n-3 essential fatty acids in Nile tilapia, Oreochromis niloticus: Bioconverting LNA to DHA is relatively efficient and the LC-PUFA biosynthetic pathway is substrate limited in juvenile fish

This study aimed to gain a better understanding of the long-chain (>= C-20) polyunsaturated fatty acid (LC-PUFA) biosynthetic metabolism, with a specific focus on the metabolic fate of linolenic acid (18:3n-3, LNA) and linoleic acid (18:2n-6, LA) and the relationship between these two substrates in the process of LC-PUFA biosynthesis in juvenile Nile tilapia, Oreochromis niloticus. Fish were fed four isoproteic (37.5%) and isolipidic (6.2%) diets formulated with fixed levels of LA (0.60%) and gradual levels of LNA (0.10, 0.63, 1.56 and 2.04% of dry weight, respectively) for 10 weeks. The whole-body fatty acid balance method was used to evaluate fish in vivo fatty acid metabolism. The results showed that most of dietary LNA was bioconverted to LC-PUFA when tilapia receiving not enough LNA for optimal growth. In contrast, when fish receiving sufficient dietary LNA, most of the LNA was beta-oxidized, with only < 10% LNA being bioconverted. The in vivo apparent Delta-5/-6 desaturase and elongase activities on n-3 and n-6 PUFA, respectively, exhibited a significant linear increase and decrease with the increasing dietary LNA levels, clearly indicating that for all of the enzyme activities, n-3 PUFA is a preferred substrate and the LC-PUFA biosynthetic pathway is substrate limited in Nile tilapia. Dietary higher inclusion of LNA could block or at least slow down n-6 LC-PUFA biosynthesis from LA, indicating that a direct substrate competition for accessing the Delta-6 desaturase and elongase exists between these two substrates. Furthermore, Nile tilapia were relatively efficient in bioconversion from LNA to DHA, and the DHA biosynthesis from EPA may be more direct or faster than the production of EPA from LNA. Despite of the active bioconversion of LNA up to DHA, this metabolic effort was still insufficient to compensate for the significant reduced dietary intake of n-3 LC-PUFA. No significant difference was observed in the gene expression of fatty acyl desaturase (fads2) and fatty acid elongase (elovl5) among all groups, which suggests that the total in vivo apparent enzyme activities are directly and positively affected by substrate availability rather than the extent of the enzyme gene expression.