Horseradish peroxidase. XL. The oxidation of native enzyme to Compound II

The oxidation of native horseradish peroxidase to Compound II by photochemically generated inorganic free radicals [Formula: see text], [Formula: see text], and [Formula: see text] was observed. The rates of Compound II accumulation for different rates of free radical generation and different reagent concentrations have been measured. The interpretation of the experimental data allowed us to estimate the rates of the reactions [Formula: see text], [Formula: see text], and [Formula: see text]: (3–6) × 106 M−1 s−1, (2.2–3.0) × 106 M−1, and [Formula: see text] respectively. Also obtained for all three oxidants were the ratios of the number of free radicals responsible for the heme oxidation to the total number of them attacking the enzyme.

Horseradish Peroxidase. XVIII. The Arrhenius Activation Energy for the Formation of Compound I

The rate of formation of compound I from the reaction of native horseradish peroxidase with hydrogen peroxide was studied from 3.7–70.0 °C. The second-order rate constants were used to construct an Arrhenius plot from which the activation energy of this reaction was calculated to be 3.5 ± 1.0 kcal/mol. The irreversibility of the reaction at 25 °C was confirmed by comparing absolute absorbance changes as recorded by the stopped-flow apparatus with the known spectra of the native enzyme and compound I.

Studies on Horseradish Peroxidase. XIII. The Kinetic Effect of Cyanide on the Oxidation–Reduction Cycle

The effect of cyanide on the steady-state oxidation of ferrocyanide by horseradish peroxidase was studied at pH values 9.00, 7.10, and 5.00. An inhibition was observed which could be explained by the binding of cyanide only to native horseradish peroxidase. Spectra in the Soret region supported these conclusions.