Abstract: Polyphenol oxidase (PPO) is the key enzyme of enzymatic browning, and its characteristics and inhibiting effects are the important involvements of studying enzymatic browning of fruit and vegetable. With spectrophotometry method and based on substrate of catechol, the characteristics of PPO and the effects of various inhibitors on PPO activity were investigated. The results showed that the optimum pH-activity is 4.5, and the optimum temperature for pear PPO activity is 34 ℃, so the high temperature short time treatment can inhibit PPO activity evidently. The kinetics of the enzyme-catalyzed reaction of PPO is in accord with Michaelis-Menten equation, whereas Km and Vmax values are respectively 0.125 mol/L and 178.57 U/min. Of the inhibitors tested, L-cysteine, sodium bisulphite or ascorbic acid is more effective than citric acid, and the most effective inhibitor is L-cysteine.

Key words: Dangshan pear, polyphenol oxidase, enzyme character, inhibiting effect