Relative reactivities of N-chloramines and hypochlorous acid with human plasma constituents

Hypochlorous acid (HOCl), the major strong oxidant produced by the phagocyt e enzyme myeloperoxidase, reacts readily with free amino groups to form N-c hloramines. Since different N-chloramines have different stabilities and re activities depending on their structures, we investigated the relative reac tivities of three model N-chloramines and HOCl with human plasma constituen ts. The N-chloramines studied were N alpha -acetyl-lysine chloramine (LysCA , a model of protein-associated N-chloramines), taurine chloramine (TaurCA, the primary N-chloramine produced by activated neutrophils), and monochlor amine (MonoCA, a lipophilic N-chloramine). Addition of these chlorine speci es (100-1000 muM 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 p lasma 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 a scorbate, the N-chloramines reacted preferentially with thiols. The chlorin e species also inactivated alpha (1)-antiproteinase, implicating oxidation of methionine residues, and ascorbate provided variable protection dependin g on the chlorine species involved. Together, our data indicate that in bio logical fluids N-chloramines react more readily with protein thiols than wi th methionine residues or ascorbate, and thus may cause biologically releva nt, selective loss of thiol groups. (C) 2001 Elsevier Science Inc.