Biosynthesis of Mammary Glycoproteins

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

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