Isolation, Purification, Structural Characterization of Acidic Polysaccharides from Brassica rapa L. Rhizomes and Their In Vitro Activity Verification in Ameliorating Glycolipid Metabolism Disorders

Acidic polysaccharides, valued for their outstanding bioactivity and physicochemical properties, represent a promising strategy for metabolic disease intervention. In this study, three acidic polysaccharide fractions (BRP-1, BRP-2, and BRP-3) were isolated from Brassica rapa L. using membrane filtration and ion-exchange chromatography. BRP-3, notable for its high galacturonic acid content (76.64%), was further purified to yield the homogeneous fraction BRP-3-1 (Mw = 22.3 kDa). Combining GC-MS, FTIR, and NMR analyses, we report for the first time the detailed structure of BRP-3-1—a heteropolysaccharide composed of rhamnose (1.687%), galacturonic acid (75.584%), galactose (14.452%), and arabinose (8.277%)—with a backbone composed with T-α-L-Araf-(1 → 5)-α-L- Araf -(1 → 4)-α-D-GalpA-(1 → 4)-α-D-2-O- GalpA Me-(1 → 4)-α-D-GalpA-(1 → 4)-α-D-GalpA-(1 → 3)-Galp-(1 → 4)-α-D-GalpA, and T-Rhap, T-Galp as well as T-GalpA for branched chain and terminals. In HepG2 insulin-resistant cells, BRP-3-1 demonstrated potent dual regulation of glucose and lipid metabolism—enhancing glucose consumption, lowering total cholesterol, and significantly reducing triglyceride levels in the high-dose group (800 μg/mL), outperforming BRP-2. This work systematically defines the structure of a highly bioactive acidic polysaccharide from B. rapa L. and confirms its metabolic regulatory effects, offering a strong scientific foundation for its application in functional foods and as an adjuvant therapeutic for metabolic disorders.