Purification of two peroxidase isoenzymes of Aloe barbadensis which oxidize p-coumaric acid

Using a combination of hydrophobicity and ion-exchange chromatography methods, one cationic (pI 9.0) and one anionic (pI 4.5) peroxidase (donor: hydrogen-peroxide oxidoreductase; EC 1.11.1.7) isoenzymes of Aloe barbadensis have been purified (the cationic peroxidase to homogeneity as judged by SDS-PAGE analysis and microsequencing). This allowed us to initiate the investigation of individual catalytic properties to be related to their respective functions in vivo. The two peroxidases have an optimal activity at pH 6.0. Apparent affinities for H2O2 range between 0.01 and 0.14 mM depending on the phenolic substrate and the isoenzyme. The apparent Km values for the phenolics (p-coumaric acid and hydroquinone) are some 25-fold lower in the anionic (around 0.02 mM) than in the cationic (around 0.77 and 0.34 mM, respectively) isoenzyme. The possible functions of the activities are discussed.