Liu Liang, Cao Shaoqian, Xie Bijun, Sun Zhida, Li Xiaoyao, Miao Wenhua
Food Science and Technology College, Central China Agriculture University, 430070, People's Republic China.
J Agric Food Chem. 2007 Aug 22;55(17):7140-3. doi: 10.1021/jf070964a. Epub 2007 Jul 25.
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 Ki 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.
以荔枝（Litchi chinensis Sonn.）果皮中的主要内源性多酚（-）-表儿茶素为底物，对荔枝果皮中的多酚氧化酶（PPO）进行了表征。PPO以（-）-表儿茶素为底物时的最适pH值为7.5，该酶在pH值低于4.5时不稳定，在pH值6.0 - 8.0范围内稳定。在4℃下分别于pH值6.0、7.5和8.0孵育67天后，PPO的残留活性分别为86.25%、86.31%和80.17%。从热稳定性研究来看，Ki值随温度升高而增加，结果表明该酶在45℃以上不稳定。此外，结果还提供了有力证据表明PPO的变性温度接近70℃。抑制研究表明，即使在低浓度下，L-半胱氨酸和谷胱甘肽也是强抑制剂，而NaF的抑制作用适中。此外，结果还表明巯基的抑制机制与卤化物盐的不同。
