Lipoproteins: Function & Diseases

Lipoproteins are plasma proteins that serve to transport fats. Six different classes of these complexes have been identified to date. Lipid metabolism disorders are a common condition in the Western world, increasing the risk of heart attacks and strokes.

What are lipoproteins?

Lipoproteins are a complex of lipids and proteins found in blood plasma. Thus, lipoproteins are among the plasma proteins. There are different classes of lipoproteins. A total of six different classes are distinguished. Classification is based on physical density. HDL lipoproteins, for example, have a high density. LDL lipoproteins, on the other hand, are of rather low density. This different density of the individual complexes results predominantly from the ratio in which proteins and lipids are contained in the respective complex. In addition to HDL and LDL, VLDL, IDL, so-called chylomicrons and lipoprotein a belong to the lipoproteins. HDL can be subdivided into further subclasses and is also called alpha lipoprotein.

Anatomy and structure

Lipoproteins are particles of lipids and proteins. They are noncovalent aggregates, or proteids, corresponding to a conjugated form of proteins. They resemble micelles in their properties, which aggregate in a medium such as water. All lipoproteins have a nonpolar core. This core consists of cholesterol esters and triglycerides. Cholesterol esters are compounds between various fatty acids and cholesterol. The shell of lipoproteins is oriented toward the aqueous phase and consists of phospholipids, protein, and some hydroxyl groups of unesterified cholesterol. The shell is hydrophilic. In contrast, the core of lipoproteins is hydrophobic. Their density ranges up to 1.21 mg/l. Like density, the proportions of cholesteryl esters, triglycerides, and cholesterol differ with the subclasses.

Function and tasks

The subclasses of lipoproteins perform various functions in the body and serve primarily to transport water-insoluble lipids or fats, cholesteryl esters, and cholesterol through the blood system. To transport the substances, lipoproteins bind with the apoproteins they contain to receptor proteins of cells. The intestinal tract absorbs triglycerides and cholesterol. Via chylomicrons, the substances migrate into the lymphatic pathway and enter the veins through the thoracic duct. When the fatty acids are released, lipases act on the chylomicrons. This leaves chylomicron remnants in muscle and fat cells, which migrate back to the liver to be broken down. VLDL particles and its metabolites LDL and IDL serve to transport endogenously synthesized cholesterol. They can also take triglycerides from the chylomicrons. Together with the synthesized cholesterol, they transport the taken triglycerides into the tissues. HDL particles, on the other hand, serve to remove cholesterol from the tissues. Through the enzyme LCAT, they take up cholesterol esterified with fatty acids and return it to the liver. The subgroup of chylomicrons forms in the intestinal wall. They transport triglycerides exclusively out of the intestinal wall. With the bloodstream, they transport them to the liver cells. Transport into the fat and muscle cells also occurs via the bloodstream. VLDL is also formed in the cells of the liver. Lipoproteins transport triglycerides from the liver, where they are stored and re-synthesized. IDL are degradation products of VLDL that have no independent function. They are usually not detectable in blood plasma. LDL particles originate from liver cells and transport cholesterol esters and cholesterol through the body. This group can be divided into two subgroups. In addition to dense and small LDL particles, there are large LDL particles that are buoyant. HDL particles also originate from liver cells. Lipoprotein a, on the other hand, is a component of blood lipids.

Diseases

Disorders of lipid metabolism are common diseases. A lipid metabolism disorder is manifested primarily by a shifted composition of blood lipids. All lipid metabolism disorders actually mean impaired metabolism of lipoproteins. Hypercholesterolemia is one of the best-known lipometabolic disorders. Here, the cholesterol in the plasma is elevated. Hypertriglyceridemia, in which only the triglycerides are elevated, is to be distinguished from this.Both phenomena belong to the group of hyperlipidemias. The opposite is hypolipidemia, in which deficiency symptoms are present in the plasma. Lowered HDL, on the other hand, indicates hypolipoproteinemia. In dyslipoproteinemia, there is simultaneously too much LDL and too little HDL in the plasma. Hyperlipoproteinemia, on the other hand, is when only elevated LDL is present. Hyperlipidemias are particularly common in the Western world. An elevated LDL concentration is also referred to as excess bad cholesterol. HDL, on the other hand, is referred to as good cholesterol and should therefore not be lowered. However, due to the dietary habits of the Western world, Western society tends to have low HDL, while LDL is often decreased. This phenomenon is usually accompanied by insulin resistance. Diabetes thus develops as part of a metabolic syndrome. More than half of Western adults suffer from abnormal cholesterol levels, usually due to both diet and lifestyle. Poor blood lipid levels can promote atherosclerosis and strokes, as well as heart attacks and coronary heart disease. Obesity and alcoholism are considered risk factors, as are thyroid dysfunction and liver dysfunction or the use of certain medications.