Metabolic activation of polycyclic aromatic hydrocarbons (PAH) can be understood in terms of two main pathways: one-electron oxidation to yield reactive intermediate radical cations and monooxygenation to produce bay- region diol epoxides. Although considerable data have been interpreted as evidence for bay-region diol epoxides as the ultimate carcinogenic metabolites of the most potent PAH, which include benzo[a]pyrene (BP), 7,12-dimethylbenz[a]anthracene and 3-methylcholanthrene, substantive results from carcinogenicity experiments do not support this hypothesis. To gain more evidence about one-electron oxidation in the metabolism, carcinogenesis and binding of PAH to DNA and protein, we plan the following studies with BP and 6=CH3BP as models for unsubstituted and methyl-substituted PAH. To demonstrate that BP radical cations are involved in the formation of 3- and 9-hydroxyBP, we will study the metabolism of 3- and 9-FBP with rat liver microsomes and analyze the formation of the respective hydroxy derivatives by high pressure liquid chromatography. We will also investigate the ability of cysteine to act as a trapping agent for BP reactive intermediates formed by cytochrome P-450 and to reduce the binding of BP to DNA and protein. We will determine the structure of BP-DNA and BP-protein adducts formed by rat liver microsomes and nuclei to determine whether or not binding involves the BP radical cation. To gain evidence that the carcinogenicity of 6- FBP and the non-carcinogenicity of 9-FBP are related to their radical cation chemistry, we will a) determine the relative reactivity at C-6 of the radical cation of 6-, 8-, and 9-FBP obtained by one-electron oxidation with manganic acetate; b) compare the extent of binding of BP, 6-, 8- and 9-FBP to DNA catalyzed by horseradish peroxidase (HRP), prostaglandin H synthase (PHS) and rat liver microsomes; c) identify the adducts formed when 6-FBP is bound to DNA by HRP, PHS and microsomes. Finally, to study the activation of a methyl-substituted PAH by one- electron oxidation, we will identify the adducts formed when 6-CH3BP is bound to a) DNA by HRP, PHS and rat liver microsomes; b) protein by rat liver microsomes; and c) histones and DNA in rat liver nuclei. The results from this project will provide further evidence on the central role of one-electron oxidation in the metabolism, binding and carcinogenesis of PAH.
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