PPO (phenolase or catechol oxidase) is a ubiquitous group of copper metalloproteins and it is found in microorganisms (bacteria and fungi), plants, and animals, where it is involved in the biosynthesis of melanins and other poly-phenolic compounds. PPO is of particular importance in the processing and marketing of horticultural products because of the brown discoloration of bruised fruits and vegetables resulting from the action of this enzyme. Not only does an undesirable color form, but browning is accompanied by a loss of nutrient quality and the development of undesirable flavors. In some tropical fruits, the action of PPO leads to major economic losses (up to 50%). Black spots in shrimp are caused by PPO-catalyzed browning; the browned shrimp are not acceptable to the consumer and they are downgraded in quality. Millions of dollars are spent each year on attempts to control PPO oxidation; to date none of the control methods are entirely successful. On the other hand, browning by PPO is a desired reaction in certain foods such as tea, coffee, cocoa, prunes, and dates.
PPO was first discovered in mid 1800s in mushrooms (2). PPO is a generic term for the group of enzymes that catalyze the oxidation of phenolic compounds at the expense of oxygen to quinones. Quinones are highly reactive, electrophilic molecules, which covalently modify and crosslink a variety of cellular constituents including nucleo-philes of proteins such as sulfhydryl, amine, amide, indol, and imidazole substituents. These quinone adducts produce brown or black color on cut surfaces of fruits and vegetables, and they are the major detrimental effect of the enzyme in postharvest physiology and food processing.
PPO is a copper-containing enzyme that belongs to a group of oxidoreductases. Although the function of PPO in plant metabolism is not well understood yet, the recent cloning of PPO presents opportunities to explore its function in plants as well as to explore the extent to which PPO expression can be manipulated. Various aspects of PPO have been reviewed recently (3-8).
Depending on the substrate specificity, International Enzyme Nomenclature has designated monophenol mono-oxygenase or tyrosinase as EC 22.214.171.124, which hydroxyl-ates monophenols to o-diphenols; diphenol oxidase or catechol oxidase or diphenol oxygen oxidoreductase as 126.96.36.199, which dehydrogenates o-dihydroxyphenols to o-quinones; and lacease or p-diphenol oxygen oxidoreductase as 188.8.131.52. The first two enzymes, tyrosinase and catechol oxidase, which many authors refer to as PPOs, occur in practically all plants.
PPO was first described by G. Bertrand in 1895 who demonstrated that the darkening of mushroom was due to the enzymatic oxidation of tyrosine (9). In addition to its general occurrence in plants, PPO is also found in some bacteria, fungi, algae, bryophytes, and gymnosperms (1013). Laccase was first observed by H. Yoshida in the latex of the Japanese lacquer tree (14). PPO catalyzes two dis tinct reactions: (1) hydroxylation of monophenols to give o-diphenols. This monophenol oxidase activity often requires priming with reducing agents or trace amounts of o-diphenol indicated as BH2,
and (2) the removal of hydrogens from o-diphenol to give o-quinone.
OH Catechol II n oxidase r\ r 2 || | + 02 ---+ 2H20
It was reported in 1944 that PPO preparations from mushrooms had the ability to catalyze oxidation of monophenols in addition to catalyzing oxidation of polyphenols (15). However, the ratio of rate of monophenol oxidation to that of polyphenol oxidation varied corisiderably from different mushroom sources. Since then many reports have shown a wide range of PPO activities in many different fruits and vegetables in different ratios. Today, it is known that enzyme preparations from many other sources possess these two activities in different ratios and that the ratios may change during isolation and purification. Most PPO preparations from potato, apple, mushroom, and bean possess both activities, whereas those from tea leaf, tobacco, mango, banana, pear, clingstone peach, and sweet cherry have been reported not to act on monohydroxy phenols (16).
Laccase is less frequently encountered as a cause of browning in fruits and vegetables than is catechol oxidase. Laccase oxidizes o- and p-diphenols,
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