Polyphenoloxidase in Bartlett Pearsa

Abstract

SUMMARYThe characteristics of polyphenoloxidase in Bartlett pears were investigated. In a citrate‐phosphate buffer containing 0.03M catechol as the substrate, activity of the pear polyphenoloxidase was greatest in the pH range 5.8‐6.4, being optimum at pH 6.2. The Michaelis constant of the enzyme was 0.048M at pH 6.2 in a citrate‐phosphate buffer. It was active only on phenolic compounds having an ortho‐diphenolic configuration. Neither the meta‐ nor para‐dihydroxy phenolic compounds nor phenol was attacked. The energy of activation for pear polyphenoloxidase on catechol was 4.9 kcal per mole. Oxygen was necessary for browning of catechol to take place in the presence of pear polyphenoloxidase, and the activity was greatly decreased when the concentration of oxygen in the reaction mixture was lowered. Diethyldithiocarbamate, a copper‐chelating agent, and phloroglucinol, a competitive inhibitor, reduced browning markedly, but ascorbic acid was most effective of all. It was noted that ascorbic acid acts as an antioxidant rather than as a true enzyme inhibitor. Iodoacet‐amide, a sulfhydryl inhibitor, had no effect on rate of browning. Methods for preventing brown discoloration in canned pears are discussed.