The substrate profiles of the acidic and slightly basic horseradish peroxidases

Abstract

1. 1. The catalytic properties of two horseradish peroxidases, isoenzymes A2 and C2, with p I 3.9 and 8.8, have been compared. 2. 2. The rate of formation of Compound I has been determined for the hydrogen, methyl, ethyl, n-propyl, and hydroxymethyl hydroperoxides and for p-nitroperoxybenzoic acid. The elongation of the alkyl group hampered the reaction with the acidic peroxidase and stimulated the reaction with the basic peroxidase. The acidic peroxidase generally reacted more slowly. Its reaction with H 2O 2 showed a higher energy of activation than that of the slightly basic peroxidase. 3. 3. The rate of reaction with hydrogen peroxide was slightly but definitely influenced by pH for both peroxidases. 4. 4. The rates of reaction of twelve hydrogen donors (phenols, aromatic amines, acidic substances) with the secondary peroxidase-peroxide complex of the two peroxidases could be arranged in three groups according to the chemical nature of the donors. The acidic peroxidase was consistently more active at pH 4.5 than at pH 7.0 whereas the slightly basic peroxidase showed the reverse behavior in the presence of phenols. 5. 5. It is concluded that the two peroxidases are kinetically different with distinct substrate profiles and that they may fulfill different physiological functions.