Nitric oxide exchange in nitrosylmyoglobin

The exchange of nitric oxide in nitrosylmyoglobin, the heme pigment of nitrite-cured meat, has been studied using nitrogen-15 labelling in aqueous solution under conditions (pH, concentration of ascorbate and nitrite) similar to those prevailing in meat during the curing process, and has been found to have a half-life of approximately 2 h at 40 degrees C. One nitric oxide molecule is coordinated to the iron(II) centre of a myoglobin molecule and, in weakly acidic aqueous solution under anaerobic conditions, the exchange rate of the bound nitric oxide is proportional to the concentration of nitrosylmyoglobin, nitrite and hydrogen ion. The rate of exchange has a moderate temperature dependence, corresponding to an activation barrier of delta H+- = 47 +/- 3 kJ.mol-1 at 25 degrees C and pH 5.9, a value dramatically lower than that found for the enthalpy of activation for the oxidation of nitrosylmyoglobin by molecular oxygen, delta H+- = 110 kJ.mol-1. The difference in temperature dependence between the exchange and the autoxidation is discussed in relation to the function of nitrosylmyoglobin as antioxidant in cured meat products.