Regio- and stereoselective metabolism of dibenz[a,h]anthracene: identification of 12 new microsomal metabolites.

Incubation of the carcinogenic polycyclic aromatic hydrocarbon dibenz[a,h]anthracene (DBA) with liver microsomes of Sprague-Dawley rats, pretreated with Aroclor 1254, yielded more than 30 metabolites. Fifteen of these could be identified, and they account for 95% of the ethyl acetate-extractable metabolites of DBA. Twelve metabolites were identified for the first time, by chromatographic and spectroscopic methods: these were DBA-5,6-oxide, 1-, 2-, 3-, 4-, 5-, 6-phenols, 3,4:12,13-bis-dihydrodiol, 1,4/2,3-tetrol, 1,3/2,4-tetrol, 3,4-catechol, and a phenol dihydrodiol derived from the 2-phenol. Quantitative determination revealed that the attack of cytochrome P-450 dependent monooxygenases occurs at the 1,2-, 3,4- and 5,6-positions of the DBA molecule in the ratio 1.7:1.9:1.0. Evidence is presented which indicates that the phenols of DBA are formed by aromatization of the initially generated arene oxides, rather than by direct hydroxylation. The index Ni obtained by refined perturbational molecular orbital calculations was found to be superior to the reactivity number Nt in predicting the predominant phenols, i.e., 2-, 4-, and 5-phenols, formed by aromatization of the corresponding arene oxides. Their enzymatic hydrolysis leads to the formation of trans-dihydrodiols, of which the 3,4-isomer dominates the microsomal metabolites of DBA accounting for more than 22% of the total metabolic conversion, compared to the 1,2-dihydrodiol with 11-16% and the 5,6-dihydrodiol with 2%. These metabolites were obtained as enantiomeric-enriched mixtures in which the R,R enantiomer of the 1,2-dihydrodiol prevailed with 84%, of the 3,4-dihydrodiol with 79% and of the 5,6-dihydrodiol with 96%. The metabolic pathway via the 1,2-dihydrodiol proceeds to the vicinal diol epoxides, as indicated by the products of hydrolysis the 1,4/2,3- and 1,3/2,4-tetrols. No evidence for the formation of vicinal dihydrodiol epoxides from the 3,4-dihydrodiol, one of the most mutagenic and carcinogenic metabolite of DBA, could be found. In this case, tetrol epoxides have been proposed as ultimate reactive metabolites. Tetrol epoxides can also be formed from DBA-5,6-dihydrodiol via the identified 3,4:12,13-bis-dihydrodiol. This unprecedented metabolic behavior of a carcinogenic polycyclic aromatic hydrocarbon could have its cause in the high molecular symmetry of DBA which permits subsequent metabolic attacks at discrete, but structurally equivalent sites of the molecule.