A diphenolase from persimmon fruits (Diospyros kaki L.), active against catechol, 4-methylcatechol, L-3,4-dihydroxyphenylalanine and 3-(3,4-dihydroxyphenyl)propionic acid, was characterized in detail in terms of pH and temperature optima, stability, kinetic parameters and inhibition behaviour toward some general PPO inhibitors. The substrate specificity of the persimmon PPO was very high for catechol and 4-methylcatechol. The enzyme was extremely stable at its optimum pH of 7.5 in the presence of 4-methylcatechol as the substrate and it retained over 90% of its original PPO activity after 24 h of incubation at 4 degreesC at that pH. Thermal properties of the persimmon enzyme, together with thermodynamic parameters, show that the persimmon enzyme is very heat-sensitive. Ascorbic acid, metabisulfite, azide and benzoic acid all inhibited the 4-methylcatechol oxidation by persimmon PPO, indicating its sensitivity toward the general PPO inhibitors, especially metabisulfite. All the data support the presence of a highly active diphenolase in persimmon fruits (Diospyros kaki L.) having similar properties to other plant polyphenoloxidases. (C) 2003 Elsevier Ltd. All rights reserved.