Chloramine Derivatives of Cystine:  Surprisingly, given the routine use of OCl^- (bleach) as a disinfectant and its involvement in human health, there is only modest information concerning the mechanism(s) by which it kills microorganisms.  This is due in part to the exceptional reactivity of OCl^- under physiological conditions.  The reactions of HOCl with thiols, thioethers, and amines are particularly facile.  With respect to biocidal mechanisms, the reactions of HOCl with amines are particularly relevant because chloramine derivatives are generally cytotoxic.   But, this is not always the case.  For example, we have recently learned that cystine and oxidized glutathione (GSSG) react preferentially with HOCl at physiologic pH at the N-centers (not the S-centers as previously proposed) to yield chloramine derivatives.  This result is consistent with our recent report that cystine reacts with HOCl to form  chloramines (Nagy, P.; Ashby, M. T., Chem. Res. Toxicol. 2005, 18, 919-923).  However, not all bacteria are capable of synthesizing GSH, nor do all bacteria possess the capability of keeping GSH in a reduced state. Those bacteria that do possess this capability employ it as a mechanism for protection from oxidative stress. E. coli possess this capability (as do most eukaryotes).  Remarkably, the bis-N-dichloro-g-L-glutamyl derivative of oxidized glutathione (ClGSSGCl) is not cytotoxic to E. coli (unpublished results).  Indeed, in addition to the usual protection against oxidative stress that is provided by GSH (Eqn 1), GSSG may provide additional protection against attack by HOCl by absorbing two additional molar equivalents of HOCl (Eqn 2), thereby providing three times the antioxidant capacity toward HOCl than expected from Eqn 1 alone.  We are presently exploring the possibility that enzymes may catalyze the reduction of ClGSSGCl.  Regardless of the fate of ClGSSGCl, the fact that it is apparently not toxic to E. coli provides an explanation for the observation that oxidized glutathione protects E. coli from oxidative stress by HOCl.  While we recognize the efficacy of chloramine itself (NH₂Cl) as a biocide, these results challenge dogma concerning the general involvement of organic chloramine derivatives in the biocidal mechanisms of HOCl.

Acknowledgement:  We are very grateful to the Petroleum Research Fund (PRF#42850-AC4) and the Oklahoma Center for the Advancement of Science and Technology (HR02-019) for their financial support of this project.