Behavior of dog lymph chylomicron lipid constituents during incubation with serum

Dog lymph chylomicrons incubated with serum showed a progressive gain in free cholesterol whereas no change took place in the cholesterol ester and triglyceride fractions. Chylomicron phospholipids were observed to suffer a net loss while at the same time exchanging with serum phospholipids. Before incubation, lymph chylomicron phospholipids contained a higher proportion of noncholine phospholipids than serum; during incubation, this fraction was largely transferred to serum. The noncholine fraction showed comparatively little exchange. Lecithin was by far the largest component of both chylomicron and serum phospholipids. In absolute amounts, the largest transfer and exchange could be accounted for by lecithin. The percentages of sphingomyelin and lysolecithin in chylomicron phospholipids were lower than in those of serum. During incubation, the sphingomyelin content of chylomicrons changed little, but active exchange of sphingomyelin took place. The amount of lysolecithin increased both in chylomicrons and in serum. Almost all the phospholipid lost from the chylomicrons appeared in the high-density lipoprotein fraction. Evidence is presented that the extra phospholipid is taken up by preexisting high-density lipoprotein. Exchange of phospholipids between chylomicrons and serum lipoprotein fractions took place about in proportion to the relative phospholipid contents of these fractions. The bearing of these results on the mechanisms whereby chylomicrons are removed from the circulation is discussed.