Characterization of polyphenol oxidase activity from medlar fruits, (Mespilus germanica L., Rosaceae)

SUMMARY Characterization of polyphenol oxidase activity from medlar fruits, (Mespilus germanica L., Rosaceae) In this work, polyphenol oxidase (PPO) activity in the extracts prepared from medlar fruits (Mespilus germanica L., Rosaceae) was investigated in detail from the point of biochemical and kinetics data. Medlar PPO enzyme activity catalyzed efficiently the oxidation of phenolic substrates, catechol, 4-methyl catechol, DOPA (3,4-dihydroxyphenyl alanine), DHPPA (3,4-dihydroxlyphenyl propionic acid) and epicatechine, and the activity was linear up to 0.96 mg protein/ml. The enzyme showed different activities at specific pH and temperature values for each substrate. Kinetic studies have shown that the medlar PPO enzyme obeys the simple Michaelis-Menten kinetics with the greatest degree of affinity for 4-methyl catechol and the order of affinity was observed to be 4-methyl catechol>DOPA>catechol>epicatechin>DHPPA. Moreover, the Hill plots for each substrate with Hill constants lower than 1 have indicated that there is only one binding site for the substrates with no cooperativity. The three of the common inhibitors of PPO enzymes, cystein, sodium azide and benzoic acid, all inhibited the activity very effectively. All the data support that the PPO enzyme activity detected in medlar fruits, although it is relatively higher, has similar biochemical and kinetic characteristics to other plant PPO enzymes. Key words: Polyphenol oxidase, diphenolase, medlar fruits (Mespilus germanica), DOPA, DHPPA V