Divalent metal-ion transporter DMT1 mediates both H+ -coupled Fe2+ transport and uncoupled fluxes

The H(+) -coupled divalent metal-ion transporter DMT1 serves as both the primary entry point for iron into the body (intestinal brush-border uptake) and the route by which transferrin-associated iron is mobilized from endosomes to cytosol in erythroid precursors and other cells. Elucidating the molecular mechanisms of DMT1 will therefore increase our understanding of iron metabolism and the etiology of iron overload disorders. We expressed wild type and mutant DMT1 in Xenopus oocytes and monitored metal-ion uptake, currents and intracellular pH. DMT1 was activated in the presence of an inwardly directed H(+) electrochemical gradient. At low extracellular pH (pH(o)), H(+) binding preceded binding of Fe(2+) and its simultaneous translocation. However, DMT1 did not behave like a typical ion-coupled transporter at higher pH(o), and at pH(o) 7.4 we observed Fe(2+) transport that was not associated with H(+) influx. His(272) --> Ala substitution uncoupled the Fe(2+) and H(+) fluxes. At low pH(o), H272A mediated H(+) uniport that was inhibited by Fe(2+). Meanwhile H272A-mediated Fe(2+) transport was independent of pH(o). Our data indicate (i) that H(+) coupling in DMT1 serves to increase affinity for Fe(2+) and provide a thermodynamic driving force for Fe(2+) transport and (ii) that His-272 is critical in transducing the effects of H(+) coupling. Notably, our data also indicate that DMT1 can mediate facilitative Fe(2+) transport in the absence of a H(+) gradient. Since plasma membrane expression of DMT1 is upregulated in liver of hemochromatosis patients, this H(+) -uncoupled facilitative Fe(2+) transport via DMT1 can account for the uptake of nontransferrin-bound plasma iron characteristic of iron overload disorders.

Functional properties of multiple isoforms of human divalent metal-ion transporter 1 (DMT1)

DMT1 (divalent metal-ion transporter 1) is a widely expressed metal-ion transporter that is vital for intestinal iron absorption and iron utilization by most cell types throughout the body, including erythroid precursors. Mutations in DMT1 cause severe microcytic anaemia in animal models. Four DMT1 isoforms that differ in their N- and C-termini arise from mRNA transcripts that vary both at their 5'-ends (starting in exon 1A or exon 1B) and at their 3'-ends giving rise to mRNAs containing (+) or lacking (-) the 3'-IRE (iron-responsive element) and resulting in altered C-terminal coding sequences. To determine whether these variations result in functional differences between isoforms, we explored the functional properties of each isoform using the voltage clamp and radiotracer assays in cRNA-injected Xenopus oocytes. 1A/IRE+-DMT1 mediated Fe2+-evoked currents that were saturable (K(0.5)(Fe) approximately 1-2 microM), temperature-dependent (Q10 approximately 2), H+-dependent (K(0.5)(H) approximately 1 muM) and voltage-dependent. 1A/IRE+-DMT1 exhibited the provisional substrate profile (ranked on currents) Cd2+, Co2+, Fe2+, Mn2+>Ni2+, V3+>>Pb2+. Zn2+ also evoked large currents; however, the zinc-evoked current was accounted for by H+ and Cl- conductances and was not associated with significant Zn2+ transport. 1B/IRE+-DMT1 exhibited the same substrate profile, Fe2+ affinity and dependence on the H+ electrochemical gradient. Each isoform mediated 55Fe2+ uptake and Fe2+-evoked currents at low extracellular pH. Whereas iron transport activity varied markedly between the four isoforms, the activity for each correlated with the density of anti-DMT1 immunostaining in the plasma membrane, and the turnover rate of the Fe2+ transport cycle did not differ between isoforms. Therefore all four isoforms of human DMT1 function as metal-ion transporters of equivalent efficiency. Our results reveal that the N- and C-terminal sequence variations among the DMT1 isoforms do not alter DMT1 functional properties. We therefore propose that these variations serve as tissue-specific signals or cues to direct DMT1 to the appropriate subcellular compartments (e.g. in erythroid cells) or the plasma membrane (e.g. in intestine).

Metal-ion-dependent biological properties of a chelator-derived somatostatin analogue for tumour targeting

Somatostatin-based radioligands have been shown to have sensitive imaging properties for neuroendocrine tumours and their metastases. The potential of [(55)Co(dotatoc)] (dotatoc =4,7,10-tricarboxymethyl-1,4,7,10-tetraazacyclododecane-1-ylacetyl-D-Phe-(Cys-Tyr-D-Trp-Lys-Thr-Cys)-threoninol (disulfide bond)) as a new radiopharmaceutical agent for PET has been evaluated. (57)Co was used as a surrogate of the positron emitter (55)Co and the pharmacokinetics of [(57)Co(dotatoc)] were investigated by using two nude mouse models. The somatostatin receptor subtype (sst1-sst5) affinity profile of [(nat)Co(dotatoc)] on membranes transfected with human somatostatin receptor subtypes was assessed by using autoradiographic methods. These studies revealed that [(57)Co(dotatoc)] is an sst2-specific radiopeptide which presents the highest affinity ever found for the sst2 receptor subtype. The rate of internalisation into the AR4-2J cell line also was the highest found for any somatostatin-based radiopeptide. Biodistribution studies, performed in nude mice bearing an AR4-2J tumour or a transfected HEK-sst2 cell-based tumour, showed high and specific uptake in the tumour and in other sst-receptor-expressing tissues, which reflects the high receptor binding affinity and the high rate of internalisation. The pharmacologic differences between [(57)Co(dotatoc)] and [(67)Ga(dotatoc)] are discussed in terms of the structural parameters found for the chelate models [Co(II)(dota)](2-) and [Ga(III)(dota)](-) whose X-ray structures have been determined. Both chelates show six-fold coordination in pseudo-octahedral arrangements.

The Influence of Different Metal-Chelators on the Biological Profil of Nanoparticles for Gallium-68 Based Molecular Imaging

The use of metal chelators is becoming increasingly important in the development of new tracers for molecular imaging. With the rise of the field of nanotechnology, the fusion of both technologies has shown great potential for clinical applications. The pharmacokinetcs of nanoparticles can be monitored via positron emission tomography (PET) after surface modification and radiolabeling with positron emitting radionuclides. Different metal ion chelators can be used to facilitate labeling of the radionuclides and as a prerequisite, optimized radiolabeling procedure is necessary to prevent nanoparticle aggregation and degradation. However, the effects of chelator modification on nanoparticle pharmacokinetic properties have not been well studied and currently no studies to date have compared the biological effects of the use of different chelators in the surface modification of nanoparticles.

Development and Validation of a Fast and Homogeneous Cell-Based Fluorescence Screening Assay for Divalent Metal Transporter 1 (DMT1/SLC11A2) Using the FLIPR Tetra.

Divalent metal ion transporter 1 (DMT1) is a proton-coupled Fe(2+) transporter that is essential for iron uptake in enterocytes and for transferrin-associated endosomal iron transport in many other cell types. DMT1 dysfunction is associated with several diseases such as iron overload disorders and neurodegenerative diseases. The main objective of the present work is to develop and validate a fluorescence-based screening assay for DMT1 modulators. We found that Fe(2+) or Cd(2+) influx could be reliably monitored in calcium 5-loaded DMT1-expressing HEK293 cells using the FLIPR Tetra fluorescence microplate reader. DMT1-mediated metal transport shows saturation kinetics depending on the extracellular substrate concentration, with a K0.5 value of 1.4 µM and 3.5 µM for Fe(2+) and Cd(2+), respectively. In addition, Cd(2+) was used as a substrate for DMT1, and we find a Ki value of 2.1 µM for a compound (2-(3-carbamimidoylsulfanylmethyl-benzyl)-isothiourea) belonging to the benzylisothioureas family, which has been identified as a DMT1 inhibitor. The optimized screening method using this compound as a reference demonstrated a Z' factor of 0.51. In summary, we developed and validated a sensitive and reproducible cell-based fluorescence assay suitable for the identification of compounds that specifically modulate DMT1 transport activity.

Characterization of major zinc containing myonecrotic and procoagulant metalloprotease 'malabarin' from non lethal trimeresurus malabaricus snake venom with thrombin like activity: its neutralization by chelating agents

A major myonecrotic zinc containing metalloprotease 'malabarin' with thrombin like activity was purified by the combination of gel permeation and anion exchange chromatography from T. malabaricus snake venom. MALDI-TOF analysis of malabarin indicated a molecular mass of 45.76 kDa and its N-terminal sequence was found to be Ile-Ile-Leu- Pro(Leu)-Ile-Gly-Val-Ile-Leu(Glu)-Thr-Thr. Atomic absorption spectral analysis of malabarin raveled the association of zinc metal ion. Malabarin is not lethal when injected i.p. or i.m. but causes extensive hemorrhage and degradation of muscle tissue within 24 hours. Sections of muscle tissue under light microscope revealed hemorrhage and congestion of blood vessel during initial stage followed by extensive muscle fiber necrosis with elevated levels of serum creatine kinase and lactate dehydrogenase activity. Malabarin also exhibited strong procoagulant action and its procoagulant action is due to thrombin like activity; it hydrolyzes fibrinogen to form fibrin clot. The enzyme preferentially hydrolyzes A? followed by B subunits of fibrinogen from the N-terminal region and the released products were identified as fibrinopeptide A and fibrinopeptide B by MALDI. The myonecrotic, fibrinogenolytic and subsequent procoagulant activities of malabarin was neutralized by specific metalloprotease inhibitors such as EDTA, EGTA and 1, 10-phenanthroline but not by PMSF a specific serine protease inhibitor. Since there is no antivenom available to neutralize local toxicity caused by T. malabaricus snakebite, EDTA chelation therapy may have more clinical relevance over conventional treatment.

Exploratory studies on coordination chemistry of a redox-active bridging ligand: synthesis, properties and solid state structures of the complexes

The explorative coordination chemistry of the bridging ligand TTF-PPB is presented. Its strong binding ability to Co(II) and then to Ni(II) or Cu(II) in the presence of hexafluoroacetylacetonate (hfac(-)), forming new mono-and dinuclear complexes 1-3, is described. X-ray crystallographic studies have been conducted in the case of the free ligand TTF-PPB as well as its complexes [Co(TTF-PPB)(hfac)(2)] (1) and [Co(hfac)(2)(mu-TTF-PPB)Ni(hfac)(2)] (2). Each metal ion is bonded to two bidentate hfac-anions through their oxygen atoms and two nitrogen atoms of the PPB moiety with a distorted octahedral coordination geometry. Specifically, nitrogen donor atoms of TTF-PPB adopt a cis-coordination but not in the equatorial plane, which is quite rare. Electronic absorption, photoinduced intraligand charge transfer ((1)ILCT), and electrochemical behaviour of 1-3 have been investigated. UV-Vis spectroscopy shows very strong bands in the UV region consistent with ligand centred pi-pi* transitions and an intense broad band in the visible region corresponding to a spin-allowed pi-pi* (1)ILCT transition. Upon coordination, the (1)ILCT band is bathochromically shifted by 3100, 6100 and 5900 cm(-1) on going from 1 to 3. The electrochemical studies reveal that all of them undergo two reversible oxidation and one reversible reduction processes, ascribed to the successive oxidations of the TTF moiety and the reduction of the PPB unit, respectively.

EpCAM-targeted delivery of nanocomplexed siRNA to tumor cells with designed ankyrin repeat proteins

Specific delivery to tumors and efficient cellular uptake of nucleic acids remain major challenges for gene-targeted cancer therapies. Here we report the use of a designed ankyrin repeat protein (DARPin) specific for the epithelial cell adhesion molecule (EpCAM) as a carrier for small interfering RNA (siRNA) complementary to the bcl-2 mRNA. For charge complexation of the siRNA, the DARPin was fused to a truncated human protamine-1 sequence. To increase the cell binding affinity and the amount of siRNA delivered into cells, DARPin dimers were generated and used as fusion proteins with protamine. All proteins expressed well in Escherichia coli in soluble form, yet, to remove tightly bound bacterial nucleic acids, they were purified under denaturing conditions by immobilized metal ion affinity chromatography, followed by refolding. The fusion proteins were capable of complexing four to five siRNA molecules per protamine, and fully retained the binding specificity for EpCAM as shown on MCF-7 breast carcinoma cells. In contrast to unspecific LipofectAMINE transfection, down-regulation of antiapoptotic bcl-2 using fusion protein complexed siRNA was strictly dependent on EpCAM binding and internalization. Inhibition of bcl-2 expression facilitated tumor cell apoptosis as shown by increased sensitivity to the anticancer agent doxorubicin.

Syntheses, characterization and crystal structures of a new functionalised TTF derivative and its Ni(II) complex

The new ligand 4,5-bis (2-pyridylmethylsulfanyl)-4',5'-bis(cyanoethylthio)tetrathiafulvalene (BPM-BCET-TTF) and its nickel(II) complex have been prepared and crystallographically characterized. The Ni(II) complex shows octahedral geometry around the metal ion with the coordination site occupied by the pyridyl nitrogen atoms, the thioether sulfur atoms of the ligand and cis coordination of the halide ions.

Iron-Promoted Nucleophilic Additions to Diimine-Type Ligands: A Synthetic and Structural Study

We report here three examples of the reactivity of protic nucleophiles with diimine-type ligands in the presence of FeII salts. In the first case, the iron-promoted alcoholysis reaction of one nitrile group of the ligand 2,3-dicyano-5,6-bis(2-pyridyl)-pyrazine (L1) permitted the isolation of an stable E-imido−ester, [Fe(L1‘)2](CF3SO3)2 (1), which has been characterized by spectroscopic studies (IR, ES-MS, Mössbauer), elemental analysis, and crystallographically. Compound 1 consists of mononuclear octahedrally coordinated FeII complexes where the FeII ion is in its low-spin state. The iron-mediated nucleophilic attack of water to the asymmetric ligand 2,3-bis(2-pyridyl)pyrido[3,4-b]pyrazine (L2) has also been studied. In this context, the crystal structures of two hydration−oxidation FeIII products, [Fe(L2‘)2](ClO4)3·3CH3CN (2) and trans-[FeL2‘‘Cl2] (3), are described. Compounds 2 and 3 are both mononuclear FeIII complexes where the metals occupy octahedral positions. In principle, L2 is expected to coordinate to metal ions through its bipyridine-type units to form a five-membered ring; however, this is not the case in compounds 2 and 3. In 2, the ligand coordinates through its pyridines and through the hydroxyl group attached to the pyrazine imino carbon after hydration, that is, in an N,O,N tridentate manner. In compound 3, the ligand has suffered further transformations leading to a very stable diamido complex. In this case, the metal ion achieves its octahedral geometry by means of two pyridines, two amido N atoms, and two axial chlorine atoms. Magnetic susceptibility measurements confirmed the spin state of these two FeIII species:  compounds 2 and 3 are low-spin and high-spin, respectively.

The Exploitation of Versatile Building Blocks for the Self-Assembly of Novel Molecular Magnets

Using molecular building blocks to self-assemble lattices supporting long-range magnetic order is currently an active area of solid-state chemistry. Consequently, it is the realm of supramolecular chemistry that synthetic chemists are turning to in order to develop techniques for the synthesis of structurally well-defined supramolecular materials. In recent years we have investigated the versatility and usefulness of two classes of molecular building blocks, namely, tris-oxalato transition-metal (M. Pilkington and S. Decurtins, in “Magnetoscience—From Molecules to Materials,” Wiley–VCH, 2000), and octacyanometalate complexes (Pilkington and Decurtins, Chimia 54, 593 (2001)), for applications in the field of molecule-based magnets. Anionic, tris-chelated oxalato building blocks are able to build up two-dimensional honeycomb-layered structural motifs as well as three-dimensional decagon frameworks. The discrimination between the crystallization of the two- or three-dimensional structures relies on the choice of the templating counterions (Decurtins, Chimia 52, 539 (1998); Decurtins et al. Mol. Cryst. Liq. Cryst. 273, 167 (1995); New J. Chem. 117 (1998)). These structural types display a range of ferro, ferri, and antiferromagnetic properties (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials”). Octacyanometalate building blocks self-assemble to afford two new classes of cyano-bridged compounds namely, molecular clusters and extended three dimensional networks (J. Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000); Pilkington et al., in preparation). The molecular cluster with a MnII9MoV6 core has the highest ground state spin value, S=51/2, reported to-date (Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000)). In the high-temperature regime, the magnetic properties are characterized by ferromagnetic intracluster coupling. In the magnetic range below 44 K, the magnetic cluster signature is lost as possibly a bulk behavior starts to emerge. The three-dimensional networks exhibit both paramagnetic and ferromagnetic behavior, since the magnetic properties of these materials directly reflect the electronic configuration of the metal ion incorporated into the octacyanometalate building blocks (Pilkington et al., in preparation). For both the oxalate- and cyanide-bridged materials, we are able to manipulate the magnetic properties of the supramolecular assemblies by tuning the electronic configurations of the metal ions incorporated into the appropriate molecular building blocks (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials,” Chimia 54, 593 (2000)).

Microscopic theory of cooperative spin crossover: Interaction of molecular modes with phonons

In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the [Fe(ptz)6](BF4)2 crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.

Thermal release rate studies of nuclear reaction products from polycrystalline metal matrices

The thermal release rate of nuclear reaction products was investigated in offline annealing experiments. This work was motivated by the search for a high melting catcher material for recoiling products from heavy ion induced nuclear fusion reactions. Polycrystalline refractory metal foils of Ni, Y, Zr, Nb, Mo, Hf, W, and Re were investigated as catcher metals. Diffusion data for various tracer/host combinations were deduced from the measured release rates. This work focuses on the diffusion and the release rate of volatile p-elements from row 5 and 6 of the periodic table as lighter homologues of the superheavy elements with Z ≥ 113 to be studied in future experiments. A massive radiation damage enhancement of the diffusion velocity was observed. Diffusion trends have been established along the groups and rows of the periodic table based on the dependence of diffusion velocity on atomic sizes.