Biosynthesis of Mammary Glycoproteins: Structural Characterization of Lipid‐Linked Glucosyloligosaccharides

The lipid‐linked glucosyloligosaccharide intermediates for the biosynthesis of asparagine‐linked glycoproteins in the lactating bovine mammary tissue were isolated from incubations of microsomes of this tissue with labeled sugar‐nucleotides. These were shown to have the general structure (Glc)_1–3(Man)₉(GlcNAc)₂. A structural characterization indicated that glucose residues in these oligosaccharides are linked as Glcα1 → 3, Glcα1 → 3Glcα1 → 3, and Glc1 → 2Glcα1 → 3Glcα1 → 3, to the tetramannosyl branch of (Man)₉(GlcNAc)₂ portion previously characterized as Manα1 → 2Manα1 → 2Manα1 → 3(Manα1 → 2Manα1 → 6[Manα1 → 2Manα1 → 3]‐Manα1 → 6)Manβ1 → 4(3)GlcNAcβ1 → 4(3)GlcNAc [Vijay, I. K., Perdew, G. H., and Lewis, D. E. (1980) J. Biol. Chem. 255, 11210–11220]. The lipid‐linked (Man)₉(GlcNAc)₂ appears to be the initial acceptor of the first glucosyl residue. Even though incubations of the bovine mammary tissue in vitro synthesize multiple isomers of a number of mannosyloligosaccharides, the glucosyloligosaccharides are all monoisomeric.