High Affinity For Cd Research Articles

Multifunctional Roles of Zinc in Cadmium Transport in Soil-Rice Systems: Novel Insights from Stable Isotope Fractionation and Gene Expression.

The effect of Zn on Cd accumulation in rice varies under flooding and drainage conditions, and the underlying mechanism during uptake and transport from the soil to grains remains unclear. Isotope fractionation and gene expression were investigated using pot experiments under distinct water regimes and with Zn addition to gain a deeper understanding of the molecular effects of Zn on Cd uptake and transport in rice. The higher OsHMA2 expression but constitutively lower expression of zinc-regulated, iron-regulated transporter-like protein (ZIP) family genes in roots under the drainage regime than the flooding regime caused the enrichment of nonheavy Zn isotopes in the shoots relative to roots but minimally affected Cd isotopic fractionation. Drainage regime seem to exert a striking effect on the root-to-shoot translocation of Zn rather than Cd, and increased Zn transport via OsHMA2. The changes in expression patterns in response to Zn addition were similar to those observed upon switching from the flooding to drainage regime, except for OsNRAMP1 and OsNRAMP5. However, soil solution-to-rice plants and root-to-shoot fractionation toward light Zn isotopes with Zn addition (Δ66Znriceplant-soilsolution = -0.49 to -0.40‰, Δ66Znshoot-root = -0.36 to -0.27‰) indicated that Zn transport occurred via nonspecific uptake pathways and OsHMA2, respectively. Accordingly, the less pronounced and minimally varied Cd isotope fractionation suggested that OsNRAMP5 and OsHMA2 are crucial for Cd uptake and root-to-shoot transport, respectively, facilitating Cd accumulation in grains. This study demonstrated that a high Zn supply promotes Cd uptake and root-to-shoot transport in rice by sharing distinct pathways, and by utilizing a non-Zn-sensitive pathway with a high affinity for Cd.

Kaolinite loaded amorphous zero-valent iron enhanced removal of cadmium (II) from aqueous solution

Amorphous materials tend to have excellent chemical reactivity, which provides us with a new idea to amorphize zero-valent iron (ZVI) to enhance its activity. Therefore, in this study, amorphous zero-valent iron (AZVI) was synthesized by introducing ethylenediamine (EDA) and loaded on kaolinite to prepare a composite material (K-AZVI) for enhancing the removal of cadmium (II) (Cd(II)) in water. SEM and XRD were used to characterize the structure and morphology of the samples, and the microstructure of the composites was studied by TEM, FTIR, and XPS. The results showed that the spherical AZVI particles were dispersed and loaded on kaolinite in a "core-shell" structure. Compared with the crystalline composite (K-ZVI), K-AZVI had a larger particle size and better electron transfer ability. The adsorption process of K-AZVI for Cd(II) conformed to the pseudo-second-order kinetic model. When K-AZVI (dosage = 1.00 g/L, EDA/Fe(II) = 2, pH = 5.0, and T = 298 K) was mixed with Cd(II) (50 mg/L), the removal rate could reach more than 90% within 20 min. The mechanism of removing Cd(II) was mainly the synergistic effect of the corrosion products of K-AZVI and Cd(II) adsorption and co-precipitation. During the reaction, more γ-FeOOH was generated on the surface of K-AZVI, which had a high affinity for Cd(II). Therefore, K-AZVI has great potential for the remediation of Cd(II) in water.

Spatial localization of cadmium and metallothionein in the kidneys of mice at the early phase of cadmium accumulation.

Chronic exposure to cadmium (Cd) leads to an accumulation of Cd in the kidneys. Metallothionein (MT) is a low-molecular-weight protein having a high affinity for Cd. Cd bound to MT in serum is filtered through the glomeruli of kidney nephrons and reabsorbed by endocytosis into the proximal tubules from the luminal side. Accumulation of Cd in renal cells induces MT synthesis, leading to long-term deposition of Cd and the suppression of Cd toxicity. Recently, many studies have investigated the tissue distribution of metals using laser ablation ICP-MS (LA-ICP-MS). However, little information has been available regarding renal Cd distribution. Hence, we dually investigated the renal distribution of Cd by LA-ICP-MS and that of MT by immunohistochemistry to clarify the dose- and time-dependent changes in the distributions of Cd and MT in mice exposed to Cd from drinking water for 1, 2, and 4 months. Both Cd and MT exhibited characteristic heterogeneous distribution patterns in the renal cortex. The accumulation of Cd and MT near the surface of the cortex suggests a preferential accumulation of Cd in the surface nephrons. MT distribution was more pronounced in the proximal tubules than in the distal tubules, and there were clear differences in MT immunostaining even among the proximal tubules. The heterogeneous localization of MT may reflect the nephron-specific accumulation of Cd. Combining elemental imaging of Cd with immunostaining of MT proved a successful strategy to reveal the characteristic renal Cd distribution, especially in the early stages of Cd accumulation.

Mechanisms for cadmium adsorption by magnetic biochar composites in an aqueous solution.

There is a demand to develop techniques for the continuous removal/immobilization of heavy metals from contaminated soil and water bodies. In this study, a unique biochar preparation method was developed for the removal of cadmium. First, conventional biochars of corn straw were produced by pyrolysis at two temperatures and then treated using one-step synthesis at different ferric nitrate ratios and different calcination temperatures to produce magnetic biochars. Second, the prepared biochars were used as adsorbents for Cd(II) removal from a solution, and the best one was selected for further evaluation. Various techniques were used to characterize the adsorbents and determine the main adsorption mechanism. The results indicated that the biochars successfully carried iron particles within, which improved the specific surface area, formed inner-sphere complexes with oxygen-containing groups, and increased the number of oxygen-containing groups. The adsorption experiments revealed that MBC800-0.6300 had a higher affinity for Cd(II) than the other adsorbents. Batch adsorption experiments were performed to explore the influence of the kinetics, isotherm, pH, thermodynamics, ionic strength, and humic acid on Cd(II) adsorption. The results indicated that the Langmuir model fit the Cd(II) adsorption best with MBC800-0.6300 having the highest adsorption capacity (46.90mgg-1). The sorption kinetics of Cd(II) on the adsorbent follows a pseudo-second-order kinetics model. Because MBC800-0.6300 is loaded with metal ions, it can be conveniently collected by a magnet. Thus, biochar modification methods with ferric nitrate impregnation provide an excellent approach to eliminating Cd(II) from aqueous solutions. The possible adsorption mechanisms include chemisorption, electrostatic interaction, and monolayer adsorption.

Enhancing the removal performance of Cd(Ⅱ) from aqueous solutions by NaA zeolite through doped thiourea reduced GO which is trapped within zeolite crystals

In this study, thiourea reduced graphene oxide/NaA zeolite composite (tGO-Zeo) was successfully prepared by a facile two-step synthetic route and used as an efficient adsorbent for removal of Cd(II) from aqueous solution. Characterization of the composite material showed that tGO was entrapped within zeolite crystalline particles and intimately interacted with the zeolite framework. Furthermore, the presence of tGO considerably reduced zeolite crystal size and altered crystal morphology, turning cubic zeolite crystals of micrometer size into nanometer irregular particles. Batch adsorption experiment for Cd(II) removal was conducted and the adsorption performance of tGO-Zeo and Zeo (zeolite) were determined. It was shown that doped tGO significantly enhanced the adsorption capacity of zeolite by 31.54%. The adsorption mechanism differs for tGO-Zeo and Zeo, and tGO-Zeo has higher affinity for Cd(II). The adsorption capacity of tGO-Zeo with a tGO doping amount of 1.26% (wt%) was estimated to be 196 mg/g (1.74 mM/g) at 30 °C and superior to most previously reported zeolite based composites, which often contain large amounts of dopants. The effects of pH and ionic strength on adsorption suggest that ion exchange and coordination are responsible for Cd(II) adsorption by tGO-Zeo. Humic acid can slightly increase Cd(II) adsorption performance of tGO-Zeo, which may be due to tGO-Zeo—humic acid—Cd(II) complexes formation. Therefore, tGO-Zeo composite is a cost-effective and eco-friendly adsorbent for efficient removal of Cd(II) ions from aqueous solutions.

Removal of cadmium(II) cations from an aqueous solution with aminothiourea chitosan strengthened magnetic biochar

ABSTRACTAn aminothiourea chitosan modified magnetic biochar composite (TMBC) was prepared for the efficient removal of Cd(II) from wastewater. The synthesized materials were characterized, and the detailed adsorption mechanisms and thermodynamics were studied. The adsorption experiments revealed that TMBC had a higher affinity for Cd(II) than the magnetic biochar composite, raw biochar, and other carbon‐based adsorbents did. The Cd(II) adsorption process fit the pseudo‐second‐order kinetic model, and the maximum adsorption capacities on the basis of the Langmuir model were 93.72, 121.9, and 137.3 mg/g at 298, 308, and 318 K, respectively. The practical efficacy of the adsorbent was also tested with a real mine water. The metal‐ion‐loaded TMBC could be conveniently collected by a magnet and could be easily regenerated with adsorption efficiencies up to 84% after five adsorption–desorption cycles. The as‐prepared TMBC might be a promising adsorbent for the treatment of heavy‐metal‐ion‐contaminated water or highly mineralized mine water. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46239.

Sorption of Cadmium(II) and Zinc(II) from Aqueous Solution by Water Treatment Residuals at Different pH Values

Here, we report on the ability of two different water treatment residues, a Fe-based (Fe-WTR) and an Al-based (Al-WTR) ones, to accumulate Cd(II) and Zn(II) from aqueous solutions at different pH values (pH 4.5, 5.5, and 7.0). Fe-WTR showed a greater Zn(II) and Cd(II) sorption capacity than Al-WTR at all the pH values investigated, in particular at pH 7.0 (e.g., ∼0.200 and ∼0.100 mmol g−1 of Me(II) sorbed by Fe- and Al-WTR at pH 7.0, respectively). The greater capacity of the Fe-WTR to accumulate Me(II) seems to be linked to its higher content of iron and manganese ions and to its higher CEC value compared to Al-WTR. The role of the inorganic and organic fractions of WTRs in metal sorption was also assessed. A higher affinity of Cd(II) with respect to Zn(II) toward functional groups of the organic matter of both WTRs was observed, while Zn(II) showed a stronger association with the inorganic phases. The sorption of both metal ions appeared mainly governed by the formation of inner-sphere surface complexes with the inorganic and organic phases of WTRs, as suggested by the sequential extraction data.

Renal cells exposed to cadmium in vitro and in vivo: normalizing gene expression data.

Cadmium (Cd) is a toxic metal with a long half-life in biological systems. This half-life is partly as a result of metallothioneins (MTs), metal-binding proteins with a high affinity for Cd. The high retention properties of the kidneys reside in proximal tubular cells that possess transport mechanisms for Cd-MT uptake, ultimately leading to more Cd accumulation. Researchers have studied MT-metal interactions using various techniques including quantitative real-time PCR (qPCR), an efficient tool for quantifying gene expression. Often a poor choice of reference genes, which is represented by their instability and condition dependency, leads to inefficient normalization of gene expression data and misinterpretations. This study demonstrates the importance of an efficient normalization strategy in toxicological research. A selection of stable reference genes was proposed in order to acquire reliable and reproducible gene quantification under metal stress using MT expression as an example. Moreover, in vitro and in vivo setups were compared to identify the influence of toxicological compounds in function of the experimental design. This study shows that glyceraldehyde-3-phosphate dehydrogenase (Gapdh), tyrosine monooxygenase/tryptophan5-monooxygenase activation-protein, zeta polypeptide (Ywhaz) and beta-actin (Actb) are the most stable reference genes in a kidney proximal tubular cell line exposed to moderate and high Cd concentrations, applied as CdCl2 . A slightly different sequence in reference gene stability was found in renal cells isolated from rats in vivo exposed to Cd. It was further shown that three reference genes are required for efficient normalization in this experimental setup. This study demonstrates the importance of an efficient normalization strategy in toxicological research.

In silico design and construction of metal‐binding hybrid proteins for specific removal of cadmium based on CS3 pili display on the surface of Escherichia coli

In this study, through a combination of bioinformatics and genetic engineering procedures, high-affinity metal-binding peptides were designed and expressed on the surface of Escherichia coli for selective Cd(2+) adsorption. Putative cadmium-binding motifs were identified by searches against the Prosite database and permissive sites in the major subunit (CstH) of the enterotoxigenic E. coli pili were predicted based on the data derived from modeling of 3D structures, secondary structure prediction and assignment, inspection of protein hydropathy and exposed regions, and also protein interaction sites. The metal-binding motifs were inserted into one permissive site of the CstH (amino acid 38) with the aid of the SOEing PCR technique. The capacity and selectivity of the recombinant bacteria displaying hybrid pili to adsorb cadmium were evaluated with the atomic absorption procedure. The levels of Cd(2+) accumulation in the recombinant E. coli strains were 13.9- and 11.33-fold higher than those in the control strain. Cd(2+) was selectively absorbed from a solution containing equal concentrations of four metals, resulting in more than 90% of the total adsorbed metals being Cd(2+) , showing a relatively high affinity for Cd(2+) over other coexisting metal ions.

Bioremediation of Ni(II) and Cd(II) from aqueous solution

Different kinds of low cost agro based fibres can be used to remove both trace and heavy metals from aqueous solutions. The removal efficiency of maize ( Zea mays ) cob, sawdust and coal was investigated in this study for the bioremediation of nickel and cadmium. Sorption experiments were carried out using batch process and changes in concentration of the adsorbates were monitored by the use of Shimadzu AA650 double beam atomic absorption/flame spectrophotometer. Effects of varying adsorbent dose, adsorbate loading concentration, pH and adsorbent surface area on the percentage removal of the adsorbates were studied. Results revealed that maize cob have higher affinity for Cd than Ni, while the reverse was the case with sawdust and coal, where Ni was found to have higher percentage removal than Cd. Thermodynamic parameters such as ΔH and ΔS were calculated from Van't Hoff plots which subsequently lead to the computation of ΔG. At 60 mg/L adsorbate concentrations, the adsorption process was found to be endothermic, spontaneous and feasible. The enthalpy, entropy and Gibbs energy changes for the removal of Ni onto maize cob were higher than those of Cd and the opposite scenario was observed for sawdust and coal.

Reduction of cadmium uptake in spinach ( Spinacia oleracea L.) by soil amendment with animal waste compost

A field experiment was conducted to evaluate the efficacy of animal waste compost (AWC) in reducing Cd uptake by spinach ( Spinacia oleracea L.). Spinach was grown in a field that had been treated by having cattle, swine, or poultry waste compost incorporated into the soil before each crop throughout 4 years of rotational vegetable production. Cadmium concentration was 34–38% lower in spinach harvested from the AWC-treated soils than in the chemical fertilizer-treated soil. Although the repeated application of swine and poultry compost caused significant P accumulation in the cropped soils, that of cattle compost did not. These results indicate that cattle compost with high affinity for Cd and low P content should be the preferred soil amendment when used to reduce Cd uptake by spinach.

Sorption of Cd(II) onto carbon-based materials—a comparative study

The sorption behaviour of Cd(II) on three carbon- based materials including activated carbon (AC), carbon nanotubes (CNTs) and carbon-encapsulated magnetic nano- particles (CEMNPs) which were prepared under similar conditions by nitric acid treatment were investigated. Gener- ally,sorptionofcadmiumonthesematerialsincreasedwiththe increase of pH. For AC and CNTs very similar results were obtained,while CEMNPs exhibits much higher affinity for Cd (II) despite of almost the same surface acidity. Thus, cadmium retention on tested sorbents was more dependent on the concentration of specific active sites than the total surface area available. The equilibrium sorption data were better fitted to Langmuir isotherm. The theoretical saturation capacity of the monolayer derived from this model at pH 8.0 for AC, CNTs and CEMNPs are 9.91 mg g �1 , 20.37 mg g �1 and 91.0 mg g �1 , respectively. The kinetic of Cd(II) sorption is fitted for pseudo-second order equation and could be described as a combination of film diffusion and intra- particle diffusion, whereby the last one dominates. The experimental parameters for preconcentration of cadmium on a microcolumn packed with CEMNPs prior to its determi- nation by flame atomic absorption spectrometry have been investigated. Cadmium can be quantitatively retained at pH 8 from sample volumes up to 150 mL and then eluted completely with 3m L of 0.5 mol L �1 HNO3.

Role of Bound Zn(II) in the CadC Cd(II)/Pb(II)/Zn(II)-responsive Repressor

The Staphylococcus aureus plasmid pI258 cadCA operon encodes a P-type ATPase, CadA, that confers resistance to Cd(II)/Pb(II)/Zn(II). Expression is regulated by CadC, a homodimeric repressor that dissociates from the cad operator/promoter upon binding of Cd(II), Pb(II), or Zn(II). CadC is a member of the ArsR/SmtB family of metalloregulatory proteins. The crystal structure of CadC shows two types of metal binding sites, termed Site 1 and Site 2, and the homodimer has two of each. Site 1 is the physiological inducer binding site. The two Site 2 metal binding sites are formed at the dimerization interface. Site 2 is not regulatory in CadC but is regulatory in the homologue SmtB. Here the role of each site was investigated by mutagenesis. Both sites bind either Cd(II) or Zn(II). However, Site 1 has higher affinity for Cd(II) over Zn(II), and Site 2 prefers Zn(II) over Cd(II). Site 2 is not required for either derepression or dimerization. The crystal structure of the wild type with bound Zn(II) and of a mutant lacking Site 2 was compared with the SmtB structure with and without bound Zn(II). We propose that an arginine residue allows for Zn(II) regulation in SmtB and, conversely, a glycine results in a lack of regulation by Zn(II) in CadC. We propose that a glycine residue was ancestral whether the repressor binds Zn(II) at a Site 2 like CadC or has no Site 2 like the paralogous ArsR and implies that acquisition of regulatory ability in SmtB was a more recent evolutionary event.

Cadmium complexation by bacteriogenic iron oxides from a subterranean environment

This study quantifies the metal sorption characteristics of subterranean bacteriogenic iron oxides (BIOS) and their organic phases (intermixed intact and fragmented bacteria). A Cd 2+ ion-selective electrode was used to generate high-resolution metal sorption data as a function of increasing pH. A multisite Langmuir model, along with a linear programming regression method (LPM), was applied to fit experimental data. This approach found two discrete Cd 2+ binding sites for the BIOS with average −log 10 equilibrium constants (p K S, j ) of 1.06±0.19 and 2.24±0.28. Three discrete sites were obtained for the bacterial fraction, with p K S, j values of −0.05±0.12, 1.18±0.02, and 3.81±0.16. This indicated that the BIOS surface had a lower affinity for Cd 2+ than that of the bacteria. p K S, j values for the BIOS were similar to those reported for pure iron oxide phases, while the organic fraction p K S, j spectrum was consistent with previous spectra for intact bacteria. Individual binding site densities of 0.04±0.01 and 0.05±0.02 and 0.29±0.05, 0.11±0.01, and 0.09±0.02 μmol/mg of BIOS corresponded to the iron oxide mixture and bacteria fraction, respectively. These values indicated high concentrations of strong affinity Cd 2+ complexing groups on the bacterial surface. Comparison of total site densities of 0.08±0.02 and 0.48±0.06 μmol/mg of BIOS for the mixture and the bacterial phase, respectively, suggested a nonadditive character for the BIOS surface reactivity. This was emphasized by a higher affinity for Cd 2+, as well as an increase in total site concentration observed for the bacterial phase. LPM was able to distinguish between the BIOS mixture and its organic fraction Cd 2+ complexation characteristics. This approach is therefore a useful tool for the study of natural sorbent materials controlling metal partitioning in contaminated and pristine environments.

Metal complexes of the mycotoxins sporidesmin A and gliotoxin, investigated by electrospray ionisation mass spectrometry.

The mycotoxin sporidesmin A (spdA), responsible for the intoxication of animals, causing facial eczema, has been investigated by electrospray ionisation mass spectrometry. Protonated [spdA+H](+) and deprotonated [spdA-H](-) ions are observed in positive and negative ion modes respectively. Reduced spdA, formed by cleavage of the disulfide bond by Na[BH(4)] gives an ion [spdA+H](-), and forms ions of the type [2spdA+M](2-) with a range of divalent metal ions M(2+)=Zn(2+), Cd(2+), Hg(2+), Sn(2+) and Fe(2+). Sodium-containing analogues [2spdA+M+Na](-) are observed, particularly at high cone voltages, where they are stable towards cone voltage-induced fragmentation, indicating appreciable stability of the (spdA)(2)M system. A competition experiment between Cd(2+) and Zn(2+) demonstrates that reduced spdA has a higher affinity for Cd(2+) ions. The related gliotoxin (gtx) forms analogous [2gtx+M](2-) and [2gtx+M+Na](-) ions. The reduction and metal complexation of spdA can be monitored by (1)H NMR spectroscopy, and results in chemical shift changes for those protons adjacent to the sulfur atoms. The isolation of a polymeric cadmium-spdA complex is also reported.

The geochemistry of Cu, Cd, Zn, Ni and Pb in sediment cores from the continental slope of the Banc d’Arguin (Mauritania)

Trace and major elements were measured in sediment cores collected at the shelf and continental slope of the Banc d’Arguin (Mauritania). Most of the sediments have a calcium carbonate content >50% and as a consequence have a low aluminium (<1.5%) and iron (<1.0%) content. The concentrations of the latter two elements increase down slope. Based on the major elemental composition, the sediments seem to be well mixed and concentration gradients are only observed on a spatial distribution. Concentrations of Cu, Cd, Zn, Ni and Pb are low and approximately equal to “natural background” values, indicating negligible anthropogenic influences. An exception is Cd for which very high values were found in the northern most part of the research area, close to Nouadhibou. A possible explanation for this high Cd content can be the existence of phosphate rich sediments with a high affinity for Cd. Small increases in the available trace metal concentrations (exchangeable fraction, 0.1 N HCl treated) observed near the sediment surface could be attributed to early diagenetic processes, and not to pollution effects. The inputs from the Saharan dust plume with low trace metal contents, the transport of small particles with higher trace metal content down slope together with the escape of redissolved trace metals from the sediment, and the absence of a supply of metal rich material to the Banc d’Arguin, has created a metal poor environment. There is clear evidence of trace metal redistributions, associated with Mn recycling. Mean enrichment factors, calculated relative to the crustal abundance, show that Zn, Pb and Cd are equal to unity and Cu and Ni below unity. These observations suggest that the Cu and Ni contents in sediments of Banc d’Arguin are depleted relative to the accepted values in the earth crust. The very low trace metal concentrations detected in these sediments can be taken as “baseline” values for unpolluted sediments in the global ocean coastal zone.

Mechanism for the decrease in the accumulation of cadmium (Cd) in Cd-resistant Chinese hamster V79 cells.

The mechanism for the decrease in the accumulation of cadmium (Cd) in Cd-resistant Chinese hamster V79 (Cdr) cells in culture was investigated in a comparison with Cd-sensitive (Cds) cells. Both Cdr and Cds cells took up Cd in a time-dependent manner but the rate of uptake of Cd by Cdr cells was about 15% of that by Cds cells. Kinetic studies of the uptake of Cd showed that the Vmax values for Cdr and Cds cells were 0.31 and 0.46 pmol Cd/h per mg protein, respectively. The Km values were 31.95 microM for Cdr cells and 3.15 microM for Cds cells. Mersalyl acid, a sulfhydryl (SH) blocker to which cells are impermeable, inhibited the uptake of Cd by Cds cells at subtoxic concentrations while Cdr cells were insensitive to inhibition by mersalyl acid, suggesting that SH groups in the plasma membrane play a role in the uptake of Cd. Uptake of Cd by Cds cells was dependent on the pH of the incubation medium and the rate of uptake was very high at pH 7.4 and pH 8.0 relative to the rates at pH 6.0 and pH 6.8. By contrast, the uptake of Cd by Cdr cells was lower at all pH values than that by Cds cells. The decrease in the rate of uptake of Cd by Cdr cells could not be ascribed to an increase in the efflux of Cd. A Cd-blotting technique was used to detect plasma membrane proteins with high affinity for Cd. Two major differences in terms of Cd-binding proteins (Cd-BPs) were observed between Cdr and Cds cells. A 110-kDa Cd-BP, detected in Cds cells, was found at a reduced level in Cdr cells, while an 82-kDa Cd-BP, which was not observed in Cds cells, was detected in Cdr cells.

Acute cadmium uptake by rabbit kidneys: Mechanism and effects

Trapping of an arterial bolus in the kidney by 40 sec aortic occlusion permitted demonstration of diffusion-limited and Zn-sensitive renal uptake of nonfiltered but diffusible Cd from plasma: mercaptoethanol (ME) or 2,3-dimercaptopropanol (BAL) had been added to the bolus to prevent sequestration of the metals by plasma protein. Cadmium taken up by the kidney under these conditions, whether from blood or glomerular filtrate, unlike Zn, did not return to blood over a period of 2–3 min. Nor was Cd thus accumulated removed by EDTA; it had presumably been transferred into cells. The diffusion dependence of this internalization shows it to be a relatively slow process, as it is in jejunum. In contrast, uptake of Cd tightly bound in a lipid-soluble complex with diethyldithiocarbamate (DDTC) is rapid and flow dependent. Slow uptake in the presence of ME and its inhibition by Zn are therefore not likely to involve movement of undissociated ME complex across the cell membranes. Instead, it is suggested that (1) Cd-binding sites on the membrane possess a relatively high affinity for Cd and can compete for it with ME, and (2) it is the resulting Cd-membrane interaction which, as in jejunum, is depressed by Zn. During transient occlusion kidneys could be loaded with up to 25 μg Cd/g cortex without evidence of immediate malfunction. Inhibition of amino acid transport, as previously described, is seen only after an initiation period of 1–2 days following CdME injection, although cortical Cd levels at that time have decreased. The finding of a slowly developing inhibition of amino acid carrier systems suggests an indirect action of Cd; by implication, unique threshold concentrations should not be defined for Cd in renal cortex following subchronic exposures without reference to the duration of exposure.

Direct actions of organophosphate anticholinesterases on nicotinic and muscarinic acetylcholine receptors.

Four nerve agents and one therapeutic organophosphate (OP) anticholinesterase (anti-ChE) bind to acetylcholine (ACh) receptors, inhibit or modulate binding of radioactive ligands to these receptors, and modify events regulated by them. The affinity of nicotinic (n) ACh receptors of Torpedo electric organs and most muscarinic (m) ACh receptors of rat brain and N1E-115 neuroblastoma cultures for the OP compounds was usually two to three orders of magnitude lower than concentrations required to inhibit 50% (IC-50) of ACh-esterase activity. However, a small population of m-ACh receptors had an affinity as high as that of ACh-esterase for the OP compound. This population is identified by its high-affinity [3H]-cis-methyldioxolane ([3H]-CD) binding. Although sarin, soman, and tabun had no effect, (O-ethyl S[2-(diisopropylamino)ethyl)] methyl phosphonothionate (VX) and echothiophate inhibited competitively the binding of [3H]-quinuclidinyl benzilate ([3H]-QNB) and [3H]-pirenzepine ([3H]-PZ) to m-ACh receptors. However, VX was more potent than echothiophate in inhibiting this binding and 50-fold more potent in inhibiting carbamylcholine (carb)-stimulated [3H]-cGMP synthesis in N1E-115 neuroblastoma cells--both acting as m receptor antagonist. All five OPs inhibited [3H]-CD binding, with IC-50s of 3, 10, 40, 100, and 800 nM for VX, soman, sarin, echothiophate, and tabun, respectively. The OP anticholinesterases also bound to allosteric sites on the n-ACh receptor (identified by inhibition of [3H]-phencyclidine binding), but some bound as well to the receptor's recognition site (identified by inhibition of [125I]-alpha-bungarotoxin binding). Soman and echothiophate in micromolar concentrations acted as partial agonists of the n-ACh receptor and induced receptor desensitization. On the other hand, VX acted as an open channel blocker of the activated receptor and also enhanced receptor desensitization. It is suggested that the toxicity of OP anticholinesterases may include their action on n-ACh as well as m-ACh receptors if their concentrations in circulation rise above micromolar levels. At nanomolar concentrations their toxicity is due mainly to their inhibition of ACh-esterase. However, at these low concentrations, many OP anticholinesterases (eg, VX and soman) may affect a small population of m-ACh receptors, which have a high affinity for CD. Such effects on m-ACh receptors may play an important role in the toxicity of certain OP compounds.

In vitro assessment of target cell specificity in cadmium carcinogenesis: interactions of cadmium and zinc with isolated interstitial cells of the rat testes.

Cadmium (Cd) induces testicular tumors of interstitial cell (IC) origin in rats which can be prevented by zinc (Zn). Zn-induced synthesis of metallothionein (MT), a metal-binding protein with a high affinity for Cd, is thought to account for tolerance to Cd in most tissues by sequestration of Cd. However, the mechanism of Zn inhibition of Cd-induced carcinogenesis in the testes is unknown. Our studies with ICs obtained by collagenase dispersion of rat testes, indicate the levels of the Cd-binding protein in ICs are unaltered by Zn. This testicular protein also was found to differ from MT in amino acid content and to have a lower affinity for Cd. Thus, MT does not seem to be involved in protection of ICs against Cd carcinogenesis. Altered Cd toxicokinetics as a possible explanation for Zn-induced tolerance was therefore explored. Cd uptake into isolated ICs had passive diffusion and nonpassive (carrier mediated or active transport or both) components. The nonpassive component of Cd accumulation was markedly reduced by addition of Zn in vitro, indicative of competition for uptake at the cellular level. These results indicate that toxicokinetic alterations leading to reduced Cd accumulation may play an important role in Zn induction of tolerance to Cd carcinogenesis in the testes.