Cholesterol

Cholesterol is an unsaturated alcohol of formula C27H45OH that has long been known to be the principal constituent of human gall stones and has received notoriety in recent years for its connection with circulatory ailments, particularly hardening of the arteries. Cholesterol, either free or in the form of esters, actually is widely distributed in the body, particularly in nerve and brain tissue, of which it makes up about one sixth of the dry weight. The function of cholesterol in the body is not understood; experiments with labeled cholesterol indicate that cholesterol in nerve and brain tissue is not rapidly equilibrated with cholesterol administered in the diet. Two things are clear: Cholesterol is synthesized in the body and its metabolism is regulated by a highly specific set of enzymes. The high specificity of these enzymes may be judged from the fact that the very closely related plant sterols, such as sitosterol, are not metabolized by the higher animals, even though they have the same stereochemical configuration of all the groups in the ring and differ in structure only near the end of the side chain:

The accepted numbering system for the steroid nucleus and attached side chains is illustrated for cholesterol in 4. The methyl groups at the junction of rings A and B (C10) and rings C and D (C13) are called angular methyls. To avoid misinterpretation of structure and stereochemistry, methyl groups and hydrogens at ring junctions should be explicitly written as CH3 or HH. The stereochemistry is specified by a solid line if the atom or group is above the ring plane (β), and by a dashed line if below the ring plane (α). Thus compound 5, 5α-cholestan-3β-ol, which is obtained by the reduction of cholesterol, implies in the name that the hydroxyl at C3 is above the ring plane and that the hydrogen at C5 is below the ring plane.