Stereoselective formations of enantiomeric K-region epoxide and trans-dihydrodiols in dibenz[a,h]anthracene metabolism

Synthesis, absolute configuration, and bacterial mutagenicity of the 8 stereoisomeric vicinal diol epoxides at the terminal benzo ring of carcinogenic dibenz[a,h]anthracene

The synthesis of the 8 possible stereoisomeric diol epoxides (DEs) at the terminal benzo ring of carcinogenic dibenz[a,h]anthracene (DBA) is reported. trans-3,4-Dihydroxy-3,4-dihydro-DBA (1) afforded the 4 bay region DEs: the enantiomeric pairs of the anti diastereomers (+)-3/(-)-3 and of the syn diastereomers (-)-4/(+)-4, respectively. trans-1,2-Dihydroxy-1,2-dihydro-DBA (2) served as precursor of the 4 reverse DEs: the enantiomeric pairs of the anti diastereomers (+)-5/(-)-5 and of the syn diastereomers (-)-6/(+)-6, respectively. The transformation of the olefinic double bond in the enantiomeric trans-dihydrodiols to epoxides was achieved by either (i) oxidation with m-chloroperoxybenzoic acid or (ii) formation of a bromohydrin with N-bromoacetamide/H(2)O followed by dehydrobromination with an anion exchange resin. Because of the pseudodiequatorial conformation of the hydroxyl groups in 1, both reactions proceeded highly stereoselectively, while the stereoselectivity was impaired by the pseudodiaxial conformation of the hydroxyl groups in 2. Diastereomers and racemic compounds were efficiently separated without derivatization by HPLC on achiral or chiral stationary phases, respectively. The absolute configurations of the DEs were deduced from the absolute configuration of 1 and 2 considering the regio- and stereoselectivity of the subsequent reactions and resulted in (+)-(1R,2S,3S,4R)-3/(-)-(1S,2R,3R,4S)-3, (-)-(1S,2R,3S,4R)-4/(+)-(1R,2S,3R,4S)-4, (+)-(1R,2S,3S,4R)-5/(-)-(1S,2R,3R,4S)-5, and (-)-(1R,2S,3R,4S)-6/(+)-(1S,2R,3S,4R)-6. The bacterial mutagenicity of the 8 stereoisomeric DEs was determined in histidine-dependent strains TA98 and TA100 of Salmonella typhimurium in the absence of a metabolizing system. In general, the bay region DEs of DBA were stronger mutagens than the reverse DEs. In strain TA98, the syn diastereomers of bay region DEs were stronger mutagens than their anti isomers, while in the case of reverse DEs the anti diastereomers were more potent than their syn isomers. In strain TA100, all syn diastereomers surpassed the bacterial mutagenicity of their anti isomers. Concerning the bay region DEs of DBA, this corresponds to the situation described for benzo[a]pyrene: of the 4 enantiomeric bay region DEs of DBA and benzo[a]pyrene, the syn diastereomer with [(R,S)-diol (R,S)-epoxide] absolute configuration is the most potent mutagen in both bacterial strains, while the anti isomer with [(S,R)-diol (R,S)-epoxide] configuration is the weakest mutagen.

Microsomal Biotransformation of Benzo[ghi]perylene, a Mutagenic Polycyclic Aromatic Hydrocarbon without a “Classic” Bay Region

Carcinogenic polycyclic aromatic hydrocarbons (PAH), e.g., benzo[a]pyrene (BaP), possess a bay region comprising an ortho-fused benzene ring. Benzo[ghi]perylene (BghiP) represents the group of PAHs lacking such a "classic" bay region and hence cannot be metabolically converted like BaP to bay region dihydrodiol epoxides considered as ultimate mutagenic and carcinogenic metabolites of PAH. BghiP exhibits bacterial mutagenicity in strains TA98 (1.3 his(+)-revertant colonies/nmol) and TA100 (4.3 his(+)-revertant colonies/nmol) of Salmonella typhimurium after metabolic activation by the postmitochondrial hepatic fraction of CD rats treated with 3-methylcholanthrene. Inhibition of microsomal epoxide hydrolase (mEH) with 1,1,1-trichloro-2-propene oxide raised the bacterial mutagenicity of BghiP in TA98 almost 4-fold indicating arene oxides as ultimate mutagens. To confirm this assumption, the biotransformation of BghiP was elucidated. Incubation of BghiP with liver microsomes of CD rats treated with Aroclor 1254 yielded 17 ethyl acetate extractable metabolic products. Twelve metabolites were identified by a combination of chromatographic, spectroscopic, and biochemical methods. The microsomal biotransformation of BghiP proceeds by two pathways: Pathway I starts with the monooxygenase attack at the 7-position leading to the 7-phenol, which is transformed to the 7,8- and 7,10-diphenols followed by oxidation to the 7,8- and 7,10-quinones. On pathway II, the K regions of BghiP are successively converted to arene oxides yielding the indirectly identified 3,4-oxide and the 3,4,11,12-bisoxides. Enzymatic hydrolysis of the 3,4-oxide leads to the trans-3,4-dihydrodiol, which is oxidized to the 3,4-quinone. Similarly, the trans-3,4-trans-11,12-bisdihydrodiols and the trans-3,4-dihydrodiol 11,12-quinone are generated from the 3,4,11,12-bisoxides. The trans-3,4-dihydrodiol and the trans-3,4-trans-11,12-bisdihydrodiols are preferentially formed as R,R and R,R,R,R enantiomers, respectively. The intrinsic bacterial mutagenicity of the 3,4,11,12-bisoxides is rather low and hardly explains the strong increase in bacterial mutagenicity of BghiP after inhibition of mEH. Thus, we believe that the 3,4-oxide plays a more important role as the ultimate mutagenic metabolite of BghiP.

Implication of cytochrome P-450 1A isoforms and the AH receptor in the genotoxicity of coal-tar fume condensate and bitumen fume condensates

During the hot application of bitumen- or coal-tar-containing materials, fumes are emitted that contain polycyclic aromatic compounds. Although workers' exposure to these fumes is low, it might lead to health problems. No study has reported the metabolic pathways involved in the genotoxicity of coal tar or bitumen fume condensates (CTFC, BFCs). We have therefore studied the DNA adducts formed by incubation of CTFC or BFCs with liver microsomes from several type of mice and with yeast microsomes expressing individual human CYP enzymes. Our results demonstrates that: (1) the aryl hydrocarbon receptor (AHR) plays an important role in the biotransformation of BFCs and to a lesser extent of CTFC; (2) for CTFC, both cytochrome P450 (CYP) 1A isoforms are involved in the formation of genotoxic compounds, and the reactive metabolites formed via CYP 1A1, are substrates for epoxide hydrolase (mEH); (3) for BFCs, the genotoxicity is partially dependent upon CYP 1A1 and the reactive metabolites are not substrates for mEH; (4) CYP 1A isoforms are not exclusively responsible for the genotoxicity of the CTFC and BFCs as other CYPs and also enzymes of the [AH] gene battery, may play an important role.

Stereoselective epoxidation and hydration at the K-region of polycyclic aromatic hydrocarbons by cDNA-expressed cytochromes P450 1A1, 1A2, and epoxide hydrolase

Stereoselective epoxidation by cytochrome P450s (P450s) and regioselective hydration by epoxide hydrolase determine the carcinogenic potency of some polycyclic aromatic hydrocarbons (PAHs). In this report, cDNA-expressed human and mouse P450s 1A1 and 1A2 and epoxide hydrolase were used to characterize the stereoselective epoxidation and regioselective hydration at the K-region of benz[a]-anthracene (BA), 7,12-dimethylbenz[a]anthracene (DMBA), chrysene (CR), benzo[a]pyrene (B[a]P), dibenz[a,h]anthracene (DB[a,h]A), and benzo[c]phenanthrene (B[c]Ph) by direct chiral stationary-phase HPLC (CSP-HPLC) analyses. Our results indicated that all P450 isoforms preferentially produced major K-region, S,R-epoxides of BA (95-98%), DMBA (94-97%), B[a]P (91-96%), DB[a,h]A (94-98%), and B[c]Ph (87-92%), and major R,S-epoxide of CR (74-85%) in the presence of 3,3,3-trichloropropylene 1,2-oxide (TCPO), an inhibitor of epoxide hydrolase, suggesting that P450 enzymes exhibited the high stereoselectivity toward one of two stereoheterotopic faces of K-region double bond of the PAHs. Epoxide hydrolase either expressed from recombinant vaccinia virus or contained in human hepatoma G2 cells (HepG2) hydrated the C-O bond of epoxy-ring at the S-carbon of major metabolically-formed K-region epoxide enantiomers of BA, CR, DMBA, B[a]P, and DB[a,h]A to yield 80-98% dihydrodiols enriched in R,R-form and that at the R-carbon of B[c]Ph epoxide to yield 77-92% dihydrodiol enriched in S,S-form, suggesting that epoxide hydrolase was highly regioselective. The various enantiomeric components of dihydrodiol products in the metabolism of PAHs were apparently due to the combined effect of stereoselectivity of the P450s and regioselectivity of epoxide hydrolase. Our results provide a clear understanding of how these enzymes catalyze overall stereoselective metabolism at the K-region of the PAHs.