Andrawis, A., Department of Food Technology, Agricultural Research Organization, The Volcani Center, P.O.B. 6, Bet Dagan, Israel
Kahn, V., Department of Food Technology, Agricultural Research Organization, The Volcani Center, P.O.B. 6, Bet Dagan, Israel

Hydrogen peroxide (H2O2) inactivates mushroom tyrosinase in a biphasic manner, with the rate being faster in the first phase than in the second one. The inactivation of the enzyme is dependent on H2O2 concentration (in the range of 0.05-5.0 mM), but independent of the pH (in the range of 4.5-8.0). The rate of inactivation of mushroom tyrosinase by H2O2 is faster under anaerobic conditions (nitrogen) than under aerobic ones (air). Substrate analogues such as L-mimosine, L-phenylalanine, p-fluorophenylalanine and sodium benzoate protect the enzyme against inactivation by H2O2. Copper chelators such as tropolone and sodium azide also protect the enzyme. Under identical conditions, apotyrosinase is not inactivated by H2O2, unlike holotyrosinase. The inactivation of mushroom tyrosinase is not accelerated by an OH-dot generating system (Fe2+-EDTA-H2O2) nor is it protected by OHdot scavengers such as mannitol, urate, sodium formate and histidine. Exhaustive dialysis or incubation with catalase does not restore the activity of H2O2-inactivated enzyme. The data suggest that Cu2+ at the active site of mushroom tyrosinase is essential for the inactivation by H2O2. The inactivation does not occur via the OHdot radical in the bulk phase but probably via an enzyme-bound OHdot. © 1985.