Asymmetric distribution of arachidonic and n-3 polyunsaturated fatty acids in rat liver microsomal membranes under a fat-free diet.

Rats adapted to a corn oil or a fish oil diet were fed a fat-free diet, and changes in phospholipid polyunsaturated fatty acids (PUFA) in the inner and outer leaflets of liver microsomal membranes were followed for 18 wk. In rats previously adapted to a corn oil diet, arachidonic acid in phosphatidylcholine and phosphatidylethanolamine in the inner and outer leaflets did not decrease quickly; rather, linoleic acid decreased more than arachidonic acid for the first three weeks of feeding the fat-free diet. Even at 18 wk, 40-50% of the beginning arachidonic acid levels were still retained. In contrast, in rats previously adapted to a fish oil diet, the n-3 PUFA were quickly decreased by the fat-free diet to only 10-30% at 18 wk. Due to the appearance and increase of n-9 eicosatrienoic acid in the replacement of the n-3 and n-6 PUFA, total PUFA did not decrease in the inner and outer phosphatidylcholine in either group, but decreased somewhat in the phosphatidylethanolamine due to the insufficient increase of the n-9. On the other hand, the overall degrees of unsaturation in phosphatidylcholine fatty acids were always higher in the outer than in the inner leaflets and were not altered by feeding the fat-free diet even for 18 wk. Thus, the results appear to reveal the physiological importance of unsaturation ratio of fatty acids and the necessity of arachidonic acid in each membrane leaflet.