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.