Structure and immunostimulating activity of a galactofuranose-rich polysaccharide from the bamboo parasite medicinal fungus Shiraia bambusicola

Shiraia bambusicola is a parasitic fungus on the twigs of bamboos. Its relatively large stroma has high medicinal value and can treat a variety of diseases such as rheumatoid arthritis, cold stomach pain, sciatica, injuries, chronic bronchitis, and infantile. It is widely distributed in many provinces in Southern China and also is also found in Japan. Medicinal fungi were important resources for bioactive polysaccharides. To explore bioactive polysaccharides from Shiraia bambusicola, a heteropolysaccharide SB2-1 was purified and obtained from S. bambusicola and its immunostimulating activity was researched. The polysaccharide from S. bambusicola was extracted and purified using enzyme assisted extraction, ethanol precipitation, anion-exchange and size-exclusion chromatography. Molecular weight of polysaccharide was estimated by high performance gel permeation chromatography. Monosaccharide compositions were determined by high performance liquid chromatography after pre-column derivatization and UV detection. Structure information was elucidated by IR spectrum, GC-MS analysis after methylation and gradual acid hydrolysis of the polysaccharide. The RAW264.7 cells were used to study the immunostimulating activity in vitro. Physicochemical and structural analyses showed that SB2-1 was a neutral heteropolysaccharide with molecular weight at 22.2 kDa and consisted of glucose, galactose and mannose at a ratio of 2.0:1.5:1.0. The structure of SB2-1 was a branched polysaccharides composed of a mannan core and side chains consisted of glucose and galactose. The mannan core was composed of (1→2)-Manp as the main chain. Glucose with (1→4)-D-Glcp, (1→2)-D-Glcp and (1→6)-D-Glcp at different degrees of polymerization were linked at C-6 and C-3 of the (1→2)-Manp as the side chains. The galactose with the linages of (1→6)-D-Galf, →2)-D-Galf(1→ and terminal D-Galf(1→ also existed in the side chain. The study on the immunostimulating activities of SB2-1 and its core structure P-2 were investigated on RAW264.7 macrophages. The results showed that SB2-1 could activate RAW264.7 macrophage and significantly improve its phagocytic ability by neutral red uptake experiment. Meanwhile, SB2-1 increased significantly higher inducible nitric oxide synthase (iNOS) production and the productions of IL-1, IL-6, IL-12 and TNF-α. The effect of SB2-1 was better than its core structure P-2 produced by gradual acid hydrolysis, which meant the side chains played an important role in the immunostimulating activities. The investigation demonstrated that the galactofuranose-containing mannogalactoglucan was characteristic polysaccharides in S. bambusicola and could enhance the activation of macrophages.