Structural characterization and immunomodulatory activity in vitro of an arabinogalactan from the pulp of Siraitia grosvenorii.

An arabinogalactan (SGP80-A1) was isolated for the first time from the pulp of Siraitia grosvenorii, with an apparent Mw of 81.3 kDa. Its structure was initially characterized using an integrated strategy combining NMR spectroscopy with database matching, allowing non-destructive identification of sugar residue types directly within intact polysaccharides. Monosaccharide analysis by PMP-HPLC further revealed that SGP80-A1 was composed of arabinose, galactose, and glucuronic acid in a molar ratio of 49.8:46.6:3.6, which was consistent with 1D 1H qNMR analysis, where arabinose accounted for approximately 49.6% and galactose together with glucuronic acid accounted for approximately 50.4%. Methylation analysis (GC-MS) was subsequently employed to determine the glycosidic linkage patterns, which supported the 2D NMR-based structural assignment. Additionally, Yariv reactivity indicated that SGP80-A1 contains β-1,3-galactan fragments. SGP80-A1 features a backbone of →6)-β-Galp-(1→, which is substituted at the O-3 position primarily by various α-arabinofuranosyl side chains, alongside a small proportion of β-Galp-(1 → 3)-β-Galp-(1 → branches. For the critical 1,3,6-β-Gal residues exhibiting highly overlapped H6b and H5 signals, a 13C-coupled HSQC-TOCSY experiment was employed to obtain accurate chemical shift assignments. In vitro bioactivity assays indicated that SGP80-A1 (40-160 μg/mL) potently stimulated immune responses in RAW264.7 macrophages following effective endotoxin removal, as evidenced by significantly enhanced production of NO, IL-6 and TNF-α, thereby confirming the reliability of the observed immunomodulatory potential of SGP80-A1. Collectively, these results not only characterize the structural features of SGP80-A1 but also highlight its potential as an immunostimulant, providing a foundation for further investigation of its in vivo efficacy and underlying mechanisms.