cholesterol(redirected from HDL cholesterol)
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cholesterol(kəlĕs`tərōl'), fatty lipidlipids,
a broad class of organic products found in living systems. Most are insoluble in water but soluble in nonpolar solvents. The definition excludes the mineral oils and other petroleum products obtained from fossil material.
..... Click the link for more information. found in the body tissues and blood plasma of vertebrates; it is only sparingly soluble in water, but much more soluble in some organic solvents. A steroidsteroids,
class of lipids having a particular molecular ring structure called the cyclopentanoperhydro-phenanthrene ring system. Steroids differ from one another in the structure of various side chains and additional rings. Steroids are common in both plants and animals.
..... Click the link for more information. , cholesterol can be found in large concentrations in the brain, spinal cord, and liver. The liver is the most important site of cholesterol biosynthesis, although other sites include the adrenal glands and reproductive organs. By means of several enzymatic reactions, cholesterol is synthesized from acetic acidacetic acid
, CH3CO2H, colorless liquid that has a characteristic pungent odor, boils at 118°C;, and is miscible with water in all proportions; it is a weak organic carboxylic acid (see carboxyl group). Glacial acetic acid is concentrated, 99.
..... Click the link for more information. ; it then serves as the major precursor for the synthesis of vitamin D3, of the various steroid hormoneshormone,
secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
..... Click the link for more information. , including cortisolcortisol
steroid hormone that in humans is the major circulating hormone of the cortex, or outer layer, of the adrenal gland. Like cortisone, cortisol is classed as a glucocorticoid; it stimulates liver glycogen formation while it decreases the rate
..... Click the link for more information. , cortisonecortisone
, steroid hormone whose main physiological effect is on carbohydrate metabolism. It is synthesized from cholesterol in the outer layer, or cortex, of the adrenal gland under the stimulation of adrenocorticotropic hormone (ACTH).
..... Click the link for more information. , and aldosteronealdosterone
, steroid secreted by the cortex of the adrenal gland. It is the most potent hormone regulating the body's electrolyte balance. Aldosterone acts directly on the kidney to decrease the rate of sodium-ion excretion (with accompanying retention of water), and to
..... Click the link for more information. in the adrenal glandsadrenal gland
or suprarenal gland
, endocrine gland (see endocrine system) about 2 in. (5.1 cm) long situated atop each kidney. The outer yellowish layer (cortex) of the adrenal gland secretes about 30 steroid hormones, the most important of which are aldosterone and
..... Click the link for more information. , and of the sex hormones progesterone, estrogen, and testosterone. Cholesterol is excreted from the liver in the form of a secretion known as bilebile,
bitter alkaline fluid of a yellow, brown, or green color, secreted, in man, by the liver. Bile, or gall, is composed of water, bile acids and their salts, bile pigments, cholesterol, fatty acids, and inorganic salts.
..... Click the link for more information. ; it sometimes crystallizes in the gall bladdergall bladder,
small pear-shaped sac that stores and concentrates bile. It is connected to the liver (which produces the bile) by the hepatic duct. When food containing fat reaches the small intestine, the hormone cholecystokinin is produced by cells in the intestinal wall and
..... Click the link for more information. to form gallstones. The insolubility of cholesterol in water is also a factor in the development of atherosclerosis (see arteriosclerosisarteriosclerosis
, general term for a condition characterized by thickening, hardening, and loss of elasticity of the walls of the blood vessels. These changes are frequently accompanied by accumulations inside the vessel walls of lipids, e.g.
..... Click the link for more information. ), the pathological deposition of plaques of cholesterol and other lipids on the insides of major blood vessels, a condition associated with coronary artery diseasecoronary artery disease,
condition that results when the coronary arteries are narrowed or occluded, most commonly by atherosclerotic deposits of fibrous and fatty tissue. Coronary artery disease is the most common underlying cause of cardiovascular disability and death.
..... Click the link for more information. . This buildup of cholesterol in the blood vessels may constrict the passages considerably and inhibit the flow of blood to and from the heart. Recent research has shown that the relative abundance of certain protein complexes, called lipoproteins, to which cholesterol becomes attached may be the real cause of cholesterol buildup in the blood vessels. High-density lipoprotein (HDL) carries cholesterol out of the bloodstream for excretion, while low-density lipoprotein (LDL) carries it back into the system for use by various body cells. Researchers believe that HDL and LDL levels in the bloodstream may be at least as important as cholesterol levels, and now measure both to determine risk for heart disease. Reducing consumption of foods containing cholesterol and saturated fat has been found to lower blood cholesterol levels. Cholesterol levels can also be reduced with drugs, most especially with HMG-CoA reductase inhibitors (commonly called "statins"), such as lovastatin (Mevacor) and atorvastatin (Lipitor), and by regular exercise.
A cyclic hydrocarbon alcohol commonly classified as a lipid because it is insoluble in water but soluble in a number of organic solvents. It is the major sterol in all vertebrate cells and the most common sterol of eukaryotes. In vertebrates, the highest concentration of cholesterol is in the myelin sheath that surrounds nerves and in the plasma membrane that surrounds all cells. See Lipid
Cholesterol can exist either in the free(unesterified) form (see structure below) or in the esterified form, in which a fatty acid is bound to the hydroxyl group of cholesterol by an ester bond. The free form is found in membranes. Cholesteryl esters are normally found in lipid droplets either within the cells of steroidogenic tissues, where it can be converted to free cholesterol and then to steroid hormones, or in the middle of spherical lipid-protein complexes, called lipoproteins, that are found in blood. See Cell membranes
Cholesterol, together with phospholipids and proteins, is important in the maintenance of normal cellular membrane fluidity. At physiological temperatures, the cholesterol molecule interacts with the fatty acids of the membrane phospholipids and causes increased packing of the lipid molecules and hence a reduction of membrane fluidity. Thus, all vertebrate cells require cholesterol in their membranes in order for the cell to function normally. Cholesterol is also important as a precursor for a number of other essential compounds, including steroid hormones, bile acids, and vitamin D. See Lipid metabolism; Steroid
Cellular cholesterol is obtained both from the diet, following its absorption in the intestine, and from synthesis within all cells of the body. Foods that are particularly high in cholesterol include eggs, red meat, and organs such as liver and brain. About 40–50% of the dietary cholesterol is absorbed from the intestine per day. In contrast, plant sterols are very poorly absorbed. Cholesterol synthesis occurs in all vertebrate cells but is highest in the liver, intestine, and skin, and in the brain at the time of myelination.
Cholesterol and cholesteryl esters are essentially insoluble in water. In order to transport these compounds around the body in the blood, the liver and intestine produce various lipid-protein complexes, called lipoproteins, which serve to solubilize them. Lipoproteins are large, complex mixtures of cholesterol, cholesteryl esters, phospholipids, triglycerides (fats), and various proteins. The major lipoproteins include chylomicrons, very low density lipoprotein (VLDL), low-density lipoprotein, and high-density lipoprotein (HDL).
Total plasma cholesterol levels of less than 200 mg/100 ml are considered desirable. Values of 200–239 or greater than 239 mg per 100 ml are considered, respectively, borderline high or high risk values, indicating the potential for a heart attack. High levels of low-density lipoprotein in the plasma are associated with increased risk of atherosclerosis, (“hardening of the arteries”), which involves deposition of cholesterol and other lipids in the artery wall. Diets low in cholesterol and saturated fats often result in a reduction in total plasma and LDL cholesterol levels. Such changes in blood cholesterol levels are thought to be beneficial and to reduce the incidence of heart attacks.
an organic compound of the steroid class; the most important sterol in animals (seeSTEROL).
Cholesterol was first isolated from a gallstone (hence the name, from the Greek chole, “bile”). It is obtained as colorless crystals, with a melting point of 149°C. It is insoluble in water but readily soluble in nonpolar organic solvents. A characteristic chemical property of cholesterol is the capacity to form molecular complexes with many salts, acids, amines, proteins, and such neutral compounds as saponins and vitamin D3 (cholecalciferol).
Cholesterol is present in virtually all living organisms, including bacteria and blue-green algae. Its content in plants is usually low except in seed oils and pollen. In vertebrate animals, large amounts of cholesterol are found in nerve tissue lipids, where it is bound to structural elements of the myelin sheath, as well as in ova and sperm cells, the liver (the principal organ of cholesterol biosynthesis), the adrenal glands, the cutaneous sebum, and erythrocyte cell walls. It is present in blood plasma in the form of complex esters of higher fatty acids (for example, oleic acid) and serves as a carrier during their transport. These esters are formed in the intestinal walls with the participation of the enzyme cholesterol esterase. Most organisms, except some microorganisms, annelid worms, mollusks, echinoderms, and sharks, can synthesize cholesterol from squalene (seeSQUALENE).
The most important biochemical function of cholesterol in vertebrates is its conversion to the hormone progesterone in the placenta, testes, corpus luteum, and adrenal glands. This conversion initiates the chain of biosynthesis of steroid sex hormones and corticosteroids. Another result of cholesterol metabolism in vertebrates is the formation of bile acids and vitamin D3. Cholesterol also participates in the regulation of cell permeability and protects erythrocytes from hemolytic toxins. In insects, the cholesterol present in their food is used to synthesize ecdysones, the molting hormones.
In some animals, the cholesterol level is controlled by a feedback mechanism: when excess dietary cholesterol is ingested, cholesterol biosynthesis is inhibited. Human beings lack such a mechanism, and consequently the blood level of cholesterol, which is normally 150–200 mg percent, may rise considerably, especially in individuals between 30 and 60 years of age whose diet is high in fat. This leads to the blockage of the bile ducts and encourages fatty infiltration of the liver, the formation of gallstones, and the deposition of atherosclerotic plaques on the walls of blood vessels containing cholesterol. Cholesterol is excreted mainly with feces, in the form of coprosterol, or coprostanol.
In the pharmaceutical industry, cholesterol is used as a starting material in the production of many steroid preparations. Its main source is the bone marrow of dead cattle.
REFERENCESBiosintez lipidov: Simpozium VII. Moscow, 1962. (Tr. V Mezhdunarodnogo biokhimicheskogo kongressa, vol. 7.)
Miasnikov, A. L. Gipertonicheskaia bolezn’ i ateroskleroz. Moscow, 1965.
Heftmann, E. Biokhimiia steroidov. Moscow, 1972. (Translated from English.)
Schwartzman, A. Cholesterol and the Heart. New York, 1965.
E. P. SEREBRIAKOV