Lu-Ying Yang

Similarities in surface lipids of chylomicrons from glyceryl and alkyl ester feeding: Major components

This study tests the hypothesis that the rat chylomicrons are assembled and released into lymph similarly regardless of the site (rough or smooth endoplasmic reticulum) or pathway (phosphatidic acid or monoacylglycerol) of triacylglycerol biosynthesis. For this purpose we determined the lipid class, fatty acid and molecular species composition of the choline, ethanolamine, inositol and serine phospholipids of lymph chylomicrons during absorption of menhaden, mustard-seed and corn oil (monoacylglycerol pathway) or the corresponding fatty acid methyl or ethyl esters (phosphatidic acid pathway). The dietary fatty acids were found to be incorporated to various extents into different phospholipid classes, the proportions of which were not affected by the nature of the dietary fat. The chylomicron phospholipids contained 80–82% choline, 8% ethanolamine and 2.5% inositol glycerophospholipids, and much smaller amounts of serine and other minor phospholipids. Administration of a meal of each dietary fat resulted in a retention of approximately 50% endogenous fatty acids in the major glycerophospholipids of the chylomicrons. A minimum of 50% of the molecular species of the choline and ethanolamine glycerophospholipids contained at least one exogenous fatty acid. No significant discrepancies were found in the fatty acid and molecular species composition of the glycerophospholipids between chylomicrons from the oil and corresponding ester feeding. It is concluded that the chylomicrons arising from the monoacylglycerol (oil feeding) and the phosphatidic acid (ester feeding) pathways of triacylglycerol biosynthesis become enveloped in surfactant monolayers containing qualitatively and quantitatively identical classes and molecular species of phospholipids.

Surface components of chylomicrons from rats fed glyceryl or alkyl esters of fatty acids: Minor components

The lipid class, fatty acid and molecular species composition of the minor polar surface components of rat lymph chylomicrons were determined during absorption of menhaden oil and corn oil or of the corresponding fatty acid ethyl esters. In addition to the previously reported minor polar lipids (sphingomyelin, phosphatidylserine, phosphatidylinositol, phosphatidic acid and lysophosphatidylcholine), we identified phosphatidylglycerol, dimethyl-phosphatidylethanolamine, ceramide and cholesteryl sulfate in the chylomicrons from both oil and ester feeding. The dietary fatty acids were found to be incorporated to a variable extent into the different phospholipid classes, the proportions of which remained the same during both types of feeding. No evidence was obtained for the presence of the minor glycerophospholipids characteristic of the lysosomal membranes (e.g., bis-phosphatidic, lysobisphosphatidic and semilysobis-phosphatidic acids), although special efforts were made to identify them. These results indicate that the chylomicrons arising from the monoacylglycerol and phosphatidic acid pathways of triacylglycerol biosynthesis become enveloped in closely similar monolayers of phospholipids. Hence, all triacylglycerols may be secreted from the villus cellsvia a common mechanism as suggested by the previously demonstrated convergence (at the 2-monoacylglycerol stage) of the monoacylglycerol and the phosphatidic acid pathways of mucosal triacylglycerol formation [Yang, Y.L., and Kuksis, A. (1991)J. Lipid Res. 32, 1173–1186].