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Any of a group of organic compounds belonging to the general class of biochemicals called lipids, which are easily soluble in organic solvents and slightly soluble in water. Additional members of the lipid class include fatty acids, phospholipids, and triacylglycerides. The unique structural characteristic of steroids is a four-fused ring system. Members of the steroid family are ubiquitous, occurring, for example, in plants, yeast, protozoa, and higher forms of life. Steroids exhibit a variety of biological functions, from participation in cell membrane structure to regulation of physiological events. Naturally occurring steroids and their synthetic analogs are used extensively in medical practice.
Each steroid contains three fused cyclohexane (six-carbon) rings plus a fourth cyclopentane ring (see illustration). Naturally occurring steroids have an oxygen-containing group at carbon-3. Shorthand formulas for steroids indicate the presence of double bonds, as well as the structure and position of oxygen-containing or other organic groups.
The most abundant steroid in mammalian cells is cholesterol. The levels and locations of planar cholesterol molecules, embedded in the phospholipid bilayers that form cell and organelle membranes, are known to influence the structure and function of the membranes. A second major function of cholesterol is to serve as a precursor of steroids acting as physiological regulators (such as the steroid hormones). Enzyme systems present in a hormone-secreting gland convert cholesterol to the hormone specific for that gland. For example, the ovary produces estrogens (such as estradiol and progesterone); the testis produces androgens (such as testosterone); the adrenal cortex produces hormones that regulate metabolism (such as cortisol) and sodium ion transport (such as aldosterone). A third major function of cholesterol is to serve as a precursor of the bile acids. These detergentlike molecules are produced in the liver and stored in the gall bladder until needed to assist in the absorption of dietary fat and fat-soluble vitamins and in the digestion of dietary fat by intestinal enzymes. See Cell membranes, Cholesterol, Sterol
Some examples of diseases treated with naturally occurring or synthetic steroids are allergic reactions, arthritis, some malignancies, and diseases resulting from hormone deficiencies or abnormal production. In addition, synthetic steroids that mimic an action of progesterone are widely used oral contraceptive agents. Other synthetic steroids are designed to mimic the stimulation of protein synthesis and muscle-building action of naturally occurring androgens. See Hormone, Lipid
a group of organic compounds belonging by virtue of their chemical nature to the isoprenoids. The various types of steroids are widely distributed throughout the biological world and are encountered in microorganisms, plants, and animals. One of the main directions in the chemical evolution of steroids has been that toward specialization as biological regulators —hormones and other substances.
In a formal sense, all steroids are derivatives of the hypothetical hydrocarbon sterane (I, where R = H); biogenetically, the steroids derive from squalene, which is converted into the immediate polycyclic steroid precursors lanosterol (in animals) or cy-cloartenol (in plants). These precursors contain 30 carbon atoms (C30). Nearly all steroids are crystalline substances that possess optical activity and dissolve more readily in organic solvents than in water.
The classification of steroids is based on chemical structure and on the nature of the physiological effect or function. There are eight groups of steroids.
The first group comprises the sterols, which contain a branched side chain R made up of 8–10 carbon atoms. Sterols are components of plant and animal lipids, and the most important sterol—cholesterol—participates in the biosynthesis of steroid hormones.
The second group—D vitamins—is made up of unsaturated isomers of sterols (with ring B open). These isomers act to regulate calcium metabolism and the formation of the skeleton in vertebrates.
The third group includes the bile alcohols and bile acids, which contain a hydroxyl or carboxylic group in the side chain (consisting of eight or five carbon atoms). These substances aid in the digestion of food in the intestines of vertebrates.
The fourth group is that of the aglycones (genins) of steroid saponins and steroid glycoalkaloids. Typical representatives of this group are diosgenin (II, where X = O) and solasodine (II, where X = NH). Both groups of aglycones are characteristic of plants of the Liliaceae, Scrophulariaceae, and Solanaceae families, and in the form of glycosides, they have surface-active and hemolytic properties.
The fifth group comprises steroid alkaloids of other types. Among them are C27 alkaloids with modified steroid skeletons (jerveratrum, ceveratrum), which stimulate the contraction of striated muscle, C21 alkaloids, which possess bactericidal and amebicidal action, and modified C21 alkaloids from the glands of amphibians (samandarine, “toad poisons”), which are cardiotox-ic, as well as highly toxic to the central nervous system.
The sixth group is that of the cardiac genins, containing a side chain in the form of an unsaturated five-membered ring (C23 car-denolides) or six-membered lactone ring (C24 bufodienolides). These substances can strengthen the contraction of cardiac muscles by inhibiting the enzyme ATPase in the membrane of the heart cells. Cardenolides are found in many plants, while bufodienolides are found primarily in the venom from the cutaneous glands of toads.
The seventh group comprises the steroid sex hormones and the products of the hormones’ conversions. These substances determine the development and operation of the sexual system in animals. They include progesterone and related C21 compounds, in which the side chain R contains two carbon atoms, as well as the male sex hormones—androgens—which contain 19 carbon atoms, and the female sex hormones—estrogens—which contain 18 carbon atoms. A hydroxyl or carbonylic group replaces the side chain in androgens and estrogens.
The last group includes the hormones of the adrenal cortex—corticosteroids (where R = COCH2OH)—which regulate the balance of electrolytes and the metabolism of carbohydrates in vertebrates. Certain triterpene antibiotics (fusidic acid, cephalosporin P]) and other triterpenes are similar to steroids.
The biosynthesis of steroids is an operation more characteristic of higher vertebrates. Insects do not produce steroids and instead obtain these compounds from their food. However, the critical function of molting is controlled by a special type of C27 sterol known as ecdysone. Derivatives of progesterone serve as a means of chemical protection for a number of species of insects. Sexual reproduction in certain lower fungi (Achlya) is also induced by steroids. Steroids may participate in the morphogenesis of vascular plants. Inhibitors of the biosynthesis of steroids prevent the reproduction of certain fungi and the flowering of higher plants.
The chemistry, biochemistry, and physiological activity of steroids are under intense study in light of the great importance of steroids in medicine, veterinary science, and animal husbandry. In industry, chemical and microbiological methods have been introduced for the partial synthesis of steroid hormones from available raw materials (sterols, bile acids, saponins), and in the 1960’s and 1970’s methods have been introduced for complete chemical synthesis from the simplest starting materials. The synthesis of “artificial” steroid hormones with specialized physiological effects (contraceptive, anabolic), in particular, fluorine-containing and nitrogen-containing analogs, is acquiring increasing importance.
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E. P. SEREBRIAKOV