Zhan D, Janssen P, Mort A J
Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater 74078-3035, USA.
Carbohydr Res. 1998 Jun;308(3-4):373-80. doi: 10.1016/s0008-6215(98)00096-2.
Commercial citrus pectin containing galacturonic acid and rhamnose in a ratio of approximately 40:1 was saponified and then exhaustively digested with endopolygalacturonase (EPG). The products were separated by ultrafiltration into low-molecular-weight (LMW) and high-molecular-weight (HMW) fractions. The LMW fraction accounted for 80% of the starting material, but for only 10% of the total rhamnose. The molar ratio of galacturonic acid to rhamnose of the LMW fraction was 236, suggesting that very few small Rha-containing oligomers were generated by the EPG digestion. No distinct Rha-containing oligomers were found by various chromatographic analyses of the LMW fraction. The HMW fraction, which only accounted for 10% by weight of the starting pectin, contained more than 85% of the rhamnose. The ratio of GalA to Rha in the HMW fraction was 1.7:1 and partial acid hydrolysis of this fraction produced a series of oligomers consisting of GalA-Rha repeating units, suggesting that it contained rhamnogalacturonan, which has a backbone composed of GalA-Rha disaccharide repeating units. The HMW fraction also contained large amounts of arabinose and galactose, which probably originated from side chains linked to some of the rhamnose residues. We propose that commercial citrus pectin is composed of two regions: the predominant region consists of chains of uninterrupted 1,4-linked alpha-D-GalA residues with between 60-70% of the residues methyl esterified; and the other region consists of rhamnogalacturonan with a backbone composed of GalA-Rha disaccharide repeating units and neutral sugar side chains.
将含有半乳糖醛酸和鼠李糖比例约为40:1的商业柑橘果胶进行皂化，然后用内切多聚半乳糖醛酸酶（EPG）进行彻底消化。产物通过超滤分离为低分子量（LMW）和高分子量（HMW）部分。LMW部分占起始原料的80%，但仅占总鼠李糖的10%。LMW部分的半乳糖醛酸与鼠李糖的摩尔比为236，这表明EPG消化产生的含鼠李糖的小寡聚物极少。通过对LMW部分的各种色谱分析未发现明显的含鼠李糖寡聚物。HMW部分仅占起始果胶重量的10%，却含有超过85%的鼠李糖。HMW部分中GalA与Rha的比例为1.7:1，对该部分进行部分酸水解产生了一系列由GalA-Rha重复单元组成的寡聚物，这表明它含有鼠李半乳糖醛酸聚糖，其主链由GalA-Rha二糖重复单元组成。HMW部分还含有大量的阿拉伯糖和半乳糖，它们可能源自与一些鼠李糖残基相连的侧链。我们认为商业柑橘果胶由两个区域组成：主要区域由不间断的1,4-连接的α-D-半乳糖醛酸残基链组成，其中60 - 70%的残基被甲酯化；另一个区域由鼠李半乳糖醛酸聚糖组成，其主链由GalA-Rha二糖重复单元和中性糖侧链组成。
