Polyphenol oxidases (PPOs), or tyrosinases, are nuclear-coded
enzymes with a dinuclear copper centre, which are able to insert
oxygen in a position ortho to an existing hydroxyl group in an aromatic
ring, followed by the oxidation of the diphenol to the corresponding
quinone. These o-quinone intermediates are highly
reactive and quickly polymerise to dark-coloured melanins (Rapeanu
et al., 2006).