Partial purification and characterization of polyphenoloxidase from peppermint (Mentha piperita)

Polyphenoloxidase (PPO) of peppermint leaves ( Mentha piperita) was isolated by (NH4)2SO4 precipitation and dialysis. Its pH and temperature optima were 7.0 and 30°C, respectively. On heat-inactivation, half of the activity was lost after 6.5 and 1.5 min of treatment at 70 and 80°C, respectively. Sucrose, (NH4)2SO4, NaCl and KCl appeared to be protective agents of peppermint PPO against thermal denaturation. Km of this enzyme ranged from 6.25×10−3 M with catechol to 9.00×10−3 M with L-dopa. The I50 values of inhibitors studied on PPO were determined by means of activity percentage (I) diagrams. Values were 1.4×10−4 M, 1.7×10−4 M, 9.7×10−5 M, 2.45×10−4 M, 2.16×10−1 M, 1.83×10−5 M, 6.5×10−5 M, 1.4×10−2 M, 7.5×10−5 M, for potassium cyanide, glutathione, ascorbic acid, thiourea, sodium azide, sodium metabisulfite, dithioerythritol, β-mercaptoethanol and sodium diethyl dithiocarbamate respectively. Therefore, sodium metabisulfite was the most effective inhibitor.