Alkali-labile oligosaccharides of brain glycoproteins

Abstract

Treatment of glycopeptides prepared from rabbit brain glycoproteins with NaOH or NaOH-NaBH 4 released approximately 10% of the carbohydrate as low molecular weight oligosaccharides. After treatment with alkali alone, a disaccharide and a trisaccharide were isolated by gel filtration and preparative paper chromatography, and have been identified as N- acetylneuraminyl-(2→6)-O-2- acetamido-2- deoxy- D- galactose (Component B-I) and di-N- acetylneuraminyl- D- galactose (Component B-III). The N-acetylgalactosamine in Component B-I was obtained as preformed Morgan-Elson chromogen, and Components B-I and B-III were not obtained when the glycopeptides were treated with alkaline NaBH 4 under conditions which prevent peeling. These results indicate that the trisaccharide (Component B-III) was originally present as a substituent at C-3 of the N-acetylgalactosamine involved in the carbohydrate-peptide linkage, and was eliminated as a result of alkali treatment. A disaccharide (Component M) was isolated after alkaline NaBH 4 treatment of the desialylated glycopeptides, and yielded equimolar amounts of galactose and galactosaminitol after hydrolysis. Smith degradation of Component M gave galactose and threosaminitol, indicating that the disaccharide was galactopyranosyl-(1→3)-O-2- acetamido-2- deoxy- D- galactitol . From the isolation of Components B-I, B-III and M after NaOH and NaOH-NaBH 4 treatment of the glycopeptides, it is concluded that the major alkali-labile oligosaccharide of brain glycoproteins is the pentasaccharide di-N- acetylneuraminyl- D- galactopyranosyl-(1→3) [N- acetylneuraminyl-(2→6)]-O-2- acetamido-2- deoxy- D- galactose .