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 the Formation of Dibenzo[a,l]pyrene−DNA Adducts

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Abstract
Metabolic activation studies of dibenzo[a,l]pyrene (DB[a,l]P) (dibenzo[def,p]chrysene), an extremely potent environmental carcinogen, have been focused on metabolism at the fjord region, a region associated with high mutagenic and carcinogenic activities of the corresponding fjord-region DB[a,l]P-11,12-diol-13,14-epoxides. DB[a,l]P is metabolized by β-naphthoflavone (BNF)- and 3-methylcholanthrene-induced rat liver microsomes and a recombinant human P450 1A1 system to two major dihydrodiols, the K-region dihydrodiol, DB[a,l]P-8,9-dihydrodiol (DB[a,l]P-8,9-diol), and the fjord-region dihydrodiol, DB[a,l]P-11,12-dihydrodiol. We have investigated the further metabolic activation of DB[a,l]P-8,9-diol by BNF-induced rat liver microsomes and a recombinant human P450 1A1 system with epoxide hydrolase to DB[a,l]P-bis-diols and to DNA adducts. (±)-trans-DB[a,l]P-8,9-diol was synthesized and resolved into its enantiomers. Racemic trans-DB[a,l]P-8,9-diol was metabolized by BNF-induced rat liver microsomes to six metabolites: two diastereomers of trans,trans-DB[a,l]P-8,9:11,12-bis-diol, two diastereomers of trans,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-diol 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,trans-DB[a,l]P-8,9:11,12-bis-diol and the same two enantiomeric mixtures of diastereomers of trans-DB[a,l]P-8,9:13,14-bis-diol. This suggested that the microsomal oxidations of (−)- and (+)-trans-DB[a,l]P-8,9-diol were stereospecific. The stereospecific formation of enantiomers of trans-DB[a,l]P-8,9-diol from DB[a,l]P was examined using both BNF-induced rat liver microsomes and a recombinant human P450 1A1 system with epoxide hydrolase. Stereospecificity was observed as both metabolic systems favored the formation of (−)-trans-DB[a,l]P-8,9-diol by 8−9-fold. DNA adduct studies were undertaken using TLC/HPLC 32P-postlabeling techniques. In the presence of a recombinant human P450 1A1 system with epoxide hydrolase, DB[a,l]P gave two groups of calf thymus DNA adducts. The group of later-eluting adducts were identified as arising 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 activation of trans-DB[a,l]P-8,9-diol. Our data indicate that, in P450 1A1-mediated 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 and 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.

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