The nasal tissues are exposed to the highest levels of air-borne xenobiotics of any body tissue, and mammalian olfactory mucosa is remarkably active int he metabolism of xenobiotics. The molecular mechanism underlying such high metabolic activity is not fully understood although the presence of high levels and unique forms of P450 has been demonstrated. P450NMa (NMa), one of the major P450s in rabbit nasal microsomes, is highly active in the metabolic activation of many known nasal toxicants and procarcinogens, including N-nitrosodiethylamine,4- (methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), phenacetin, hexamethylphosphoramide, 2,6-dichlorobenzonitrile (DCBN), and aflatoxin B1 (AFB1). Nevertheless, the significance of the preferential expression of NMa in the nasal tissues in the tissue-specific toxicity of these xenobiotics in rodents and in risk assessment in humans is not conclusive, since most toxicity studies have been conducted in rodents and little is known about xenobiotic metabolism in human olfactory mucosa. Thus, my hypothesis is that P450s structurally similar to NMa are present in human and rodent olfactory tissues and play important roles in the metabolic activation of many olfactory toxicants.
My specific aims are: 1. Identification and cDNA cloning of P450 isozymes expressed in human and rat olfactory mucosa with particular emphasis on isoforms that are structurally similar to rabbit P450 2A10 and 2A11 (NMa); 2. Heterologous expression of rat and human 2A-related cDNAs (obtained in aim 1) in E. coli or COS1 cells, and examination of the activity of the recombinant and microsomal P450s in the bioactivation of two olfactory toxicants, DCBN (herbicide) and AFB1 (food contaminant); and 3. Immunochemical and molecular characterization of 2A-related P450 isozymes expressed in rat olfactory bulb so as to determine whether the central olfactory tissues are vulnerable to the toxicity associated with metabolic activation in situ. If time permits, olfactory expression of other P450 genes will be examined wand the role of rodent and human olfactory P450s in the metabolic activation or detoxification of additional toxicants will be studied. With a long-term objective of understanding the impact of environmental toxicants on the olfactory system, future studies will examine the likely genetic polymorphism of 2A-related olfactory P450s in human populations and the developmental expression of these and other P450s to explore the potential genetic predisposition and age-related vulnerability of the olfactory system to xenobiotic toxicity.
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