Graduate Thesis Or Dissertation
 

Polyphenol oxidase of D'Anjou pears (Pyrus communis L.)

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  • Stability, substrate and inhibitor specificity and electrophoretic properties of a crude polyphenol oxidase (PPO) preparation extracted from d'Anjou pears (Pyrus communis L.) were investigated. Levels of polyvinylpyrrolidone and pH of buffer for extraction were found to affect the specific activity of the extracted enzyme. An extract prepared with 1.5 g insoluble PVP per 15 g fresh tissue in acetate buffer (pH 5.6) resulted in the highest PPO specific activity of the crude extract. Addition of PVP did not affect the electrophoretic patterns of PPO isozymes. The pH optimum of PPO occurs at 7.0. Heat inactivation of PPO followed first order kinetics and approximately 50% of PPO activity was inactivated after heating for 11.7, 6.25, 2.25 and 1.1 min at temperature of 70°,75°,80° and 85°C, respectively. The crude PPO enzyme was active towards o-dihydroxyphenols, but inactive towards monophenols. Disc electrophoresis on 7% polyacrylamide gels revealed eight active isozymes towards catechol, 4-methylcatechol, chlorogenic acid, caffeic acid, dopamine, d-catechin and DL-dopa. Similar electrophoretic patterns were observed with all substrates. No differences in the band patterns were observed between a fresh crude PPO preparation, a frozen crude extract and a dialyzed extract when catechol was used as substrate. L-cysteine, diethyldithiocarbamate, thiourea, metabisulfite, cyanide, mercaptoethanol and ascorbic acid inhibited the enzyme activity. L-cysteine and diethyldithiocarbamate were the most effective inhibitors.
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