Structural characteristics and hypoglycemic activity of a polysaccharide from an edible bolete Phlebopus portentosus

Phlebopus portentosus, a bolete renowned for its exceptional flavor, delightful taste, and pleasant texture, has been reported to exhibit hypoglycemic activity attributed to its polysaccharides. However, the structural characteristics of these polysaccharides and the mechanisms of action remain unclear. In this study, a neutral polysaccharide (PPP-0A) was isolated and purified from its fruiting bodies using DEAE-52 cellulose and Sephacryl S-400 HR propylene dextran gel column chromatography. The structural characteristics of PPP-0A were elucidated at multiple levels, employing SEC-MALLS-RI, GC-MS, IR, methylation analysis, NMR, SEM, and CD. PPP-0A is identified as an α-pyranoside with a molecular weight of 13.2 kDa, comprising galactose, fucose, glucose, mannose, and xylose in the ratios of 62.26:16.96:15.23:4.68:0.87. Methylation and NMR analyses suggest that the backbone of PPP-0A mainly consists of →6)-α-D-Galp-(1→, →2,6)-α-D-Galp-(1→, and →3)-α-D-Glcp-(1→. The branches are formed by α-L-Fucp-(1→ linked to the sugar residue at the O-2 position of →2,6)-α-D-Galp-(1→. Furthermore, we confirmed the potent hypoglycemic activity of PPP-0A through enzyme activity assays, inhibition kinetics, and in vitro insulin resistance experiments using HepG2 cells. This study clearly outlines the structural features of purified polysaccharides from P. portentosus under specific processing conditions, establishing a foundation for further investigation into the structure-activity relationship of these bolete polysaccharides.