Soluble and ionically bound peroxidases were extracted from apple peel and apple pulp, and activities were measured; most occurred in the soluble fraction of peel. The soluble peroxidases consisted of both cationic and anionic types of isoenzymes, whereas those ionically bound were solely cationic. Five different isoperoxidases were found by isoelectric focussing. The soluble extract of apple pulp contained the most heat stable peroxidases. After heat treatment of extracts some activity regenerated. The pH optima for the mixed peroxidases in extracts were pH 5.0 to 6.0. A commercial fresh apple juice was found to contain a small amount of peroxidase activity when assayed at pH 6.0.

Three peroxidase isoenzymes, two anionic and one cationic, were isolated from extracts of apple pulp using gel filtration and ion-exchange chromatography. The homogeneity of the purified isoenzymes was established by isoelectric focusing and staining for peroxidase activity. The isoenzymes varied in their heat stability: the anionic isoenzymes were found to be more heat stable than the cationic isoenzyme. The measured enzymic activity of the isoenzymes was greatest when o-dianisidine was used as the test substrate. After heating solutions of the anionic isoenzymes at 80-100°C for 2 min, up to 80% of the original enzymic activity was observed to regenerate when the heat-treated isoenzymes were held at 30°C for 1 hr. Regeneration of enzymic activity following heat inactivation was not observed for the cationic isoenzyme.

Soluble and ionically-bound peroxidases have been obtained from the pulp and peel of Conference pears. The peel soluble fraction contained the highest level of peroxidase activity. The peel peroxidases, both soluble and ionically bound, were particularly heat stable. Following heat inactivation, regeneration of enzymic activity was observed for the peel peroxidases and soluble pulp enzymes. Plots of the percentage of heat inactivation for the pear peroxidases against heating time were non-linear.