In vitro metabolism of 7,12-dimethylbenz[a]anthracene by rainbow trout liver microsomes and trout P450 isoforms

Abstract

Liver microsomes from juvenile trout metabolized DMBA to unknown highly polar metabolites (X) and to DMBA-t-5,6-diol, DMBA-t-8,9-diol, 7-OHM-12-MBA, 7M-12-OHMBA, 2-OH-DMBA, 4-OH-DMBA, and trace amounts of DMBA-t-3,4-diol. Treatment of trout with beta-naphthoflavone (BNF) and isosafrole (ISF) increased the formation of these products except for the hydroxymethyl derivatives of DMBA. The production of DMBA-t-3,4-diol, 2-OH-DMBA, and 4-OH-DMBA was much greater in BNF-induced liver microsomes than that in ISF-induced liver microsomes. In contrast, the yield of DMBA-t-8,9-diol and 7-OHM-12-MBA was greater in ISF-induced microsomes than that in BNF-induced microsomes. Trout CYP1A1 (P450 LM4b) purified from BNF-treated trout catalyzed the formation of the same metabolites generated by BNF-induced microsomes in the presence of added human microsomal EH. The constitutive forms of P450 isolated from untreated trout such as P450s LMC3, LMC4, and LMC5, CYP2M1 (P450 LMC1), and CYP2K1 (P450 LMC2) did not produce any of the DMBA metabolites (except for DMBA-t-8,9-diol by CYP2K1) generated by the trout microsomes. Generation of DMBA-DNA and DMBA-protein adducts in vitro was enhanced by treatment of trout with BNF and by ISF to a lesser extent. Formation of adducts and DMBA diols by BNF-induced liver microsomes and by trout CYP1A1 was completely blocked by the CYP1A inhibitor ellipticine (100 microM). These results suggest that the BNF-inducible trout P450 (CYP1A), not the constitutive P450s, is the major catalyst for the biotransformation of DMBA to metabolites that bind to macromolecules.