Both polyphenol oxidase and peroxidase have been implicated in enzymic browning of plant tissues (Vamos-Vigyazo, 1981).
Polyphenol oxidase is widely distributed in the plant kingdom (Mayer and Harel, 1979).
It is a coppercontaining enzyme which catalyses the ortho-hydroxylation of monophenols and the oxidation of o-diphenols to o-quinones (Mayer and Harel, 1979).
The highly reactive quinones thus formed can polymerize to form the red, black, and brown pigments associated with the browning of plant tissues.
Normally, PPO is separated from its phenolic substrates, which are located in the vacuole, so
that browning only occurs when cells are damaged and compartmentation is lost.