Occurrence of 2- O-sulphated d-glucuronic acid in rat liver heparan sulphate

Abstract

Heparan sulphates (HS) are a complex and heterogeneous family of glycosaminoglycans with a polysaccharide chain composed of alternating units of o-glucosamine and uranic acid (D-glucuronic or L-iduronic) with a (1 + 4) linkage. Sulphate substituents occur as N-sulphate (at C-2 of the o-glucosamine residues) or as O-sulphate groups (at C-6 of the o-glucosamine or at C-2 of the L-iduronic acid units). In our previous studiesle3 we have isolated and characterized a highly sulphated HS from rat liver tissues, Ll. In spite of its high degree of sulphation (2.0 sulphate groups/disaccharide), only 50% of the hexosamine residues bear Nsulphate groups, and r_-iduronic acid accounts for 25% of the total uranic acids. The high sulphate content of Ll could not be explained on the basis of the sulphation pattern normally observed in heparin-like polysaccharides. As the polymer is only partially N-sulphated, the high degree of sulphation is presumably due to the O-sulphate groups. The low proportion of L-iduronic acid suggests that positions other than C-2 or C-6 of o-glucosamine are substituted. In order to determine the location of the remaining O-sulphate groups, we continued our structural studies on Ll through 13C NMR and methylation analysis, which allowed us to detect the presence of 2-O-sulphated o-glucuronic acid residues in Ll.