Involvement of a cytosolic carrier protein fraction in the microsomal metabolism of benzo(a)pyrene in rat liver.

Rat liver cytosol contains two protein fractions capable of serving as benzo(a)pyrene (BP) carrier in in vitro oxidation of BP (O. Hanson-Painton, M.J. Griffin, and J. Tang, Biochem. Biophys. Res. Commun., 101: 1364-1371, 1981). The role of the smaller carrier fraction in in vitro BP transport and oxidation was studied. Using Sephadex G-100 chromatography, [14C]BP bound to the carrier was shown to preferentially transfer to the microsomes. In the presence of excess BP in suspension, the carrier protein fraction transferred levels of BP many-fold in excess of its BP-binding capacity. The carrier fraction delivered BP to liposomes prepared from microsomal lipids. However, incubation of fresh liver homogenate with protein-bound BP resulted in the transfer of over 80% of the BP to the microsomes, indicating that BP is transferred selectively. Isolated microsomes containing [14C]BP oxidized bound BP upon the addition of NADPH. Sephadex G-100 chromatography and high-performance liquid chromatography analysis of bound radioactivity indicated that oxidized products of BP were preferentially transferred back to the carrier protein fraction. Thus, the carrier fraction is capable of both transferring BP to the microsomes and accepting oxidized BP from microsomes. The transfer of oxidized BP from microsomes to the carrier fraction was inhibited by an epoxide hydrolase inhibitor, alpha-bromo-4-nitroacetophenone.