Xanthine oxidase inhibitory activity of quercetin and its derivatives: Interaction mechanism and evaluation methods

Quercetin (QU) and its derivatives exhibited good potential in exerting xanthine oxidase (XO) inhibitory activity. Herein, this study comprehensively investigated the interaction mechanism of QU and its derivatives in inhibiting XO activity by the molecular simulation, inhibition kinetics, ultraviolet–visible spectra, circular dichroism spectrum, and fluorescence spectrum methods. Results showed that glycosylation and hydrogenation of QU were unfavorable for the XO inhibitory activity, while methylation and hydroxylation were beneficial. The types of hydrogen bonds between flavonoids and amino acid residues played an important role in the XO inhibitory activity. Meanwhile, the mixed inhibition type with XO was considered more effective than noncompetitive. Moreover, QU and its derivatives broke the secondary structure and hydrogen networks and quenched the intrinsic fluorescence of XO. The formation of XO-flavonoids complex was endothermic and spontaneous, and H-bonding and van der Waals forces might play a dominant role in this process. Absorption, distribution, metabolism, excretion, and toxicity (ADMET) prediction showed that QU and its derivatives possessed good pharmacokinetic properties. Furthermore, molecular dynamics simulation, inhibition types, ultraviolet–visible spectrum, fluorescence quenching, and thermodynamic parameters could quantitatively evaluate the XO inhibitory effect of flavonoids with different structures. However, there was a deviation in the quantitative evaluation of the XO inhibitory activity of flavonoids by molecular docking and circular dichroism spectrum.