The Malaysian Mahseer, Tor tambroides possess all required biosynthesis enzymes for the conversion of C18 polyunsaturated fatty acids to long-chain polyunsaturated fatty acids

Abstract

The Malaysian mahseer (Tor tambroides) is an iconic and highly-priced freshwater large cyprinid with a natural dietary preference towards riverine fruits. Previous works indicate that dietary C18 polyunsaturated fatty acids can fulfill mahseer's essential fatty acid requirements. Given the need to reduce the use of fish meal and fish oil in aquafeed, there is considerable interest to sustain healthy levels of the beneficial n-3 long-chain polyunsaturated fatty acids (LC-PUFA) in farmed fish. A prerequisite for a functional LC-PUFA biosynthesis pathway is an entire repertoire of desaturase (Fads) and elongase (Elovl) enzymes required for specific steps along the path. We showed here the presence of a Fads2 and two elongases, Elovl5 and Elovl2, in mahseer. These enzymes function collectively to convert C18 PUFA to the C20 LC-PUFA through Δ6 and Δ5 desaturation processes working in tandem with the Elovl5 elongation activities. Subsequently, further conversion to docosahexaenoic acid (DHA; 22:6n-3) involves Δ6 desaturation and Elovl2 and Elovl5. These enzymes displayed a typical expression pattern of freshwater teleost at the mRNA level, with significant transcripts in liver, intestine, and brain tissues. Under limited dietary LC-PUFA intakes, the expression of these genes in the liver was elevated. Tissue fatty acid composition also suggests the conversion of dietary α-linolenic acid (ALA; 18:3n-3) and linoleic acid (LA; 18:2n-6) to their respective LC-PUFA for deposition in brain and muscle tissues. Although the highest concentration of n-3 LC-PUFA is obtained using fish oil-based diet, our results showed considerable accumulation of EPA and DHA with dietary ALA provision. Taken together, we show the existence of complete machinery for the biosynthesis of LC-PUFA from C18 PUFA in mahseer and validates the mahseer's capacity to utilize dietary vegetable oils (VO) both growth and n-3 LC-PUFA deposition.