Structural characterization and inhibitions on α-glucosidase and α-amylase of alkali-extracted water-soluble polysaccharide from Annona squamosa residue

A novel acidic polysaccharide, named as AWPA, was extracted form Annona squamosa residue by 0.1 M NaOH alkaline solution and purified by DEAE-cellulose and Sephadex G-150. HPLC analysis indicated that AWPA was a homogeneous polysaccharide with molecular weight of 3.08 × 103 kDa. The monosaccharide composition of AWPA, determined by ion chromatography, was consisted of L-arabinose, D-galactose, d-glucose, D-mannose, D-galacturonic acid in a percentage of 15.58:13.48:60.14:9.02:1.78, respectively. The results of FT-IR, methylation and NMR showed that the sugar residue of AWPA were mainly composed of α-L-Araf-(1→, →4)-α-D-Glcp-(1→, →4)-β-D-Galp-(1→, →6)-β-D-Glcp-(1→, →4,6)-β-D-Galp(1→, →3,6)-α-D-Manp-(1→, respectively. The Congo red experiment on AWPA showed that there was helix conformation. The microstructure of AWPA was detected by scanning electron microscopy, showing that the shape of AWPA was reticular and its structure was irregular. AWPA had effectively α-glucosidase inhibitory activity and α-amylase inhibitory activity with IC50 of 0.667 mg/mL and 1.360 mg/mL, respectively. The inhibitory effects of AWPA on α-glucosidase and α-amylase were both reversible with mixed type and competitive type competition, respectively. The significance of manuscript was not only to avoid the waste of Annona squamosa residue, but provided alternative in the developments of inhibitors of α-glucosidase and α-amylase.