Specific features of radical generation in the reaction of thiols with hydrogen peroxide
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Specific features of radical generation in the reaction of thiols with hydrogen peroxide K. M. Zinatullina,a O. T. Kasaikina,a M. V. Motyakin,a,b I. S. Ionova,a E. N. Degtyarev,a,b and N. P. Khrameevab аSemenov
Federal Research Center of Chemical Physics, Russian Academy of Sciences, 4 ul. Kosygina, 119991 Moscow Russian Federation. Fax: +7 (495) 651 2191. E-mail: [email protected] bN. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, 4 ul. Kosygina, 119991 Moscow, Russian Federation
Using the method of inhibitors it was found that the reduction of Н2О2 with natural thiols in aqueous solutions is accompanied by the formation of radicals. The reaction of the nitroxyl radical TEMPO with glutathione (GSH) and Н2О2 was studied together and separately. It allowed us to determine the optimal conditions for using the spin trap method in the presence of thiols. In the case of 5,5´-dimethyl-1-pyrroline N-oxide the reaction of GSH and synthetic thiol N-acetylcysteine with hydrogen peroxide at pH < 7 leads to the formation of thiyl and hydroxyl radicals. Key words: glutathione, acetylcysteine, hydrogen peroxide, hydroxyl radicals, thiyl radicals, EPR, DMPO, TEMPO.
Natural endogenous thiols, glutathione (GSH), cysteine (CSH), and homocysteine (HSH), perform a variety of antioxidant functions: they interact with hydroxyl radicals, reduce hydroperoxides and disulfide bonds, and also prevent protein oxidation.1—4 In living organisms, the reduction of reactive oxygen species is carried out by glutathione peroxidases, which are enzymes specific for organs and tissues. Its use GSH as a substrate and efficiently reduce not only H2O2, but also organic hydroperoxides, including those of membrane polyunsaturated fatty acids. Glutathione, which exists in cells of living organisms in two forms (oxidized GSSG and reduced GSH), performs many physiological functions, as well as regulates protein functions, gene expression through thiol-disulfide exchange reactions, lymphocyte functions, and immune responses.5—10 All biological tissues and fluids contain large amounts of GSH (from 0.1 to 10 mmol L–1), which is significantly higher than the concentrations of other bioantioxidants. The synthetic thiol N-acetylcysteine (NSH) was used as a drug since the late 1980s, which, like GSH, is able not only to reduce disulfide bonds in proteins, but also exerts an antioxidant effect due to the presence of a free —SH group and the stimulation of glutathione biosynthesis.11,12 The redox pair GSH/GSSG and H2O2 play central roles in the determination of redox homeostasis and redox signaling.13—18 Studying the kinetics and mechanism of the reaction between GSH and H2O2, most researchers1,18—23 noted that the reaction of GSH and H2O2 proceeds stoichiometrically with the formation of a disulfide:
2 GSH + H2O2
GSSG + 2 H2O.
The rate of the reduction of Н2О2 with glutathione is of the first order in the GSH concentration and depends on the ratio of the starting components. Our studies of the kinetics of the reaction of GSH with Н2О2
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