Multiple genes for functional Δ6 fatty acyl desaturases (Fad) in Atlantic salmon (Salmo salar L.): Gene and cDNA characterization, functional expression, tissue distribution and nutritional regulation

Fish are the primary source in the human food basket of the n-3 long-chain polyunsaturated fatty acids, eicosapentaenoate (EPA; 20:5n-3) and docosahexaenoate (DHA: 22:6n-3), that are crucial to the health of higher vertebrates. Atlantic salmon are able to synthesize EPA and DHA from 18:3n-3 through reactions catalyzed by fatty acyl desaturases (Fad) and elongases of very long chain fatty acids. Previously, two cDNAs encoding functionally distinct Delta 5 and Delta 6 Fads were isolated, but screening of a genomic DNA library revealed the existence of more putative fad genes in the Atlantic salmon genome. In the present study, we show that there are at least four genes encoding putative Fad proteins in Atlantic salmon. Two genes, Delta 6fad_a and Delta 5fad, corresponded to the previously cloned Delta 6 and Delta 5 Fad cDNAs. Functional characterization by heterologous expression in yeast showed that the cDNAs for both the two further putative fad genes, Delta 6fad_b and Delta 6fad_c, had only Delta 6 activity, converting 47% and 12% of 18:3n-3 to 18:4n-3, and 25 and 7% of 18:2n-6 to 18:3n-6, for Delta 6Fad_b and Delta 6fad_c, respectively. Both Delta 6fad_a and Delta 6fad_b genes were highly expressed in intestine (pyloric caeca), liver and brain, with Delta 6fad_b also highly expressed in gill, whereas Delta 6fad_c transcript was found predominantly in brain, with lower expression levels in all other tissues. The expression levels of the Delta 6fad_a gene in liver and the Delta 6fad_b gene in intestine were significantly higher in fish fed diets containing vegetable oil compared to fish fed fish oil suggesting up-regulation in response to reduced dietary EPA and DHA. In contrast, no significant differences were found between transcript levels for Delta 6fad_a in intestine, Delta 6fad_b in liver, or Delta 6fad_c in liver or intestine of fish fed vegetable oil compared to fish fed fish oil. The observed differences in tissue expression and nutritional regulation of the fad genes are discussed in relation to gene structures and fish physiology. (C) 2010 Elsevier B.V. All rights reserved.