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 NO.2 from NO-2 by a peroxidase action and NO.2 oxidizes oxyhemoglobin. However, in N,M-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 O-2.