The interactions of zinc, nickel, and cadmium with Xenopus transcription factor IIIA, assessed by equilibrium dialysis

Zinc binding of a Cys2His2-type zinc finger protein is enhanced by the interaction with DNA

Zinc finger proteins specifically recognize DNA sequences and, therefore, play a crucial role in living organisms. In this study the Zn(II)-, and DNA-binding of 1MEY#, an artificial zinc finger protein consisting of three finger units was characterized by multiple methods. Fluorimetric, circular dichroism and isothermal calorimetric titrations were applied to determine the accurate stability constant of a zinc finger protein. Assuming that all three zinc finger subunits behave identically, the obtained thermodynamic data for the Zn(II) binding were ΔH_{binding site} =  - (23.5 - 28.0) kcal/mol (depending on the applied protonation state of the cysteines) and logβ'_pH 7.4 = 12.2 ± 0.1, being similar to those of the CP1 consensus zinc finger peptide. The specific DNA binding of the protein can be characterized by logβ'_{pH 7.4} = 8.20 ± 0.08, which is comparable to the affinity of the natural zinc finger proteins (Sp1, WT1, TFIIIA) toward DNA. This value is ~ 1.9 logβ' unit higher than those determined for semi- or nonspecific DNA binding. Competitive circular dichroism and electrophoretic mobility shift measurements revealed that the conditional stability constant characteristic for Zn(II) binding of 1MEY# protein increased by 3.4 orders of magnitude in the presence of its target DNA sequence.

Interactions of an Artificial Zinc Finger Protein with Cd(II) and Hg(II): Competition and Metal and DNA Binding

Cys2His2 zinc finger proteins are important for living organisms, as they—among other functions—specifically recognise DNA when Zn(II) is coordinated to the proteins, stabilising their ββα secondary structure. Therefore, competition with other metal ions may alter their original function. Toxic metal ions such as Cd(II) or Hg(II) might be especially dangerous because of their similar chemical properties to Zn(II). Most competition studies carried out so far have involved small zinc finger peptides. Therefore, we have investigated the interactions of toxic metal ions with a zinc finger proteins consisting of three finger units and the consequences on the DNA binding properties of the protein. Binding of one Cd(II) per finger subunit of the protein was shown by circular dichroism spectroscopy, fluorimetry and electrospray ionisation mass spectrometry. Cd(II) stabilised a similar secondary structure to that of the Zn(II)-bound protein but with a slightly lower affinity. In contrast, Hg(II) could displace Zn(II) quantitatively (logβ′ ≥ 16.7), demolishing the secondary structure, and further Hg(II) binding was also observed. Based on electrophoretic gel mobility shift assays, the Cd(II)-bound zinc finger protein could recognise the specific DNA target sequence similarly to the Zn(II)-loaded form but with a ~0.6 log units lower stability constant, while Hg(II) could destroy DNA binding completely.

Regulation of zinc-dependent enzymes by metal carrier proteins

Zn²⁺ ions are essential in many physiological processes, including enzyme catalysis, protein structural stabilization, and the regulation of many proteins. The affinities of proteins for Zn²⁺ ions span several orders of magnitude, with catalytic Zn²⁺ ions generally held more tightly than structural or regulatory ones. Metal carrier proteins, most of which are not specific for Zn²⁺, bind these ions with a broad range of affinities that overlap those of catalytic, structural, and regulatory Zn²⁺ ions and are thought to be responsible for distributing the metal through most cells, tissues, and fluid compartments. While little is known about how many proteins obtain or release these ions, there is now considerable experimental evidence suggesting that metal carrier proteins may be responsible for transferring metals to and from some Zn²⁺-dependent proteins, thus serving as a major regulatory factor for them. In this review, the biological roles of Zn²⁺ and structures of Zn²⁺ binding sites are examined, and experimental evidence demonstrating the direct participation of metal carrier proteins in enzyme regulation is discussed. Mechanisms of metal ion transfer are also offered, and the potential physiological significance of this phenomenon is explored.

Ni(II), Hg(II), and Pb(II) Coordination in the Prokaryotic Zinc-Finger Ros87

Zinc ion binding is a principal event in the achievement of the correct fold in classical zinc finger domains since the motif is largely unfolded in the absence of metal. In the case of a prokaryotic zinc finger, the larger βββαα domain contributes to the folding mechanism with a larger hydrophobic core. For these reasons, following the great amount of attention devoted to unveiling the effect of xenobiotic metal ion replacement in zinc fingers and in zinc-containing proteins in general, the prokaryotic zinc finger domain appears to be an interesting model for studying metal ion interaction with metalloproteins. Here, we explore the binding of Ni(II), Hg(II), and Pb(II) to Ros87, the DNA binding domain of the prokaryotic zinc finger protein Ros. We measured Ros87-metal ion dissociation constants and monitored the effects on the structure and function of the domain. Interestingly, we found that the protein folds in the presence of Ni(II) with important structural perturbations, while in the presence of Pb(II) and Hg(II) it does not appear to be significantly folded. Accordingly, an overall strong reduction in the DNA binding capability is observed for all of the examined proteins. Our data integrate and complement the information collected in the past few years concerning the functional and structural effects of metal ion substitution in classical zinc fingers in order to contribute to a better comprehension of the toxicity of these metals in biological systems.

The prokaryotic zinc-finger: structure, function and comparison with the eukaryotic counterpart

Classical zinc finger (ZF) domains were thought to be confined to the eukaryotic kingdom until the transcriptional regulator Ros protein was identified in Agrobacterium tumefaciens. The Ros Cys2 His2 ZF binds DNA in a peculiar mode and folds in a domain significantly larger than its eukaryotic counterpart consisting of 58 amino acids (the 9-66 region) arranged in a βββαα topology, and stabilized by a conserved, extensive, 15-residue hydrophobic core. The prokaryotic ZF domain, then, shows some intriguing new features that make it interestingly different from its eukaryotic counterpart. This review will focus on the prokaryotic ZFs, summarizing and discussing differences and analogies with the eukaryotic domains and providing important insights into their structure/function relationships.

Zinc to cadmium replacement in the prokaryotic zinc-finger domain

Given the similar chemical properties of zinc and cadmium, zinc finger domains have been often proposed as mediators of the toxic and carcinogenic effects exerted by this xenobiotic metal. The effects of zinc replacement by cadmium in different eukaryotic zinc fingers have been reported. In the present work, to evaluate the effects of such substitution in the prokaryotic zinc finger, we report a detailed study of its functional and structural consequences on the Ros DNA binding domain (Ros87). We show that this protein, which bears important structural differences with respect to the eukaryotic domains, appears to structurally tolerate the zinc to cadmium substitution and the presence of cadmium does not affect the DNA binding activity of the protein. Moreover, we show for the first time how zinc to cadmium replacement can also take place in a cellular context. Our findings both complement and extend previous results obtained for different eukaryotic zinc fingers, suggesting that metal substitution in zinc fingers may be of relevance to the toxicity and/or carcinogenicity mechanisms of this metal.

NMR characterization of foldedness for the production of E3 RING domains

We summarize the use of NMR spectroscopy in the production and the screening of stability and foldedness of protein domains, and apply it to the RING domains of E3 ubiquitin-ligases. RING domains are involved in specific interactions with E2 ubiquitin-conjugating enzymes and thus play an essential role in the ubiquitination pathway. Protein production of the Zn(2+) containing and cysteine rich RING domains for molecular studies frequently turns out to be problematic. We compared the expression and solubility of 14 E3 RING/U-box domains fused to the N-terminal tags of His(6), His(6)-GB1, His(6)-Trx and His(6)-GST at small scale and analyzed, by NMR spectroscopy, their correct folding after purification. The addition of GST, Trx or GB1 to the N-terminal His(6) tag significantly improved both the expression and solubility of target proteins as compared to His(6) tag alone. More importantly most of the immobilized metal affinity chromatography (IMAC) purified proteins were largely unfolded as judged by analysis of the (1)H-(15)N HSQC spectra. We demonstrate that imidazole causes a concentration dependent decrease in stability of RING proteins ascribed to metal depletion and resulting in unfolding or precipitation. In contrast, using glutathione affinity chromatography, the His(6)-GST fused RING and U-box domains were purified as correctly folded proteins with high yields. Our data clearly demonstrate that IMAC should be avoided and that GST-fusion affinity chromatography is generally applicable for expression and purification of Zn(2+) containing proteins.

Zn-, Cd-, and Pb-transcription factor IIIA: properties, DNA binding, and comparison with TFIIIA-finger 3 metal complexes

Properties of the metal ion binding sites of Zn-transcription factor IIIA (TFIIIA) were investigated to understand the potential of this type of zinc finger to undergo reactions that remove Zn(2+) from the protein. Zn-TFIIIA was purified from E. coli containing the cloned sequence for Xenopus laevis oocyte TFIIIA and its stoichiometry of bound Zn(2+) was shown to depend on the details of the isolation process. The average dissociation constant of Zn(2+) in Zn-TFIIIIA was 10(-7). The dissociation constant for Zn-F3, the third finger from the N-terminus of TFIIIA, was 1.0 x 10(-8). The reactivity of Zn-TFIIIA with a series of metal binding ligands, including 2-carboxy-2'-hydroxy-5'-sulfoformazylbenzene (zincon), 4-(2-pyridylazo)-resorcinol (PAR), and 3-ethoxy-2-oxo-butyraldehyde-bis-(N(4)-dimethylthiosemicarbazone) (H(2)KTSM(2)) revealed similar kinetics. The reactivity of PAR with Zn-TFIIIA declined substantially when the protein was bound to the internal control region (ICR) of the 5S ribosomal DNA. Both Cd(2+) and Pb(2+) disrupt TFIIIA binding to its cognate DNA sequence. The Pb(2+) dissociation constant of Pb-F3 was measured as 2.5 x 10(-8). According to NMR spectroscopy, F3 does not fold into a regular conformation in the presence of Pb(2+).

Interaction of Cd2+ with Zn finger 3 of transcription factor IIIA: structures and binding to cognate DNA

Finger 3 of transcription factor IIIA of Xenopus laevis was synthesized and constituted with Zn(2+) or Cd(2+). The C-block element of the internal control region of the promoter of the 5S rRNA gene binds to the Zn-F3 and Cd-F3 with dissociation constants of 2.6 x 10(-5) and 1.5 x 10(-4) M, respectively. According to NMR spectroscopy, Zn-F3, as well as Cd-F3, exists as a conformational equilibrium that is not susceptible to structural analysis by NMR methods. To restrict the observed conformational flexibility, a mutant F3 (mF3), which differs from F3 in the number and type of amino acids between the cysteine and the histidine ligands, was synthesized. The affinity of Zn-mF3 for the C-block DNA was greatly reduced relative to Zn-F3. Nevertheless, the metal ion dissociation constants of the Zn- and Cd-mF3 complexes remain similar to those of the native structures at 4.5 x 10(-9) and 3.2 x 10(-8) M, respectively. Zn-mF3 is more thermally stable than Cd-mF3, but both adopt similar conformations according to two-dimensional (1)H NMR spectroscopy. Each peptide displays a betabetaalpha fold for its backbone that is typical of this class of zinc finger domains. The(113)Cd ion in (113)Cd-mF3 is coupled to the protons of two cysteine and two histidine residues and characterized by a chemical shift of 567 ppm.

Co(II) and Cd(II) Substitute for Zn(II) in the Zinc Finger Derived from the DNA Repair Protein XPA, Demonstrating a Variety of Potential Mechanisms of Toxicity

XPA is one of the key members of the protein complex of the nucleotide excision repair (NER) pathway of DNA repair. The CCCC zinc finger domain of XPA is involved in the interactions with other NER proteins. To study the possible molecular mechanisms of XPA inhibition, we previously investigated Zn(II) and Ni(II) interactions with the synthetic 37 amino acid peptide (XPAzf), AcDYVICEECGKEFMDSYLMNHFDLPTCDNCRDADDKHKam, representing the XPA zinc finger sequence (Bal, W., Schwerdtle, T., and Hartwig, A. (2003) Mechanism of nickel assault on the zinc finger of DNA repair protein XPA. Chem. Res. Toxicol. 16, 242-248). In this work, we extended these studies on other carcinogenic metal ions, Co(II) and Cd(II). The binding constants and complex geometries were determined using UV-vis and CD spectroscopies, and oxidative damage to XPAzf was studied with HPLC. The conditional binding constants determined for Co(II) and Cd(II) in 50 mM phosphate buffer, pH 7.4, are 10(7.4)+/-(0.4) and 10(12.8)+/-(0.5), respectively, yielding binding constant ratios Zn(II)/Co(II) of 100 and Zn(II)/ Cd(II) of 0.001, which are the lowest values reported for zinc fingers so far. The Co(II) ion forms a tetrahedral complex with the sulfurs of XPAzf, which is isostructural with the native zinc finger. The Cd(II) complex is somewhat less structured. The oxidation of Zn(II)-saturated XPAzf by H2O2 is accelerated in the presence of Co(II), but the concentration profile of this effect indicates the formation of an active Co(II) complex external to the metal-sulfur center. The Cd(II)-saturated XPAzf is very resistant to oxidation by H2O2. Overall, our results indicate that XPAzf can undergo Co(II) and Cd(II) assault under specific conditions.

Interprotein metal exchange between transcription factor IIIa and apo-metallothionein

Zn(2+) and Cd(2+) ion exchange between transcription factor IIIA (TFIIIA) and apo-metallothionein (MT) were studied using a combination of methods including chromatography, ultrafiltration and UV spectroscopy. Under near stoichiometric conditions, apoMT was able to remove most if not all of the zinc ions from TFIIIA, whether or not the TFIIIA was bound to the 5S DNA internal control region (ICR), and concomitantly inhibit its DNA-binding activity as indicated by an electrophoretic mobility shift assay. The kinetics of the two processes were similar. The rate of the metal exchange reaction increased with the concentrations of both reactants. A second-order rate constant of 30+/-10 M(-1)s(-1) was calculated. Similar observations were made for the reaction between apoMT and Cd-substituted TFIIIA, which proceeded without observable intermediates according to a spectrophotometric analysis. A very slow metal ion exchange occurred between Cd-TFIIIA and Zn-MT, but not between Cd-MT and Zn-TFIIIA. Comparative studies on the reaction of TFIIIA with a small competing ligand, ethylenedinitrilo-tetraacetic acid (EDTA), were also conducted. Although EDTA reacts with free Zn-TFIIIA, under similar conditions it failed to compete for Zn(2+) bound as Zn-TFIIIA-ICR.

Mechanism of nickel assault on the zinc finger of DNA repair protein XPA

Xeroderma pigmentosum group A complementing protein (XPA) is a member of the protein complex of the nucleotide excision repair (NER) pathway of DNA repair, participating in the assembly of the incision complex. The 4S zinc finger domain of XPA is involved the interactions with other NER proteins. As demonstrated previously, the activity of XPA is compromised by several metal ions implicated in DNA repair inhibition, including Ni(II), Cd(II), and Co(II) (Asmuss, M., Mullenders, L. H. F., Elker, A., and Hartwig, A. (2000) Carcinogenesis 21, 2097-2104). To study the possible molecular mechanisms of XPA inhibition, we investigated Zn(II) and Ni(II) interactions with the synthetic 37 peptide (XPAzf), representing the XPA zinc finger sequence AcDYVICEECGKEFMDSYLMNHFDLPTCDNCRDADDKHKam. The binding constants were determined using fluorescence and UV-vis spectroscopies, structural insights were provided by CD, and oxidative damage to XPAzf was studied with HPLC. The binding constants for Zn(II) and Ni(II) are (8.5 +/- 1.5) x 10(8) (log value 8.93(7)) and (1.05 +/- 0.07) x 10(6) M(-)(1) (6.02(3)), respectively, in 10 mM phosphate buffer, pH 7.4, and (6 +/- 4) x 10(9) (9.8(2)) and (2.9 +/- 0.5) x 10(6) M(-)(1) (6.46(8)) in 50 mM phosphate buffer, pH 7.4, yielding binding constant ratios Zn(II)/Ni(II) of 800 +/- 100 and 2300 +/- 500, respectively. The Ni(II) ion forms a square planar complex with the sulfurs of XPAzf, opposed to the tetrahedral structure of the native Zn(II) complex. Consequently, the overall zinc finger structure is lost in the Ni(II)-substituted peptide. Zn(II)-saturated XPAzf is remarkably resistant to air oxidation and is only slowly oxidized by 0.01 mM, 0.1 mM, and 1 mM H(2)O(2) in a concentration-dependent fashion. However, the presence of just 10-fold molar excess of Ni(II) is sufficient to accelerate this process for all three H(2)O(2) concentrations tested. Overall, our results indicate that XPAzf can undergo Ni(II) assault in specific conditions.

Zinc finger proteins as potential targets for toxic metal ions: differential effects on structure and function

Zinc finger structures are frequently found in transcription factors and DNA repair proteins, mediating DNA-protein and protein-protein binding. As low concentrations of transition metal compounds, including those of cadmium, nickel, and cobalt, have been shown to interfere with DNA transcription and repair, several studies have been conducted to elucidate potential interactions of toxic metal ions with zinc-binding protein domains. Various effects have been identified, including the displacement of zinc, e.g., by cadmium or cobalt, the formation of mixed complexes, incomplete coordination of toxic metal ions, as well as the oxidation of cysteine residues within the metal-binding domain. Besides the number of cysteine and/or histidine ligands, unique structural features of the respective protein under investigation determine whether or not zinc finger structures are disrupted by one or more transition metals. As improper folding of zinc finger domains is mostly associated with the loss of correct protein function, disruption of zinc finger structures may result in interference with manifold cellular processes involved in gene expression, growth regulation, and maintenance of the genomic integrity.

Inhibition of rainbow trout (Oncorhynchus mykiss) estrogen receptor activity by cadmium

This study was conducted to determine if the cadmium-mediated inhibition of vitellogenesis observed in fish collected from contaminated areas or undergoing experimental exposure to cadmium correlated with modification in the transcriptional activity of the estrogen receptor. A recombinant yeast system expressing rainbow trout (Oncorhynchus mykiss) estradiol receptor or human estradiol receptor was used to evaluate the direct effect of cadmium exposure on estradiol receptor transcriptional activity. In recombinant yeast, cadmium reduced the estradiol-stimulated transcription of an estrogen-responsive reporter gene. In vitro-binding assays indicated that cadmium did not affect ligand binding to the receptor. Yeast one- and two-hybrid assays showed that estradiol-induced conformational changes and receptor dimerization were not affected by cadmium; conversely, DNA binding of the estradiol receptor to its cognate element was dramatically reduced in gel retardation assay. This study provides mechanistic data supporting the idea that cadmium is an important endocrine disrupter through a direct effect on estradiol receptor transcriptional activity and may affect a number of estrogen signaling pathways.

Cadmium and lead interactions with transcription factor IIIA from Xenopus laevis: a model for zinc finger protein reactions with toxic metal ions and metallothionein

Zinc finger proteins comprise the largest class of eukaryotic transcription factors. The metal binding sites in these proteins have been proposed as plausible targets for exchange reactions between zinc and toxic metal ions that lead to the alteration of function of the proteins in gene transcription. According to the present work, both Cd2+ and Pb2+ displace Zn2+ from transcription factor IIIA (TFIIIA). Neither product binds to the internal control region (ICR) of the 5 S rRNA gene, the normal binding site for Zn-TFIIIA. Furthermore, the adduct of Zn-TFIIIA with ICR is also reactive with Cd2+ and Pb2+, leading to the dissociation of the DNA-protein complex. Cd-TFIIIA reacts with apometallothionein (apoMT) to form Cd-MT and apoTFIIIA. Similarly, Cd2+ and Zn2+ can be exchanged in the reaction of Cd-TFIIIA with Zn-MT. Zn-finger 3 of TFIIIA has also been examined to compare the reactivity of a single finger motif with fingers in the holoprotein. Zn-finger 3 reacts with much faster kinetics than the holoprotein.

Diverse effects of metal chelating agents on the neuronal cytotoxicity of zinc in the hippocampus

Abnormal metabolism of metal ions such as zinc may contribute to neuropathology. Complexing zinc could reduce this pathology. Thus, to examine the effectiveness of metal chelating agents in vivo, a model system was used. This involved determining the ability of chelating agents to prevent neuronal death caused by zinc chloride injected into the rat hippocampus. Significant protection against zinc toxicity was obtained with pyrithione, inositol hexakisphosphate, ethylenediamine tetraacetate (EDTA) and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). The affinity of these agents for zinc varied between 106 M-1 and 1018 M-1. Thus, the affinity for zinc within this range does not appear to be a major factor affecting the ability of chelators to provide neuroprotection. While almost complete protection was found with EDTA and TPEN given simultaneously with zinc chloride, poor protection was obtained if TPEN was given before or after zinc chloride. Other agents either did not protect against zinc-induced neuronal death (zincon), or exacerbated zinc toxicity (BTC-5N and about 40% of rats injected with a combination of zinc chloride and diethylenetriamine pentaacetate [DTPA]). Rats showing increased damage after zinc plus BTC-5N or DTPA suffered wet dog-like shakes (WDS), suggesting that these zinc chelate complexes can induce seizures resulting in seizure-related damage. In contrast, in the 60% of rats treated with zinc chloride and DTPA that had no WDS, there was about an 80% reduction in the size of the zinc-induced lesion. The ability of chelators to cross cell membranes was examined by determining whether Timm's staining for vesicular zinc was reduced following the injection of a chelator into the hippocampus. TPEN and pyrithione reduced Timm's staining for zinc. However, cell permeability was not necessary for a chelator to protect against zinc toxicity.

Zinc and Alzheimer's disease: is there a direct link?

Zinc is an essential trace element in human biology, but is neurotoxic at high concentrations. Several studies show that zinc promotes aggregations of beta-amyloid protein, the main component of the senile plaques typically found in Alzheimer's disease brains. In other neurological disorders where neurons appear to be dying by apoptosis (gene-directed cell death), chelatable zinc accumulates in the perikarya of neurons before, or during degeneration. As there is evidence for apoptotic death of neurons in Alzheimer's disease, an involvement of zinc in this process needs to be investigated. Zinc interacts with enzymes and proteins, including transcription factors, which are critical for cell survival and could be linked to apoptotic processes. While controversial, some studies indicate that total tissue zinc is markedly reduced in several brain regions of Alzheimer's patients. At face value, it seems that a paradox exists between reports of a decrease in zinc in the Alzheimer's brain and the putative link to aberrant high zinc levels promoting plaque formation. An hypothesis to explain this inconsistency is presented. Neuropathological changes mediated by endogenous or exogenous stressors may be relevant factors affecting abnormal zinc metabolism. This paper reviews current investigations that suggest a role of zinc in the etiology of Alzheimer's disease.

Reversible zinc exchange between metallothionein and the estrogen receptor zinc finger

We report here the first demonstration that reversible metal exchange occurs between metallothionein (MT) and full-length estrogen receptor (ER). Specific binding of ER to estrogen response element is inhibited in the presence of 40 microM thionein and restored by 120 microM zinc. Moreover, ER in metal-depleted nuclear extracts exhibits reduced DNA binding which can be restored by 140 microM native MT. Hence, thionein inhibits DNA binding by abstracting zinc from functional ER while native MT is capable of restoring binding to metal-depleted extracts by donating metal to ER. This indicates MT may be an important physiological regulator of intracellular zinc and/or other metals.

Effects of teratogenic exposures to Zn2+, Cd2+, Ni2+, Co2+, and Cu2+ on metallothionein and metallothionein-mRNA contents of Xenopus embryos

Xenopus laevis embryos were analyzed for metallothionein by silver-saturation assay and metallothionein-mRNA by reverse transcriptase/polymerase chain reaction following exposures to the following metal chlorides at levels that caused > 95% malformations and < 7% mortality: Zn2+ (300 microM); Cd2+ (18 microM); Ni2+ (56 microM); Co2+ (1,800 microM); and Cu2+ (5.6 microM). At the beginning of the exposure (stages 8), metallothionein-mRNA and metallothionein levels averaged 2.0 x 10(6) copies/embryo and 19 pmol/embryo, respectively. In control embryos at stages 26, 36, 42, and 46, metallothionein-mRNA content averaged 9, 37, 104, and 97 copies x 10(6)/embryo, and metallothionein content averaged 6, 11, 15, and 18 pmol/embryo. In Zn(2+) -exposed embryos at the same stages, metallothionein-mRNA content averaged 116*, 11,400*, 3,210*, and 14 copies x 10(6)/embryo and metallothionein content averaged 10, 18*, 46*, and 90* pmol/embryo; in Cd(2+)-exposed embryos, metallothionein-mRNA content averaged 22, 7,170*, 1,783*, and 240 copies x 10(6)/embryo and metallothionein content averaged 8, 14, 33*, and 56* pmol/embryo, respectively (*P < 0.05 versus controls). Exposure-response curves (Cd2+, 1-18 microM; Zn2+, 3-300 microM) indicated that Cd2+ was 3- to 5-times more potent than Zn2+, based on metallothionein-mRNA response at stage 36 and metallothionein response at stage 46. In Ni(2+)-, Co(2+)-, or Cu(2+)-exposed embryos, metallothionein-mRNA and metallothionein contents did not differ significantly from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

The 40 kDa 63Ni(2+)-binding protein (pNiXc) on western blots of Xenopus laevis oocytes and embryos is the monomer of fructose-1,6-bisphosphate aldolase A

A Ni(2+)-binding protein (pNiXc, 40 kDa), present in Xenopus laevis oocytes and embryos, was isolated from mature oocytes by chromatography on DEAE-cellulose and cellulose phosphate, followed by FPLC on Ni-iminodiacetate-Agarose, or reverse-phase HPLC on a C-4 column. Size-exclusion HPLC showed that intact pNiXc is approximately 155 kDa, consistent with tetrameric structure. After cleavage with Lys-C proteinase or cyanogen bromide, six peptides were separated by HPLC and sequenced by Edman degradation, providing sequence data for 83 residues. Data-base search showed similarity of pNiXc to eukaryotic aldolases, with 96% identity to human aldolase A. pNiXc demonstrated aldolase activity with fructose 1,6-bisphosphate as substrate (Km, 30 microM Vmax 26 mumol min-1 mg-1); the aldolase activity was inhibited non-competitively by Cu2+, Cd2+, Co2+, or Ni2+. Equilibrium dialysis showed high affinity binding (Kd, 7 microM) of 1 mole of Ni per mole of 40 kDa subunit. Based on metal-blot competition assays, the abilities of metals to compete with 63Ni2+ for binding to pNiXc were ranked: Cu2+ >> Zn2+ > Cd2+ > Co2+. This study identifies pNiXc as the monomer of fructose-1,6-bisphosphate aldolase A, and raises the possibility that aldolase A is a target enzyme for metal toxicity.