Metmyoglobin and Methemoglobin Catalyze the Isomerization of Peroxynitrite to Nitrate^†

Hemoproteins, in particular, myoglobin and hemoglobin, are among the major targets of peroxynitrite in vivo. The oxygenated forms of these proteins are oxidized by peroxynitrite to their corresponding iron(iii) forms (metMb and metHb). This reaction has previously been shown to proceed via the corresponding oxoiron(iv) forms of the proteins. In this paper, we have conclusively shown that metMb and metHb catalyze the isomerization of peroxynitrite to nitrate. The catalytic rate constants were determined by stopped-flow spectroscopy in the presence and absence of 1.2 mM CO₂ at 20 and 37 °C. The values obtained for metMb and metHb, with no added CO₂ at pH 7.0 and 20 °C, are (7.7 ± 0.1) × 10⁴ and (3.9 ± 0.2) × 10⁴ M^-1 s^-1, respectively. The pH-dependence of the catalytic rate constants indicates that HOONO is the species that reacts with the iron(iii) center of the proteins. In the presence of 1.2 mM CO₂, metMb and metHb also accelerate the decay of peroxynitrite in a concentration-dependent way. However, experiments carried out at pH 8.3 in the presence of 10 mM CO₂ suggest that ONOOCO₂^-, the species generated from the reaction of ONOO^- with CO₂, does not react with the iron(iii) center of Mb and Hb. Finally, we showed that different forms of Mb and Hb protect free tyrosine from peroxynitrite-mediated nitration. The order of efficiency is metMbCN < apoMb < metHb < metMb < ferrylMb < oxyHb < deoxyHb < oxyMb. Taken together, our data show that myoglobin is always a better scavenger than hemoglobin. Moreover, the globin offers very little protection, as the heme-free (apoMb) and heme-blocked (metMbCN) forms only partly prevent nitration of free tyrosine.