Lipoproteins

These are particles composed of lipids and proteins connected by non-covalent bonds, called apoproteins. Blood plasma lipoproteins are made up of a hydrophobic core of triglycerides and cholesterol esters, surrounded by an outer hydrophilic layer containing amphipathic lipids – free cholesterol and phospholipids. They are a form of exogenous and endogenous lipid transport from their formation and absorption sites to their utilization and storage sites. Lipoproteins supply tissues with lipids and transport their excess from tissues; moreover, they carry fat-soluble hormones and vitamins. Lipoproteins are a complex supramolecular complex held together by non-covalent bonds, hence called a “particle” rather than a “molecule.” Lipoproteins perform important functions: they transport exogenous (dietary) and endogenous (synthesized in the liver, adipose tissue, and the wall of the small intestine) lipids to circulation, utilization, and storage systems; they capture “spent” cholesterol from peripheral tissue cells and transport it to the liver, where it is converted into bile acids and excreted with bile; they transport fat-soluble vitamins, hormones, and other biologically active substances. The ratio of hydrophilic and hydrophobic parts in lipoproteins depends on the class of lipoproteins. The classification of lipoproteins is based on the behavior of lipoproteins in a gravitational field during ultracentrifugation. Using salt solutions of various densities, chylomicrons (ChM), very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) are separated. They differ in their properties and functions. Some contain a lot of triglycerides, others – cholesterol, and still others – phospholipids. The significance of various lipoprotein fractions for the risk of ischemic heart disease is not the same. A relationship has been established between total cholesterol concentration and ischemic heart disease. It has been proven that reducing cholesterol by one percent reduces the risk of heart and vascular diseases by 2 percent. The most atherogenic is LDL cholesterol. Conversely, there is an inverse relationship between HDL concentration and atherosclerosis – higher HDL concentration protects against ischemic heart disease. HDL is involved in reverse cholesterol transport, collecting excess “spent” cholesterol from the periphery and transporting it to the liver, where it is excreted with bile. Hypertriglyceridemia usually occurs with decreased HDL and the effect depends more on the decreased HDL concentration.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė