Fatty acid metabolism in marine fish: low activity of fatty acyl delta5 desaturation in gilthead sea bream (Sparus aurata) cells.

Marine fish have an absolute dietary requirement for C20 and C22 highly unsaturated fatty acids. Previous studies using cultured cell lines indicated that underlying this requirement in marine fish was either a deficiency in fatty acyl delta5 desaturase or C18-20 elongase activity. Recent research in turbot cells found low C18-20 elongase but high delta5 desaturase activity. In the present study, the fatty acid desaturase/elongase pathway was investigated in a cell line (SAF-1) from another carnivorous marine fish, sea bream. The metabolic conversions of a range of radiolabeled polyunsaturated fatty acids that comprised the direct substrates for delta6 desaturase ([1-14C]18:2n-6 and [1-14C]18:3n-3), C18-20 elongase ([U-14C]18:4n-3), delta5 desaturase ([1-14C]20:3n-6 and [U-14C]20:4n-3), and C20-22 elongase ([1-14C]20:4n-6 and [1-14C]20:5n-3) were utilized. The results showed that fatty acyl delta6 desaturase in SAF-1 cells was highly active and that C18-20 elongase and C20-22 elongase activities were substantial. A deficiency in the desaturation/elongation pathway was clearly identified at the level of the fatty acyl delta5 desaturase, which was very low, particularly with 20:4n-3 as substrate. In comparison, the apparent activities of delta6 desaturase, C18-20 elongase, and C20-22 elongase were approximately 94-, 27-, and 16-fold greater than that for delta5 desaturase toward their respective n-3 polyunsaturated fatty acid substrates. The evidence obtained in the SAF-1 cell line is consistent with the dietary requirement for C20 and C22 highly unsaturated fatty acids in the marine fish the sea bream, being primarily due to a deficiency in fatty acid delta5 desaturase activity.