HALIDE CATALYSIS OF EPOXIDE/DNA ADDUCT FORMATION

点击翻译按钮获取中文摘要

Polycyclic aromatic hydrocarbons (PAH) and many other important chemical carcinogens are metabolically activated to electrophilic epoxides that alkylate DNA. Mutational activation of proto-oncogenes by these reactions constitutes one of the initial steps in chemical carcinogenesis. The view that epoxides are direct alkylating agents has dominated our concepts of tumor imitation for our two decades. However, we have discovered that halides catalyze both the formation of benzo[a]pyrene diol epoxide (BPDE)-DNA adducts and tetrols. Preliminary in vitro studies demonstrate that cholorhydrins derived from the epoxides are intermediates. What we propose to do in this application is to determine (i) whether halide catalysis generally occurs with PAH and non- PAH epoxides, (ii) whether other halides (iodide and bromide) catalyze adduct formation, (iii) if halide catalysis occurs in vivo, and (iv) whether steric properties of carcinogenic epoxides contribute to the extent that they undergo halide catalysis. In the absence of halide ions, the acid-catalyzed formation of BPDE-DNA adducts and tetrols yields almost exclusively trans products (with reference to the 9,10-positions of the hydrocarbon moiety). Halide catalysis results in a substantial increase in cis product formation which is diagnostic for the involvement of these anions. We will test whether halide catalysis is generally involved in carcinogenic epoxide reactions by studying a series of hydrocarbons (BADE, DMBADE, MCDE, and styrene oxide) and determining the extent to which halides catalyze their reactions. We will determine whether iodide and bromide are, in addition to chloride, capable of catalyzing adduct formation. Product ratios for hydrolysis will be determined in media and inside cells as well as for adduct formation in CHO cells. Chlorohydrins of the carcinogenic epoxides. These data will allow us to determine whether chlorohydrins are formed in cell culture media and whether they are involved in adduct formation in vivo. Finally, we will determine the role of steric crowding in the stability and properties of chlorohydrins by studying a series of PAH epoxides with varying degrees of hindrance in the bay region. Halide catalysis in the reactions of carcinogenic epoxides has several important implications. Previous studies on DNA binding in vitro will need to be re-interpreted depending on the amount of chloride used. This will be particularly important where substantial amounts of cis adducts were obtained. Furthermore, our view of carcinogenesis initiation mechanisms will have to be altered if significant amounts of halide catalysis occur in vivo. One of the long term goals of this study and other work in my laboratory is to develop a complete understanding of initiation mechanisms since this will be essential if we are to discover rational approaches to preventing carcinogenic damage and the tumors that result from it.