A β-Mannosidic Linkage in the Unit A Oligosaccharide of Bovine Thyroglobulin

An endo-β-N-acetylglucosaminidase purified to homogeneity from the cultural filtrate of Streptomyces griseus was found to release an oligosaccharide, most probably the A unit, from bovine thyroglobulin. This oligosaccharide, containing 8 mannose residues and 1 N-acetylglucosamine, with the latter on the reducing end, was digested with jack bean meal α-mannosidase. An α-mannosidase-resistant disaccharide was isolated following the release of about 7 mannose residues. Based on the results obtained from gas chromatography, from Morgan-Elson color assay, and following treatment with hen oviduct β-mannosidase, the disaccharide appears to be O-β-d-Man(1→4)-d-GlcNAc. The structure of this disaccharide is consistent with that reported to be present in the core glycosyl asparagine unit of a number of glycoproteins. These findings are in contrast to those reported by Arima and Spiro ((1972) J. Biol. Chem. 247, 1836–1848) who indicated that all of the mannose residues in the unit A oligosaccharide of thyroglobulin were α-linked.