A kinetic study of p-cresol oxidation by quince fruit polyphenol oxidase

The monophenolase activity of quince pulp polyphenol oxiclase was characterized by extracting samples using a combination of a two-phase partition step in Triton X-114, followed by a PEG 8000/ phosphate partition step, and a final ammonium sulfate fractionation between 30 and 75%. The purification method avoids the loss of cresolase activity described in another quince pulp polyphenol oxiclase. The activity was characterized by a lag period, whose duration depended on the substrate concentration, the pH, and the presence of catalytic amounts of o-diphenol. By increasing the concentration of o-diphenols, it was possible to evaluate the enzyme activation constant, K-act, which showed a value of 4.5 muM for 4-methylcatechol. A general kinetic mechanism for this enzyme is used to explain the loss of activity that normally occurs during quince pulp polyphenol oxidase purification.