H2S signals via protein persulfidation. To be regulatory the modification will have to be reversible. Using a new method for persulfide detection, we discover this missing link and show that thioredoxin system acts as depersulfidase in vivo.
Hydrogen sulfide (H2S) elicits pleiotropic physiological effects ranging from modulation of cardiovascular to CNS functions. A dominant method for transmission of sulfide-based signals is via posttranslational modification of reactive cysteine thiols to persulfides. However, the source of the persulfide donor and whether its relationship to H2S is as a product or precursor is controversial. The transsulfuration pathway enzymes can synthesize cysteine persulfide (Cys−SSH) from cystine and H2S from cysteine and/or homocysteine. Recently, Cys−SSH was proposed as the primary product of the transsulfuration pathway with H2S representing a decomposition product of Cys−SSH. Our detailed kinetic analyses demonstrate a robust capacity for Cys−SSH production by the human transsulfuration pathway enzymes, cystathionine beta-synthase and γ-cystathionase (CSE) and for homocysteine persulfide synthesis from homocystine by CSE only. However, in the reducing cytoplasmic milieu where the concentration of reduced thiols is significantly higher than of disulfides, substrate level regulation favors the synthesis of H2S over persulfides. Mathematical modeling at physiologically relevant hepatic substrate concentrations predicts that H2S rather than Cys−SSH is the primary product of the transsulfuration enzymes with CSE being the dominant producer. The half-life of the metastable Cys−SSH product is short and decomposition leads to a mixture of polysulfides (Cys−S−(S)n−S−Cys). These in vitro data, together with the intrinsic reactivity of Cys−SSH for cysteinyl versus sulfur transfer, are consistent with the absence of an observable increase in protein persulfidation in cells in response to exogenous cystine and evidence for the formation of polysulfides under these conditions.
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 (15)N 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.
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