DNA Adducts from syn 7-Methylbenz[A]anthracene 3,4-Dihydrodiol 1,2-Epoxide

Abstract

DNA adducts derived from reaction of the racemic bay region syn 7-methylbenz[a]anthracene 3,4-dihydrodiol 1,2-epoxide with calf thymus DNA in vitro were tentatively identified. Eight markers (four deoxyguanosine and four deoxyadenosine adducts) were obtained from separate reactions of the racemic syn dihydrodiol epoxide with deoxyguanylic and deoxyadenylic acids. The nucleoside of origin of individual DNA products was established by comparing HPLC retention times and UV spectra of DNA adducts with those of the adduct markers. All DNA adducts eluted with retention times and had UV spectra that corresponded to the purine nucleoside adducts. Circular dichroism spectra of the marker adducts allowed assignment of S and R configuration at C1, the site of attachment of the hydrocarbon residue to the nucleoside. The CD spectra were comprised of four pairs of spectra that were mirror images of one another. Each pair consisted of the two cis or two trans products resulting from epoxide ring opening of each dihydrodiol epoxide enantiomer by the exocyclic amino group of the deoxyguanosine or deoxyadenosine residues. Since the adducts obtained were insufficient for NMR studies, assignment of cis and trans structures was made by comparison of elution sequence and circular dichroism data with those of the known syn benz[a]anthracene dihydrodiol epoxide adducts. The ratio of dGuo/dAdo adducts in DNA was 1.5, close to the ratio reported for syn benz[a]anthracene dihydrodiol epoxide. DNA adducts accounted for 27% of the original dihydrodiol epoxide used. The ratios of dGuo/dAdo modifications were 58/42 and 69/31 for the 3S,4R-dihydrodiol 1S,2R-epoxide and the 3R,4S-dihydrodiol 1R,2S-epoxide enantiomers, respectively. Approximately 75% of total DNA adducts was derived from the 3S,4R-dihydrodiol 1S,2R-epoxide enantiomer and these were largely cis adduets.