Galangin competitively inhibits xanthine oxidase by a ping-pong mechanism

Galangin is a natural flavonol isolated from plants with potent biological activities. Galangin was found to significantly inhibit xanthine oxidase (XOD) activity in a competitive manner with the generation of superoxide radical (O₂^-) in the enzyme catalysis, but galangin showed insignificant scavenging activity on 1,1-diphenyl-2-picryhydrazyl (DPPH) and O₂^- radicals. These results demonstrated that inhibition of O₂^- radical generation by galangin may be due to the competitive inhibition of uric acid formation by a ping-pong mechanism. XOD had one high affinity binding site for galangin with a binding constant of 3.60×10⁴Lmol^-1 at 298K. Hydrogen bond and hydrophobic interaction dominated the binding process on account of the negative enthalpy and positive entropy changes. The binding of galangin to XOD induced an increase in α-helix and random coil contents and a decrease in β-sheet and β-turn contents of XOD. Further molecular docking study validated that galangin can competitively bind to the site in the molybdenum atomic (Mo) domain, occupying the catalytic center to avoid the entrance of the substrate xanthine, resulting in the inhibition of XOD activity. These findings have provided new insights into the two-substrate kinetics of galangin on XOD and useful information for functional research of galangin in the treatment of gout and oxidative damage.