Cholesterol


Cholesterol

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 the 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 perhy-drocyclopentanophenanthrene 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, a young German chemist by the name of Adolf Windaus decided to concentrate on finding the molecular composition of the compound. Windaus, sometimes referred to as the Father of Steroid Chemistry, eventually worked out a detailed structure for cholesterol, an accomplishment that was partially responsible for his earning the 1928 Nobel Prize in chemistry.

Late research showed that the structure proposed by Windaus was in error. By the early 1930s, however, addi-

tional evidence from x-ray analysis allowed Windaus' longtime colleague Heinrich Wieland (among 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, the American chemist Robert B. Woodward 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) + 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.

But cholesterol 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, 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, 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 it production of the compound. If one's intake of cholesterol greatly exceeds the body's needs, excess cholesterol may then precipitate out of 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 Anitschow in the 1910s. An-itschow 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 Anitschow'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 on-going Framing-ham 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 fat and protein components, 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.

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