Metabolic activation of racemic and enantiomeric trans-8,9-dihydroxy-8,9-dihydrodibenzo[a,l]pyrene (dibenzo[def,p]chrysene) to dibenzo[a,l]pyrene-bis-dihydrodiols by induced rat liver microsomes and a recombinant human P450 1A1 system: The role of the K-region-derived metabolic intermediates in theformation of dibenzo[a,l]pyrene-DNA adducts

Metabolic activation studies of dibenzo[a,l] pyrene (DB[a,l]P) (dibenzo[def ,p] chrysene), an extremely potent environmental carcinogen, have been focu sed on metabolism at the fjord region, a region associated with high mutage nic and carcinogenic activities of the col rer;ponding fjord-region DB[a,l] P-11,12-diol-13,14-epoxides. DB[a,l]P is metabolized by beta-naphthoflavone (BNF)- and 3-methylcholanthrene-induced rat liver microsomes and a recombi nant human P450 1A1 system to two major dihydrodiols, the K-region dihydrod iol, DB[a,l]P-8,9-dihydrodiol (DB[a,l]P-8,9-diol), and the fjord-region dih ydrodiol, DB[a,l]P-11,12-dihydrodiol. We have investigated the further meta bolic activation of BB[a,l]P-8,9-diol by BNF-induced rat liver microsomes a nd a recombinant human P450 1A1 system with epoxide hydrolase to DB[a,l]P- bis-diols and to DNA adducts. (+/-)-trans-DB[a,;]P-8,9-diol was synthesized and resolved into its enantiomers. Racemic trans-DB[a,l]P-8,9-diol was met abolized by BNF-induced rat liver microsomes to six metabolites: two diaste reomers of trans,trans-DB[a,;l]P-8,9:11,12-bis-diol, two diastereomers of t rans, cis-DB[a,l]P-8,9:11,12-bis-diol, and two diastereomers of trans-DB- [ a,l]P-8,9:13,14-bis-diol as characterized by NMR, MS, and UV spectroscopy. Metabolic studies using both enantiomeric (-)- and (+)-trans-DB[a,l]P-8,9-d iol further demonstrated that each diastereomer of trans,trans-DB[a,l]P-8,9 :11,12-bis-diol and trans-DB[a,l]P-8,9:13,14-bis-diol was comprised of two enantiomers. Similarly, incubations of enantiomeric or racemic trans-DB[a,l ]P-8,9-diol with a recombinant human P450 1A1 system and epoxide hydrolase also gave the same two enantiomeric mixtures of diastereomers of trans,tran s-DB[a,l]P-8,9:11,12-bis-diol and the same two enantiomeric mixtures of dia stereomers of trans-DB[a,l]P-8,9:13,14-bis diol. This suggested that the mi crosomal oxidations of (-)- and (+)-trans-DB[a,l]P-8,9-diol were stereospec ific. The stereospecific formation of enantiomers of trans-DB[a,l]P-8,9-dio l from DB[a,l]P was examined using both BNF-induced rat liver microsomes an d a recombinant human P450 1A1 system with epoxide hydrolase, Stereospecifi city was observed as both metabolic systems favored the formation of (-)-tr ans-DB[a,l]P-8,9-diol by 8-9-fold. DNA adduct studies were undertaken using TLC/HPLC P-32-postlabeling techniques. In the presence of a recombinant hu man P450 1A1, system with epoxide hydrolase, DB[a,l]P gave two groups of ca lf thymus DNA adducts. The group of later-eluting adducts were identified a s at ising from syn- and anti-DB[a,l]P-11,12-diol-13-14-epoxides, while the more polar early-eluting adducts were derived, in part, from the further a ctivation of trans-DB[a,l]P-8,9-diol. Our data indicate that, in P450 1A1-m ediated microsomal incubations, DB[a,l]P is metabolized to trans-DB[a,l]P-8 -9-diol which is further metabolized to DB[a,l]P-bis-diols. trans-DB[a,l]P- 8,9-diol is metabolically activated to intermediates that can bind to DNA a nd give DNA adducts similar to those observed with DB[a,l]P. These results indicate that DB[a,l]P can be metabolically activated by both fjord-region and K-region pathways.