Structural characterization of an intestinal immune system-modulating arabino-3,6-galactan-like polysaccharide from the above-ground part of Astragalus membranaceus (Bunge)

Arabino-3,6-galactan (AMA -I b PS2), an intestinal immunomodulatory compound, was purified from the above-ground portion of Astragalus membranaceus (Bunge). Its structure was characterized using sequential enzymatic digestion with exo-alpha-L-arabinofuranosidase (AFase) and exo-beta-D-(1> 3)-galactanase (GNase), producing small amounts of intermediate-sized and shorter oligosaccharide (AF-PS2-G2 and AF-PS2-G3) fractions, and a large GNase-resistant fraction (AF-PS2-G1). Simultaneous AFase and GNase digestion of the enzyme-resistant fraction produced two long fragments (AF3-P52-G1-1-1 and AF3-P52-G1-1-2). Products of GNase digestion of the upper fractions showed decreased intestinal immunomodulatory activity; the GNa se-re si stant fraction (AF-1352-G1) retained significant activity. Sugar component, methylation, and FAB-MS analyses indicated that the oligosaccharides consisted of hexosyl tri- to hexa-decasaccharides and hexosyl di- to hepta-saccharides mainly comprising 6-linked Gal(f) and Gal(p); some were partially mono- or di-arabinosylated. These oligosaccharide fractions were attached to the non-reducing terminus of the beta-D-(1 -> 3)-galactan backbone as side chains at position 6. AMA-1-b-P52 likely modulates both the systemic and gastric mucosal immune systems. (c) 2015 Elsevier Ltd. All rights reserved.