Chemical Composition and Biological Activity of Polysaccharides from Mulberry Leaves Grown in Cold Regions

This study investigated the structure and functional properties including in vitro hypoglycemic, antioxidant, and immunoenhancing activities of water-soluble mulberry leaf polysaccharides (MLP) from Morus abla L. cv. Longsang 1. The results showed that the yield of MLP was (16.16 ± 0.30) mg/g with sugar content of (57.16 ± 4.00)% and uronic acid content of (30.97 ± 2.06)%. The molecular mass distribution of MLP, as measured by gel permeation chromatography (GPC), was mainly concentrated at 21.74 kDa. Monosaccharide composition analysis showed that MLP mainly contained glucose, rhamnose, galactose, xylose, uronic acid (including glucuronic acid, galacturonic acid and mannuronic acid), and mannose, with a molar ratio of 59.17:22.31:10.44:2.65:2.28:1.07. Fourier transform infrared (FT-IR) spectroscopy and atomic force microscopy (AFM) indicated that MLP was a pyranose with intertwined branches and helical aggregation. MLP inhibited α-glucosidase and α-amylase with concentration for 50% of maximal effect (EC₅₀) of 1.81 mg/mL and 2.91 mg/mL, respectively, and its inhibitory activities were 49.17% and 37.80% as high as those of acarbose, respectively. MLP scavenged 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical, and hydroxyl radical with EC₅₀ of 0.54, 0.60 and 1.77 mg/mL, respectively, and the radical scavenging capacities were 4.77%, 38.03%, and 19.94% as high as those of ascorbic acid, respectively. When applied to RAW264.7 mouse macrophages, MLP increased the secretion of the proinflammatory cytokines NO, tumor necrosis factor-α (TNF-α), interleukin (IL)-6, and IL-1β and enhanced macrophage proliferation and phagocytosis, thereby exerting an immunoenhancing effect. The research results can provide experimental data for the application and functional evaluation of mulberry leaves grown in cold regions.