title: Epoxidation reactions catalyzed by rat liver cytochromes P-450 and P-448 occur at different faces of the 8,9-double bond of 8-methylbenz[a]anthracene.
abstract: 8-Methylbenz[a]anthracene (8-MeBaA) transdihydrodiol metabolites were isolated by reversed-phase and normal-phase HPLCs from incubations of 8-MeBaA with liver microsomes or a reconstituted system containing purified cytochrome P-448 and epoxide hydrolase. Regardless of the enzyme source, the metabolically formed 8-MeBaA trans-3,4- and -5,6-dihydrodiols were found to be enriched in one enantiomeric isomer and differed only in the degree of optical purity. The 8-MeBaA trans-8,9-dihydrodiol formed by liver microsomes from either untreated or phenobarbital-treated rats was enriched with the (+)-enantiomer. In contrast, the 8-MeBaA trans-8,9-dihydrodiol formed either by liver microsomes from 3-methylcholanthrene-treated rats or by the reconstituted rat liver enzyme system containing cytochrome P-448 and epoxide hydrolase was enriched with the (-)enantiomer. These results indicate that, in catalyzing the formation of 3,4- and 5,6-epoxide intermediates, the interaction with the unsubstituted 3,4- and 5,6-double bonds of 8-MeBaA by the different forms of cytochrome P-450 occur preferentially on the same face of the aromatic plane and they differ only in the degree of stereoselectivity. However, different forms of cytochrome P-450 may interact with different faces of the aromatic plane at the methyl-substituted 8,9-double bond of 8-MeBaA, resulting in the formation of trans-8,9-dihydrodiols enriched in different enantiomeric forms. This demonstrates that different forms of cytochrome P-450 may catalyze the epoxidation reaction preferentially at different sides of the methyl-substituted double bond of a planar polycyclic hydrocarbon molecule. These properties may be used to further classify and to understand the enzyme-substrate interactions of the different forms of cytochrome P-450 in the drug-metabolizing enzyme systems.
mesh_terms: D000818:Animals; D001551:Benz(a)Anthracenes / Q000378:metabolism*; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D019388:Cytochrome P-450 CYP1A2; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism*; D004851:Epoxide Hydrolases / Q000378:metabolism*; D004852:Epoxy Compounds / Q000378:metabolism; D008862:Microsomes, Liver / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D001551:Benz(a)Anthracenes; D003580:Cytochromes; D004852:Epoxy Compounds; D003577:Cytochrome P-450 Enzyme System; C017170:Cyp1a2 protein, rat; D019388:Cytochrome P-450 CYP1A2; D004851:Epoxide Hydrolases; C031255:8-methylbenzanthracene