Crude extracts prepared from medlar fruits (Mespilus germanica L., Rosaceae) possess a diphenolase activity toward catechol, 4-methyl catechol, L-3,4-dihydroxyphenylalanine, epicatechin and 3-(3,4-dihydroxyphenyl)propionic acid. Among the substrates employed, the greatest substrate specificity was observed with 4-methylcatechol. The pH-activity optimum for the enzyme, in the presence of this substrate, was 6.5 and the pH-stability profile for the enzyme showed that 80% of the PPO activity was retained at physiological pH values. The temperature-activity optimum, for the enzyme in the presence 4-methyl catechol, was 35 degreesC. The enzyme was stable for 30 min at its optimum temperature and moderately stable at 60 degreesC. At higher temperatures, heat denaturation of the enzyme occurred after 10 min of incubation. Thermal inactivation parameters indicate that the medlar enzyme is very heat-labile. Moreover, the medlar PPO activity was very sensitive to some common PPO inhibitors, especially to cysteine and metabisulfite. All data indicate that medlar fruits have highly active PPO enzymes which possess similar biochemical and kinetic characteristics to other plant PPO enzymes. (C) 2002 Elsevier Science Ltd. All rights reserved.