BIOCHEMICAL-CHARACTERIZATION OF THE HUMAN LIVER CYTOCHROME-P450 ARACHIDONIC-ACID EPOXYGENASE PATHWAY

Human liver microsomal fractions metabolized arachidonic acid in the p resence of NADPH yielding epoxyeicosatrienoic acids and their hydratio n products, dihydroxyeicosatrienoic acids, as the principal reaction p roducts. Inhibition studies using polyclonal antibodies prepared again st recombinant CYP2C8, an abundant human liver cytochrome P450 epoxyge nase, demonstrated 85-90% inhibition of arachidonic acid epoxide forma tion, Both epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acids were detected in large amounts in human liver using gas chromatograph y/mass spectrometry, Chiral analysis of the endogenous liver epoxides demonstrated a preference for the 14(R),15(S)-, 11(R),12(S)-, and 8(S) ,9(R)-epoxyeicosatrienoic acid enantiomers, Importantly, the chirality of liver epoxyeicosatrienoic acids matched that previously reported f or recombinant CYP2C8 (Zeldin et al, (1995) Arch, Biochem, Biophys, 32 2, 76-86), Incubations of both human liver cytosolic and microsomal fr actions with synthetic epoxyeicosatrienoic acids revealed a 10-fold hi gher rate of dihydroxyeicosatrienoic acid formation with cytosolic rel ative to microsomal fractions, Recombinant human liver cytosolic epoxi de hydrolase rapidly and regioselectively hydrated epoxyeicosatrienoic acids, We conclude, based on these data, that CYP2C8 is one of the pr imary, constitutive hepatic arachidonic acid epoxygenases responsible for formation of epoxyeicosatrienoic acids and that cytosolic epoxide hydrolase is the principal liver enzyme which forms the dihydroxyeicos atrienoic acids. We speculate that these biologically active eicosanoi ds may be important in maintaining homeostasis in the liver. (C) 1996 Academic Press, Inc.