Two alternative pathways for docosahexaenoic acid (DHA, 22:6n-3) biosynthesis are widespread among teleost fish

Docosahexaenoic acid (DHA) plays important physiological roles in vertebrates. Studies in rats and rainbow trout confirmed that DHA biosynthesis proceeds through the so-called Sprecher pathway, a biosynthetic process requiring a Delta 6 desaturation of 24::5n-3 to 24:6n-3. Alternatively, some teleosts possess fatty acyl desaturases 2 (Fads2) that enable them to biosynthesis DHA through a more direct route termed the Delta 4 pathway. In order to elucidate the prevalence of both pathways among teleosts, we investigated the Delta 6 ability towards C-24 substrates of Fads2 from fish with different evolutionary and ecological backgrounds. Subsequently, we retrieved public databases to identify Fads2 containing the YXXN domain responsible for the Delta 4 desaturase function, and consequently enabling these species to operate the Delta 4 pathway. We demonstrated that, with the exception of Delta 4 desaturases, fish Fads2 have the ability to operate as Delta 6 desaturases towards C-24 PUFA enabling them to synthesise DHA through the Sprecher pathway. Nevertheless, the Delta 4 pathway represents an alternative route in some teleosts and we identified the presence of putative Delta 4 Fads2 in a further 11 species and confirmed the function as Delta 4 desaturases of Fads2 from medaka and Nile tilapia. Our results demonstrated that two alternative pathways for DHA biosynthesis exist in teleosts.