Physiological roles of fatty acyl desaturases and elongases in marine fish: Characterisation of cDNAs of fatty acyl Δ6 desaturase and elovl5 elongase of cobia (Rachycentron canadum)

Abstract

In the present paper, we investigated the expression of fatty acyl desaturase and elongase genes in a marine teleost, cobia, a species of great interest due to its considerable aquaculture potential. A cDNA was cloned that, when expressed in yeast, was shown to result in desaturation of 18:3n-3 and 18:2n-6, indicating that it coded for a Δ6 desaturase enzyme. Very low desaturation of 20:4n-3 and 20:3n-6 indicated only trace Δ5 activity. Another cloned cDNA enabled elongation of 18:4n-3, 18:3n-6, 20:5n-3 and 20:4n-6 in the yeast expression system, indicating that it had C18-20 and C20-22 elongase activity. Sequence comparison and phylogenetic analysis confirmed that it was homologous to human ELOVL5 elongase. However, the cobia Elovl5 elongase also had low activity toward C24 HUFA. The cobia Δ6 desaturase had a preference for 18:3n-3, but the elongase was generally equally active with both n-3 and n-6 substrates. Expression of both genes was 1-2 orders of magnitude greater in brain than other tissues suggesting an important role, possibly to ensure sufficient docosahexaenoic acid (DHA, 22:6n-3) synthesis in neural tissues through elongation and desaturation of eicosapentaenoic acid (EPA; 20:5n-3).3

Omega-3 long-chain polyunsaturated fatty acids and aquaculture in perspective

“…Therefore, the differences between freshwater/salmonid and marine species in the EPA pathway are probably a consequence of the differing levels of LC-PUFA in the respective environments (higher in marine) leading to different evolutionary pressures, and are reflected in their qualitative EFA requirements (Leaver et al, 2008b;Carmona-Antoñanzas et al, 2013). DHA production from EPA is more conserved although there are few data, but this pathway is likely important particularly for brain development and function and, again, an evolutionary adaptation (Zheng et al, 2009). In species with complete LC-PUFA pathways such as salmonids, endogenous biosynthesis is sufficient to satisfy the relatively low levels of their normal physiological requirements, and there has been no biochemical/metabolic driver or evolutionary pressure to synthesise excess EPA and DHA just to be deposited in TAG and stored (Leaver et al, 2008b;Castro et al, 2012).…”

“…However, the DHA pathway from EPA is probably functional in most teleost fish, including marine species, at least in some tissues such as brain Zheng et al, 2009;Monroig et al, 2011a). Salmonids and many freshwater species have complete pathways and can produce both EPA and DHA from LNA (Tocher, 2010;Monroig et al, 2011a).…”

Investigating long-chain polyunsaturated fatty acid biosynthesis in teleost fish: Functional characterization of fatty acyl desaturase (Fads2) and Elovl5 elongase in the catadromous species, Japanese eel Anguilla japonica

“…proteins from fish and terrestrial vertebrates grouped together, separately from Elovl2 255 and Elovl4, other PUFA elongases with roles in the biosynthesis of LC-PUFA in 256 vertebrates including fish (Monroig et al, 2009(Monroig et al, , 2010b. 257…”

Long-chain polyunsaturated fatty acid synthesis in fish: Comparative analysis of Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.) Δ6 fatty acyl desaturase gene promoters

“…Several FADs have been characterised in various fish species, and all are Δ6 FADs (Zheng et al, 2004(Zheng et al, , 2005a(Zheng et al, , 2009Tocher et al, 2006), except for a zebrafish bifunctional Δ6/Δ5 FAD (Hastings et al, 2001) and an Atlantic salmon (Salmo salar) Δ5 FAD (Hastings et al, 2005). To date no Δ5 FAD has been isolated from a marine fish species and, supported by biochemical studies in fish cell lines (Ghioni et al, 1999;Tocher and Ghioni, 1999), this has led to the hypothesis that some groups of fish may be unable to biosynthesise LC-PUFA because they lack specific genes in the pathway (Leaver et al, 2008a).…”