Comparative DNA binding of 7, 12-dimethylbenz[a]anthracene and some of its metabolites in mouse epidermis in vivo as revealed by the 32P-postlabeling technique

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Abstract
The binding of some mouse skin metabolites and related derivatives of the tumor initiator 7,12-dimethylbenz[a]-anthracene (DMBA) was investigated by 32P-postlabeling analysis after its topical administration. DMBA and trans-3,4-dihydro3,4-dihydroxy-DMBA (DMBA-3,4-dihydro-diol) both led to the formation of four DNA adducts, which showed a very similar pattern of spots on thin-l ayer chromato-grams. With trans-8,9-Ktihdbro-8,9-dihydroxy-7,12-dimethyl-benz[a]anthracene (DMBA-8,9-dihydrodiol) one major adduct was obtained which was chromatographically indistinguishable from one of the DMBA adducts. In contrast, 7-hydroxymethyl-12-methylbenz[a]anthracene (7-OHM-12-MBA) gave rise to two major adducts which were separable from DMBA adducts. 3-hydroxy-7, 12-dimethyIbenz[a]anthra-cene (3-OH-DMBA) and 7,12-dimethylbenz[a]anthracene-7, 12-epoxide (DMBA-O2) did not lead to detectable amounts of adducts. Quantitative determination of DNA binding showed that an initiating dose (i = 100 nmol) of DMBA yielded ˜12 adducts/107 normal nucleotides. Adduct formation with the same dose of DMBA-3,4-dihydrodiol was 7-8 times higher. At a 4-fold higher dose level, DMBA-8,9-dihydrodiol exhibited a 3- to 6-times weaker binding and 7-OHM-12-MBA a slightly stronger binding than DMBA. Chromatography of the DMBA and DMBA-3,4-dihydrodiol adducts with a solvent containing borate showed a decreased mobility of two out of four adducts in each case. These adducts were also sensitive to oxidation by periodate. The results suggest that two DMBA adducts carried vicinal cis-hydroxyl groups and thus were probably derived from the anti3,4-dihydrodiol1,2-oxide(s) of DMBA. The other two adducts were probably derived from the syn-stereoisomer(s). When the DNA-modifying capabilities and initiating activities of the more prominent mouse-skin metabolites are considered in relation to DMBA, DMBA-3,4-dihydrodiol is postulated to be a proximate and DMBA-3,4-dihydrodiol-1,2-oxide(s) to be ultimate initiators.