The 75% Isopropanol-Soluble Polysaccharides from the Endosperm of the Legume Seed of Gleditsia Triacanthos

Abstract

The 75% isopropanol-soluble material from the endosperm of the legume-seed of Gleditsia triacanthos was isolated. The material extracted with boiling water was fraction-ated by ion-exchange chromatography and characterized. Besides minor amounts of galac-tomannans, major proportions of arabinans and/or arabinogalactans appear.

Introduction

The galactomannans from the endosperm of the seed of the legume Gleditsia triacanthos have been widely studied in our lab [1]. The system of carbohydrates of this seed also comprise low-molecular weight galactomannans, soluble in 85% ethanol, extractable at room temperature [2]. Herein is re-ported the extraction, purification and characterization of an arabinose-rich product extracted with boiling water, and soluble at high alcohol concentrations.

Experimental

The endosperm of seeds of Gleditsia triacanthos was milled and extracted exhaustively with water at room temperature, then at 50° and then at 95°. Extractions were aided with mechanical stirring. The residues were centrifuged off, and the extracts precipitated with 3 vol. of isopropanol. The supernatants were concentrated, and the final material was obtained by freeze-drying.

Analyses (total carbohydrates, proteins, etc.) were carried out following reported procedures [1,2]. The constituting monosaccharides were quantitated after hydrolysis with 2M TFA (90 min, 120°C) by HPLC-AEC. Anion-exchange chromatography was performed on DEAE Sephadex A-50. Exhaustive methylation was carried out with the technique of Ciucanu and Kerek [3]; the permethylated product was hydrolyzed and analyzed by GC of the alditol acetates.

Results and Discussion

The 75% isopropanol-soluble material (S) of the extract (95°C) from the endosperm from Gleditsia triacanthos was obtained with a yield of 2.3% (endosperm dry weight basis). Its analysis indicated the presence of carbohydrates (70%) and proteins (26%). The main constituent sugars were galactose, mannose and arabinose. Fractionation of S by anion-exchange chromatography resulted in two frac-tions: one eluted with water (N, 23% yield), and another eluted with 0.2M ammonium carbonate (C, 50% yield). Both reveal similar analyses: 80-85% carbohydrates, with arabinose as their major moo-saccharide (63% in C, 45% in N); although C is richer in protein content.

The ¹³C-NMR spectra of both fractions are similar: three anomeric signals corresponding to fura-nose sugars appear at 110.2, 109.9 and 109.1 ppm. However, in the hexopyranose anomeric region, while C shows a major signal at 105.8 ppm, possibly originated in a β-galactose moiety, N shows two signals, at 102.7 and 101.4 ppm, characteristic of the Gal/Man moieties of a galactomannan. Analysis of permethylated C and N indicate the presence of galactomannans (2,3,4,6-tetra-O-methylGal, 2,3-di-O-mehyl and 2,3,6-tri-O-methylMan), concurrently with arabinans or arabinogalactans, as, besides the above mentioned products, derivatives of arabinose methylated at 2,3,5-, 2,3- and 3- appear as major components, together with minor amounts of other derivatives of this sugar.