REGIOSELECTIVITY AND STEREOSELECTIVITY IN THE METABOLISM OF BENZO[C]PHENANTHRENE MEDIATED BY GENETICALLY-ENGINEERED V79 CHINESE-HAMSTER CELLS EXPRESSING RAT AND HUMAN CYTOCHROMES P450

Regio- and stereoselective metabolism mediated by cytochrome P450 (CYP ) and metabolite-dependent cytotoxicity of benzo[c]phenanthrene (B[c]P h) and its trans-3,4-dihydrodiol, the metabolic precursor of the carci nogenic fjord-region B[c]Ph-3,4-dihydrodiol 1,2-epoxides (B[c]PhDE), w ere investigated with V79 Chinese hamster cells genetically engineered for three rat and six human CYP isoforms. The order of the capabiliti es of the CYP isoforms to metabolize B[c]Ph was as follows: h1A1 > r1A 1 > r1A2 > h1B1 > h1A2 > r2B1 > > h2E1 > h2A6 > h3A4. Regardless of th e species, all individual CYP isoforms preferentially catalyzed the ox idation of B[c]Ph at the 5,6-position (K-region) except human CYP1A1 a nd human CYP1A2, which oxidized both the 5,6- and the 3,4-position wit h similar efficiency. While human CYP1A1, rat CYP1A1 and rat CYP1A2 fo rmed almost exclusively the(-)-B[c]Ph-3R,4R-dihydrodiol, human CYP1A2 produced both the (-)-3R,4R- and the (+)-3S,4S-dihydrodiol enantiomers in a ratio of 2:1. Stereoselective activation of B[c]Ph, the ( +/-)-B [c]Ph-3,4-dihydrodiol and its (-)-3R,4R-enantiomer to the fjord-region (-)-anti-B[c]PhDE occurred upon incubation with rat CYP1A1 and rat CY P1A2 as indicated by the formation of two stereoisomeric tetraols, the hydrolysis products of the labile anti-B[c]PhDE. The formation of tet raols in the culture medium was accompanied by a concentration-depende nt increase in cytotoxicity indicating that this effect was mediated b y the fjord-region (-)-anti-B[c]PhDE formed as reactive intermediate. All human and rat CYP-expressing V79 cell lines investigated did not s how any significant capacity to metabolize the(+)-3S,4S-dihydrodiol. T he present study indicates that the human CYP isoforms 1A1 and 1B1 hav e complementary catalytic properties to activate B[c]Ph to its fjord-r egion B[c]PhDE, whereas other human isoforms play a minor role. Activa tion of B[c]Ph by human CYP1A1 and 1B1 is less efficient than by rat C YP1A1 or rat CYP1A2, but proceeds with similar stereoselectivity via t he (-)-3R,4R-dihydrodiol to the strong carcinogen (-)-anti-B[c]PhDE wi th (R,S,S,R)-configuration. (C) 1998 Elsevier Science B.V. All rights reserved.