Structure and immunobiological activity of a new polysaccharide from Bletilla striata

A new polysaccharide (designated BSPF2), with a molecular weight of 2.35 × 105 Da, was isolated from the tubers of Bletilla striata. It contained mannose, glucose, and galactose in a molar ratio of 9.4:2.6:1.0. The acetyl content of BSPF2 was estimated to be 2.9%, and acetyl groups were located in positions 3 and 6 of mannosyl residues. The structural features were elucidated by a combination of monosaccharide composition analysis, periodate oxidation, partial acid hydrolysis, acetolysis, and methylation analysis. The results indicated that the backbone of BSPF2 consisted of (1 → 4)-linked mannosyl residues and (1 → 4)-linked glucosyl residues in a molar ratio of 2:1. About three fifths of glucosyl residues in the backbone were branched at O-6 position, and the terminal sugar residues were composed of mannosyl residues. Immunological assay results demonstrated that BSPF2 significantly induced the spleen cell proliferation in a dose-dependent manner.