Inhibition of apple phenoloxidase and peroxidase by natural phenolics of tea leaves

Proanthocyanidins were isolated from green leaves of tea (Camellia sinensis). The inhibiting effects of these natural antioxidants on the activities of apple phenoloxidase and peroxidase were studied. Apple peroxidase wasinhibited by much lower concentrations of the antioxidative tea polyphenols than the phenoloxidase. By the action of these natural inhibitors the pH optima of both apple fruit enzymes were slightly more alkaline and the interaction resulted in the formation of a non-dissociable enzyme - inhibitor complex. The dependence of the initial rate of the apple phenoloxidase reaction catalyzing the oxidation of phenolic substrates in tissue-damaged apples with and without addition of the tea leaf inhibitors followed the Michaelis-Menten kinetics, causing a 1.4-fold decrease of V m a x when tea proanthocyanidins have been added. In the presence of these natural inhibitors also a positive cooperation between peroxidase and apple substrate was observed. The value of the Hill coefficient n H was found to be 3.8 and the affinity of the enzyme towards the substrate was also decreased indicating the complex nature of the peroxidase inhibition. Therefore, it is suggested that these natural phenolic inhibitors from tea leaves may effectively be used to improve an alternative technology of apple juice production.