The requirement for glutathione S-transferase in the conjugation of activated aflatoxin B 1 during aflatoxin hepatocarcinogenesis in the rat

Abstract

The formation of an aflatoxin B 1-reduced glutathione (AFB 1-GSH) conjugate in in vitro systems has been examined. AFB 1 was activated by a chicken liver microsomal system and factors affecting the subsequent conversion to the AFB 1-dihydrodiol or conjugation with GSH were investigated by HPLC. A requirement for glutathione S-transferase in the formation of the AFB 1-GSH conjugate was observed. Studies using CM-cellulose columns showed the fractions containing glutathione S-transferase B activity were the most effective in catalysing the formation of the AFB 1-GSH conjugate. The possibility of changes in the level of AFB 1-GSH conjugate production in the liver during carcinogenesis by AFB 1 has been examined. It has been found, using freshly isolated rat hepatocytes, that low level feeding with AFB 1 in vivo increases the production of the conjugate in vitro. Further increases in the production of the conjugate by hepatocytes in vitro, accompanying increases in the preneoplastic lesions, are achieved by partially hepatectomising the AFB 1-fed animals. Partial hepatectomy of control-fed animals yielded no similar changes. The AFB 1/partial hepatectomy treatment resulted in increased levels of all the glutathione S-transferase activities fractionated on CM-cellulose. Macromolecular binding of AFB 1 and/or of its metabolites was detected in the fractions containing glutathione S-transferase activity, but there was no evidence for a greater binding in the glutathione S-transferase B/ligandin containing fractions. Furthermore fractionation on Sephadex G-75 indicated a predominance of binding of AFB 1 to proteins of a higher molecular weight than the glutathione S-transferases, although some binding in the molecular weight range of the latter was observed.