Modulation of microsome-mediated aflatoxin B₁ binding to exogenous and endogenous DNA by cytosolic glutathione S-transferases in rat and hamster livers

Abstract

Microsome mediated aflatoxin B1 (AFB1) binding to exogenous and endogenous DNA and its modulation by cytosolic glutathione (GSH) S-transferases have been examined in rat and hamster livers. Kinetic studies over a wide range of cytosol concentrations indicate that cytosol from the hamster is several-fold more effective than that from the rat in inhibiting AFB1 binding to exogenous calf thymus DNA mediated by microsomes from either species. Low concentrations of GSH (0.1-0.2 mM) are required for 50% inhibition of AFB1-DNA binding by cytosol. With exogenous DNA, combined microsome-cytosol fractions from the hamster give more AFB1-DNA binding than those from the rat. However, with nuclei as a source of endogenous DNA, AFB1-DNA binding is less with combined microsome-cytosol fractions from the hamster than those from the rat. Cytosolic inhibition of AFB1-DNA binding is almost completely reversed in the presence of 1 mM levels of either trichloropropene oxide or styrene oxide. Quantitation of AFB1-DNA binding and AFB1-GSH conjugation indicate an inverse relationship between these two processes. Cytosol from the rat has less capacity than that from the hamster to form an AFB1-GSH conjugate. Hepatic GSH levels are about equal (6-7 mM) in both species. I.p. administration of [14C]AFB1 2 h before sacrifice gives more AFB1 binding to hepatic nuclear DNA in rats than in hamsters. However, depletion of hepatic GSH levels by 80% by i.p. administration of diethylmaleate (600 mg/kg) increases AFB1-DNA binding 2- to 3-fold in both species. The role of cytosolic GSH S-transferases in modulating hepatic AFB1-DNA binding in rats and hamsters is discussed.