Structure identification of a polysaccharide in mushroom Lingzhi spore and its immunomodulatory activity

Ganoderma lucidum spore serves as a well-known immunomodulatory functional food in Asia. The polysaccharides in G. lucidum spore are responsible for the claimed immunomodulatory activity. However, the structural information of polysaccharides remains unclear. In this work, the leading water-soluble polysaccharide in G. lucidum spore (GLSP-I) with a molecular weight of 128.0 kDa was isolated and purified. The monosaccharide composition analysed by gas chromatography indicated that GLSP-I was a glucan. Three side chains, including Glc-(1 → 3)-Glc-(1 → 3)-Glc-(1 → 6)-Glc, Glc-(1 → 6)-Glc-(1 → 6)-Glc-(1 → 6)-Glc and Glc-(1 → 3)-Glc-(1 → 3)-Glc-(1 → 3)-Glc-(1 → 3)-Glc, were identified by UPLC-MS/MS. The structural characteristics were further identified by NMR spectra. The results indicated that the backbone of GLSP-I was (1 → 3)-β-D-glucan, with side chains linking at O-6. The proposed structure was drawn as below. The immunomodulatory activity assay indicated that GLSP-I could activate macrophages in a dose-dependent manner.