Kinetic studies of the reactions of benzo[a]pyrene-7,8-diol 9,10-epoxides in aqueous solutions of human serum albumin and nonionic micelles.

Abstract

The rates of reactions of the cis and trans (benzylic 7-hydroxyl group relative to epoxide oxygen) bay-region 7,8-diol 9,10-epoxides of benzo[a]pyrene (DE-1 and DE-2, respectively) in solutions containing human serum albumin (HSA) and a model system of micelles of the nonionic detergent Tween-80 have been determined as a function both of HSA or Tween-80 concentrations and of pH. The effect of increasing concentrations of both HSA and Tween-80 is to retard substantially the rates of reaction of DE-1 and DE-2 over the pH range 5-7. The rate data are consistent with a mechanism in which the diol epoxides physically associate with HSA or Tween-80, and the rates of reaction of the association complexes are reduced compared to those of free diol epoxides. The limiting rate constants for reaction of the diol epoxide.HSA and diol epoxide.Tween-80 complexes are dependent on pH. The rate data are best accommodated by a mechanism in which the complexes react by two competing pathways, one whose rate is proportional to hydronium ion activity, and the second whose rate is pH-independent. These reactions of the association complexes are therefore kinetically analogous to the acid-catalyzed and spontaneous reactions of free diol epoxide. The spontaneous reaction of the (DE-2).HSA complex results in approximately 10% covalent binding of diol epoxide to the protein, whereas the acid-catalyzed reaction of the complex results in significantly less covalent binding. The balance of the products consists of tetraols formed by hydrolysis of the diol epoxide.