The Balancing of Peroxynitrite Detoxification between Ferric Heme-Proteins and CO: The Case of Zebrafish Nitrobindin.

Nitrobindins (Nbs) are all-β-barrel heme proteins and are present in prokaryotes and eukaryotes. Although their function(s) is still obscure, Nbs trap NO and inactivate peroxynitrite. Here, the kinetics of peroxynitrite scavenging by ferric Nb (-Nb(III)) in the absence and presence of CO is reported. The -Nb(III)-catalyzed scavenging of peroxynitrite is facilitated by a low pH, indicating that the heme protein interacts preferentially with peroxynitrous acid, leading to the formation of nitrate (91%) and nitrite (9%). The physiological levels of CO dramatically facilitate the spontaneous decay of peroxynitrite, overwhelming the scavenging activity of -Nb(III). The effect of -Nb(III) on the peroxynitrite-induced nitration of L-tyrosine was also investigated. -Nb(III) inhibits the peroxynitrite-mediated nitration of free L-tyrosine, while, in the presence of CO, -Nb(III) does not impair nitro-L-tyrosine formation. The comparative analysis of the present results with data reported in the literature indicates that, to act as efficient peroxynitrite scavengers in vivo, i.e., in the presence of physiological levels of CO, the ferric heme protein concentration must be higher than 10 M. Thus, only the circulating ferric hemoglobin levels appear to be high enough to efficiently compete with CO/HCO in peroxynitrite inactivation. The present results are of the utmost importance for tissues, like the eye retina in fish, where blood circulation is critical for adaptation to diving conditions.