PEROXIDASE ACTIVITY OF HEMOPROTEINS

II. METMYOGLOBIN AND CYTOCHROME C

抄録

The peroxidase activities and their pH dependencies of metmyoglobin and cytochrome c were measured with leuco malachite green as the hydrogen donor and in terms of both k₁ and k₄, in which the former is the rate constant for the reaction with H₂O₂ and the latter for the reaction with the hydrogen donor. The optimum pH values for ki and k4 were found to be 4.9 and 5.1 for metmyoglobin, and 3.4 and 4.2-4.4 for cytochrome c, respectively. Measurements of the sedimantation coefficients revealed that the activities observed at the acidic optimum pH are those of the native molecules. This result contrasts with the mechanism of activation of hemiglobin that the activation results from the dissociation of the native molecule into its subunits. The change of the Soret bands with pH elucidated that the activity drop observed below the optimum pH was caused by the dissociation of the heme group from the apoprotein in the case of metmyoglobin and was caused by unknown change of the state of heme in cytochrome c. The absolute values of k₄ in M^-1sec.^-1 at the optimum pH were of the order of 10⁴ and were higher than would be expected from the activity data so far obtained with no separate observation of k₁ and k₄. However, the values were lower than those for common peroxidases such as horseradish peroxidase. The k₁ values were considerably smaller than the k₄ values and were of the order of 10-10². The significance of the results obtained is disussed in comparison with the activity data of various other hemoproteins.