Nutritional Regulation of the Rainbow Trout Long Chain‐Polyunsaturated Fatty Acid Biosynthetic Enzymes

Abstract

Rainbow trout (Oncorhynchus mykiss) is a commercially important aquaculture species. The use of the vegetable oil‐derived omega‐3 polyunsaturated fatty acid (PUFA) α‐linolenic acid (18:3n‐3; ALA) instead of fish oil as a sustainable lipid source in feeds is being investigated. The metabolism of ALA to eicosapentaenoic acid (20:5n‐3; EPA) and docosahexaenoic acid (22:6n‐3; DHA) found in fish oil requires three desaturation and three elongation steps performed by the enzymes Δ6desaturase, Δ5desaturase, Elovl5 and Elovl2. However, the biosynthetic capabilities of rainbow trout are largely unknown. The objectives of this study were to (a) functionally characterise the elongases, Elovl5 and Elovl2, using a yeast expression system and (b) investigate the nutritional regulation of the desaturase and elongase enzymes in juvenile rainbow trout fed diets rich in ALA, EPA or DHA and any combination thereof. The Elovl5 activity was specific to C18–20 PUFA substrates and not C22 PUFA. In contrast, the Elovl2 activity was specific to C20‐22 PUFA only, with no activity for C18 PUFA. Furthermore, we determined the change in Δ6desaturase, Δ5desaturase, elovl5 and elovl2 gene expression in response to dietary omega‐3 PUFA. Our study was the first to identify an Elovl2 in rainbow trout and it is only the third Elovl2 to be functionally characterised from fish. This study has elucidated that rainbow trout is dependent on Elovl2 for docosapentaenoic acid (22:5n‐3; DPA) to 24:5n‐3 synthesis and ultimately DHA synthesis.