steroid
Any of a class of natural or synthetic organic compounds with a molecular core, or nucleus, of 17 carbon atoms in a characteristic three-dimensional arrangement of four rings. The configuration of the nucleus, the nature of the groups attached to it, and their positions distinguish different steroids. Hundreds have been found in plants and animals and thousands more synthesized or made by modifying natural steroids. Steroids are important in biology, chemistry, and medicine. Examples include many hormones (including the sex hormones), bile acids, sterols (including cholesterol), and oral contraceptives (see contraception). Digitalis was the first steroid widely used in Western medicine. Corticosteroids (see cortisone) and their synthetic analogs are used to treat rheumatism and other inflammatory ailments. See also anabolic steroid.
steroid
Steroid
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.
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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
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