Cholesterol
Cholesterol
Cholesterol (sometimes also known as cholesterin) is the principle sterol (an ester of a fatty acid and an aromatic [ring structure] complex alcohol) found in all animal tissues. Cholesterol is found in humans in both free and esterified forms. It was first isolated from humans in gallstones. Cholesterol is now commercially obtained from cattle spinal cords or from lanosterol—the fatty coating of sheep wool. High levels of cholesterol have been implicated in some forms of atherosclerosis (a disease caused by the thickening of arterial walls).
Cholesterol is a complex organic compound with the molecular formula C27H46O. It is a member of the biochemical family of compounds known as the lipids. Other lipids, such as waxes, fats, and oils, share not a structural similarity (as is the case with most families of compounds), but a physical property; they are all insoluble in water, but are soluble in organic liquids.
Cholesterol belongs more specifically to a class of compounds known as the steroids. Most steroids are naturally occurring compounds that play critical roles in plant and animal physiology and biochemistry. Other steroids include sex hormones, certain vitamins, and adrenocorticoid hormones. All steroids share a common structural unit, a four-ring structure known as the perhydrocyclopentanophenanthrene ring system or, more simply, the steroid nucleus.
History
Although cholesterol had been isolated as early as 1770, productive research on its structure did not begin until the twentieth century. Then, in about 1903, German chemist Adolf Windaus (1876–1959) decided to concentrate on finding the molecular composition of the compound. Windaus eventually worked out a detailed structure for cholesterol, an accomplishment that was partially responsible for his earning the 1928 Nobel Prize in chemistry.
Later research showed that the structure proposed by Windaus was in error. By the early 1930s, however, additional evidence from x-ray analysis allowed Windaus’ long-time colleague, German chemist Heinrich Wieland, (1877–1957), along with others, to determine the correct structure for the cholesterol molecule.
The next step in understanding cholesterol, synthesizing the compound, was not completed for another two decades. In 1951, American chemist Robert B. Woodward (1917–1979) completed that line of research when he synthesized cholesterol starting with simple compounds. For this accomplishment and his other work in synthesizing large molecule compounds, Woodward was awarded the 1965 Nobel Prize in chemistry.
Properties and occurrence
Cholesterol crystallizes from an alcoholic solution as pearly white or pale yellow granules or plates. It is waxy in appearance and has a melting point of 299.3°F (148.5°C) and a boiling point of 680°F (360°C) (with some decomposition). It has a specific gravity of 1.067. Cholesterol is insoluble in water, but slightly soluble in alcohol and somewhat more soluble in ether and chloroform.
Cholesterol occurs in almost all living organisms with the primary exception of microorganisms. Of the cholesterol found in the human body, about 93% occurs in cells and the remaining 7% in the circulatory system. The brain and spinal cord are particularly rich in the compound. About 10% of the former’s dry weight is due to cholesterol. An important commercial source of the compound is spinal fluid taken from cattle. Cholesterol is also found in myelin, the material that surrounds nerve strands. Gallstones are nearly pure cholesterol.
The concentration of cholesterol in human blood varies rather widely, from a low of less than 200 mg/dL (milligrams per deciliter) to a high of more than 300 mg/dL. It is also found in bile, a source from which, in fact, it gets its name: chole (Greek for bile) and stereos (Greek for solid).
Cholesterol in the human body
Cholesterol is a critically important compound in the human body. It is synthesized in the liver and then used in the manufacture of bile, hormones, and nerve tissue.
It is also a part of the human diet. A single egg yolk for example, contains about 250 mg of cholesterol. Organ meats are particularly rich in the compound. A 3-oz (85 g) serving of beef liver, for example, contains about 372 mg of cholesterol, and a similar-size serving of calves’ brain has about 2, 700 mg of the compound. Because diets differ from culture to culture, the amount of cholesterol an individual consumes differs widely around the world. The average European diet includes about 500 mg of cholesterol a day, but the average Japanese diet includes only about 130 mg a day. The latter fact reflects a diet in which fish rather than meat tends to predominate.
The human body contains a feedback mechanism that keeps the serum concentration of cholesterol approximately constant. The liver itself manufactures about 600 mg of cholesterol a day, but that output changes depending on the intake of cholesterol in the daily diet. As a person consumes more cholesterol, the liver reduces its production of the compound. If one’s intake of cholesterol greatly exceeds the body’s needs, excess cholesterol may then precipitate out of the blood and be deposited on arterial linings.
Cholesterol and health
Some of the earliest clues about possible ill effects of cholesterol on human health came from the research of Russian biologist Nikolai in the 1910s. Anitschkow (1885–1964) fed rabbits a diet high in cholesterol and found that the animals became particularly susceptible to circulatory disorders. Post-mortem studies of the animals found the presence of plaques (clumps) of cholesterol on their arterial walls.
Since Anitschkow’s original research, debate has raged over the relationship between cholesterol intake and circulatory disease, particular atherosclerosis (the blockage of coronary arteries with deposits of fatty material). Over time, it has become increasingly obvious that high serum cholesterol levels do have some association with such diseases. A particularly powerful study in forming this conclusion has been the ongoing Framingham Study, conducted since 1948 by the National Heart Institute in the Massachusetts town that has given its name to the research. Among the recommendations evolving out of that study has been that a reduced intake of cholesterol in one’s daily diet is one factor in reducing the risk of heart disease.
The cholesterol-heart disease puzzle is not completely solved. One of the remaining issues concerns the role of lipoproteins in the equation. Since cholesterol is not soluble in water, it is transported through the blood stream bound to molecules containing both
KEY TERMS
Bile —A greenish yellow liquid, secreted by the liver and stored in the gall bladder, that aids in the digestion of fats and oils in the body.
Circulatory system —The body system that circulates blood pumped by the heart through the blood vessels to the body tissues.
Lipid —A family of biochemical compounds soluble in many organic solvents, but not in water.
Steroids —A group of organic compounds that belong to the lipid family and that include many important biochemical compounds including the sex hormones, certain vitamins, and cholesterol.
Synthesis —A chemical process by which some new substance is produced by reacting other substances with each other
fat and protein components, called lipoproteins. These lipoproteins are of two kinds, high-density lipoproteins (HDLs) and low-density lipoproteins (LDLs). For some time, researchers have thought that LDL is particularly rich in cholesterol and, therefore, bad, while HDL is low in cholesterol and, therefore, good. While this analysis may be another step in the right direction, it still does not provide the final word on the role of cholesterol in the development of circulatory diseases.
See also Lipid; Nervous system.
Resources
BOOKS
Betteridge, John. Clinicians’ Guide to Lipids and Coronary Heart Disease. London, UK, and Arnold, NY: Oxford University Press, 2003.
Teaque, Michael L. Your Health Today: Choices in a Changing Society. Boston, MA: McGraw Hill, 2007.
OTHER
American Heart Association. “Cholesterol.” <http://www.americanheart.org/presenter.jhtml?identifier=4488> (accessed October 5, 2006).
National Heart, Lung, and Blood Institute. “Live Healthier, Live Longer: Cholesterol Counts For Everyone.” <http://www.nhlbi.nih.gov/chd/> (accessed October 5, 2006).
David E. Newton