Carcinogen treatment increases glutathione hydrolysis by gamma-glutamyl transpeptidase.

Abstract

The effect of carcinogen treatment on gamma-glutamyl transpeptidase (GGT)-mediated hydrolysis of GSH to glutamate and cysteinylglycine in the blood and bile compartments was investigated in livers perfused in situ. Treatment of rats with 40 p.p.m. diethylnitrosamine (DEN) in the drinking water or 0.02% 2-acetylaminofluorene (AAF) in the diet for 50-60 days increased GGT activity in liver homogenates by 100 and 800% respectively. Bile flow and the sum of glutamate and glutathione (GSH) efflux into the bile of perfused livers was not affected by carcinogen treatment. However, the ratio of GSH to glutamate in bile was 2.1, 1.1 and 0.2 in livers from control, DEN- and AAF-treated rats respectively. Pretreatment with L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT125) decreased GGT activity in liver homogenates by about 85% and elevated the ratio of GSH to glutamate in the bile to 3.2 in all groups. Thus, the hydrolysis of GSH to glutamate in the bile of perfused livers correlated with the degree of induction of GGT by DEN and AAF treatments. Exogenous GSH (10 microM) infused into the portal vein of perfused livers from control, DEN- and AAF-treated rats was recovered completely in the effluent perfusate. Pretreatment with AT125 had no effect on the recovery of exogenous GSH in the effluent perfusate. Thus, metabolism of GSH in the blood space was not detected after short-term carcinogen treatment. To increase the possible hydrolysis of GSH in the perfusate, rats were treated for 130-180 days with DEN and GSH (60 microM) was infused into the hepatic artery of livers perfused simultaneously via the hepatic artery and portal vein. Only 50% of the infused GSH was recovered in the effluent perfusate of perfused livers from DEN-treated rats. In contrast, significantly more GSH (80-90%) was recovered from livers from control rats or DEN-treated rats that had received AT125 pretreatment. In addition AT125 pretreatment increased the basal rates of GSH efflux in livers from DEN-treated rats. Thus, DEN-induced GGT metabolizes GSH entering the liver via the hepatic artery. Furthermore, GGT may act to decrease the net efflux of GSH from perfused livers by causing the intraorgan recycling of GSH and its constituent amino acids.