Use of plasma induced modification of biomolecules (PLIMB) to evaluate hemin dissociation from fish and bovine methemoglobins.

Hemin dissociation occurs much faster from fish methemoglobin (metHb) compared to mammalian metHb yet the mechanism remains poorly understood. This may involve enhanced solvent access to His(E7) of fish metHbs by a protonation mechanism. Plasma induced modification of biomolecules (PLIMB) produces free radicals that covalently modify solvent accessible residues of proteins, and so can provide insight regarding accessibility of hydronium ions to protonate His(E7). PLIMB-induced modifications to heme crevice sites of trout IV and bovine metHb were determined using tandem mass spectrometry after generating peptides with Trypsin/Lys-C. αHis(CE3) was more modified in trout attributable to the more dynamic nature of bovine αHis(CE3) from available crystal structures. Although His(E7) was not found to be more modified in trout, aspects of trout peptides containing His(E7) hampered modification determinations. An existing computational structure-based approach was also used to estimate protonation tendencies, suggesting His(E7) of metHbs with low hemin affinity are more protonatable.

Elucidating the binding properties of methemoglobin in red blood cell to cyanide, hydrosulfide, and azide ions using artificial red blood cell.

Methemoglobin (metHb), the oxidized form of hemoglobin, lacks the ability of reversible oxygen binding; however, it has a high binding affinity to toxic substances such as cyanide, hydrosulfide, and azide. This innate property of metHb offers the clinical option to treat patients poisoned with these toxins, by oxidizing the endogenous hemoglobin in the red blood cells (RBCs). The binding properties of naked metHb (isolated from RBC) with these toxins has been studied; however, the binding behaviors of metHb under the intracellular conditions of RBC are unclear because of the difficulty in detecting metHb status changes in RBC. This study aimed to elucidate the binding properties of metHb in RBC under physiological and poisoned conditions using artificial RBC, which was hemoglobin encapsulated in a liposome. The mimic-circumstances of metHb in RBC (metHb-V) was prepared by oxidizing the hemoglobin in artificial RBC. Spectroscopic analysis indicated that the metHb in metHb-V exhibited a binding behavior different from that of naked metHb, depending on the toxic substance: When the pH decreased, (i) the cyanide binding affinity of metHb-V remained unchanged, but that of naked metHb decreased (ii) the hydrosulfide binding affinity was increased in metHb-V but was decreased in naked metHb. (iii) Azide binding was increased in metHb-V, which was similar to that in naked metHb, irrespective of the pH change. Thus, the binding behavior of intracellular metHb in the RBC with cyanide, hydrosulfide, and azide under physiological and pathological conditions were partly elucidated using the oxidized artificial RBC.

Pharmaceutical stability of methemoglobin-albumin cluster as an antidote for hydrogen sulfide poisoning after one-year storage in freeze-dried form.

Long-term stability during storage is an important requirement for pharmaceutical preparations. The methemoglobin (metHb)-albumin cluster, in which bovine metHb is covalently enveloped with an average of three human albumin molecules, is a promising antidote for hydrogen sulfide (H2S) poisoning. In this study, we investigated the pharmaceutical stability of metHb-albumin cluster after storage for one year in solution and as freeze-dried powder. The lyophilized powder of metHb-albumin cluster stored for one year was readily reconstituted in sterile water for injection, yielding a homogeneous brown solution. Physicochemical measurements revealed that the overall structure of the metHb-albumin cluster was still maintained after preservation. Results of the pharmacological study showed that 100 % of the H2S-poisoned mice survived after treatment with the reconstituted solution of metHb-albumin cluster powder. Furthermore, the solution did not cause any toxic reactions. The antidotal efficacy of metHb-albumin cluster for H2S poisoning was preserved in freeze-dried powder form for at least one year.

Methemoglobin-albumin clusters for cyanide detoxification.

Sodium nitrite (NaNO2) is a universal antidote for patients with cyanide poisoning. However, its use has serious drawbacks in terms of efficacy and safety. Herein, we present a promising antidote: methemoglobin (metHb)-albumin clusters. The metHb-albumin cluster is made by a metHb core wrapped by covalently bound human serum albumin. Spectral analyses proved that the metHb-albumin clusters possessed cyanide-binding properties similar to those of naked metHb. In vitro cell experiments showed that metHb-albumin clusters prevented the cyanide-induced inhibition of cytochrome c oxidase activity, resulting in a strong cytoprotective effect. In mice subjected to cyanide poisoning, metHb-albumin clusters reduced mortality and alleviated metabolic acidosis, while maintaining the activity of cytochrome c oxidase in organs; their efficacy was better than that of NaNO2. Furthermore, the oxygen carrying capacity was maintained in poisoned mice treated with metHb-albumin clusters and was low in those treated with NaNO2. These results indicate that metHb-albumin clusters could be a more effective and safer antidote against cyanide poisoning than NaNO2.

Protective Effect of Dinitrosyl Iron Complexes Bound with Hemoglobin on Oxidative Modification by Peroxynitrite.

Dinitrosyl iron complexes (DNICs) are a physiological form of nitric oxide (•NO) in an organism. They are able not only to deposit and transport •NO, but are also to act as antioxidant and antiradical agents. However, the mechanics of hemoglobin-bound DNICs (Hb-DNICs) protecting Hb against peroxynitrite-caused, mediated oxidative modification have not yet been scrutinized. Through EPR spectroscopy we show that Hb-DNICs are destroyed under the peroxynitrite action in a dose-dependent manner. At the same time, DNICs inhibit the oxidation of tryptophan and tyrosine residues and formation of carbonyl derivatives. They also prevent the formation of covalent crosslinks between Hb subunits and degradation of a heme group. These effects can arise from the oxoferryl heme form being reduced, and they can be connected with the ability of DNICs to directly intercept peroxynitrite and free radicals, which emerge due to its homolysis. These data show that DNICs may ensure protection from myocardial ischemia.

Hydration of methemoglobin studied by in silico modeling and dielectric spectroscopy.

The hemoglobin concentration of 35 g/dl of human red blood cells is close to the solubility threshold. Using microwave dielectric spectroscopy, we have assessed the amount of water associated with hydration shells of methemoglobin as a function of its concentration in the presence or absence of ions. We estimated water-hemoglobin interactions to interpret the obtained data. Within the concentration range of 5-10 g/dl of methemoglobin, ions play an important role in defining the free-to-bound water ratio competing with hemoglobin to recruit water molecules for the hydration shell. At higher concentrations, hemoglobin is a major contributor to the recruitment of water to its hydration shell. Furthermore, the amount of bound water does not change as the hemoglobin concentration is increased from 15 to 30 g/dl, remaining at the level of ∼20% of the total intracellular water pool. The theoretical evaluation of the ratio of free and bound water for the hemoglobin concentration in the absence of ions corresponds with the experimental results and shows that the methemoglobin molecule binds about 1400 water molecules. These observations suggest that within the concentration range close to the physiological one, hemoglobin molecules are so close to each other that their hydration shells interact. In this case, the orientation of the hemoglobin molecules is most likely not stochastic, but rather supports partial neutralization of positive and negative charges at the protein surface. Furthermore, deformation of the red blood cell shape results in the rearrangement of these structures.

Atomistic Simulations of Heme Dissociation Pathways in Human Methemoglobins Reveal Hidden Intermediates.

Heme dissociations disrupt function and structural integrity of human hemoglobin and trigger various cardiovascular complications. These events become significant in methemoglobins that have undergone autoxidation of ferrous into ferric heme. We have structurally characterized the heme disassociation pathways for adult tetrameric methemoglobins using all-atom molecular dynamics simulations. These reveal that bis-histidine hemichromes, characterized here by the coordination of heme iron to both the F8 (proximal) and E7 (distal) histidines, are seen as intermediates following dissociation of the water molecule distally bound to each heme iron. Later, the breaking of coordination between heme iron and proximal histidine disrupts the F helix and pushes it away from the heme cavity, enabling both bulk solvent penetration and disruption of tetramer interface interactions. The interactions inhibiting heme dissociation were then seen to be (i) either a direct or a water-molecule-mediated interaction between distal histidine and heme iron and (ii) stacking between heme and the αCE1/ßCD1 phenylalanine residue. These interactions are less important in the ß than in α subunits due to a more flexible ß subunit CE loop region. The absence of a distal histidine interaction in the H(E7)L mutant and increased heme cavity volume in the V(E11)A mutant both promoted heme escape from the protein interior. Adult and fetal hemoglobins were seen to share a general heme disassociation pathway and intermediates due to the conservation of key heme pocket residues. The intermediates seen here are analyzed in light of experimental studies of heme dissociation and pathways of certain hemoglobinopathies.

Dielectric spectra broadening as a signature for dipole-matrix interactions. V. Water in protein solutions.

In this paper, the fifth of our series focused on the dielectric spectrum symmetrical broadening of water, we consider the solutions of methemoglobin (MetHb) in pure water and in phosphate-buffered saline (PBS). The universal character of the Cole-Cole dielectric response, which reflects the interaction of water dipoles with solute molecules, was described in Paper I [E. Levy et al., J. Chem. Phys. 136, 114502 (2012)]. It enables the interpretation of the dielectric data of MetHb solutions in a unified manner using the previously developed 3D trajectory method driven by the protein concentration. It was shown that protein hydration is determined by the interaction of water dipoles with the charges and dipoles located on the rough surfaces of the protein macromolecules. In the case of the buffered solution, the transition from a dipole-charged to a dipole-dipole interaction with the protein concentration is observed {see Paper III [A. Puzenko et al., J. Chem. Phys. 137, 194502 (2012)]}. A new approach is proposed for evaluating the amount of hydration water molecules bounded to the macromolecule that takes into account the number of positive and negative charges on the protein's surface. In the case of the MetHb solution in PBS, the hydration of the solvent ions and their interaction with charges on the protein's surface are also taken into consideration. The difference in hydration between the two solutions of MetHb is discussed.

In Silico Insight into the Reductive Nitrosylation of Ferric Hemeproteins.

A combination of in silico methods was used to extend the experimental description of the reductive nitrosylation mechanism in ferric hemeproteins with the molecular details of the role of surrounding amino acids. The computational strategy consisted in the estimation of potential energy profiles for the transition process associated with the interactions of the coordinated N(NO) moiety with O(H2O) or O(OH-) as nucleophiles, and with distal amino acids as proton acceptors or affecting the stability of transition states. We inspected the reductive nitrosylation in three representative hemeproteins -sperm whale metmyoglobin, α subunit of human methemoglobin and nitrophorin 4 of Rhodnius prolixus. For each case, classical molecular dynamics simulations were performed in order to obtain relevant reactive conformations, and a potential energy profile for the reactive step was obtained using adiabatic mapping or nudged elastic band approaches at the QM/MM level. Specifically, we report the role of a charged Arg45 of myoglobin in destabilizing the transition state when H2O acts as nucleophile, differently to the neutral Pro43 of the hemoglobin subunit. The case of the nitrophorin is unique in that the access of the required water molecules is scarce, thus, preventing the reaction.

Novel X-ray sequences and crystal structures of Persian and Starry sturgeon methemoglobins: Highlighting the role of heme pocket waters in causing autoxidation.

Although there is a high sequence similarity between mammalian and fish hemoglobin (Hb), the oxidation and heme loss rates can vary greatly between them such that fish Hbs oxidise much more rapidly than mammalian Hbs. There is to date no sequence or structural data for any sturgeon Hb to reveal the level of autoxidation in these fish. In this study, novel high resolution X-ray sequences and crystal structures of methemoglobin (Met-Hb) from two sturgeon fish including Persian sturgeon (Acipenser percisus) and Starry sturgeon (Acipenser stellatus) belonging to the Caspian sea has been determined. A comprehensive sequence and structure comparison between these sturgeon Met-Hbs and a number of non-sturgeon and normal and sickle cell anaemia human Hb in varying heme states has been carried out highlighting (i) the structural variability in the heme propionate groups; (ii) the existence of certain residues or their displacement and shift in the heme pocket allowing entry of water molecules into the heme pocket; (iii) the importance of the number of water molecules in the heme pocket; (iv) the hydrogen bonding between oxygens of A and D propionate groups and that of waters in the heme pocket; and (v) the role of heme binding waters causing oxidative stress and heme autoxidation.

Impact of hemoglobin breakdown products in the spectral analysis of burn wounds using spatial frequency domain spectroscopy.

Burn wounds and wound healing invoke several biological processes that may complicate the interpretation of spectral imaging data. Through analysis of spatial frequency domain spectroscopy data (450 to 1000 nm) obtained from longitudinal investigations using a graded porcine burn wound healing model, we have identified features in the absorption spectrum that appear to suggest the presence of hemoglobin breakdown products, e.g., methemoglobin. Our results show that the calculated concentrations of methemoglobin directly correlate with burn severity, 24 h after the injury. In addition, tissue parameters such as oxygenation (StO2) and water fraction may be underestimated by 20% and 78%, respectively, if methemoglobin is not included in the spectral analysis.

Engineering tyrosine residues into hemoglobin enhances heme reduction, decreases oxidative stress and increases vascular retention of a hemoglobin based blood substitute.

Hemoglobin (Hb)-based oxygen carriers (HBOC) are modified extracellular proteins, designed to replace or augment the oxygen-carrying capacity of erythrocytes. However, clinical results have generally been disappointing due to adverse side effects, in part linked to the intrinsic oxidative toxicity of Hb. Previously a redox-active tyrosine residue was engineered into the Hb ß subunit (ßF41Y) to facilitate electron transfer between endogenous antioxidants such as ascorbate and the oxidative ferryl heme species, converting the highly oxidizing ferryl species into the less reactive ferric (met) form. We inserted different single tyrosine mutations into the α and ß subunits of Hb to determine if this effect of ßF41Y was unique. Every mutation that was inserted within electron transfer range of the protein surface and the heme increased the rate of ferryl reduction. However, surprisingly, three of the mutations (ßT84Y, αL91Y and ßF85Y) also increased the rate of ascorbate reduction of ferric(met) Hb to ferrous(oxy) Hb. The rate enhancement was most evident at ascorbate concentrations equivalent to that found in plasma (< 100 µM), suggesting that it might be of benefit in decreasing oxidative stress in vivo. The most promising mutant (ßT84Y) was stable with no increase in autoxidation or heme loss. A decrease in membrane damage following Hb addition to HEK cells correlated with the ability of ßT84Y to maintain the protein in its oxygenated form. When PEGylated and injected into mice, ßT84Y was shown to have an increased vascular half time compared to wild type PEGylated Hb. ßT84Y represents a new class of mutations with the ability to enhance reduction of both ferryl and ferric Hb, and thus has potential to decrease adverse side effects as one component of a final HBOC product.

A second-derivate fitting algorithm for the quantification of free hemoglobin in human plasma.

BACKGROUND: Assessment of hemolysis in vivo is becoming increasingly relevant in critical care. Current methods (Harboe, 1959) for quantifying the free hemoglobin (fHb) content produce unsatisfactory results in case of hyperbilirubinemia, a frequent condition in patients at risk for intravascular hemolysis. METHODS: A novel evaluation method based on second-derivative fitting to quantify fHb content was developed. The method uses spectrophotometric data from 350 to 650 nm recorded with standard instruments as input. To evaluate the power of the new method, plasma of patients and non-icteric plasma of healthy volunteers were spiked with fHb concentrations up to 2000 mg/L and compared to methods described in the literature by Harboe, Noe and Fairbanks. All measurements were done in compliance with the bioanalytical method validation protocol from the European Medicines Agency. RESULTS: Both the second-derivative fitting algorithm as well as the methods of Harboe, Noe and Fairbanks quantified fHb accurately in non-icteric samples, with inaccuracy and imprecision below 10%. For icteric specimen, false high results were obtained with the established formulas for fHb concentrations below 700 mg/L. In contrast, no interference was found with the second-derivate fitting method for bilirubin concentrations up to 465 µmol/L. The lower limits of quantifications for the second-derivative fitting algorithm were specified in agreement with the EMA guideline with 25 mg/L fHb for both non-icteric and icteric specimens. CONCLUSIONS: A user-friendly, computer-based algorithm is reported that allows the accurate quantification of fHb concentrations in the presence of high bilirubin concentrations. The new method allows for uniform sample preparation with only a single dilution step and can be readily implemented in any laboratory on standard spectrophotometers using the provided supplementary Microsoft Excel macro.

Stability of postmortem methemoglobin: Artifactual changes caused by storage conditions.

Hemoglobin is the protein in red blood cells that carries and distributes oxygen to the body. Methemoglobinemia is a blood disorder in which an abnormal amount of methemoglobin (MetHb), a form of hemoglobin (Hb), is produced from either inadequate MetHb reductase activity or too much MetHb production or by exposure to oxidizing agents. This could lead to anoxia and death if it is not treated. However, this parameter has not been investigated as a valid post-mortem indicator because random MetHb levels have been observed in various studies: MetHb increases can be observed due to autoxidation during storage, and MetHb decreases can be observed due to MetHb reductase or microbial activity in post-mortem samples. MetHb variations can also come from the blood state and can interfere in the optical measurements of MetHb. We have studied the post-mortem MetHb concentrations according to various storage conditions. Based on our results, both the post-mortem delay and the delay before analysis should be reduced whenever possible to avoid changes in MetHb. If the analysis is delayed for a short period of time (two weeks), the blood sample taken at autopsy should not be frozen but collected in EDTA preservative and stored under refrigeration (4-6°C) until analysis. If the analysis is delayed for a longer period (more than two weeks), the blood sample should be frozen with cryoprotectant at -80°C or -196°C.

Methemoglobin Forming Effect of Dimethyl Trisulfide in Mice.

Dimethyl trisulfide (DMTS) is a natural organic trisulfide that has been patented as a promising antidotal candidate against cyanide (CN). The primary mode of action of DMTS is as a sulfur donor that enables the conversion of CN to thiocyanate. Recently, it was discovered that DMTS is capable of oxidizing hemoglobin (Hb) to methemoglobin (MetHb) in vitro. The goal of these experiments was to measure the extent of DMTS-induced MetHb formation in vivo. In these experiments, intramuscular (IM) injections of formulated DMTS were administered to mice. Following the IM injection, blood was drawn and analyzed for MetHb using a rapid spectrophotometric method. Methemoglobin levels peaked in a dose-dependent manner between 20 and 30 min., and then began dropping. The highest MetHb levels measured for the 50, 100, 200 and 250 mg/kg doses of DMTS were respectively 3.28, 6.12, 9.69, and 10.76% MetHb. These experiments provide the first experimental evidence that IM administered DMTS generates MetHb in vivo and provide additional evidence for the presence of a secondary therapeutic pathway for DMTS - CN scavenging by DMTS-generated MetHb.

Determination of the formal redox potentials of the cyanhaemoglobin/cyanmethaemoglobin and the myoglobin/metmyoglobin couples at neutral pH.

Determination of a representative formal redox potential of the Fe(II)/Fe(III) redox couple in cyanhaemoglobin, at pH=7 and related to the state in solution, was the objective of this work. It was achieved at low concentrations of the protein (5µM) to circumvent undesired adsorption. Square-wave voltammetry instead of classical cyclic voltammetry was applied because this method is more sensitive and provides information on the formal redox potential and reversibility, even for rapid processes. We obtained E°'=-0.12±0.01V for cyanhaemoglobin and E°'=-0.10±0.01V, vs. SHE, for myoglobin in comparison. These values differ by only 20mV because the two Fe(II)/Fe(III) redox centres are embedded in closely resembling chemical environments. The small difference is probably owed to the additional axially coordinating cyanide ligand in cyanmethaemoglobin which slightly favours the Fe(III) state in the haem macrocycle.

In vivo evaluation of electron mediators for the reduction of methemoglobin encapsulated in liposomes using electron energies produced by red blood cell glycolysis.

Earlier studies have clarified that NADH and NADPH, re-energized repeatedly by red blood cell (RBC) glycolysis, can be used in extracellular chemical reactions, where electron energies are extracted by electron mediators, such as methylene blue (MB). The electron mediators, which are reduced by NAD(P)H, permeate both the membranes of RBC and phospholipid bilayer of liposomes encapsulating haemoglobin (Hb-vesicles, HbV) and reduce autoxidized ferric methemoglobin (metHb) in HbV to ferrous Hb. Moreover, in vitro screening study clarified some other potential electron mediators with comparable capacity to reduce metHb. Given this background, eight of these compounds: MB, 1,9-dimethyl MB, azure A, azure B (AB), azure, toluidine blue, brilliant cresyl blue and toluylene blue, were evaluated in both in vitro and in vivo studies in this work. Compared with MB as a reference, in vitro experiments demonstrated that most compounds caused effective metHb reduction of HbV in the presence of RBC. However, in vivo experiments of bolus injection of autoxidized HbV to rats (10 mL HbV/kg body weight) followed by injection of the dye (1.53 mL/kg body weight, 2.6 mM) led to some differences from in vitro results. Effective metHb reduction was found for the combination of AB. To evaluate AB effectiveness further, a haemorrhagic shock and resuscitation model was used, where the rats were resuscitated with HbV. When the level of metHb increased to 50%, a dye solution was injected. Again, AB caused sufficient reduction of metHb. Through these in vivo experiments, this study clarified that AB is a suitable electron mediator to prolong the functional lifetime of HbV.

Potential role of ferric hemoglobin in MS pathogenesis: Effects of oxidative stress and extracellular methemoglobin or its degradation products on myelin components.

There is a well-documented relationship between cerebral vasculature and multiple sclerosis (MS) lesions: abnormal accumulations of iron have been found in the walls of the dilated veins in cerebral MS plaques. The source of this iron is unknown, but could be related to the recognized phenomenon of capillary and venous hemorrhages leading to blood extravasation. In turn, hemorrhaging leading to hemolysis results in extracellular release of hemoglobin, a reactive molecule that could induce local oxidative stress, inflammation, and tissue damage. Our previous studies with a reduced form of hemoglobin (oxyHb) have demonstrated its ability to cause extensive lipid and protein oxidation in vitro, which would result in membrane destabilization. Here, we investigated in further detail the mechanism by which the more abundant oxidized form of extracellular hemoglobin (metHb), and dissociated hemin, cause direct oxidative damage to myelin components, specifically membrane-mimetic lipid vesicles and myelin basic protein (MBP), a highly-abundant protein in the CNS. Oxidation of lipids was assessed by the formation of conjugated diene/triene and malondialdehyde, and oxidation of MBP was demonstrated by the bityrosine formation and by the change in protein mass. Our results show that metHb causes oxidative damage to MBP and myelin lipids, partly by transferring its hemin moiety to protein and lipid, but mostly as an intact protein possibly via formation of a ferryl radical. These results elucidating the mechanism of extracellular hemoglobin-induced oxidative damage to myelin components support the need for further research into vascular pathology in MS pathogenesis, to gain insight into the role of iron deposits and/or in stimulation of different comorbidities associated with the disease.

Reaction of Dimethyl Trisulfide with Hemoglobin.

Dimethyl trisulfide (DMTS) is a promising antidotal candidate for cyanide intoxication. DMTS acts as a sulfur donor in the conversion of cyanide to the less-toxic thiocyanate. The alternate reaction pathways of DMTS in the blood are not well understood. We report changes in the hemoglobin absorption spectrum upon reaction with DMTS. These changes closely match those induced by the known methemoglobin former, sodium nitrite. The kinetics of methemoglobin formation with DMTS is slower than with sodium nitrite. These results support the hypothesis that a potentially significant side-reaction of the therapeutically administered DMTS is the oxidization of hemoglobin to methemoglobin.

S-Transnitrosation reactions of hydrogen sulfide (H2S/HS-/S2-) with S-nitrosated cysteinyl thiols in phosphate buffer of pH 7.4: Results and review of the literature.

Cysteine (CysSH) and its derivatives including N-acetylcysteine (NAC) and glutathione (GSH), and cysteine residues in proteins and enzymes are nitrosated with nitric oxide (NO) reaction products such as N2O3 to form S-nitrosated cysteine thiols (RCysSNO). RCysSNO undergo with cysteine thiols (RCysSH) S-transnitrosation reactions, thereby transferring reversibly their nitrosyl (+NO) group to RCysSH to form RCysSNO. •NO release from RCysSNO and S-transnitrosation are considered the most important features and signalling pathways of RCysSNO. Hydrogen sulfide (H2S: pKa1, 7; HS-: pKa2, 12.9) is an endogenous product of cysteine metabolism. We hypothesized that RCysSNO would also undergo S-transnitrosation reaction with H2S/HS-/S2- to form thionitrite (ONS-), the smallest S-nitrosated thiol. This article describes spectrophotometric and mass spectrometric investigations of S-transnitrosation reactions in phosphate buffered saline (PBS) of pH 7.4 between H2S/HS-/S2- (supplied as Na2S) and S-nitrosoglutathione (GSNO), S-nitroso-l-cysteine (CysSNO), S-nitroso-N-acetyl-l-cysteine (SNAC), and the synthetic S-nitroso-N-acetyl-l-cysteine ethyl ester (SNACET). For comparison, we also investigated the reactions of H2S/HS-/S2- with NO+BF4- and NO2+BF4-, direct ON+ and O2N+ donors, respectively, and assumed formation of ONS- and thionitrate (O2NS-), respectively. Addition of Na2S (at 1 mM) to buffered RCysSNO solutions resulted in decreases of the absorbance at 340 nm and concomitant increases in the absorbance at 410 nm depending upon the nature and concentration of RCysSNO (range, 25-1000 µM). The reactivity order of RCysSNO against H2S/HS-/S2- was: CysSNO > SNACET > GSNO > SNAC. Our spectrophotometric and GC-MS analyses indicate that H2S/HS-/S2- and RCysSNO undergo multiple reactions. Major final reaction products were found to be nitrite and nitrate. ONS- and O2NS- were not detected by GC-MS, suggesting rapid and complete S/O-exchange from water at pH 7.4. GC-MS analyses of ethyl acetate extracts of reaction mixtures suggested formation of tetrasulfur (S4), the precursor of elemental sulfur (S8). The broad absorbance around 410 nm and the turbidity occurred in RCysSNO/Na2S reaction mixtures support formation of polysulfides (polysulfanes) and colloidal sulfur. The reaction of NO+BF4- and NO2+BF4- with H2S/HS-/S2- differed from the S-transnitrosation reactions of RCysSNO, with NO+BF4- being more reactive than NO2+BF4-. In this article, we also briefly review and discuss recent published work dealing with the reaction of H2S/HS-/S2- with low- and high-molecular-mass S-nitrosated thiols. This research area is highly challenging and controversial with respect to the primarily formed reaction products. The synthesis of structurally well-characterized, pure stable-isotope labelled species of putative reaction products, including ONS-, O2NS- and ONSS-, and their use in combination with mass spectrometry coupled to chromatography, e.g. GC-MS and LC-MS/MS, are indispensable in exploring the complex interaction of the two gasotransmitters, H2S and •NO.