NASAL CYTOCHROME-P450 2A - IDENTIFICATION, REGIONAL LOCALIZATION, ANDMETABOLIC-ACTIVITY TOWARD HEXAMETHYLPHOSPHORAMIDE, A KNOWN NASAL CARCINOGEN

Two members of the cytochrome P450 2A subfamily, CYP2A10 and 2A11, are abundant nasal enzymes previously characterized in rabbit olfactory m icrosomes. Rabbit CYP2A is active toward a number of nasal toxicants, including the rat nasal procarcinogen hexamethylphosphoramide (HMPA). While P450s immunochemically related to the rabbit CYP2As have been de tected in rat and human nasal mucosa, confirmation of these enzymes as members of the CYP2A subfamily and efforts to characterize their abil ity to bioactivate toxicants have been limited. In the present study, the regional distribution and cell-specific expression of CYP2A in the rat nasal cavity were examined using an antibody to rabbit CYP2A10/11 . In sections of the anterior nose, immunoreactive CYP2A was present i n ciliated cells of the nasal respiratory epithelium and cuboidal epit helial cells of the nasal transitional epithelium, but was absent in s quamous epithelial cells. The most intense immunostaining was observed in the posterior nose. Olfactory sustentacular cells and Bowman's gla nd cells in sections posterior to the nasal papilla stained most inten sely. Western blot analysis revealed that anti-CYP2A10/11 recognized a sharp band of approximately 50 kDa in nasal respiratory and olfactory microsomes, supporting the premise that the antibody is reacting with a cytochrome P450 enzyme. The nasal expression of CYP2A6 mRNA-a membe r of the human CYP2A subfamily having a high degree of homology to rab bit 2A10 and 2A11-was examined in human surgical patients. Middle turb inectomy tissues-largely composed of nasal respiratory epithelia-from 11 patients were analyzed for the presence of CYP2A6 using reverse tra nscription - polymerase chain reaction (RT-PCR). Identification of CYP 2A6 was confirmed by DNA sequencing of RT-PCR products. CYP2A6 mRNA wa s detected in all of the human samples analyzed. In additional experim ents, human CYP2A6 metabolized HMPA to formaldehyde, suggesting that t his compound might cause nasal toxicity in humans. The identification of CYP2A cytochromes in rat and human nasal tissues may have important implications for risk assessment of inhaled xenobiotics. (C) 1997 Aca demic Press.