Fatty acid metabolism in marine fish: Low activity of fatty acyl Delta 5 desaturation in gilthead sea bream (Sparus aurata) cells

Marine fish have an absolute dietary requirement for C-20 and C-22 highly u nsaturated fatty acids. Previous studies using cultured cell lines indicate d that underlying this requirement in marine fish was either a deficiency i n fatty acyl Delta 5 desaturase or C18-20 elongase activity. Recent researc h in turbot cells found low C18-20 elongase but high Delta 5 desaturase act ivity. 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 polyunsatur ated fatty acids that comprised, the direct substrates for Delta 6 desatura se ([1-C-14]18:2n-6 and [1-C-14]18:3n-3), C18-20 elongase ([U-C-14]18:4n-3) , Delta 5 desaturase ([1-C-14]20:3n-6 and [U-C-14]20:4n-3), and C20-22 elon gase ([1-C-14]20:4n-6 and [1-C-14]20:5n-3) were utilized. The results showe d that fatty acyl Delta 6 desaturase in SAF-1 cells was highly active and t hat C18-20 elongase and C20-22 elongase activities were substantial. A defi ciency in the desaturation/elongation pathway was clearly identified at the level of the fatty acyl Delta 5 desaturase, which was very law, particular ly with 20:4n-3 as substrate. In comparison, the apparent activities of Del ta 6 desaturase, C18-20 elongase, and C20-22 elongase were approximately 94 -, 27-, and 16-fold greater than that for Delta 5 desaturase toward their r espective n-3 polyunsaturated fatty acid substrates. The evidence obtained in the SAF-1 cell line is consistent with the dietary requirement for C-20 and C-22 highly unsaturated fatty acids in the marine fish the sea bream, b eing primarily due to a deficiency in fatty acid Delta 5 desaturase activit y.