Plasma lipoproteins chemical composition

The studies that led to the lipid hypothesis measured plasma total cholesterol concentration. Cholesterol is insoluble in aqueous solution and needs to be combined with protein for transport in blood. These plasma lipoproteins are large heterogeneous aggregates that have different physical properties, such as density, chemical composition and metabolic function (Gurr et al., 2002). 

The compositional and metabolic heterogeneity of operationally defined plasma lipoproteins necessitates the introduction of a classification system based on apolipoproteins as specific markers for identifying discrete lipoprotein families or particles. According to the chemical classification system, plasma lipoproteins consist of discrete simple and complex lipoproteins. Simple lipoproteins contain a single apolipoprotein, whereas complex lipoproteins contain two or more apolipoproteins. 

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. 

PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF HUMAN PLASMA LIPOPROTEINS... 

Table 4. Chemical Composition of Normal Human Plasma Lipoproteins...

To study the chemical composition and the physical properties of lipoproteins it is necessary to isolate them from plasma and separate them from each other. This will also be useful in many metabolic studies. There are three basic methods for their isolation separation in the ultracentrifuge gel filtration or precipitation. Ultracentrifugation is costly and time... 

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