Lipoproteins chemical composition

L14. Lindgren, F. T., Nichols, A., and Freeman, N. K., Physical and chemical composition studies on the lipoproteins of fasting and heparinized human sera. /. Phys. Chem. 59, 930-938 (1955). 

This structure (Fig. 7.14) has received much attention due to the role it plays in the hatching process (see below). It is not a typical unit membrane but resembles a membranous lamina and consists of a layer of regularly arranged granules bounded on both sides by a number of lamina (442). Its chemical composition has not been determined but, in taeniid cestodes, there is some histochemical evidence that it may be a lipoprotein (442). It is apparently formed by the delamination of the inner part of the inner envelope, detaching from it as a thin, separate layer. 

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). 

This author will now explain what he believes, at the moment, is the primary quantum conversion process that goes on in the layer of chlorophyll (and other pigments) in the lattice. We know something about the chemical composition of the chloroplast itself, i.e., it is a lipoprotein together with pigments. There are a number of specific molecules present in the chloroplast, for example, chlorophyll and... 

Little is known of the chemical composition of the nuclear membrane most of our knowledge is based on indirect evidence. In a few instances, clean preparations of nuclear membranes were obtained and analyzed chemically. The nuclear membrane presumably contains lipoproteins because most nuclear lipids are phospholipids, compounds that are likely to be structural constituents moreover, few nuclear lipids are found in other nuclear structures that can be readily prepared. 

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. 

Table 10.2 Density, size and chemical composition of lipoproteins. ...

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,... 

H. A. Eder, and J. H. Bragdon The distribution and chemical composition of ultra-centrifugally separated lipoproteins in human serum. J. din. Invest. 34,1345—1353 (1955). 

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