The desaturation and elongation of (14)C-labelled polyunsaturated fatty acids by pike (Esox lucius L.) in vivo.

To examine the ability of pike (Esox lucius L.) to modify exogenous PUFA by desaturation and elongation, (14)C-labelled 18:2(n-6), 18:3(n-3), 20:4(n-6) and 20:5(n-3) were injected intraperitoneally and the distribution of radioactivity in tissue lipid classes and liver PUFA measured. In all tissues examined, radioactivity from all (14)C-PUFA was recovered in many classes of acyl lipids and the level of recovery generally reflected the relative abundance of the lipid classes. Triacylglycerols, CGP and EGP usually contained high levels of all incorporated (14)C-PUFA. PI contained higher levels of radioactivity from (14)C-20:4(n-6) than from other injected substrates. In liver lipid, the Δ6 desaturation products of (14)C-18:2(n-6) and (14)C-18:3(n-3) contained no measurable radioactivity although the elongation products of the Δ6 desaturation products were labelled, as were the direct elongation products of these injected substrates. No radioactivity from (14)C-18:2(n-6) or (14)C-18:3(n-3) was detected in C20 or C22 products of Δ5 and Δ4 desaturation. Almost all radioactivity from injected (14)C-20:4(n-6) was recovered in this PUFA. Of the total radioactivity from (14)C-20:5(n-3) incorporated into liver lipid, 7% was present as 24:5 and 16.4% was recovered in hexaenoic fatty acids. In liver, 24:5(n-3) and 24:6(n-3) each accounted for 1% of the mass of total fatty acids and were located almost exclusively in triacylglycerols. The presence of radioactivity in these C24 PUFA suggests that in pike the synthesis of 22:6(n-3) from 20:5(n-3) may proceed without Δ4 desaturase via the pathway which involves chain shortening of 24:6(n-3). It is concluded that under the circumstances employed in this study pike, do not exhibit Δ5 desaturase activity and are unable to synthesize 20:4(n-6) and 20:5(n-3) from 18:2(n-6) and 18:3(n-3), respectively. This suggests that pike may require 20:4(n-6) and 20:5(n-3) preformed in the diet.