RSH Groups Research Articles

Repeated sprint training in hypoxia induces specific skeletal muscle adaptations through S100A protein signaling.

Athletes increasingly engage in repeated sprint training consisting in repeated short all-out efforts interspersed by short recoveries. When performed in hypoxia (RSH), it may lead to greater training effects than in normoxia (RSN); however, the underlying molecular mechanisms remain unclear. This study aimed at elucidating the effects of RSH on skeletal muscle metabolic adaptations as compared to RSN. Sixteen healthy young men performed nine repeated sprint training sessions in either normoxia (FIO2 = 0.209, RSN, n = 7) or normobaric hypoxia (FIO2 = 0.136, RSH, n = 9). Before and after the training period, exercise performance was assessed by using repeated sprint ability (RSA) and Wingate tests. Vastus lateralis muscle biopsies were performed to investigate muscle metabolic adaptations using proteomics combined with western blot analysis. Similar improvements were observed in RSA and Wingate tests in both RSN and RSH groups. At the muscle level, RSN and RSH reduced oxidative phosphorylation protein content but triggered an increase in mitochondrial biogenesis proteins. Proteomics showed an increase in several S100A family proteins in the RSH group, among which S100A13 most strongly. We confirmed a significant increase in S100A13 protein by western blot in RSH, which was associated with increased Akt phosphorylation and its downstream targets regulating protein synthesis. Altogether our data indicate that RSH may activate an S100A/Akt pathway to trigger specific adaptations as compared to RSN.

Adding heat stress to repeated-sprint training in hypoxia does not enhance performance improvements in canoe/kayak athletes.

The present study investigated the effects of adding heat stress to repeated-sprint training in hypoxia on performance and physiological adaptations in well-trained athletes. Sixteen canoe/kayak sprinters conducted 2weeks of repeated-sprint training consisting of three sets of 5 × 10s sprints with 20s active recovery periods under conditions of either normobaric hypoxia (RSH, FiO2: 14.5%, ambient temperature: 18 ℃, n = 8) or combined heat and normobaric hypoxia (RSHH, FiO2: 14.5%, ambient temperature: 38 ℃, n = 8). Before and after training, the 10 × 10s repeated-sprint ability (RSA) test and 500m time trial were performed on a canoe/kayak ergometer. Peak and average power outputs during the RSA test were significantly improved after training in both RSH (peak power: + 21.5 ± 4.6%, P < 0.001; average power: + 12.5 ± 1.9%, P < 0.001) and RSHH groups (peak power: + 18.8 ± 6.6%, P = 0.005; average power: + 10.9 ± 6.8%, P = 0.030). Indirect variables of skeletal muscle oxygen extraction (deoxygenated hemoglobin) and blood perfusion (total hemoglobin) during the RSA test were significantly increased after training in the RSH group (P = 0.041 and P = 0.034, respectively) but not in the RSHH group. In addition, finish time during the 500m time trial was significantly shortened after the training only in the RSH group (RSH: -3.9 ± 0.8%, P = 0.005; RSHH: -3.1 ± 1.4%, P = 0.078). Adding heat stress to RSH does not enhance performance improvement and may partially mask muscle tissue adaptation.

Association between Long Working Hours and Psychological Distress: The Effect Modification by Request to Stay Home When Sick in the Workplace during the COVID-19 Pandemic.

The effect of workplace infection control measures required by the COVID-19 pandemic on the association between long working hours and psychological distress has not yet been fully revealed. This study investigated the effect of requesting to stay home when sick (RSH) on the association between long working hours and psychological distress. We conducted a cross-sectional survey in December 2020 among participants who had previously registered with a Japanese web survey company. A total of 27,036 workers completed a self-administered questionnaire which assessed usual daily overtime work hours. RSH was assessed using an original single-item scale, while psychological distress was measured with the K6 scale. After the interaction effect of overtime work hours and RSH on psychological distress was tested, we conducted stratified analyses using RSH. The statistical analysis demonstrated a significant interaction effect (p for interaction < 0.001). When we conducted stratified analyses, the odds ratios increased with longer working hours, both with and without RSH groups; however, the risk of long working hours causing psychological distress was greater in the latter group (odds ratio = 1.95 [95% confidence interval: 1.62–2.36] than in the former group (odds ratio = 1.73 [95% confidence interval: 1.55–1.93]). We found that working without RSH could strengthen the association between long working hours and psychological distress. Our findings contribute to preventing the deterioration of mental health during the COVID-19 pandemic.

Anaerobic RSH-dependent tellurite reduction contributes to Escherichia coli tolerance against tellurite

BackgroundTellurium is a rare metalloid that exerts high toxicity on cells, especially on bacteria, partly due to reactive oxygen species (ROS) generation. Moreover, it has also been observed that tellurite can target free cell thiols groups (RSH) (i.e. reduced glutathione (GSH)), enhancing the cellular redox imbalance. Additionally, in vitro experiments have suggested that several enzymes can reduce tellurite (IV) to its elemental form (0); where RSH present on their active sites may be responsible for the process. Nevertheless, the mechanisms implemented by bacteria for tellurite reduction and its role in resistance have not been evaluated in vivo.ResultsThis work shows that tellurite reduction to elemental tellurium is increased under anaerobic conditions in E. coli cells. The in vivo tellurite reduction is related to the intracellular concentration of total RSH, in the presence and absence of oxygen. This metabolization of tellurite directly contributes to the resistance of the bacteria to the oxyanion.ConclusionsWe demonstrated that in vivo tellurite reduction is related to the intracellular thiol concentration, i.e. large availability of cellular RSH groups, results in a more significant reduction of tellurite. Furthermore, we observed that, when the bacterium exhibits less resistance to the oxyanion, a decreased tellurite reduction was seen, affecting the growth fitness. Together, these results let us propose that tellurite reduction and the intracellular RSH content are related to the oxyanion bacterial resistance, this tripartite mechanism in an oxygen-independent anaerobic process.

Irregular-Shaped Hematoma Predicts Postoperative Rehemorrhage After Stereotactic Minimally Invasive Surgery for Intracerebral Hemorrhage.

Background and PurposeMinimally invasive surgery (MIS) is performed to treat patients with intracerebral hemorrhage (ICH) with favorable results. However, postoperative rehemorrhage is a significant risk. The present study retrospectively analyzed the association of irregular-shaped hematoma with postoperative rehemorrhage following stereotactic MIS (sMIS).MethodsWe enrolled 548 patients with spontaneous ICH who underwent sMIS. Based on the hematoma shape, the patients were assigned to the regular-shaped hematoma group (RSH group; 300 patients) or irregular-shaped hematoma group (ISH group; 248 patients). Logistic regression analysis was performed to identify the predictors of postoperative rehemorrhage after sMIS for ICH evacuation. The functional outcome was assessed using the modified ranking scale (mRS) score at discharge. A receiver operating characteristic (ROC) curve was used to confirm the results.ResultsAmong 548 patients with ICH who underwent sMIS, 116 developed postoperative rehemorrhage. Postoperative rehemorrhage occurred in 30.65% of patients with ISH and 13.30% with RSH (P < 0.01), with a significant difference between the ISH and RSH groups. Among 116 patients with postoperative rehemorrhage, 76 (65.52%) showed ISH on CT scan. In 432 patients without postoperative rehemorrhage, only 39.81% displayed ISH. The logistic regression analysis demonstrated that ISH could independently predict postoperative rehemorrhage. The sensitivity, specificity, positive predictive value, and negative predicative value were 0.655, 0.398, 0.655, and 0.602, respectively. The ROC analysis confirmed the value of ISH in predicting postoperative rehemorrhage with an area under the curve of 0.629.ConclusionsIrregular-shaped hematoma was an independent predictor of postoperative rehemorrhage after sMIS.

Effect of hypoxia and nitrate supplementation on different high-intensity interval-training sessions.

To test the hypothesis that interval-training (IHT) would be impaired by hypoxia to a larger extent than repeated-sprint training (RSH) and that dietary nitrate (NO3-) would mitigate the detrimental effect of hypoxia to a larger extent during IHT than RSH. Thirty endurance-trained male participants performed IHT (6 × 1min at 90%∆ with 1min active recovery) and RSH (2 sets of 6 × 10s "all-out" efforts with 20s active recovery) on a cycle ergometer, allocated in one of three groups: normobaric hypoxia (~ 13% FiO2) + NO3--HNO, n = 10; normobaric hypoxia + placebo-HPL, n = 10; normoxia (20.9% FiO2) + placebo-CON, n = 10. Submaximal oxygen uptake ([Formula: see text]O2), time spent above 90% of maximal [Formula: see text]O2 (≥ 90 [Formula: see text]O2max) and heart rate (≥ 90 HRmax) were compared between IHT and RSH sessions and groups. Additionally, mean power output (MPO), decrement score and % of power associated with [Formula: see text]O2max (%p[Formula: see text]O2max) in RSH sessions were analyzed. [Formula: see text]O2 at sub-maximal intensities did not differ between training protocols and groups (~ 27mlkg-1min-1). ≥ 90 HRmax was significantly higher in IHT compared to RSH session (39 ± 8 vs. 30 ± 8%, p = 0.03) but only in HNO group. MPO (range 360-490W) and decrement score (10-13%) were similar between groups although %p[Formula: see text]O2max was significantly higher (p = 0.04) in CON (166 ± 16W) compared with both HPL (147 ± 15W) and HNO (144 ± 10W) groups. IHT responses were neither more impaired by hypoxia than RSH ones. Moreover, dietary NO3- supplementation impacted equally IHT and RSH training responses' differences between hypoxia and normoxia.

On the Use of the Repeated-Sprint Training in Hypoxia in Tennis.

PurposeTo examine physiological and technical responses to repeated-sprint training in normobaric hypoxia at ∼3,000 m (RSH, n = 11) or in normoxia (RSN, n = 11) compared to a control group (CON, n = 8) in well-trained tennis players. Participants were 28.8 ± 5.9 years old without any previous experience of training in hypoxia.MethodsIn addition to maintaining their usual training (CON), both RSH and RSN groups completed five tennis specific repeated-shuttle sprint sessions (4 × 5 × ∼8 s maximal sprints with ∼22 s passive recovery and ∼5 min rest between sets) over 12 days. Before (Pre), the week after (Post-1) and 3 weeks after Post-1 (Post-2), physical/technical performance during Test to Exhaustion Specific to Tennis (TEST), repeated-sprint ability (RSA) (8 × ∼20 m shuttle runs—departing every 20 s) and heart rate variability (HRV) were assessed.ResultsFrom Pre to Post-1 and Post-2, RSH improved TEST time to exhaustion (+18.2 and +17.3%; both P < 0.001), while the “onset of blood lactate accumulation” at 4 mmol L–1 occurred at later stages (+24.4 and +19.8%, both P < 0.01). At the same time points, ball accuracy at 100% V̇O2max increased in RSH only (+38.2%, P = 0.003 and +40.9%, P = 0.007). Markers of TEST performance did not change for both RSN and CON. Compared to Pre, RSA total time increased significantly at Post-1 and Post-2 (−1.9 and −2.5%, P < 0.05) in RSH only and this was accompanied by larger absolute Δ total hemoglobin (+82.5 and +137%, both P < 0.001). HRV did not change either supine or standing positions.ConclusionFive repeated sprint training sessions in hypoxia using tennis specific shuttle runs improve physiological and technical responses to TEST, RSA, and accompanying muscle perfusion responses in well-trained tennis players.

Effect of Different Substrates on Soil Microbial Community Structure and the Mechanisms of Reductive Soil Disinfestation.

Reductive soil disinfestation (RSD) has recently attracted much attention owing to its effectiveness for controlling pathogens. In this study, we aimed to evaluate the effects of different C/N substrates on RSD and to explore the changes in microbial community structure during RSD treatment. The experimental set up included 10 groups, as follows: CK, without substrates; RSD treatments with alfalfa (Medicago sativa L.)[AL], maize (Zea mays Linn. Sp.) straw [MS], and rice (Oryza sativa L.) straw [RS], with three levels of addition (0.5% [L], 2% [M], and 5% [H]), yielding ALL, ALM, ALH, MSL, MSM, MSH, RSL, RSM, and RSH groups. Compared with CK, RSD treatments significantly increased the content of -N, and effectively eliminated the accumulated -N in the soil. The relative abundances of organic acid producers, including Clostridium, Coprococcus, and Oxobacter, in all RSD groups were significantly higher than those in the CK by day 21. Moreover, on day 21, Aspergillus and Fusarium in all RSD groups were significantly lower than those in the CK. In summary, RSD treatments clearly increased the relative abundances of organic acid generators and effectively inhibited pathogens; however, when the C/N was too low and the amount of addition too high, ammonia poisoning and rapid growth of some microorganisms (e.g., Pseudallescheria and Arthrographis) may occur.

Bi- and polynuclear coordination compounds of d-elements as catalysts of homogeneous selective oxidation of thiol groups

This research aims to study the catalytic cycle reactions for homogeneous selective oxidation of thiol RSH groups to RSSR with the participation of coordination compounds for d-element ions-NiII, PdII, Pt II, CuI , CuII. We used the DFT M06, PBE0 / Def2-TZVP methods to build the quantum chemical models of the reactions. We have developed a mechanism for the functioning of the catalytic system in which primary active centers are either binuclear {M(-OH)2M}n+ or polynuclear {M(μ-OH)2M(μ-OH)2M}2+ sites. Catalysts under consideration should retain stable spatial complementarity at all stages of the process. The main interrelated functions of the binuclear catalysts are the spatial approaching of anions RS– in the inner sphere of the bridged coordination compound required for the disulfide (–S–S–) cross-linking and the two-electron redox transfer during the transformation of these anions into disulfide (СH3)2S2

Thermodynamics of Hg(II) Bonding to Thiol Groups in Suwannee River Natural Organic Matter Resolved by Competitive Ligand Exchange, Hg LIII-Edge EXAFS and 1H NMR Spectroscopy.

A molecular level understanding of the thermodynamics and kinetics of the chemical bonding between mercury, Hg(II), and natural organic matter (NOM) associated thiol functional groups (NOM-RSH) is required if bioavailability and transformation processes of Hg in the environment are to be fully understood. This study provides the thermodynamic stability of the Hg(NOM-RS)2 structure using a robust method in which cysteine (Cys) served as a competing ligand to NOM (Suwannee River 2R101N sample) associated RSH groups. The concentration of the latter was quantified to be 7.5 ± 0.4 μmol g-1 NOM by Hg LIII-edge EXAFS spectroscopy. The Hg(Cys)2 molecule concentration in chemical equilibrium with the Hg(II)-NOM complexes was directly determined by HPLC-ICPMS and losses of free Cys due to secondary reactions with NOM was accounted for in experiments using 1H NMR spectroscopy and 13C isotope labeled Cys. The log K ± SD for the formation of the Hg(NOM-RS)2 molecular structure, Hg2+ + 2NOM-RS- = Hg(NOM-RS)2, and for the Hg(Cys)(NOM-RS) mixed complex, Hg2+ + Cys- + NOM-RS- = Hg(Cys)(NOM-RS), were determined to be 40.0 ± 0.2 and 38.5 ± 0.2, respectively, at pH 3.0. The magnitude of these constants was further confirmed by 1H NMR spectroscopy and the Hg(NOM-RS)2 structure was verified by Hg LIII-edge EXAFS spectroscopy. An important finding is that the thermodynamic stabilities of the complexes Hg(NOM-RS)2, Hg(Cys)(NOM-RS) and Hg(Cys)2 are very similar in magnitude at pH values <7, when all thiol groups are protonated. Together with data on 15 low molecular mass (LMM) thiols, as determined by the same method ( Liem-Ngyuen et al. Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory . Environ. Chem. 2017 , 14 , ( 4 ), 243 - 253 .), the constants for Hg(NOM-RS)2 and Hg(Cys)(NOM-RS) represent an internally consistent thermodynamic data set that we recommend is used in studies where the chemical speciation of Hg(II) is determined in the presence of NOM and LMM thiols.

Cellulose and chitosan derivatives for enhanced sorption of erbium(III)

Cellulose (Cell) was chemically modified by grafting thiourea (Thio-Cell) and glutaraldehyde cross-linked chitosan (GLA-Chit) was functionalized by poly(aminocarboxymethylation) (PCM-Chit); material was also prepared as a composite material that incorporates a magnetic core (Magn PCM-Chit). The sorption properties of these materials have been compared to non-modified cellulose and GLA-Chit for Er(III) uptake. Sorption increases with progressive deprotonation of reactive groups, such as ROH, RSH, amine and carboxylic acid groups. The chemical modification significantly increases sorption performance and more specifically the poly(aminocarboxymethylation): sorption capacities increase up to 117–145mgEr(III)g−1. Sorption capacities also increase with temperature: the sorption is endothermic and spontaneous. The spontaneity of the reaction significantly increases with chemical modification of chitosan-based sorbent. The entropy of the system is negative for GLA-Chit, Cell and Thio-Cell and positive for PCM-Chit materials. Acidic solutions of thiourea efficiently desorb Er(III) and allows the recycling of the sorbents for a minimum of 5 sorption/desorption cycles. FT-IR spectrometry, XRD, TGA, elemental analyses and SEM observations have been used for characterizing the materials. The magnetic properties of Magn PCM-Chit were also characterized by VSM.

Modeling of the structure-specific kinetics of abiotic, dark reduction of Hg(II) complexed by O/N and S functional groups in humic acids while accounting for time-dependent structural rearrangement

Dark reduction of Hg(II) to Hg(0) in deep waters, soils and sediments accounts for a large part of legacy Hg recycling back to the atmosphere. Natural organic matter (NOM) plays a dual role in the process, acting as an electron donor and complexation agent of Hg(II). Experimental determination of rates of dark Hg(II) reduction is complicated by the simultaneously ongoing kinetics of Hg(II) rearrangement from the abundant, relatively weakly bonding RO/N (carboxyl, amino) groups in NOM to the much stronger bonding RSH (thiol) group. In this study, kinetics of the rearrangement are accounted for and we report rates of dark Hg(II) reduction for two molecular structures in presence of humic acids (HA) extracted from three different sources. Values on the pseudo first-order rate constant for the proposed structure Hg(OR)2 (kred Hg(OR)2) were 0.18, 0.22 and 0.35h−1 for Peat, Coal and Soil HA, respectively, and values on the constant for the proposed structure RSHgOR (kred RSHgOR) were 0.003 and 0.006h−1 for Peat and Soil HA, respectively. The Hg(SR)2 structure is the thermodynamically most stable, but the limited time of the experiment (53h) did not allow for a determination of the rate of the very slow reduction of Hg(II) in this structure. For two out of three HA samples the concentration of RSH groups optimized by the kinetic model (0.6×10−3 RSH groups per C atoms) was in good agreement with independent estimates provided by sulfur X-ray absorption near-edge spectroscopy (S XANES). Experiments conducted at varying concentrations of Hg(II) and HA demonstrated a positive relationship between Hg(II) reduction and concentrations of specific Hg(II) structures and electron donor groups, suggesting first order in each of these two components. The limitation of the Hg(II) reduction by electron donating groups of HA, as represented by the native reducing capacity (NRC), was demonstrated for the Coal HA sample. Normalization to NRC resulted in pseudo second-order rate constants (kred Hg(OR)2/NRC=0.012h−1, kred RSHgOR/NRC=1.9−2.1×10−4h−1) that were very similar for Soil and Peat HA.

Aislamiento y caracterización parcial de miosina del manto de calamar gigante (Dosidicus gigas)

In the present work the myosin molecule from jumbo squid mantle (Dosidicus gigas) was isolated and characterized with the aim to evaluate the influence of ionic strength on its gelling properties. It was found that the myosin molecule possesses different chemical and structural characteristics than other vertebrates and invertebrates species, such as some cephalopods, which might explain differences in gelation in comparison to those from other organisms. A lower content of total sulfhydryl groups (TSH) possibly caused an improvement in the myosin molecule flexibility when the ionic strength increased (p ˂ 0.05). The aforementioned possibly affected (p ˂ 0.05) the enzyme activity, surface hydrophobicity, viscosity and RSH groups. The results demonstrate that the myosin molecule from jumbo squid is structurally different from the rest of the marine species.

Beneficial effects of rutin andl-arginine coadministration in a rat model of liver ischemia-reperfusion injury

Reperfusion following liver ischemia results in oxidative stress leading to liver injury. The aim of this study was to investigate the combined effects of two antioxidant agents, rutin and L-arginine, in rat liver ischemia-reperfusion (I/R). Male Wistar rats were divided into five groups: 1) sham operated, 2) I/R, 3) I/R+rutin, 4) I/R+L-arginine, and 5) I/R+rutin+L-arginine. Plasmatic and hepatic levels of alanine transaminase (ALT), aspartate transaminase (AST), lipid peroxides (LOOH), and thiol groups (RSH) were examined, as well as DNA fragmentation and liver histopathology. Furthermore, to elucidate the pathophysiological processes involved in the antioxidant mechanism(s) of rutin and L-arginine, we assessed the expression of inducible (iNOS) and endothelial nitric oxide synthase (eNOS) isoforms and heme oxygenase-1 (HO-1), both playing key roles in the biochemical cascade of liver injury. Significant increase in plasmatic ALT and AST activities were observed in untreated I/R rats compared with sham-operated animals, whereas treatment with rutin or L-arginine in I/R rats reduced hepatic damage. Interestingly, combined therapy with rutin and L-arginine resulted in a further reduction of plasmatic ALT and AST activities compared with rutin or L-arginine alone. These results were further confirmed by the analysis of DNA fragmentation, LOOH, RSH groups, and liver histopathology, which showed the highest protective effects following the coadministration of rutin and L-arginine. Finally, the combined therapy protocol resulted in a significant induction of liver HO-1 and a concomitant reduction of iNOS expression that may both be responsible for the beneficial effects of the proposed pharmacological protocol.

Complexation of cadmium to sulfur and oxygen functional groups in an organic soil

Cadmium (Cd) is a toxic trace element and due to human activities soils and waters are contaminated by Cd both on a local and global scale. It is widely accepted that chemical interactions with functional groups of natural organic matter (NOM) is vital for the bioavailability and mobility of trace elements. In this study the binding strength of cadmium (Cd) to soil organic matter (SOM) was determined in an organic (49% organic C) soil as a function of reaction time, pH and Cd concentration. In experiments conducted at native Cd concentrations in soil (0.23 μg g −1 dry soil), halides (Cl, Br) were used as competing ligands to functional groups in SOM. The concentration of Cd in the aqueous phase was determined by isotope-dilution (ID) inductively-coupled-plasma-mass-spectrometry (ICP-MS), and the activity of Cd 2+ was calculated from the well-established Cd–halide constants. At higher Cd loading (500–54,000 μg g −1), the Cd 2+ activity was directly determined by an ion-selective electrode (ISE). On the basis of results from extended X-ray absorption fine structure (EXAFS) spectroscopy, a model with one thiolate group (RS −) was used to describe the complexation (Cd 2+ + RS − ⇆ CdSR +; log K CdSR) at native Cd concentrations. The concentration of thiols (RSH; 0.047 mol kg −1 C) was independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. Log K CdSR values of 11.2–11.6 (p K a for RSH = 9.96), determined in the pH range 3.1–4.6, compare favorably with stability constants for the association between Cd and well-defined thiolates like glutathione. In the concentration range 500–54,000 μg Cd g −1, a model consisting of one thiolate and one carboxylate (RCOO −) gave the best fit to data, indicating an increasing role for RCOOH groups as RSH groups become saturated. The determined log K CdOOCR of 3.2 (Cd 2+ + RCOO − ⇆ CdOOCR +; log K CdOOCR; p K a for RCOOH = 4.5) is in accordance with stability constants determined for the association between Cd and well-defined carboxylates. Given a concentration of reduced sulfur groups of 0.2% or higher in NOM, we conclude that the complexation to organic RSH groups may control the speciation of Cd in soils, and most likely also in surface waters, with a total concentration less than 5 mg Cd g −1 organic C.

Combined XANES and EXAFS study on the bonding of methyl mercury to thiol groups in soil and aquatic organic matter

We combined x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy to determine the coordination chemistry of methyl mercury (CH3Hg) in natural organic matter from an organic soil (OS), potentially soluble organic matter extracted from the soil (PSOS) and in a humic stream draining the soil (SOS). The sum of concentrations of highly reduced organic S groups (Org-SRED), such as thiol (RSH), sulfide (RSR) and disulphide (RSSR) was 39, 42 and 47% of total S in SOS, PSOS and OS, respectively. No traces of inorganic sulfides were detected. Hg LIII-edge EXAFS analysis were determined on samples added CH3Hg to yield CH3Hg/Org-SRED ratios in the range 0.01–1.62. At low ratios Hg was associated to one C atom (the methyl-group) at an average distance of 2.03 ± 0.02 Å and to one S atom at an average distance of 2.34 ± 0.03 Å in the first coordination shell. At higher CH3Hg/Org-SRED ratios RSH groups were saturated and O and/or N atoms gradually took part in the bonding. Based on the assumption that RSH is the only S group accounting for the very strong complexation of CH3Hg, approximately 17% of total organic S consisted of RSH groups in the organic soil, 14% in potentially soluble organic matter extracted from soil and 9% in aquatic organic matter.

Bonding of ppb Levels of Methyl Mercury to Reduced Sulfur Groups in Soil Organic Matter

The strong binding of CH3Hg+ to natural organic matter (NOM) in soils and waters determines the speciation of CH3Hg under aerobic conditions and indirectly its bioavailability and rates of demethylation. In lab experiments, halides (Cl, Br, I) were used as competing ligands to determine the strength of CH3Hg+ binding to solid-phase soil organic carbon (SOC) and to dissolved soil organic carbon (DOC) as a function of time, pH, and concentration of halide. Experiments were conducted with native concentrations of CH3Hg (1.7-9.8 ng g(-1)) in organic soils, and equilibrium concentrations of CH3Hg were determined by species-specific-isotope-dilution (SSID) gas-chromatography-induced-coupled-plasma-mass-spectrometry (GC-ICP-MS). A simple model (RS- + CH3Hg+ = CH3HgSR; log KCH3HgSR) was used to simulate the binding to SOC and DOC, in which the binding sites (RSH) were independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. The pKa values of RSH groups were fixed at 8.50 and 9.95, reflecting the two major thiol groups in proteins. Log KCH3HgSR values determined for SOC and DOC were similar, showing a range of 15.6-17.1 for all experiments covering a pH range of 2.0-5.1. Despite large differences in affinities between Cl, Br, and I for CH3Hg+, determined constants were independent of type and concentration of halide used in the experiments (log KCH3HgSR = 16.1-16.7 at pH 3.5-3.6). Even if our log KCH3HgSR values were conditional in that they decreased with pH above 3.5, they were in fair agreement with stability constants determined for the association between CH3Hg+ and thiol groups in well-defined organic molecules (log K1 = 15.7-17.5). Speciation calculations based on our results show that, in absence of substantial concentrations of inorganic sulfides, neutral chloro-complexes (CH3HgCl) and free CH3Hg+ reach concentrations on the order of 10(-17)-10(-18) M at pH 5 in soil solutions with 3 x 10(-5) M of chloride.

Bonding of methyl mercury to reduced sulfur groups in soil and stream organic matter as determined by x-ray absorption spectroscopy and binding affinity studies

We combined synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and binding affinity studies to determine the coordination, geometry, and strength of methyl mercury, CH 3Hg (II), bonding in soil and stream organic matter. Samples of organic soil (OS), potentially soluble organic substances (PSOS) from the soil, and organic substances from a stream (SOS) draining the soil were taken along a short “hydrological transect.” We determined the sum of concentrations of highly reduced organic S groups (designated Org-S RED), such as thiol (RSH), disulfane (RSSH), sulfide (RSR), and disulfide (RSSR), using sulfur K-edge XANES. Org-S RED varied between 27% and 64% of total S in our samples. Hg L III-edge EXAFS analysis were determined on samples added CH 3Hg (II) to yield CH 3Hg (II)/Org-S RED ratios in the range 0.01–1.62. At low ratios, Hg was associated to one C atom (the methyl group) at an average distance of 2.03 ± 0.02 Å and to one S atom at an average distance of 2.34 ± 0.03 Å, in the first coordination shell. At calculated CH 3Hg(II)/Org-S RED ratios above 0.37 in OS, 0.32 in PSOS, and 0.24 in SOS, the organic S sites were saturated by CH 3Hg +, and O (and/or N) atoms were found in the first coordination shell of Hg at an average distance of 2.09 ± 0.01 Å. Based on the assumption that RSH (and possibly RSSH) groups take part in the complexation of CH 3Hg +, whereas RSSR and RSR groups do not, approximately 17% of total organic S consisted of RSH (+ RSSH) functionalities in the organic soil. Corresponding figures for samples PSOS and SOS were 14% and 9%, respectively. Competitive complexation of CH 3Hg + by halide ions was used to determine the average binding strength of native concentrations of CH 3Hg (II) in the OS sample. Using data for Org-S RED, calculated surface complexation constants were in the range from 10 16.3 to 10 16.7 for a model RSH site having an acidity constant of mercaptoacetic acid. These values compare favorably with identically defined stability constants (log K 1) for the binding of methyl mercury to thiol groups in well-defined organic compounds.

Tellurite-mediated thiol oxidation in Escherichia coli.

The oxyanion of tellurium, tellurite (TeO3(2-)), is toxic to most micro-organisms, particularly gram-negative bacteria. The mechanism of tellurite toxicity is presently unknown. Many heavy metals and oxyanions, including tellurite, interact with reduced thiols (RSH). To determine if tellurite interaction with RSH groups is involved in the toxicity mechanism, the RSH content of Escherichia coli cultures was assayed. After exposure to tellurite, cells were harvested and lysed in the presence of the RSH-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid). Upon exposure of tellurite-susceptible cells to TeO3(2-), the RSH content decreased markedly. Resistance to potassium tellurite (Te(r)) in gram-negative bacteria is encoded by plasmids of incompatibility groups IncFI, IncP alpha, IncHI2, IncHI3 and IncHII, as well as the tehAtehB operon from the E. coli chromosome. When cells harbouring a Te(r) determinant were exposed to TeO3(2-), only a small fraction of the RSH content became oxidized. In addition to tellurite-dependent thiol oxidation, the resistance of E. coli mutants affected in proteins involved in disulfide-bond formation (dsb) was investigated. Mutant strains of dsbA and dsbB were found to be hypersensitive to tellurite (MIC 0.008-0.015 microg K2TeO3 ml(-1) compared to wild-type E. coli with MICs of 1-2 microg K2TeO3 ml(-1)). In contrast, dsbC and dsbD mutants showed no hypersensitivity. The results suggest that hypersensitivity to tellurite is reliant on the presence of an isomerase activity and not the thiol oxidase activity of the Dsb proteins. The results establish that the Te(r) determinants play an important role in maintaining homeostasis of the intracellular reducing environment within gram-negative cells through specific reactions with either TeO3(2-) or thiol:tellurium products.