Biosynthesis of Mammary Glycoproteins

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 Glc alpha 1 leads to 3, Glc alpha 1 leads to 3Glc alpha 1 leads to 3, and Glc1 leads to 2Glc alpha 1 leads to 3Glc alpha 1 leads to 3, to the tetramannosyl branch of (Man)9(GlcNAc)2 portion previously characterized as Man alpha 1 leads to 2Man alpha 1 leads to 2Man alpha 1 leads to 3(Man alpha 1 leads to 2Man alpha 1 leads to 6[Man alpha 1 leads to 2Man alpha 1 leads to 3]-Man alpha 1 leads to 6)Man beta 1 leads to 4(3)GlcNAc beta 1 leads to 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.