Polyunsaturated fatty acid metabolism in a marine teleost, Nibe croaker Nibea mitsukurii: Functional characterization of Fads2 desaturase and Elovl5 and Elovl4 elongases

To reduce the requirement for fish oil in marine aquaculture, it would be advantageous to endow marine fish species with the capability for the endogenous biosynthesis of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). For this purpose, we have previously produced transgenic Nibe croaker (Nibea mitsukurii) carrying an elongase of very-long-chain fatty acids 2 (elovl2) gene isolated from Masu salmon (Oncorhynchus masou). However, fatty acid analysis revealed that 24:5n-3 accumulated in the liver of the transgenic fish, whereas the DHA level did not differ between non-transgenic and transgenic fish. Therefore, to select more effective enzymes for successful transgenic synthesis of DHA, understanding the endogenous DHA biosynthetic pathway in the Nibe croaker is considered to be important. The present study aimed to investigate the biochemical functions of the Elovl5, Elovl4 and Fads2 enzymes involved in the DHA biosynthetic pathway in the Nibe croaker. The results showed that both Elovl5 and Elovl4 were able to elongate C18 fatty acids to C22 fatty acids and that Fads2 had Δ6 desaturase activity toward C18 fatty acids and weak Δ8 desaturase activity toward C20 fatty acids. On the other hand, Fads2 was found to lack the ability to convert 24:5n-3 to 24:6n-3, a fatty acid that can directly be converted to DHA via β-oxidation.