Kinetics of superoxide-initiated reaction of nitroxides with cysteine
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2097
Kinetics of superoxide-initiated reaction of nitroxides with cysteine I. V. Tikhonov, L. I. Borodin, V. D. Sen´, and E. M. Pliss P. G. Demidov Yaroslavl State University, 14 ul. Sovetskaya, 150000 Yaroslavl, Russian Federation. Fax: +7(4852) 79 7751. E-mail: [email protected] Kinetics of the reduction of nitroxides with cysteine in the presence of a source of superoxide radicals was studied. The reactivity of nitroxides in this process is determined by the reduction potential of the N-oxoammonium cation / nitroxide pair. The rate-limiting step of the reaction is the nitroxide oxidation by the hydroperoxyl radical to the N-oxoammonium cation. Key words: nitroxides, cysteine, superoxide.
Depending on the conditions, redox active nitroxide radicals (>NO•) act as either reducing agents (antioxidants) or oxidants (pro-oxidants), the relevant reactions being often catalytic. As applied to biology, of most interest are the anioxidant properties of nitroxides, which make them capable of suppressing the oxidative stress. Nitroxides would scavenge the reactive oxygen species and thus protect the vital molecules of proteins, lipids,1 and DNA.2 These properties are due to the absence of significant barriers to one- and two-electron transformations over the redox triad N-oxoammonium cation (>N+=O)—nitroxide (>NO•)—hydroxylamine (>NOH).3,4 Similarly to the superoxide dismutase enzyme, nitroxides catal O •– + H+ disproportionation via oxidyze the HO2• 2 ation of >NO• with the HO2• radicals to >N+=O and reduction of >N+=O to >NO• upon the reaction with O2•–.5,6 In biological media, an important role belongs to the reactions of >NO• with endogenous antioxidants: ascorbate anion,7 glutathione, and the thiol groups of proteins.
R = H (1, TEMPO), OH (2), Cl (3), C(O)OH (4), CN (5), CH2C(O)NH2 (6), NH2 (7), NHAc (8)
However, nitroxides >NO• cannot be directly reduced with thiols, for example, with cysteine.8 Meanwhile, in the presence of a source of O2•– radical anions, >NO• is reduced with cysteine at a considerable rate, this rate being higher for six-membered cyclic >NO• than for fivemembered ones.8,9 The kinetics of this reaction was not studied in the cited publications; therefore, the observed dependences cannot be interpreted. This communication addresses the kinetics of cysteine reduction of >NO• radicals 1—10, which considerably differ in the redox properties.4 Experimental Nitroxide radicals (>NO•) 1—10 were synthesized as reported.3 Salts >N+=O (11) and >NOH (12) were obtained by known procedures.4 The kinetics of consumption of >NO• in the superoxideinitiated reaction with cysteine (RSH) at pH 7.4 and at 298 K was studied by ESR spectroscopy using an X-range Adani CMS 8400 spectrometer, field center of 3480 G, modulation frequency of 100 kHz, modulation amplitude of 2 G, and microwave power attenuation of 5 mW. The buffer solution was prepared by mixing 0.02 mol L–1 solutions of NaH2PO4 and Na 2HPO 4 additionally purified from traces of transition metals using the Chelex-100 ion exchange resin (Bio-Rad). UV sp
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