An antioxidative galactomannan–protein complex isolated from fermentation broth of a medicinal fungus Cs-HK1

A protein-containing polysaccharide, EPS2BW, was fractionated from the exopolysaccharide (EPS) produced by a medicinal fungus Cordyceps sinensis (Cs-HK1). EPS2BW was mainly composed of galactomannan with about 16% (w/w) protein and 50 kDa average molecular weight. The galactomannan part consisted of mannose and galactose at 1.7:1.0 molar ratio, and the protein segments were composed of sixteen amino acids with 12.5% proline and 16.6% threonine (mol%) being the most abundant. Based on analytical results from NMR, methylation analysis, partial acid hydrolysis and GC–MS, the galactomannan structure was elucidated as a (1 → 2)-α-d-mannopyranosyl (Manp) backbone with O-6-linked galactopyranosyl (Galp) branches. EPS2BW exhibited a high antioxidant capacity in both chemical and cell culture assays, with a Trolox equivalent radical scavenging activity of 44.7 μmol Trolox/mg, a Fe3+ reducing power of 38.9 μmol Fe2+/mg, and significant cytoprotective effect against H2O2-induced PC12 cell death at 50–250 μg/mL.