γ-Glutamyltransferase (γ-GT) is a key enzyme in the γ-glutamyl cycle and catalyzes glutathione degradation. It transfers γ-glutamyl groups from glutathione or related compounds to acceptors and plays an important role in extracellular glutathione metabolism. γ-GT catalyzes the transfer of the γ-glutamyl group from γ-glutamyl-p-nitroanilide to glycylglycine, releasing p-nitroaniline with characteristic absorbance at 405 nm. Enzyme activity is calculated from the absorbance increase rate.