Structure of pectic polysaccharides isolated from onion Allium cepa L. using a simulated gastric medium and their effect on intestinal absorption.

The polysaccharide fraction extracted with simulated gastric juice from onion bulbs contained a mixture of galactan with short-length sugar chains, pectic polysaccharides and evident content of proteinaceous material. Galacturonan and rhamnogalacturonan were the main constituents of the linear regions of the sugar chains of the pectic polysaccharides. The ramified regions included rhamnogalacturonan-I. NMR data revealed that the side chains of the ramified region contained mainly 1,4-linked β-D-galactopyranose residues and lesser content of 1,3-linked β-D-galactopyranose and 1,5-linked α-L-arabinofuranose residues. Furthermore, the proteinaceous material was determined to be partly linked to the sugar chains. The polysaccharide fraction was found to decrease absorption of ovalbumin (OVA) to the blood from the gut lumen. The serum OVA level was threefold lower in mice fed with OVA mixed with the onion pectins compared with the control group, which was administered OVA alone. Protein removal failed to abolish the inhibitory effect of the onion polysaccharides, confirming that the polysaccharide chains are the active component of onion gastric juice extract.