Mechanism of fisetin suppressing superoxide anion and xanthine oxidase activity

Excessive activity of xanthine oxidase (XO) will promote the formation of uric acid and superoxide anion (O2−), which may cause hyperuricemia and oxidative damage. Fisetin, a dietary flavonoid, was found to insert into flavin adenine dinucleotides (FAD) center of XO, which might hinder the diffusion of O2− out of the FAD site, leading to the significant decrease of uric acid formation. The promotion of reduced XO and the suppression of uric acid formation resulting from the inhibition of XO by fisetin led to a decrease in O2− radical. Hydrogen bonding and hydrophobic interactions facilitated the formation of fisetin–XO complex. The chemical modification of XO confirmed the results of molecular docking that three polar amino acid residues Gln, Arg and Lys played a crucial role in the binding process. Furthermore, the isobolographic analysis showed that the joint use of fisetin and allopurinol exhibited a synergistic inhibition on XO activity.