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RESEARCH PRODUCT

Metabolic Activation of the (+)-S,S- and (−)-R,R-Enantiomers of trans-11,12-Dihydroxy-11,12-dihydrodibenzo[a,l]pyrene:  Stereoselectivity, DNA Adduct Formation, and Mutagenicity in Chinese Hamster V79 Cells

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Polycyclic aromatic hydrocarbons require metabolic activation in order to exert their biological activity initiated by DNA binding. The metabolic pathway leading to bay or fjord region dihydrodiol epoxides as ultimate mutagenic and/or carcinogenic metabolites is thought to play a dominant role. For dibenzo[a,l]pyrene, considered as the most potent carcinogenic polycyclic aromatic hydrocarbon, the formation of the fjord region syn- and/or anti-11,12-dihydrodiol 13,-14-epoxide (DB[a,l]PDE) diastereomers has been found to be the principal metabolic activation pathway in cell cultures leading to DNA adducts. In order to further elucidate the stereoselectivity involved in this activation pathway via the formation of the trans-11,12-dihydrodiol, we have synthesized the enantiomerically pure 11,12-dihydrodiols of dibenzo[a,l]-pyrene and investigated their biotransformation in rodents. Incubations with liver microsomes of Sprague-Dawley rats and CD-1 mice pretreated with Aroclor 1254 revealed that the enzymatic conversion to the fjord region DB[a,l]PDE strongly depends on the absolute configuration of the 11,12-dihydrodiol enantiomers. While oxidation at the 13,14-position of the (+)-(11S,12S)-dihydrodiol is limited to a small extent, the (-)-11R,12R-enantiomer is metabolized to its fjord region dihydrodiol epoxides in considerably higher amounts. Moreover, this substrate is transformed with high stereoselectivity to the corresponding (-)-anti-dihydrodiol epoxide by liver microsomes of Aroclor 1254-treated rodents. The metabolism results were in good accordance with the extent of stable adduct formation in calf thymus DNA as investigated by the 32P-postlabeling technique and with the mutagenicity in Chinese hamster V79 cells of the two enantiomeric 11,12-dihydrodiols mediated by hepatic postmitochondrial preparations of Aroclor 1254-treated rats. The results indicate that both genotoxic events occurred predominantly by the stereoselective activation of the (-)-(11R,12R)-dihydrodiol to the (-)-anti-DB[a,l]PDE with R,S,S,R-configuration.