Investigating glucosyltransferase inhibitory activities of polyphenols from kapur (Dryobalanops sp.) heartwood extracts

Abstract

The inhibitory effects of 50% aqueous ethanol extracts obtained from 36 tropical woody plants species on glucosyltransferase (GTase) activity were studied. Out of the 36 species examined, those obtained from kapur (Dryobalanops sp.), a species growing in Kalimantan (Indonesia), showed the highest level of GTase inhibition. Kapur extracts were further subjected to fractionation using column chromatography (LH-20 gel, cellulose and C-18 silica gel column). LH-20 gel provided the most successful method of fractionation. The separated fractions showed positive with Folin-Ciocalteau’s reagent and negative with vanillin-HCl reagent, indicating that the main constituents of the active fractions were polyphenols but not proanthocyanidin (condensed tannins). Results of the assay for protein precipitating ability with bovine serum albumin (BSA) solution suggested these polyphenols have strong protein-precipitating ability. The predominant compound produced after acid hydrolysis was ellagic acid, indicating that the GTase-inhibitory components were mainly ellagitannins. Two polyphenolic compounds referred to as compounds 1 and 2 were isolated from the water eluate fraction with LH-20 gel column, and these compounds showed comparatively strong GTase-inhibitory activities and relatively low molecular weight. Using a combination of two-dimensional, 1H and 13C nuclear magnetic resonance analysis, compound 1 was identified as 4-methoxy-2-[tetrahydro-3,4,5-trihydroxy-6-(hydroxymethyl)pyran-2-yl]-α-resorcylic acid δ–lactone (bergenin), and 2 was identified as 4-O-(α-rhamnopyranosyl) ellagic acid (eschweilenol C). Bergenin has been previously isolated from the roots of Bergenia crassifolia, and eschweilenol C has been isolated from the bark of Eschweilera coriacea. Both compounds were found in kapur for the first time.