Mechanisms of reaction of benzo(a)pyrene-7,8-diol-9,10-epoxide with DNA in aqueous solutions.

Abstract

The physical and chemical reaction pathways of the metabolite model compound benzo(a)pyrene-7,8-diol-9,10-epoxide (BPDE) in aqueous (double-stranded) DNA solutions was investigated as a function of temperature (0-30 degrees C), pH (7.0-9.5), sodium chloride concentration (0-1.5M) and DNA concentration in order to clarify the relationships between the multiple reaction mechanisms of this diol epoxide in the presence of nucleic acids. The reaction pathways are (1) noncovalent intercalative complex formation with DNA, characterized by the equilibrium constant K, and Xb the fraction of molecules physically bound; (2) accelerated hydrolysis of BPDE bound to DNA; (3) covalent binding to DNA; and (4) hydrolysis of free BPDE(kh). The DNA-induced hydrolysis of BPDE to tetraols and the covalent binding to DNA are parallel pseudo-first-order reactions. Following the rapid (millisecond time scale) noncovalent complex formation between BPDE and DNA, a much slower (approximately minutes) H+-dependent (either specific or general acid catalysis) formation of a DNA-bound triol carbonium ion (rate constant k3) occurs. At pH 7.0 the activation energy of k3 is 8.7 +/- 0.9 kcal/mol, which is lower than the activation energy of hydrolysis of free BPDE in buffer solution (14.2 +/- 0.7 kcal/mol), and which thus partially accounts for the acceleration of hydrolysis of BPDE upon complexation with DNA. The formation of the triol carbonium ion is followed by a rapid reaction with either water to form tetraols (rate constant kT), or covalent binding to DNA (kc). The fraction of BPDE molecules which undergo covalent binding is fcov approximately equal to kc/(kc + kT) = 0.10 and is independent of the overall BPDE reaction rate constant k = kh(1 - Xb) + k3Xb if Xb----1.0, or is independent of Xb as long as k3Xb much greater than kh(1 - Xb). Thus, at Xb = 0.9, fcov is independent of pH (7.0-9.5) even though k exhibits a 70-fold variation in this pH range and k----kh above pH 9 (k3 = kh). Similarly, fcov is independent of temperature (0-30 degrees C), while k varies by a factor of approx. 3. In the range of 0-1.5 M NaCl, fcov decreases from 0.10 to 0.04. These variations are attributed to a combination of salt-induced variations in the factors k3, Xb and the ratio kc/kT.