Association of structural characteristics with immunostimulatory activity of pectic polysaccharides prepared from Tartary buckwheat sprouts by enzymatic degradation and partial acid hydrolysis.

Although Tartary buckwheat sprout pectic polysaccharides (YM) have been reported to exhibit immunostimulatory activity, their structure-activity relationship remains poorly understood. We therefore compared the structural characteristics and immunostimulatory activities of three YM fractions to explore structure-activity associations. YM was enzymatically digested and partially acid-hydrolyzed to yield two derivatives, YME and YMA, with distinct structures. YME showed an increased proportion of the galactose-rich rhamnogalacturonan-I (RG-I) domain (from 68.66 to 72.65 mol%) and a moderate molecular mass reduction (from 1.26 × 105 to 0.85 × 105 Da), whereas YMA was completely depleted of arabinose-side chains in RG-I (from 16.97 to 0 mol%), with increased galacturonic acid (from 19.96 to 34.85 mol%) and a drastic molecular mass reduction (from 1.26 × 105 to 0.16 × 105 Da). Both YME and YMA showed higher immunostimulatory activity than YM. YMA significantly increased body mass, thymus and spleen indices, cytokines interleukin-6 (IL-6), interferon-γ, immunoglobulins (IgA, IgG), tight-junction proteins (ZO-1, occludin), and short-chain fatty acids (SCFAs), particularly butyrate, whereas YME only improved spleen index, IL-6, IgG, ZO-1, occludin, and butyrate (all p < 0.05). These results suggest that YM's immunostimulatory activity is closely associated with galactose-rich RG-I domains, unesterified uronic acids, and molecular mass. This enhanced efficacy is associated, at least in part, with an enrichment of beneficial bacterial taxa (e.g., Muribaculaceae, Bacteroides, Rikenellaceae_RC9_gut_group), leading to improved SCFAs and enhanced gut barrier function and immune responses. These findings advance the understanding of YM's structure-immunostimulatory activity relationship, highlighting the role of galactose-rich RG-I domains in determining its activity.