The DNA adducts of 7,12-dimethylbenz[a]anthracene (DMBA) previously identified in vitro and in vivo are stable adducts formed by reaction of the bay-region diol epoxides of DMBA with dG and dA. In this paper we report identification of several new DMBA-DNA adducts formed by one-electron oxidation, including two adducts lost from DNA by depurination, DMBA bound at the 12-methyl to the N-7 of adenine (Ade) or guanine (Gua) [7-methylbenz[a]anthracene (MBA-12-CH2-N7Ade or 7-MBA-12-CH2-N7Gua, respectively]. The in vitro systems used to study DNA adduct formation were DMBA activated by horseradish peroxidase or 3-methylcholanthrene-induced rat liver microsomes. The biologically-formed depurination adducts were identified by high-pressure liquid chromatography and by fluorescence line narrowing spectroscopy. Stable DMBA-DNA adducts were analyzed by the 32P-postlabeling method. Quantitation of DMBA-DNA adducts formed by microsomes showed about 99% as depurination adducts: 7-MBA-12-CH2-N7Ade (82%) and 7-MBA-12-CH2-N7Gua (17%). Stable adducts (1.4% of total) included one adduct spot that may contain adduct(s) formed from the diol epoxide (0.2%) and unidentified adducts (1.2%). Activation of DMBA by horseradish peroxidase afforded 56% of stable unidentified adducts and 44% of depurination adducts, with 36% of 7-MBA-12-CH2-N7Ade and 8% of 7-MBA-12-CH2-N7Gua. Adducts containing the bond to the DNA base at the 7-CH3 group of DMBA were not detected. These results show that with activation by cytochrome P-450 the DMBA-DNA adducts are predominantly formed by one-electron oxidation and lost from DNA by depurination, and the adducts formed by the diol epoxide pathway are very minor. The 12-CH3 group is shown to be critical in metabolic activation of DMBA, a finding consistent with the results of carcinogenicity studies in rodents.
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