Biochemical insight into physiological effects of H₂S: reaction with peroxynitrite and formation of a new nitric oxide donor, sulfinyl nitrite

The reaction of hydrogen sulfide (H₂S) with peroxynitrite (a key mediator in numerous pathological states) was studied in vitro and in different cellular models. The results show that H₂S can scavenge peroxynitrite with a corresponding second order rate constant of 3.3±0.4×10³ M⁻¹·s⁻¹ at 23°C (8±2×10³ M⁻¹·s⁻¹ at 37°C). Activation parameters for the reaction (ΔH^‡, ΔS^‡ and ΔV^‡) revealed that the mechanism is rather associative than multi-step free-radical as expected for other thiols. This is in agreement with a primary formation of a new reaction product characterized by spectral and computational studies as HSNO₂ (thionitrate), predominantly present as sulfinyl nitrite, HS(O)NO. This is the first time a thionitrate has been shown to be generated under biologically relevant conditions. The potential of HS(O)NO to serve as a NO donor in a pH-dependent manner and its ability to release NO inside the cells has been demonstrated. Thus sulfide modulates the chemistry and biological effects of peroxynitrite by its scavenging and formation of a new chemical entity (HSNO₂) with the potential to release NO, suppressing the pro-apoptotic, oxidative and nitrative properties of peroxynitrite. Physiological concentrations of H₂S abrogated peroxynitrite-induced cell damage as demonstrated by the: (i) inhibition of apoptosis and necrosis caused by peroxynitrite; (ii) prevention of protein nitration; and (iii) inhibition of PARP-1 [poly(ADP-ribose) polymerase 1] activation in cellular models, implying that a major part of the cytoprotective effects of hydrogen sulfide may be mediated by modulation of peroxynitrite chemistry, in particular under inflammatory conditions.