Sterol

Sterols from a group of lipid resistant to saponification found in an appreciable quantity in all animal and vegetal tissues. Such unsaponifiable lipids may include one or more of a variety of molecules belonging to C27-C30 crystalline alcohols (in Greek, stereos, solid). These lipids can be classed as triterpenes as they derive from squalene giving directly by cyclization, unsaturation and 3b-hydroxylation, lanosterol in animals or cycloartenol in plants. They form an important group among the streroids.

In the tissues of vertebrates, the main sterol is the C27 alcohol cholesterol (Greek, chole, bile), particularly abundant in adrenals (10%, w/w), nervous tissues (2%,w/w), liver (0.2%,w/w) and gall stones, its fundamental carbon structure being a cyclopentanoperhydrophenanthrene ring (also called sterane). It was the first isolated sterol around 1770 by Poulletier de La Salle from gall stones. The correct formula (C27H46O) was proposed in 1888 by F. Reinitzer but structural studies from 1900 to 1932, mainly by H.O. Wieland "on the constitution of the bile acids and related substances" (Nobel Prize Chemistry 1927) and by A.O.R. Windaus on "the constitution of sterols and their connection with the vitamins" (Nobel Prize Chemistry 1928), led to the exact steric representation of cholesterol. In 1936, Callow and Young have designated steroids all compounds chemically related to cholesterol. In animals, another sterol, lanosterol (C30 compound) is found as a major constituent of the unsaponifiable portion of wool fat (lanoline) and was shown to be an intermediate in the biosynthesis of cholesterol. Animal tissues contain in addition to cholesterol small amounts of 7-dehydrocholesterol which , on UV irradiation, is converted to vitamin D3 (cholecalciferol). In higher plants, the first sterols were isolated by Hesse (1878) from the Calabar beans (Phytostigma venenosum) and coined the term "phytosterine". This substance was later named stigmasterol (Windaus and Hault, 1906) from the plant genus. The denomination "phytosterol" was proposed in 1897 (Thoms H) for all sterols of vegetal origin. Chemically, these sterols have the same basic structure as cholesterol but differences arise from the lateral chain on carbon 17. Most phytosterols are compounds having 28 to 30 carbon atoms and one or two carbon-carbon double bonds, typically one in the sterol nucleus and sometimes a second in the alkyl side chain. All phytosterols were shown to derive from cycloartenol, direct product of the cyclization of squalene in plants.