Capacity for eicosapentaenoic acid and arachidonic acid biosynthesis in silver barb (Barbonymus gonionotus): Functional characterisation of a Δ6/Δ8/Δ5 Fads2 desaturase and Elovl5 elongase

The silver barb (Barbonymus gonionotus), a widely distributed cyprinid in Southeast Asia, has a reasonable deposition of the physiologically important omega-3 (n-3) long-chain polyunsaturated fatty acids (LC-PUFA), implying the existence of a functional LC-PUFA biosynthesis pathway. In relation to this, the cloning and functional characterisation of a fatty acyl desaturase (Fads) and fatty acyl elongase (Elovl) were examined to elucidate the capacity of silver barb in converting α-linolenic acid (ALA; 18:3n-3) into eicosapentaenoic acid (EPA; 20:5n-3) and subsequently docosahexaenoic acid (DHA; 22:6n-3), and linoleic acid (LA; 18:2n-6) into arachidonic acid (ARA; 20:4n-6). Sequence analysis showed that the cloned cDNAs correspond to fads2 and elovl5. In terms of function, the Fads2 showed Δ6, Δ5 and Δ8 desaturation activities, while the Elovl5 showed activities towards polyunsaturated fatty acid (PUFA) substrates at preference of C18 > C20 > C22. Both enzymes showed higher activities towards n-3 PUFAs as compared to n-6 PUFAs. Taken together, these results showed the presence of all the required enzymes for biosynthesis of EPA and ARA from ALA and LA, respectively in silver barb fish. Both genes are highly expressed in brain and liver. Juvenile fish fed with ALA-rich diet showed higher expression of hepatic fads2 and elovl5 than to those fed with LC-PUFA-rich diet. Dietary ALA also contributed to the biosynthesis and deposition of EPA and DHA in fish tissues, albeit lower than levels achieved with the fish oil (FO)-based diet. Overall, this study shows the presence of functional Fads2 and Elovl5 in a farmed cyprinid species, which contributes to LC-PUFA biosynthesis during limited dietary LC-PUFA intake.