The antithrombin-binding sequence in heparin. Identification of an essential 6-O-sulfate group.

An octasaccharide with high affinity for antithrombin, isolated after partial deaminative cleavage of heparin, was previously found to have an L-iduronosyl-N-acetylglucosaminyl-6-O-sulfate nonreducing terminal disaccharide unit. After digestion of this octasaccharide with alpha-L-iduronidase and N-acetylglucosamine-6-sulfate sulfatase, two fractions, with high and low affinity for antithrombin, respectively, were isolated by affinity chromatography on antithrombin-Sepharose. Structural analysis showed that the high affinity fraction contained intact octasaccharide, whereas the low affinity fraction consisted of the expected heptasaccharide, lacking a 6-sulfate group on the terminal N-acetylglucosamine residue. Digestion of the octasaccharide with alpha-L-iduronidase only yielded heptasaccharide which was identical with the low affinity species except for the presence of this 6-sulfate group. This less degraded heptasaccharide retained high affinity for antithrombin. It is concluded that the 6-sulfate group on the N-acetylglucosamine residue is of critical importance to the interaction between heparin and antithrombin.