The Dermatan Sulfate-Protein Linkage Region

Abstract

A study was made to identify the components of the region of linkage between polysaccharide and protein of dermatan sulfate from umbilical cord. The sulfated mucopolysaccharides were obtained after digestion of the tissue with pepsin and trypsin, and a dermatan sulfate-polypeptide preparation was isolated and purified. In addition to uronic acid and galactosamine, this material contained approximately 39 µmoles of galactose and 18 µmoles of xylose per g. Treatment of the dermatan sulfate polypeptide with alkali resulted in the cleavage of the mucopolysaccharide from the peptide moiety, with the preferential destruction of the serine from the peptide. When this reaction was performed in the presence of sodium borohydride, there was an increase in alanine concurrent with the destruction of serine and a conversion of the xylose to xylitol. The galactose remained unchanged. These results indicate that the polysaccharide is linked to the protein via the hydroxyl oxygen of serine; this linkage is known to be easily cleaved with alkali by a β elimination reaction. The appearance of alanine upon treatment with alkaline borohydride confirms this mechanism, while the reduction of the xylose to xylitol indicates that xylose is the unit linked to the serine. Digestion of the dermatan sulfate polypeptide with Pronase yielded a product in which the amino acid composition corresponded to a dipeptide of serine and glycine. Serine was found to be the NH2-terminal amino acid, indicating that the peptide structure is serylglycine. This dermatan sulfate dipeptide is comparatively stable to alkali, and it is proposed that β-elimination from serine is minimal when its amino group is free.