Metabolism of deuterium-labeled linoleic, 6,9,12-octadecatrienoic, 8,11,14-eicosatrienoic, and arachidonic acids in the rat

Male weanling rats were fed a diet that contained 2.1% ethyl oleate, 1% ethyl linoleate, and 0.2% ethyl linolenate. After 4 weeks all the linoleate was replaced by the deuterium-labeled analog and the animals were killed 4 days later. The molar fraction of total 20:4(n-6) in liver, heart, and kidney phospholipids containing deuterium was 33.9, 8.9, and 13.3%, respectively. Second; animals were preconditioned by incorporating either 0.2% of 18:3(n-6), 20:3(n-6), or 20:4(n-6) into the above diet and again after 4 weeks all the linoleate was replaced with the labeled analog. Now the molar fraction of labeled 20:4(n-6) in liver phospholipids from these three groups of animals was reduced from 33.9 to 27.1, 23.9, and 24.1% respectively. In contrast, there was little change in the specific activity of 20:4(n-6) in heart and kidney phospholipids. The third protocol was a direct crossover study in that again unlabeled linoleate was fed during the entire period. Four days prior to killing the unlabeled 18:3(n-6), 20:3(n-6), and 20:4(n-6) were replaced with the deuterium-labeled analogs. The mole % of total esterified 20:4(n-6) in liver phospholipids was now 24.6, 32.0, and 26.2%, respectively. Even though 18:3(n-6), 20:3(n-6), and 20:4(n-6) were all fed at only 20% of the level of 18:2(n-6), it can be calculated that the molar fraction of esterified 20:4(n-6) in liver phospholipids was between 65 to 77% of that found when 18:2(n-6) was the only dietary (n-6) acid as under these conditions 33.9 mol % of the 20:4(n-6) was labeled. Interestingly, when deuterium-labeled 18:3(n-6), 20:3(n-6), or 20:4(n-6) was fed, the specific activity of esterified 20:4(n-6) in kidney and heart phospholipids was always equal to or greater than what was derived from deuterium-labeled 18:2(n-6). The results show that under steady-state dietary conditions, (n-6) dietary fatty acids are processed in different ways by liver, heart, and kidney.