The carbohydrate structure of the asparagine-linked oligosaccharides of rat plasma thiostatin.

The complete carbohydrate structure of the asparagine-linked oligosaccharides of rat plasma thiostatin was elucidated through chemical and enzymatic methods including gas chromatography-mass spectrometry (GC-MS) and lectin affinity chromatography. Pronase digestion of thiostatin yielded a major glycopeptide fraction with asparagine the most abundant amino acid present. Based on one mole of aspartic acid, the following molar ratios obtained for the four major amino acids: aspartic acid (1.0), threonine (0.53), glycine (0.48) and serine (0.30). Neutral sugar analysis yielded a 3:2 molar ratio for mannose to galactose based on an assigned value to mannose of 3. On this basis, the fraction also contained 3 residues of sialic acid and, on average, 0 to 1 residue of fucose. GC-MS of partially methylated alditol acetates from the glycopeptide fraction identified the presence of biantennary and triantennary structure. Analyses of the neutral sugar and amino-acid composition, together with methylation data, support a biantennary N-linked structure for this major glycopeptide fraction and a triantennary N-linked structure as a lesser component. Sequencing of the desialyated 14C-labelled glycopeptide fraction by sequential exoglycosidase digestion and lectin affinity chromatography uncovered the following saccharide order: terminal galactose, N-acetylglucosamine and pentasaccharide inner core. This sequence is consistent with the N-linked glycan structures demonstrated by methylation and compositional analyses.