Biochemical properties and potential endogenous substrates of polyphenoloxidase from chufa (Eleocharis tuberosa) corms

Polyphenoloxidase (PPO) was partially purified from chufa corms through ammonium sulphate precipitation and dialysis. Biochemical properties of chufa PPO were analysed using exogenous substrate catechol. Optimal pH and temperature for PPO activity were 5 and 45 °C. Ethylenediaminetetraacetic acid disodium salt and l-cysteine could not inhibit the PPO activity. However, sodium thiosulphate pentahydrate exhibited the strongest inhibiting effect, followed by ascorbic acid and anhydrous sodium sulphite. Except for K⁺, other metal ions such as Zn²⁺, Cu²⁺, Fe³⁺, Ca²⁺, Fe²⁺ and Na⁺ accelerated the enzymatic reaction between catechol and PPO. Kinetic analysis showed that the apparent K_m and V_max values were around 10.77 mM and 82 units/ml min. In addition, (−)-gallocatechin gallate, (−)-epicatechin gallate and (+)-catechin gallate isolated and identified from chufa corms were supposed to be the potential endogenous PPO substrates due to their ortho-diphenolic or pyrogallolic structures. These polyphenols might be catalysed by PPO, resulting in the browning of chufa corms after fresh-cut processing.