IN-VIVO METABOLISM OF [1-C-14]LINOLENIC ACID (18 3(N-3)) AND [1-C-14]EICOSAPENTAENOIC ACID (20/5(N-3)) IN A MARINE FISH - TIME-COURSE OF THE DESATURATION/ELONGATION PATHWAY/

The metabolism (via the desaturation/elongation pathways) of [1-C-14]1 8:3(n - 3) and [1-C-14]20:5(n - 3) in a marine fish, gilthead sea brea m (Sparus aurata L.), were investigated over 8 days to determine the t ime-courses for the production of Delta(6) and Delta(5)-desaturase pro ducts and 22:6(n - 3). Fish were starved for 1 week prior to, and duri ng, the period of the experiment. The recovery of radioactivity from [ 1-C-14]20:5(n - 3) in tissue lipids exceeded that of [1-C-14]18:3(n - 3) at all time points. The recoveries of both fatty acids decreased by 85-89% between days 2 and 8, indicating that substantial loss of radi oactivity due to P-oxidation occurred. Incorporation of 18:3(n - 3) an d 20:5(n - 3) was predominantly into triacylglycerol but during the ti me-course of the experiment there were decreased percentages of radioa ctivity from both labelled fatty acids recovered in triacylglycerol wi th concomitant increased percentages recovered in phospholipids indica ting preferential oxidation of fatty acids in triacylglycerol and/or r edistribution of incorporated fatty acids. Recovery of radioactivity i n 22:6(n - 3) was 10-fold greater with [1-C-14]20:5(n - 3) than with [ 1-C-14]18:3(n - 3). However, there were few consistently significant t rends in the levels of components of the desaturation/elongation pathw ays during the time-course of the experiment. In particular, the relat ive recovery of radioactivity in 22:6(n - 3) did not increase during t he experiment with either substrate. Substantial amounts of radioactiv ity were found in 24:5(n - 3) and 24:6(n - 3), particularily after inj ection with [1-C-14]20:5(n - 3), indicating that the conversion of 20: 5(n - 3) to 22:6(n - 3) in sea bream may occur by a pathway utilizing Delta(6)-desaturase activity rather than by a Delta(4)-desaturation.