Kinetic study of the oxidation of quercetin by mushroom tyrosinase

The kinetic behavior of mushroom tyrosinase in the presence of the flavonol quercetin was studied. This flavonol was oxidized by mushroom tyrosinase and the reaction was followed by recording spectral changes over time. The spectra obtained during the reaction showed two isosbectic points, indicating a stable o-quinone. When quercetin was oxidized by tyrosinase in the presence of cysteine and 3-methyl-2-benzothiazolone hydrazone (Besthorn's hydrazone, MBTH) isosbestic points were also observed indicating a definite stoichiometry. From the data analysis of the initial rate) in the presence of MBTH, the kinetic parameters: V-max(app) = (16.2 +/- 0.6) muM/min, K-m(app) = (0.12 +/- 0.01) mM, (V-max(app)/K-m(app)) = (V-max/K-S') = (13.5 +/- 1.4) x 10(-2) min(-1), k(cat)(app) (6.2 +/- 0.6) s(-1) were determined. We propose that quercetin acts simultaneously as a substrate and a rapid reversible inhibitor of mushroom tyrosinase, depending on how it binds to the copper atom of the enzyme active site. Thus, if the binding occurs through the hydroxylic groups at the C3' and C4' positions, quercetin acts as a substrate, while if it occurs through the hydroxylic group at the C3 position of the pyrone ring, quercetin acts as an inhibitor.