N-3 ESSENTIAL FATTY ACIDS IN NILE TILAIA, Oreochromis niloticus: DIETARY LINOLENIC ACID HAS GREAT EFFECTS ON LINOLEIC ACID BIOCONVERSION

Qing Pan*, Chen Cuiying, Guan Wutai, Wang Yadi, Bi Yingzuo
Department of Aquaculture, College of Animal Science, South China Agricultural University, Guangzhou 510642, China (qpan@scau.edu.cn )

This study was conducted to gain a better understanding of the biosynthetic pathways of long chain polyunsaturated fatty acids (LC-PUFA) in Nile tilapia, Oreochromis niloticus. Fish were fed with four purified isoproteic and isolipidic diets which were formulated with fixed levels of linoleic acid (LA, 18:2n-6) and gradual levels (0.10, 0.63, 1.56 and 2.04% of dry weight, respectively) of α-linolenic acid (LNA, 18:3n-3) for 10 weeks. The results showed that no significant differences were observed in apparent digestibility coefficents of fatty acids in all groups (P>0.05). With the dietary LNA levels increased, significant differences were found between the 0.1% LNA group and other groups in β-oxidation, elongation and desaturation of n-3 PUFA and n-6 PUFA (P<0.05). When the dietary LNA level was higher than 0.10%, 78.7-81.9% of the net intake of dietary LNA was oxidised, 7.7-11.2% was deposited and only 6.8-13.7% was bioconverted to LC-PUFA. Fish fed the diets with increased dietary LNA showed both significant increased desaturase (△-6 and △-5) and elongase activities on n-3 PUFA bioconverion, while significant decreases were recorded on n-6 PUFA bioconverion. Irrespective of the substrate availabilities of LNA and LA, there were greater activities of desaturases and elongase on n-3 PUFA than those on n-6 PUFA. The expression of △-6 desaturase (FD6D) and PUFA elongase (ELOVL5) genes were highest in fish fed the diet with 2.04% of LNA, but no significant differences were observed among all groups. The results indicated that the bioconversion of LA towards n-6 LC-PUFA was competitively inhibited by LNA, while in turn LA has only trivial effects on the bioconversion of LNA towards n-3 LC-PUFA. Moreover, the apparent in vivo enzyme activity was directly and positively affected by substrate availability but not the enzyme transcription rate.