Oxidation of the flavonoid eriodictyol by tyrosinase

A pathway is proposed for the oxidation of the flavonoid eriodictyol by mushroom tyrosinase. In it, the enzymatic oxidation of eriodictyol leads to the formation of eriodictyol-o-quinone, which undergoes the nucleophilic attack of another eriodictyol unit to yield a dimer. This dimer is then oxidized by the eriodictyol-o-quinone. The reaction was followed by recording the time course of formation of this second o-quinone at 475 nm. Progress curves at this wavelength showed the appearance of a lag, the length of which varied with enzyme and substrate concentrations, and which must have been caused by the chemical reactions taking place after the enzymatic reaction. When eriodictyol oxidation was studied in the presence of 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH), which competes with the substrate in the reaction with eriodictyol-o-quinone, the lag disappeared. The kinetic parameters were similar with and without MBTH. Eriodictyol oxidation was inhibited by tropolone, which behaved as a slow-binding inhibitor.