Antimicrobial and cytotoxic activity of hypochlorous acid: interactions with taurine and nitrite

Objective. HOCl, a major bactericidal product of neutrophil MPO - halide sy stem reacts with taurine to form taurine chloramine (TauCl), a less toxic a nti-inflammatory mediator. Recently, it has been reported that HOCl may als o react with nitrite (NO2-), a major end-product of nitric oxide (NO) metab olism, to form very active oxidant, nitryl chloride (NO2Cl). The present st udy was conducted to elucidate the effect of nitrite on bactericidal and so me immunoregulatory properties of HOCl and TauCl. Materials: TauCl was prepared from NaOCl and taurine. The reaction was carr ied out at pH 5.0 and pH 7.4, in the presence or absence of nitrite. All re actions were monitored by UV absorption spectra. Methods: Bactericidal activity of HOCl and TauCl in the presence of nitrite was tested by incubation off. coli with the compounds and determined by th e pour-plate method. To test the effect of the compounds on activity of inf lammatory cells, murine peritoneal neutrophils (PMN) and macrophages were u sed. The cells were activated in vitro with either LPS, IFN-gamma or zymosa n and the production of following mediators was measured: reactive oxygen s pecies using luminol-dependent chemiluminescence; nitric oxide by Griess re action; TNF-alpha using capture ELISA. In addition, we tested the effect of HOCl and TauCl on activity of myeloperoxidase (MPO). Results: At physiological pH nitrite reacts with HOCl but not with TauCl. T his reaction was abolished in the presence of taurine. Nitrite prevented HO Cl-mediated bacterial killing, inhibition of MPO activity, cellular cytotox icity and inhibition of TNF-alpha Produktion. Nitrite did not affect any ac tivity of TauCl. Conclusion. We have shown that nitrite may react in vitro with HOCl but not with TauCl, to form new biologically active product(s). We did not confirm the hypothesis that a product of HOCl reaction with nitrite is more toxic than HOCl. To the contrary, we found that nitrite diminished bactericidal a nd immunoregulatory properties of HOCl. In vivo, nitrite will also compete with taurine for reaction with PMN-released HOCl. Nevertheless, due to high concentration of taurine in PMN cytosol, formation of TauCl will be a majo r regulatory mechanism of MPO-halide-system.