Insights into the inhibitory mechanism of purpurogallin on xanthine oxidase by multiple spectroscopic techniques and molecular docking

Purpurogallin, a benzotropolone containing natural compound found in nutgall and oak bark, was investigated for its inhibitory mechanism on xanthine oxidase (XO) by multispectroscopic methods and molecular docking analysis. The enzyme kinetic analysis showed that purpurogallin possessed a strong inhibition on XO activity in a reversible mixed type manner with IC50 value of (5.60 ± 0.13) × 10−6 mol.L−1. The results of fluorescence titration indicated that purpurogallin presented a strong fluorescence quenching effect through a spontaneous and exothermic static quenching procedure, and the interaction was predominately driven by hydrogen bonds and van der Waals forces. The synchronous fluorescence confirmed that purpurogallin increased the polarity of tyrosine and tryptophan microenvironment. Analysis of circular dichroism demonstrated that purpurogallin induced the conformational change of XO with increases in α-helix and reductions in β-strand and random coil structures. Further molecular docking showed that purpurogallin occupied molybdenum atomic domain of XO, and formed hydrogen bonding with amino acid residues (Arg880, Thr1010, Val1011 and Glu1261). These findings provide comprehensive insights into understanding the inhibitory mechanism of purpurogallin on XO.