Polyphenol oxidases (PPOs) are a group of coppercontaining
enzymes, which are able to catalyze some specific
phenolic substrates to form their corresponding quinones [1].
PPOs can be divided into different types such as monophenol
oxidase (tyrosinase, EC 1.14.18.1), catechol oxidase (EC
1.10.3.2) and laccase (EC 1.10.3.1) according to the variability
of their substrates [1]. PPOs were first discovered in lacquer
tree by Yoshida in 1883 [2], and later on they were found to
widely exist in animals, plants and microorganisms. PPOs have
been reported to be involved in the biosynthetic processes of
certain plant secondary metabolites (e.g. betalain) and also
play a role in plants' resistance to stress, pathogens and
herbivores [1]. However, what the researchers concern more
about PPOs is that PPOs can catalyze browning reactions in our
agricultural products. Enzymatic browning occurring in fruits
and vegetables often leads to significant deterioration of food
quality, which eventually results in tremendous economic lose
for the agricultural and food industry [3].Moreover, tyrosinase