Rainbow trout (Oncorhynchus mykiss) Elovl5 and Elovl2 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 Elovl5 or Elovl2, but further elongation of DPA to 24:5n - 3, the penultimate precursor of DHA, is limited to Elovl2, at least in mammals. Elovl5 enzymes have been characterised from seventeen fish species but Elovl2 enzymes have only been characterised in two of these fish. The essentiality of Elovl2 for DHA synthesis is unknown in fish. This study is the first to identify an Elovl2 in rainbow trout (Oncorhynchus mykiss) and functionally characterise the Elovl5 and Elovl2 using a yeast expression system. Elovl5 was active with C_18-20 PUFA substrates and not C₂₂ PUFA. In contrast, Elovl2 was active with C_20-22 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 elovl5 was significantly higher than elovl2 in liver. Elucidating this dependence on Elovl2 to elongate DPA and the low elovl2 gene expression compared with elovl5 are critical findings in understanding the potential for rainbow trout to synthesize DHA.