Kinetics of the Formation of p-670 and of the Decay of Compound III of Horseradish Peroxidase

The kinetics of the decay of compound III of horseradish peroxidase formed from compound II and excess hydrogen peroxide and of the formation of p-670 from this preparation were studied as a function of pH at 25°C and constant ionic strength of 0.11. Compound III undergoes an irreversible formation of p-670 and a slow, spontaneous decay to ferriperoxidase via compound II and the rate of compound III decay is less than that of p-670 formation at all pH values. Both reactions follow pseudo first order kinetics at all pH values. The observed rate constant for compound III decay decreases with increase in pH with a minimum at pH 8.0. Also the observed rate constant for p-670 formation increases with increase in pH with a maximum also at pH 8.0. The slow decay of our preparation of compound III and its unique formation of p-670 as compared to oxyperoxidase prepared from ferrous horseradish peroxidase and molecular oxygen (J. B. Wittenberg, R. W. Noble, B. A. Wittenberg, E. Antonini, M. Brunori, and J. Wyman (1967)J. Biol. Chem.242, 625–634) are discussed in terms of the presence of excess hydrogen peroxide in our preparation. Ferriperoxidase and p-670 are formed from compound III in two parallel reactions.