DEXAMETHASONE INDUCES BISALLYLIC HYDROXYLATION OF POLYUNSATURATED FATTY-ACIDS BY RAT-LIVER MICROSOMES

Human, monkey, and rat liver microsomes catalyze bisallylic hydroxylat ions of arachidonic and linoleic acids. The cytochrome P450 gene famil y of these hydroxylases has not been determined. We examined whether i nducers of cytochrome P450 could augment the bisallylic hydroxylation activity of male rat liver microsomes, The microsomes were incubated w ith [C-14]linoleic acid and NADPH and the monohydroxy metabolites were characterized. Microsomes prepared from control rats yielded mainly 1 8-hydroxyoctadecadienoic acid (18-HODE) and 17-HODE and microsomes fro m clofibrate-treated rats 18-HODE. Microsomes from beta-naphthoflavone -treated rats hydroxylated linoleic acid without position specificity, i.e., at carbons 8, 11, 14, 16, 17, and 18. 11-HODE, 17-HODE, and 18- HODE were major metabolites. Microsomes from rats treated with phenoba rbital, isopropanol, imidazole, or acetone also formed these three pro ducts along with many other hydroxy metabolites. The synthetic glucoco rticoid dexamethasone increased the biosynthesis of 11-HODE selectivel y. Microsomes hom male Sprague-Dawley and Fischer rats treated with de xamethasone mainly formed 11-HODE and 18-HODE. The biosynthesis of 11- HODE was increased 10-fold and troleandomycin (50 mu M) inhibited the biosynthesis of 11-HODE by 90%. The bisallylic hydroxylases were also investigated with C-14-labeled arachidonic and eicosapentaenoic acids as substrates. Microsomes from rats treated with dexamethasone convert ed 20:4n-6 to 13-hydroxyeicosatetraenoic acid (13-HETE), 10-HETE, 7-HE TE, 19-HETE, and 20-HETE, Induction by acetone yielded the same produc ts, Microsomes from dexamethasone-treated rats metabolized 20:5n-3 to 16-hydroxyeicosapentaenoic acid (16-HEPE), 13-HEPE, 10-HEPE, 19-HEPE, and 20-HEPE as major products, while microsomes from control and aceto ne-treated rats mainly formed 19-HEPE and 20-HEPE. We conclude that mi crosomes from dexamethasone-treated rats catalyze bisallylic hydroxyla tions of 18:2n-6, 20:4n-6, and 20:5n-3, possibly by induction of bisal lylic hydroxylases of the CYP3A subfamily. (C) 1996 Academic Press, In c.