omega-Hydroxylation of oleic acid in Vicia sativa microsomes: inhibition by substrate analogs and inactivation by terminal acetylenes

Oleic acid (18:1) is hydroxylated exclusively on the terminal methyl by a microsomal cytochrome P-450-dependent system (omega-OAH) from clofibrate-induced Vicia sativa L. (var minor) seedlings (F. Pinot, J.-P. Salaun, H. Bosch, A. Lesot, C. Mioskowski, F. Durst [1992] Biochem Biophys Res Commun 184: 183-193). This reaction was inactivated by two terminal acetylenes: (Z)-9-octadecen-17-ynoic acid (17-ODCYA) and the corresponding epoxide, (Z)-9,10-epoxyoctadecan-17-ynoic acid (17-EODCYA). Inactivation was mechanism-based, with an apparent binding constant of 21 and 32 micromolar and half-lives of 16 and 19 min for 17-ODCYA and 17-EODCYA, respectively. We have investigated the participation of one or more omega-hydroxylase isoforms in the oxidation of fatty acids in this plant system. Lauric acid (12:0) is omega-hydroxylated by the cytochrome P-450 omega-hydroxylase omega-LAH (J.-P. Salaun, A. Simon, F. Durst [1986] Lipids 21: 776-779). Half-lives of omega-OAH and omega-LAH in the presence of 40 micromolar 17-ODCYA were 23 and 41 min, respectively. inhibition of oleic acid omega-hydroxylation was competitive with linoleic acid (18:2), but noncompetitive with lauric acid (12:0). In contrast, oleic acid did not inhibit omega-hydroxylation of lauric acid. Furthermore, 1-pentadecyltriazole inhibited omega-hydroxylation of oleic acid but not of lauric acid. These results suggest that distinct monooxygenases catalyze omega-hydroxylation of medium- and long-chain fatty acids in V. sativa microsomes