GGT present in the sample catalyzes the transfer of the glutamyl group from the substrate y-glutamyl-3-carboxy-p-nitroanilide to glycylglycine by Gamma-GT(Gamma-Glutamyl-Transferase). The amountof p-nitroaniline formed is directly proportional to the activity of GST present in the sample and can be measured kinetically at (400-420) nm.