Partial Characterization of Polyphenoloxidase Extracted from `Anna' Apple

Abstract

Polyphenoloxidase (PPO) was studied in extracts from `Anna' apples (Malus domestica Borkh.) produced under the desert conditions of the state of Sonora, Mexico. A crude extract of PPO was prepared in phosphate buffer from an acetone powder of pulp from ripe apples. Optimum conditions for enzyme activity were pH 5.36 at 35C. Specificity toward substrates, in order of decreasing activity, was 4-methylcatechol, chlorogenic acid, catechol, caffeic acid, and 3,4-dihydroxyphenylalanine. Activation energy for the reaction with 4-methylcatechoI was 6543 J·mol -1 (1563 cal·mol -1 ). The PPO from `Anna' was similar to that described from other apple cultivars, except that it was more thermostable between 35 and 60C. A single peak of PPO activity was achieved by hydrophobic chromatography of the crude extract, resulting in a 300-fold purification. `Anna' apples contained high levels of phenolic substances, 1.16% on a fresh-weight basis. The high phenolic content and PPO thermostability help explain the browning susceptibility of `Anna' apples during postharvest handling.