Effect of manipulation of γ-glutamyl transpeptidase levels on biliary excretion of aflatoxin B1 conjugates

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Abstract
The reactive intermediate of aflatoxin B1 (AFB1) forms a glutathione conjugate (AFB1-GSH) and this has been shown to be a substrate for γ-glutamyl transpeptidase (GGT) in vitro. This study describes the biliary excretion of AFB1-GSH and the product of GGT activity, the cysteinylglycyl conjugate (AFB1-Cys-Gly), following i.v. injection of AFB1 (5 μmol kg−1) in control male rats and in rats that had been maintained on a toxic diet containing 4 p.p.m. AFB1 for 10–12 weeks prior to the experiment. AFB1 metabolites in the bile were analyzed by reverse phase h.p.l.c. and GGT activity in the liver was assessed histochemically and by quantitative fluorimetric assay. In the control male rats (n = 6) AFB1-GSH and AFB1-Cys-Gly together were detected as 4.2 ± 2.3% of the i.v. dose over the first two hours of bile collection (AFB1-GSH: AFB1-Cys-Gly, 5.5:1). GGT activity (38.2 + 7.9 nmol product formed/g liver) was located in the bile duct epithelium. The group maintained on a toxic diet (n = 2) showed higher levels of AFB1-Cys-Gly (AFB1 GSH: AFB1-Cys-Gly, 1:1). GGT activity was elevated (5–10 x control levels) and located in numerous foci throughout the liver. The involvement of GGT in the biliary excretion of AFB1-Cys-Gly was demonstrated by in vivo inhibition of GGT by administering AT125 to a group of animals (n = 3) 15 min prior to the injection of AFB1. Histochemical and quantitative estimation of GGT confirmed total inhibition throughout the liver and conversion of AFB1-GSH to AFB1-Cys-Gly was almost completely blocked. Female Fischer 344 rats (n = 3) showed slightly elevated AFB1-Cys-Gly excretion and higher GGT activity (79.3 + 26.3 nmol/min/g liver) compared to control male rats.