Structure characterization of a highly branched galactan from the slug Vaginulus alte and its utilization by human gut microbiota.

The slug Vaginulus alte is used as folk medicine in China, but the structure and activities of its galactan components remain to be clarified. Here, the galactan from V. alte (VAG) was purified. The Mw of VAG was determined as ~28.8 kDa. Chemical composition analysis showed that VAG was composed of d-galactose (75 %) and l-galactose (25 %). To elucidate its precise structure, a series of disaccharides and trisaccharides were purified from mild acid hydrolyzed VAG and their structures were characterized by 1D/2D NMR spectroscopy. Based on methylation analysis and structural analysis of oligosaccharides, VAG was elucidated as a highly branched polysaccharide and mainly composed of (1 → 6)- or (1 → 3)-linked β-d-galactose, and distinct (1 → 2)-linked α-l-galactose. The investigation of probiotic effects in vitro revealed that VAG could promote the growth of B. thetaiotaomicron and B. ovatus, while had no effect on the growth of L. acidophilus, L. rhamnosus, B. longum subsp. infantis and B. animalis subsp. lactis, but dVAG-3 with Mw ~1.0 kDa could promote the growth of L. acidophilus. These results will provide insights into specific structures and functions of polysaccharides from the V. alte.