Structural characterization of polysaccharides from Saposhnikovia divaricata and their antagonistic effects against the immunosuppression by the culture supernatants of melanoma cells on RAW264.7 macrophages

Two purified native polysaccharides, namely, SDNP-1 and SDNP-2 with apparent molecular weight of 67.9 × 103 and 5.2 × 103, separately, were isolated from the water extract of Saposhnikovia divaricata by combined ion-exchange and gel permeation chromatography. The structures of these polysaccharides were characterized by high-performance gel permeation chromatography, chemical derivative analysis, GC–MS, FT-IR spectroscopy, and NMR spectroscopy. Results showed that SDNP-1 and SDNP-2 were composed of arabinose and galactose with molar ratio of approximately 1:1. The polysaccharides were deduced to be AGII-type arabinogalactans with distinct molecular weights and backbone chains. The backbone chain of SDNP-2 mainly consisted of (1 → 3)-linked and (1 → 6)-linked Galp residues with an approximate molar ratio of 1:3. Approximately half of the (1 → 6)-linked Galp residues were substituted at O-3, and small amounts of (1 → 6)-linked Galp were substituted at O-3 and O-4 by branches that mainly consisted of terminal and (1 → 5)-linked Araf residues. Bioactivity tests showed that SDNP-2 exhibited significant antagonistic effect against immunosuppression as shown by the cell viability of the culture supernatants of melanoma cells on RAW264.7 macrophages. However, SDNP-1 showed no effect. Thus, molecular weight and backbone chain may be the key factors for the antagonistic effects against immunosuppression.