Studies on the Peroxidase Effect of Cytochrome c. IV. The Influence of pH and Certain Anions on the Over-all Reaction.

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Abstract
The peroxidation of pyrogallol to purpurogallin, catalyzed by ferricytochrome c, was studied in the presence of different anions in the pH range 2.3 to 6. 8. The pH-activity curves in citrate, acetate, chloride, and sulfate were of very similar form, differing from each other primarily in the position of and the height at the apparent pH optimum. The pH optima with the different anions were obtained within 0.2 pH unit; the highest value (approx. 3. 5) was found in citrate buffer. In a double logarithmic plot the pH dependence of the catalytic activity revealed the dissociation of 2 functional groups with pK1[less than or equal to] 3.13 and pK2[less than or equal to]3. 78 (25[degree]C, 40 m[image] citrate buffer). Evidence is presented in support of the view that these groups are the hemochromogen-forming groups of the cytochrome c molecule, and their nature is discussed. The peroxidase activity of cytochrome c is greatly influenced by the presence of anions, which seems to be essential in order to obtain measurable rates. The change in activity with the anion concentration follows ordinary, simple dissociation curves, indicating that ferricytochrome c is dissociably combined with the anions. The degree of activation increased in the following order: chloride, citrate, and acetate. The mechanism of the anion activation is discussed. The influence of pH and anions on the peroxidase activity of ferricytochrome c is paralleled by changes in the light absorption of the pure hemoprotein, indicating a common mechanism for both these effects.