| The overall goal was to identify the molecular basis of species and tissue-specific toxicity of xenobiotic compounds. The central hypothesis is that the mouse olfactory mucosa-specific cytochrome P450 enzyme, CYP2G1, plays a critical role in target tissue metabolism of sex steroid hormones and olfactory toxicants. The specific aims are to understand the metabolic basis of coumarin-induced tissue-specific toxicity; to identify the mechanisms of the olfactory mucosa-specific expression of CYP2G1; and to establish a novel knockout mouse model for future studies of the biological and toxicological roles of mouse CYP2G1 in vivo.; We have found that rat and mouse olfactory mucosal microsomes have much higher activity in coumarin bioactivation than liver microsomes, suggesting that target tissue activation is critical in tissue-selective toxicity. CYP2G1 seems to play a major role in the metabolic activation of coumarin, whereas CYP2A5 is mainly responsible for detoxification. Experiments with human P450s indicated, for the first time, a potential risk of coumarin toxicity in humans.; To identify regulatory regions important for its tissue-specific expression, the mouse Cyp2g1 gene was characterized. Analysis of transgenic mice demonstrated that a 3.6-kilobase 5′-flanking region of the Cyp2g1 gene was sufficient for tissue-selective and proper developmental expression of a LacZ reporter gene. However, additional regulatory sequences are necessary for correct cell type-selectivity within the olfactory mucosa.; CYP2G1-null mice were generated. Homozygous Cyp2g1 −/− mice showed no phenotypic abnormalities. In addition, deletion of the Cyp2g1 gene did not cause compensatory increases in the expression of other biotransformation enzymes in the olfactory mucosa. In vitro metabolic studies demonstrated that CYP2G1 is important for the metabolism of testosterone and progesterone in olfactory mucosal microsomes. However, CYP2G1 seems not to play a major role in the bioactivation of a known olfactory toxicant, 2,6-dichlorobenzonitrile.; These findings broaden our knowledge of the mechanisms of chemical-induced tissue-specific toxicity. Future studies based on these results and using the animal model developed in this study should have a significant impact on risk assessment in humans. |
