The kinetics of the reaction of nitrogen dioxide with iron(II)- and iron(III) cytochrome c |
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Institution: | 1. Faculty of Chemistry, University of Wroc?aw, Joliot-Curie 14, 50–383 Wroc?aw, Poland;2. Department of Animal Physiology and Development, Institute of Experimental Biology, Adam Mickiewicz University, Umultowska 89, 61–614 Poznań, Poland;3. Department of Systematic Zoology, Adam Mickiewicz University, Umultowska 89, 61–614 Poznań, Poland;1. Faculty of Chemistry, University of Wroc?aw, Joliot-Curie 14, 50-383 Wroc?aw, Poland;2. Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznań, Poland;3. Department of Biotechnology, Chemistry and Pharmacy Via Aldo Moro, 2-53100 Siena, Italy |
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Abstract: | The reactions of NO2 with both oxidized and reduced cytochrome c at pH 7.2 and 7.4, respectively, and with N-acetyltyrosine amide and N-acetyltryptophan amide at pH 7.3 were studied by pulse radiolysis at 23 °C. NO2 oxidizes N-acetyltyrosine amide and N-acetyltryptophan amide with rate constants of (3.1±0.3)×105 and (1.1±0.1)×106 M?1 s?1, respectively. With iron(III)cytochrome c, the reaction involves only its amino acids, because no changes in the visible spectrum of cytochrome c are observed. The second-order rate constant is (5.8±0.7)×106 M?1 s?1 at pH 7.2. NO2 oxidizes iron(II)cytochrome c with a second-order rate constant of (6.6±0.5)×107 M?1 s?1 at pH 7.4; formation of iron(III)cytochrome c is quantitative. Based on these rate constants, we propose that the reaction with iron(II)cytochrome c proceeds via a mechanism in which 90% of NO2 oxidizes the iron center directly—most probably via reaction at the solvent-accessible heme edge—whereas 10% oxidizes the amino acid residues to the corresponding radicals, which, in turn, oxidize iron(II). Iron(II)cytochrome c is also oxidized by peroxynitrite in the presence of CO2 to iron(III)cytochrome c, with a yield of ~60% relative to peroxynitrite. Our results indicate that, in vivo, NO2 will attack preferentially the reduced form of cytochrome c; protein damage is expected to be marginal, the consequence of formation of amino acid radicals on iron(III)cytochrome c. |
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Keywords: | Peroxynitrite Protein radical Amino acid radical Tyrosine radical Tryptophan radical Kinetics Rate constant Pulse radiolysis Free radicals |
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