The human xenobiotic-metabolizing enzyme arylamine N-acetyltransferase 1 (NAT1) is irreversibly inhibited by inorganic (Hg) and organic mercury (CH3Hg): Mechanism and kinetics

Human arylamine N-acetyltransferase 1 (NAT1) is a xenobiotic-metabolizing enzyme that biotransforms aromatic amine chemicals. We show here that biologically-relevant concentrations of inorganic (Hg²⁺) and organic (CH₃Hg⁺) mercury inhibit the biotransformation functions of NAT1. Both compounds react irreversibly with the active-site cysteine of NAT1 (half-maximal inhibitory concentration (IC₅₀) = 250 nM and k_inact = 1.4 × 10⁴ M⁻¹ s⁻¹ for Hg²⁺ and IC₅₀ = 1.4 μM and k_inact = 2 × 10² M⁻¹ s⁻¹ for CH₃Hg⁺). Exposure of lung epithelial cells led to the inhibition of cellular NAT1 (IC₅₀ = 3 and 20 μM for Hg²⁺ and CH₃Hg⁺, respectively). Our data suggest that exposure to mercury may affect the biotransformation of aromatic amines by NAT1.     

Removal of Hg2+ and methylmercury in waters by functionalized multi-walled carbon nanotubes: adsorption behavior and the impacts of some environmentally relevant factors

Adsorption of Hg²⁺ and methylmercury (MeHg) to multi-walled carbon nanotubes (MWCNTs) modified, respectively, with hydroxyl, amine and carboxyl groups was studied. The effect of various factors like the initial pH, natural organic matter (NOM), Cl^- and adsorbent dose on the sorption efficiency were evaluated. It was found that amine-modified MWCNTs showed a strong adsorption capacity to Hg²⁺ and MeHg, and the removal efficiency could reach up to 92% (0.5 g/L MWCNTs, and 100 μg/L Hg²⁺ and MeHg) which is independent of pH. NOM had complex effects on the adsorption of Hg²⁺ and MeHg to MWCNTs. Cl^- inhibited the adsorption of Hg²⁺ and MeHg to MWCNTs. The adsorption of Hg²⁺ and MeHg was found to be inhomogeneous and homogeneous chemisorption, respectively. Our results suggested that MWCNTs modified with different functional groups can efficiently adsorb both Hg²⁺ and MeHg in aqueous environment.     

Skeletal muscle glycogen phosphorylase is irreversibly inhibited by mercury: Molecular,cellular and kinetic aspects

Muscle glycogen phosphorylase (GP) plays an important role in muscle functions. Mercury has toxic effects in skeletal muscle leading to muscle weakness or cramps. However, the mechanisms underlying these toxic effects are poorly understood. We report that GP is irreversibly inhibited by inorganic (Hg²⁺) and organic (CH₃Hg⁺) mercury (IC₅₀ = 380 nM and k_inact = 600 M⁻¹ s⁻¹ for Hg²⁺ and IC₅₀ = 43 μM and k_inact = 13 M⁻¹ s⁻¹ for CH₃Hg⁺) through reaction of these compounds with cysteine residues of the enzyme. Our data suggest that the irreversible inhibition of GP could represent one of the mechanisms that contribute to mercury-dependent muscle toxicity.     

Biotransformation of mercury in pH-stat cultures of eukaryotic freshwater algae

Eukaryotic algae were studied to determine their ability to biotransform Hg^II under aerated and pH controlled conditions. All algae converted Hg^II into β-HgS and Hg⁰ to varying degrees. When Hg^II was administered as HgCl₂ to the algae, biotransformation by species of Chlorophyceae (Selenastrum minutum and Chlorella fusca var. fusca) was initiated with β-HgS synthesis (K _1/2 of hours) and concomitant Hg° evolution occurred in the first hour. Hg° synthesis was impeded by the formation of β-HgS and this inhibition was released in C. fusca var. fusca when cellular thiols were oxidized by the addition of dimethylfumarate (DMF). The diatom, Navicula pelliculosa (Bacillariophyceae), converted a substantially greater proportion of the applied Hg^II into Hg⁰, whereas the thermophilic alga, Galdieria sulphuraria (Cyanidiophyceae), rapidly biotransformed as much as 90% of applied Hg^II into β-HgS (K _1/2 ≈ 20 min). This thermophile was also able to generate Hg⁰ even after all exogenously applied HgCl₂ had been biotransformed. The results suggest that β-HgS may be the major dietary mercurial for grazers of contaminated eukaryotic algae.     

The influence of organic soil amendments on sulfate adsorption and sulfur availability in a Brazilian Oxisol

Soil management practices that involve additions of organic materials may influence plant sulfur availability in highly-weathered, acid soils. This study evaluated the effects of organic additions on sulfate adsorption and sulfur availability in a limed (3,4 t ha^-1) and unlimed Typic Haplustox soil of the Cerrado Region of Brazil. In unlimed soil, the proportion of applied sulfate (600 kg S ha^-1 as gypsum) that was adsorbed temporarily decreased over two cropping seasons by incorporation of 10 t dry matter ha^-1 crop^-1 of guinea grass (Panicum maximum Jacq.) but not when a similar quantity of a tropical legume, feijâo de porco (Canavalia ensiformis L.), was added. Liming reduced sulfate adsorption and resulted in sulfate leaching to a depth of 30 to 45 cm. Both plant materials temporarily reduced sulfate adsorption in laboratory studies when added to an unlimed soil at a rate equivalent to 40 t ha^-1. Analysis of soil properties affected by organic additions and liming showed significant correlations between sulfate adsorption and soil pH, extractable aluminum, calcium and magnesium, and surface charge. Maize dry matter yields increased by 1.3 to 3.5 t ha^-1 with addition of both organic materials. However, only the feijâo de porco treatment resulted in increases in sulfur uptake for the years in which organic materials were applied. Determining the effects of organic material additions on plant sulfur availability is complicated by the combined effects of sulfur mineralization, sulfate adsorption, and the plant's ability to utilize adsorbed subsoil sulfate.Joint contribution of Cornell University and CPAC-EM- BRAPA. This research was supported by USAID through the Title XII CRSP subgrant SM-CRSP-10 from North Carolina State University     

Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment

Concentrations of methyl mercury, CH₃Hg (II), total mercury, Hg_tot = CH₃Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH₃Hg (II)/Hg_tot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH₃Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hg_tot in soil solution. This, and the fact that the CH₃Hg (II)/Hg_tot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-S_RED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-S_RED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS₂ (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH₃Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH₃Hg⁺ and Hg²⁺ ions in soil solution and stream were on the order of 10^–18 and 10^–32M, respectively, at pH 5. For CH₃Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10^–9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).     

Mercury inputs and outputs at a small lake in northern Minnesota

Storages and cycling of total mercury (Hg_T), methylmercury (MeHg), and Hg⁰ are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of Hg_T and MeHg. Atmospheric deposition was the main input of Hg_T; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. Hg_T outputs were dominated by burial (67%), and Hg⁰ evasion accounted for 26% of Hg_T outputs. The watershed of Spring Lake is small (3.7× lake surface area), and accordingly, bog and upland runoff were minor contributors to both Hg_T and MeHg inputs. Wet deposition was ∼9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed ∼3× the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and Hg_T in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg²⁺ to Hg⁰ was greater than the reverse reaction (Hg⁰ photooxidation), and the residence time of Hg⁰ in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (Hg_T, MeHg, and Hg⁰) and the importance of MeHg photolysis and photo-redox processes.     

Performance of mango seed adsorbents in the adsorption of anthraquinone and azo acid dyes in single and binary aqueous solutions

In this study the husk of mango seed and two carbonaceous adsorbents prepared from it were used to study the adsorption behavior of eight acid dyes. The adsorbed amount in mmol m⁻² decayed asymptotically as the molecular volume and area increased. The interaction between the studied dyes and the mesoporous carbon was governed by the ionic species in solution and the acidic/basic groups on the surface. Less than 50% of the external surface of the microporous carbon became covered with the dyes molecules, though monolayer formation demonstrating specific interactions only with active sites on the surface and the adsorption magnitudes correlated with the shape parameter of the molecule within a particular dye group. The adsorption behavior in mixtures was determined by the molecular volume of the constituents; the greater the molecular volume difference, the greater the effect on the adsorbed amount. We also demonstrated that the raw husk of the mango seed can be used to remove up to 50% from model 50 mg l⁻¹ solutions of the studied acid dyes.     

Effect of Cd and Hg on the Activity of Na/K-ATPase and Mg-ATPase Adsorbed on Polystyrene Microtiter Plates

In the present study a polystyrene microtiter plate was tested as a support material for synaptic plasma membrane (SPM) immobilization by adsorption. The adsorption was carried out by an 18-h incubation at +4°C of SPM with a polystyrene matrix, at pH 7.4. Evaluation of the efficiency of the applied immobilization method revealed that 10% protein fraction of initially applied SPM was bound to the support and that two SPM enzymes, Na⁺/K⁺-ATPase and Mg²⁺-ATPase, retained 70–80% activity after the adsorption. In addition, adsorption stabilizes Na⁺/K⁺-ATPase and Mg²⁺-ATPase, since the activities are substantial 3 weeks after the adsorption. Parallel kinetic analysis showed that adsorption does not alter significantly the kinetic properties of Na⁺/K⁺-ATPase and Mg²⁺-ATPase and their sensitivity to and mechanism of Cd²⁺- or Hg²⁺-induced inhibition. The only exception is the “high affinity” Mg²⁺-ATPase moiety, whose affinity for ATP and sensitivity toward Cd²⁺ were increased by the adsorption. The results show that such system may be used as a practical and comfortable model for the in vitro toxicological investigations.     

siRNA-mediated knockdown of CiGRP78 gene expression leads cell susceptibility to heavy metal cytotoxicity

Heavy metal ion is one of the critical environmental pollutants accumulated in living organisms and causes toxic or carcinogenic effects once passed threshold levels. As an important member of Hsp70 (heat shock protein 70) family, the 78-kDa glucose-regulated protein (GRP78) can enhance cell survival rates remarkably under thermal stress. Recent studies also demonstrated that the expression of GRP78 enhances the cell survival under heavy metal stress. In this study, three most representative heavy metal ions, Pb^2 +, Hg^2 + and Cd^2 +, were used to stimulate Ctenopharyngodon idella kidney (CIK) cells. The results showed that cell viability under Pb^2 +, Hg^2 + and Cd^2 + stress decreased significantly. The longer and the greater the concentrations of stimulation from heavy metal ions, the higher the rate of cell death was observed. Among them, Hg^2 + is the most hazardous to cells. Under the same stress condition, Hg^2 + resulted in 50% of cell death, Cd^2 + (or Pb^2 +) led to 45% (or 35%) of cell death, respectively. Western immunoblotting indicated that C. idella GRP78 (CiGRP78) protein expression level was enhanced obviously in CIK cells under Pb^2 +, Hg^2 + and Cd^2 + stress, meaning CiGRP78 is involved in heavy metal cytotoxicity. To further study the role of CiGRP78 in cytoprotection, we designed the siRNA against CiGRP78 (from nucleotides + 788 to + 806) and transfected it into CIK cells to silence endogenous CiGRP78. The viability rate of CIK cells transfected with or without siRNA incubated with HgCl₂ for 12 h showed a significant decrease from 50% to 21%. Our results showed that CiGRP78 protects cells against heavy metal stimuli to some extent.     

Adsorption thermodynamics to clean up wastewater; critical review

The current review paper deals with critical evaluation of the thermodynamic parameters of adsorption equilibrium uptakes of heavy metals, dyes and other organic pollutants from the wastewater using low-cost adsorbents, activated carbon derived from agricultural waste, industrial wastes and other important adsorbent materials. Present review paper focus the spontaneity of adsorption processes when the thermodynamic parameters were assessed with the compensation effects of standard enthalpy change (ΔH^o) and standard entropy change (ΔS^o) with temperature. This paper also investigates the spontaneity of adsorption on the basis of thermodynamic criteria of spontaneity and the temperature range to predict feasibility of the adsorption processes.     

Structural characterization of intramolecular Hg(2+) transfer between flexibly linked domains of mercuric ion reductase

The enzyme mercuric ion reductase MerA is the central component of bacterial mercury resistance encoded by the mer operon. Many MerA proteins possess metallochaperone-like N-terminal domains (NmerA) that can transfer Hg²⁺ to the catalytic core domain (Core) for reduction to Hg⁰. These domains are tethered to the homodimeric Core by ∼ 30-residue linkers that are susceptible to proteolysis, the latter of which has prevented characterization of the interactions of NmerA and the Core in the full-length protein. Here, we report purification of homogeneous full-length MerA from the Tn21 mer operon using a fusion protein construct and combine small-angle X-ray scattering and small-angle neutron scattering with molecular dynamics simulation to characterize the structures of full-length wild-type and mutant MerA proteins that mimic the system before and during handoff of Hg²⁺ from NmerA to the Core. The radii of gyration, distance distribution functions, and Kratky plots derived from the small-angle X-ray scattering data are consistent with full-length MerA adopting elongated conformations as a result of flexibility in the linkers to the NmerA domains. The scattering profiles are best reproduced using an ensemble of linker conformations. This flexible attachment of NmerA may facilitate fast and efficient removal of Hg²⁺ from diverse protein substrates. Using a specific mutant of MerA allowed the formation of a metal-mediated interaction between NmerA and the Core and the determination of the position and relative orientation of NmerA to the Core during Hg²⁺ handoff.     

Mercury dynamics in Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River

The accumulation of total mercury (Hg_T) andmethylmercury (MeHg) was evaluated in sediments ofTivoli South Bay, a freshwater tidal mudflat wetlandin the Hudson River National Estuarine ResearchReserve system. Hg_T concentrations in sedimentcores were measured to evaluate the spatialvariability of Hg_T deposition, and to establisha chronology of Hg_T accumulation. Cores takenfrom the northern, middle, and southern sections ofthe bay had similar distribution patterns andconcentrations of Hg_T, suggesting a common sourceof Hg_T throughout the bay. Sedimentconcentrations ranged from 190 to 1040 ng Hg g^–1,2 to 10 times greater than concentrations expected insediments from non-anthropogenic sources. Hg_Tdeposition rates were similar in different regions ofthe bay, and increased from 200 ng Hg cm^–2yr^–1in the 1930s to a maximum of 300 ngHg cm^–2 yr^–1 in the 1960s. Deposition rateshave steadily declined since the 1970s and arecurrently at 80 ng Hg cm^–2 yr^–1. Transportof Hg_T by tidal waters from the Hudson River islikely the main source of Hg_T in the bay.Distribution patterns and absolute concentrations ofMeHg in sediment cores were similar throughout thebay, with concentrations ranging from 0.43 to 2.95ng g^–1. Maxima in MeHg concentration profilesoccurred just below the sediment-water interface andat a depth of 30 cm. The maximum at 30 cm wascoincident with maximum Hg_T concentrations. MeHgconcentrations in suspended particulate matter (SPM)from the Hudson River suggest that MeHg in the baycould be derived from riverine SPM rather than formedin situ.     

Influence of organic matter and solute concentration on nitrate sorption in batch and diffusion-cell experiments

Nitrate sorption potentials of three surface soils (soils-1-3) were evaluated under different solute concentrations, i.e. 1-100 mg L⁻¹. Batch and diffusion-cell adsorption experiments were conducted to delineate the diffusion property and maximum specific nitrate adsorption capacity (MSNAC) of the soils. Ho’s pseudo-second order model well fitted the batch adsorption kinetics data (R² > 0.99). Subsequently, the MSNAC was estimated using Langmuir and Freundlich isotherms; however, the best-fit was obtained with Langmuir isotherm. Interestingly, the batch adsorption experiments over-estimated the MSNAC of the soils compared with the diffusion-cell tests. On the other hand, a proportionate increase in the MSNAC was observed with the increase in soil organic matter content (OM) under the batch and diffusion-cell tests. Therefore, increasing the soil OM by the application of natural compost could stop nitrate leaching from agricultural fields and also increase the fertility of soil.     

Comparative Aspects of Sulfur Mineralization in Sediments of a Eutrophic Lake Basin

The net mineralization of organic sulfur compounds in surface sediments of Wintergreen Lake was estimated from a mass-balance budget of sulfur inputs and sediment sulfur concentrations. The net mineralization of organic sulfur inputs is <50% complete, which is consistent with the dominance of organic sulfur (>80% of total sulfur) in sediment. Although sediment sulfur is predominantly organic, sulfate reduction is the most significant process in terms of the quantities of sulfur transformed in surface sediments. Rates of sulfate reduction in these sediments average 7 mmol/m² per day. On an annual basis, this rate is 19-fold greater than net rates of organic sulfur mineralization and 65-fold greater than sulfate ester hydrolysis.     

A carbonothioate‐based highly selective fluorescent probe with a large Stokes shift for detection of Hg2+

Mercury (Hg) is one of the heavy metal pollutants in the environment. Even a very small amount of mercury can cause serious harm to human beings. Herein, we reported a new carbonothioate‐based fluorescent probe for the detection of Hg²⁺ without interference from other metal ions. This probe possessed a very large Stokes shift (192 nm), which could improve the detection sensitivity by minimizing the interferences resulted from self‐absorption or auto‐fluorescence. With the addition of Hg²⁺ to the probe solution, considerable fluorescence enhancement was observed. Additionally, the Hg²⁺ concentration of 0–16 μM and fluorescence intensity showed a good linear relationship (y = 22106× + 53108, R² = 0.9955). Finally, the proposed probe was used to detect Hg²⁺ in real water samples, and its result was satisfactory. Therefore, our proposed probe would provide a promising method for the determination of Hg²⁺ in the environment.     

Stimulating denitrification in a stream mesocosm with elemental sulfur as an electron donor

The heavy use of fertilizers in agricultural lands can result in significant nitrate (NO₃⁻) loadings to the aquatic environment. We hypothesized that biological denitrification in agricultural ditches and streams could be enhanced by adding elemental sulfur (S^o) to the sediment layer, where it could act as a biofilm support and electron donor. Using a bench-scale stream mesocosm with a bed of S^o granules, we explored NO₃⁻ removal fluxes as a function of the effluent NO₃⁻ concentrations. With effluent NO₃⁻ ranging from 0.5 mg N L⁻¹ to 4.1 mg N L⁻¹, NO₃⁻ removal fluxes ranged from 228 mg N m⁻² d⁻¹ to 708 mg N m⁻² d⁻¹. This is as much as 100 times higher than for agricultural drainage streams. Sulfate (SO₄²⁻) production was high due to aerobic sulfur oxidation. Molecular studies demonstrated that the S^o amendment selected for Thiobacillus species, and that no special inoculum was required for establishing a S^o-based autotrophic denitrifying community. Modeling studies suggested that denitrification was diffusion limited, and advective flow through the bed would greatly enhance NO₃⁻ removal fluxes. Our results indicate that amendment with S^o is an effective means to stimulate denitrification in a stream environment. To minimize SO₄²⁻ production, it may be better to place S^o deeper in the sediment layer.     

Using lignimerin (a recovered organic material from Kraft cellulose mill wastewater) as sorbent for Cu and Zn retention from aqueous solutions

Adsorption of copper and zinc in lignimerin (an organic material mainly composed by lignin, carbohydrate fragments and some extractives) and its acid derivative (H-lignimerin), recovered from Kraft cellulose mill wastewater was examined. A Box–Behnken experiment design, used to optimize lignimerin recovery process, revealed that the type of solvent used for precipitation is a determining factor in the amount of substance obtained. Conversely, batch adsorption studies at pH 4.0 revealed that the maximum adsorption capacities, modeled by the Langmuir equation, were 666.7 and 370.4 mmol kg⁻¹ for Cu(II) and Zn(II), respectively in lignimerin and 232.6 and 312.5 mmol kg⁻¹ for Cu(II) and Zn(II), respectively in H-lignimerin. The adsorption of Cu(II) and Zn(II) through deprotonated hydroxyl and carboxylic groups was the dominant mechanism that may explain the adsorption in both materials. The adsorption capacities indicated that lignimerin, with a molecular mass between 50 and 70 kDa, has a potential use as an organic sorbent for removing copper and zinc from liquid resources.     

A coumarin‐based fluorescent probe for Hg2+ and its application in living cells and zebrafish

Mercury (Hg) is a heavy metal with high toxicity and easy migration; it can be enriched through the food chain, and cause serious threats to the natural environment and human health. So, the development of a method that can be used to detect mercury ions (Hg²⁺) in the environment, in cells, and in organisms is very important. Here, a new 7‐hydroxycoumarin‐derived carbonothioate‐based probe ( CC‐Hg ) was designed and synthesized for detection of Hg²⁺. After addition of Hg²⁺, a large fluorescence enhancement was observed due to the formation of 7‐hydroxyl, which reinforced the intramolecular charge transfer process. The CC‐Hg probe had good water solubility and selectivity. Moreover, the probe was able to detect Hg²⁺ quantitatively over the concentration range 0–2 μM and with a detection limit of 7.9 nM. Importantly, we successfully applied the probe to detect Hg²⁺ in water samples, in living cells, and in zebrafish. The experimental results demonstrated its potential value in practical applications.     

An integrated system of pyrene and rhodamine-6G for selective colorimetric and fluorometric sensing of mercury(II)

A pyrene and rhodamine-6G functionalized simple chemosensor L is studied toward sensing of metal ions in solution extensively. L shows selective color change from colorless to pink in the presence of Hg²⁺ in acetonitrile and the UV-Vis study shows peak at 525 nm with a ε value of 5.2 × 10⁴ M⁻¹ cm⁻¹ due to selective ring opening of rhodamine spirolactam moiety. The selective sensing of Hg²⁺ by L in the presence of other metal ions and reversible nature of “OFF-ON-OFF” functionality of L by Hg²⁺ and EDTA, respectively, are also established. The fluorescence study of L in the presence of Hg²⁺ shows emission at 550 nm when excited at 525 nm (ring opened rhodamine wavelength) or 340 nm (pyrene wavelength) in dry CH₃CN. Thus L acts as a selective colorimetric and fluorometric probe (dual probe) for the Hg²⁺ in solution. Metal ion sensing ability of L is also carried out in water as well as in aqueous Hepes buffer. These studies suggest that the fluorescence output of L in presence of Hg²⁺ in aqueous environment is apparently due to the generation of acid upon addition of Hg²⁺ salt in water.