In several enzymatic systems, selenium (an essential element commonly incorporated as sodium selenite to the human diet) is bonded through a mercapto group of a cysteine residue. The reaction of Se IV (as selenite) with different thiols in aqueous media was studied to increase the information about the mechanism of the enzyme reaction. The influence of the reagents concentration on the reaction rate and temporal spectral changes was analyzed. A complex with a UV maximum between 270 and 290 nm was formed, which quickly decayed when the thiol concentration was increased. The proposed reaction mechanism includes a fast Se IV -thiol association step followed by two parallel steps (one is first-order in the complex and the other one is globally second-order). Kinetics and equilibrium constants were obtained, and the rate dependence of the thiol oxidation on the Se IV concentration was modelled for all thiols tested. Very good agreement between the experimental and calculated curves was observed. The role of selenium as an essential element was demonstrated in 1954 when it was observed that low ingestion levels were associated with cardiopathies, specially in children, and with diseases in cattle [7]. It was reported that this alterations were reverted by addition of sodium selenite [8] to the diet, and numerous commercial products exist today to supply this element.Selenium is incorporated in the proteins [9] glutathione peroxidase (25%), selenoprotein P (68%), whose biological function is not yet completely understood [10] [11], and albumin A (7%) [12]. Glutathione peroxidase [13] is the bestcharacterized enzymatic system containing selenium. It is formed by four subunits bonded to a Se-atom as a selenocysteine residue. This enzyme transforms glutathione into the oxidized form, with the corresponding reduction of hydrogen peroxide and peroxides formed during the lipid oxidation in membranes. Therefore, it is nowadays used in antioxidant formulations.It is well known that the paths followed by selenate and selenite in the organism are different. Both can be used as a nutritional supplement, but whereas selenite,
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