Scientific studies of foreign and domestic scientists convincingly proved, that selenium is a vitally necessary microelement with a wide spectrum of biological action. Despite the small concentrations in the body, selenium has unique multi-functional functions - catalytic, structural, regulatory, - in the process of which he activates the action of many enzymes, vitamins, hormones and thus ensure the normal functioning of various biological systems, the implementation of numerous physiological and biochemical reactions in the living organism. The biochemical functions of selenium are not determined by the trace element itself, but by selenium proteins that contain the selenocysteine residue as an integral part of their active center. For today more than thirty such specific selenoproteins have been identified and identified in their pure form, the main of which are glutathione peroxidase, thioredoxine reductase, 5-iodothyronine deiodinase, selenoprotein P, and others. Glutathione peroxidases are the main antioxidant enzymes involved in peroxide oxidation of lipids in biological membranes, catalyze the reduction of hydrogen peroxide and almost all organic hydroperoxides, thus protecting the body's cells from reactive oxygen species. Thioredoxin reductase restores the SH-group in a specific thioredoxin protein, which is responsible for maintaining the oxidative-reduction potential in the cell, affects the products of eicosanoids, modulates the processes of inflammation and chemotaxis. Dayon dentiases of the I-III type are responsible for the exchange of thyroid hormones, are involved in the transformation of the "prohormone" of thyroxine into a more active form of influence - triiodothyronine, regulate and maintain a constant concentration of other thyroid hormones. Selenoprotein P is involved in the transport of selenium to various tissues, acts as an agent that helps neutralize toxic effects and remove heavy metals (Pt, Hg), protecting Leydig cells from active forms of oxygen, indirectly affects the synthesis of testosterone and the processes of spermatogenesis. Selenium plays an important role in the functioning of a nonspecific and specific immune system, prevents mutation of viruses and the emergence of new high-pathogenic strains. Selenium inhibits the formation of hyperplastic and enzyme-altered cells caused by aflatoxin B1 or T2 toxin, capable of destroying molds producing aflatoxins. Selenium is involved in the formation of mechanisms that determine the reproductive function, in particular, stimulates the hormonal function of the fetoplacental complex towards increasing the synthesis of estrogens, activates childbirth and postnatal involutional processes in the organs of the reproductive system, maintains the active function and structural integrity of the sperm, and thereby ensures their high mobility, preservation, and active penetration into the egg cell. The discovery of the biological properties of selenium has become the basis for its use initially in the prophylaxis and treatment of many illnesses and pain symptoms associated with the deficiency of this trace element, and subsequently - in order to increase the productivity of farm animals and poultry, improve the quality of their products and produce dietary food products of a functional purpose, with bio-correcting effect. Despite the considerable amount of conducted research, mechanisms of action of selenium on some metabolic processes in the body are still not fully clarified.
