Cholesterol ester is a fatty acid-esterified cholesterol molecule. Cholesterol is a polycyclic alcohol with an enzyme that attaches a fatty acid molecule to the hydroxyl group with elimination of water. The cholesterol ester is a transport form of cholesterol, which can be easily broken down as an ester in the liver.
In the body, 75% of cholesterol is cholesterol ester. It serves as an intermediate and storage substance in the metabolism of the organism. That is why it is also an integral part of the human diet. The cholesterol esters are found mainly in animal food products. The most common fatty acids found in cholesterol esters are oleic acid, palmitic acid and linoleic acid. For the esterification of cholesterol, the enzyme lecithin-cholesterol acyltransferase is responsible. This enzyme is located in the HDL particles and also controls the esterification of cholesterol.
The HDL particles consist of cholesterol, triglycerides and lipoproteins. Bound to the lipoproteins, the water-insoluble cholesterol esters are made transportable and transported via the HDL particles from the organs to the liver. Esterification raises the density of these particles to become high-density lipoproteins (densely packed lipoproteins).
Cholesterol esters are only a transport form of cholesterol. They are bound to the lipoproteins in HDL, LDL or VLDL. Thus, cholesterol is both free and esterified with fatty acids. However, esterified cholesterol is more easily degradable by the liver. It can be transported very well using lipoproteins in the blood.
Its formation takes place in the High Density Lipoproteins. These are highly concentrated lipoproteins. The HDL transports cholesterol from the extrahepatic organs (organs outside the liver) to the liver, where it is then broken down into bile acids. The bile acids are excreted into the intestine via the bile and at the same time emulsify the fats, cholesterol and cholesterol esters absorbed by the diet. Over 90 percent of cholesterol converted into bile acid is reconverted and re-circulates. Cholesterol and cholesterol esters are the starting materials for the formation of hormones such as sex hormones, mineral corticosteroids and glucocorticoids (cortisone). Furthermore, they serve to form bile acids and vitamin D.
Cholesterol esters are formed in the HDL using the enzyme lecithin-cholesterol acyltransferase. The HDL or the high-density lipoproteins are responsible for the transport of cholesterol from the tissues of the body to the liver. This transport is also referred to as reverse cholesterol transport. In the liver, the esterified cholesterol can be broken down well. However, HDL can also transfer cholesterol esters to LDL or VLDL in exchange with triglycerides. Therefore, cholesterol esters are also found in the LDL and VLDL.
Generally, HDL is called good cholesterol because it transports tissue-derived cholesterol to the liver for breakdown. It has been found that the risk of developing atherosclerosis and cardiovascular disorders is lower at a high HDL concentration. In some cases even a slight regression of arteriosclerotic changes could be observed. For the breakdown of cholesterol in the liver, it is first necessary to dissolve the ester linkage between cholesterol and fatty acid. This requires a hormone-sensitive lipase. The ingested cholesterol esters are broken down into their individual components by a bile salt-activated lipase. So both the fatty acids and the cholesterol are released.
Within a cell, free cholesterol is also esterified by the so-called sterol-O-acyltransferase and converted into its storage form as a cholesterol ester. Thus, in the cytosol unwanted effects are avoided by free cholesterol. However, accumulations of cholesterol esters in macrophages or smooth muscle indicate incipient atherosclerosis.
In the cells, a balance develops between cholesterol synthesis and release of cholesterol from cholesterol esters. The degradation of cholesterol esters is accomplished by the so-called acid lipase. There are two very rare syndromes associated with loss of function or decreased activity of acid lipase.
Thus, a genetic defect on chromosome 10 affects a gene encoding the acidic lipase. With complete failure of this enzyme, cholesterol esters in the lysosomes can no longer be broken down. There is a decreased concentration of cholesterol in the cytoplasm of the cell. As a result, the loop is interrupted and there is an uncontrolled production of cholesterol. This also increases LDL receptor activity. The cell is now overcharged with cholesterol, eventually leading to cell death. This disease is usually fatal in the earliest childhood (at the age of three to six months). A much milder form of the disease is cholesterol ester storage disease (CEST).
Again, the same gene is affected. However, the acid lipase still has a residual function here, so that it affects the disease only in the liver. Due to this residual function, cholesterol ester degradation can still take place in most cells. However, due to the high metabolic activity in the liver, the slowed down degradation has an effect there. For example, elevated levels of cholesterol ester are stored in the liver. The disease usually manifests itself after the age of 18 years with an enlarged liver and an increased risk of developing atherosclerosis.Tags: