Stereochemical specificity in the metabolic activation of benzo(c)phenanthrene to metabolites that covalently bind to DNA in rodent embryo cell cultures.

Benzo(c)phenanthrene (BcPh) has only weak carcinogenic activity in rodent bioassays. However, bay-region diol-epoxides of BcPh have the highest tumor-initiating activities of all hydrocarbon diol-epoxides tested to date. To determine whether BcPh is metabolically activated to bay-region diol-epoxides that bind to DNA in cells, Sencar mouse, Syrian hamster, and Wistar rat embryo cell cultures were exposed to [5-3H]-BcPh, and the BcPh-deoxyribonucleoside adducts formed were analyzed by immobilized boronate chromatography and reverse-phase high-performance liquid chromatography. Greater than 74% of the BcPh-deoxyribonucleoside adducts formed in all 3 species resulted from reaction of (4R,3S)-dihydroxy-(2S,1R)-epoxy-1,2,3,4-tetrahydro-BcPh [(-)-BcPhDE-2] with DNA to yield deoxyadenosine and deoxyguanosine adducts in a ratio of 3:1. A much smaller proportion of BcPh-deoxyribonucleoside adducts were formed by reaction of (4S,3R)-dihydroxy-(2S,1R)-epoxy-1,2,3,4-tetrahydro-BcPh [(+)-BcPhDE-1] with deoxyadenosine. No BcPh-deoxyribonucleoside adducts arising from either (+)-BcPhDE-2 or (-)-BcPhDE-1 were detected. The absence of adducts from these isomers of BcPhDE was not due to failure of these isomers to react with DNA in cells, for reaction of (+/-)-BcPhDE-1 or (+/-)-BcPhDE-2 with DNA in solution or in hamster embryo cell cultures resulted in the formation of DNA adducts from both the (+)- and (-)-enantiomers of each BcPhDE. These results indicate that both the (+)- and (-)-3,4-dihydrodiols of BcPh are formed and that their metabolic activation to diol-epoxides occurs with high stereospecificity in cells from all 3 species of rodents. The finding that the major DNA-binding metabolite is (-)-BcPhDE-2, the diol-epoxide with the (R,S)-diol-(S,R)-epoxide absolute configuration that is associated with high carcinogenic activity of diol-epoxides of other hydrocarbons, demonstrates that these cells are able to activate BcPh to an ultimate carcinogenic metabolite. The fact that a high proportion of the BcPh-DNA adducts are deoxyadenosine adducts suggests that BcPh has DNA-binding properties similar to those of the potent carcinogen 7,12-dimethylbenz(a)anthracene. The stereospecificity observed in the metabolic activation of BcPh to DNA-binding metabolites and the reaction of these metabolites with both deoxyguanosine and deoxyadenosine suggest that studies of the interactions of BcPh with DNA in vivo may be a valuable approach for establishing the role of specific activation pathways and DNA adducts in tumor induction.