Locust bean gum galactomannan hydrolyzed by thermostable β-d-mannanase may reduce the secretion of pro-inflammatory factors and the release of granule constituents

Locust bean gum (LBG) galactomannan has been claimed to have applications in the biopharmaceutical field. However, the effects of LBG galactomannan on immunomodulatory aspects are not yet clear. The purpose of this study was to over-express thermostable β-d-mannanase from the thermophilic actinomycete Thermobifida fusca BCRC 19214 using a Pichia pastoris expression system. The maximum intracellular β-d-mannanase activity obtained from the cell-free extract was approximately 40.0 U/mL after 72 h of cultivating a P. pastoris transformant (pPICZ-man) induced with methanol. Hydrolysis of native LBG galactomannan with 8 U/mL β-d-mannanase for 24 h significantly decreased the weight-average molecular weight of LBG galactomannan from 5,580,010 to 3188. Native and hydrolyzed LBG galactomannan in a range of 0-0.2% did not trigger significant cytotoxicity after 24 h of treatment compared with the control. The native LBG galactomannan stimulated RAW 264.7 cells to produce cytokine TNF-α dose-dependently, but there was no significant IL-1β or nitric oxide production. The native LBG galactomannan also stimulated β-hexosaminidase secretion in RBL-2H3 cells. After the native LBG galactomannan was hydrolyzed with β-d-mannanase, all of the immunological properties disappeared. These results suggest the possible immunomodulatory effects of native LBG galactomannan.