Lipoprotein complexes

Polyunsaturated fatty acids and especially arachidonic acid are highly susceptible to lipid peroxidation, which leads to the generation of lipid hydroperoxides, which then undergo carbon-carbon bond cleavage giving rise to the formation of short chain, unesterified aldehydes and aldehydes still esterified to the parent lipid, termed core-aldehydes (Esterbauer et al. 1987). Considerable progress has been made in recent years in dissecting the molecular structures of OxPL, which consequently allowed for the experimental use of defined compounds rather than complex lipoproteins and lipid mixtures. 

The lipids are among the most important components of human diet and occur widely in nature. However, it is the biochemical role of lipids as the basic components of various cellular membranes and the lipid-protein complexes (lipoproteins) that bring them into the focus of highly important scientific activities. Major clinical interest has concentrated on blood lipid chemistry as related to atherosclerosis, lipid storage diseases, diabetes, and other metabolic conditions. 

Pulmonary surfactant is a complex lipoprotein material (King, 1974 Harwood et al.y 1975) which is absorbed at the alveolar air/liquid interface and assists in maintaining alveolar stability. It lowers the surface tension to about 10nNm and reduces the contractile force of the surface and the work of lung expansion. It also prevents atelectasis because during expiration it forms a solid film on the alveolar surface. 

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. 

Tlie Apo A- and Apo B-containing lipoproteins are two major groups of plasma lipoproteins. Each of these two groups consists of simple and complex lipoprotein families. The major Apo A-containing lipoprotein families are LP-A-I andLP-A-I A-II. The major Apo B-containing lipoprotein families include LP-B, LP-B C E, LP-B E, LP-A-II B C D E, andLP-B C. 

When one considers the multiplicity of proteins in the membrane postulated for excitable membranes, or indeed for cell membranes in general, it is perhaps hardly surprising that the membrane should show a complex dielectric behaviour. Each ionic channel is believed to be a large and complex lipoprotein structure, part of which probably permanently spans the membrane and so must be subject in some way to the electric field within the membrane. These proteins cannot be non-polar because their potential sensitivity must depend on some alteration of their structure or position. Membranes have a multiplicity of functions of which regulating ionic currents are only among the most obvious and most obviously potential regulated. 

FAT AND HEALTH PROBLEMS. Fats either cause heart disease or cancer—or so it seems. As for coronary heart disease, the blame on fats arises primarily from two factors (1) atherosclerotic deposits in blood vessels are composed of cholesterol and other fatty substances and (2) increases in certain fat components of the blood contribute to atherosclerosis. Fats are transported in the blood in the form of lipid-protein complexes—lipoproteins. It is the blood levels of cholesterol, triglycerides, and certain lipoproteins which are considered risk factors in the development of heart disease. 

Fats or lipids, which are not soluble in a water medium such as blood, are transported in the blood in the form of water-soluble, fat-protein complexes— lipoproteins. All of these lipoproteins contain phospholipids, triglycerides, and cholesterol, but in varying amounts. 

The first three are classical categories of supramolecular structure research in molecular biology, and as a result one might expect a rationalization of lipoprotein structure by some kind of static picture. However, from all we know about natural and artificial lipid - protein complexes, lipoproteins cannot be considered as static particles Mobility of components within the particles is a well-established fact, and exchange of components aimong lipoproteins and with cell membranes is part of their biological function. In search of structure-function relationships, therefore, considerations of structural dynamics are certainly of paramount importance. 

Lipoproteins. The lipid moiety of lipoproteins is quite variable both qualitatively and quantitatively. The a-lipoprotein of serum contains glyceride, phosphatide and cholesterol to about 30 -40% of the total complex. The -lipoprotein of serum contains some glyceride but the phosphatide and cholesterol account for nearly 75% of the total. 

Cholesterol is a principal component of animal cell plasma membranes, and much smaller amounts of cholesterol are found in the membranes of intracellular organelles. The relatively rigid fused ring system of cholesterol and the weakly polar alcohol group at the C-3 position have important consequences for the properties of plasma membranes. Cholesterol is also a component of lipoprotein complexes in the blood, and it is one of the constituents oiplaques that form on arterial walls in atherosclerosis. 

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

Transport of Many Lipids Occurs via Lipoprotein Complexes... 

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

HDL and VLDL are assembled primarily in the endoplasmic reticulum of the liver (with smaller amounts produced in the intestine), whereas chylomicrons form in the intestine. LDL is not synthesized directly, but is made from VLDL. LDL appears to be the major circulatory complex for cholesterol and cholesterol esters. The primary task of chylomicrons is to transport triacylglycerols. Despite all this, it is extremely important to note that each of these lipoprotein classes contains some of each type of lipid. The relative amounts of HDL and LDL are important in the disposition of cholesterol in the body and in the development of arterial plaques (Figure 25.36). The structures of the various... 

Lipoprotein (Chapter 27 Focus On) A complex molecule with both lipid and protein parts that transports lipids through the body. 

Cholesterol, which is water-insoluble, is transported through the blood in the form of soluble protein complexes known as lipoproteins. 

Heterogeneous Non-uniform composition, 4 catalysis, 305-306 equilibrium, 329-331 High-density lipoprotein, 603 High-spin complex A complex that, for a particular metal cation, has the largest number of unpaired electrons, 419 Hindenburg, 111... 

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