Gama-Glutamyl Transpeptidase and Glutathione Function

γ-Glutamyl transpeptidase (5-L-glutamyl-peptide: amino acid 5-glutamyl transferase; EC 2.3.2.2; abbreviated GGT), is a plasma membrane-associated enzyme that is located on the outer surface of the cells of secretory tissues [1]. It plays a central role in the metabolism of glutathione that is widely in various mammals [2]. The enzyme catalyzes the degradation of the extracellular glutathione in a well identified cycle known as the γ-glutamyl cycle. In addition, GGT plays a role in the formation of mercapturic acid and the slow reacting substance, leukotriene [3,4]. This enzyme in serum from normal subjects and liver disease patients was separated into multiple molecular forms, which differ in electric charge and relative molecular mass but have similar kinetic behavior [5]. GGT is usually assayed by using γ-glutamyl p-nitroanilide as a synthetic γ-glutamyl donor substrate. It is preferred more than the natural substrate glutathione due to the greater analytical convenience of the former compared to glutathione and the much greater solubility of the synthetic substrate [6].