In vivo metabolism of [1-14C]linolenic acid (18:3(n-3)) and [1-14C]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 [1-14C]18:3(n-3) and [1-14C]20:5(n-3) in a marine fish, gilthead sea bream (Sparus aurata L.), were investigated over 8 days to determine the time-courses for the production of delta 6 and delta 5-desaturase products and 22:6(n-3). Fish were starved for 1 week prior to, and during, the period of the experiment. The recovery of radioactivity from [1-14C]20:5(n-3) in tissue lipids exceeded that of [1-14C]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 radioactivity due to beta-oxidation occurred. Incorporation of 18:3(n-3) and 20:5(n-3) was predominantly into triacylglycerol but during the time-course of the experiment there were decreased percentages of radioactivity from both labelled fatty acids recovered in triacylglycerol with concomitant increased percentages recovered in phospholipids indicating preferential oxidation of fatty acids in triacylglycerol and/or redistribution of incorporated fatty acids. Recovery of radioactivity in 22:6(n-3) was 10-fold greater with [1-14C]20:5(n-3) than with [1-14C]18:3(n-3). However, there were few consistently significant trends in the levels of components of the desaturation/elongation pathways during the time-course of the experiment. In particular, the relative recovery of radioactivity in 22:6(n-3) did not increase during the experiment with either substrate. Substantial amounts of radioactivity were found in 24:5(n-3) and 24:6(n-3), particularly after injection with [1-14C]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.