Partial purification and some properties of polyphenol oxidase extracted from litchi fruit pericarp

Litchi (Litchi chinensis Sonn.) fruit peel polyphenol oxidase (PPO) was partially purified 21 fold by ammonium sulfate fractionation and gel filtration. Pyrogallol, catechol, and 4-methylcatechol were good substrates for the enzyme; with no activity observed with chlorogenic acid, p-cresol, resorcinol, or tyrosine. The optimal pH for PPO activity was 7.0 with 4-methylcatechol, with the enzyme being most stable at pH 7.4. The enzyme was relatively temperature stable with maximum activity at 70 °C and requiring a little less than 10 min at 90 °C for 50% loss of activity. The K_m and V_max for the enzyme, with 4-methylcatechol, were 10 mM and 1.47 × 10⁴ units/min per mg protein, respectively. The enzyme was not activated by SDS. Reduced glutathione, -cysteine, tropolone, 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 and control commercially, litchi fruit peel browning.