Fluorometric assay of serum gamma-glutamyltransferase in solution and on a semi-solid surface

We describe an enzymatic, fluorometric method for measuring the activity of serum gamma-glutamyltransferase (EC 2.3.2.2) in solution and on silicone rubber pads. N-gamma-L-Glutamyl-alpha-naphthylamide in tris(hydroxymethyl)aminomethane buffer is used as substrate. In solution, measurements are performed at 37 degrees C with use of a reaction volume of 3.0 ml, in Pyrex cuvets. Measurements on silicone rubber pads are also made at 37 degrees C, after establishing a stable substrate film by lyophilizing the reagent for the gamma-glutamyltransferase assay on the surface of the pads. Only 10-50 mul of buffered substrate and serum containing the enzyme to make a total volume of 60 mul are necessary for each assay. The rate of appearance of alpha-naphthylamine fluorescence (lambda ex equals 344 nm; lambda em equals 445 nm), liberated from N-gamma-L-glutamyl-alpha-naphthylamide by the enzymatic action of gamma-glutamyl-transferase, is measured and equated to its activity in serum. Calibration plots of the change in fluorescence per min vs. enzyme concentration for measurements in solution and on pads show a good proportionality in the range of 26.5-265 U/liter, and indicate the usefulness of these methods.