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-Araf-(1 ->, -> 3)-L-Araf-(1 ->, -> 5)-L-Araf-(1 ->, -> 3,5)-L-Araf-(1 ->, -> 2,5)-L-Araf-(1 ->, D-Manp-(1 ->, -> 2)-D-Manp-(1 ->, -> 4)-D-Manp-(1 ->, D-Galp-(1 ->, -> 4)-D-Galp-(1 ->, -> 4,6)-D-Galp-(1 ->, -> 6)-D-Glcp-(1 ->, -> 4,6)-D-Glcp-(1 ->, -> 3,6)-D-Glcp-(1 ->. The linkages of RGP70-1-2 is including -> 5)-L-Araf-(1 ->, -> 3,5)-L-Araf-(1 ->, -> 4)-D-Manp-(1 ->, -> 3,6)-D-Manp-(1 ->, D-Galp-(1 ->, -> 6)-D-Galp-(1 ->, D-Glcp-(1 ->, -> 6)-D-Glcp-(1 ->, -> 4,6)-D-Glcp-(1 ->. Furthermore, RGP70-1-1 and RGP70-1-2 can inhibit alpha-glucosidase and alpha-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.