Polyphenol oxidase (PPO) is an enzyme that catalyzes the hydroxylation of monophenols into ortho-diphenols (cresolase activity) and the oxidation of o-diphenols into quinones (catecholase activity) (Figure 1). These quinones spontaneously polymerize to form dark-colored phytomelanins, most often seen in the browning of damaged plant tissue. PPO activity can be easily assayed in crude protein extracts from English walnut (Juglans regia) leaves and from many other plant tissue extracts. PPO activity is most commonly measured by spectrophotometric assay, in which the rate of phytomelanin production is quantified, or by oxygen electrode assay, in which the consumption of oxygen by the enzyme is quantified (Figure 1). Though simpler, the utility of the spectrophotometric assay is limited by variation in the absorption maxima of phytomelanins generated from different phenolic substrates. The oxygen electrode assay is generally considered the “gold standard” for measurement of PPO activity, but it is more time consuming and difficult to implement with monophenol substrates, since cresolase activity is typically quite low compared to catecholase activity. This protocol will describe crude protein extraction from walnut leaves, the spectrophotometric assay, and the oxygen electrode assay for determining PPO activity.
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