Two polysaccharides from Rehmannia glutinosa : isolation, structural characterization, and hypoglycemic activities.

Rehmannia glutinosa (RG) as a Chinese herbal medicine can be used both in medicine and food. As the main component of RG, the polysaccharides have a hypoglycemic effect, however, the hypoglycemic activity of RG homopolysaccharides remains unknown. We isolated and purified two polysaccharides, RGP70-1-1 and RGP70-1-2 (4.9 kDa and 2.8 kDa) from RG. The structural characteristics, including monosaccharide composition, linkage, and configuration were analyzed by FT-IR, HPLC, GC-MS, NMR spectroscopy, Congo test, and SEM. RGP70-1-1 and RGP70-1-2 consist of four monosaccharides (glucose, mannose, arabinose, and galactose). RGP70-1-1 contains 14 connection modes, with the linkages including l-Ara f -(1→, →3)-l-Ara f -(1→, →5)-l-Ara f -(1→, →3,5)-l-Ara f -(1→, →2,5)-l-Ara f -(1→, d-Man p -(1→, →2)-d-Man p -(1→, →4)-d-Man p -(1→, d-Gal p -(1→, →4)-d-Gal p -(1→, →4,6)-d-Gal p -(1→, →6)-d-Glc p -(1→, →4,6)-d-Glc p -(1→, →3,6)-d-Glc p -(1→. The linkages of RGP70-1-2 is including →5)-l-Ara f -(1→, →3,5)-l-Ara f -(1→, →4)-d-Man p -(1→, →3,6)-d-Man p -(1→, d-Gal p -(1→, →6)-d-Gal p -(1→, d-Glc p -(1→, →6)-d-Glc p -(1→, →4,6)-d-Glc p -(1→. Furthermore, RGP70-1-1 and RGP70-1-2 can inhibit α-glucosidase and α-amylase. RGP70-1-1 stimulated GLP-1 secretion in STC-1 cells and was related to the up-regulation of PI3K and p-AKT protein expression. The findings revealed a natural product with potential hypoglycemic activity, which may be used as a GLP-1 secretagogue and a beneficial functional food ingredient for T2D.