Fast dereplication of xanthine oxidase-inhibiting compounds in alfalfa using comparative metabolomics

Xanthine oxidase (XO) inhibition is a major strategy for preventing hyperuricemia and associated comorbidities, such as gout. Alfalfa extract has been demonstrated to possess XO-inhibiting activity; however, the elaborate conventional fraction-by-fraction analyses hindered the identification of the active components. In this study, we established a streamlined approach to rapidly screen, identify, and characterize XO-interacting compounds in alfalfa, by incorporating protein-subtraction, mass profiling, and molecular docking analysis. Crude extract was incubated with or without XO protein before UPLC-ESI-Q-TOF-MS/MS composition profiling. By dereplicating the component profile of XO-subtracted extract from that of untreated extract, the targets were rapidly narrowed down to twelve XO-interacting compounds, regarded as potential xanthine oxidase inhibitors (XOIs). Molecular docking analysis revealed that nine of these compounds, namely salicylic acid, tricin 7-O-glucuronopyranoside, chrysoeriol-7-glucoside, ferulic acid, apigenin 7-O-β-glucuronopyranoside, apigenin, tricin, chrysoeriol, and liquiritigenin, exhibited high affinity with XO, and depicted the possible mechanisms of inhibition. In vitro bioassay further verified the XO inhibitory activities of selected compounds, among which apigenin, chrysoeriol and liquiritigenin were more potent XO inhibitors (XOIs), with IC50 of 0.25, 0.5 and 1 µM, respectively, compared to allopurinol (IC50 = 1.41 µM), the well-known XO-inhibiting drug. Together, the results demonstrated that alfalfa is a promising natural source for potent XOIs which might be applied for nutraceuticals development and that the approach used is applicable for efficient screening, identification, and mechanistic analyses of enzyme-inhibiting compounds from plant-based resources.