Molecular and functional characterisation of a putative elovl4 gene and its expression in response to dietary fatty acid profile in Atlantic bluefin tuna (Thunnus thynnus)

Elongation of very long-chain fatty acid 4 (Elovl4) proteins are involved in the biosynthesis of very long-chain (> C24) fatty acids and in many teleost fish species they are key enzymes in the pathway for the production of docosahexaenoic acid (DHA; 22:6n-3) from eicosapentaenoic acid (EPA; 20:5n-3). Therefore, Elovl4 may be particularly important in Atlantic bluefin tuna (ABT; Thunnus thynnus) characterised by having high DHA to EPA ratios. The present study cloned and characterised both the function and expression of an elovl4 cDNA from ABT. The Elovl4 had an open reading frame of 915 base pairs encoding a putative protein of 304 amino acids. Alignment and phylogenetic analyses indicated that the Elovl4 isoform identified in the present study was an Elovl4b. Functional characterisation demonstrated that the Elovl4b enzyme had elongase activity towards all the polyunsaturated fatty acid (PUFA) substrates assayed. The ABT Elovl4b contributed to DHA biosynthesis by elongation of EPA and DPA to 24:5n-3, the latter being desaturated to 24:6n-3 by the action of fads2 (Δ6 desaturase). Additionally, the ABT Elovl4b has a role in the biosynthesis of very long-chain PUFA up to C34, compounds of key structural roles in neural tissues such as eye and brain, which had high levels of elovl4b transcripts. Surprisingly, while the relative expression of fads2, required for the production of DHA from EPA, was increased in liver of ABT fed a diet with reduced levels of EPA and DHA, expression of elovl4b was reduced. Results indicated that ABT has enzymes necessary for endogenous production of DHA from EPA and demonstrate that Elovl4b can effectively compensate for absence of Elovl2