FOOD CHEMISTRY

Polyphenoloxidase (PPO) of peppermint leaves (Mentha piperita) was isolated by (NH4)(2)SO4 precipitation and dialysis. Its pH and temperature optima were 7.0 and 30 degreesC, respectively. On heat-inactivation, half of the activity was lost after 6.5 and 1.5 min of treatment at 70 and 80 degreesC, respectively. Sucrose, (NH4)(2)SO4, NaCl and KCl appeared to be protective agents of peppermint PPO against thermal denaturation. Km of this enzyme ranged from 6.25 x 10(-3) M with catechol to 9.00 x 10(-3) M with L-dopa. The I-50 values of inhibitors studied on PPO were determined by means of activity percentage (I) diagrams. Values were 1.4 x 10(-4) M, 1.7 x 10(-4) M, 9.7 x 10(-5) M, 2.45 x 10(-4) M, 2.16 x 10(-1) M, 1.83 x 10(-5) M, 6.5 x 10(-5) M, 1.4x 10(-2) M, 7.5 x 10(-5) M, for potassium cyanide, glutathione, ascorbic acid, thiourea, sodium azide, sodium metabisulfite, dithioerythritol, beta -mercaptoethanol and sodium diethyl dithiocarbamate respectively, Therefore, sodium metabisulfite was the most effective inhibitor. (C) 2001 Elsevier Science Ltd. All rights reserved