The activity and kinetic parameters of the enzyme polyphenol oxidase (PPO) in crude extracts of the fruits of apple (Pyrus malus L.), cucumber (Cucumis sativus L.), squash (Cucurbita pepo L.), pomegranate rind and seeds (Punica granatum L.), and the leaf of cactus (Opuntia ficus-indica (L.) were investigated. The highest browning intensity was found in apple, followed by pomegranate seeds and cucumber fruits. The optimum activity of the enzyme was shown at pH 7.0 for apple and cucurbita, but it was found at pH 6.0 for the rind and seeds of the pomegranate. The PPO of cactus leaf and cucumber fruits showed two peaks at pH 4.0 and pH 7.0. On the other hand, the optimal temperature found for PPO activity in these plants was around 40ºC. The relative activity of PPO was ranging from 45% in cactus leaf to 72% in cucumber fruit at 30oC. A strong correlation with value (R) = 0.9485 between the browning intensity and PPO activity in all studied samples was obtained. These results indicated that the studied activity of PPO on different plants could serve as a guideline for predicting the conduct of enzymes to effectively prevent the browning of plants during grading and storage.
Key words: Kinetic parameters, Browning, optimum activity, Relative activity, Activation energy