Fatty acid esterification and chylomicron formation during fat absorption: 2. Phospholipids

Abstract

Studies were conducted on the over-all fatty acid specificity of the mechanisms involved in chylomicron lecithin formation during fat absorption in the rat. Synthetic mixtures of fatty acids containing trace quantities of C14-labeled fatty acids were administered by gastric intubation to rats with cannulated thoracic ducts. The chylomicron lipids were first separated on silicic acid columns into cholesterol ester, glyceride, and phospholipid fractions. The results obtained with the sterol ester and glyceride fractions are described in the accompanying paper. The phospholipids were rechromatographed to obtain pure lecithin, and determinations were made of the total fatty acid radioactivity, total mass, and distribution of mass and radioactivity, among the several fatty acids of lecithin, by gas-liquid chromatography. Different fatty acids fed to the rat were not incorporated to equal extents into chylomicron lecithin. The incorporation showed a marked relative specificity for stearic acid and a lesser specificity for linoleic acid. Oleic acid was incorporated least of all into lecithin. The incorporation of different dietary fatty acids varied with the nature of the diet, but the addition of fatty acids from endogenous sources was of such magnitude as to make this change less evident, so that the over-all fatty acid pattern of lecithin was relatively constant and independent of the composition of the diet. The endogenous contribution to lecithin fatty acids was greater than the endogenous contribution to the sterol ester or triglyceride fatty acids of the same chylomicron sample. This endogenous contribution to lecithin also varied from fatty acid to fatty acid, being greatest with palmitic acid and least with oleic acid. The general features of chylomicron formation are discussed in detail. Lecithin carried a relatively large share of stearic acid, and relatively vcry little oleic acid, within the chylomicron.