Chylomicron chemical composition

Approximately 95% of total lipids of LD from bovine heart are constituted by TAG. The amount of protein present in these LD is about 5% of total mass, and the amount of phospholipids varies from 3 to 7% of total lipid. The major phospholipids forming the monolayer of these LD are phosphatidylcholine (ca. 50%) and phosphatidylethanolamine (30-40%). The NEFA content is very low. The chemical composition of LD in beef heart with their high TAG content and the rather small amount of phospholipids resembles the composition of chylomicrons. A striking difference, however, is the lack of cholesterol and cholesteryles-ters in LD from beef heart whereas these lipid species occur at approximately 1-2% of total lipid in chylomicrons. Furthermore, the protein content of LD is two to three times higher than of chylomicrons. In contrast to bovine heart LD, those of stellate cells from the rat liver consist of retinylesters, TAG, free cholesterol and a small amount of phospholipids [142]. A general characteristic of LD regardless of the cell type appears to be the content of approximately 5% phospholipids of the total mass (reviewed in Ref. [143]). 

The cholesterylesters present on plasma lipoproteins are partly secreted into the plasma on nascent hpoproteins (chylomicrons and VLDL) and partly synthesized by the plasma enzyme LCAT. In some species, including man, active lipid transfer proteins (LTP) circulate in plasma. Both LCAT and LTP are synthesized in the liver (for reviews, see [25, 26]). Cholesterylester transfer protein (CETP) catalyses a transfer/exchange of cholesterylesters between HDL and the other lipoproteins. This process is important for the turnover of plasma cholesterol because, depending on the amount of active CETP and the chemical composition of the circulating plasma hpoproteins, a variable part of the HDL-cholesterylesters are transferred by CETP to hpoprotein classes of lower density, or vice versa. The presence of active CETP seems to provide a link between VLDL/IDL/LDL metabolism on one hand, and HDL metabohsm on the other. In addition LTP may directly influence the hepatic uptake of cholesterylesters from lipoproteins by as yet unknown mechanisms. 

Normal concentrations in the blood plasma are in the range 1200-2200 mg/1. Some 30 per cent of this is in the free state, the remainder being bound to lipoproteins. These are complexes of proteins and lipids held together by non-co-valent bonds. Each has a characteristic size, molecular weight, chemical composition and density. They are classified on the basis of their density. The five classes, of which one, the chylomicrons, occurs only in the post-absorptive state, are shown in Table 3.5. 

The chemical composition of human and dog chylomicrons is described in Table 2. Significant variations in composition, especially protein content, were obtained when chylomicrons were separated in sucrose gradients (Yokoyama and Zn.VEBSMiT 1965). The origin and nature of chylomicron protein is an intriguing and as yet unsolved problem. High density lipoproteins were first suggested as components of the chylomicron molecule (Rodbell and Fbedbickson 1959,... 

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