Characterization of novel galactooligosaccharides derived from lactulose

Abstract

Galactooligosaccharides (GOS) are considered non-digestible carbohydrates with well-recognised prebiotic properties. They are synthesized from lactose by a transgalatosylation reaction, catalysed by ß-galactosidase, giving rise to oligosaccharides of different glycosidic linkages and molecular weights. By using commercial enzymatic preparations, novel GOS derived from lactulose have been obtained recently. In vitro fermentation studies of these carbohydrates with human faecal slurries have shown to promote the growth of bifidobacteria as well as the production of acetic acid. Several factors such as type of glycosidic linkage, monomeric composition and carbohydrate chain length are likely to affect the prebiotic activity of these oligosaccharides. Therefore, the molecular characterization of these oligosaccharides is critical in order to gain knowledge and understanding of structure-prebiotic function relationships for further exploitation. In this work, we have characterised novel GOS from lactulose by GC-MS, HPLC-ESI MS and MALDI-ToF MS. In adidition, chemical structures of GOS derived from lactose were studied for comparative purposes. Degrees of polymerization (DP) of GOS obtained from lactulose and lactose by transgalactosylation reactions catalysed by ß-galactosidase from commercial enzymatic extracts were determined by MALDI-ToF MS, using 2,5-dihydroxybenzoic acid as matrix. HPLC-ESI MS analyses at positive mode were carried out using a graphitized charcoal column (100 mm x 2.1 mm, 3 μm; Hypercarb, Thermo Scientific) with H2O:methanol as mobile phase. GC-MS analyses were carried out using a HT5 column (25 m x 0.22 μm id x 0.1 μm df) from SGE, coated with 5% phenyl polysiloxane-carborane following the method of Hernández et al. Oligosaccharides were converted into their trimethylsilyl oximes previous their GC-MS analysis as described before. MALDI-ToF MS analyses of GOS from lactulose revealed the existence of oligosaccharides from DP2 to DP8. HPLC-MS analyses confirmed these data, being di- and trisaccharides the most abundant fractions. Galactosyl-galactoses and galactosyl-fructoses were detected as major components in the disaccharide fraction, and glycosidic linkages 1-6 and 1-4 prevailed over the others. Further studies to evaluate their prebiotic activity are in progress.