Beyond H₂S and NO Interplay: Hydrogen Sulfide and Nitroprusside React Directly to Give Nitroxyl (HNO). A New Pharmacological Source of HNO

Hydrogen sulfide (H₂S) has been increasingly recognized as an important signaling molecule that regulates both blood pressure and neuronal activity. Attention has been drawn to its interactions with another gasotransmitter, nitric oxide (NO). Here, we provide evidence that the physiological effects observed upon the application of sodium nitroprusside (SNP) and H₂S can be ascribed to the generation of nitroxyl (HNO), which is a direct product of the reaction between SNP and H₂S, not a consequence of released NO subsequently reacting with H₂S. Intracellular HNO formation has been confirmed, and the subsequent release of calcitonin gene-related peptide from a mouse heart has been demonstrated. Unlike with other thiols, SNP reacts with H₂S in the same way as rhodanese, i.e., the cyanide transforms into a thiocyanate. These findings shed new light on how H₂S is understood to interact with nitroprusside. Additionally, they offer a new and convenient pharmacological source of HNO for therapeutic purposes

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO_(g) to Nitrite Conversion Utilizing μ‑oxo Heme-Fe^III–O–Cu^II(L) Constructs

While nitric oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO₂^–) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a₃/Cu_B active site of cytochrome <i>c</i> oxidase. In this report, the oxidation of NO_(g) to nitrite is shown to occur efficiently in new synthetic μ-oxo heme-Fe^III–O–Cu^II(L) constructs (L being a tridentate or tetradentate pyridyl/alkylamino ligand), and spectroscopic and kinetic investigations provide detailed mechanistic insights. Two new X-ray structures of μ-oxo complexes have been determined and compared to literature analogs. All μ-oxo complexes react with 2 mol equiv NO_(g) to give 1:1 mixtures of discrete [(L)­Cu^II(NO₂^–)]⁺ plus ferrous heme-nitrosyl compounds; when the first NO_(g) equiv reduces the heme center and itself is oxidized to nitrite, the second equiv of NO_(g) traps the ferrous heme thus formed. For one μ-oxo heme-Fe^III–O–Cu^II(L) compound, the reaction with NO_(g) reveals an intermediate species (“intermediate”), formally a bis-NO adduct, [(NO)­(porphyrinate)­Fe^II–(NO₂^–)–Cu^II(L)]⁺ (λ_max = 433 nm), confirmed by cryo-spray ionization mass spectrometry and EPR spectroscopy, along with the observation that cooling a 1:1 mixture of [(L)­Cu^II(NO₂^–)]⁺ and heme-Fe^II(NO) to −125 °C leads to association and generation of the key 433 nm UV–vis feature. Kinetic-thermodynamic parameters obtained from low-temperature stopped-flow measurements are in excellent agreement with DFT calculations carried out which describe the sequential addition of NO_(g) to the μ-oxo complex

Does Perthionitrite (SSNO^–) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization

Hydrogen sulfide (H₂S) and nitric oxide (NO) are important signaling molecules that regulate several physiological functions. Understanding the chemistry behind their interplay is important for explaining these functions. The reaction of H₂S with <i>S</i>-nitrosothiols to form the smallest <i>S</i>-nitrosothiol, thionitrous acid (HSNO), is one example of physiologically relevant cross-talk between H₂S and nitrogen species. Perthionitrite (SSNO^–) has recently been considered as an important biological source of NO that is far more stable and longer living than HSNO. In order to experimentally address this issue here, we prepared SSNO^– by two different approaches, which lead to two distinct species: SSNO^– and dithionitric acid [HON­(S)­S/HSN­(O)­S]. (H)­S₂NO species and their reactivity were studied by ¹⁵N NMR, IR, electron paramagnetic resonance and high-resolution electrospray ionization time-of-flight mass spectrometry, as well as by X-ray structure analysis and cyclic voltammetry. The obtained results pointed toward the inherent instability of SSNO^– in water solutions. SSNO^– decomposed readily in the presence of light, water, or acid, with concomitant formation of elemental sulfur and HNO. Furthermore, SSNO⁻ reacted with H₂S to generate HSNO. Computational studies on (H)­SSNO provided additional explanations for its instability. Thus, on the basis of our data, it seems to be less probable that SSNO^– can serve as a signaling molecule and biological source of NO. SSNO^– salts could, however, be used as fast generators of HNO in water solutions