Myricetin,an antioxidant flavonol,is a substrate of polyphenol oxidase

The present study demonstrates the antiradical efficiency of myricetin, a flavonol widely distributed in fruits and vegetables, by testing its ability to react with two different free radicals, ABTS^+· and DPPH^·. The polyphenolic nature of myricetin led us to consider the possibility of its oxidation by polyphenol oxidase (PPO). The results reported show that myricetin can be oxidised by PPO extracted and partially purified from broad bean seeds. The reaction was followed by recording spectral changes with time, maximal spectral changes being observed at 372 nm. The presence of two isosbestic points (at 274 and 314 nm) suggested that only one absorbing product was formed. The spectral changes were not observed in the absence of PPO. The oxidation rate varied with the pH, reaching its highest value at pH 5.5. The myricetin oxidation rate increased in the presence of SDS, an activing agent of polyphenol oxidase. Maximal activity was obtained at 1.3 mM SDS. The kinetic parameters were also determined: V _m = 1.35 µM min⁻¹, K _m = 0.3,mM , V _m/ K _m = 4.5 × 10⁻³ min⁻¹. Flavonol oxidation was inhibited by a selective PPO inhibitor such as cinnamic acid (K_I = 1 mM ). The results reported show that myricetin oxidation was strictly dependent on the presence of polyphenol oxidase. © 1999 Society of Chemical Industry