REGIOSELECTIVE AND STEREOSELECTIVE METABOLISM OF PHENANTHRENE BY 12 CDNA-EXPRESSED HUMAN, RODENT, AND RABBIT CYTOCHROMES P-450

The regio- and stereoselective metabolism of phenanthrene (PA) by seven cDNA-expressed human and five rodent and rabbit cytochromes P-450 has been examined using reverse-phase and chiral stationary phase high-pressure liquid chromatography (HPLC). Turnover numbers ranged from 0.2 to 55 nmol/min per nmol. Using vaccinia virus expression of P-450 enzymes in Hep G2 cells, m1A1 and m1A2 were found to be the most active P-450s. Of the human P-450s, 1A2 and 2B6 have the highest activity and 2C9 has moderate activity. Using cytochrome P-450s expressed in a lymphoblastoid cell line in presence of epoxide hydrolase (EH), human 1A1 had approximately twice the activity of human 1A2. Regioselectivities for PA metabolism were found to be both isozyme and species-dependent. Stereochemical analysis revealed that the P-450s 1A1, m1A1, m1A2, r2A1, r2B1, PB- and 3MC-treated rat liver microsomes preferentially formed 3R,4R-diol enantiomer (88-97%), whereas rabbit 4B formed the 3S,4S-diol enantiomer (72%). Eleven P-450s, 3MC and PB microsomes preferentially formed 1R,2R-diol enantiomer (80-96%). This is the same stereochemistry as the precursors to some diol epoxides that are potent carcinogens.