Structural diversity of N-sulfated heparan sulfate domains: Distinct modesof glucuronyl C5 epimerization, iduronic acid 2-O-sulfation, and glucosamine 6-O-sulfation

The N-sulfated regions (NS domains) represent the modified sequences of hep aran sulfate chains and mediate interactions of the polysaccharide with pro teins. We have investigated the relationship between the type/extent of pol ymer modification and the length of NS domains in heparan sulfate species f rom human aorta, bovine kidney, and cultured NMuMG and MDCK cells. C5 epime rization of D-glucuronic acid to L-iduronic acid was found to be extensive and essentially similar in all heparan sulfate species studied, regardless of domain size, whereas the subsequent 2-O-sulfation of the formed iduronic acid residues varies appreciably. In aorta heparan sulfate, up to 90% of t he formed iduronate residues were 2-O-sulfated, whereas in kidney heparan s ulfate 2-O-sulfation occurred only in less than or equal to 50% of the idur onate residues. The degree of 2-O-sulfation was consistently increased with increasing NS domain length, suggesting a correlation between 2-O-sulfatio n efficiency and length of the polymeric substrate during heparan sulfate b iosynthesis. By contrast, 6-O-sulfation of glucosamine units did not correl ate to domain size. 6-O-Sulfation exceeded 2-O-sulfation in NS domains from kidney heparan sulfate, but was very low in aorta heparan sulfate. Remarka bly, total O-sulfation of NS domains, i.e., the sum of 2-O- and 6-O-sulfate groups, was highly similar in all heparan sulfate samples investigated. Th e results reveal marked tissue-specific variation in the sulfation patterns of NS domains and indicate previously unrecognized distinctions in the coo rdination of the three polymer modification reactions during heparan sulfat e biosynthesis.