Mechanisms underlying the xanthine oxidase inhibitory effects of dietary flavonoids galangin and pinobanksin

Xanthine oxidase (XOD) inhibitory activities of five dietary flavonoids pinobanksin, galangin, pinocembrin, pinocembrin-7-O-β- d -glucopyranoside and glabranin were evaluated. Enzyme kinetic studies and molecular docking simulation were conducted to investigate the mechanisms underlying the inhibitory activities. The results showed that these flavonoids exhibited excellent inhibitory activities (which were ranked in the order of pinobanksin > galangin > pinocembrin > pinocembrin-7-O-β- d -glucopyranoside > glabranin). Competitive inhibition and a mixed-type of competitive–noncompetitive inhibition were observed. The mode of inhibition was dependent on the type and concentration of the substrate and inhibitor. Fluorescence quenching data suggested that these flavonoids could interact with XOD at more than one binding site. The docking simulation revealed that galangin and pinobanksin could enter into the active site of XOD and form hydrogen bonding with amino acid residues (such as Ser-876, Asn-768, Glu-1261 and Thr-1010) and sandwiching aromatic interactions (π–π interactions) around the active site of XOD.