Identification of major α-glucosidase inhibitors in Radix Astragali and its human microsomal metabolites using ultrafiltration HPLC–DAD–MSn

Radix Astragali is one of the most popular traditional medicinal herbs with α-glucosidase inhibitory activity, however, more comprehensive information regarding α-glucosidase inhibition of Radix Astragali and its metabolites is yet unknown. Here, an ultrafiltration HPLC-DAD-MS(n) was developed to rapidly and selectively screen and identify major α-glucosidase ligands from Radix Astragali and its human microsomal metabolites. The developed method showed high selectivity and specificity to directly screen α-glucosidase ligands from complex system by testing mixtures of positive ((+)-catechin) and negative (salicylic acid) controls in the optimized conditions. As a result, thirteen prototype isoflavonoids and one monohydroxylated metabolic isoflavonoid with α-glucosidase binding activity were observed. Their structures were elucidated by combination of high-resolution MS, linear ion trap MS(n), in-source collision-induced dissociation (CID) fragmentation and NMR data. Particularly, except for calycosin and formononetin, the other twelve isoflavonoids were found as new α-glucosidase ligands. The activity of eight aglycones among fourteen ligands (glycosides were almost deglycosylated in vivo) was evaluated and confirmed using in vitro enzymatic assay. The results indicated that the proposed ultrafiltration HPLC-DAD-MS(n) method was a powerful tool for the discovery of α-glucosidase inhibitors from complex matrix, and these findings would enhance understanding of the real biochemical profiles of Radix Astragali.