Showing papers on "Zinc published in 1994"

Zinc and carbon co-limitation of marine phytoplankton

Abstract: PROCESSES that control carbon uptake by marine phytoplankton are important in the global carbon cycle11–3. Uptake of CO2 itself may be limited by diffusion4. Bicarbonate uptake may be limited by zinc as HCO−3 transport appears to involve the zinc metallo-enzyme carbonic anhydrases5,6and the concentration of inorganic zinc in seawater7is low enough to limit the growth of certain phytoplankton in culture8,9. Here we show that HCO–3 uptake by the marine diatom Thalassiosira weissflogii is modulated by the partial pressure of CO2 and by the concentration of inorganic Zn (for which Cd and Co may substitute in carbonic anhydrase). This result leads naturally to a 'zinc hypothesis' which, like the standing 'iron hypothesis10, posits that Zn (Fe) may limit oceanic production and influence the global carbon cycle. Because of the large13C enrichment of HCO−3 over CO2, our results may be important for the interpretation of δ13C measurements in seawater and sediments.

Zinc: health effects and research priorities for the 1990s.

Abstract: This review critically summarizes the literature on the spectrum of health effects of zinc status, ranging from symptoms of zinc deficiency to excess exposure. Studies on zinc intake are reviewed in relation to optimum requirements as a function of age and sex. Current knowledge on the biochemical properties of zinc which are critical to the essential role of this metal in biological systems is summarized. Dietary and physiological factors influencing the bioavailability and utilization of zinc are considered with special attention to interactions with iron and copper status. The effects of zinc deficiency and toxicity are reviewed with respect to specific organs, immunological and reproductive function, and genotoxicity and carcinogenicity. Finally, key questions are identified where research is needed, such as the risks to human health of altered environmental distribution of zinc, assessment of zinc status in humans, effects of zinc status in relation to other essential metals on immune function, reproduction, neurological function, and the cardiovascular system, and mechanistic studies to further elucidate the biological effects of zinc at the molecular level.

Regulation of metallothionein genes by heavy metals appears to be mediated by a zinc-sensitive inhibitor that interacts with a constitutively active transcription factor, MTF-1.

Abstract: A construct, MRE-beta Geo, with five metal response elements fused to a selectable reporter gene was transfected into BHK cells and a stable clone that could be induced up to 100-fold by zinc, cadmium, bismuth, silver, cobalt, copper, mercury, or nickle was isolated. Some, and perhaps all, of these metals induce MRE-beta Geo by displacing zinc. Transfection of these cells with a construct encoding the transcriptional activator MTF-1 resulted in constitutive expression of MRE-beta Geo, whereas expression of an antisense MTF-1 construct in these cells prevented induction by all of the metals. A variant cell line with high constitutive expression in the absence of added metals was isolated; normal regulation was restored by cell fusion. These results suggest that regulation of metallothionein genes by metals is mediated by MTF-1 interacting with metal response elements and that zinc functions to release MTF-1 from an inhibitor.

Oxidative metal release from metallothionein via zinc-thiol/disulfide interchange

Abstract: Mammalian metallothionein has been postulated to play a pivotal role in cellular zinc distribution. All seven of its metal atoms are bound with high thermodynamic stability in two clusters buried deeply in the molecule. If the protein is to function in metal delivery, there must be a biological mechanism to facilitate metal release. One means to achieve this would be a labilization of the clusters by interaction of metallothionein with an appropriate cellular ligand. To search for such a mediator, we have designed a rapid radiochromatographic method that can detect changes in the zinc content of 65Zn-labeled metallothionein in response to other biomolecules. Using this methodology, we have established that rabbit liver metallothionein 2 interacts with glutathione disulfide with concomitant release of zinc. Under conditions of pseudo-first-order kinetics, the monophasic reaction depends linearly on the concentration of glutathione disulfide in the range from 5 to 30 mM with a second-order rate constant k = 4.9 x 10(-3)s-1.M-1 (pH 8.6; 25 degrees C). Apparently, zinc release does not involve direct access of glutathione disulfide to the inner coordination sphere of the metals. Rather it appears that the solvent-accessible zinc-bound thiolates in two clefts of each domain of metallothionein [Robbins, A. H., McRee, D. E., Williamson, M., Collett, S. A., Xuong, N. H., Furey, W. F., Wang, B. C. & Stout, C. D. (1991) J. Mol. Biol. 221, 1269-1293] participate in a thiol/disulfide interchange with glutathione disulfide. This rate-limiting initial S-thiolation, which occurs with indistinguishable rates in both clusters, then causes the clusters to collapse and release their zinc. Such a mechanism of metal release would link the control of the metal content of metallothionein to the cellular glutathione redox status and raises important questions about the physiological implications of this observation with regard to a role of glutathione in zinc metabolism and in making zinc available for other biomolecules.

Metallothionein III is expressed in neurons that sequester zinc in synaptic vesicles

Abstract: MT-III, a brain-specific member of the metallothionein gene family, binds zinc and may facilitate the storage of zinc in neurons. The distribution of MT-III mRNA within the adult brain was determined by solution and in situ hybridization and compared to that of MT-I mRNA. MT-III mRNA is particularly abundant within the cerebral cortex, hippocampus, amygdala, and nuclei at base of the cerebellum. Transgenic mice generated using 11.5 kb of the mouse MT-III 5' flanking region fused to the E. coli lacZ gene express beta-galactosidase in many of the same regions identified by in situ hybridization. MT-III mRNA was present in readily identifiable neurons within the olfactory bulb, hippocampus, and cerebellum, and beta-galactosidase activity was localized to neurons throughout the brain, but not to glia, as determined by costaining with X-Gal and neural- and glia-specific antibodies. There is marked correspondence between the neurons that are rich in MT-III mRNA and those neurons that store zinc in their terminal vesicles. MT-III is found complexed with zinc in vivo and its expression in cultured cells leads to the intracellular accumulation of zinc and enhanced histochemical detection of zinc. These results are discussed in light of the possibility that MT-III may participate in the utilization of zinc as a neuromodulator.

Video Image Analysis of Labile Zinc in Viable Pancreatic Islet Cells Using a Specific Fluorescent Probe for Zinc.

Abstract: We used an intracellular zinc-specific fluorophore, Zinquin, in conjunction with fluorescence video image analysis, to reveal labile zinc in pancreatic islet cells, which concentrate this metal for use in synthesis, storage, and secretion of insulin. Zinquin vividly demonstrated zinc in the islet cell secretory granules, which formed a brightly labeled crescent in the cytoplasm between one side of the nucleus and the plasma membrane. Lower but still appreciable amounts of zinc were detected in the remaining cytoplasm, but there was little labeling in the nucleus. Fluorescence intensity varied among islet cells, suggesting differences in zinc content. Their average fluorescence intensity greatly surpassed that of the surrounding pancreatic acinar cells in frozen sections of pancreas and in all other types of cell studied, including lymphocytes, neutrophils, fibroblasts, and erythrocytes. Less labile zinc was detected in cells of the mouse insulinoma cell line NIT-1, regardless of whether they were maintained in long-term culture in the presence or absence of exogenous extracellular zinc. Exposure of islet or insulinoma cells to a high concentration of glucose or other secretagogue decreased the content of labile zinc. Zinquin should be a useful probe for revealing changes in zinc homeostasis in islet B-cells that may be important in their dysfunction and death during diabetes.

Zinc mining for protein domains

Abstract: Three newly-determined structures testify to the widespread use of zinc for folding protein domains

Specific binding of HIV-1 nucleocapsid protein to PSI RNA in vitro requires N-terminal zinc finger and flanking basic amino acid residues.

Abstract: The nucleocapsid (NC) protein of human immunodeficiency virus HIV-1 (NCp7) is responsible for packaging the viral RNA by recognizing a packaging site (PSI) on the viral RNA genome. NCp7 is a molecule of 55 amino acids containing two zinc fingers, with only the first one being highly conserved among retroviruses. The first zinc finger is flanked by two basic amino acid clusters. Here we demonstrate that chemically synthesized NCp7 specifically binds to viral RNA containing the PSI using competitive filter binding assays. Deletion of the PSI from the RNA abrogates this effect. The 35 N-terminal amino acids of NCp7, comprising the first zinc finger, are sufficient for specific RNA binding. Chemically synthesized mutants of the first zinc finger demonstrate that the amino acid residues C-C-C/H-C/H are required for specific RNA binding and zinc coordination. Amino acid residues F16 and T24, but not K20, E21 and G22, located within this zinc finger, are essential for specific RNA binding as well. The second zinc finger cannot replace the first one. Furthermore, mutations in the basic amino acid residues flanking the first zinc finger demonstrate that R3, 7, 10, 29 and 32 but not K11, 14, 33 and 34 are also essential for specific binding. Specific binding to viral RNA is also observed with recombinant NCp15 and Pr55Gag. The results demonstrate for the first time specific interaction of a retroviral NC protein with its PSI RNA in vitro.

Studies on the uptake of lead and zinc by lignin obtained from black liquor - a paper industry waste material

Abstract: Lignin extracted from black liquor ‐ a paper industry waste material, has been characterized and used for the removal of lead and zinc metals. The uptake of lead is found to be greater than the uptake of zinc, and the sorption capacity increases with increasing pH. Adsorption on lignin follows the Freundlich and Langmuir models. Isotherms have been used to obtain the thermodynamic parameters of the process. Several polyhydric phenols and other substituted analogues may be involved in the uptake of metal ions. In comparison to other low‐cost adsorbents, the sorption capacity of the material, under investigation is found to be significantly high for the two metal ions.

A Novel Catalytic Effect of Lead on the Reduction of a Zinc Carbenoid with Zinc Metal Leading to a Geminal Dizinc Compound. Acceleration of the Wittig-Type Olefination with the RCHX2-TiCl4-Zn Systems by Addition of Lead

Abstract: A catalytic amount of lead promotes further reduction of zinc carbenoid (ICH 2 ZnI) with zinc in THF to give a geminal dizinc compound (CH 2 (ZnI) 2 ), which is a key intermediate for the methylenation of carbonyl compounds with a CH 2 I 2 , zinc, and TiCl 4 system

Immobilized Cyanobacteria for Online Trace Metal Enrichment by Flow Injection Atomic Absorption Spectrometry

Abstract: Cyanobacteria (Spirulina platensis) immobilized on controUed pore glass preconcentrate Cu(II), Zn(II), Cd(II), Pb(II), and Fe(III) from aqueous solution with high efficiency as ascertained using an on-line flow injection atomic absorption spectrometry system. The degree of metal binding depends on the pH of the solution. Quantitative retention of copper, zinc, and cadmium occurred at a wide range of pH values, while the retention for lead and iron was pH-dependent. The latter metals were adsorbed strongly only at pH 6 and 7, respectively. The breakthrough capacity was determined from the breakthrough curve, with values of 0.00035, 0.0008, 0.0011, 0.0028, and 0.0017 ng/mL for Cu, Zn, Cd, Pb, and Fe, respectively, being obtained

Compartmentation and transport of zinc in barley primary leaves as basic mechanisms involved in zinc tolerance

Abstract: The heavy metal zinc was administered to barley seedlings by increasing its concentration in the hydroponic medium. The most dramatic effect was a severe inhibition of root elongation with little effect on root biomass production. The growth of primary leaves was little affected although the zinc content of the primary leaves increased several-fold. A detailed compartment analysis was performed for 10-d-old barley primary leaves. Under low zinc nutrition (2mmol m −3), highest zinc contents were observed in the cytoplasm of mesophyll protoplasts. At inhibitory zinc concentrations in the hydroponic medium (400 μmol m −3), zinc levels dramatically and preferentially increased in the apoplastic space. Elevated zinc levels were also observed in the epidermal cells, and to a lesser extent, in mesophyll vacuoles. The cytoplasmic content of mesophyll protoplasts was unchanged, indicating perfect zinc homeostasis within the leaf. In order to understand the transport mechanisms underlying the steady-state distribution profile, we used 65Zn to conduct uptake experiments with leaves whose lower epidermis had been stripped. The leaves were placed on zinc solutions of varying concentrations containing 65Zn for 5 min to 6 h. After the incubation, the leaves were fractionated into mesophyll and epidermis protoplasts and residue, the latter mainly representing cell wall. Adsorption of Zn to the extracellular matrix was 100 times faster than Zn uptake into the cells. By far the largest portion taken up into the mesophyll protoplasts rapidly appeared in the vacuolar compartment. These results demonstrate the importance of compartmentation and transport as homeostatic mechanisms within the leaves to handle high, possibly toxic, zinc levels in the shoot.

Structure analysis of coprecipitated ZnFe2O4 by extended x‐ray‐absorption fine structure

Abstract: Though the anomalous antiferromagnetism of zinc ferrite was a subject of intensive studies in the past, the unusually high magnetization of coprecipitated zinc ferrite at low temperatures has drawn renewed interest among researchers. The local structures of zinc ferrites around Fe and Zn atoms are measured using extended x‐ray‐absorption fine structure and the results are discussed in correlation with their magnetic properties. The structure around the Zn atom was found to differ between zinc ferrite produced by ceramic and coprecipitation techniques. The position of the second‐nearest neighbor of the Zn atom for coprecipitated zinc ferrite was shorter than the one produced by the ceramic method. This suggests the possible occupation of the Zn atoms in the octahedral sites and the cause for the unusually high magnetization in coprecipitated zinc ferrite. Furthermore, the intensity of the peak is weak compared to the one produced by the ceramic method. This is thought to be due to the deformation induced b...

Induction of cytokines by zinc ions in human peripheral blood mononuclear cells and separated monocytes.

Abstract: Zinc plays an important role in the maintenance of immune functions. The immunological mechanisms triggered by zinc, however, are still poorly understood. In our experiments Zn2+ ions, added as ZnSO4, stimulated PBMC to produce IFN-gamma, IL-1 beta, IL-6, TNF-alpha, and sIL-2R in a concentration-dependent manner. CuSO4 and CaCl2 as cation and anion controls had no effect. The optimal concentration of zinc was 0.5 mM for monokine induction and 0.25 mM for induction of IFN-gamma and sIL-2R. The highest IL-1 beta and IL-6 levels were found on day 2 and maximum TNF-alpha after 16 h. IFN-gamma and sIL-2R production were optimum after 6 and 7 days, respectively. Monokines could be induced in autologous serum as well as in fetal calf serum and serum-free medium. Enriched monocytes and the human monocyte cell line Mono Mac 6 also released IL-1 beta after zinc challenge. Anti-IL-6 reduced IFN-gamma secretion whereas anti-IL-1 beta inhibited it. These data suggest that zinc acts primarily on monocytes by inducing monokine secretion and that T-cell activation represents a secondary effect in the cytokine cascade.

Serum copper and zinc levels and copper/zinc ratio in patients with breast cancer

Abstract: Serum copper, zinc levels, and the Cu/Zn ratio were evaluated in 31 patients with breast cancer and 35 healthy controls. Copper and zinc were determined by atomic absorbtion spectrophotometry. The mean serum copper level and the mean Cu/Zn ratio in patients with breast cancer were significantly higher than the control group (p < 0.001 and p < 0.001). In addition, the mean serum zinc level in patients with breast cancer was significantly lower than the control group (p < 0.001). Neither serum copper and zinc levels nor the Cu/Zn ratio were of value in discriminating of the disease activity and severity. Interestingly, the Cu/Zn ratio in premenopausal patients was higher than postmenopausal patients (p < 0.05) and this was not related to age. The further combined biological and epidemiological studies are necessary to investigate the roles of copper and zinc in breast cancer.

Adsorption of cadmium and zinc from aqueous solution on natural and activated bentonite

Abstract: The adsorption of cadmium and zinc ions on natural bentonite heat-treated at 110°C or at 200°C and on bentonite acid-treated with H 2 SO 4 (concentrations: 0.5 mol/dm 3 and 2.5 mol/dm 3 ), from aqueous solution at 30°C has been studied. The adsorption isotherms corresponding to cadmium and zinc may be classified respectively as H and L types of the Giles classification which suggests the samples have respectively a high and a medium affinity for cadmium and zinc ions. The experimental data points have been fitted to the Langmuir equation in order to calculate the adsorption capacities (X m ) and the apparent equilibrium constants (K a ) of the samples; X m and K a values range respectively for 4.11 mg/g and 1.90 dm 3 /g for the sample acid-treated with 2.5 mol/dm 3 H 2 SO 4 [(B-A(2.5)] up to 16.50 mg/g and 30.67 dm 3 /g for the natural sample heat-treated at 200°C [B-N-200], for the adsorption process of cadmium, and from 2.39 mg/g and 0.07 dm 3 /g, also for B-A(2.5), up to 4.54 mg/g and 0.45 dm 3 /g [B-N-200], for the adsorption process of zinc. X m and K a values for the heat-treated natural samples were higher than those corresponding to the acid-treated ones. The removal efficiency (R) has also been calculated for every sample; R values ranging respectievly from 65.9% and 8.2% [B-A(2.5)] up to 100% and 19.9% [B-N-200], for adsorption of cadmium and zinc

Crystallographic evidence for dual coordination around zinc in the T3R3 human insulin hexamer.

Abstract: The T3R3 human insulin hexamer (T and R referring to extended and alpha-helical conformations, respectively, of the first eight residues of the B-chain), complexed to two zinc ions, crystallizes in space group R3 with hexagonal cell constants a = 8064 and c = 3778 A The structure has been refined to a residual of 0172 using 9225 independent data to 16-A resolution The asymmetric unit consists of a TR dimer, and the insulin hexamer is generated by the action of the crystallographic 3-fold axis The conformation of one insulin trimer is nearly identical to that of the T6 hexamer, while the other trimer approximates that of the R6 hexamer, except for the three N-terminal B-chain residues that adopt an extended rather than an alpha-helical conformation Each of the two zinc ions, which lie on the crystallographic 3-fold axis and exhibit two different, disordered coordination geometries, is coordinated by the imidazole groups of three symmetry-related B10 histidine residues The coordination sphere of the zinc in the T3 trimer is either tetrahedral, with the fourth site filled by a chloride ion, or octahedral, completed by three water molecules The coordination of the zinc in the 12-A narrow channel in the R3 trimer is tetrahedral, with either a second chloride ion or a water molecule completing the coordination sphere The putative off-axial zinc binding sites that result from the T-->R transition of monomer II do not contain zinc ion, but instead are filled with clusters of ordered water molecules(ABSTRACT TRUNCATED AT 250 WORDS)

Functional characterization of human carbonic anhydrase II variants with altered zinc binding sites.

Abstract: Carbonic anhydrase II (CAII) contains a conserved His3 zinc polyhedron which is essential for catalysis. Removal of any one of the His ligands by replacement with Ala decreases (approximately 10(5)-fold), but does not abolish, zinc binding and increases the rate constant for zinc dissociation. CAII variants with a His ligand substituted with Cys, Asp, or Glu bind zinc only approximately 10-fold better than a His2 zinc polyhedron in CAII. The large decrease in zinc affinity (approximately 5 kcal/mol) in these variants compared to the wild-type His3 site reflects mainly unfavorable compensatory protein structural rearrangements observed in the X-ray crystallographic structures of some of these CAII variants, described by Ippolito and Christianson (following paper in this issue). However, the zinc affinity of these sites is still higher than zinc polyhedra designed de novo. Substitution of the His zinc ligands with negatively charged amino acids both increases the pKa of the zinc-bound water by > or = 1.6 pH units, confirming that neutral ligands maintain the low zinc-water pKa, and decreases the pH-independent kcat/KM for ester hydrolysis (3-30-fold) and CO2 hydration (approximately 10(3)-10(5)-fold). Additionally, decreases in the dissociation constant (approximately approximately 10(2)-10(5)-fold) for the transition state analog acetazolamide correlate with the decreased catalytic efficiency and increased pKa of these CAII variants. These data indicate that the histidine ligands, although not essential for catalysis, are conserved to maximize electrostatic stabilization of both the ground-state zinc-hydroxide and the negatively charged transition state. These studies provide valuable insights into the functional consequences of engineering a catalytic zinc site in a metalloenzyme.