NO• release from MbFe(II) NO and HbFe(II) NO after oxidation by peroxynitrite

In this work, we showed that the reaction of peroxynitrite with MbFe(II) NO, in analogy to the corresponding reaction with HbFe(II) NO (Herold, S. Inorg. Chem. 2004, 43, 3783-3785), proceeds in two steps via the formation of MbFe(III) NO, from which NO center dot dissociates to produce iron(III) myoglobin (Mb = myoglobin; Hb = hemoglobin). The second-order rate constants for the first steps are on the order of 10(4) and 10(3) M-1 s(-1), for the reaction of peroxynitrite with MbFe(II) NO and HbFe(II) NO, respectively. For both proteins, we found that the values of the second-order rate constants increase with decreasing pH, an observation that suggests that HOONO is the species responsible for oxidation of the iron center. Nevertheless, it cannot be excluded that the pH-dependence arises from different conformations taken up by the proteins at different pH values. In the presence of 1.2 mM CO2, the values of the second-order rate constants are larger, on the order of 10(5) and 10(4) M-1 s(-1), for the reaction of peroxynitrite with MbFe(II) NO and HbFe(II) NO, respectively. The pH-dependence of the values for the reaction with MbFe(II) NO suggests that ONOOCO2- or the radicals produced from its decay (CO3 center dot-/NO2 center dot-) are responsible for the oxidation of MbFe(II) NO to MbFe(III) NO. In the presence of large amounts of nitrite (in the tens and hundreds of millimoles range), we observed a slight acceleration of the rate of oxidation of HbFe(II) NO by peroxynitrite. A catalytic rate constant of 40 +/- 2 M-1 s(-1) was determined at pH 7.0. Preliminary studies of the reaction between nitrite and HbFe(II) NO showed that this compound also can oxidize the iron center, albeit at a significantly slower rate. At pH 7.0, we obtained an approximate second-order rate constant of 3 x 10(-3) M-1 s(-1).