Chain elongation of polyunsaturated fatty acids by vascular endothelial cells: studies with arachidonate analogues

This study has utilized radiolabeled analogues of arachidonic acid to study the substrate specificity of elongation of long-chain polyunsaturated fatty acids. Human umbilical vein endothelial cells were incubated for 2-72 hr in medium supplemented with 0.9-2.6 microM [14C]fatty acid, and cellular glycerolipids were analyzed by gas-liquid chromatography with radioactivity detection. Elongation of naturally occurring C20 polyunsaturated fatty acids occurred with eicosapentaenoate (20:5(n-3] greater than Mead acid (20:3(n-9] greater than arachidonate (20:4(n-6]. Chain length markedly influenced the extent of elongation of 5,8,11,14-tetraenoates (18:4 greater than 19:4 greater than 20:4 greater than 21:4); effects of initial double bond position were also observed (6,9,12,15-20:4 greater than 4,7,10,13-20:4. Neither 5,8,14- nor 5,11,14-20:3 was elongated to the extent of 5,8,11-20:3. Differences between polyunsaturated fatty acids were observed both in the initial rates and in the maximal percentages of elongation, suggesting that the content of cellular C20 and C22 fatty acids may represent a balance between chain elongation and retroconversion. Umbilical vein endothelial cells do not exhibit significant desaturation of either 22:4(n-6) or 22:5(n-3). By contrast, incubation with 5,8,11,14-[14C]18:4(n-4) resulted in formation of both [14C]20:5(n-4) and [14C]22:5(n-4). The respective time courses for the appearances of [14C]22:5(n-4) and [14C]20:5(n-5) suggests delta 6 desaturation of [14C]22:4(n-4) rather than delta 4 desaturation of [14C]20:4(n-4).