The affinity of human erythrocyte porphobilinogen synthase for Zn2+ and Pb2+

Single-Cell Study Unveils Lead Lifespan in Blood Cell Populations Follows a Universal Lognormal Distribution with Individual Skewness

Lead is a widespread environmental hazard that can adversely affect multiple biological functions. Blood cells are the initial targets that face lead exposure. However, a systematic assessment of lead dynamics in blood cells at single-cell resolution is still absent. Herein, C57BL/6 mice were fed with lead-contaminated food. Peripheral blood was harvested at different days. Extracted red blood cells and leukocytes were stained with 19 metal-conjugated antibodies and analyzed by mass cytometry. We quantified the time-lapse lead levels in 12 major blood cell subpopulations and established the distribution of lead heterogeneity. Our results show that the lead levels in all major blood cell subtypes follow lognormal distributions but with distinctively individual skewness. The lognormal distribution suggests a multiplicative accumulation of lead with stochastic turnover of cells, which allows us to estimate the lead lifespan of different blood cell populations by calculating the distribution skewness. These findings suggest that lead accumulation by single blood cells follows a stochastic multiplicative process.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Cd(II)- and Pb(II)-Induced Self-Assembly of Peripheral Membrane Domains from Protein Kinase C

Cd²⁺ and Pb²⁺ are xenobiotic heavy metal ions that use ionic mimicry to interfere with the cellular function of biomacromolecules. Using a combination of SAXS, electron microscopy, FRET, and solution NMR spectroscopy, we demonstrate that treatment with Cd²⁺ and Pb²⁺ causes self-assembly of protein kinase C regulatory domains that peripherally associate with membranes. The self-assembly process successfully competes with ionic mimicry and is mediated by conserved protein regions that are distinct from the canonical Ca²⁺-binding motifs of protein kinase C. The ability of protein oligomers to interact with anionic membranes is enhanced compared to the monomeric species. Our findings suggest that metal-ion-dependent peripheral membrane domains can be utilized for generating protein-metal-ion nanoclusters and serve as biotemplates for the design of sequestration agents.

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.

High affinity interactions of Pb2+ with synaptotagmin I

Lead (Pb) is a potent neurotoxin that disrupts synaptic neurotransmission. We report that Synaptotagmin I (SytI), a key regulator of Ca2+-evoked neurotransmitter release, has two high-affinity Pb2+ binding sites that belong to its cytosolic C2A and C2B domains. The crystal structures of Pb2+-complexed C2 domains revealed that protein-bound Pb2+ ions have holodirected coordination geometries and all-oxygen coordination spheres. The on-rate constants of Pb2+ binding to the C2 domains of SytI are comparable to those of Ca2+ and are diffusion-limited. In contrast, the off-rate constants are at least two orders of magnitude smaller, indicating that Pb2+ can serve as both a thermodynamic and kinetic trap for the C2 domains. We demonstrate, using NMR spectroscopy, that population of these sites by Pb2+ ions inhibits further Ca2+ binding despite the existing coordination vacancies. Our work offers a unique insight into the bioinorganic chemistry of Pb(ii) and suggests a mechanism by which low concentrations of Pb2+ ions can interfere with the Ca2+-dependent function of SytI in the cell.

Lead(II) Binding in Natural and Artificial Proteins

This article describes recent attempts to understand the biological chemistry of lead using a synthetic biology approach. Lead binds to a variety of different biomolecules ranging from enzymes to regulatory and signaling proteins to bone matrix. We have focused on the interactions of this element in thiolate-rich sites that are found in metalloregulatory proteins such as Pbr, Znt, and CadC and in enzymes such as δ-aminolevulinic acid dehydratase (ALAD). In these proteins, Pb(II) is often found as a homoleptic and hemidirectic Pb(II)(SR)3- complex. Using first principles of biophysics, we have developed relatively short peptides that can associate into three-stranded coiled coils (3SCCs), in which a cysteine group is incorporated into the hydrophobic core to generate a (cysteine)3 binding site. We describe how lead may be sequestered into these sites, the characteristic spectral features may be observed for such systems and we provide crystallographic insight on metal binding. The Pb(II)(SR)3- that is revealed within these α-helical assemblies forms a trigonal pyramidal structure (having an endo orientation) with distinct conformations than are also found in natural proteins (having an exo conformation). This structural insight, combined with 207Pb NMR spectroscopy, suggests that while Pb(II) prefers hemidirected Pb(II)(SR)3- scaffolds regardless of the protein fold, the way this is achieved within α-helical systems is different than in β-sheet or loop regions of proteins. These interactions between metal coordination preference and protein structural preference undoubtedly are exploited in natural systems to allow for protein conformation changes that define function. Thus, using a design approach that separates the numerous factors that lead to stable natural proteins allows us to extract fundamental concepts on how metals behave in biological systems.

Structural studies of substrate and product complexes of 5-aminolaevulinic acid dehydratase from humans,Escherichia coliand the hyperthermophilePyrobaculum calidifontis

A number of X-ray analyses of an enzyme involved in a key early stage of tetrapyrrole biosynthesis are reported. Two structures of human 5-aminolaevulinate dehydratase (ALAD), native and recombinant, have been determined at 2.8 Å resolution, showing that the enzyme adopts an octameric quaternary structure in accord with previously published analyses of the enzyme from a range of other species. However, this is in contrast to the finding that a disease-related F12L mutant of the human enzyme uniquely forms hexamers [Breiniget al.(2003),Nature Struct. Biol.10, 757–763]. Monomers of all ALADs adopt the TIM-barrel fold; the subunit conformation that assembles into the octamer includes the N-terminal tail of one monomer curled around the (α/β)8barrel of a neighbouring monomer. Both crystal forms of the human enzyme possess two monomers per asymmetric unit, termedAandB. In the native enzyme there are a number of distinct structural differences between theAandBmonomers, with the latter exhibiting greater disorder in a number of loop regions and in the active site. In contrast, the second monomer of the recombinant enzyme appears to be better defined and the active site of both monomers clearly possesses a zinc ion which is bound by three conserved cysteine residues. In native human ALAD, theAmonomer also has a ligand resembling the substrate ALA which is covalently bound by a Schiff base to one of the active-site lysines (Lys252) and is held in place by an ordered active-site loop. In contrast, these features of the active-site structure are disordered or absent in theBsubunit of the native human enzyme. The octameric structure of the zinc-dependent ALAD from the hyperthermophilePyrobaculum calidifontisis also reported at a somewhat lower resolution of 3.5 Å. Finally, the details are presented of a high-resolution structure of theEscherichia coliALAD enzyme co-crystallized with a noncovalently bound moiety of the product, porphobilinogen (PBG). This structure reveals that the pyrrole side-chain amino group is datively bound to the active-site zinc ion and that the PBG carboxylates interact with the enzymeviahydrogen bonds and salt bridges with invariant residues. A number of hydrogen-bond interactions that were previously observed in the structure of yeast ALAD with a cyclic intermediate resembling the product PBG appear to be weaker in the new structure, suggesting that these interactions are only optimal in the transition state.

Similarities between N-Acetylcysteine and Glutathione in Binding to Lead(II) Ions

N-Acetylcysteine is a natural thiol-containing antioxidant, a precursor for cysteine and glutathione, and a potential detoxifying agent for heavy metal ions. However, previous accounts of the efficiency of N-acetylcysteine (H2NAC) in excretion of lead are few and contradicting. Here, we report results on the nature of lead(II) complexes formed with N-acetylcysteine in aqueous solution, which were obtained by combining information from several spectroscopic methods, including (207)Pb, (13)C, and (1)H NMR, Pb LIII-edge X-ray absorption, ultraviolet-visible (UV-vis) spectroscopy, and electro-spray ionization mass spectrometry (ESI-MS). Two series of solutions were used containing CPb(II) = 10 and 100 mM, respectively, varying the H2NAC/Pb(II) mole ratios from 2.1 to 10.0 at pH 9.1-9.4. The coordination environments obtained resemble those previously found for the Pb(II) glutathione system: at a ligand-to-lead mole ratio of 2.1, dimeric or oligomeric Pb(II) N-acetylcysteine complexes are formed, while a trithiolate [Pb(NAC)3](4-) complex dominates in solutions with H2NAC/Pb(II) mole ratios >3.0.

Cross-species coherence in effects and modes of action in support of causality determinations in the U.S. Environmental Protection Agency's Integrated Science Assessment for Lead

The peer-reviewed literature on the health and ecological effects of lead (Pb) indicates common effects and underlying modes of action across multiple organisms for several endpoints. Based on such observations, the United States (U.S.) Environmental Protection Agency (EPA) applied a cross-species approach in the 2013 Integrated Science Assessment (ISA) for Lead for evaluating the causality of relationships between Pb exposure and specific endpoints that are shared by humans, laboratory animals, and ecological receptors (i.e., hematological effects, reproductive and developmental effects, and nervous system effects). Other effects of Pb (i.e., cardiovascular, renal, and inflammatory responses) are less commonly assessed in aquatic and terrestrial wildlife limiting the application of cross-species comparisons. Determinations of causality in ISAs are guided by a framework for classifying the weight of evidence across scientific disciplines and across related effects by considering aspects such as biological plausibility and coherence. As illustrated for effects of Pb where evidence across species exists, the integration of coherent effects and common underlying modes of action can serve as a means to substantiate conclusions regarding the causal nature of the health and ecological effects of environmental toxicants.

Lead(II) binding to the chelating agent D-penicillamine in aqueous solution

A spectroscopic investigation of the complexes formed between the Pb(II) ion and D-penicillamine (H2Pen), a chelating agent used in the treatment of lead poisoning, was carried out on two sets of alkaline aqueous solutions with CPb(II) ≈ 10 and 100 mM, varying the H2Pen/Pb(II) molar ratio (2.0, 3.0, 4.0, 10.0). Ultraviolet-visible (UV-vis) spectra of the 10 mM Pb(II) solutions consistently showed an absorption peak at 298 nm for S(-) → Pb(II) ligand-to-metal charge-transfer. The downfield (13)C NMR chemical shift for the penicillamine COO(-) group confirmed Pb(II) coordination. The (207)Pb NMR chemical shifts were confined to a narrow range between 1806 ppm and 1873 ppm for all Pb(II)-penicillamine solutions, indicating only small variations in the speciation, even in large penicillamine excess. Those chemical shifts are considerably deshielded, relative to the solid-state (207)Pb NMR isotropic chemical shift of 909 ppm obtained for crystalline penicillaminatolead(II) with Pb(S,N,O-Pen) coordination. The Pb LIII-edge extended X-ray absorption fine structure (EXAFS) spectra obtained for these solutions were well-modeled with two Pb-S and two Pb-(N/O) bonds with mean distances 2.64 ± 0.04 Å and 2.45 ± 0.04 Å, respectively. The combined spectroscopic results, reporting δ((207)Pb) ≈ 1870 ppm and λmax ≈ 298 nm for a Pb(II)S2NO site, are consistent with a dominating 1:2 lead(II):penicillamine complex with [Pb(S,N,O-Pen)(S-HnPen)](2-n) (n = 0-1) coordination in alkaline solutions, and provide useful structural information on how penicillamine can function as an antidote against lead toxicity in vivo.

Curcumin as a protector for normal cells during bortezomib therapy

This study evaluated the protective potential of curcumin on the possible side effects of bortezomib (Bt) therapy on normal cells in mice. The mice were segregated into three groups ( n = 10) that included normal control, Bt-treated, and Bt + curcumin-treated groups. The Bt treatment resulted in significant decrease in the enzyme activity of erythrocyte δ-aminolevulinic acid dehydratase (ALAD). Also a significant decrease in the hemoglobin (Hb) was also noticed. On the other hand, curcumin co-treatment improvised enzyme activity of erythrocyte ALAD as well as Hb values. The study, therefore, concludes that curcumin co-treatment with Bt has a potential to take care of possible side effects of Bt therapy on normal cells.

pH-dependent coordination of Pb2+ to metallothionein2: structures and insight into lead detoxification

Lead is a toxic heavy metal whose detoxification in organisms is mainly carried out by its coordination with some metalloproteins such as metallothioneins (MTs). Two Pb-MT complexes, named as Pb7-MT2(I) and Pb7-MT2(II), form under neutral and weakly acidic conditions, respectively. However, the structures of the two complexes, which are crucial for a better understanding of the detoxification mechanism of Pb-MTs, have not been clearly elucidated. In this Work, coordination of Pb(2+) with rabbit liver apo-MT2, as well as with the two individual domains (apo-αMT2 and apo-βMT2) at different pH, were studied by combined spectroscopic (UV-visible, circular dichroism, and NMR) and computational methods. The results showed that in Pb7-MT2(I) the Pb(2+) coordination is in the trigonal pyramidal Pb-S3 mode, whereas the Pb7-MT2(II) complex contains mixed trigonal pyramidal Pb-S3, distorted trigonal pyramidal Pb-S2O1, and distorted quadrilateral pyramidal Pb-S3O1 modes. The O-donor ligand in Pb7-MT2(II) was identified as the carboxyl groups of the aspartic acid residues at positions 2 and 56. Our studies also revealed that Pb7-MT2(II) has a greater acid tolerance and coordination stability than Pb7-MT2(I), thereby retaining the Pb(2+) coordination at acidic pH. The higher flexibility of Pb7-MT2(II) renders it more accessible to lysosomal proteolysis than Pb7-MT2(I). Similar spectral features were observed in the coordination of Pb(2+) by human apo-MT2, suggesting a commonality among mammalian MT2s in the Pb(2+) coordination chemistry.

Lead(II) complex formation with glutathione

A structural investigation of complexes formed between the Pb(2+) ion and glutathione (GSH, denoted AH(3) in its triprotonated form), the most abundant nonprotein thiol in biological systems, was carried out for a series of aqueous solutions at pH 8.5 and C(Pb(2+)) = 10 mM and in the solid state. The Pb L(III)-edge extended X-ray absorption fine structure (EXAFS) oscillation for a solid compound with the empirical formula [Pb(AH(2))]ClO(4) was modeled with one Pb-S and two short Pb-O bond distances at 2.64 ± 0.04 and 2.28 ± 0.04 Å, respectively. In addition, Pb···Pb interactions at 4.15 ± 0.05 Å indicate dimeric species in a network where the thiolate group forms an asymmetrical bridge between two Pb(2+) ions. In aqueous solution at the mole ratio GSH/Pb(II) = 2.0 (C(Pb(2+)) = 10 mM, pH 8.5), lead(II) complexes with two thiolate ligands form, characterized by a ligand-to-metal charge-transfer band (LMCT) S(-) → Pb(2+) at 317 nm in the UV-vis spectrum and mean Pb-S and Pb-(N/O) bond distances of 2.65 ± 0.04 and 2.51 ± 0.04 Å, respectively, from a Pb L(III)-edge EXAFS spectrum. For solutions with higher mole ratios, GSH/Pb(II) ≥ 3.0, electrospray ionization mass spectroscopy spectra identified a triglutathionyllead(II) complex, for which Pb L(III)-edge EXAFS spectroscopy shows a mean Pb-S distance of 2.65 ± 0.04 Å in PbS(3) coordination, (207)Pb NMR spectroscopy displays a chemical shift of 2793 ppm, and in the UV-vis spectrum, an S(-) → Pb(2+) LMCT band appears at 335 nm. The complex persists at high excess of GSH and also at ∼25 K in frozen glycerol (33%)/water glasses for GSH/Pb(II) mole ratios from 4.0 to 10 (C(Pb(2+)) = 10 mM) measured by Pb L(III)-edge EXAFS spectroscopy.

The importance of stereochemically active lone pairs for influencing Pb(II) and As(III) protein binding

The toxicity of heavy metals, which is associated with the high affinity of the metals for thiolate rich proteins, constitutes a problem worldwide. However, despite this tremendous toxicity concern, the binding mode of As(III) and Pb(II) to proteins is poorly understood. To clarify the requirements for toxic metal binding to metalloregulatory sensor proteins such as As(III) in ArsR/ArsD and Pb(II) in PbrR or replacing Zn(II) in δ-aminolevulinc acid dehydratase (ALAD), we have employed computational and experimental methods examining the binding of these heavy metals to designed peptide models. The computational results show that the mode of coordination of As(III) and Pb(II) is greatly influenced by the steric bulk within the second coordination environment of the metal. The proposed basis of this selectivity is the large size of the ion and, most important, the influence of the stereochemically active lone pair in hemidirected complexes of the metal ion as being crucial. The experimental data show that switching a bulky leucine layer above the metal binding site by a smaller alanine residue enhances the Pb(II)  binding affinity by a factor of five, thus supporting experimentally the hypothesis of lone pair steric hindrance. These complementary approaches demonstrate the potential importance of a stereochemically active lone pair as a metal recognition mode in proteins and, specifically, how the second coordination sphere environment affects the affinity and selectivity of protein targets by certain toxic ions.

Quantum-mechanical study of lead coordination in sulfur-rich proteins: mode and structure recognition in UV resonance Raman spectra

Resonance Raman spectra are computed applying the weighted gradient methodology with CIS and DFT gradients to determine the characteristic spectral patterns for Hg(II) and Pb(II) loaded sulfur-rich proteins while excited to a characteristic LMCT electronic transition band. A framework of structure-spectrum relationships is established to assess lead coordination modes via vibrational spectroscopy. Illustrative calculations on Hg(II) complexes agree with experimental data demonstrating reliability and accuracy of the applied methodology. In contrast to Hg(II) complexes, a unique 3-center-4-electron hypervalent C(β)H···S interaction present in lead-sulfur complexes was established and suggested to play a key role in the strong preference for lead versus other metal ions in lead specific proteins such as PbrR691. The characteristic Pb-S symmetric stretching bands, predicted without additional refinements such as scaling of a force field or frequencies, are found around 238 cm(-1) for 3-coordinated lead-sulfur domains and around 228 cm(-1) for 4-coordinated lead-sulfur domains. These results present an experimental challenge for clear detection of lead coordination via solely UVRR spectroscopy. In addition to predicted UVRR spectra, UVRR excitation profiles for relevant vibrational bands of lead-sulfur domains are presented.

Pb-207 NMR spectroscopy reveals that Pb(II) coordinates with glutathione (GSH) and tris cysteine zinc finger proteins in a PbS3 coordination environment

207Pb NMR spectroscopy can be used to monitor the binding of Pb(II) to thiol rich biological small molecules such as glutathione and to zinc finger proteins. The UV/visible (UV/Vis) absorption band centered at 334 nM and the observed 207Pb-signal in 207Pb NMR (δ~5750 ppm) indicate that glutathione binds Pb(II) in a trigonal pyramidal geometry (PbS3) at pH 7.5 or higher with a 1:3 molar ratio of Pb(II) to GSH. While previous studies using UV/Vis and extended X-ray absorption fine structure (EXAFS) spectroscopy were interpreted to show that the zinc binding domain from HIV nucleocapsid protein (HIV-CCHC) binds Pb(II) in a single PbS3 environment, the more sensitive 207Pb NMR spectra (at pH 7.0, 1:1 molar ratio) provide compelling evidence for the presence of two PbS3 structures (δ=5790 and 5744 ppm), one of which is more stable at high temperatures. It has previously been proposed that the HIV-CCHH peptide does not fold properly to afford a PbS2N motif, because histidine does not bind to Pb(II). These predictions are confirmed by the present studies. These results demonstrate the applicability of 207Pb NMR to biomolecular structure determination in proteins with cysteine binding sites for the first time.

Zinc finger proteins as templates for metal ion exchange and ligand reactivity. Chemical and biological consequences

Zinc finger reactions with inorganic ions and coordination compounds are as diverse as the zinc fingers themselves. Use of metal ions such as Co(2+) and Cd(2+) has given structural, thermodynamic and kinetic information on zinc fingers and zinc-finger-DNA/RNA interactions. It is a general truism that alteration of the coordination sphere in the finger environment will disrupt the recognition with DNA/RNA and this has implications for mechanism of toxicity and carcinogenesis of metal ions. Structural zinc fingers are susceptible to electrophilic attack and the recognition that the coordination sphere of inorganic compounds may be modulated for control of electrophilic attack on zinc fingers raises the possibility of systematic studies of zinc fingers as drug targets using inorganic chemistry. Some inorganic compounds such as those of As(III) and Au(I) may exert their biological effects through inactivation of zinc fingers and novel approaches to specifically attack the zinc-bound ligands using Co(III)-Schiff bases and Platinum(II)-Nucleobase compounds have been proposed. The genomic importance of zinc fingers suggests that the "coordination chemistry" of zinc fingers themselves is ripe for exploration to design new targets for medicinal inorganic chemistry.

Structural changes of human serum albumin in response to a low concentration of heavy ions

Lead ions in solution interact strongly with human serum albumin and modify the properties and function of albumin molecules. In the present study, we used optical spectroscopic techniques to explore the binding sites of lead, present in albumin. Structural and chemical analysis of albumin molecules using fluorescence and Raman spectroscopy, predicted the modification of two major amino acids in albumin due to lead binding. No secondary structural changes are observed in the protein molecule, which is further confirmed using circular dichroism absorption measurements. The results indicate that loss of charge from the binding site of albumin by the charged lead ions, give rise to dipole interaction which acts as the major contributor to promote protein agglomeration.

Interactions of diorganolead(IV) with 3-(2-thienyl)-2-sulfanylpropenoic acid and/or thiamine: chemical and in vitro and in vivo toxicological results

The reactions of PbR(2)(OAc)(2) (R = Me, Ph) with 3-(2-thienyl)-2-sulfanylpropenoic acid (H(2)tspa) in methanol or ethanol afforded complexes [PbR(2)(tspa)] that electrospray ionization-mass spectrometry (ESI-MS) and IR data suggest are polymeric. X-ray studies showed that [PbPh(2)(tspa)(dmso)] x dmso, crystallized from a solution of [PbPh(2)(tspa)] in dmso, is dimeric, and that [HQ](2)[PbPh(2)(tspa)(2)] (Q = diisopropylamine), obtained after removal of [PbPh(2)(tspa)] from a reaction including Q, contains the monomeric anion [PbPh(2)(tspa)(2)](2-). In the solid state the lead atoms are O,S-chelated by the tspa(2-) ligands in all these products, and in the latter two have distorted octahedral coordination environments. NMR data suggest that tspa(2-) remains coordinated to PbR(2)(2+) in solution in dmso. Neither thiamine nor thiamine diphosphate reacted with PbMe(2)(NO(3))(2) in D(2)O. Prior addition of H(2)tspa protected LLC-PK1 renal proximal tubule cells against PbMe(2)(NO(3))(2); thiamine had no statistically significant effect by itself, but greatly potentiated the action of H(2)tspa. Administration of either H(2)tspa or thiamine to male albino Sprague-Dawley rats dosed 30 min previously with PbMe(2)(NO(3))(2) was associated with reduced inhibition of delta-ALAD by the organolead compound, and with lower lead levels in kidney and brain, but joint administration of both H(2)tspa and thiamine only lowered lead concentration in the kidney.

Zinc improves antioxidative enzymes in red blood cells and hematology in lithium-treated rats

The present study was designed to evaluate the protective role of zinc in attenuating the adverse effects induced by lithium in blood of female Wistar rats. Female Wistar rats received lithium in the form of lithium carbonate in diet at a dose level of 1.1 g/kg diet, zinc alone in the form of zinc sulfate in drinking water at a dose level of 227 mg/L drinking water, or lithium plus zinc treatments in the combined group for a total duration of 2 months. Effects of the treatments were studied on antioxidant defense system, various hematologic parameters, and percentage of (65)Zn-specific activity. Lithium treatment resulted in a significant increase in lipid peroxidation levels but caused a significant decrease in reduced glutathione levels and the activities of catalase, glutathione S-transferase, and superoxide dismutase. Lithium treatment also caused a significant decrease in the activities of aminolevulinic acid dehydratase and Na(+) K(+) adenosine triphosphatase. However, it resulted in a significant increase in total leukocyte counts, neutrophils, and lymphocyte counts as well as zinc protoporphyrin levels, whereas a significant decrease in counts of monocytes, eosinophils, and percentage specific activity of (65)Zn in blood and its various fractions was noticed. Furthermore, lithium treatment caused a significant decrease in serum zinc levels. However, zinc supplementation to lithium-treated rats effectively raised the reduced glutathione levels and also normalized lipid peroxidation and the activities of antioxidative enzymes, which included catalase, glutathione S-transferase, and superoxide dismutase. Moreover, zinc supplementation could raise the activities of the enzymes aminolevulinic acid dehydratase and Na(+) K(+) adenosine triphosphatase as well as the percentage uptake values of (65)Zn in blood and its fractions. The study suggests that zinc, as a nutritional supplement, has the potential in attenuating most of the adverse effects induced by lithium in rat blood.

131I induced hematological alterations in rat blood: protection by zinc

The present study was planned to determine the potential of zinc in attenuating the toxicity induced by 131I in rat blood. Female wistar rats were segregated into four main groups. Animals in Group I served as normal controls; Group II animals were administered a dose of 3.7 Mbq of 131I (carrier free) intraperitoneally, Group III was supplemented with Zinc in the form of ZnSo4.7H2O (227 mg/l drinking water), and Group IV was given a combined treatment of Zinc as well as 131I, in a similar way as was given to Groups IV and II animals, respectively. The effects of different treatments were studied on various parameters in rat blood including hemoglobin (Hb) levels, % hematocrit, zinc protoporphyrins (ZPP), activities of enzymes which included aminolevulinic acid dehydratase (delta-ALAD) and Na+ K+ ATPase and uptake of 65Zn in blood. The study revealed an increase in the levels of hemoglobin, % hematocrit, activities of delta-ALAD, Na+ K+ ATPase and uptake of 65Zn, 7 days after the 131I treatment. On the contrary, the levels of ZPP were found to be significantly decreased after 131I treatment. However, zinc treatment to 131I-treated animals significantly attenuated the various biochemical and hematological indices. Moreover, zinc treatment to the 131I-treated animals could significantly decrease the uptake of 65Zn, which was increased after 131I treatment. Based upon these data, the present study suggests that zinc has the potential to attenuate 131I induced toxicity by restoring the altered hematological indices and biochemical changes.

The effects of lead and cadmium on GATA-1 regulated erythroid gene expression

Lead (Pb) and cadmium (Cd) are heavy metal toxins that cause many pathophysiologic effects, including anemia. Previous in vitro studies have shown that these metals are able to replace coordinated Zinc (Zn) atoms in the Zn fingers of transcription factors and that this can alter the structure and DNA-binding characteristics of these proteins. This has lead to the hypothesis that one mechanism underlying the toxic effects of Pb and Cd is their ability to alter Zn finger transcription factor function resulting in aberrant target gene expression. A recent report that Pb is able to replace Zn in the Zn fingers of the hematopoietic transcription factor GATA-1 prompted us to address this hypothesis in the setting of MEL cell differentiation. If Pb or Cd is able to inhibit GATA-1 function, this should be detectable through alterations in chemically induced erythroid differentiation and GATA-1-dependent gene expression. Despite a strong rationale for this hypothesis, we have found no significant change in MEL differentiation, the expression of several GATA-1 target genes, or of in vitro and in vivo GATA-1 binding to DNA at concentrations well above those associated with toxic effects in humans. These results argue against the hypothesis that Pb or Cd significantly alters GATA-1 function in vivo.

Reversal of aminolevulinic acid dehydratase (ALAD) inhibition and reduction of erythrocyte protoporphyrin levels by Vitamin C in occupational lead exposure in Abeokuta, Nigeria

In order to investigate the toxic effects of lead during occupational exposure to this metal, the activity of aminolevulinic acid dehydratase (ALAD) and the concentrations of erythrocyte protoporphyrin (EPP) were determined in blood of various artisans in Abeokuta, Nigeria and these were related to blood lead levels. ALAD activity in the artisans was inhibited to varying extents. ALAD activity was inhibited to the tune of 77% in petrol station attendants while the lowest inhibition of 36% was obtained in the welders. EPP was also significantly increased in the artisans (p<0.001). The highest EPP level of 241.57±89.27μg/100ml of red blood cell was observed in upholsterers. A significant (p<0.001) negative correlation was observed between ALAD activity and blood lead levels on one hand (r=-0.631) and between ALAD activity and EPP on the other hand (r=-0.461). Administration of a daily dose of 500mg ascorbic acid for 2 weeks reversed the lead-induced inhibition of ALAD. Increased EPP levels observed in the artisans also responded positively to the ascorbic acid supplementation. A significant reduction (p<0.001) in blood lead was also observed in the artisans at the end of the 2-week ascorbic acid therapy. Our findings indicate that ascorbic acid may offer more advantages over the conventional agents for the treatment of lead poisoning, especially in cases where the subjects cannot be removed from the source of lead exposure.

Spectroscopic and functional determination of the interaction of Pb2+ with GATA proteins

GATA proteins are transcription factors that bind GATA DNA elements through Cys4 structural zinc-binding domains and play critical regulatory roles in neurological and urogenital development and the development of cardiac disease. To evaluate GATA proteins as potential targets for lead, spectroscopically monitored metal-binding titrations were used to measure the affinity of Pb2+ for the C-terminal zinc-binding domain from chicken GATA-1 (CF) and the double-finger domain from human GATA-1 (DF). Using this method, Pb2+ coordinating to CF and DF was directly observed through the appearance of intense bands in the near-ultraviolet region of the spectrum (250-380 nm). Absorption data collected from these experiments were best fit to a 1:1 Pb2+ -CF model and a 2:1 Pb2+ -DF model. Competition experiments using Zn2+ were used to determine the absolute affinities of Pb2+ for these proteins. These studies reveal that Pb2+ forms tight complexes with cysteine residues in the zinc-binding sites in GATA proteins, beta1Pb = 6.4 (+/- 2.0) x 10(9) M(-1) for CF and beta2 = 6.3 (+/- 6.3) x 10(19) M(-2) for Pb(2+)2-DF, and within an order of magnitude of the affinity of Zn2+ for these proteins. Furthermore, Pb2+ was able to displace bound Zn2+ from CF and DF. Upon addition of Pb2+, GATA shows a decreased ability to bind to DNA and subsequently activate transcription. Therefore, the DNA binding and transcriptional activity of GATA proteins are most likely to be targeted by Pb2+ in cells and tissues that sequester Pb2+ in vivo, which include the brain and the heart.

Blood lead is a predictor of homocysteine levels in a population-based study of older adults

Lead and homocysteine are both associated with cardiovascular disease and cognitive dysfunction. We evaluated the relations among blood lead, tibia lead, and homocysteine levels by cross-sectional analysis of data among subjects in the Baltimore Memory Study, a longitudinal study of 1,140 randomly selected residents in Baltimore, Maryland, who were 50-70 years of age. Tibia lead was measured by (superscript)109(/superscript)Cd K-shell X-ray fluorescence. The subject population had a mean +/- SD age of 59.3 +/- 5.9 years and was 66.0% female, 53.9% white, and 41.4% black or African American. Mean +/- SD blood lead, tibia lead, and homocysteine levels were 3.5 +/- 2.4 microg/dL, 18.9 +/- 12.5 microg/g, and 10.0 +/- 4.1 micromol/L, respectively. In unadjusted analysis, blood lead and homocysteine were moderately correlated (Pearson's r = 0.27, p < 0.01). After adjustment for age, sex, race/ethnicity, educational level, tobacco and alcohol consumption, and body mass index using multiple linear regression, results revealed that homocysteine levels increased 0.35 micromol/L per 1.0 microg/dL increase in blood lead (p < 0.01). The relations of blood lead with homocysteine levels did not differ in subgroups distinguished by age, sex, or race/ethnicity. Tibia lead was modestly correlated with blood lead (Pearson's r = 0.12, p < 0.01) but was not associated with homocysteine levels. To our knowledge, these are the first data to reveal an association between blood lead and homocysteine. These results suggest that homocysteine could be a mechanism that underlies the effects of lead on the cardiovascular and central nervous systems, possibly offering new targets for intervention to prevent the long-term consequences of lead exposure.

X-ray structure of a putative reaction intermediate of 5-aminolaevulinic acid dehydratase

The X-ray structure of yeast 5-aminolaevulinic acid dehydratase, in which the catalytic site of the enzyme is complexed with a putative cyclic intermediate composed of both substrate moieties, has been solved at 0.16 nm (1.6 A) resolution. The cyclic intermediate is bound covalently to Lys(263) with the amino group of the aminomethyl side chain ligated to the active-site zinc ion in a position normally occupied by a catalytic hydroxide ion. The cyclic intermediate is catalytically competent, as shown by its turnover in the presence of added substrate to form porphobilinogen. The findings, combined with those of previous studies, are consistent with a catalytic mechanism in which the C-C bond linking both substrates in the intermediate is formed before the C-N bond.

The x-ray structure of yeast 5-aminolaevulinic acid dehydratase complexed with substrate and three inhibitors

The structures of 5-aminolaevulinic acid dehydratase (ALAD) complexed with substrate (5-aminolaevulinic acid) and three inhibitors: laevulinic acid, succinylacetone and 4-keto-5-aminolaevulinic acid, have been solved at high resolution. The ligands all bind by forming a covalent link with Lys263 at the active site. The structures define the interactions made by one of the two substrate moieties that bind to the enzyme during catalysis. All of the inhibitors induce a significant ordering of the flap covering the active site. Succinylacetone appears to be unique by inducing a number of conformational changes in loops covering the active site, which may be important for understanding the co-operative properties of ALAD enzymes. Succinylacetone is produced in large amounts by patients suffering from the hereditary disease type I tyrosinaemia and its potent inhibition of ALAD also has implications for the pathology of this disease. The most intriguing result is that obtained with 4-keto-5-amino-hexanoic acid, which seems to form a stable carbinolamine intermediate with Lys263. It appears that we have defined the structure of an intermediate of Schiff base formation that the substrate forms upon binding to the P-site of the enzyme.

The X-ray structure of yeast 5-aminolaevulinic acid dehydratase complexed with two diacid inhibitors

The structures of 5-aminolaevulinic acid dehydratase complexed with two irreversible inhibitors (4-oxosebacic acid and 4,7-dioxosebacic acid) have been solved at high resolution. Both inhibitors bind by forming a Schiff base link with Lys 263 at the active site. Previous inhibitor binding studies have defined the interactions made by only one of the two substrate moieties (P-side substrate) which bind to the enzyme during catalysis. The structures reported here provide an improved definition of the interactions made by both of the substrate molecules (A- and P-side substrates). The most intriguing result is the novel finding that 4,7-dioxosebacic acid forms a second Schiff base with the enzyme involving Lys 210. It has been known for many years that P-side substrate forms a Schiff base (with Lys 263) but until now there has been no evidence that binding of A-side substrate involves formation of a Schiff base with the enzyme. A catalytic mechanism involving substrate linked to the enzyme through Schiff bases at both the A- and P-sites is proposed.

The biological chemistry of lead

Recent biophysical studies on the interactions between lead and recombinant proteins and peptides that naturally bind zinc or calcium have provided unparalleled insights into the biological chemistry and molecular toxicology of lead. These studies lay the foundation for the rational design of improved methods for detecting and treating lead poisoning.

Stimulatory and inhibitory effects of inorganic lead on calcineurin

Calcineurin is a phosphatase with activity dependent on both Ca(2+)/calmodulin binding to the catalytic A subunit and Ca(2+) binding to the regulatory B subunit. We have previously shown that Pb(2+) activates calmodulin with a threshold of about 100 pM free Pb(2+), and that Pb(2+) and Ca(2+) are roughly additive in calmodulin activation (Kern et al., NeuroToxicology 21, 353-364 (2000)). In the present study, we evaluated the effects of Pb(2+), with and without Ca(2+) and calmodulin, on calcineurin activity. In calmodulin-containing, Ca(2+)-free solutions, Pb(2+) activated calcineurin with a threshold of about 100 pM free Pb(2+). Maximum calcineurin activity (comparable to that induced by 10 microM Ca(2+)) was reached at about 200 pM free Pb(2+). Higher Pb(2+) concentrations reduced activity, although some activity remained even at 2000 pM free Pb(2+). Combined with subsaturating Ca(2+) concentrations, as little as 20 pM free Pb(2+) enhanced calcineurin activity, but free Pb(2+) concentrations greater than 200 pM still reduced activity below maximum. Extremely high Ca(2+) concentrations (10 microM) completely reversed the inhibition of activity by 2000 pM free Pb(2+). In the absence of calmodulin, Ca(2+) slightly stimulated calcineurin activity. Pb(2+) did not substitute for Ca(2+) in calmodulin-free activation; in fact, high concentrations of Pb(2+) inhibited Ca(2+)-mediated activation. We tentatively conclude that low concentrations of free Pb(2+) activate calcineurin by activating calmodulin. Higher concentrations reduce calcineurin activity, perhaps by binding to the B subunit.

An artificial gene for human porphobilinogen synthase allows comparison of an allelic variation implicated in susceptibility to lead poisoning

Porphobilinogen synthase (PBGS) is an ancient enzyme essential to tetrapyrrole biosynthesis (e.g. heme, chlorophyll, and vitamin B(12)). Two common alleles encoding human PBGS, K59 and N59, have been correlated with differential susceptibility of humans to lead poisoning. However, a model for human PBGS based on homologous crystal structures shows the location of the allelic variation to be distant from the active site with its two Zn(II). Previous microbial expression systems for human PBGS have resulted in a poor yield. Here, an artificial gene encoding human PBGS was constructed by recursive polymerase chain reaction from synthetic oligonucleotides to rectify this problem. The artificial gene was made to resemble the highly expressed homologous Escherichia coli hemB gene and to remove rare codons that can confound heterologous protein expression in E. coli. We have expressed and purified recombinant human PBGS variants K59 and N59 in 100-mg quantities. Both human PBGS proteins purified with eight Zn(II)/octamer; Zn(II) binding was shown to be pH-dependent; and Pb(II) could displace some of the Zn(II). However, there was no differential displacement of Zn(II) by Pb(II) between K59 and N59, and simple Pb(II) inhibition studies revealed no allelic difference.

Environmental lead exposure and activity of delta-aminolevulinic acid dehydratase (ALA-D) in maternal and cord blood

The hypothesis that environmental lead exposure measured from blood (Pb-B) inhibits delta-aminolevulinic acid dehydratase activity (ALA-D) from whole blood was tested in 241 urban mothers and their newborns. Geometric means and (5th and 95th Percentiles) for maternal and cord Pb-B were 6.4 microg dl(-1) (3.4-11.9) and 4.6 microg dl(-1) (2.8-9.2). Spearman correlations between mother and cord Pb-B and ALA-D were all negative but statistically significant only for cord Pb-B and mother ALA-D. A potential lead threshold, was identified between 3.2 and 4.8 microg dl(-1), above which ALA-D may be inhibited by lead, and below which ALA-D may be insensitive or even activated. In conclusion, low environmental exposure to lead is responsible for a demonstrable biochemical effect. This potential ALA-D inhibition may lead to neurotoxic effects, especially in newborns who have high level of neurogenesis.

The Schiff base complex of yeast 5-aminolaevulinic acid dehydratase with laevulinic acid

The X-ray structure of the complex formed between yeast 5-aminolaevulinic acid dehydratase (ALAD) and the inhibitor laevulinic acid has been determined at 2.15 A resolution. The inhibitor binds by forming a Schiff base link with one of the two invariant lysines at the catalytic center: Lys263. It is known that this lysine forms a Schiff base link with substrate bound at the enzyme's so-called P-site. The carboxyl group of laevulinic acid makes hydrogen bonds with the side-chain-OH groups of Tyr329 and Ser290, as well as with the main-chain >NH group of Ser290. The aliphatic moiety of the inhibitor makes hydrophobic interactions with surrounding aromatic residues in the protein including Phe219, which resides in the flap covering the active site. Our analysis strongly suggests that the same interactions will be made by P-side substrate and also indicates that the substrate that binds at the enzyme's A-site will interact with the enzyme's zinc ion bound by three cysteines (133, 135, and 143). Inhibitor binding caused a substantial ordering of the active site flap (residues 217-235), which was largely invisible in the native electron density map and indicates that this highly conserved yet flexible region has a specific role in substrate binding during catalysis.

Molecular mechanisms of lead neurotoxicity

Epidemiological studies have shown a strong relationship between the level of lead in blood and bone as assessed by performance on IQ tests and other psychometric tests. Approximately 1 out of 10 children in the United States have blood lead levels above 10 microg/dl, which has been established as the level of concern. Studies on experimental animals exposed to lead after birth have shown learning deficits at similar blood lead levels. Since learning requires the remodeling of synapses in the brain, lead may specifically affect synaptic transmission. Although the molecular targets for lead are unknown, a vast amount of evidence accumulated over many years has shown that lead disrupts processes that are regulated by calcium. Our laboratory has been studying the effect of lead on protein kinase C, a family of isozymes some of which require calcium for activity. We and others have shown that picomolar concentrations of lead can replace micromolar concentrations of calcium in a protein kinase C enzyme assay. Furthermore, lead activates protein kinase C in intact cells and induces the expression of new genes by a mechanism dependent on protein kinase C. We propose that the learning deficits caused by lead are due to events regulated by protein kinase C that most likely occur at the synapse.

X-ray structure of 5-aminolaevulinate dehydratase, a hybrid aldolase

5-Aminolaevulinate dehydratase (ALAD) is a homo-octameric metallo-enzyme that catalyses the formation of porphobilinogen from 5-aminolaevulinic acid. The structure of the yeast enzyme has been solved to 2.3 A resolution, revealing that each subunit adopts a TIM barrel fold with a 39 residue N-terminal arm. Pairs of monomers wrap their arms around each other to form compact dimers and these associate to form a 422 symmetric octamer. All eight active sites are on the surface of the octamer and possess two lysine residues (210 and 263), one of which, Lys 263, forms a Schiff base link to the substrate. The two lysine side chains are close to two zinc binding sites one of which is formed by three cysteine residues (133, 135 and 143) while the other involves Cys 234 and His 142. ALAD has features at its active site that are common to both metallo- and Schiff base-aldolases and therefore represents an intriguing combination of both classes of enzyme. Lead ions, which inhibit ALAD potently, replace the zinc bound to the enzyme's unique triple-cysteine site.

Lead binding to delta-aminolevulinic acid dehydratase (ALAD) in human erythrocytes

Over 99% of the lead present in blood is usually found in erythrocytes. To investigate the nature of this selective accumulation of lead in erythrocytes, the specific binding of lead to proteins in human erythrocytes was studied using liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). The principal lead-binding protein had a mass of approximately 240 kDa, and adsorption to specific antibodies showed that protein was delta-aminolevulinic acid dehydratase (ALAD). Thus, the previous notion that lead in erythrocytes was bound primarily to haemoglobin has to be revised. Furthermore, in lead-exposed workers, the percentage of lead bound to ALAD was influenced by a common polymorphism in the ALAD gene. Specifically, in seven carriers of the ALAD2 allele, 84% of the protein-bound lead recovered was bound to ALAD compared to 81% in seven homozygotes for the ALAD1 allele whose erythrocytes were matched for blood-lead concentration. The small difference was statistically significant in Wilcoxon matched-pairs signed-rank test (P = 0.03). No ALAD allele-specific difference in ALAD-bound lead was found among 20 unexposed controls. Perhaps the difference in ALAD-bound lead can provide an explanation for the previously reported finding of higher blood-lead levels among carriers of the ALAD2 allele than among ALAD1 homozygotes in lead-exposed populations.