Existence of a Rhamnogalacturonan II-like Region in Bioactive Pectins from Medicinal Herbs

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Abstract
Unusual component sugars such as 2-methylfucose (2-Me-Fuc), 2-methylxylose (2-Me-Xyl), apiose (Api), and aceric acid (AceA) are contained in the bioactive pectins from Bupleurum falcatum, Glycyrrhiza uralensis, and Angelica acutiloba, but not in the other bioactive pectic heteroglycans and arabinogalactans from Chinese and Japanese herbs tested. Each pectin was digested with endo-α-(1→4)-polygalacturonase, and gave two enzyme-resistant fractions, PG-1 (rhamnogalacturonan core with neutral sugar side-chains) and PG-2, and an oligogalacturonide fraction (PG-3) by gel filtration on Bio-gel P-30. The PG-2 fractions commonly consisted of unusual sugars such as 2-Me-Fuc, 2-Me-Xyl, Api, AceA, 3-deoxy-D-lyxo-heptulosaric acid (Dha), and 3-deoxy-D-manno-2-octulosonic acid (Kdo) in addition to Rha, Fuc, Ara, Xyl, Man, Gal, Glc, GalA, and GlcA. Lithium degradation of each PG-2 gave a pentosyl→6-deoxyhexosyl→6-deoxyhexosyl→pentitol fragment as a major oligosaccharide in addition to some neutral di- to trisaccharide alditols. Methylation analysis of the lithium degradation products from each PG-2 also indicated that these oligosaccharide alditols mainly consisted of terminal Rha, Araf, Fuc, Xyl, and Gal, 4-linked Rha, 3-linked Fuc, and 3′-linked Api. HPLC analysis showed that PG-2 had molecular heterogeneity. These results indicate that the bioactive pectins from medicinal herbs commonly consist of the minor KDO-containing region which resembles the rhamnogalacturonan II in plant cell walls.

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