Relative reactivities ofN-chloramines and hypochlorous acid with human plasma constituents

Hypochlorous acid (HOCl), the major strong oxidant produced by the phagocyte enzyme myeloperoxidase, reacts readily with free amino groups to formN-chloramines. Since different N-chloramines have different stabilities and reactivities depending on their structures, we investigated the relative reactivities of three modelN-chloramines and HOCl with human plasma constituents. TheN-chloramines studied were Nα-acetyl-lysine chloramine (LysCA, a model of protein-associatedN-chloramines), taurine chloramine (TaurCA, the primaryN-chloramine produced by activated neutrophils), and monochloramine (MonoCA, a lipophilic N-chloramine). Addition of these chlorine species (100–1000 μM each) to plasma resulted in rapid loss of thiols, with the extent of thiol oxidation decreasing in the order TaurCA = LysCA > MonoCA = HOCl. The single reduced thiol of albumin was the major target. Loss of plasma ascorbate also occurred, with the extent decreasing in the order HOCl > LysCA > TaurCA > MonoCA. Experiments comparing equimolar albumin thiols and ascorbate showed that while HOCl caused equivalent loss of thiols and ascorbate, theN-chloramines reacted preferentially with thiols. The chlorine species also inactivated α1-antiproteinase, implicating oxidation of methionine residues, and ascorbate provided variable protection depending on the chlorine species involved. Together, our data indicate that in biological fluids N-chloramines react more readily with protein thiols than with methionine residues or ascorbate, and thus may cause biologically relevant, selective loss of thiol groups.