Pathways for the Decay of Organic Dichloramines and Liberation of Antimicrobial Chloramine Gases

When neutrophils phagocytose bacteria, they generate the cytotoxic agent hypochlorous acid (HOCl). The specific role that HOCl lays in bacterial killing is unclear. In the phagosome, it should react with neutrophil proteins to form protein chloramines and dichloramines. We investigated the stability of model dichloramines that are likely to be formed on N-terminal amino acids and Lys residues of proteins contained within phagosomes. Dichloramines were much more unstable than their analogous monochloramines. The stability was affected by substituents on the alpha-carbon. Amino acid dichloramines were extremely unstable, indicating that an alpha-carboxyl group facilitated decomposition. In general, the absence of a substituent enhanced stability. The carboxyl group on N-terminal Glu residues favored break down, but this effect was not apparent with Asp residues. Unstable dichloramines that contained a substituent on their alpha-carbon were cytotoxic and killed 50% of 10(5) Staphylococcus aureus (LD50) at a dose of approximately 2.5 nmol. Their cytotoxicity declined with time. The dichloramines of N-alpha-acetyl Lys and taurine were not bactericidal up to 10 nmol per 10(5) S. aureus. None of the analogous monochloramines were cytotoxic at this dose. Dichloramines decomposed to yield chlorimines, aldehydes, and the inorganic gases ammonia monochloramine (NH2Cl) and ammonia dichloramine (NHCl2). The LD50 values were determined for NH2Cl (0.37 +/- 0.14 nmol), NHCl2 (0.08 +/- 0.02 nmol), and HOCl (0.14 +/- 0.04 nmol). Stable products formed during the breakdown of dichloramines were not bactericidal. We propose a potential antimicrobial mechanism that explains in part how HOCl can react mainly with neutrophil components but still promote killing of phagocytosed bacteria. HOCl produced in phagosomes will react with amine groups on neutrophil proteins to form unstable dichloramines that will liberate cytotoxic NH2Cl and NHCl2. These gases will contribute to killing of ingested bacteria.