N-heterocyclic aromatics are environmentally important pollutants. The studies on the potentially harmful biological effects of N-heterocyclics produced by incomplete combustion have been limited and the mechanism(s) of action of these compounds is unknown. The objectives of the study are to investigate the metabolic activation and DNA binding of an N- heterocyclic aromatic that has been found to be a potent carcinogen in mouse lung, liver, and skin, namely dibenzo(c,g) carbazole (DBC) with respect to dibenz(a,j)-acridine (DBA) a moderate to weak carcinogen in mouse lung and skin. Since the structural difference between the well characterized polycyclic aromatic hypdrocarbons and the N-heterocyclic analogs is the existence of a nitrogen atom in the aromatic ring system of latter, it is hypothesized that any difference in the metabolism, metabolic activiation, DNA binding, or the carcinogenic potency between the two classes is due not only to the presence of a nitrogen atom but to the aromaticity of the heteroatom containing ring.
The specific aims of this proposal will be to characterize the metabolic activation and DNA binding of DBC and DBA in mouse skin as follows: 1) synthesize sufficient quantities of the major DBC and DBA metabolites, 2) determine the mutagenicity of DBC and DBA metabolites in both a mammalian epithelial cell and a bacterial system, 3) determine the initiating ability of selected metabolites in an initiation-promotion study using CD-1 mice and TPA as a promoter, and 4) characterize the covalent binding of DBC and DBA to DNA. Identification of the major metabolites will be accomplished by chemical analyses of DBC and DBA products isolated from liver microsomal incubation and mouse skin and by comparison of the products obtained by chemical synthesis. Proposals for the pathway of activation will be generated in an integrated short-term test approach by determining the mutatgenic and initiating ability of the major metabolites. Lastly, in order to identify the possible ultimate metabolites of DBC and DBA involved in covalent binding, mouse skin will be used to characterize DNA adducts using DNA isolation procedures and chromatographic and fluorescence techniques. Adducts generated in liver microsomal preparations will be used as standards for in vivo mouse skin experiments. N- heterocyclics are an important class of environmental and occupational carcinogens. The approach defined in this proposal should provide valuable information concerning the disposition of an important class of carcinogens and will lead directly to the understanding and clarification of the mechanism(s) of action of N-heterocyclic aromatic carcinogenesis.
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