Essential fatty acid deficiency in freshwater fish: the effects of linoleic, alpha-linolenic, gamma-linolenic and stearidonic acids on the metabolismof [1-C-14]18 : 3n-3 in a carp cell culture model

The desaturation of [1-C-14]18:3n-3 to 20:5n-3 and 22:6n-3 is enhanced in a n essential fatty acid deficient cell line (EPC-EFAD) in comparison with th e parent cell line (EPC) from carp. In the present study, the effects of co mpeting, unlabeled C-18 polyunsaturated fatty acids (PUFA), linoleic (18:2n -6), alpha-linolenic (18:3n-3), gamma-linolenic (18:3n-6) and stearidonic ( 18:4n-3) acids, on the metabolism of [1-C-14]18:3n-3 were investigated in E PC-EFAD cells in comparison with EPC cells. The incorporation of [1-C-14]18 :3n-3 in both cell lines was significantly reduced by competing C-18 PUFA, with the rank order being 18:4n-3 > 18:3n-3 = 18:2n-6 > 18:3n-6. In the abs ence of competing PUFA, radioactivity from [1-C-14]18:3n-3 in EPC cells was predominantly recovered in phosphatidylethanolamine followed by phosphatid ylcholine. This pattern was unaffected by competing n-6PUFA, but n-3PUFA re versed this pattern as did essential fatty acid deficiency in the presence of all competing PUFA. The altered lipid class distribution was most pronou nced in cells supplemented with 18:4n-3. Competing C-18 PUFA significantly decreased the proportions of radioactivity recovered in 22:6n-3, pentaene a nd tetraene products, with the proportions of radioactivity recovered in 18 :3n-3 and 20:3n-3 increased, in both cell lines. However, the inhibitory ef fect of competing C-18 PUFA on the desaturation of [1-C-14]18:3n-3 was sign ificantly greater in EPC-EFAD cells. The magnitude of the inhibitory effect s of C-18 PUFA on [1-C-14]18:3n-3 desaturation was dependent upon the speci fic fatty acid with the rank order being 18:4n-3 > 18:3n-3 > 18:2n-6, with 18:3n-6 having little inhibitory effect on the metabolism of [1-C-14]18:3n- 3 in EPC cells. The differential effects of the C-18 PUFA on [1-C-14]18:3n- 3 metabolism were consistent with mass competition in combination with incr eased desaturation activity in EPC-EFAD cells and the known substrate fatty acid specificities of desaturase enzymes. However, the mechanism underpinn ing the greater efficacy with which the unlabeled C(18)PUFA competed with [ 1-C-14]18:3n-3 in the desaturation pathway in EPC-EFAD cells was unclear.