Biosynthesis of mammary glycoproteins. Structural characterization of different isomers of lipid-linked hexa- and heptasaccharides

A previous study indicated that incubation of a membrane preparation from the lactating bovine mammary tissue with UDP-[³H]-GlcNAc and GDP-[¹⁴C]Man synthesized a series of lipid-linked saccharides that appeared to be related to one another as precursor products in the biosynthesis of asparagine-linked glycoproteins of this tissue. The structure of monoisomeric species of tri-, tetra-, penta-, and undecasaccharide were characterized. Utilizing the substrate specificity of endo-β-N-acetylglucosaminidase D and H for oligomannosylchitobiose type of saccharides, two isomers in the hexa- and at least three isomers in the heptasaccharide fraction could be sorted out. Based on digestion with α-mannosidase, endo-β-N-acetylglucosaminidase L, and β-mannosidase, and on Smith periodate degradation, acetolysis, and methylation analysis, the proposed structures of two isomers of the lipid-linked hexasaccharide are Manα1→3Manα1→6(Manα1→3)Manβ1→4(3)GlcNAc β1→4(3)GlcNAc (Hexa-I) and Manα1→2Manα1→3(Manα1→6)Manβ1→4(3)GlcNAcβ1→4(3)GlcNAc (Hexa-II). Both of the hexasaccharides appear to be biosynthetic intermediates rather than products of a lipid-linked saccharide processing. The two isomers of lipid-linked heptasaccharide that were characterized are Manα1→2Manα1→3Manα1→6(Manα1→3)Manβ1→4(3)GlcNAcβ1→4(3)GlcNAc(Hepta-I) and Manα1→2Manα1→2Manα1→3(Manα1→6)Manβ1→4(3)GlcNAcβ1-4(3)GlcNAc (Hepta-II). Hepta-II, presumably derived from Hexa-II, appears to be the intermediate in the major dolichol phosphate-mediated pathway for the lipid-linked assembly of the saccharides; Hepta-I could be a biosynthetic product of Hexa-I in a minor pathway, or might result from lipid-linked processing of higher saccharides. The precise structure and origin of a third, heterogeneous fraction of heptasaccharide isomer(s) is presently unclear.