Lignan Constituents from the Fruits of Viburnum macrocephalum f. keteleeri and Their α-Amylase, α-Glucosidase and PTP1B Inhibitory Activities

Eight previously undescribed lignan glycosides, viburmacrosides A-H (1-8), and seven known analogues (9-15) were isolated from Viburnum macrocephalum f. keteleeri fruits through bioactivity-guided fractionation. Their structures and absolute configurations were elucidated by extensive spectroscopic analyses and chemical evidences. Using the well-recognized carbohydrate-hydrolyzing enzymes α-amylase and α-glucosidase as well as the promising protein tyrosine phosphatase 1B (PTP1B) as inhibitory targets, all isolated compounds were tested for their anti-diabetic potential in vitro. Compound 4 displayed potent inhibitory activities with IC50 values of 9.9 ± 0.6 and 8.9 ± 0.5 μM against α-glucosidase and PTP1B, respectively. The results from enzymatic kinetics suggested that compound 4 competitively inhibited α-glucosidase while it suppressed α-amylase and PTP1B in the mixed-type manner. These findings supported that V. macrocephalum f. keteleeri fruits may be a new functional food resource with anti-diabetic potential.