Rainbow trout (Oncorhynchus mykiss) Elov15 and Elov12 differ in selectivity for elongation of omega-3 docosapentaenoic acid.

The synthesis of the omega-3 long-chain polyunsaturated fatty acids (LCPUFA) eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n--3) from dietary α-linolenic acid (ALA; 18:3n--3) requires three desaturation and three elongation steps in vertebrates. The elongation of EPA to docosapentaenoic acid (DPA; 22:5n-3) can be catalysed by the elongase enzymes Elov15 or Elov12, but further elongation of DPA to 24:5n-3, the penultimate precursor of DHA, is limited to Elov12, at least in mammals. Elov15 enzymes have been characterised from seventeen fish species but Elov12 enzymes have only been characterised in two of these fish. The essentiality of Elov12 for DHA synthesis is unknown in fish. This study is the first to identify an Elov12 in rainbow trout (Oncorhynchus mykiss) and functionally chafacterise the Elovl5 and Elovl2 using a yeast expression system. Elovl5 was active with C18-20 PUFA substrates and not C22 PUFA. In contrast, Elovl2 was active with C20-22 PUFA substrates and not C18 PUFA. Thus, rainbow trout is dependent on Elovl2 for DPA to 24:5n--3 synthesis and ultimately DHA synthesis. The expression of elov15 was significantly higher than elov12 in liver. Elucidating this dependence on Elov12 to elongate DPA and the low elov12 gene expression compared with elovl5 are critical findings in understanding the potential for rainbow trout to synthesize DHA.