Conversion of metmyoglobin to NO myoglobin in the presence of nitrite and reductants.

Formation of NO myoglobin through the reaction of horse heart metmyoglobin with NADH in the presence of nitrite was observed optically at pH 5.5. Superoxide generation during the reaction was demonstrated using the ESR spin trap, 5,5-dimethyl-1-pyrroline-1-oxide. A weak optical spectrum corresponding to oxymyoglobin appeared transiently and the spectrum of NO myoglobin then developed. The conversion to NO myoglobin was eliminated in the presence of catalase, SOD or 5,5-dimethyl-1-pyrroline-1-oxide. The kinetics of NADH oxidation and oxygen consumption catalyzed by myoglobin showed an initial lag phase, indicating a chain reaction. When the oxygen was exhausted, the NO form emerged. The duration of the lag phase was prolonged by an increase in the concentration of catalase, SOD or 5,5-dimethyl-1-pyrroline-1-oxide, whereas it disappeared in the presence of H2O2. The spectral change from metmyoglobin to NO myoglobin was also observed under anaerobic conditions though the rate was slower than that obtained under aerobic conditions, while the spectral change was accelerated in the presence of H2O2. Nitric oxide (NO) was derived through the reaction of nitrite with NADH. The formation of NO myoglobin from metmyoglobin is explained in terms of the NADH-oxidase reaction catalyzed by myoglobin. Ascorbate and GSH also serve as reductants though NO myoglobin was formed slowly.