Polyphenol oxidases are copper-containing enzymes that are naturally found in certain vegetables and fruits, where they catalyse the oxidation of polyphenols and cause browning.
There are two types of inhibitor of PPO, those competitive to oxygen in the copper site of the enzyme and those competitive to the phenolic compounds. Examples of inhibitors of these enzymes are: Tropolone, potassium metabisulfite, dithiothreitol and sodium metabisulfite, and similar sulfur-containing compounds including sodium dithionite and cysteine, in addition to ascorbic acid (vitamin C).
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