Chylomicron plasma composition

The plasma lipoproteins are spherical macromolecular complexes of lipids and specific proteins (apolipoproteins or apoproteins). The lipoprotein particles include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They differ in lipid and protein composition, size, and density (Figure 18.13). Lipoproteins function both to keep their component lipids soluble as they transport them in the plasma, and also to provide an efficient mechanism for transporting their lipid contents to (and from) the tissues. In humans, the transport system is less perfect than in other animals and, as a result, humans experience a yradual deposition of lipid—especially cholesterol—in tissues. This is a potentially life-threat-en ng occurrence when the lipid deposition contributes to plaque formation, causing the narrowing of blood vessels (atherosclerosis). 

The different compositions of the plasma lipoproteins give a clue to their function. Essentially, those lipoproteins rich in TAGs are synthesized by the liver (VLDL) and small intestine (chylomicrons) and deliver the neutral fat to extrahepatic tissues (particularly adipose tissue). The fat-depleted lipoproteins have a higher density, and are involved in essential cholesterol transfers. 

The lipoproteins are macromolecules with varying complexes of lipids where the hydrophobic lipid portions—cholesterol esters and triglycerides—are localized at the core of the molecules. The amphipathic surface layers surrounding the core contain the apolipoproteins and phospholipids. The lipoproteins vary in size, density, lipid composition, and apolipoprotein constituents, and they ean be classihed by size, the flotation rate determined by ultracentrifugation, or their electrophoretic mobilities. Put simply, the density of a lipoprotein particle is determined by the relative amounts of lipid and protein contained in the particle. Chylomicrons and very low density lipoproteins have the highest lipid content and the lowest protein content thus, very excessive amounts of chylomicrons float on the surface of plasma. In descending order of size, the broad lipoprotein fractions (with their electrophoretic mobility) are... 

Apart from their operational distinction on the basis of density and lipid composition the plasma lipoprotein classes are also distinguished by the nature of the polypeptides present. The apoprotein species have been characterized and designated family names from A to H. The human plasma lipoprotein apoprotein distribution is shown in Table 12.3. Although the distribution, interchange and metabolism of lipoprotein apoproteins is extremely complicated some overall aspects of the dynamic situation can be discerned (see Schaefer etal, 1978). For example, the apoprotein content of newly synthesized VLDL and chylomicrons changes drastically as these lipoproteins enter the circulation and interact with the other lipoproteins present. Both... 

The lipoproteins present in plasma may be operationally defined according to their density, as low or very low density lipoproteins and high density lipoproteins, but other, more functional definitions may be more appropriate in connection with in vivo metabolism. Examples are the hpoprotein classes chylomicrons and chylomicron remnants. Alaupovic has suggested a classification based on the apolipoprotein composition [1]. As apolipoproteins often determine the metabolic fate of lipoprotein particles this is a logical approach. Lipoprotein particles with specific apolipoprotein compositions exist in all density fractions of human plasma, as well as plasma from a variety of animal species. 

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 Lipid Composition of Htnnan Plasma Chylomicrons J. Lipid Res. 5(3) 225-231 (1964) ... 

Plasma total cholesterol is usually increased the degree of elevation of this lipid fraction depends on both the kind and degree of hyperglyceridemia. Only mildly elevated levels (2—3 times normal) are frequently found in hyperchylomicronemia as would be expected from the lipid composition of chylomicrons. Subnormal levels... 

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