Extraction, Purification, Structural Characterization, and Biological Activity of Polysaccharides from Mulberry Leaves in Cold Regions

To investigate the structural characteristics and functional activity of mulberry leaf polysaccharide (MLP) from Morus abla L. cv. Longsang 1, DEAE-52 cellulose column and Sephadex G-100 gel chromatographies were used to purify and separate MLP into three fractions, named MLP-1, MLP-2, and MLP-3. The structural properties as well as the in vitro antioxidant, hypoglycemic, and lipid-lowering activities of the purified polysaccharides were analyzed. The results showed that the yields of MLP-1, MLP-2, and MLP-3 were 12.75%, 5.52%, and 2.63%, respectively. Compositional analysis showed that the total sugar content of MLP-1, MLP-2, and MLP-3 decreased in that order, while the contents of uronic acid, protein, and polyphenol exhibited the opposite trend. Gel permeation chromatography (GPC) revealed that MLP-1 and MLP-3 were homogeneous polysaccharides with molecular masses of 625.69 and 7.25 kDa, respectively, whereas MLP-2 was a mixture of polysaccharides with molecular masses of 769.05 and 41.35 kDa. Monosaccharide composition analysis showed that the three polysaccharides were composed of fructose, mannose, glucose, and galactose at different molar ratios. Fourier transform infrared spectroscopy (FT-IR), iodine-potassium iodide assay and atomic force microscopy (AFM) indicated that all three polysaccharide components were pyranoses, characterized by long side chains and multiple branches interwoven with each other. Congo red staining showed that MLP-1 and MLP-2 possessed a triple helix structure, while MLP-3 did not. Physicochemical analyses indicated that MLP-1 significantly outperformed the other polysaccharide components in terms of water-holding and water-retention capacities, while MLP-2 performed the best in terms of water solubility and waterabsorbing capacity. In terms of swelling capacity and oil-holding capacity, MLP-3 performed the best. Rheological analysis indicated that MLP-1 and MLP-2 exhibited superior apparent viscosity and solid elastic properties. Moreover, MLP-1 was more prone to phase transition at low temperatures. The free radical scavenging capacities of the polysaccharides decreased in the order: MLP-1 > MLP > MLP-2 > MLP-3, the inhibitory activity against α-glucosidase, α-amylase, and pancreatic lipase in the order: MLP-1 > MLP-2 > MLP3, the bile salt binding capacity in the order: MLP-1 > MLP-3 > MLP-2, and the cholesterol adsorption capacity in the order: MLP-3 > MLP-2 > MLP-1. The results of this study can provide experimental data for the application and functional evaluation of mulberry leaves in cold regions.