Characterization of polyphenol oxidase from litchi pericarp using (-)-epicatechin as substrate

Polyphenol oxidase (PPO) from litchi (Litchi chinensis Sonn.) pericarp was characterized using (-)-epicatechin, which was the major endogenous polyphenol in litchi pericarp as a substrate. The optimum pH for PPO activity with (-)-epicatechin was 7.5, and the enzyme was unstable below pH 4.5 and stable in the pH range of 6.0-8.0. Residual activities of PPO were 86.25, 86.31, and 80.17% after 67 days of incubation at 4 degrees C at pH 6.0, 7.5, and 8.0, respectively. From thermostability studies, the K-i value increased with temperature and the results suggested that the enzyme was unstable above 45 degrees C. Moreover, the results also provided strong evidence that the denaturalization temperature of PPO was near 70 degrees C. The inhibition studies indicated that L-cysteine and glutathione were strong inhibitors even at low concentrations while NaF inhibited moderately. In addition, the results also indicated that the inhibition mechanisms of thiol groups were different from those of halide salts.