Physical interactions of isomeric benzo [a] pyrene diol-epoxides with DNA

The physical interactions of two diol-epoxides derived from benzo [a] pyrene (B[a]P), 7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydro B[a]P (BPDE) and 9,10-dihydroxy-7,8-oxy-7,8,9,10-tetrahydro B[a]P, (reverse BPDE) with DNA have been studied in a simple, in vitro system. The effects of DNA on the rates of hydrolysis of BPDE and reverse BPDE were studied by fluorescence spectroscopy. For both compounds, interaction with DNA was indicated by an increase in the rate of hydrolysis in the presence of DNA. This increased hydrolysis was more marked for BPDE than for reverse BPDE. Direct confirmation of physical binding was obtained by u.v. spectroscopy, where a 10 nm redshift in absorbance maxima characteristic of polycyclic aromatic hydrocarbon.DNA intercalation complexes was observed. Using absorbance changes to monitor binding, association constants of 6580 L/mol and 5080 L/mol were determined for BPDE and reverse BPDE, respectively. Consistent with the intercalation model, binding was inhibited by low concentrations of Mg2+. The enhancement of hydrolytic rate by DNA for BPDE and reverse BPDE was also inhibited by low concentrations of Mg2+, suggesting involvement of intercalation complexes in the mechanism of enhanced hydrolysis.