Characterization of a Novel Sugar Sequence from Rat‐Brain Glycoproteins Containing Fucose and Sialic Acid

The structure of a major fucose‐containing sugar sequence of rat brain glycoproteins was investigated. Glycopeptides obtained by proteolytic digestion of the lipid‐free residue of whole rat brain were fractionated by affinity chromatography on Sepharose bound to concanavalin A and wheat‐germ agglutinin. An N ‐glycosidic glycopeptide fraction corresponding to about 65% of the protein‐bound fucose was obtained. Methylation analysis before and after treatment with α‐fucosidase or mild acid demonstrated that a major proportion of fucose is linked to the C‐3 of N ‐acetylglucosamine. Studies involving neuraminidase digestion, partial acid hydrolysis, chromium trioxide oxidation and uronic acid degradation revealed that the fucose‐containing sugar sequence has the structure:The structure of the Gal(β1–4) [Fuc(α1–3)]GlcNAc sequence was also confirmed by gas‐liquid chromatography and mass spectrometry after preparation of an oligosaccharide derivative by specific degradation with nitrous acid deamination. The sugar sequence, which has not previously been described in glycoproteins, is a sialosylated derivative of the so‐called X antigen structure and it accounts for most of the fucose present in brain glycoproteins. The sugar sequence occurs mainly in membrane‐bound glycoproteins, but is also found in the soluble fraction.