Quantitation and structures of oligosaccharide chains in human trachea mucin glycoproteins

Abstract

Human respiratory mucin glycoproteins from patients with cystic fibrosis were purified and oligosaccharide chains were released by treatment with alkaline borohydride. A neutral oligosaccharide alditol fraction was isolated from mucin obtained from a patient with A blood group determinant by chromatography on DEAE-cellulose and individual oligosaccharide chains were then isolated by gel filtration on BioGel P-6 columns and high performance liquid chromatography with gradient and isocratic solvent systems. The structures of the purified oligosaccharides were determined by methylation analysis, sequential glycosidase digestion and 'H-NMR spectroscopy. The amount of each chain was determined by compositional analysis. A wide array of discrete branched oligosaccharide structures that contain from 3 to 22 sugar residues were found. Many of the oligosaccharides are related and appear to be precursors of larger chains. The predominant branched oligosaccharides which accumulate contain terminal blood group H (Fuc alpha 2Ga1 beta 4) or blood group A (Fuc alpha 2(Ga1NAc alpha 3) (Ga1 beta 4) determinants which stop further branching and chain elongation. The elongation of oligosaccharide chains in respiratory mucins occurs on the beta 3-linked G1cNAc at branch points, whereas the beta 6-linked G1cNAc residue ultimately forms short side chains with a Fuc alpha 2(Ga1NAc alpha 3) Ga1 beta 4 G1cNAc beta 6 structure in individuals with A blood group determinant. The results obtained in the current studies further suggest that even higher molecular weight oligosaccharide chains with analogous branched structures are present in some human respiratory mucin glycoproteins. Increasing numbers of the repeating sequence shown in the oligosaccharide below is present in the higher molecular weight chains. [formula: see text] This data in conjunction with our earlier observations on the extensive branching of these oligosaccharide chains helps to define and explain the enormous range of oligosaccharide structures found in human and swine respiratory mucin glycoproteins. Comparison of the relative concentrations of each oligosaccharide chain suggest that these oligosaccharides represent variations of a common branched core structure which may be terminated by the addition of alpha 2-linked fucose to the beta 3/4 linked galactose residue at each branch point. These chains accumulate and are found in the highest concentrations in these respiratory mucins.