A preferential incorporation of dietary arachidonic acid (20:4, n-6) into chyle lipoprotein phospholipids, a relative resistance of 20:4 esters of chyle triacylglycerol (TG) to hydrolysis by lipoprotein lipase, a preferential utilization of 20:4 for phospholipid acylation, and a low rate of oxidation of 20:4 are factors that may contribute to the differences seen in the incorporation into tissue lipids between absorbed 20:4 and the predominant dietary 16-18 carbon fatty acids. In this study we fed [C-14]20:4 and [H-3]eicosapentaenoic acid (20:5, n-3) as free fatty acids in a fish oil emulsion to rats and analyzed the radioactivity in different tissue lipids after 1, 2, and 4 h. The purpose was to examine the degree of similarity in the fate of the two major eicosanoid precursors during the absorption of a fish oil meal. The recovery after 2 and 4 h of C-14 exceeded that of H-3 in lipids of small intestine, serum, liver, heart, kidneys, and spleen. The differences increased with time, e.g., the liver contained 9.7 (+/-0.7)% H-3 and 17.9 (+/-1.4)% of the C-14 (P < 0.001), and the upper half of the small intestine 10.0 (+/- 0.8)% of the H-3 and 22.8 (+/-1.1)% of the C-14 (P < 0.001) after 4 h. The C-14 and 3H radioactivity per g tissue after 4 h ranked as follows: liver and brown adipose tissue > kidneys > heart, lungs, spleen, and serum > colon > white adipose tissue and testes, the differences between tissues being up to 50-fold. There were up to fourfold variations in the C-14/H-3 ratios between tissues after 4 h, the highest value being observed in the heart and the lowest in white adipose tissue. Of the radioactivity retained in liver and intestine, more C-14 and H-3 was in phospholipids and less in triacylglycerol (TG), the differences being largest in the liver, e.g., after 4 h 57.6 (+/-0.8)% of the C-14 and 29.9 (+/-0.9)% of the H-3 (P < 0.001) in the liver was in phosphatidylcholine (PC). In both intestine and liver the highest C-14/H-3 ratios were found in phosphatidylinositiol (PI). Also phosphatidylethanolamine (PE) contained more C-14 than H-3 but the quantitative differences were relatively small after 4 h. In heart the proportions of H-3 and C-14 found in PE and PI did not differ, whereas more of the C-14 was in PC and more of the H-3 was in cardiolipin and phosphatidylserine. In serum more of the H-3 appeared in TG, free fatty acids, and diacylglycerols and less in phospholipids as compared to C-14. The relative distribution of H-3 and C-14 between TG, free fatty acids, and diacylglycerols did, however, not differ suggesting that 20:4 and 20:5 esters were metabolized similarly. Large proportions of both [H-3]20:5 and [C-14]20:4 were in cholesteryl esters, indicating that rat lecithin:cholesterol acyltransferase (LCAT) is as active against 20:5 as against 20:4. The interconversion of [C-14]20:4 to other fatty acids was examined by high pressure liquid chromatography of fatty acid methyl esters. The interconversion of [C-14]20:4 to other fatty acids was negligible. The proportion of H-3 in docosapentaenoic (22:5, n-3) and docosahexaenoic (22:6, n-3) acids did not exceed 7% in intestine and 11% in the liver after 4 h; the values observed at 1 h were below 4%. The values given thus reflect the distribution of [C-14]20:4, whereas in the case of[H-3]20:5 the formation of interconversion products may influence the data to a small extent. The study shows that dietary 20:5 is metabolized according to a pattern similar to that of 20:4. 20:5 does, however, exhibit a lower preference for incorporation into phospholipids, particularly PC and PI, and more appears in TG of intestine, serum, and liver. 20:4 and 20:5 esters of chyle TG and PC are similarly metabolized by lipases and by LCAT.
