Biochemical Functions of Selenium

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Abstract
The metabolic role of selenium is largely determined by the reactions catalyzed by selenium-dependent enzymes. At present, two Secontaining glutathione peroxidases are known, one of which exclusively reacts with H2O2 and with a large variety of other hydroperoxides, however not with esterified phospholipid hydroperoxides. The second GSH peroxidase is an interfacial enzyme acting on lipid hydroperoxides including phosphatidyl-choline and cholesterol-hydroperoxide. Recently, a 5-deiodinase activity has been identified as a selenoenzyme which is responsible for the production of active thyroid hormone (T3) from its pro-hormone (T4). A plasma selenoprotein P has been isolated and sequenced which contains as many as 10 selenocysteine residues per molecule. Its function is still unknown. In spite of this increasing information, selenium deficiency in animals and man cannot be simply explained on an enzymological basis. Animal experiments showed that selenium is depleted from these proteins with different rates. Upon resupplementation with selenium, enzyme activities are restored with a different kinetics and a different selenium requirement. Also metabolic changes affecting protein turnover, CO2 exhalation and protein processing have been described in animals. The pattern of inflammatory mediators released upon a stimulus is also much greater in selenium-deficient white blood cells than in controls. These observations suggest a delicate network of interactions of selenium-dependent processes in the metabolism of animals which is by far more complex than previously anticipated.