Microsomes isolated from rabbit olfactory epithelium catalyzed the conversion of arachidonic acid to several metabolites at a rate of 692 ± 106 pmol/min/nmol P-450. The major metabolite was the ω-hydroxylated metabolite, 20-hydroxyeicosatetraenoic acid, accounting for 57% of the total metabolite produced. A putative ω-1 hydroxylated metabolite was also formed to a lesser extent. Epoxyeicosatrienoic acids were not detected with microsomal incubations, although metabolites corresponding to the dihydroxyeicosatrienoic acids were observed and represented about 20% of the total metabolite produced. The metabolism of arachidonic acid was also studied in a reconstituted system with six purified P-450 isoforms that are known to be expressed in rabbit olfactory mucosa. These included P-450NMa, P- 450 2G1 (NMb), P-450 1A2, P-450 2B4, P-450 2E1, P-450 3A6 and a partially purified preparation of P-450 4A isoforms. The P-450 4A forms were the only enzymes to produce significant amounts of the ω-hydroxylated metabolite of arachidonic acid. The other isoforms were either inactive (P-450NMa and P-450 3A6) or produced metabolites other than the 20-hydroxyeicosatetraenoic acid and, thus, cannot account for the majority of the microsomal metabolism of arachidonic acid. Immunoblot analysis with goat anti-rat P-450 4A1 identified one major and a second minor protein of the P-450 4A gene family in olfactory microsomes. The same antibody identified two proteins in rabbit renal tissue that were significantly induced by pretreatment with clofibric acid and were present in a partially purified preparation of P-450 4A from rabbit renal cortex. The presence of P-450 4A gene products in nasal microsomes was consistent with the high level of 20-hydroxyeicosatetraenoic acid formation. Several odorants and steroids (anisole, benzylbenzoate, 3,7-dimethyl-1- octanol, androstenone, progesterone and testosterone) affected olfactory microsomal arachidonic acid metabolism to different degrees. Androstenone and testosterone stimulated dihydroxyeicosatrienoic acid formation while benzylbenzoate and 3,7-dimethyl-1-octanol inhibited formation of 20- hydroxyeicosatetraenoic acid. The formation of vasoactive metabolites of arachidonic acid and the modulation of the formation of these metabolites by odorants in nasal tissue may be important in the regulation of nasal blood flow and ultimately the rate of absorption of xenobiotics in the nasal cavity.
|Number of pages
|Journal of Pharmacology and Experimental Therapeutics
|Published - 1992
ASJC Scopus subject areas
- Molecular Medicine