The desaturation and elongation of 14C-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, ¹⁴C-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 ¹⁴C-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 ¹⁴C-PUFA. PI contained higher levels of radioactivity from ¹⁴C-20:4(n-6) than from other injected substrates. In liver lipid, the 6 desaturation products of ¹⁴C-18:2(n-6) and ¹⁴C-18:3(n-3) contained no measurable radioactivity although the elongation products of the 6 desaturation products were labelled, as were the direct elongation products of these injected substrates. No radioactivity from ¹⁴C-18:2(n-6) or ¹⁴C-18:3(n-3) was detected in C₂₀ or C₂₂ products of 5 and 4 desaturation. Almost all radioactivity from injected ¹⁴C-20:4(n-6) was recovered in this PUFA. Of the total radioactivity from ¹⁴C-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 4 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 5 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.Abbreviations CGP choline glycerophospholipids - CL cardiolipin - EGP ethanolamine glycerophospholipids - PG phosphatidylglycerol - PI phosphatidylinositol - PS phosphatidylserine - PUFA polyunsaturated fatty acids - SM sphingomyelin - TLC thin-layer chromatography