Antioxidant and immunostimulant β-glucan from edible mushroom Russula albonigra (Krombh.) Fr

•A β-glucan (PS) was isolated from the mushroom Russula albonigra (Krombh.) Fr.•Its structure was established by chemical and 1D/2D NMR studies.•This molecule showed immunostimulatory properties in vitro.•It also showed potent antioxidant activities. A water soluble β-glucan (PS) with an average molecular weight ∼1.95×105Da was isolated from the alkaline extract of ectomycorrhizal edible mushroom, Russula albonigra (Krombh.) Fr. and found to consist of terminal, (1→3)-, (1→6)-, and (1→3,6)-linked β-d-glucopyranosyl moieties in a ratio of nearly 1:2:2:1. The structure of this PS was elucidated on the basis of total hydrolysis, methylation analysis, Smith degradation, partial hydrolysis, and 1D/2D NMR experiments. On the basis of these experiments, the repeating unit of the PS was found to contain a backbone of three (1→6)-β-d-glucopyranosyl residues, one of which was branched at O-3 position with the side chain consisting of two (1→3)-β-d-glucopyranosyl and a terminal β-d-glucopyranosyl residue. This PS showed in vitro macrophage activation by NO production as well as splenocytes and thymocytes proliferation. Moreover, it also exhibited potent antioxidant activities.