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Peroxynitrite is a strong oxidant involved in cell injury. In tissues, most of peroxynitrite reacts preferentially with CO₂ or hemoproteins, and these reactions affect its fate and toxicity. CO₂ promotes tyrosine nitration but reduces the lifetime of peroxynitrite, preventing, at least in part, membrane crossing. The role of hemoproteins is not easily predictable, because the heme intercepts peroxynitrite, but its oxidation to ferryl species and tyrosyl radical(s) may catalyze tyrosine nitration. The modifications induced by peroxynitrite/CO₂ on oxyhemoglobin were determined by mass spectrometry, and we found that αTyr42, βTyr130, and, to a lesser extent, αTyr24 were nitrated. The suggested nitration mechanism is tyrosyl radical formation by long-range electron transfer to ferrylhemoglobin followed by a reaction with ^•NO₂. Dityrosine (α24−α42) and disulfides (β93−β93 and α104−α104) were also detected, but these cross …