STEROID

any of a class of natural or synthetic organic chemical compounds characterized by a molecular structure of 17 carbon atoms arranged in four rings. In the parent structure (named gonane and referred to as the steroid nucleus), the carbon atoms are bonded to 28 hydrogen atoms. Steroids are important in biology, chemistry, and medicine. They include the sex hormones, adrenal cortical hormones, bile acids, sterols, anabolic agents, and oral contraceptives. The steroid nucleus is three dimensional. Steroids vary from one another not only in the nature of the attached groups but also in the configuration of the steroid nucleus and the position of the groups. Small modifications in the molecular structures of steroids can produce remarkable differences in their biological activities. Chemists have isolated hundreds of steroids from plants and animals. Thousands more have been made by treating natural steroids chemically or by synthesis. Organic solvents are used to isolate steroids from natural sources. Sterols, the most abundant of the steroids, are treated with an alkali and then extracted by means of water-immiscible solvents, such as hexane or ether. Highly purified steroids can be obtained in the laboratory by these methods. Commercially large amounts of steroids are usually purified by repeated crystallization from solvents. In plants and animals, steroids appear to be biosynthesized by similar reactions, beginning with acetic acid, assisted by a type of enzyme. The isoprenoid hydrocarbon called squalene, which occurs widely in nature, is thought to be the starting material from which all steroids are made. Enzymatic transformation of squalene produces lanosterol in animals and cycloartenol in plants, which yield cholesterol in both animals and plants. Cholesterol is then converted to bile acids and steroid hormones in animals and to steroids such as alkaloids in plants. The commercial and laboratory synthesis of steroids usually begins with a one-ring starting material such as quinone, upon which other rings are built. Total synthesis of steroids has proved commercially feasible, but it is often more practical to prepare them by modifying other steroids that are naturally abundant. Certain microbes can transform parts of the steroid molecule, and industrial steroids are often made by a combination of chemical and microbiological techniques. All the sex hormones and corticosteroids, which originate in the adrenal cortex, are derived from one of the most widely occuring steroidsnamely, cholesterol. Corticosteroids play an essential role in maintaining life through a variety of hormonal functions that help to balance the ionic composition of the body fluids. Bile acids, a type of steroid found in mammals, play a role in emulsifying fats during digestion. Cholesterol, in addition to its role as a precursor of steroid hormones, is an important component of cell membrane. Unfortunately, in some persons excess serum cholesterol contributes to the formation of deposits on the arterial walls, which leads to atherosclerosis. The first therapeutic use of steroids goes back to the 18th century when foxglove extracts were found to be beneficial for some heart conditions. The active ingredient in these preparations, digitalis, is still used today. It is a steroid glycoside, a molecule in which a steroid is linked to a sugar residue. Many plant steroids are cardiac glycosides, which in large doses can be fatal and may be used by the plant to ward off predatory insects. Some toads secrete steroid glycosides that may also act as a defense mechanism. Corticosteroids and their synthetic analogues, such as prednisone and dexamethasone, are used therapeutically to control rheumatism and other inflammatory ailments. Anabolic steroidswhich increase constructive metabolismare sometimes administered to postoperative and geriatric patients to promote muscle growth and tissue regeneration. In recent years, a growing number of amateur and professional athletes have made use of synthetic analogues of testosterone to accelerate muscular development and to improve strength. Medical researchers have determined that anabolic steroids can have harmful effects, particularly in young people who are still developing physically. Continued and prolonged use may lead to heart disease, sexual and reproductive disorders, immune deficiencies, liver damage, stunted growth (in teenagers and young adults), and aggressive, violent behaviour. The most widely employed steroid drugs, however, are undoubtedly the oral contraceptives, which were introduced in the early 1960s. These synthetic materials, which act by suppressing ovulation, are made chiefly from diosgenin, a plant steroid obtained from wild yams.