Kinetic studies on the reduction of metmyoglobins by ascorbic acid

The kinetics of the reduction of metmyoglobins by ascorbic acid (H2A) were studied under a nitrogen atmosphere at 25 degrees C, at an ionic strength of 0.30 M (NaCl), and between pH 7.18 and 8.09. Neither Tris-HCl nor phosphate buffers had any effect on the reduction of metmyoglobin. Imidazol and 1-methylimidazol accelerated this reaction, but N3- and CN- ions inhibited it. It is concluded that the reduction of imidazolmetmyoglobin or 1-methylimidazolmetmyoglobin is faster than that of aquametmyoglobin and that neither azidometmyoglobin nor cyanometmyoglobin can be reduced by ascorbate under the present experimental conditions. The second-order rate constants were determined for the reductions of aqua-, imidazol-, and 1-methylimidazolmetmyoglobin by ascorbate (HA- and A2-). The higher reactivity of imidazolmetmyoglobin with ascorbate may be due to the easy transfer of an electron of ascorbate to partially exposed imidazol or porphyrin ring because of expansion of the heme pocket induced by the coordination of imidazol.