The capacity to oxidize the terminal carbon of aliphatic chains is a highly conserved enzymatic activity of cytochrome P450s in plant and animal species. In mammals, the -hydroxylation of fatty acids provides a means to remove potentially toxic excess concentrations of free fatty acids. This is the first step in the formation of dicarboxylic acids that can be more readily excreted or are further degraded by peroxisomal ␤-oxidation. The -hydroxylases also provide pathways for the degradation of signaling molecules such as prostanoids and leukotrienes. On the other hand, -hydroxylation of arachidonic acid forms 20-hydroxyeicosatetraenoic acid (20-HETE), 2 which has been implicated in the regulation of vascular tone and blood pressure (1-3).Cytochrome P450 4F2 (P450 4F2) is a fatty acid -hydroxylase that is expressed in human liver and kidney and contributes to free fatty acid catabolism and the conversion of arachidonic acid to 20-HETE. In humans, the predominant -hydroxylases in liver and kidney are microsomal P450s 4A11, 4F2, and 4F3B (4). The -hydroxylation of saturated fatty acids is generally associated with P450 4A enzymes in non-human species, and human P450 4A11 catalyzes the -hydroxylation of lauric acid Ͼ palmitic acid Ͼ arachidonic acid (5). Selective disruption of the murine Cyp4a10 (6) and Cyp4a14 (7) genes leads to hypertensive phenotypes that are thought to reflect changes in renal 20-HETE production arising through distinct mechanisms. Interestingly, an allelic variant encodes a human CYP4A11 with reduced catalytic activity that is associated with an increased risk for hypertension in studies of three independent cohorts (8, 9). P450s 4F2 and 4F3B are highly similar in amino acid sequence and display overlapping catalytic activity profiles. Both enzymes efficiently oxidize arachidonic acid (10). An alternative transcript of the CYP4F3 gene, 4F3A, arises from differential promoter use leading to the incorporation of an alternative exon that alters substrate preferences to favor the -hydroxylation of leukotrienes (11). P450 4F3A is predominantly expressed in human leukocytes (12). Antibody inhibition experiments (13, 14) suggest that P450 4F2/4F3B in human liver and kidney microsomes contribute ϳ66% to the formation of 20-HETE. Recently, P450s 4F2 and 4F3B were found to -hydroxylate very long chain saturated fatty acids (15) as well as phytanic acid (16). Our preliminary experiments using com-* This work was supported by National Institutes of Health Grant HD004445(to E. F. J.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.