Polysaccharide–Protein Complex from Sargassum fusiforme: Fractionation, Characterization, and Hypoglycemic Activity

The present study aimed to isolate a polysaccharide–protein complex (SFPr-PS3A) from the edible brown seaweed Sargassum fusiforme and to evaluate its hypoglycemic activity. SFPr-PS3A exhibited strong inhibitory activity against α-glucosidase (87.81 ± 2.41%). Protease degradation of its protein fraction markedly reduced the bioactivity (8.04 ± 2.01%), highlighting the essential role of the protein in the complex. Methylation and nuclear magnetic resonance (NMR) analyses revealed that the polysaccharide backbone of SFPr-PS3A was mainly composed of the following repeating units: →[2)-β-d-Xylp-(1]3 → 3)-α-d-Rhap-(1 → [3)-α-d-Rhap-(1]2 → 4)-α-d-Glcp-(1 → 4)-α-d-Galp-(1→, with branches of β-d-Xylp-(1→ and α-d-Fucp-(1→ linked at specific positions. In the activity assessment, SFPr-PS3A exhibited strong α-glucosidase inhibitory activity with an IC50 value of 5.23 ± 0.85 μg/mL. After simulated gastrointestinal digestion, its inhibitory activity remained stable with no significant decrease. In an insulin-resistant (IR) HepG2 cell model, treatment with SFPr-PS3A effectively promoted glucose uptake and glycogen synthesis, concomitant with the upregulation of the protein expression of Akt and GLUT2.