THE IN-VIVO INCORPORATION AND METABOLISM OF [1-C-14] LINOLENATE (18 3N-3) IN LIVER, BRAIN AND EYES OF JUVENILES OF RAINBOW-TROUT ONCORHYNCHUS-MYKISS L AND GILTHEAD SEA BREAM SPARUS-AURATA L/

Accumulation of docosahexaenoic acid (DHA; 22:6n-3) in brain and eyes during development has been demonstrated in fish but it is not clear w hether liver or neural tissues themselves are of greater importance in the biosynthesis of DHA from dietary 18:3n-3. In the present study, w e investigated the in vivo metabolism of intraperitoneally injected [1 -C-14]18:3n-3 in liver, brains and eyes of young juvenile fish. Metabo lism was followed over a 48h time-course in order to obtain dynamic in formation that could aid the elucidation of the roles of the different tissues in the biosynthesis and provision of DHA from dietary 18:3n-3 . The study was performed in both a freshwater fish, rainbow trout Onc orhynchus mykiss L and a marine fish, gilthead sea bream Sparus aurata L to determine the effect that low or limiting Delta 5-desaturase act ivity may have in this process. As expected, the results showed that a lthough the sea bream incorporated more 18:3n-3 into its lipids, metab olism of the incorporated fatty acid by desaturation and elongation wa s generally greater in the trout. In liver, the percentages of radioac tivity recovered in tetraene and pentaene products were greater in tro ut than in sea bream although there was no difference in hexaenes. In contrast, the recovery of radioactivity in DHA was significantly great er in brain in trout compared to sea bream. In both species, the perce ntage of radioactivity recovered in desaturated/elongated products was much lower in liver than in brains and eyes, but that percentage incr eased over the 48h time-course. In trout though, the highest percentag es of desaturated products in brain and eye were observed after 12 and 24h, respectively. However in sea bream the highest percentages of de saturated products in the neural tissues were observed after 24-48h. R adioactivity was recovered in 24:5n-3 and 24:6n-3, intermediates in th e Delta 4-independent (''Sprecher shunt'') pathway for the synthesis o f DHA, in both species, especially in the brain and eyes. Overall, alt hough the results cannot eliminate a role for liver in the biosynthesi s and provision of DHA for developing neural tissues in fish, they sug gest that DHA can be synthesised in fish brain and eye in vivo.