Dexamethasone induces bisallylic hydroxylation of polyunsaturated fatty acids by rat liver microsomes

Abstract

Human, monkey, and rat liver microsomes catalyze bisallylic hydroxylations of arachidonic and linoleic acids. The cytochrome P450 gene family of these hydroxylases has not been determined. We examined whether inducers of cytochrome P450 could augment the bisallylic hydroxylation activity of male rat liver microsomes. The microsomes were incubated with [14C]linoleic acid and NADPH and the monohydroxy metabolites were characterized. Microsomes prepared from control rats yielded mainly 18-hydroxyoctadecadienoic acid (18-HODE) and 17-HODE and microsomes from 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 phenobarbital, isopropanol, imidazole, or acetone also formed these three products along with many other hydroxy metabolites. The synthetic glucocorticoid dexamethasone increased the biosynthesis of 11-HODE selectively. Microsomes from male Sprague-Dawley and Fischer rats treated with dexamethasone mainly formed 11-HODE and 18-HODE. The biosynthesis of 11-HODE was increased 10-fold and troleandomycin (50 microM) inhibited the biosynthesis of 11-HODE by 90%. The bisallylic hydroxylases were also investigated with 14C-labeled arachidonic and eicosapentaenoic acids as substrates. Microsomes from rats treated with dexamethasone converted 20:4n-6 to 13-hydroxyeicosatetraenoic acid (13-HETE), 10-HETE, 7-HETE, 19-HETE, and 20-HETE. Induction by acetone yielded the same products. 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 acetone-treated rats mainly formed 19-HEPE and 20-HEPE. We conclude that microsomes from dexamethasone-treated rats catalyze bisallylic hydroxylations of 18:2n-6, 20:4n-6, and 20:5n-3, possibly by induction of bisallylic hydroxylases of the CYP3A subfamily.