Oxidation of heparan sulphate by hypochlorite: role of N-chloro derivatives and dichloramine-dependent fragmentation

Abstract

Activated phagocytes release the haem enzyme MPO (myeloperoxidase) and produce superoxide radicals and H2O2 via an oxidative burst. MPO uses H2O2 and Cl− to form HOCl, the physiological mixture of hypochlorous acid and its anion present at pH 7.4. As MPO binds to glycosaminoglycans, oxidation of extracellular matrix and cell surfaces by HOCl may be localized to these materials. However, the reactions of HOCl with glycosaminoglycans are poorly characterized. The GlcNAc (N-acetylglucosamine), GlcNSO3 (glucosamine-N-sulphate) and GlcNH2 [(N-unsubstituted) glucosamine] residues of heparan sulphate are potential targets for HOCl. It is shown here that HOCl reacts with each of these residues to generate N-chloro derivatives, and the absolute rate constants for these reactions have been determined. Reaction at GlcNH2 residues yields chloramines and, subsequently, dichloramines with markedly slower rates, k2∼3.1×105 and 9 M−1·s−1 (at 37 °C) respectively. Reaction at GlcNSO3 and GlcNAc residues yields N-chlorosulphonamides and chloramides with k2∼0.05 and 0.01 M−1·s−1 (at 37 °C) respectively. The corresponding monosaccharides display a similar pattern of reactivity. Decay of the polymer-derived chloramines, N-chlorosulphonamides and chloramides is slow at 37 °C and does not result in major structural changes. In contrast, dichloramine decay is rapid at 37 °C and results in fragmentation of the polymer backbone. Computational modelling of the reaction of HOCl with heparan sulphate proteoglycans (glypican-1 and perlecan) predicts that the GlcNH2 residues of heparan sulphate are major sites of attack. These results suggest that HOCl may be an important mediator of damage to glycosaminoglycans and proteoglycans at inflammatory foci.