Structural Study on the Carbohydrate Moiety of Human Placental Alkaline Phosphatase

Alkaline phosphatase purified from human placenta contains a single asparagine-linked sugar chain in one molecule. The sugar chain was quantitatively liberated as radioactive oligosaccharides from the polypeptide moiety by hydrazinolysis followed by N-acetylation and NaB3H4 reduction, and separated by paper electrophoresis into one neutral and two acidic fractions. By a combination of sequential exoglycosidase digestion and methylation analysis, the structures of oligosaccharides in the neutral fraction were confirmed to be as follows: Galβ1→4GlcNAcβ1→2Manαl→6(Galβ1→4GlcNAcβ1→2Manα1→3)Manα1→4GlcNAcα1→4(±Fucα1→6)GlcNAc. The acidic oligosaccharide fractions were mixtures of mono- and disialyl derivatives of the neutral fraction. All the sialic acid residues of the sugar chains occur as the NeuAcα2→3Gal group. In the case of monosialyl derivatives, the N-acetylneuraminic acid was exclusively linked to the Manal1→3 arm.