Structural Characterization and In Vitro α-Amylase Inhibitory and Bile Acid-Binding Properties of a Glucan Isolated From Fermented Cowpea Natto.

Cowpea natto is a fermented legume food with potential functional properties. However, the effects of fermentation on cowpea polysaccharides remain insufficiently understood. In this study, cowpea was fermented with Bacillus subtilis natto, and polysaccharides extracted from unfermented cowpea (JD) and fermented cowpea natto (JD-F) were comparatively characterized. Fermentation increased the polysaccharide yield by 35.60%. A homogeneous polysaccharide, designated JDFE1-F, was isolated and purified from JD-F and had a weight-average molecular weight (M w) of 5.175 kDa. Monosaccharide composition analysis showed that JDFE1-F was predominantly composed of glucose (Glc, 98.22%). Methylation analysis and nuclear magnetic resonance (NMR) spectroscopy revealed that JDFE1-F possessed a backbone mainly composed of →4)-α-D-Glcp-(1→ residues, with limited substitution at the O-6 position. Its branched regions contained terminal α-D-Glcp-(1→ residues and minor α-D-Glcp-(1 → 5)-α-L-Araf-(1→ structural motifs. Following purification, the total carbohydrate content of JDFE1-F increased to 85.12% ± 3.09%. JDFE1-F exhibited an α-amylase IC50 of 2.668 ± 0.281 mg/mL, which was significantly lower than that of JD (p < 0.05). At 4 mg/mL, JDFE1-F bound 58.09% ± 2.58% of sodium glycodeoxycholate and 60.79% ± 2.87% of sodium taurodeoxycholate. The concentration required for 50% sodium glycodeoxycholate binding was significantly lower for JDFE1-F than for JD (p < 0.05), whereas no significant differences among the three preparations were observed for sodium taurodeoxycholate binding. These findings indicate that fermentation with Bacillus subtilis natto increased the polysaccharide yield and was associated with changes in the compositional and in vitro functional characteristics of cowpea polysaccharides. The low molecular weight, glucose-rich composition, and branched α-glucan structure of JDFE1-F may be associated with its observed in vitro activities, providing a basis for further investigation of its structure-function relationships and physiological relevance.