The Linkage of Corneal Keratan Sulfate to Protein

The linkage of corneal keratan sulfate to protein has been investigated. After exhaustive digestion of bovine corneas with papain and pronase, a product was obtained in which aspartic acid was the predominant amino acid and constituted 59% of the total amino acids. A carbohydrate-protein linkage fragment was isolated from this preparation by a relatively simple procedure involving the following steps: (1) partial acid hydrolysis, adsorption of glycopeptides and other cationic material on Dowex 50-X2 (H+) and elution with 0.25 M HCl: (2) paper electrophoresis of the eluted fraction at pH 6.5 and pH 1.9; (3) paper chromatography; and (4) final purification by column chromatography on Aminex A"-5 resin. The structure of the linkage fragment was established as 2-acetamido-1-(L-beta-aspartamido)-1,2-dideoxy-beta-D-glucose (Asn-GlcNAc). Evidence for this structure was obtained from qualitative and quantitative analyses as well as from the migration characteristics in several chromatographic anc electrophoretic systems. Further support for the identity of the isolated compound was provided by treatment with beta-aspartyl N-acetylglucosyl-amine amidohydrolase which specifically cleaves Asn-GlcNAc or asparaginyl-oligosaccharides. It is concluded that corneal keratan sulfate is bound to protein via a N-glycosylamine linkage between N-acetylglucosamine and asparagine: this type of linkage is common to many glycoproteins.