Structural elucidation and immunomodulatory activities in vitro of type I and II arabinogalactans from different origins of Astragalus membranaceus

Astragalus membranaceus polysaccharide (APS) possesses excellent immunomodulatory activity. However, there are several studies on the structural characterization of APS. Here, we aimed to elucidate the repeating units of polysaccharides (APS1, 106.5 kDa; APS2, 114.5 kDa) obtained from different Astragalus membranaceus origins and further investigated their immunomodulatory activities. Based on structural analysis, types of the two polysaccharides were identified as arabinogalactan-I (AG-I) and arabinogalactan-II (AG-II), and co-elution of arabinogalactans (AGs) and α-glucan was observed. The backbone of AG-I was 1,4-linked β-Galp occasionally substituted by α-Araf at O-2 and/or O-3. AG-II was a highly branched polysaccharide with long branches of α-Araf, which were attached to the O-3 of 1,6-linked β-Galp of the backbone. The presence of AGs in A. membranaceus was confirmed for the first time. The two polysaccharides could promote the expression of IL-6, IL-1β and TNF-α in RAW264.7 cells via MAPKs and NF-κB signaling pathways. The constants for APS1 and APS2 binding to Toll-like receptor 4 (TLR4) were 1.83 × 10-5 and 2.08 × 10-6, respectively. Notably, APS2 showed better immunomodulatory activity than APS1, possibly because APS2 contained more AGs. Hence, the results suggested that AGs were the vital components of APS in the immunomodulatory effect.