Steam-Driven Structural Remodeling of Polygonatum sibiricum Polysaccharides: Correlating Macromolecular Changes with Enhanced Renal Cell Protection

The rhizome of Polygonatum sibiricum (PR) is a widely used medicinal and edible herb valued worldwide. Traditional processing by repeated steaming and sun-drying empirically enhances its therapeutic properties, primarily through alterations in its bioactive polysaccharides. This study aimed to elucidate the structure–activity relationship by systematically investigating the structural features and renoprotective activities of polysaccharides from PR and processed PR (PPR). Polysaccharides from PR and PPR (designated PRP and PPRP, respectively) were isolated and identified as five fractions (PRP-1, PRP-2, PPRP-1, PPRP-2, and PPRP-3). PPRP exhibited a broader molecular weight distribution (1.9–21.5 kDa) compared to PRP (2.6 kDa), indicating degradation and repolymerization. 1D/2D NMR analysis revealed that PRP-1 contained a backbone of →1)-β-D-Fruf-(2→, while processing introduced an additional →5)-α-L-Araf-(1→ linkage into PPRP-1. The acidic fractions, PPRP-2 and PPRP-3, showed a partial overlap of motifs, including →4)-α-D-GalAp-(1→ (in both methylated and non-methylated forms), →4)-β-D-Galp-(1→, →5)-α-L-Araf-(1→, and →2,4)-α-L-Rhap-(1→. Notably, PPRP-2 demonstrated superior renoprotective activity in high glucose-induced NRK-52E cells. These structural transformations provide a molecular basis for the enhanced bioactivity and functional-food potential of polysaccharides.