Globins are generally considered as carriers of diatomic gaseous ligands (e.g., O(2) and NO) in metazoa. Recently, the (pseudo-)enzymatic activity of globins towards reactive nitrogen and oxygen species has been elucidated. In particular, some globins (e.g., hemoglobin and myoglobin) catalyze the enzymatic scavenging of NO and peroxynitrite in the presence of H(2)O(2). Indeed, H(2)O(2) oxidizes some globins leading to the formation of water and of the heme-protein ferryl derivative, which, in turn, oxidizes NO and peroxynitrite leading to the formation of the globin ferric species, NO(2)(-), and NO(3)(-). Here, we hypothesize that NO, peroxynitrite, and H(2)O(2) are co-substrates for the peroxidase activity of some globins, this catalytic activity was reported in 1900 for the first time, even though the substrates have never been identified firmly up to now.