Structural studies of endopolygalacturonase-resistant fragments of an antiulcer pectin from the roots of Bupleurum falcatum L.

Abstract

The endoα-(1 → 4)-polygalacturonase-resistant fractions (PG-1, PG-2, and PG-3) from an antiulcer pectin (Bupleuran 2IIc), isolated from the roots of Bupleurum falcatum, L., were further analysed by lithium degradation. The results indicated that PG-1 contained a small proportion of long, branched arabinosyl chains and a large proportion of short, neutral oligosaccharide chains. GLC-MS analysis showed that, after methylation the short, neutral oligosaccharide fraction consisted of at least 22 kinds of di- to tetra-saccharide alditols, such as Gal-(1 → 4)-Rha-ol (a major component), Ara-(1 → 4)-Rha-ol, Glc-(1 → 4)-Rha-ol, Ara → Ara →a Ara-ol, and Ara → Ara → Ara → Ara-ol (minor components) in addition to heteroglycosyl alditols. After deesterification, PG-2 and PG-3 were digested with endo-α-(1 → 4)-polygalacturonase again, and the enzyme-resistant intermediate size fraction (PG-2′) was purified. Component sugar analysis indicated that PG-2′ contained 2-Me-Fuc, 2-Me-Xyl, apiose (Api), aceric acid (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 PG-2′ gave mainly a pentosyl → 6-deoxyhexosyl → 6-deoxyhexosyl → pentitol fragment, with some neutral di- and tri-saccharide alditols, including a pentosysl → deoxyhexitol. Methylation analysis of these degradation products indicated that they contained terminal Rha, Ara f, Fuc, Xyl, and Gal, 4-linked Rha, 3-linked Fuc, 3-linked Ara, and 3′-linked Api. Bupleuran 2IIc was eluted as essentially a single peak on gel filtration on Sepharose CL-6B. The neutral sugar content of the successive fractions increased with increasing molecular weight, but each fraction also contained, in addition to Rha, Ara, and Gal, 2-Me-Fuc, 2-Me-Xyl, and Api.