Apoprotein classes

Lipoprotein class Lipid components Main apoprotein components Enzymes present Role... 

The classes of Upoproteins and the important apoproteins associated with their functions are summarized in Table 1-15-1 and Figure 1-15-4. 

One approach for identifying the type of iron center in a protein has been to remove the center, intact, by ligand exchange between the protein and an exogenous acceptor ligand (reviewed by Berg and Holm, 1982). The removed, or extruded, center is identified by comparison of its spectral properties (usually absorption spectrum) to known model compounds. Alternatively, the extruded center can be inserted into a second apoprotein which has been previously determined to accept only one type of iron center. The reconstructed standard protein is then analyzed by EPR. The latter method, interprotein cluster transfer, requires that acceptor apoproteins for all known classes of centers are included in the reaction mixture and that the reconstituted reporter ... 

Two classes of binding sites are also indicated by studies with the cobalt enzyme. The first two cobalt ions bound by the apoprotein do not produce an active enzyme. In the absence of evidence to the contrary, it is assumed that these two sites are the same as the unnecessary zinc sites. The absorption spectrum of the two-cobalt enzyme is similar... 

Properties of the Apoproteins of the Major Human Lipoprotein Classes... 

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. 

Human lipoproteins exist in several sizes and densities with differing lipid to protein ratios. These various lipoproteins have different origins in the body, different destinations and different functions (10). Thus, chylomicrons are extremely large low density particles formed in the intestine and designed to deliver dietary fat to adipose tissue. Very low density lipoproteins (VLDL), on the other hand, are smaller, more dense particles designed to deliver lipids from the liver to adipose and other tissues. Low density lipoproteins (LDL), formed from VLDL or produced in the liver or intestine deliver cholesterol to peripheral tissue, while high density lipoproteins (HDL) function to return cholesterol from peripheral tissues to the liver for catabolism. There is a complex exchange of lipids and apoproteins between the lipoprotein classes. 

There are six major classes of apoproteins several sub-classes and hundreds of genetic polymorphisms have been described (Table 5.1). 

Polyclonal antibodies are widely used in clinical laboratories for the measurement of plasma protein concentrations. However, immunoassays are often sensitive to the nature of the antibody used. The development of polyclonal antibodies is affected by several factors, such as the purity and dose of the antigen used, the species of host animal, and the immunization procedure. Monoclonal antibodies are viewed as a viable alternative to alleviate these problems. However, the expression of particular epitopes varies with the hpoprotein particles and among individuals in addition, the apohpoproteins themselves are polymorphic in nature. Therefore the use of a single monoclonal antibody might not detect a particular variant. If a monoclonal antibody is used in the determination of an apohpoprotein, it should he directed to an epitope that is expressed on all polymorphic forms of that particular apoprotein. Furthermore, the epitope should be equally reactive to the antibodies regardless of which hpoprotein class contains it. Alternatively a mixture of monoclonal antibodies directed at different epitopes of the apohpoprotein may also be used. Such mixtures are referred to as panmonoclonal antibodies. 

Initially recorded with rare paraproteins (B23), mainly of the IgA class, this has also been associated with IgG and IgM. Usually the total serum cholesterol is raised, but in one case it has been within normal limits curiously it is subnormal in most patients with IgA-myelomatosis without xanthoma (S7). The xanthomata are typically of the soft eruptive variety and contain complexes of the paraprotein and /8-Iipo-protein. Beaumont (B8) has collected evidence suggesting that the paraproteins are antibodies to the 8-apoprotein. Occasionally excess complexes can result in viscosity syndrome (Section 7.5.6). If a lipid stain is used, the paraprotein band is positive. In such cases, regrettably. Potter has been unable to relate the antibody activity to phosphoryl-choline (P13). Cytotoxic treatment can reduce the serum levels of lipid and paraprotein. 

The laws of mass action govern the interactions of lipids and most apoproteins in lipoproteins, so that as the affinities between surface components change dining lipoprotein metabolism, apoproteins may dissociate from one particle and bind to another. In fact, all of the apoproteins, with the possible exception of apoprotein B (apo B), can change their lipoprotein associations. The reason for the unique behavior of apo B remains a mystery. On the basis of their principal transport function, lipoproteins may be divided into two classes according to the composition of their major core lipids. The principal triacylglycerol carriers are chylomicrons and very-low-density lipoproteins (VLDLs), whereas most cholesterol transport occurs via LDLs and HDLs. 

Fig. 2. The apoproteins of the major classes of lipoproteins separated on sodium dodecyl-polyacrylamide gels. Reproduced with permission of the authors and publisher, from Mahley and Innerarity [12].

These lipoproteins vary in size, density, relative composition of triglycerides, phospholipids, cholesterol, cholesterol ester, and proteins. In addition, each of these classes of lipoproteins may contain different apoproteins. These specific apoproteins are important for transformations within the lipid particle, and are signals for cellular receptors allowing uptake and/or endocytosis. 

Cholesterol travels in the bloodstream via lipoprotein complexes called Chylomicrons, VLDL, IDL, LDL, and HDL. Of the five lipoprotein classes, LDL is by far the richest in cholesterol. Cholesterol in plasma lipoproteins exists both as the free sterol and esterified at its hydroxyl position with a long-chain fatty acid, usually unsaturated (see also Table 18.1). The LDL particle contains a single molecule of apoprotein B-lOO (Mr = 513,000) as its primary protein component. Because cholesterol biosynthesis is confined primarily to the liver with some occurring also in intestine, LDL plays an important role in delivering cholesterol to other tissues. Cholesterol esters are too hydrophobic to traverse cell membranes by themselves and must be transported into cells via specialized LDL receptors. 

Low-density lipoprotein complexes (LDLs), which are the primary means of transporting cholesterol in the blood, are readily oxidized. These oxidations include peroxidation of unsaturated fatty acids, hydroxylation of cholesterol, and oxidation of amino acid residues in the apoprotein. A class of white blood cells recognizes the oxidation and absorbs the LDL through its scavenger receptor. After a white blood cell has absorbed numerous LDLs containing cholesterol, it becomes engorged and is referred to as a foam cell. Foam cells attract other white blood cells, which leads to accumulation of more cholesterol. Ultimately, this accumulation of cholesterol becomes one of the chief chemical constituents of the atherosclerotic plaque that forms at the site. 

Lipoproteins are macromolecular assemblies that contain proteins and lipids, including free and esterified cholesterol, triglycerides, and phospholipids. The protein components, known as apoUpoprotems, provide structural stability to the lipoproteins, and also may function as ligands in hpoprotein-receptor interactions or as cofactors in enzymatic processes that regulate lipoprotein metabolism. In aU Upoprotems, the most water-insoluble lipids (cholesteryl esters and triglycerides) are core components, and the more polar, water-soluble components (apoproteins, phospholipids, and unesterified cholesterol) are located on the surface. The major classes of lipoproteins and their properties are presented in Table 35-1. Table 35-2 describes apoproteins that have well-defined roles in plasma lipoprotein metabolism. 

Lipoprotein Class Density of Flotation, g/mL Major Lipid Constituent TG Chol Ratio Significant Apoproteins Site of Synthesis Mechanism(s) of Catabolism... 

Class Buoyant density Plasma concentration (mg/ml) Protein (%of total) Lipid (% of total) Phospholipid (% of total) Ch-l-ChE (% of total) Triglyceride (% of total) Apoproteins present... 

The composition of the various lipoprotein fractions are given in Table 4.2 and it can be seen that the proportion of lipid decreases from the VLDL to the HDL, but the total plasma concentrations of the three classes increases in the order VLDL (1%), LDL (16%) to HDL (80%). The LDLiHDL ratio of approximately 1 5 is much lower than in mammals, where it is around unity (Hermier et al, 1984). The protein component of the lipoproteins, known as the apoproteins, are very important in the functioning of the lipoproteins. Apart from maintaining the lipid in a soluble form, the proteins have important binding... 

Lipoprotein class Apoprotein Density (g/cm ) Diameter (nm) Function... 

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