This article is available online at http://www.jlr.org Cytochrome P450 (CYP) enzymes catalyze the formation of biologically active epoxy-and hydroxy-metabolites of long-chain PUFAs ( 1 ). Traditionally, and in line with the prevalence of n-6 PUFAs in the "Western diet", arachidonic acid (AA) (20:4 n-6) has been considered as the main precursor and the corresponding metabolites were categorized as a subclass of eicosanoids ( 2 ). CYP-eicosanoid formation is also known as the "third branch of the AA cascade," complementary to the previously discovered cyclooxygenase (COX)-and lipoxygenase (LOX)-initiated pathways of prostanoid and leukotriene formation ( 3, 4 ).Physiologically important AA-derived CYP-eicosanoids include a set of regio-and stereoisomeric epoxyeicosatrienoic acids (EETs) and 20-HETE ( 2, 5 ). EETs and 20-HETE play partially opposing roles in the regulation of vascular, renal, and cardiac function ( 6-9 ). The contribution of EETs to cardiovascular function is infl uenced by the soluble epoxide hydrolase (sEH) that metabolizes EETs to less potent dihydroxyeicosatrienoic acids (DHETs) ( 10 ). Imbalances in CYP-eicosanoid formation are linked to the development of endothelial dysfunction and hypertension; ischemia-induced injury of the heart, kidney and brain; infl ammatory disorders; and atherosclerosis (11)(12)(13)(14)(15)(16)(17).Recent studies demonstrated that the same CYP isoforms that epoxidize or hydroxylate AA, also effi ciently metabolize