The oxidation of L-ascorbic acid catalysed by pear tyrosinase

The ability of partially purified pear tyrosinase (PPO) to catalyse the oxi dation of L-ascorbic acid (AA) has been reported here for the first time. T he ascorbate oxidase activity of PPO was studied by oxymetric assays. The a ctivity was linearly related to the enzyme concentration with a Michaelis c onstant (K-m) for AA of 0.55 +/- 0.03 mM at pH 7. The stoichiometry was fou nd to be 1:2 (O-2:AA). The action of the PPO inhibitors tropolone and sodiu m chloride was studied to exclude a possible interference of endogenous pea r ascorbate oxidase in the oxidation of AA. A possible role of the 'AA/PPO' system in the browning of pears is proposed.