Noncovalent Binding of 7β,8α-Dihydroxy-9α,10α-epoxytetrahydrobenzo[a]pyrene to Deoxyribonucleic Acid and Its Catalytic Effect on the Hydrolysis of the Diol Epoxide to Tetrol

In the presence of native DNA the hydrolysis of benzo[a]pyrene-7,8-diol 9,10-epoxide (BPDE) to tetrols (BPT) is markedly accelerated (by a factor of up to ∼80 at 25 °C, pH 7.0, in 5 mM sodium cacodylate buffer solution). When stopped-flow kinetic techniques are utilized, it is shown that the pseudo-first-order hydrolysis rate constant k_H is smaller by a factor of ∼3 in the presence of equivalent concentrations of denatured DNA, by a factor of 8–25 in the presence of nucleotides, and by a factor of 35–45 in the presence of nucleosides (depending on the nucleotide or nucleoside). In the presence of native DNA, k_H increases with increasing DNA concentration and reaches a limiting value of k_H = 0.684 ± 0.04 s⁻¹ at DNA concentrations in excess of ∼5 × 10⁻⁴ M (expressed in concentration of nucleotides). A kinetic model based on (1) rapid formation of a noncovalent BPDE-DNA complex followed by (2) slower hydrolysis of BPDE to BPT at these binding sites is consistent with the experimental data. It is shown furthermore that the DNA concentration dependence of k_H and of noncovalent intercalative binding of BPDE to DNA is similar and that addition of magnesium ions (which is known to reduce intercalative binding of planar aromatic molecules to DNA) also reduces k_H. These results suggest, but do not necessarily prove, that the DNA binding sites at which the hydrolysis of BPDE (to BPT) is catalyzed are intercalative in nature.