Influence of Na+ on DNA reactions with aromatic epoxides and diol epoxides: Evidence that DNA catalyzes the formation of benzo[a]pyrene and benz[a]anthracene adducts at intercalation sites

Reactions of the benzo[a]pyrene (BP) and benz[a]anthracene (BA) metabolites, (+/-)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-BP (BPDE), (+)-trans-3,4-dihydroxy-anti-1,2-epoxy-1,2,3,4-tetrahydro-BA (BADE), (+/-)-BP-4,5-oxide (BPO), and (+/-)-BA-5,6-oxide (BAO), were examined under pseudo-first-order conditions at varying Na+ (2.0-100 mM) and native, calf thymus DNA (ctDNA) concentrations. In 0.2 mM ctDNA and 2.0 mM Na+, at a pH of 7.3, most BPDE, BADE, BPO, and BAO (87-95%) undergo DNA catalyzed hydrolysis or rearrangement. For BPDE and BPO, overall, pseudo-first-order rate constants, k, in 2.0 mM Na+ and 0.2 mM ctDNA are 21-72 times larger than values obtained without DNA. For BADE and BAG, the rate constants are less strongly influenced by DNA; k values in 0.2 mM ctDNA are only 9-12 times larger than values obtained without DNA. Kinetic data for BPDE, BPO, BADE, and BAO and DNA intercalation association constants (K-A) for BP and BA diols which are model compounds indicate that K-A values for BPDE and BPO in 2.0 mM Na+ are 6.6-59 times larger than those of BADE and BAG. The greater DNA enhancement of rate constants for BPDE and BPO, versus BADE and BAG, correlates with the larger K-A values of the BP metabolites. DNA adducts, which account for less than 10% of the yields, also form. For BPDE in 0.20 mM ctDNA, k decreases 5.1 times as the Na+ concentration increases from 2.0 to 100 mM. Nevertheless, the DNA adduct level remains constant over the range of Na+ concentrations examined. These results provide evidence that, for BPDE in 0.20 mM DNA and 2.0 mM Na+, ctDNA adduct formation follows a mechanism which is similar to that for DNA catalyzed hydrolysis. The pseudo-first-order rate constant for adduct formation, k(Ad), given approximately by K(Ad)approximate to(k(cat,Ad)K(A)[DNA])/(1+K-A[DNA]), where k(cat,Ad) is a catalytic rate constant. For BADE, BPO, and BAG, the influence of varying DNA and Na+ concentrations on k values is similar to that for BPDE, and provides evidence that the formation of adducts follows the same rate law.