Apolipoprotein properties

In contrast to MDA and hydroxynonenai, other aldehyde products of lipid peroxidation are hydrophobic and remain closely associated with LDL to accumulate to mil-limolar concentrations. Aldehydes at these elevated levels react with the protein portion of the LDL molecule, apolipoprotein B (apoB). Accumulated aldehydes bind the free amino groups from lysine residues in addition to other functional groups (-OH, -SH) on the apoB polypeptide. Consequently, the protein takes on a net negative charge and complete structural rearrangement results in the formation of ox-LDL. ox-LDL is no longer recognized by the LDL receptor, and has several pro-inflammatory properties (discussed below). 

Lipoproteins are globular, micelle-like particles consisting of a hydrophobic core of triacylglycerols and cholesterol esters surrounded by an amphipathic coat of protein, phospholipid and cholesterol. The apolipoproteins (apoproteins) on the surface of the lipoproteins help to solubilize the lipids and target the lipoproteins to the correct tissues. There are five different types of lipoprotein, classified according to their functional and physical properties chylomicrons, very low density lipoproteins (VLDLs), intermediate density lipoproteins (IDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). The major function of lipoproteins is to transport triacylglycerols, cholesterol and phospholipids around the body. 

The properties of the apolipoproteins in solution have been extensively studied. Two points are relevant to the present discussion. 

Plasma HDL constitute a heterogeneous group of lipoproteins their common property is that they have a hydrated density in the 1.063-1.21 g/ml interval. HDL subfractions are usually described in terms of their hydrated density, but it should not be forgotten that a number of functions may be the property of particular subsets of HDL which can be defined better in terms of their apolipoprotein composition than their hydrated density. A preparation of HDL isolated by flotation in the ultracentrifuge, d between 1.063 and 1.21 g/ml, may contain almost all the major apolipoproteins (except perhaps apoA-IV and apoB-48), together with LCAT and lipid transfer protein. 

F22. Fukushima, D., Yokoyama, S., Kroon, D. J., Kezdy, F. J., and Kaiser, E. T., Chain length-function correlation of amphiphilic peptides synthesis and surface properties of a tetratetracontapeptide segment of apolipoprotein A-I. J. Biol. Chem. 255, 10651-10657 (1980). 

J6. Jackson, R. L., Morrisett, J. D., Pownall, H. J., and Gotto, A. M., Jr., Human high density lipoprotein, apoliprotein glutamine II. The immunochemical and lipid-binding properties of apolipoprotein glutamine II derivatives. J. Biol. Chem. 248, 5218-5224 (1973). 

Steele, J. C. H., Jr., and Reynolds, J. A., Molecular weight and hydrodynamic properties of apolipoprotein B in guanidine hydrochloride and sodium dodecyl sulfate solutions. J. Biol. Chem. 254, 1639-1643 (1979). 

Several types of proteins are associated with lipoproteins. These are termed apolipoproteins, or simply apoproteins. Table 19.2 shows the various apolipo-proteins (Apos), their chemical properties and occurrence, and their function, which is discussed later. Note that the A apoproteins are found largely in HDL, the B-100 is found largely in LDL, VLDL, and IDL, and C apoproteins are largely seen in chylomicrons. Nevertheless, there is a large degree of apoprotein overlap among the various lipoprotein classes. 

Apolipoproteins Methg and Methgo Loss of lipid-carrier properties (Mil)... 

Of the approximately 14 plasma apolipoproteins that have been described to date, apolipoprotein E (apoE) is one of the best characterized in terms of its structural and functional properties. In general, plasma apolipoproteins serve to regulate lipoprotein metabolism and to control the transport and redistribution of lipids among tissues and cells. Specifically, apolipoproteins can perform one of three major roles. First, because of their ability to bind lipid, a property they all share, apolipoproteins stabilize the pseudomicellar structure of lipoprotein parti-... 

Furthermore, as will be described later, a direct comparison of the properties of these two groups with those of class A has enabled us to hypothesize and identify many hitherto unknown properties of apolipoproteins and other class A amphipathic helical peptide analogs. Class M is included because it has significant lipid affinity (although it is not surface active), and class G is included because it is similar to certain types of nonclass A amphipathic helices also found in apolipoproteins. 

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