Regulation of Dietary Linoleic Acid and Arachidonic Acid on LC-PUFA Biosynthesis and Non-Specific Immunity in Sea Cucumber, Apostichopus japonicus

In order to verify the synthesis pathway of linoleic acid (LA) to generate arachidonic acid (ARA), the functions of Δ6 FAD and Elovl 5 in Apostichopus japonicus were tested by heterologous expression in Pichia pastoris. A 60-day feeding experiment was conducted to evaluate the effects of dietary LA and ARA on growth, polyunsaturated fatty acids (PUFA) biosynthesis and immune function of A. japonicus. Seven diets containing graded levels of LA or ARA were formulated, and one diet without PUFA was applied as a control. The results confirmed that Δ6 FAD from A. japonicus has a double desaturation ability of Δ6 and Δ5 for PUFA, and it works together with Elovl 5 enzyme play important roles in biosynthesis of ARA from LA. With increasing dietary LA from 4.9 to 12.1 g/kg, the expressions of Δ6 FAD and Elovl 5, levels of LA, ARA and EPA in tissues increased, and the activities of ACP, AKP and CAT enzymes in intestine initially increased and then decreased. Additionally, by increasing dietary ARA from 0.7 to 3.8 g/kg, the contents of ALA, EPA and DHA decreased, while the Δ6 FAD expression and the activities of ACP, AKP and CAT in intestine increased significantly. Therefore, appropriate dietary levels of LA (12.1 g/kg) or ARA (3.8 g/kg) build a reasonable composition of n-3/n-6 PUFA in A. japonicus, and improved its growth, antioxidant capacity and immunological defenses.