The onset of olfactory transduction has been extensively studied, but considerably less is known about the molecular basis of olfactory signal termination. It has been suggested that the highly active cytochrome P450 monooxygenases of olfactory neuroepithelium are termination enzymes, a notion supported by the identification and molecular cloning of olfactory-specific cytochrome P450s (refs. 13-16). But as reactions catalysed by cytochrome P450 (refs 17, 18) often do not significantly alter volatility, lipophilicity or odour properties, cytochrome P450 may not be solely responsible for olfactory signal termination. In liver and other tissues, drug hydroxylation by cytochrome P450 is frequently followed by phase II biotransformation, for example by UDP glucuronosyl transferase (UGT), resulting in a major change of solubility and chemical properties. We report here the molecular cloning and expression of an olfactory-specific UGT. The olfactory enzyme, but not the one in liver microsomes, shows preference for odorants over standard UGT substrates. Furthermore, glucuronic acid conjugation abolishes the ability of odorants to stimulate olfactory adenylyl cyclase. This, together with the known broad spectrum of drug-detoxification enzymes, supports a role for olfactory UGT in terminating diverse odorant signals.

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Previously, we described two olfactory-specific cytochromes P-450 : rat cytochrome P-45001fl (IIGl), identified by cDNA cloning, and bovine cytochrome P-45001f2 (IIA), identified by peptide microsequencing of a transmembranal polypeptide (p52). Here we describe the preparation of polyclonal antisera against peptide sequences of these proteins and their use in the immunolocalization of cytochromes P-45001fl and P-45001f2 in rat olfactory mucosa. Immunoreactivities related to both enzymes are found in the subepithelial Bowman's glands of olfactory mucosa. Practically no immunoreactivity was found in other rat tissues, including liver, lung, kidney and respiratory mucosa. In addition, double-labeling experiments demonstrated that cytochromes P-45001fl and P-45001f2 are present in the same population of Bowman's glands. The olfactory-specific localization of cytochromes P-45001fl and P-45001f2 is consistent with a role for these enzymes in the modification or clearance of odorants from the chemosensory tissue.Cytochrome-P-450-type enzymes catalyze the metabolism and detoxification of myriad exogenous and endogenous compounds (reviewed in [I -21). Although found predominantly in the liver, cytochromes P-450 are also present in extrahepatic sites, such as lung, kidney, small intestine, and the nasal mucosa, tissues that are constantly exposed to xenobiotic chemicals [3 -51. A major group of cytochromes P-450 are regarded as phase-I biotransformation enzymes, which generally catalyze hydroxylation reactions. The products of these reactions are then acted upon by phase-I1 enzymes, such as UDPglucuronosyl transferase and glutathione transferase [6, 71. The combined action of these enzymes results in the production of hydrophilic metabolites that can be readily excreted [l, 61. The action of cytochromes P-450 may sometimes result in the production of metabolites that are more toxic or carcinogenic than the parent compounds [8].The nasal mucosa exhibits cytochrome P-450 activity that is comparable to, or in some cases greater than, the activity in the liver [9 -111. In addition, the nasal enzymes appear to differ from those in the liver in their substrate specificities and mechanisms of action [12-141. Using antisera against liver cytochromes P-450, several studies have identified cross-reacting counterparts in the olfactory mucosa [3-5, 15, 161 (reviewed in [11, 171). These were localized in an olfactory epithelial superficial layer and in the subepithelial Bowman's glands [3, 5, 151.Recently, we described two cytochrome P-450 enzymes that appear to be specific to the olfactory system [18, 191. cDNA coding for cytochrome P-45001fI (P-450IIG1) was isolated from a rat olfactory mucosal cDNA library using rat liver cytochrome P-450b (P-450IIB1) cDNA as a probe [18]. It is expressed in the olfactory mucosa, but not in respiratory mucosa, liver, lung, or heart. Cytochrome P-45001f2 (P-450IIA) was identified as an olfactory-specific, major transmembranal protein of bovine olfactory membranes [19]. To further understand the...

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