The Shigella flexneri O-antigenic polysaccharide chain. Nature of the biological repeating unit

The sequence of monosaccharides in the biological repeating tetrasaccharide unit of Shigella flexneri variant Y O-antigenic polysaccharide chain was determined by subjecting three oligosaccharides of the polysaccharide, obtained by phage-Sf6-mediated enzymatic hydrolysis, to methylation analysis and proton nuclear magnetic resonance spectroscopy. The smallest saccharide was shown to be a tetrasaccharide with the structure alpha-L-Rhap-(1-2)-L-Rha. The next saccharide, an octasaccharide, was shown to be a dimer of the tetrasaccharide with the L-Rha residues linked alpha 1.3. The longest saccharide was shown to be a decasaccharide with the following structure: alpha-L-Rhap-(1-2)-alpha-L-Rhap-(1-3)-alpha-L-Rhap-(1- 3)-beta-D-GlcpNAc-(1-2)-alpha-L-Rhap-(1-2)-alpha-L-Rhap++ + +-(1-3)-alpha-L-Rhap-(1-3)-beta-D-GlcpNAc-(1-2)-alpha-L-R hap-(1-2)-L-Rha. Thus the decasaccharide differed from the octasaccharide and tetrasaccharide by having the alpha-L-Rhap-(1-2)-L-Rhap disaccharide added in the terminal non-reducing end of the saccharide chain. This shows that the alpha-L-Rhap-(1-2)-alpha-L-Rhap-(1-3)-alpha-L-Rhap-(1- 3)-D-GlcpNAc tetrasaccharide is the biological repeating unit of the O chain and that the repeating units are joined through a beta-D-GlcpNAc-(1-2)-L-Rhap linkage. Inhibition experiments utilizing the enzyme-linked immunosorbent assay (ELISA) with S. flexneri Y lipopolysaccharide/S. flexneri Y rabbit antiserum showed that the decasaccharide was the best inhibitor (threefold as active as the octasaccharide and sixtyfold as active as the tetrasaccharide); this supports the postulated structure of the biological repeating unit.