Molecular Cloning and Functional Characterization of Fatty Acyl Desaturase and Elongase cDNAs Involved in the Production of Eicosapentaenoic and Docosahexaenoic Acids from ?-Linolenic Acid in Atlantic Salmon (Salmo salar)

Abstract

Fish are the only major dietary source for humans of omega-3 highly unsaturated fatty acids (HUFA) and, with declining fisheries, farmed fish such as Atlantic salmon (Salmo salar) constitute an increasing proportion of the fish in the human diet. However, the current high use of fish oils, derived from wild capture marine fisheries, in aquaculture feeds is not sustainable in the longer term, and will constrain continuing growth of aquaculture activities. A greater understanding of how fish metabolise and biosynthesise HUFA may lead to effective use of more sustainable aquaculture diets. The study described here contributes to an effort to determine the molecular genetics of the HUFA biosynthetic pathway in salmon, with the overall aim being to determine mechanisms for optimising the use of vegetable oils in Atlantic salmon culture. In this paper we Upon expression in yeast, SalElo showed broad substrate specificity for polyunsaturated fatty acids with a range of chain lengths, with the rank order being C 18 > C 20 > C 22 . Thus, all fatty acid elongase activities required for the biosynthesis of docosahexaenoic acid (22:6n-3) from 18:3n-3 are displayed by this one polypeptide product.

Characterization and comparison of fatty acyl Δ6 desaturase cDNAs from freshwater and marine teleost fish species

“…This was as expected based upon the functional characterisation of the previously reported fish desaturase genes, the zebrafish Δ6/Δ5 and the salmon Δ5, and all mammalian Δ6 and Δ5 desaturases reported to date (Hastings et al, 2001(Hastings et al, ,2004Pereira et al, 2003). That no Δ4 desaturase has been cloned or characterised in higher animals is, therefore, consistent with the hypothesis that the synthesis of DHA from EPA in both mammals and fish proceeds via elongation to 24:5n-3 followed by a Δ6 desaturation rather than via Δ4 desaturation of 22:5n-3 (Buzzi et al, 1996(Buzzi et al, ,1997Wallis et al, 2002).…”

“…This was consistent with all of the fish cDNAs having close homology with mammalian Δ6 desaturases. However, they also have close homology with the zebrafish and Atlantic salmon desaturases previously characterised (Hastings et al, 2001(Hastings et al, ,2004.…”

“…These were the novel, bifunctional zebrafish cDNA showing both Δ6 and Δ5 desaturase activity (Hastings et al, 2001) and an Atlantic salmon desaturase that was shown to be predominantly an n-3 Δ5 desaturase (Hastings et al, 2004). That neither of these cDNAs were unifunctional Δ6 desaturases was surprising as they both had closer sequence homology to mammalian Δ6 desaturases than to Δ5…”

“…desaturases (Hastings et al, 2001(Hastings et al, , 2004. Therefore, there was a gap in our knowledge in that single function Δ6 desaturases had not been identified in fish, which has particular importance as Δ6 desaturase is regarded as the rate-limiting step in the HUFA synthesis pathway, at least in humans and probably mammals in general (Horrobin, 1993).…”

“…Heterologous expression in yeast indicated that the zebrafish desaturase gene was unique in that it encoded an enzyme having both Δ6 and Δ5 desaturase activities, prompting us to speculate that it may represent a component of a prototypic vertebrate HUFA biosynthesis pathway. However, we recently cloned a desaturase cDNA from Atlantic salmon (Salmo salar) which, as shown by heterologous expression in yeast, was predominantly an omega-3 Δ5 desaturase, with only very low levels of Δ6 activity (Hastings et al, 2004). Thus this salmon desaturase was primarily a single function enzyme, similar to the mammalian desaturases, questioning the interpretation of the bifunctional zebrafish desaturase as a prototypic enzyme.…”