an oxidizing enzyme that is widespread in nature. Polyphenoloxidase is a protein of variable molecular weight (for example, 34,500 in mushrooms and 144,000 in tea leaves) that contains 0.2–0.3 percent copper. It catalyzes the oxidation of o-diphenols, as well as phenols, triphenols, and polyphenols, to produce the corresponding quinones; molecular oxygen acts as the hydrogen acceptor in this process.

Polyphenoloxidase apparently takes part in the respiration of plant cells (the reversible oxidation of polyphenols forms the intermediate stage for hydrogen transfer from a substrate to O2 in plants). By oxidizing the amino acid tyrosine in animals and humans, it plays a role in the formation of pigments called melanins, which are present in the skin, hair, and iris. Polyphenoloxidase also oxidizes tannins in tea leaves and causes the color of rye bread.