In Silico and In Vitro Evaluation of Quercetin Metabolites Binding to Inflammatory Target Proteins

Background/Objectives: The most abundant flavonoid, quercetin, which is mostly found as glycosides, is widely distributed in plants. Quercetin is rapidly metabolized, having a short half-life in the blood circulation, and forms its conjugates by undergoing ring cleavage of the benzopyranone ring system. Despite its fast clearance in the body, quercetin was demonstrated to have clinically anti-inflammatory, cardioprotective, antidiabetic, and anti-obesity activities. This study aimed to determine whether quercetin itself or its metabolites are responsible for these activities. Methods: We performed molecular docking of 27 metabolites, including quercetin itself, against ten inflammation-related proteins in silico. We then conducted microscale thermophoresis (MST) of selected metabolites towards the NLRP3 inflammasome. Results: Overall, Phase II metabolites yielded better binding energies compared to the metabolites formed by degradation. MST results revealed that isorhamnetin, the 4-O-methylated metabolite of quercetin, gave the best results, with a binding affinity (KD value) of 16.12 ± 5.16 µM, even better than quercetin itself, which has a binding affinity of 44.84 ± 4.21 µM. Glucuronide metabolites of quercetin (isorhamnetin 3-O-glucuronide, quercetin 7-O-glucuronide, and quercetin 3-O-glucuronide) were found to bind to the inflammasome protein with low binding affinities, whereas small degradation products (hippuric acid and 3,4-dihydroxytoluene) did not bind at all. Conclusions: These results suggest that Phase II metabolites, specifically isorhamnetin, may contribute more significantly to the biological activity of quercetin than the parent compound, however, degradation products appear inactive.