Heparan sulfate degradation via reductive homolysis of its N-chloro derivatives

The highly basic heme enzyme myeloperoxidase (MPO), which is released by activated phagocytes, catalyzes the production of the potent oxidant hypochlorite (HOCl) from H2O2 and chloride ions (Cl-). Heparan sulfate proteoglycans are key components of the extracellular matrix and cell surfaces and are known to bind MPO avidly via their negatively charged heparan sulfate chains. Reaction of heparan sulfate with HOCl generates polymer-derived N-chloro derivatives (chloramines, dichloramines, N-chlorosulforiamides, and chloramides). In this study, it is shown that heparan sulfate N-chloro derivatives are decomposed in the presence of redox-active transition-metal ions and superoxide (O-2(center dot-)). These processes initiate polymer modification/fragmentation. Radical intermediates in these processes have been identified by EPR spectroscopy and spin trapping. Evidence has been obtained that the N-chloro derivatives undergo reductive homolysis to nitrogen-centered (aminyl, N-chloroaminyl, sulfonamidyl, and amidyl) radicals that generate carbon-centered radicals via rapid, intramolecular hydrogen atom abstraction reactions (1,2and/or 1,5-shifts). In the case of the sulfonamidyl radicals, rearrangement via 1,2-shifts and beta-sclssion of the resultant C-2 carbon-centered radicals to yield SO3 center dot- and C-2 imines is near quantitative based on the yield of SO42-, the decomposition product of SO3 center dot-. The formation of strand breaks and chromophores during these reactions is attributed to the formation and subsequent heterolytic rearrangement of the C-2 imines. The degradation of heparan sulfate via reductive homolysis of its N-chloro derivatives may be of significance at sites of inflammation, where MPO-derived HOCl is produced in high concentration and transition-metal ions and O-2(center dot-) are known to be present or generated.