Unravelling the mechanism of intracellular oxidation of thiols by (N‐Cl)‐Taurine

Myeloperoxidase, a heme enzyme stored in high amounts in neutrophils and monocytes, produces reactive oxygen species that are involved in the innate mammalian response to infection; among these, the most reactive is HOCl, generated by oxidation of Cl- in the presence of H2O2. This powerful oxidant reacts very fast with protein side chains and peptide bonds. Taurine (-O3SCH2CH2NH3+), present in high concentrations in human neutrophils, traps the HOCl to yield (N-Cl)-Taurine, much less toxic and reactive; it is a long-lived oxidant which contributes to the killing of pathogenic organisms and plays a role in the inflammatory response. Kinetic data suggest that (N-Cl)-Tau reacts with cysteine and glutathione, two relevant thiols in mammals, via two pathways: direct oxidation (chlorination) of the thiolate by (N-Cl)-Tau involving general-acid catalysis, and, the major one, (N-Cl)-Tau hydrolysis followed by fast chlorine transfer from the so-formed HOCl/ClO- mixture to the corresponding thiolate, despite the highly unfavorable hydrolysis equilibrium (K-H =10(-8)M). Scavenging of HOCl by strong nucleophiles like thiolates opens an effective reactive channel. Theoretical calculations, at the B3LYP/6-311++G(d,p) level of theory and using the SMD model to simulate aqueous salvation, agree with the experimental behavior, predicting that both the chlorine and the proton are almost half transferred at the transition state in the direct, general-acid catalyzed, chlorination of the thiolate by (N-Cl)-Tau. Under physiological conditions, the hydrolysis of (N-Cl)-Tau is the main responsible of the oxidation of intracellular thiols far away from the site of HOCl enzymatic production. Copyright (c) 2013 John Wiley & Sons, Ltd.