Abstract: We undertook this study to evaluate the structure and immunomodulatory activity of polysaccharides from dried Siraitia grosuenorii fruits. The crude polysaccharides were obtained by water extraction followed by alcohol precipitation and purified and separated into two homogeneous fractions: SGP-C2 and SGP-D1 through sequential Cellulose DE-52 and Sephadex G-200 column chromatography. The monosaccharide compositions and molecular masses of SGP-C2 and SGP-D1 were analyzed by pre-column derivatization with 1-phenyl-3-methyl-5-pyrazolone (PMP) followed by high performance liquid chromatography (HPLC) equipped with a refractive index detector (RID). Their structures were characterized by Fourier transform infrared spectrometer (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. Their immunomodulatory activity was evaluated by the neutral red colorimetric assay, their activity to induce the release of reactive oxygen species (ROS) from RAW264.7 cells was tested using 2’,7’-dichlorodihydrofluorescein diacetate (DCFH-DA) as the fluorescent probe, and their activity to induce NO production and the secretion of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in RAW264.7 cells was examined using a Griess kit and an enzyme-linked immunosorbent assay (ELISA) kit, respectively. SGP-C2 and SGP-D1 were heteropolysaccharides composed of glucuronic acid, rhamnose, galacturonic acid, glucose, galactose and arabinose with mole ratios of 0.78:1.84:26.57:1.31:0.49:0.06 and 1.37:2.00:20.79:0.61:0.12:0.25, respectively. Their relative molecular masses were 5.0 × 105 and 7.6 × 105 Da, respectively. NMR analysis showed that the backbone chain structure of SGP-C2 was →4)-α-D-GalpA-(1→ and →4)-α-D-GalpAMe-(1→, while that of SGP-D1 was →4)-α-D-GalpA-(1→. In the concentration range of 6.25–25 µg/mL, both polysaccharides promoted macrophage phagocytosis, NO production, and the secretion of TNF-α and IL-6. Besides, SGP-D1 significantly promoted the release of ROS. These results showed that SGP-C2 and SGP-D1 had immune-enhancing effects in vitro. This study provides the scientific basis for the development and application of polysaccharides from S. grosuenorii.

Key words: Siraitia grosuenorii; polysaccharide; purification; structural characterization; immunomodulatory