Nonenzymatic Reduction and Oxidation of Myoglobin and Hemoglobin by Nicotinamide Adenine Dinucleotides and Flavins

The nonenzymatic reduction of metmyoglobin and methemoglobin by NADH or NADPH in the presence of ethylene-diaminetetraacetate can occur at rates greater than most of those previously reported for enzymatic reduction. The reduction rate is considerably enhanced by the mediation of various flavins, including FAD, flavin mononucleotide, and riboflavin. The system is generally more active at pH 6.4 than at pH 7.4. The presence of Amytal is without effect. Coenzyme Q6 and menadione do not function as mediators of the reduction. Myoglobins from mammalian and fish sources behave similarly, but tuna metmyoglobin requires a mediator for reduction. Flavins, particularly flavin mononucleotide, in the presence of ethylenediaminetetraacetate can catalyze the photooxidation of oxymyoglobin, as well as the photoreduction of metmyoglobin. However, the flavin-mediated reduction of metmyoglobin by NADH or NADPH just mentioned takes place equally well in the dark.