Transfer Of Zinc Research Articles

The Transfer of Zinc in Two Linked Trophic Levels in Fresh Water and Its Effect on the Reproduction of Daphnia magna

ABSTRACT The green algae, Pseudokirchneriella subcapitata, was cultured in five different Zn concentrations (0, 50, 100, 500, and 1000 nM) for 4 d. Accumulation and toxicity of Zn in the algae increased with increasing Zn concentration, and highest and lowest algal growth rates occurred in medium containing 50 nM and 1000 nM Zn, respectively. There was a negative relationship (r = −0.89, P<0.05) between Zn accumulation and algal growth. Diets prepared from these cultures were fed to Daphnia magma up to 7 d to investigate the Zn transfer between the two trophic levels. Feeding rate, growth, protein levels, and offspring production of the clad- were investigated. Adult D. magma accumulated Zn from algae, and there was a significant positive relationship (r = 0.64, P = 0.01) between Zn in algae and Zn in D. magna. Accumulated Zn was depurated rapidly from D. magna during a 7 d elimination period. Carapace lengths of animals were not significantly altered by dietary Zn exposures (P>0.05). Likewise, the amount of algae consumed by the animals was not significantly affected by dietary Zn exposures. However, numbers of offspring produced were significantly (P<0.01) affected by different feeding regimes. Dietary Zn had a stimulating effect on the juvenile production. Dietary Zn also had a significant (P<0.01) influence on total protein levels of individual animals. Zn and protein levels showed a positive relationship (r = 0.78, P<0.01).

Metallothionein transfers zinc to mitochondrial aconitase through a direct interaction in mouse hearts

Previous studies have shown that in a cell-free system, metallothionein (MT) releases zinc when the environment becomes oxidized and the released zinc is transferred to a zinc-binding protein if such a protein is present. However, it is unknown whether and how zinc transfers from MT to other proteins in vivo. The present study was undertaken to test the hypothesis that if zinc transfer from MT to other proteins occurs in vivo, the transfer would proceed through a direct interaction between MT and a specific group of proteins. The heart extract obtained from MT-null mice was incubated with 65Zn-MT or 65ZnCl 2 and the proteins receiving 65Zn were separated by blue-native PAGE (BN-PAGE) or sodium dodecyl sulfate–PAGE (SDS–PAGE), and detected by autoradiography. A unique 65Zn-binding band was observed from the 65Zn-MT-incubated, but not the 65ZnCl 2-incubated preparation. The analysis using matrix assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry revealed that mitochondrial aconitase (m-aconitase) was among the proteins accepting Zn directly from Zn-MT. The m-aconitase, not the cytosolic aconitase (c-aconitase), was co-immunoprecipitated with MT. This study demonstrates that MT transfers zinc to m-aconitase through a direct interaction.

Influence of Soil Amendments on Heavy Metal Accumulation in Crops on Polluted Soils of Bangladesh

Pot experiments with soils from three contaminated sites and an additional field experiment were conducted. The aim of the experiments was to test different organic and inorganic soil amendments to heavy metal uptake and to alleviate toxicity in different agricultural crops. Elements in the extracts were measured by plasma emission spectroscopy (ICP‐AES). Cadmium in the extracts was measured by atomic absorption spectroscopy (AAS), with a heated graphite‐tube system (HGA). The results of the experiment were statistically evaluated by the LSD test. Almost all treatments had positive effects on crop productivity or reduced heavy metal uptake. Organic manures especially reduced manganese (Mn), chromium (Cr), and nickel (Ni) uptake. Iron (Fe) oxides contained in red mud, a by-product of the aluminum industry, reduced soil to plant transfer of zinc (Zn), Ni, cadmium (Cd), and Cr. The results from these experiments show that it is necessary to select and combine amendments taking into account both site and crop characteristics.

On the Competition for Available Zinc

Extended x-ray absorption fine structure (EXAFS) spectroscopy was combined with thermodynamic and kinetic approaches to investigate zinc binding to a zinc finger (C2H2) and a tetrathiolate (C4) peptide. Both peptides represent structural zinc sites of proteins and rapidly bind a single zinc ion with picomolar dissociation constants. In competition with EDTA the transfer of peptide-bound zinc ions proved to be 6 orders of magnitude faster than predicted for a dissociation-association mechanism thus requiring ligand exchange mechanisms via peptide-zinc-EDTA complexes. EXAFS spectra of C2H2 showed the expected Cys2His2-ligand geometry when fully loaded with zinc. For a 2-fold excess of peptide, however, the existence of zinc-bridged peptide-peptide complexes with dominating sulfur coordination could be clearly shown. Whereas zinc binding kinetics of C2H2 appeared as a simple second order process, the suggested mechanism for C4 comprises a zinc-bridged Zn-(C4)2 species as well as a Zn-C4 species with less than 4 metal-bound thiolates, which is supported by EXAFS results. A rapid equilibrium of bound and unbound states of individual ligands might explain the kinetic instability of zinc-peptide complexes, which enables fast ligand exchange during the encounter of occupied and unoccupied acceptor sites. Depending on relative concentrations and stabilities, this results in a rapid transfer of zinc ions in the virtual absence of free zinc ions, as seen for the zinc transfer to EDTA, or in the formation of zinc-bridged complexes, as seen for both peptides with excess of peptides over available zinc.

Atmospheric leaching of EAF dust with diluted sulphuric acid

In this study the hydrometallurgical processing of EAF steelmaking dust (Paul Wurth S.A.) is investigated on a laboratory scale under normal temperature and pressure conditions. The behaviour of zinc and iron under the influence of sulphuric acid as the leaching agent is discussed. The dependency between the temperature and the liquid/solid ratio is investigated. The main aim is the transfer of zinc into the solution while iron ought to remain as a solid residue. The hydrometallurgical recovery of zinc from EAF dust is feasible with a relatively high recovery yield, while iron mostly remains in the solid phase. Calcium, too, is transferred to the solution with a relatively high recovery yield. This results from the use of a low concentration of sulphuric acid. In such a way, it is possible to set up the conditions for the leaching of EAF dust, namely sufficient sulphuric acid concentration in order to achieve an optimum zinc yield to the solution without an iron dissolution. However, the problem is that the chemical and mineralogical composition of each steelmaking dust is individual. Therefore, it is necessary to study the processing conditions for each one of them.

Mechanisms Underlying Iron and Zinc Transport to Axis Organs in Grain During Early Seedling Development of Maize

The role of both acidity release and ferric reduction by scutellum was investigated in relation to iron (Fe) mobilization from endosperm and transport to axis organs (shoot + root) during early seedling development of maize (Zea mays L.). Effect of scutellum zinc (Zn) transfer to growing axis organs was also evaluated. Acidity release and translocation of Fe or Zn from an agar medium to axis organs were decreased when scutellum dorsal sides of intact seedlings were embedded into a pH buffered agar medium. Across genotypes root growth during at least 3 days was correlated to scutellar acidity release. Orthovanadate or pyrazole (1 mM) did not inhibit Fe translocation to axis organs. Thus, Fe translocation was not closely associated with scutellum plasma membrane H+-ATPases or alcohol dehydrogenase. Addition of Fe2+-citrate facilitated Fe translocation from agar medium to axis organs by 20%, compared to FeSO4. Extracellular reduction of Fe2+ to Fe3+ was associated with release of reductants from scutellum, which was dramatically inhibited by low temperature (7°C). This process was not affected by solution pH within range of 3.5–6.5 units or by copper (Cu2+), Zn2+, and manganese (Mn2+) added to assay-solution Fe.

Separation of Zinc(II) from Hydrochloric Acid Solutions in a Double Lewis Cell

A double Lewis cell (bulk liquid membrane) was used to study the extraction of zinc(II) from model 10% HCl and industrial spent pickling liquor containing iron(II) and iron(III). Water was used for stripping. Tributyl phosphate (TBP), Cyanex® 923, HOE® F2562, and AMBERLITE® LA‐2 were used as carriers. The extraction–stripping process from model HCl solution was successfully carried out when undiluted or highly concentrated TBP was used as the membrane phase. CYANEX® 923 and AMBERLITE® LA‐2 were too strong as carriers to achieve successful stripping with water. TBP did not act as a phase‐transfer catalyst when added to the membrane phase containing CYANEX® 923. The process needed additionally 3 M NaOH for stripping. An interfacial gel layer was formed, especially on the strip side, when HOE® F2562 was used as the extractant. It seemed important to select the appropriate carriers for the membrane processes, because the interface could be easily blocked if trace impurities were present in industrial reagents. When TBP was used to recover zinc(II) from actual spent pickling solution, an undesired transfer of iron(III), first to the membrane and then to the strip phase, was observed. The dilution of TBP with kerosene decreased the co‐transfer of hydrochloric acid but did not improve the selectivity of zinc(II) transfer with respect to iron. The reduction of iron(III) to iron(II) and/or extraction of iron(III) with a small volume of TBP, carried out just before the process, enabled the selective extraction of zinc(II). The organic phase loaded with iron(III) needed quick stripping to avoid both the hydrolysis of TBP and then the undesired transfer of iron(II) with partial esters. The selective recovery of zinc(II) from industrial spent pickling solution is possible, but special care must be taken to avoid any oxidation during the process.

Multiple Metal Resistant Transferable Phenotypes in Bacteria as Indicators of Soil Contamination with Heavy Metals (6 pp)

Environmental contamination by heavy metals affects microbial communities. The number of single and multiple heavy metal resistant bacteria may be an indictor of the level of contamination. This paper details the isolation and characterisation of metal resistant microorganisms isolated from rhizosphere/soil samples obtained from an abandoned zinc, lead and copper mine and a local unaffected site. This data was compared to the level of heavy metal in the soils to establish the effect of metals on the microbial community and to determine the relationship between pollutant levels and resistant strains. This paper outlines the diversity of transferable resistance determinants between both sites and details the levels of heavy metal resistant bacteria and those expressing transferable multiple heavy metal tolerance. The sample sites were located in Co. Galway, Ireland. The first sample site (site A) was a former lead, zinc and copper mine, which was closed in 1961 due to exhaustion of ore. The second site (site B) was located two and a half kilometres from the mining site and was not affected by the mining operations. Composite soil samples were characterised for general soil matrix composition, organic content, pH and general chemical parameters. The soil was also enumerated for the total viable heterotrophic counts and tested on Pseudomonas selective agar (PSA) for total Pseudomonas counts and Sucrose Asparagine (SA), which is semi-selective for fluorescent Pseudomonas. Samples from both site A and site B were analysed by atomic absorption spectrophotometry for the presence of heavy metals. In the case of copper, which has a Dutch list recommended minimum permissible level of 190 µg/Kg dry weight, the levels detected at site A were 1270 µg/Kg dry weight while site B was detected at 36 µg/Kg. The arsenic levels detected at site A were eight times the permissible level (416 µg/Kg) while only half the permissible level was found at site B (13 µg/Kg). Zinc concentrations were also high at site A (4460 µg/Kg) while at site B (553 µg/Kg) they were well below the Dutchlist guidelines (720 µg/Kg). A large number of heavy metal tolerant strains were isolated from both sites. 270 isolates (site A (170) and site B (100)) were screened against 8 metals to examine the extent of multiple resistance. 82% of the strains from site A were found to be resistant to 5 metals. A total of 18% showed resistance to all 8 metals and of those examined only 4% were resistant to only one metal. In contrast isolates from site B showed no multiple resistance to more than 5 metals, while 62% showed resistance to individual metals only. Site A had a higher level of multiple heavy metal resistance strains. Stains isolated from site A had 23 (14%) isolates resistant to zinc, copper, nickel, arsenic and cobalt and site B had no bacteria resistant to all five of these selected metals. The transferability of heavy metal resistance was investigated in the case of 60 multiple heavy metal resistant isolates taken from site A and 50 multiple resistance isolates from site B. Transfer was only detected in isolates from site A, 13% showed transfer and expression of copper, zinc and arsenic resistance determinants. In most cases the transconjugants only expressed resistance to copper, zinc and arsenic, which were the metals, used for selection. This co-transfer of all three determinants suggests a genetic link between these resistance determinants. Heavy metal resistant bacteria are present in both sites, however, the number and presence of multiple transferable resistance phenotypes are confined to the isolates from the heavy metal contaminated site. The presence of high levels of heavy metals selects these multiple resistance phenotypes. Within these communities there seems to be little diversity between the microorganisms, which provides a hugely preferable environment for gene transfer of such metal resistant determinants. The experiments have shown a microbes ability to mobilize heavy metal determinants and a relationship between heavy metal resistance and metal contamination has been identified These multiple heavy metal resistant bacteria could eventually be used for detection and qualification of the level of heavy metal-polluted soil/water environments.

Migration of inorganic contaminants into dry food from packaging made from recycled paper and board

A radiotracer method was applied to the measurement of inorganic contaminants migrating into food. The method was applied to the study of 10 samples of recycled paper and board. The only elements detected in the static migration test were zinc and iron, at concentrations close to the detection limit. Samples of rice, oats, custard powder, mushrooms and fries showed that traces of these elements had migrated from the packaging. The migration was limited to less than 1%, with the exception of the mushrooms, where the decomposition of the food had resulted in a 12.5% transfer of zinc from the mushroom bag after 8 days, equivalent to migration of 0.022 mg dm−2. Shaking had no effect on migration, except in one case of custard powder where migration occurred at a level equivalent to 0.002 mg dm−2. The maximum migration determined in the present study was 4.8 µg zinc and 2.6 µg iron, which is negligible when compared with the recommended daily intakes of 15 and 14 mg, respectively. When Tenax was tested as a food simulant, no zinc was detected (<0.001 mg dm−2), although slight migration (0.001 mg dm–2) occurred into food.

The transfer of cadmium, mercury, methylmercury, and zinc in an intertidal rocky shore food chain

We examined the transfer of cadmium (Cd), inorganic mercury [Hg(II)], methylmercury (MeHg), and zinc (Zn) in an intertidal rocky shore food chain, namely from marine phytoplankton to suspension-feeding rock oysters ( Saccostrea cucullata) and finally to predatory whelks Thais clavigera. The uptake of metals from the dissolved phase was also concurrently quantified in the oysters and the whelks. Metal uptake by the oysters was not directly proportional, whereas metal uptake by the whelks was directly proportional to metal concentration in the water. The order of uptake was MeHg>Hg(II)>Zn>Cd, and was much higher in the oysters than in the whelks. The relative uptake of Zn and Cd was comparable between oysters and whelks, whereas MeHg and Hg(II) showed disproportionally higher uptake in oysters than in whelks as compared to Zn and Cd. The assimilation efficiencies (AEs) were in the order of MeHg>Zn>Cd=Hg(II) in oysters, whereas the AEs were highest for MeHg and comparable for Zn, Cd, and Hg(II) in the whelks. Pre-exposure of the oysters to different dissolved concentrations of Cd significantly elevated the AEs of Cd and Hg(II) but not of Zn, in association with the induction of metallothioneins in the oysters. The whelks significantly assimilated Cd and Zn from various prey (barnacles, oysters, mussels, and snails) with contrasting strageties of metal sequestration and storage. There was no significant relationship between the metal AE and the metal partitioning in the soluble fraction (including metallothionein-like proteins, heat stable protein, and organelles). The insoluble fraction of metals was also available for metal assimilation. Our calculations show that the dietary uptake of metals can be dominant in the overall bioaccumulation in the oysters and whelks, and the trophic transfer factor was >1 for all metals. Thus, the four metals have a high potential of being biomagnified in the intertidal rocky shore food chain. MeHg possessed the highest and Hg(II) and Cd the lowest potential of trophic transfer among the four metals considered.

Mass transfer of zinc(II) extraction from hydrochloric acid solution in the Lewis cell

Extraction of zinc(II) from model hydrochloric acid solutions was studied by Lewis cell mixing the bulks of both phases and keeping a stable interfacial surface. It was found that Lewis cell experiments, although dynamic in nature, support the extracting of ZnCl2∗2 TBP and H2ZnCl4∗2TBP chlorocomplex from model solutions containing 5 M Cl− and 0.55 MH+. The initial fluxes of zinc(II), HCl, Cl−, chloride present in the zinc(II) complex and water depend upon the concentration of tributyl phosphate and the mixing rates. The diffusion step in the organic phase gave a higher resistance to the mass transfer than the diffusion step in the aqueous phase. The physical transfer of small water molecules was less sensitive on the mixing rates than the transfer of bulky zinc(II) chlorocomplex.

Experimental study and modeling of the transfer of zinc in a low reactive sand column in the presence of acetate

Nowadays, it is necessary to understand and identify the reactions governing the fate of heavy metals introduced into the environment with low complexing organic compounds, particularly when they are transferred through soils in urban areas. In this work the concomitant influence of pH and acetate on the fate of zinc on siliceous sand was studied in batch and non-saturated column experiments. Total zinc concentrations varied between 2 and 20 mg/l, and total acetate concentrations were fixed at 22, 72, 132, and 223 mM to obtain solution pHs of 4, 5, 6 and 7, respectively. Natural sand (diameter, 0.3–2 mm), mainly constituted of silica, was used. In batch adsorption experiments, zinc adsorption is insignificant at pH 4, low and linear at pH 5, and increasingly nonlinear, of the Langmuir type, at pH 6 and 7 indicating near-saturation conditions of surface sites at these high pH values. In column experiments, Zn retardation increases and the maximum outlet concentration of Zn decreases with rising pH and acetate concentrations. Previous column tracer experiments revealed the occurrence of regionalized water transport in the column. Modeling these data was based on a non-electrostatic approach. Batch and column data modeling was based on the PHREEQC code that allows concomitant resolution of chemical speciation and regionalized water transport. The speciation calculation indicates that the ZnAcetate + species is the dominant Zn species in the solutions used. Batch experimental curves are correctly modeled assuming the formation of the three surface species SiOZn +, SiOH–Zn Acetate + and SiO–Zn(Acetate) 2 −. The column data could be adequately modeled assuming a two-region water transport and the formation of the same three species with the same thermodynamic constants determined in the batch experiments. The hypothesis of the modeling leads to a slight overestimation of the quantities of zinc eluted (10%) at pH 6 and 7, mostly in the desorption phase. These results show that the methodology used facilitates the correct modeling of both batch and transport experiments and formulation of the hypothesis on the interactions between the low reactive sand and a complex solution.

Influence of Zinc Intercalation on Processes in Nickel Oxide Electrode and on Service Life of Nickel-Zinc Battery

The dependence of capacity and service life of a nickel-zinc battery on the mechanism of zinc transfer and the rate of zinc intercalation into nickel oxide electrodes differing in the conductivities of the active paste and porous structures was studied.

Transfer of cadmium and zinc from sewage sludge amended soil through a plant–aphid system to newly emerged adult ladybirds ( Coccinella septempunctata)

An agricultural soil was amended with sewage sludge at rates equivalent to 0, 10 and 30 t (dry solids) ha −1 and the subsequent transfer of zinc and cadmium through a soil–plant–arthropod system was investigated. Zinc concentration in soil, wheat and aphids increased significantly with sludge amendment rate. Zinc was biomagnified during transfer along the pathway, resulting in concentrations in the aphids four times greater than in the soil. Cadmium concentration in the soil was also significantly elevated by the addition of sludge, but there was no significant difference in cadmium concentration in the shoots of wheat plants. Cadmium concentration in aphids followed the pattern found in plants, but again, differences between treatments were not significant. Aphids collected from the plants were subsequently fed to fourth instar Coccinella septempunctata. Consumption of these aphids did not result in significant differences between treatments in the body burden of newly emerged adult C. septempunctata for either metal. Sequestration of cadmium in the pupal exuviae had a greater effect on the body burden of newly emerged adult ladybirds than for zinc. Results are discussed in relation to possible risks posed by the transfer of trace metals via the soil–plant–arthropod system to predatory arthropods.

Iron and transition metal transport into erythrocytes mediated by nifedipine degradation products and related compounds

The aim of this investigation was to determine the mechanism of action of the nitrosophenylpyridine derivative of nifedipine (“nitrosonifedipine”, NN) on Fe(II) transport into erythrocytes. Nifedipine is rapidly degraded to NN by daylight. We used rabbit erythrocytes, NN, and several chemically related substances, and examined their effects on the transfer of iron and other transition metals (cadmium, cobalt, manganese, nickel, zinc) into and out of the cells. NN mediated the transfer of iron and zinc but not the other metals into the cell cytosol. The transfer of Fe(II) was not affected by changes in cell membrane potential and could not be ascribed to free radical production. Two hydroxamic acid compounds chemically related to NN also stimulated iron and zinc uptake, but no evidence was obtained for cell-induced transformation of NN to them. In vivo, NN is probably converted to a lactam derivative. This compound was found to have no effect on iron uptake by the cells. It is concluded that NN has a relatively high specificity for the transport of iron compared with other transition metals, and small changes in its structure markedly affect this action. Also, because the lactam to which NN is converted in vivo is inactive, it is unlikely that nifedipine will affect iron metabolism after therapeutic administration.

Processes in a Nickel-Zinc Battery with Nickel Oxide Electrode Fabricated from Spherical Nickel Hydroxide

Processes occurring in a cycled nickel-zinc battery with positive nickel oxide electrode fabricated from spherical nickel hydroxide on foamed-nickel support and the influence exerted by its physicochemical parameters on changes in the electrolyte composition and on inter- and intraelectrode mass transfer of zinc was studied.

The NHR2/CO2 system as a metal ion extraction reagent from aqueous solution into hydrocarbons: copper(II) and zinc(II)

A substantially quantitative transfer of copper(II) or zinc(II) salts from aqueous solution into a hydrocarbon (heptane or toluene) promptly occurs under carbon dioxide in the presence of a dialkylamine (NHR2, R=Bu, Bz). Recovery of the metal complexes from the organic phase affords copper dialkylcarbamates of the formula Cu(O2CNR2)2(NHR2)2 or μ-oxo tetranuclear zinc carbamato complexes, Zn4(μ4-O)(O2CNR2)6, respectively, in high yield and purity. An X-ray diffraction study on a single crystal of Cu(O2CNBz2)2(NHBz2)21 has shown the compound to be mononuclear with tetracoordinated copper in an almost perfect square-planar geometry. The zinc derivative has the well-established oxo-centred tetranuclear structure (R=Bu, 2).

Effect of gestational age on cord blood plasma copper, zinc, magnesium and albumin.

The transport of essential trace elements from mother to fetus varies throughout gestation, and the role of transport proteins in the neonate and the mother may change during pregnancy. Magnesium, often used as tocolytic agent, may reach the fetus and appear in cord blood at higher than normal concentrations. To determine cord blood plasma zinc, copper and magnesium concentrations, as well as plasma albumin in premature and full-term newborns, and correlate these values with those of maternal blood plasma at birth. Also, to examine whether cord blood plasma concentration of these elements varies with gestational age. The 35 mother-infant pairs included: 11 in the 38-42-week gestational age (GA), 9 in the 34-37-week GA, 11 in the 29-33-week GA group and 4 in the 24-28-week GA. Magnesium for tocolysis was given to five of the mothers in the 29-33-week GA cohort and two of the women giving birth at 24-28-week GA. Trend analysis showed that while cord plasma zinc decreased with GA at birth, the reverse was observed for copper. There were no differences with GA either in maternal plasma zinc or copper. However, maternal ceruloplasmin tended to decrease with GA (P=0.0174). Maternal and cord blood plasma magnesium exhibited a strong correlation (r=0.942, P<0.001), as well as between cord plasma magnesium and zinc (r=0.448, P<0.01). While the vigorous mother-to-fetus uphill zinc transfer is clear throughout the last trimester, copper remains in cord blood plasma at much lower concentrations than in the mother, suggesting that prematurity may place the newborn infant at a greater risk than the term infant to copper deficiency. This situation, together with a reduced synthesis in the fetus of the transport protein ceruloplasmin, creates another potential challenge in the nutritional support of the premature infant.

Food Chain Transfer of Zinc Within the Ecosystems of Old and Modern Metalliferous Mine Wastes

Concentrations of zinc in mine waste, vegetation, invertebrate and small mammals from abandoned mines and a modern mine site have been determined in order to assess and compare environmental risk. Very high concentrations of zinc in mine waste from all sites were reflected in vegetation zinc levels but not in invertebrates and mammals. Physiological regulatory control mechanisms operate to optimise zinc retention, and maintain homeostasis in animals by enhancing absorption of zinc from diets low in the trace element, and by buffering the potential accumulative effects of high dietary levels of the metal. Zinc is not considered to be an acute toxicity factor precluding colonisation of these mine sites by invertebrates and mammals though it may be a selective force influencing the species composition and relative abundance in animal communities.

Brain-specific metallothionein-3 has higher metal-binding capacity than ubiquitous metallothioneins and binds metals noncooperatively.

Zinc metabolism in the cells is largely regulated by ubiquitous small proteins, metallothioneins (MT). Metallothionein-3 is specifically expressed in the brain and is down regulated in Alzheimer's disease. We demonstrate by mass spectrometry that MT-3, in contrast to common MTs, binds Zn(2+) and Cd(2+) in a noncooperative manner and can also bind higher stoichiometries of metals than seven. MT-3 reconstituted with seven metals exists in a dynamic equilibrium of different metalloforms, where the prevalent metalloform is Me(7)MT-3, but metalloforms with 6, 8, and even 9 metals are also present. The results from pH and stability studies demonstrate that the heterogeneity of metalloforms originates from the N-terminal beta-cluster, whereas the C-terminal alpha-cluster of MT-3 binds four metal ions such as that of common MTs. Experiments with EDTA demonstrate that the beta-cluster of ZnMT-3 has a higher metal transfer potential than the beta-cluster of Zn(7)MT-2. Moreover, ZnMT-3 loses metals during ultrafiltration. MT-3, reconstituted with an excess of Zn(2+) or Cd(2+), exists as a dynamic mixture of metalloforms with higher than 7 metal stoichiometries (8-11). Such forms of ZnMT-3 are unstable and decompose partly already during a rapid gel filtration, whereas CdMT-3 forms are more stable. Extra metal ions may bind to the beta-cluster region as well as to the carboxylates of MT-3. The specific metal-binding properties of MT-3 could be functionally implemented for buffering of fluctuating concentrations of zinc in zincergic neurons and for transfer of zinc to synaptic vesicles.