Oxidation of the flavonol quercetin by polyphenol oxidase.

Because direct oxidation of flavonols by polyphenol oxidase (PPO) has not previously been reported and, given the importance of flavonols, the ability of broad bean seed PPO to oxidize the flavonol quercetin was studied. The reaction was followed by recording spectral changes with time. Maximal spectral changes were observed at 291 nm (increase) and at 372 nm (decrease). The presence of two isosbectic points (at 272 and 342 nm) suggested the formation of only one absorbent product. These spectral changes were not observed in the absence of PPO. The oxidation rate, which varied with pH, was highest at pH 5.0. The following kinetic parameters were also determined: V(m) = 11 microM/min, K(m) = 646 microM, V(m)/K(m) = 17 x 10(-)(2) min(-)(1). Flavonol oxidation was efficiently inhibited (K(I) = 3.5 microM) by specific PPO inhibitors such as 4-hexylresorcinol. The results obtained showed that quercetin oxidation was strictly dependent on the presence of PPO.