Kinetic and spectral properties of pea cytosolic ascorbate peroxidase

Sufficient highly purified native pea cytosolic ascorbate peroxidase was obtained to characterize some of its kinetic and spectral properties, Its rate constant for compound I formation from reaction with H2O2 is 4.0 x 10(7) M(-1) s(-1), somewhat faster than is typical for peroxidases, Compound I has the typical optical spectrum of an iron(IV)-porphyrin-pi-cation radical, despite considerable homology with yeast cytochrome c peroxidase. The rate constant for compound I reduction by ascorbate is extremely fast (8.0 x 10(7) M(-1) s(-1) at pH 7.8), again in marked contrast to the behavior of the yeast enzyme, The pH-rate profile for compound I formation indicates a pK(a) value of 5.0 for a group affecting the active site reaction.