Elongation of long-chain fatty acids in rabbitfish Siganus canaliculatus: Cloning, functional characterisation and tissue distribution of Elovl5- and Elovl4-like elongases

Abstract

Elongases of very long-chain fatty acids (Elovl) catalyse the rate-limiting step of the elongation pathway that results in net 2 C elongation of pre-existing fatty acyl chains. As the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA) is particularly relevant in fish, Elovl involved in the pathway have been investigated in various studies. Here we report the molecular cloning, functional characterisation and tissue distribution of two distinct elovl-like cDNAs isolated from the herbivorous marine teleost Siganus canaliculatus. Unlike the carnivorous marine fish previously investigated, we hypothesise that the rabbitfish has an enhanced LC-PUFA biosynthetic capability as previously anticipated in a former study on fatty acyl desaturases (Fad). The results of the present study showed that rabbitfish expresses at least two elovl cDNAs, which have high homology in sequence and function to Elovl5 and Elovl4 elongases that have been investigated previously in other fish species. Furthermore, the results confirm that the activities of the Elovl5 and Elovl4 enzymes enable rabbitfish to perform all the elongation reactions required for the biosynthesis of the physiologically essential C20-22 LC-PUFA including eicosapentaenoic (20:5n-3), arachidonic (20:4n-6) and docosahexaenoic (22:6n-3, DHA) acids, as well as the less common very long-chain fatty acids (greater than C24). Rabbitfish is thus the first marine teleost in which genes encoding Fad and Elovl enzymes, with all the activities required for the production of DHA from C18 PUFA, have been characterised. Highlights Rabbitfish possess at least two fatty acid elongases. The elongases were functionally characterised as Elovl5 and Elovl4 types.  The elongases possess all the activities necessary for the biosynthesis of DHA.  Elovl4 is capable of synthesising very long-chain fatty acids up to C36 in length.  Rabbitfish represent a marine fish not dependent upon dietary EPA and DHA