A comparison of reaction conditions for the automated determination of gamma-glutamyltransferase activity in serum.

The methods of Rosalki and Tarlow and the Scandinavian Committee on Enzymes for the determination of gamma-glutamyltransferase activity in serum have been compared, with an automatic reaction-rate analyzer in which the reaction is initiated by the addition of a concentrated solution of gamma-glutamyl-p-nitroanilide. Results are approximately 4% lower by the Scandinavian method, because of its lower substrate concentration. However, both methods are of comparable reproducibility. The greater stability of the more dilute substrate solution specified in the Scandinavian method has been confirmed, but attempts to stabilize solutions of gamma-glutamyl-p-nitroanilide at the higher concentrations recommended by Rosalki and Tarlow by the addition of organic solvents were accompanied by some enzyme inhibition. The comparative instability of the substrate solutionof Rosalki and Tarlow is unlikely to lead to erroneous results; however, the Scandinavian formulation offers the advantage of a more economical use of reagents, at the expense of a small and probably unimportant reduction in sensitivity.