Purification and some properties of polyphenol oxidase of longan fruit

Polyphenol oxidase (PPO) was isolated from longan (Dimocarpus longan Lour.) fruit peel, with a 46-fold purification of PPO by ammonium sulfate, Sephadex G-200 and Phenyl Sepharose being achieved. Pyrogallol, 4-methylcatechol, and catechol were good substrates for the enzyme, and activity with chlorogenic acid, p-cresol, resorcinol, or tyrosine was not observed. The optimal pH for PPO activity was 6.5 with 4-methylcatechol. The enzyme had a remarkably temperature optimum (35°C) and was relatively stable, requiring a little more than 20 min at 50°C for 50% loss of activity. Reduced glutathione, l-cysteine, thiourea, FeSO4 and SnCl2 markedly inhibited PPO activity, whereas MnSO4 and CaCl2 enhanced PPO activity. Data obtained in this study might help to better understand longan fruit peel browning.