Action mechanisms and interaction of two key xanthine oxidase inhibitors in galangal: Combination of in vitro and in silico molecular docking studies

Galangal extract (GE)-based hypouricemic functional food is under-developed due to ambiguous quality control standard that is closely associated with action mechanisms and interaction of key xanthine oxidase (XO) inhibitors (kaempferide and galangin) in GE. In terms of kinetics analysis, fluorescence quenching and molecular docking, kaempferide and galangin showed similar docking posture to xanthine in molybdopterin center, and formed flavonol-XO complexes driven by hydrogen bonding, hydrophobic interaction and van der Waals force, competitively inhibiting XO. Kaempferide, had stronger binding affinity for XO and three more hydrogen bonds with XO than galangin, interacting with critical amino acid residues (Arg880 and Glu802) in catalysis reaction of XO and showing stronger XO inhibitory activity than galangin. The combination of kaempferide and galangin enhanced their binding affinities for XO, showing synergistic inhibition on XO at optimal molar ratio 1:4 that could be quality control standard for GE. This study provided new insights into structure-XO inhibitory activity relationship of methoxylated flavonoids and quality control standard for GE-based hypouricemic functional food.