Base-sequence dependence of noncovalent complex formation and reactivity of benzo[a]pyrenediol epoxide with polynucleotides

Abstract

The base-sequence selectivity of the noncovalent binding of (.+-.)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene (BPDE) to a series of synthetic polynucleotides in aqueous solutions (5mM sodium cacodylate buffer, 20 mM NaCl, pH 7.0,22.degree. C) was investigated. The magnitude of a red-shifted absorbance at 353 nm, attributed to intercalative complex formation, was utilized to determine values of the association constant Kic. Intercalation in the alternating pyridine-purine polymers poly(dA-dT) .cntdot. (dA-dT) (Kic = 20000 M-1), poly(dG-dC) .cntdot. (dG-dC) (4200 M-1), and poly(dA-dC) .cntdot. (dG-dT) (9600 M-1) is distinctly favored over intercalation in their nonalternating counterparts poly(dA) .cntdot. (dT) (780 M-1), poly(dG) .cntdot. (dC) (1800 M-1), and poly(dA-dG) .cntdot. (dT-dC) (5400 M-1). Methylation at the 5-position of cytosine gives rise to a significant enhancement of intercalative binding, and Kic is 22000 M-1 in poly(dG-m5dG) .cntdot. (dG-m5dC). In a number of these polynucleotides, values of Kic for pyrene qualitatively follow those exhibited by BPDE, suggesting that the pyrenyl residue in BPDE is a primary factor in determining the extent of intercalation. Both BPDE and pyrene exhibit a distinct preference for intercalating within dA-dT and dG-m5dC sequences. The catalysis of the chemical reactions of BPDE (hydrolysis to tetrols and covalent adduct formation) is enchanced significantly in the presence of each of the polynucleotides studied, particularly in the dG-containing polymers. A model in which catalysis is mediated by physical complex formation accounts well for the experimentally observed enhancement in reaction rates of BPDE in the alternating polynucleotides; however, in the nonalternating polymers a different or more complex catalysis mechanism may be operative. Finally, complex formation and catalysis at different binding sites are related to the level of covalent binding, one of the critical factors in the expression of the mutagenic and tumorigenic potentials of BPDE and related compounds in cells.