Fluorescence quenching study of quercetin interaction with bovine milk xanthine oxidase.

Quercetin is a natural flavonoid with many important therapeutic properties. The interaction of this polyphenolic compound bovine milk xanthine oxidase as one of its major target proteins was studied using fluorescence quenching method for the first time. It was found that the fluorescence quenching of xanthine oxidase occurs through a static mechanism. The results revealed the presence of a single binding site on xanthine oxidase with the binding constant value equals to 1.153 x 10(4) l mol(-1) at 310 K and pH 7.4. The thermodynamic parameters were also calculated at different temperatures. The enthalpy and entropy changes were found as -10.661 kJ mol(-1) and +43.321 J mol(-1) K(-1) indicating that both hydrogen binding and hydrophobic are involved in the interaction of this polyphenolic natural compound with xanthine oxidase. The results may provide a ground for further studies with different flavonoids to find a safe alternative for allopurinol, the only xanthine oxidase inhibitor with clinical application.