Trace Elements(redirected from microelements)
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chemical elements present in organisms in low concentrations (usually thousandths of a percent or less). The term is also used to designate a number of chemical elements contained in soils, rock, minerals, and water. Precise quantitative criteria for distinguishing trace elements from major elements have not been established. A number of major elements of soils and rock, such as aluminum and iron, are trace elements for most animals and plants, as well as for humans.
Living organisms. Certain trace elements had been discovered in living organisms as early as the beginning of the 19th century, but their physiological significance remained unknown. V. I. Vernadskii established that trace elements are not random components of living organisms and that their distribution in the biosphere is determined by a number of principles. According to modern data, more than 30 trace elements are considered necessary for the vital activity of plants and animals. Most trace elements are metals (iron, copper, manganese, zinc, molybdenum, and cobalt); some are nonmetals (iodine, selenium, bromine, fluorine, and astatine).
In the organism, trace elements are found as constituents of various biologically active compounds, such as enzymes (for example, zinc in carbonic anhydrase, copper in polyphenol oxidase, manganese in arginase, and molybdenum in xanthine oxidase; about 200 metal enzymes are known), vitamins (cobalt in vitamin B12), hormones (iodine in thyroxine and zinc and cobalt in insulin), and respiratory pigments (iron in hemoglobin and other ferriferous pigments; copper in hemocyanin).
The action of trace elements that are components of the compounds listed above or that affect their functions is mainly evident in a change in the activity of metabolic processes in organisms. Some trace elements affect growth (manganese, zinc, and iodine in animals; boron, manganese, zinc, and copper in plants), reproduction (manganese and zinc in animals; manganese, copper, and molybdenum in plants), hematopoiesis (iron, copper, and cobalt), tissue respiration (copper and zinc), and cell metabolism. The quantitative distribution in tissues and organs of a number of trace elements found in organisms (scandium, zirconium, niobium, gold, and lanthanum) is unknown, and their biological role has not been clarified.
Soils. In soils, trace elements are constituents of various compounds, most of which are represented by insoluble and poorly soluble forms and only to a small extent by mobile forms that may be assimilated by plants. The mobility of trace elements and their availability to plants are greatly affected by the acidity of the soil, the humidity, and the content of organic matter. The content of trace elements is not the same in different types of soil. For example, chernozems are rich in mobile forms of boron and copper (0.4–1.5 and 4–30 mg/kg, respectively); soddy podzols are poor in them (0.02–0.6 and 0.1–6.7 mg/kg, respectively). There is an insufficiency of molybdenum in light soddy podzols, of cobalt in acidic soddy podzols, of manganese in chernozems, and of zinc in brown and chestnut soils. An insufficiency or excess of trace elements in soil leads to an insufficiency or excess of the elements in plant and animal organisms. In addition, there is a change in the nature of accumulation (deposition), an increase or decrease in the synthesis of biologically active compounds, a reorganization of processes of intermediate metabolism, and a development of new adaptations or the appearance of disturbances leading to endemic diseases of humans and animals. Thus, endemic ataxia results from an insufficiency of copper or a certain excess of molybdenum, sulfates, and possibly also lead. Endemic goiter in humans and animals results from an iodine insufficiency; acobaltosis results from a deficiency of cobalt in the soil. Boric enteritis complicated by pneumonia (in sheep) is due to an excess of boron. In various biogeochemical provinces, endemic diseases affect 5–20 percent of the livestock or the population of some other species.
An insufficiency or excess of trace elements is also harmful to plants. For example, a deficiency of molybdenum suppresses the blooming of cauliflower and some legumes. Copper insufficiency disrupts grain production of cereals and fruition of citrus trees. Harmful effects resulting from boron deficiency are poorly developed receptacles and the absence of flowering in peanuts, the withering of buds in apple and pear trees, drying of racemes on grapevines, and withering of peanuts and cabbage. Excess boron leads to rotting of roots, chlorosis, and widespread formation of galls.
In regions where the concentration of individual trace elements does not reach the minimum threshold level, endemic diseases may be prevented and treated by addition of the corresponding trace elements to livestock feed and the application of micronutrient plant fertilizers.
Livestock. Trace elements in animal feed are also used to increase livestock productivity. Salts of trace elements or aqueous solutions are added to silage and to concentrated and coarse feeds. Trace elements are components of many concentrates produced by the mixed-feed industry.
REFERENCESVinogradov, A. P. Geokhimiia redkikh i rasseiannykh khimicheskikh elementov v pochvakh, 2nd ed. Moscow, 1957.
Shaw, D. M. Geokhimiia mikroelementov kristallicheskikh porod. Lenin-grad, 1969. (Translated from French.)
Shkol’nik, M. la. Znachenie mikroelementov v zhizni rastenii i v zemledelii. Moscow-Leningrad, 1950.
Katalymov, M. V. Mikroelementy i mikroudobreniia. Moscow-Leningrad, 1965.
Evdokimov, P. D., and V. I. Artem’ev. Vitaminy, mikroelementy, biostimuliatory i antibiotiki v zhivotnovodstve. Leningrad, 1967. Berzin’, la. M., and V. T. Samokhin. Mikroelementy v zhivotnovodstve. Moscow, 1968.
Koval’skii, V. V., and G. A. Andrianova. Mikroelementy v pochvakh SSSR. Moscow, 1970.
Koval’skii, V. V., lu. I. Raetskaia, and T. I. Gracheva. Mikroelementy v rasteniiakh i kormakh. Moscow, 1971.
Zhiznevskaia, G. la. Med’ molibden i zhelezo v azotnom obmene bobovykh rastenii. Moscow, 1972.
A. R. VAL’DMAN and G. IA. ZHIZNEVSKAIA
Humans. The major source of intake of trace elements for humans is food of plant and animal origin. Drinking water covers only 1–10 percent of the daily requirements of such trace elements as iodine, copper, zinc, manganese, cobalt, and molybdenum and is a major source only for fluorine and strontium. The content of various trace elements in the diet depends on the geochemical conditions of the locality in which the individual’s foodstuffs were obtained, as well as the selection of food products in the diet. In modern dietary practice for the population of developed countries, characteristic diets include a variety of food products, a significant portion of which is produced far from the site of consumption, thus eliminating conditions that foster the effects of local geochemical factors on an individual.
Only two trace elements may be truly given as etiological factors of endemic human disease: iodine, a deficiency of which permits the development of endemic goiter, and fluorine, an excess of which results in fluorosis and a deficiency of which leads to caries.
Trace elements are unevenly distributed in the organism. An increase in their accumulation in a certain organ is to a significant extent related to the physiological role of the element and the specific activity of the organ (for example, the accumulation of zinc in the gonads and its effect on reproductive function). In other cases, trace elements act on organs and affect functions unrelated to the site of their accumulation in the body.
The content of many trace elements (aluminum, titanium, chlorine, lead, fluorine, strontium, and nickel) increases with age. The increase is relatively rapid during the period of growth and development but slows down or ceases at age 15–20. There is evidence that the content of cobalt, copper, and nickel in the blood and strontium in the skeleton are somewhat lower at age 50–60 than at age 20–25. The absolute level of trace elements in organs and tissues may vary markedly, depending on place of residence, usual diet, and other factors that determine the level of intake and accumulation of a given trace element, as well as on individual traits.
The concentration of a number of elements in the blood has been found to be maintained at a relatively stable level (cobalt at 4–8 microgram [jutg] percent, copper at 80–140 ju,g percent, and iron at 45–60 fig percent), whereas others, such as strontium, lead, and fluorine, are not subject to such regulation, and their content in the blood varies with the level of intake of the element.
In the blood, most trace elements are bound to proteins (copper in cuproproteins and ceruloplasmin, zinc in carbonic anhydrase, cobalt as a component of vitamin B12 and in protein-bound form, and iron in transferrin). Some elements, such as lithium, are present in the blood in ionic form; about 50 percent of strontium and fluorine are found in the mineral structures of bone, enamel, and dentin.
According to their physiological function in the body, a distinction is made between required trace elements (cobalt, iron, copper, zinc, manganese, iodine, fluorine, and bromine) and probably required trace elements (aluminum, strontium, molybdenum, selenium, and nickel); the role of bismuth and silver, which are regularly found in tissues, remains unclarified.
The functions of trace elements in the body are very important and variegated. For example, small amounts of manganese stimulate hematopoiesis and immune reactions, whereas larger amounts suppress them. An increase in the concentration of fluorine in drinking water to 1.0–1.5 milligrams per liter (mg/l ) leads to a decrease in caries, whereas an increase to 2–3 mg/l results in fluorosis.
Interaction of the trace elements in the body is noted. Cobalt has a positive effect on hematopoiesis only in the presence of sufficient quantities of iron and copper in the body; manganese increases the absorption of copper, whereas copper in certain aspects is an antagonist of molybdenum; and fluorine affects strontium metabolism.
The use of trace elements in clinical medicine is still limited. Preparations of cobalt, iron, copper, and manganese are used effectively in the treatment of some types of anemia. Bromine and iodine are also used as pharmacological agents. The advances of public health in the use of trace elements are considerable (iodized salt and bread for the prevention of endemic goiter and fluoridation of water to reduce caries). In cases where excess fluorine is present in natural waters, defluoridizing devices are used.
REFERENCESVoinar, A. O. Biologicheskaia rol’ mikroelementov v organizme zhivotnykh i cheloveka, 2nd ed. Moscow, 1960.
Mikroelementy [collection of articles]. Moscow, 1962. (Translated from English.)
Mikroelementy v se’skom khoziaistve i meditsine. Kiev, 1963.
Babenko, G. A. Mikroelementy v eksperimental’noi i klinicheskoi meditsine. Kiev, 1965.
Shustov, V. la. Mikroelementy v gematologii. Moscow, 1967.
Azizov, M. A. O kompleksnykh soedineniiakh nekotorykh mikroelementov s bioaktivnymi veshchestvami, 2nd ed. Tashkent, 1969.
Kolomiitseva, M. G., and R. D. Gabovich. Mikroelementy v meditsine. Moscow, 1970. (References.)
V. A. KNIZHNIKOV [16–732-1; updated]