Terminal alpha (1 leads to 4)-linked N-acetylglucosamine: a characteristic constituent of duodenal-gland mucous glycoproteins in rat and pig. A high-resolution 1H-NMR study.

The structure of the carbohydrate chains of mucous glycoproteins from the gastro-intestinal tract was examined for species- and tissue-specificity. To this purpose, oligosaccharides were released from purified glycoprotein preparations of rat and pig gastric, duodenal-gland and small-intestinal mucus, by alkaline borohydride reductive cleavage. Based on the results of 500-MHz 1H-NMR spectroscopy and of sugar analysis of the total oligosaccharide fractions, terminal GlcNAc, alpha (1 leads to 4)-linked to galactose, appears to be a characteristic constituent of duodenal-gland oligosaccharides. Similarly, NeuAc in alpha (2 leads to 3)-linkage to galactose turns out to be a typical constituent of small-intestinal mucous glycoproteins. In general, glycoproteins from gastric mucus possess larger and more-branched carbohydrate chains than those from duodenal-gland and small-intestinal mucus. Comparing rat and pig, oligosaccharide structures for corresponding tissues are less complex for the former. After fractionation, the rat duodenal-gland oligosaccharides could be characterized by application of 1H-NMR spectroscopy as being branched tetra- up to hexa-saccharide chains, all sharing the italicized trisaccharide element. The chains exhibit microheterogeneity as to the termination by fucose in alpha (1 leads to 2)- or by GlcNAc in alpha (1 leads to 4)-linkage to galactose. The following structures can be proposed for the most abundant rat duodenal-gland oligosaccharides: (table; see text).