Practical Points Regarding Routine Determination of γ -Glutamyl Transferase (γ-GT) in Serum with a Kinetic Method at 37°C

Kinetic data at 37°C on γ-GT activity in serum with γ-glutamyl-4-nitroanilide and glycylglycine as substrates are reported. The apparent Km for the two substrates were 0.95 and 9.5 mmol/l, respectively. Without glycylglycine the enzymatic formation of 4-nitroaniline was only 5 per cent of that with both substrates. 4-nitroaniline showed a slight inhibitory effect on the γ-GT activity. On the basis of the results an optimized modification at 37°C sof the traditional method of Szasz (17) is described. The method is well suited for automation, e.g. in LKB 8600 Reaction Rate Analyzer. Somewhat higher reference values for men (9-35 U/l) than women (6-25 U/l) were found. Data on purity, solubility, and stability of the reagents, and on the precision of the method are given.