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Lipoproteins

Lipoproteins are complexes of proteins with lipids.

The composition and properties of lipoproteins vary widely in relation to the amount and nature of proteins (see), and lipid components (see Lipids). The most pronounced ability to form complexes with lipids has various globulins, which constitute the predominant part of the proteins found in the composition of lipoproteins. The lipid part of the complex usually contains triglycerides, phospholipids (lecithins and kefalins), free cholesterol and its esters. The content of proteins in plasma lipoproteins varies from 2.5 to 46.4%, the content of free cholesterol varies from 2 to 7.5%, cholesterol esters from 6 to 39.4%, and phospholipids from 7.1 to 26.1 % and triglycerides - from 8.1 to 81.3%. The composition of lipoproteins of cells of various tissues varies in the same range.

The nature of the relationship between proteins and lipids in lipoproteins has been studied very little. In various lipoproteins, these bonds are not equally strong and stable, however, those lipoproteins, in which the bonds are most strong, relatively easily exchange individual lipid components for the corresponding or other lipids of the medium, as established by experiments with lipids labeled with radioactive isotopes.

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Lipoproteins are relatively easily soluble in various saline solutions. Lipoproteins of blood plasma and tissue extracts can be divided into separate fractions using electrophoresis or ultracentrifugation. During electrophoresis on paper, the position of the individual lipoprotein fractions is determined by staining the lipid components with the appropriate dyes (Sudan, etc.). The main fractions of lipoproteins are α- and β-lipoproteins (containing α- or β-globulins). During ultracentrifugation in NaCl solution (with a specific gravity of 1.063), lipoproteins are divided into two fractions, one of which settles (lipoproteins with a density from 1.063 to 1.210), the other, on the contrary, floats (lipoproteins with a density from 1.006 to 1.063). The last fraction but the degree of floatation (flotation) can be divided into several subfractions, which are denoted in units of Svedberg (S) with the addition of the letter f, which indicates that it is not a sedimentation rate, but a flotation rate. Normally, the subfraction Sf 0-1.2 prevails in the serum, i.e. the most severe subfraction of low-density lipoproteins. In atherosclerosis in people prone to myocardial infarction, and in a number of other diseases (lipoid nephrosis), there is often a sharp increase in Sf 12-20 subfraction and other, even lighter, subfractions.

Lipoproteins form specific layers in the membranes of all cells and their structural elements (mitochondria, etc.), causing the permeability of these membranes for a number of compounds. Lipoproteins are also carriers of triglycerides, cholesterol and its esters from organs to organs.

A lot of data suggests that changes in this function can be of great importance in the pathogenesis of atherosclerosis and other diseases.

Histochemical determination of lipoproteins in tissues . Lipoproteins are not detected by the usual histochemical reactions to lipids (see).

For histochemical detection of lipoproteins, it is necessary to preliminarily free them from binding to the protein by additional effects leading to rupture of the complex or to the destruction of the protein component.

Noll (A. Noll) recommends using pepsin (0.1 g of pepsin in 100 ml of 0.3% HCl) to identify associated lipids. Good results can also be obtained when exposed to tissue solutions of neutral salts (15% ammonium chloride or 5% magnesium sulphate), as well as 2% hydrochloric acid or 1% potassium hydroxide solution. SS Weil and E. K. Berezina used 15% ammonium chloride solution to identify the associated lipids. Berenbaum (M.G. Berenbaum) indicates that bound lipids can be detected by prolonged (several hours) washing paraffin sections in water. GG Nepryakhin recommends specially prepared alcohol and xylene solutions of Sudan dyes for the detection of lipoproteins. Associated lipids are partially detected by the method of J. B. Goldman (stained with Sudan III solution containing α-naphthol).