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

Abstract

Ethnopharmacological relevanceP. cuspidatum is a popular Chinese medicinal herb, having a long history of usage in traditional Chinese medicine for the treatment of several inflammatory diseases in the form of powders and decoctions. Similarly there are many reports that P. cuspidatum has antibacterial and anti-inflammatory effects, both of which are properties associated with compounds having activity against bacterial neuraminidase (BNA). Aim of the studyWe investigated whether P. cuspidatum's metabolites exhibited BNA inhibition. Consistent with our hypothesis, we found several inhibitors from the methanol extract of this plant, and then fully characterized their inhibitory mechanisms. Materials and methodsActivity guided separation of methanol extract led to isolation of individual constituents, and subsequently their structures were elucidated by spectroscopic analysis. Detailed kinetic behaviors of BNA inhibitors were explored by showing the changes of Km and Vmax, the ratios of KI/KIS and Kik/Kiv, and fluorescence quenching effect. Results and conclusionThis 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 (KSV) 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.