Sequence dependence of benzo[a]pyrene diol epoxide-DNA adduct conformer distribution: a study by laser-induced...

Low-temperature laser-induced fluorescence techniques in combination with polyacrylamide gel electrophoresis (LIF/PAGE) were used to study the binding of (-)-anti- and (+)-anti-benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxide (anti-BPDE) to several sequence-defined duplex oligomers. Two of the oligomers contain central 5'-RAGGAR-3' sequences (R = purine) which appear to be frequently mutated by racemic (+/-)-anti-BPDE in endogenous genes of cells cultured in vitro. Two contain a central 5'-CCGG-3' or 5'-TGGT-3' sequence which are strongly preferred for covalent binding but appear to be not so frequently mutated. Binding of the two enantiomers to the latter two sequences yielded a distribution of BPDE-N2-dG adduct conformations similar to those from binding to highly polymerized, random sequence DNA in vitro which, for (+/-)-anti-BPDE, means that the helix-external conformation of the N2-dG adduct is dominant. Binding of (-)-anti-BPDE to the 5'-RAGGAR-3' sequences yielded more partially base-stacked and less base-stacked (quasi-intercalated) conformer than observed for random sequence DNA. Importantly, the (+)-anti-BPDE in binding to the more mutagenically inclined 5'-RAGGAR-3' sequences yielded little external-type adduct in comparison to the other two sequences and random sequence DNA. Moreover, an unusually high proportion of the (+)-anti-BPDE adducts formed with the 5'-RAGGAR-3' sequences result from cis stereoaddition, which yields a partially base-stacked configuration. Since the (+)-anti-BPDE appears to be the more mutagenic, this result suggests a possible role of internal adduct conformations in mutagenesis.