Reaction mechanisms in melanosis formation. Basically, enzymatic
browning or melanosis takes place in the presence of oxygen when PPO and
their polyphenolic substrates are mixed. Polyphenol oxidase catalyzes two
fundamental reactions: hydroxylation to the o-position adjacent to an existing
hydroxyl group of the phenolic compounds (monophenolase activity), and
oxidation of diphenol to o-benzoquinones (diphenolase activity).
The subunits of PPO has been reported to differ with respect to
chemical, physical and kinetic properties, which were believed to be
responsible for relative affinities of the enzymes for both mono- and
diphenolic substrate (Kim et al 2002).