Microwave-assisted degradation of β-D-glucan from Ganoderma lucidum and the structural and immunoregulatory properties of oligosaccharide fractions

Microwave-assisted degradation of β-(1 → 3,1 → 6)-D-glucan from Ganoderma lucidum and correlated immunoregulatory activities were investigated in this study. The optimal temperature and degradation time for microwave hydrothermal hydrolysis were 140 °C and 40 min, respectively. Under these conditions, a high yield of degradation rate (98.4 %) and abundant β-oligosaccharide products (GLOS) with different degrees of polymerization (DP 2–24) were obtained. Four fractions including F1 (DP 2–8), F2 (DP 6–19), F3 (DP 8–24) and F4 (high DPs) with different average ratios of β-(1 → 3) to β-(1 → 6)-linked glucose units were isolated from GLOS. The structures of oligosaccharides with DP (2–6) in F1 were identified as linear β-(1 → 3)-linked glucooligosaccharides without or with β-(1 → 6)-linked glucose residues based on MS/MS analysis. The immunoregulation activity of β-glucooligosaccharides was correlated with their DPs and the average ratios of β-(1 → 3) to β-(1 → 6)-linked glucose units. F4 fraction with high DPs and ratio of 3.29:1 exhibited higher immunoenhancing activity on inducing NF-κB activation through binding to dectin-1. Surface plasmon resonance (SPR) analysis indicated that β-glucooligosaccharides could bind to Dectin-1 directly and the binding affinity increased with the increase of DPs and the ratios of β-(1 → 3)-linked glucose.