Sterols

STEROLS, substances belonging to the group of lipoids, entering into the composition of the unsaponifiable portion of fats and having wide distribution in the animal (zoosterols) and plant (phytosterols) world. Various S. accompany each other, which complicates the isolation of individual S. in a pure state. In their structure, S. are close to polyterpenes and represent highly atomized secondary alcohols; some of them are unsaturated compounds; individual S. differ from each other mainly in the number and position of double bonds. Chemical properties: in water, acids, and alkalis S. are insoluble; they dissolve in ether, alcohol, benzene, carbon disulfide, and other fat solvents; S. give insoluble compounds with digitonin, characteristic color reactions (see Cholesterol); they exhibit antihemolytic action with respect to saponins. They are optically active substances. S. are an integral part of most tissues and fluids of the body; the latter apparently possesses the ability¹ to synthesize S. S. introduced with food are partially absorbed through the intestinal wall, and partially undergo change in the intestine, since in the feces of carnivores their content is insignificant, while in the feces of herbivores they are completely absent. In the body S. are found either in the free state or in the form of esters; phytosterols can also be in the form of glycosides. The most studied and of greatest physiological interest among zoosterols is cholesterol (see) and coprosterol. Coprosterol is found in feces (absent with a milk diet), and is apparently a product of bacterial reduction of cholesterol in the intestine. It has the empirical formula C₂₇H₄₆O; contains no double bonds: it was artificially obtained by Windaus by catalytic reduction of cholesterol in the presence of Ni at 200°. It melts at 99-100°, [α]_D = +240. It gives the Salkowski test in a somewhat modified form. Lower animals contain, in addition to cholesterol, other S., its isomers; e.g., in the cocoons of the silkworm there is bombicesterol, in sponges—spongosterol and cliopasterol. Among plant S. belongs sitosterol, C₂₇H₄₆O, isolated from germinating wheat and rye seeds, linseed oil, from fat of Calabar beans, as well as other plants; it has two double bonds, gives color reactions similar to cholesterol. Stigmasterol, C₂₈H₄₈O, or C₃₀H₅₀O, together with sitosterol is found in Calabar beans; unlike it, it gives a tetrabromide derivative of acetate. Brassicasterol, C₂₈H₄₆O, is found together with other S. in rapeseed oil. Fungosterol and ergosterol are a component of various fungi, yeasts, and other plants; ergosterol has three double bonds, is most interesting from a physiological point of view, since 7-dehydro ergosterol acquires antirachitic action after irradiation (see Irradiated preparations). M. Karyagina. STERCOBILIN (from Latin stercus—dung, excrement and bilis—bile), reduced bilirubin, excreted with the feces. The main mass of the bile pigment entering the intestine undergoes reduction in it. Part of this reduced bilirubin is absorbed by the intestinal wall and in the liver is again converted into bilirubin, while another part is excreted outward with the feces. In pathological conditions, the absorbed hydrobilirubin passes from the blood into the urine, where it is called urobilin. Since the amount of bilirubin excreted by the bile ducts is an indicator of the breakdown of red blood cells, the total amount of S. excreted per day with the feces can be used to determine the degree of breakdown of red blood cells in the body. On the other hand, the presence or absence of S. in the feces serves as proof of the patency or closure of the external bile ducts. The amount of S. excreted with the feces per day, along with the amount of urobilin excreted, is a criterion for the intensity of blood pigment metabolism (see Pigment metabolism). An increase in S. excretion occurs with intensified hemolysis, primarily with hemolytic jaundice, with malaria, as well as with numerous poisonings by hemolytic poisons and with the resorption of hemorrhages in the body. S. completely disappears from the feces with complete obstruction of the bile ducts by a stone, tumor, or some other compressive factor (see Schmidt's tests). The reappearance of the reaction to S. in the feces in these cases has great diagnostic significance, as it indicates restored patency of the bile ducts. On the connection between S. and urobilin, see Urobilin, Jaundice, Hemolytic jaundice.