A comparative analysis of property of lychee polyphenoloxidase using endogenous and exogenous substrates

Lychee polyphenoloxidase (PPO) was extracted and partially purified using ammonium sulphate precipitation and dialysis. The comparative analysis of PPO property was performed using its endogenous substrate (–)-epicatechin and exogenous substrate catechol. The pH optima for activity and activation temperature profiles of lychee PPO were very different when the enzyme reacted with endogenous and exogenous substrates. The addition of ethylenediaminetetraacetic acid disodium salt into the endogenous or exogenous substrate–enzyme system exhibited the same lowest inhibition of the PPO activity. However, l-cysteine was most effective in inhibiting enzymatic activity in the endogenous substrate–enzyme system while ascorbic acid was the best inhibitor in the exogenous substrate–enzyme system. Fe²⁺ greatly accelerated the enzymatic reaction between endogenous substrate and PPO, but Cu²⁺ exerted the same effect on the reaction between exogenous substrate and PPO. Based on the kinetic analysis, lychee PPO could strongly bind endogenous substrate but it possessed a higher catalytic efficiency to exogenous substrate.