Metabolism and DNA binding of 3-methylcholanthrene.

Rat liver microsomes metabolize 3-methylcholanthrene (3MC) to oxygenated forms which alkylate DNA. Analysis of the DNA-bound products by high-pressure liquid chromatography revealed at least 13 deoxyribonucleoside adducts. The two major adducts contained guanine and had fluorescence spectra consistent with saturation at carbon atoms 7, 8, 9, and 10. Microsome-catalyzed binding of 3MC to double-stranded and single-stranded DNA yielded quantitatively similar patterns, suggesting that specific adducts do not arise from orientation of the hydrocarbon into the DNA helix prior to metabolic activation. The adducts produced in vitro varied with time of incubation; the more polar adducts appeared after longer time periods. Seven of the adducts were also detected in DNA from the lung and liver of C57BL/6J mice that had been given i.v. injections of [3H]-3MC; the relative amounts of each adduct most closely resembled the pattern which was observed after long incubation of microsomes with DNA in vitro . Microsome-mediated metabolism of [3H]-3MC was also studied. 2-Hydroxy-3-methylcholanthrene rather than 1-hydroxy-3-methylcholanthrene was the major metabolite, the latter being almost undetectable. Other metabolites included trans -9,10- and trans -11,12-dihydrodiols and a variety of unidentified derivatives which may bear modifications on either the saturated five-membered ring or on the 3-methyl group. These derivatives were reincubated with microsomes and DNA. After such incubations, seven 3MC metabolites bound to DNA with higher efficiency than did the parent hydrocarbon; trans -9,10-dihydrodiol had the highest efficiency in this regard. The other six metabolites which subsequently showed high binding efficiency appeared to be derivatized on carbon 1, carbon 2, or 3-methyl. Although the major binding to DNA may be through a vicinal diol-epoxide, the complexity surrounding the aliphatic five-membered ring probably also contributes to the large number of adducts detected.