An Acidic Arabinogalactan-Protein from the Roots of Baptisia tinctoria

Abstract

An acidic arabinogalactan-protein (AGP) isolated from an aqueous extract of the roots of wild indigo [Baptisia tinctoria (L.) R. Br.] by precipitation with β-glucosyl Yariv reagent consists of L-arabinose (34.0 %) and D-galactose (58.7 %) (1 : 1.7), minor amounts of D-glucuronic acid (4.0 %) and traces of D-rhamnose (2.3 %) and D-glucose (<1 %). The protein part of the AGP mainly contains asparagine/aspartic acid (11.3 %), glutamine/glutamic acid (10.8 %), alanine (8.0 %), serine (8.0 %), leucine (7.0 %) and hydroxyproline (6.3 %). Methylation analysis revealed that the carbohydrate moiety of the glycoprotein has a highly branched structure. The core consists of 3-linked β-D-galactopyranose units carrying side chains of 6-linked β-D-galactopyranose in position C(O)6, partly substituted in position C(O)3 by side chains of 5- and 3-linked α-L-arabinofuranosyl residues and 4-linked β-D-galactopyranose units. Galactose and arabinose as well as glucose, rhamnose and glucuronic acid occur in terminal positions. The presence of these glycosyl linkage types was confirmed by ¹³C-NMR data. The AGP was separated into two fractions by SEC. The major peak corresponded to a hydrodynamic volume of 5.6 × 10⁴ Da (AGP-F2) and the minor peak to a hydrodynamic volume of 1.2 × 10⁶ Da (AGP-F1). MALLS revealed apparent weight average molecular masses of 1.4 × 10⁵ Da and >1.0 × 10⁷ Da, respectively. Both fractions show corresponding carbohydrate compositions and structural features with regard to the carbohydrate moiety. Mild acid hydrolysis of the AGP leads to loss of terminal arabinofuranosyl units. The residual galactan backbone does not react with the Yariv reagent in gel diffusion tests while the total AGP still interferes with the reagent following reduction of terminal glucuronic acid residues.

Key words

Baptisia tinctoria - Fabaceae - arabinogalactan-protein - Yariv reagent - methylation analysis - hydroxyproline

References

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