Structures of O-glycosidically linked carbohydrate units of herring egg sialoglycoprotein

One of the major fractions of sialoglycoprotein isolated from herring eggs has been shown to contain both alkali-labile and alkali-stable carbohydrate units. Homogeneous preparations of reduced disaccharide and trisaccharide were obtained. Their structures, as determined by methylation analysis coupled with GLC-mass spectrometry, NMR spectroscopy and digestion with glycosidases, were beta-D-galactopyranosyl-(1 leads to 3)-N-acetyl-D-galactosaminitol and beta-D-galactopyranosyl-(1 leads to 3)-[alpha-N-acetylneuraminyl-(2 leads to 6)]-N-acetyl-D-galactosaminitol, respectively. No other oligosaccharide was found among the products of alkaline borohydride treatment. Glycopeptide fractions were obtained from the pronase digest of the glycoprotein. The carbohydrate components of small glycopeptide fractions were either galactose and N-acetylgalactosamine or galactose, N-acetylgalactosamine and N-acetylneuraminic acid and accounted for 80% of reduced oligosaccharides obtained by alkaline borohydride treatment of the glycoprotein. Threonine was the only amino acid that occurred in molar ratios relative to N-acetylgalactosamine of more than unity in these small glycopeptide fractions. It was concluded that, on average, 4 carbohydrate units are attached to each peptide chain through O-glycosidic linkages between the N-acetylgalactosamine and threonine residues.