Investigating the long-chain polyunsaturated fatty acid biosynthesis of the African catfish Clarias gariepinus (Burchell, 1822)

Abstract

Investigating the biosynthesis of long-chain (C20–24) polyunsaturated fatty acids (LC-PUFA), physiologically important compounds including arachidonic acid (ARA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in fish is crucial to identify dietary requirements for essential fatty acids (EFA). Moreover, knowledge of the C20–24 LC-PUFA biosynthetic capability of farmed fish species enables us to understand their ability to utilise commonly used raw materials such as vegetable oils, which naturally lack LC-PUFA but include C18 PUFA that are metabolic precursors of LC-PUFA. Studies have shown that the potential of a species for LC-PUFA biosynthesis is associated with the complement and function of fatty acyl desaturase (fads) and elongase of very long chain fatty acid (elovl) genes existing in that species. The present study therefore aimed to investigate these genes in the African catfish (Clarias gariepinus), the most commercially important farmed fish in sub-Saharan Africa. A fads2, a fads6 and four elovl (elovl2, elovl4a, elovl4b, elovl8) cDNAs were cloned and functionally characterised by heterologous expression in yeast. The Fads2 was a bifunctional desaturase enzyme with ∆6∆5 and ∆8 activities, and thus catalysing all the desaturation reactions required for ARA and EPA biosynthesis from C18 precursor fatty acids. Moreover, the C. gariepinus Fads2 enzymes also desaturated 24:5n-3 to 24:6n-3, a ∆6 desaturation required for the biosynthesis of DHA through the so-called “Sprecher pathway”. Functional characterisation of Fads6 by heterologous expression in yeast did not reveal its function. With regards to elongases, the C. gariepinus Elovl2 demonstrated the ability to elongate C20 and C22 PUFA and thus complements the Elovl5 with elongase capability towards C18 and C20 PUFA. The Elovl8 was capable of only limited elongation of C18 and C20 PUFA. Elovl4a and Elovl4b, enable the biosynthesis of very long-chain (>C24) fatty acids, compounds with major roles in vision and fertility of vertebrates. The present study confirmed that C. gariepinus possess all the enzymatic capabilities required for the biosynthesis of ARA, EPA and DHA and, therefore, its physiological EFA requirements could be satisfied with dietary provision of C18 PUFA.