Phosphorylated oligosaccharides in lysosomal enzymes: identification of α-N-acetylglucosamine (1) phospho(6)mannose diester groups

In human fibroblasts, the recognition of lysosomal enzymes by cell surface receptors is mediated by mannose 6-phosphate residues located on oligosaccharides that can be cleaved by endo-β-N-acetylglucosaminidase H. About half of these oligosaccharides, as isolated from β-hexosaminidase and cathepsin D secreted by human skin fibroblasts, are anionic. Most of these are resistant to alkaline phosphatase. The resistance is due to α-N-acetylglucosamine residues linked to mannose 6phosphate by a phosphodiester bond. The major phosphorylated oligosaccharides contain one and two and possibly three phosphate groups blocked by N-acetylglucosamine. Besides the blocked phosphate groups these oligosaccharides contain a common inner core consisting of Manα1,6- (Manαl,3)Manαl,6(Manαl,3)ManβαGlcNAc and either one or two αl,2-linked mannose residues.