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 S-nitrosothiols to form the smallest S-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 N-15 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.