Comparative study on the structural characterization and α-glucosidase inhibitory activity of polysaccharide fractions extracted from Sargassum fusiforme at different pH conditions

Sargassum fusiforme polysaccharides (SFPs), including SFP-3-40, SFP-3-60, SFP-3-80, SFP-7-40, SFP-7-60, SFP-7-80, SFP-10-40, SFP-10-60, and SFP-10-80, were extracted at different pH (3, 7, and 10), and then precipitated with graded precipitation of 40%, 60% and 80% (v/v) ethanol solution, respectively. Their physicochemical properties and α-glucosidase inhibitory activity were determined. Results showed that SFPs significantly differed in the contents of total sugar, protein, uronic acid, sulfate, the zeta potential, and molecular weight distribution. SFPs, including SFP-10-40, SFP-10-60, and SFP-10-80, had bigger absolute zeta potential value and higher respective average molecular weight in the same ethanol concentration precipitate. All samples were mainly composed of fucose, glucuronic acid, and mannose with different molar ratios. The extraction pH and precipitation ethanol solution concentration caused little changes in functional groups, but significantly altered surface morphology of SFPs. Congo red test revealed that all polysaccharides were not helical polysaccharides. Rheological measurements indicated that SFPs were pseudoplastic fluids and showed elastic behavior of the gel. Except SFP-3-40 and SFP-3-60, all other samples had a stronger α-glucosidase inhibitory activity than that of acarbose. The inhibition type of SFPs against α-glucosidase varied owing to different extraction pH and precipitation ethyl concentration. This study shows that extraction pH can significantly affect the structure and hypoglycemic activity of SFPs and provide a data support for the scientific use of Sargassum fusiforme in industrial production.