Retroconversion of docosapentaenoic acid (n-6): an alternative pathway for biosynthesis of arachidonic acid in Daphnia magna.
The aim of this study was to assess metabolic pathways for arachidonic acid (20:4n-6) biosynthesis in Daphnia magna. Neonates of D. magna were maintained on [(13)C] enriched Scenedesmus obliquus and supplemented with liposomes that contained separate treatments of unlabeled docosapentaenoic acid (22:5n-6), 20:4n-6, linoleic acid (18:2n-6) or oleic acid (18:1n-9). Daphnia in the control treatment, without any supplementary fatty acids (FA) containing only trace amounts of 20:4n-6 (~0.3% of all FA). As expected, the highest proportion of 20:4n-6 (~6.3%) was detected in Daphnia that received liposomes supplemented with this FA. Higher availability of 18:2n-6 in the diet increased the proportion of 18:2n-6 in Daphnia, but the proportion of 20:4n-6 was not affected. Daphnia supplemented with 22:5n-6 contained ~3.5% 20:4n-6 in the lipids and FA specific stable isotope analyses validated that the increase in the proportion of 20:4n-6 was due to retroconversion of unlabeled 22:5n-6. These results suggest that chain shortening of 22:5n-6 is a more efficient pathway to synthesize 20:4n-6 in D. magna than elongation and desaturation of 18:2n-6. These results may at least partially explain the discrepancies noticed between phytoplankton FA composition and the expected FA composition in freshwater cladocerans. Finally, retroconversion of dietary 22:5n-6 to 20:4n-6 indicates Daphnia efficiently retain long chain n-6 FA in lake food webs, which might be important for the nutritional ecology of fish.
Department of Biology, University of Eastern Finland, PO Box 111, 80101, Joensuu, Finland, ursula.strandberg@uef.fi.
Ursula
U
Strandberg
Sami J
SJ
Taipale
Martin J
MJ
Kainz
Michael T
MT
Brett
D000818:Animals; D016718:Arachidonic Acid / Q000096:biosynthesis*; D053898:Biosynthetic Pathways; D003621:Daphnia / Q000378:metabolism*; D005231:Fatty Acids, Unsaturated / Q000378:metabolism*; D006865:Hydrogenation; D050356:Lipid Metabolism; D008081:Liposomes
D005231:Fatty Acids, Unsaturated; D008081:Liposomes; D016718:Arachidonic Acid; C026219:docosapentaenoic acid