Mechanism of autocatalytic oxidation of oxyhemoglobin by nitrite

Oxidation of oxyhemoglobin by nitrite is characterized by the presence of a lag phase followed by the autocatalysis. In phosphate buffer, an asymmetric ESR signal is detected at g = 2.005 (hereafter referred to as the g = 2 radical) during the oxidation which is similar to that of the methemoglobin free radical generated from methemoglobin and H₂O₂. Catalase and KCN prolong the oxidation, indicating the involvement of H₂O₂ and methemoglobin in the reaction. Superoxide dismutase, on the other hand, does not modify the oxidation. The present results suggest a chain reaction mechanism for the oxidation in which the g = 2 radical catalyzes the formation of NO₂ from NO₂ ^- by a peroxidase action and NO₂ oxidizes oxyhemoglobin. However, in N,N-bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane (bistris) buffer, superoxide dismutase markedly elongates the lag phase and accelerates the autocatalysis: bistris scavenges the g = 2 radical and a radical derived from bistris reduces O₂ to O₂ ^-.