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)

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).