Among the cellular components that can react directly with peroxynitrite in the presence of physiological CO(2) concentrations are sulfur-, selenium-, and metal-containing proteins, in particular hemoproteins. We have previously shown that the reactions of peroxynitrite with oxymyoglobin (MbFeO(2)) and oxyhemoglobin proceed via the corresponding ferryl species, which, in a second step, are reduced to the iron(III) forms of the proteins (metMb and metHb). In this study, we have investigated the influence of the concentration of added CO(2) on the kinetics and the mechanism of the peroxynitrite-mediated oxidation of MbFeO(2). We found that this reaction proceeds in two steps via the formation of MbFe(IV)=O also in the presence of millimolar amounts of CO(2). As compared to the values measured in the absence of added CO(2), the second-order rate constant for the first reaction step in the presence of 1.2 mM CO(2) is significantly larger [(4.1 +/- 0.7) x 10(5) M(-1) s(-1), at pH 7.5 and 20 degrees C], whereas that for the peroxynitrite-mediated reduction of MbFe(IV)=O to metMb is almost unchanged [(3.2 +/- 0.2) x 10(4) M(-1) s(-1), at pH 7.5 and 20 degrees C]. Finally, we show that because of the parallel decay of peroxynitrite, 8-25 equiv are needed to completely oxidize MbFeO(2) to metMb, with larger amounts required to oxidize diluted MbFeO(2) solutions in the presence of CO(2). Simulation of the reaction in the absence and presence of CO(2) was carried out to get a better understanding of the mechanism. The results suggest that CO(3)*(-) and NO(2)* may be involved in the reaction and interact with MbFeO(2) and MbFe(IV)=O, respectively.

The strong oxidizing and nitrating agent peroxynitrite has been shown to diffuse into erythrocytes and oxidize oxyhemoglobin (oxyHb) to metHb. Because the value of the second-order rate constant for this reaction is on the order of 10(4) M(-)(1) s(-)(1) and the oxyHb concentration is about 20 mM (expressed per heme), this process is rather fast and oxyHb is considered a sink for peroxynitrite. In this work, we showed that the reaction of oxyHb with peroxynitrite, both in the presence and absence of CO(2), proceeds via the formation of oxoiron(iv)hemoglobin (ferrylHb), which in a second step is reduced to metHb and nitrate by its reaction with NO(2)(*). In the presence of physiological relevant amounts of CO(2), ferrylHb is generated by the reaction of NO(2)(*) with the coordinated superoxide of oxyHb (HbFe(III)O(2)(*)(-)). This reaction proceeds via formation of a peroxynitrato-metHb complex (HbFe(III)OONO(2)), which decomposes to generate the one-electron oxidized form of ferrylHb, the oxoiron(iv) form of hemoglobin with a radical localized on the globin. CO(3)(*)(-), the second radical formed from the reaction of peroxynitrite with CO(2), is also scavenged efficiently by oxyHb, in a reaction that finally leads to metHb production. Taken together, our results indicate that oxyHb not only scavenges peroxynitrite but also the radicals produced by its decomposition.

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* 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*-/NO2*) 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).

The reactions of carbonate radical anion [systematic name: trioxidocarbonate(•1-)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N 2 O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions of CO 3 •with metMb and metHb involve only amino acid residues of the globin and no oxidation of the iron is observed. The second-order rate constants measured are (4.7 ( 0.3) × 10 7 and (1.9 ( 0.3) × 10 8 M -1 s -1 , for metMb and metHb, respectively. The carbonate radical anion-mediated oxidation of oxyHb proceeds in two steps: First, CO 3 •generates radical(s) in the globin which then, over a longer time scale, oxidize the iron center to finally produce ∼40% of metHb. The rate constants obtained for the two steps are (2.1 ( 0.1) × 10 8 and (1.0 ( 0.2) × 10 2 s -1 , respectively. For the reaction between CO 3 •and oxyMb, at all wavelengths studied we obtained kinetic traces that could be fitted to a single-exponential expression. Two distinct two step mechanisms were proposed to explain the kinetic data. The reaction of CO 3 •with oxyMb proceeds either according to a mechanism identical to that observed for the reaction with oxyHb but with a significantly faster rate of electron transfer from the globin radical(s) to the iron (>6 × 10 4 s -1 ) or according to a concurring mechanism in which CO 3 •oxidizes directly both ∼50% of the iron center and amino acid residue(s) of the globin.

The reactions of carbonate radical anion [CO3*-, systematic name: trioxidocarbonate*1-] with nitrosyl(II)hemoglobin (HbFe(II)NO) and nitrosyl(II)myoglobin (MbFe(II)NO) were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions proceed in two steps: outer-sphere oxidation of the nitrosyliron(II) proteins to their corresponding nitrosyliron(III) forms and subsequent dissociation of NO*. The second-order rate constants measured for the first reaction steps were (4.3 +/- 0.2) x 10(8) and (1.5 +/- 0.3) x 10(8) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. The reactions between nitrogen dioxide and MbFe(II)NO or HbFe(II)NO were studied by pulse radiolysis in N2O-saturated 0.1 M phosphate buffer pH 7.4 containing 5 mM nitrite. Also for the reactions of this oxidant with the nitrosyliron(II) forms of Mb and Hb a two-step reaction was observed: oxidation of the iron was followed by dissociation of NO*. The second-order rate constants measured for the first reaction steps were (2.9 +/- 0.3) x 10(7) and (1.8 +/- 0.3) x 10(7) M(-1) s(-1), for MbFe(II)NO and HbFe(II)NO, respectively. Both radicals appear to be able to oxidize the iron(II) centers of the proteins directly. Only for the reactions with HbFe(II)NO it cannot be excluded that, in a parallel reaction, CO3*- and NO2* first react with amino acid(s) of the globin, which then oxidize the nitrosyliron(II) center.