“Ai-Geok”, A Unique Gel-Forming Polysaccharide of Ficus awkeotsang: Structure, Gelation Mechanism and Utilization of The Oligosaccharides

Abstract

A Water-extract of the seed of Ficus awkeotsang forms spontaneously an edible gel, known as “AI-GEOK”. A gel-forming polysaccharide located on the surface of seed was purified from the water-extract cf heat-treated seeds. Structural analysis showed that it is a highly methylesterified(DE, 63.6%), α-l,4-galacturonan(M.Wt, 3.4 x 10⁵). The intrinsic pectinesterase(EC 3.1.1.11), responsible for gelation of AI-GEOK, located solely in the red tepals, was purified by column chromatography. The purified enzyme showed monomelic nature with M.Wt. 4.2 x 10⁴, pI 4.4, and optimal pH 5.4, which sifted to 7-8 in the presence of K⁺ When 0.5% AI-GEOK solution was incubated with the enzyme and mixture of metal ions, containing Ca²⁺, Mg²⁺, K⁺ and Na⁺ it gradually formed a gel. These results confirmed that the gel formation mechanism must be involved in, a) de-esterification of highly methylesterified galacturonan by the intrinsic esterase, and b) gradual formation of “egg-bo”xstructure by ionic interaction between Ca²⁺ and regularly arranged α-1,4-linked galacturonan. Apart from the gelation problem, AI-GEOK appears to be a good sourse of phytoatexin-elicitor. Among oligogalacturonides prepared by enzymic degradation, 4,5-unsaturated hexa-galacturonide was found to exhibit a potent activity as well as deca-galacturonide.