Microbial transformation and cordycepin co-administration enhanced incretin-related and prebiotic potentials of galactomannans from two herbs.

Regulation of gut microbiota composition by polysaccharides has been regarded as a potential therapeutic strategy. Accordingly, to evaluate the interaction between a specific type of polysaccharides (galactomannans) and intestinal bacteria and their in vitro activities, a total of twenty-three polysaccharides were isolated from two medicinal herbs rich in galactomannans or glucogalactomannans, Trigonella foenum-graecum and Cordyceps militaris. Two fractions from C. militaris,CM-1a02 and CF-1b03, exhibited significant effects on two important in vitro incretin-related anti-hyperglycemic targets (IC₅₀ = 75.0 μg/mL or 0.8 μM against DPP-4 and 2.4-fold in GLP-1 secretion, respectively), and promoted the growth of a representative gut microbe (Bacteroides ovatus, BO) for the digestion of galactomannans. In addition, TF-5, the main fraction from T. foenum-graecum, also showed similar bioactivity (2.1-fold in insulin secretion and IC₅₀ = 134.4 μg/mL or 4.0 μM against DPP-4). Molecular docking confirmed that TF_BO (the metabolite of TF-5 by BO degradation) binds to a DPP-4 catalytic pocket, providing structural insights into its mechanism of action. Notably, co-administration of CM-1a02 or CF-1b03 with cordycepin enhanced the inhibition of DPP-4. Chemical and spectroscopic identification of the four active fractions (TF-5, TF_BO, CM-1a02 and CF-1b03) resulted in the elucidation of their possible repeat units or partial fragments. These findings reveal the structure-function relationships between the results of the three incretin-based in vitro screenings and the interaction between a key gut microbe and the tested fractions.