A mechanism for the conversion of oxyhemoglobin to methemoglobin by nitrite

Abstract

Each mole of oxyhemoglobin iron converted to methemoglobin causes the oxidation of 1.5 mol of nitrite to nitrate and consumes 1 mol of protons. No oxygen is liberated. The overall reaction has two simultaneously occurring parts. In the beginning the rate-limiting reaction converting O2Hb to metHb is directly proportional to H+ and NO2- concentrations and is independent of metHb. The second portion accounts in major part for the stoichiometry and rate of the overall reaction. In this portion O2Hb tetramers and metHbNO2- are the reactants. Essentially no reaction takes place in the presence of CN-, which displaces nitrite from the metHbNO2-, nor in the presence of 0.5 mol/liter Nal, which converts the O2Hb to alphabeta-dimers. The autocatalytic nature of the overall reaction in the presence of excess nitrite is the result of metHb, which is formed in both parts of the reaction, associating with nitrite to increase the concentration of one reactant of the cyanide-sensitive part. The reaction rates at constant pH in excess nitrite are porportional to the product of the O2Hb concentration and the square of the metHb concentration. The rate increases up to about 66% conversion of O2Hb followed by a decrease as the O2Hb becomes limiting. The dissociation constant of metHbNO2- at 25 degrees C and pH = 6.4 was found to be 1.11+/-0.11 mmol/liter.