Specific conversion of d-galactose into d-galacturonic acid residues in glycoproteins: a facile method for carbohydrate linkage-analysis

The terminal d-galactopyranosyl residues of asialoglycopeptides isolated from human α₁-acid glycoprotein were oxidized in nearly quantitative yield to the corresponding uronic acid residues by a two-step sequence employing d-galactose oxidase followed by treatment with Tollens reagent, Ag(NH₃)₂⁺. Mild acid hydrolysis of the oxidized glycopeptides led to the isolation of the corresponding aldobiuronic acid(s). Structural and colorimetric analysis revealed that only one aldobiuronic acid, 2-amino-2-deoxy-4-O-(β-d-galactopyranosyluronic acid)-d-glucose, was isolated from the oxidized glycopeptides of α₁-acid glycoprotein. This method can readily distinguish between the (1→3), (1→4), and (1→6) isomers of the corresponding aldobiuronic acids.