Fractionation and characterization of water-soluble polysaccharides from Astragalus membranaceus
Date Issued
2008
Date
2008
Author(s)
Chen, Ying-Chi
Abstract
The dried root of Astragalus membranaceus (Fisch.) Bunge, also called Astragali radix, is an important traditional Chinese medicine. It has the functions of anti-virus, anti-tumor, and immuno-modulating activities. Polysaccharide is one of the active components of this herb. The focus of this study is to investigate the characteristics of water-soluble polysaccharides from A. membranaceus. The α-(1,4; 1,6)-D-glucans, the starch-like polysaccharides showing blue color under iodine-staining, in the hot-water extracted crude polysaccharides were digested and removed by α-amylase and amyloglucosidase digestion. The non-starch polysaccharides, the non-digestible portion, were fractionated by using selective precipitation with cetyl trimethyl ammonium bromide, CTAB, into acidic and neutral polysaccharides fractions. The α-(1,4; 1,6)-D-glucans was found to be the most abundant component in the crude polysaccharides and its content was 87.9％. The non-starch polysaccharides contained 42.1% of uronic acids and the sugar compositions of the neutral portion were arabinose, galactose, glucose, mannose and fucose in the molar ratio of 47.5: 33.6: 8.9: 8.3:1.7 %. The CTAB could react with the polysaccharides with high ratio of uronic acids to form precipitates and successfully separated non-starch polysaccharides into acidic and neutral polysaccharides fractions with ratio of 21.3% and 78.7%, respectively. The acidic polysaccharides fraction consisted of arabinose (29.8%), galactose (21.3%), galacturonic acid (37.7%), and glucuronic acid (2.7%). The weight-average molecular weight of acidic polysaccharides fraction was 130 kDa. The neutral polysaccharides fraction consisted of arabinose (54.4%), galactose (34.3%), mannose (9.7%) and glucose (1.5%). The weight-average molecular weight of neutral polysaccharides fraction was 189 kDa with wide range of distribution. This fraction can be further divided into three sub-fractions by anion- exchange chromatography. Both acidic and neutral polysaccharide fractions showed positive reaction with periodate-thiobarbituric acid and β-D-glucosyl Yariv antigen assays indicated the existing of KDO (2-keto-3-deoxy-D-manno-octulosonic acid) and branched (1,3; 1,6)-arabinogalactans. These results indicated existing of hairy domains of pectic polysaccharides in the acidic polysaccharides and low-uronic acid containing polysaccharides in neutral polysaccharides fraction of A. membranaceus.
Subjects
Astragalus membranaceus (Fisch.)Bge.
polysaccharides
pectic polysaccharides
cetyl trimethyl ammonium bromide
Type
thesis
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