Location of N-unsubstituted glucosamine residues in heparan sulfate

Abstract

Functional properties of heparan sulfate (HS) are generally ascribed to the sulfation pattern of the polysaccharide. However, recently reported functional implications of rare N-unsubstituted glucosamine (GlcNH2) residues in native HS prompted our structural characterization of sequences around such residues. HS preparations were cleaved with nitrous acid at either N- sulfated or N-unsubstituted glucosamine units followed by reduction with NaB3H4. The labeled products were characterized following complementary deamination steps. The proportion of GlcNH2 units varied from 0.7-4% of total glucosamine in different HS preparations. The GlcNH2 units occurred largely clustered at the polysaccharide-protein linkage region in intestinal HS, also more peripherally in aortic HS. They were preferentially located within N-acetylated domains, or in transition sequences between N-acetylated and N-sulfated domains, only 20-30% of the adjacent upstream and downstream disaccharide units being N-sulfated. The nearest downstream (toward the polysaccharide-protein linkage) hexuronic acid was invariably GlcUA, whereas the upstream neighbor could be either GlcUA or IdoUA. The highly sulfated but N-unsubstituted disaccharide unit, -IdoUA2S-GlcNH26S-, was detected in human renal and porcine intestinal HS, but not in HS from human aorta. These results are interpreted in terms of a biosynthetic mechanism, whereby GlcNH2 residues are formed through regulated, incomplete action of an N-deacetylase/N-sulfotransferase enzyme.