Enhancing Effect of Glutamate on Apparent Serum γ-Glutamyltranspeptidase Activity

Free glutamate increases the apparent serum γ-glutamyltranspeptidase (γ-GTP) activity in the normal range by 100%, and in the 10-fold normal range by 20%. This increase is greatest, and is the same, at two very different concentrations, 60 and 1000 µmol of glutamate per liter of the final reaction mixture. Glutamate concentrations greater than 10 mmol/liter inhibit serum γ-GTP. Elevated serum glutamate concentrations or glutamate released by substrate breakdown will therefore give false and possibly variable results. To minimize variations in activity, the assay should be run in the presence of 1.0 mmol of glutamate per liter. This concentration, which is 100 times greater than that which normal serum would contribute (with respect to free glutamate) to the assay system, thus masks any effect of extraneous glutamate.