Globins protect from the oxidative and nitrosative cell damage. Here, kinetics of peroxynitrite scavenging by ferryl sperm whale myoglobin (Mb-Fe(IV)=O) and human hemoglobin (Hb-Fe(IV)=O), between pH 5.8 and 8.3 at 20.0 degrees C, are reported. In the absence Of CO2, values of the second-order rate constant for peroxynitrite scavenging by Mb-Fe(IV)=O and Hb-Fe(IV)=O (i.e., for Mb-Fe(III) and Hb-Fe(III) formation; k(on)) are 4.6 x 10(4) M-1 s(-1) and 3.3 x 10(4) M-1 s(-1), respectively, at pH 7.1. Values of k(on) increase on decreasing pH with pK(a) values of 6.9 and 6.7, this suggests that the ONOOH species reacts preferentially with Mb-Fe(IV)=O and Hb-Fe(IV)=O. In the presence Of CO2 (=1.2 x 10(-3) M), values of k(on) for peroxynitrite scavenging by Mb-Fe(IV)=O and Hb-Fe(IV)=O are essentially pH-independent, the average k(on) values are 7.1 x 10(4) M-1 s(-1) and 1.2 x 10(5) M-1 s(-1), respectively. As a whole, Mb-Fe(IV)=O and Hb-Fe(IV)=O, obtained by treatment with H2O2, undertake within the same cycle H2O2 and peroxynitrite detoxification. (C) 2009 Elsevier Inc. All rights reserved