Kinetic and mechanistic studies of the first step of the reaction between thiols and selenite
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Kinetic and mechanistic studies of the first step of the reaction between thiols and selenite Ilia A. Dereven’kov1 · Pavel A. Molodtsov1 · Sergei V. Makarov1 Received: 5 August 2020 / Accepted: 20 September 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract Here, we report the results of kinetic and mechanistic studies of the first step of reaction between thiols (viz., cysteine, N-acetylcysteine, mercaptoethanol, thioglycolic acid and 5-thio-2-nitrobenzoic acid (TNB)) and selenite producing thiol-S-Selenites (RS-SeO2−) in aqueous solutions of various acidity. In this set of aliphatic thiols, their reactivity toward selenite in neutral medium is directly dependent on the acidity of thiol group, whereas the stability of RS-SeO2− is affected by the interaction with groups adjacent to S–Se bond. In the case of TNB, the reaction with selenite in neutral medium proceeds much slower than processes involving aliphatic thiols due to low nucleophilicity of TNB thiol group. Using Ellman’s reagent, we demonstrated that selenite binds substantially weaker to thiol group of cysteine residue in bovine serum albumin than to thiol group of free cysteine. Keywords Selenium · Selenite · Thiols · Serum albumin · Kinetics
Introduction Selenium is an ultra-microelement, which plays important roles in vivo [1]. In nature it is found in organic (e.g., selenocysteine, selenomethionine, methyl selenides, selenosugars) and inorganic (e.g., selenate, selenite, hydrogen selenide, selenocyanate) forms [1, 2]. Its main biological effects are predominantly associated with cycling of selenoenzymes, i.e. glutathione peroxidase (GPx), thioredoxin reductase (TrxR), iodothyronine deiodinase (DIO), and others, whose active sites contains selenocysteine, a selenium analogue of cysteine [3, 4]. However, high concentrations of Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1114 4-020-01877-7) contains supplementary material, which is available to authorized users. * Ilia A. Dereven’kov [email protected] 1
Department of Food Chemistry, Ivanovo State University of Chemistry and Technology, Sheremetevskiy str. 7, 153000 Ivanovo, Russia
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Reaction Kinetics, Mechanisms and Catalysis
selenium species are toxic [5], that can be partially explained by their prooxidant effect [6]. One of the most toxic selenium species is selenite (SeO32−). Its concentration increases in the environment because of anthropogenic activity [5, 7, 8], e.g. mining and metal refining, as well as evolution of flue gases. The pathway of SeO32− detoxification includes reduction to slightly toxic elemental selenium, Se(0), which is an important step in biogeochemical selenium cycling [9]. Se(0) was detected in sediments [10, 11], although it can be further metabolized by microorganisms [9, 12, 13]. Further Se(0) reduction produces H2Se, which combines with metal ions to give insoluble metal selenides found in mineral phase of sediments [9]. Reduction of SeO32− occurs via number of biotic and
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