Some Biochemical Properties of Polyphenoloxidase from Spearmint (Mentha arvensis)

Polyphenoloxidase (PPO; EC 1.14.18.1) extracted from Mentha arvensis leaves was isolated by (NH4)2SO4 precipitation and extensive dialysis. Its optimum pH and temperature varied with the substrate. The PPO showed activity with various diphenols. Km values were found 0.825, 0.928 and 7.41mM for caffeic acid, 4-methylcatechol and catechol, respectively. On heat-inactivation, half of the activity was lost after 60 and 15 sec at 70 and 75°C, respectively. Measuring of residual activity showed a stabilizing effect of sucrose at various temperatures with activation energy (Ea) for inactivation increasing with sucrose concentration from 0 to 40% (w/w). Ea values of 78.13; 80.37; 82.79 and 81.00 kJ/Mol were found for 0, 15; 30 and 40% sucrose, respectively. PPO was inhibited by ascorbic, benzoic, cinnamic, ferulic, p-coumaric, protocatechuich acids, sodium metabisulfite, pyrogallol and resorcinol. The Ki values showed that ascorbic acid was the most effective inhibitor. The type inhibition was determined for each inhibitor.