Potent bacterial neuraminidase inhibitors, anthraquinone glucosides from Polygonum cuspidatum and their inhibitory mechanism

Abstract

This study attempted to isolate the responsible metabolites and elucidate the BNA inhibitory mechanism. The principal BNA inhibitory compounds (2-6) were identified as emodin (2), physcion-8-O-β-D-glucopyranoside (3), emodin-8-O-β-D-glucopyranoside (4), emodin-1-O-β-D-glucopyranoside (5), and 2-methoxy-6-acetyl-7-methyljuglone (6). Unexpectedly, anthraquinone glucosides (3-5) were much more potent than their corresponding aglycones (1 and 2). For example, emodin (2) had an IC₅₀=5.4μM, whereas its glucosides (4 and 5) had IC₅₀=0.85μM and 0.43μM respectively. A similar trend was observed with physcion (1, IC₅₀>200μM) and its glucoside (3, IC₅₀=6.2μM). The anthraquinone (2) was mixed type I inhibitor, whereas its glucosides (4 and 5) were noncompetitive. In addition, the fluorescence quenching study showed that the affinity constants (K_SV) of inhibitors increased in proportion to their inhibitory potencies. Furthermore, we quantified the major and minor metabolites through UPLC-PDA-Q-TOF/MS, and revealed that the most potent inhibitors were the major constituents. This result contributes to our understanding of P. cuspidatum utility as functional food stuff and widely used herbal medicine.