details of the kinetic model for gamma-glutamyltransferase when assayed with gamma-glutamyl-3-carboxy-4-nitroanilide and glycylglycine as substrates. Data from several sets of initial velocity measurements were fitted by nonlinear regression to a set of different kinetic models. gamma-Glutamyltransferase acts by a "ping-pong bi-bi" mechanism. A model encompassing transfer and autotransfer, competitive inhibition by the acceptor substrate, and no inhibition by the donor substrate gives the best fit to the experimental data. Effects of spectrophotometric nonlinearity may simulate noncompetitive inhibition by the donor substrate. The nonlinearity is dependent on the absorption of the incubation mixture and therefore is related to the concentration of the donor substrate and the wavelength (405-412 nm) used to monitor the reaction. With decreasing pH the autotransfer fraction decreases and the binding of the donor substrate to the acceptor site increases, simulating an increased competitive inhibition by the donor substrate. These results are of importance when elaborating optimum assay conditions for gamma-glutamyltransferase in serum.