Kinetic studies on apple polyphenol oxidase.

The effect of oxygen concentration on the activity of purified apple polyphenol oxidase (PPO) was studied with 4-methylcatechol, chlorogenic acid, or (+)-catechin as phenolic substrate. For these three phenolic compounds, the kinetic pattern was consistent with a sequential mechanism for the binding of oxygen and phenol. Benzoic acid is a competitive inhibitor with respect to 4-methylcatechol and uncompetitive with respect to oxygen, which suggests that apple PPO combines with oxygen before it does with the phenolic substrate. Oxygen uptake of a two-phenolic compound mixture by apple PPO was studied in air-saturated solutions. An equation that contains only the kinetic parameters Km and Vm of each phenol was developed. For the two mixtures tested, 4-methylcatechol/caffeic acid and (+)-catechin/ chlorogenic acid, the experimental values of oxygen uptake were in excellent agreement with the caiculated curves. The equation can be extended to a multicomponent solution and therefore can be used for the study of oxidability of mixtures of phenolic compounds in model solutions by PPO.

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