Isolation and characterization of the gel-forming polygalacturonide from seeds of Ficus awkeotsang.

Abstract

To examine the spontaneous gel-forming property of the polysaccharide of seeds of Ficus awkeotsang Makino, seeds were extracted sequentially with cold and hot water, and then aqueous ammonium oxalate. The major acidic polysaccharide ([α]25D+239°; DE = 63.6%; M.W. 3.4 × 105), which was responsible for gel-formation, was obtained from the surface of heat-treated seeds and found to be composed mainly of galacturonic acid (96%). On fractionation of the major acidic polysaccharide by anion-exchange column chromatography, three kinds of methyl esterified components were obtained (P-1, DE=69.3%; P-2, 49.2% and P-3, 22.8%). Methylation analysis of the carboxylreduced polysaccharide indicated that the acidic polysaccharide is an essentially linear polysaccharide containing α-(1→4)-linked D-galacturonic acid residues and devoid of L-rhamnose residues, which confirmed it to be a polygalacturonide. Comparison of the IR spectra and the degrees of methyl esterification of the polygalacturonide extracted from heat-treated and heat-untreated seeds suggested that the methyl ester groups in the polygalacturonide must have been released during the seed extraction at room temperature. As a model test, furthermore, when the acid polysaccharide prepared from heat-treated seeds was incubated with pectinesterase from lemon peel in the presence of 1mM CaCl2-2H2O, the gel-forming behavior observed was closely similar to the spontaneous gel-formation of the awkeotsang extract. These results may support the possibility that the high amount of calcium ions (4.0-5.6 %) in an aqueous extract of the seeds contributes to the egg-box formation, by being placed between the de-esterified structurally regular polygalacturonide chains.