黃耆水溶性多醣之劃分與分子特徵

Abstract

中藥黃耆為豆科植物黃耆Astragalus membranaceus (Fisch.)Bunge的乾燥根，具有抗病毒、抗腫瘤、免疫調節、促進機體代謝等功效，多醣為其主要活性成分之ㄧ。本研究之目的為探討黃耆水溶性多醣之特徵與組成。結果顯示，黃耆水溶性多醣中的類澱粉多醣，α-(1,4；1,6)-D-glucans，佔了黃耆水溶性多醣之87.9％，利用碘呈色反應呈現藍紫色，可利用α-amylase、amyloglucosidase兩澱粉酵素水解並去除。非澱粉多醣為不被α-amylase、amyloglucosidase水解之部分，佔了水溶性多醣之12.1％，可利用四級銨鹽(cetrl trimethyl ammonium bromide, CTAB)選擇性錯合沉澱反應劃分成中性與酸性多醣區分。非澱粉多醣含有42.1％之醛醣酸，中性醣組成主要有阿拉伯糖、半乳糖，葡萄糖、甘露糖與岩藻糖，相對莫耳百分比為47.5：33.6：8.9：8.3：1.7％。進一步利用四級銨鹽（CTAB，cetyl trimethyl ammonium bromide）帶正電荷之特性選擇性錯合沉澱具高醛糖酸含量之非澱粉多醣，當中有21.3％可被四級銨鹽錯合沉澱，稱之為酸性多醣區分，不被四級銨鹽錯合沉澱者稱之為中性多醣區分。酸性多醣區分其單醣組成主要有阿拉伯糖（29.8％）、半乳糖（21.3％）、半乳醣醛酸（37.7％）及葡萄醣醛酸（2.7％），重量平均分子量為130kDa。中性多醣區分之主要單醣組成有阿拉伯糖（54.4％）、半乳糖（34.3％）、甘露糖（9.7％）與葡萄糖（1.5％），重量平均分子量為189kDa，為一廣分佈之多醣。利用陰離子交換樹脂可將中性多醣區分劃分成三區塊。中性與酸性多醣區分利用periodate-thiobarbituric acid method和β-D-glucosyl Yariv antigen皆呈現正反應，證明了KDO (2-keto-3-deoxy-D-manno-octulosonic acid)和arabinogalactan Type Ⅱ之存在。結果顯示經此一流程可獲得高醛醣酸含量、具有果膠多醣特徵之黃耆多醣。

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.