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Any of a group of naturally occurring or synthetic organic compounds with a steroid ring structure, having a hydroxyl (—OH) group, usually attached to carbon-3. This hydroxyl group is often esterified with a fatty acid (for example, cholesterol ester). The hydrocarbon chain of the fatty-acid substituent varies in length, usually from 16 to 20 carbon atoms, and can be saturated or unsaturated. Sterols commonly contain one or more double bonds in the ring structure and also a variety of substituents attached to the rings. Sterols and their fatty-acid esters are essentially water insoluble. For transport in an aqueous milieu (for example, the bloodstream of mammals), sterols and other lipids are bound to specific proteins, forming lipoprotein particles. These particles are classified based on their composition and density. One lipoprotein class is abnormally high in the blood of humans prone to heart attacks.
Sterols are widely distributed in nature. Modifications of the steroid ring structure are made by specific enzyme systems, producing the sterol characteristic for each species, such as ergosterol in yeast. The major regulatory step in the sterol biosynthetic pathway occurs early in the process. Drugs that lower blood cholesterol levels in humans are designed to inhibit this regulatory enzyme. In addition to their conversion to sterols, several intermediates in the pathway are precursors of other important biological compounds, including chlorophyll in plants, vitamins A, D, E, and K, and regulators of membrane functions and metabolic pathways.
A universal role of sterols is to function as part of membrane structures. In addition, some insects require sterols in their diets. Cholesterol also serves as a precursor of steroid hormones (estrogens, androgens, glucocorticoids, and mineralocorticoids) and bile acids. See Cholesterol, Steroid
any one of the cyclic alcohols of the steroid group. Sterols are widely distributed throughout the biological world. The majority are optically active crystalline substances that are soluble in organic solvents but insoluble in water. The common precursor of sterols in microorganisms, plants, and animals is the hydrocarbon squalene, which is cyclyzed to lanosterol (C30H50O) or to lanosterol’s isomer cycloartenol; from these substances, various sterols, containing from 27 to 29 carbon atoms, are formed. The most important sterol in animals is cholesterol, and the most common sterol in fungi, including yeasts, is ergosterol. The most common of the broad group of plant sterols (phytoster-ols) are (β-sitosterol and stigmasterol.
In higher animals, sterols are contained in nerve tissue, liver, and sperm cells, where, forming esters with higher fatty acids, they serve as transmitting agents of the acids in the organism. In plants, sterols are found in the free state or in compounds with fatty acids (sterids), carbohydrates (phytosterolins), or both fatty acids and carbohydrates. Insects, which do not have the enzymes required for the first steps of the biosynthesis of sterols, use the sterols obtained from food for the synthesis of hormones. Sterols also include the hormones for the molting of insects and the substance antheridiol, which induces sexual reproduction in certain lower fungi. Substances similar to sterols include vitanolids (unsaturated lactones) and cucurbitazines (unsaturated bitter substances from cucurbitaceous plants), as well as sapogenins of marine invertebrates and the lanosterol and other triterpenes with a sterane carbon skeleton that are constituents of lanolin and waxes. The sterol functions that have been most thoroughly studied are the transformations of cholesterol into steroid hormones and of ergosterol into D vitamins under the action of ultraviolet light. Sterols are structural components of biological membranes. Readily available sterols (cholesterol, ergosterol, β-sitosterol) are used as raw materials in the industrial production of steroid hormones and D vitamins.
REFERENCESFieser, L., and M. Fieser. Steroidy. Moscow, 1964. (Translated from English.)
Haslewood, G. A. D. “Steroids in Marine Organisms.” Annals of the New York Academy of Sciences, 1960, vol. 90, p. 877.
Bean, G. A. “Phytosterols.” Advances in Lipid Research, 1973, vol. 11.
E. P. SEREBRIAKOV