First-Row Transition Metal Complexes Incorporating the 2-(2'-pyridyl)quinoxaline Ligand (pqx), as Potent Inflammatory Mediators: Cytotoxic Properties and Biological Activities against the Platelet-Activating Factor (PAF) and Thrombin.

Inflammatory mediators constitute a recently coined term in the field of metal-based complexes with antiplatelet activities. Our strategy targets Platelet-Activating Factor (PAF) and its receptor, which is the most potent lipid mediator of inflammation. Thus, the antiplatelet (anti-PAF) potency of any substance could be exerted by inhibiting the PAF-induced aggregation in washed rabbit platelets (WRPs), which internationally is a well-accepted methodology. Herein, a series of mononuclear (mer-[Cr(pqx)Cl3(H2O]) (1), [Co(pqx)Cl2(DMF)] (2) (DMF = N,N'-dimethyl formamide), [Cu(pqx)Cl2(DMSO)] (3) (DMSO = dimethyl sulfoxide), [Zn(pqx)Cl2] (4)) and dinuclear complexes ([Mn(pqx)(H2O)2Cl2]2 (5), [Fe(pqx)Cl2]2 (6) and [Ni(pqx)Cl2]2 (7)) incorporating the 2-(2'-pyridyl)quinoxaline ligand (pqx), were biologically evaluated as inhibitors of the PAF- and thrombin-induced aggregation in washed rabbit platelets (WRPs). The molecular structure of the five-co-ordinate analog (3) has been elucidated by single-crystal X-ray diffraction revealing a trigonal bipyramidal geometry. All complexes are potent inhibitors of the PAF-induced aggregation in WRPs in the micromolar range. Complex (6) displayed a remarkable in vitro dual inhibition against PAF and thrombin, with IC50 values of 1.79 µM and 0.46 µM, respectively. Within the series, complex (5) was less effective (IC50 = 39 µM) while complex (1) was almost 12-fold more potent against PAF, as opposed to thrombin-induced aggregation. The biological behavior of complexes 1, 6 and 7 on PAF's basic metabolic enzymatic pathways reveals that they affect key biosynthetic and catabolic enzymes of PAF underlying the anti-inflammatory properties of the relevant complexes. The in vitro cytotoxic activities of all complexes in HEK293T (human embryonic kidney cells) and HeLa cells (cervical cancer cells) are described via the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. The results reveal that complex 3 is the most potent within the series.

Sulfide and transition metals - A partnership for life.

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron­sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron­sulfur, heme and zinc proteins is also addressed.

Molecular Basis of Ca(II)-Induced Tetramerization and Transition-Metal Sequestration in Human Calprotectin.

Human calprotectin (CP, S100A8/S100A9 oligomer, MRP8/MRP14 oligomer) is an abundant innate immune protein that contributes to the host metal-withholding response. Its ability to sequester transition metal nutrients from microbial pathogens depends on a complex interplay of Ca(II) binding and self-association, which converts the αß heterodimeric apo protein into a Ca(II)-bound (αß)2 heterotetramer that displays enhanced transition metal affinities, antimicrobial activity, and protease stability. A paucity of structural data on the αß heterodimer has hampered molecular understanding of how Ca(II) binding enables CP to exert its metal-sequestering innate immune function. We report solution NMR data that reveal how Ca(II) binding affects the structure and dynamics of the CP αß heterodimer. These studies provide a structural model in which the apo αß heterodimer undergoes conformational exchange and switches between two states, a tetramerization-incompetent or "inactive" state and a tetramerization-competent or "active" state. Ca(II) binding to the EF-hands of the αß heterodimer causes the active state to predominate, resulting in self-association and formation of the (αß)2 heterotetramer. Moreover, Ca(II) binding causes local and allosteric ordering of the His3Asp and His6 metal-binding sites. Ca(II) binding to the noncanonical EF-hand of S100A9 positions (A9)D30 and organizes the His3Asp site. Remarkably, Ca(II) binding causes allosteric effects in the C-terminal region of helix αIV of S100A9, which stabilize the α-helicity at positions H91 and H95 and thereby organize the functionally versatile His6 site. Collectively, this study illuminates the molecular basis for how CP responds to high extracellular Ca(II) concentrations, which enables its metal-sequestering host-defense function.

Speciation of Transition-Metal-Substituted Keggin-Type Silicotungstates Affected by the Co-crystallization Conditions with Proteinase K.

We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts Na14[(A-α-SiW10O37)2{Co4(OH)2(H2O)2}]·37H2O (Na{SiW10Co2}2) and Na14[(A-α-SiW10O37)2{Ni4(OH)2(H2O)2}]·77.5H2O (Na{SiW10Ni2}2), which were prepared by applying a new synthesis protocol and characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Proteinase K was applied as a model protein and the polyoxotungstate (POT)-protein interactions of Na{SiW10Co2}2 and Na{SiW10Ni2}2 were studied side by side with the literature-known K5Na3[A-α-SiW9O34(OH)3{Co4(OAc)3}]·28.5H2O ({SiW9Co4}) featuring the same number of transition metals. Testing the solution behavior of applied POTs under the crystallization conditions (sodium acetate buffer, pH 5.5) by time-dependent UV/vis spectroscopy and electrospray ionization mass spectrometry speciation studies revealed an initial dissociation of the sandwich POTs to the disubstituted Keggin anions HxNa5-x[SiW10Co2O38]3- and HxNa5-x[SiW10Ni2O38]3- ({SiW10M2}, M = CoII and NiII) followed by partial rearrangement to the monosubstituted compounds (α-{SiW11Co} and α-{SiW11Ni}) after 1 week of aging. The protein crystal structure analysis revealed monosubstituted α-Keggin POTs in two conserved binding positions for all three investigated compounds, with one of these positions featuring a covalent attachment of the POT anion to an aspartate carboxylate. Despite the presence of both mono- and disubstituted anions in a crystallization mixture, proteinase K selectively binds to monosubstituted anions because of their preferred charge density for POT-protein interaction.

Transition metals and metal complexes in autophagy and diseases.

Transition metals refer to the elements in the d and ds blocks of the periodic table. Since the success of cisplatin and auranofin, transition metal-based compounds have become a prospective source for drug development, particularly in cancer treatment. In recent years, extensive studies have shown that numerous transition metal-based compounds could modulate autophagy, promising a new therapeutic strategy for metal-related diseases and the design of metal-based agents. Copper, zinc, and manganese, which are common components in physiological pathways, play important roles in the progression of cancer, neurodegenerative diseases, and cardiovascular diseases. Furthermore, enrichment of copper, zinc, or manganese can regulate autophagy. Thus, we summarized the current advances in elucidating the mechanisms of some metals/metal-based compounds and their functions in autophagy regulation, which is conducive to explore the intricate roles of autophagy and exploit novel therapeutic drugs for human diseases.

Transition metal-mediated O-O bond formation and activation in chemistry and biology.

Oxygen evolution and reduction reactions are fundamental processes in biological energy conversion schemes, which represent an attractive method for artificial energy conversion for a world still largely depending on fossil fuels. A range of metalloenzymes achieve these challenging tasks in biology by activating water and dioxygen using cheap and abundant transition metals, such as iron, copper, and manganese. High-valent metal-oxo/oxyl, metal-superoxo, and/or metal-(hydro)peroxo species are common reactive intermediates that are found in the O-O bond formation and activation reactions. The transient nature of the metal-oxygen intermediates has, however, prevented their isolation and characterization in most cases. As a consequence, unambiguous mechanistic assignments in the O-O bond formation and cleavage processes in biological and chemical entries remain elusive, especially for the intermediates and mechanisms involved in the O-O bond formation reactions. This viewpoint article aims at summarizing the information obtained to date in enzymatic and biomimetic systems that fuels the debate regarding the nature of the active oxidants and the mechanistic uncertainties associated with the transition metal-mediated O-O bond formation and cleavage reactions.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction.

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

Multi-metal nutrient restriction and crosstalk in metallostasis systems in microbial pathogens.

Transition metals from manganese to zinc function as catalytic and structural cofactors for an amazing diversity of proteins and enzymes, and thus are essential for all forms of life. During infection, inflammatory host proteins limit the accessibility of multiple transition metals to invading pathogens in a process termed nutritional immunity. In order to respond to host-mediated metal starvation, bacteria employ both protein and RNA-based mechanisms to sense prevailing transition metal concentrations that collectively regulate systems-level strategies to maintain cellular metallostasis. In this review, we discuss a number of recent advances in our understanding of how bacteria orchestrate the adaptive response to host-mediated multi-metal restriction, highlighting crosstalk among these regulatory systems.

TiO2 exposure alters transition metal ion quota in Rhodococcus ruber GIN-1.

After exposure to micron-sized TiO2 particles, anatase and/or rutile, Rhodococcus ruber GIN-1 accumulates an increased concentration (2.2 ± 0.2 mg kg-1) of mobilized Ti into its biomass with concomitant decreases in cellular biometals Fe, Zn, and possibly Mn, while levels of Cu and Al are unaffected.

Effect of Transition-Metal Ions on the Conformation of Encephalin Investigated by Hydrogen/Deuterium Exchange and Theoretical Calculations.

We have studied the effects of different 3d orbitals in divalent transition-metal ions [G2+ = Mn2+ (d5), Fe2+ (d6), Co2+ (d7), Ni2+ (d8), Cu2+ (d9), or Zn2+ (d10)] on the conformations of leucine encephalin (LE) and methionine encephalin (ME) in the gas phase using hydrogen/deuterium exchange mass spectrometry (HDX-MS) and theoretical calculations at the molecular level. The HDX-MS reveals a 1:1 stoichiometric monovalent complex of [LE/ME + G - H]+ and observed that the different HDX reactivities follow the trend Fe2+ < Co2+ < Ni2+ < Mn2+ < Cu2+ ≈ Zn2+ and that [ME + Mn/Cu/Zn - H]+ > [LE + Mn/Cu/Zn - H]+, while [LE + Fe/Co/Ni - H]+ > [ME + Fe/Co/Ni - H]+. We cross-correlated the collision-induced dissociation energies of the complexes with the HDX results and found that the more stable the complex, the harder it is for it to undergo HDX. Furthermore, we used theoretical calculations to optimize the favorable conformations of the complexes and found the same interaction structure of G2+ coordination with the five carbonyl oxygens of LE/ME that have different bond lengths. Finally, we calculated the proton affinity (PA) values of the optimized complexes in order to interpret the HDX observations that the higher the PA values, the more difficult it is for the complex to undergo HDX. Overall, both the experiments and the theoretical calculations show that the six metal ions have different effects on the LE/ME conformation, with the low-energy stability of the G2+ 3d orbitals corresponding to more dramatic effects on the LE/ME conformation. In addition, the hardness of the ionic acid corresponding to the fully filled Mn2+ and half-filled Zn2+ orbitals also contributes strongly to the coordination effect; the conformation effect of Fe2+/Co2+/Ni2+ on LE is greater than that on ME, whereas the conformation effect of Mn2+/Cu2+/Zn2+ on ME is greater than that on LE.

Interaction of iron ions with melanin.

One of the antioxidant roles of melanin is binding redox-active transition metal ions. The aim of this study was to examine the redox reactions accompanying iron binding by melanin. Two kinds of synthetic eumelanin were mixed with iron (II) and iron (III) in the presence and absence of citrate and ADP in the aerobic and anaerobic system. The iron binding was examined by electron paramagnetic resonance (EPR) spectroscopy and thiocyanate assay. Obtained results indicate that although melanin reduces iron (III) that is unbound to this polymer, binding of iron (II) is accompanied by its oxidation by melanin.

New Artificial Biomimetic Enzyme Analogues based on Iron(II/III) Schiff Base Complexes: An Effect of (Benz)imidazole Organic Moieties on Phenoxazinone Synthase and DNA Recognition.

Elucidation of the structure and function of biomolecules provides us knowledge that can be transferred into the generation of new materials and eventually applications in e.g., catalysis or bioassays. The main problems, however, concern the complexity of the natural systems and their limited availability, which necessitates utilization of simple biomimetic analogues that are, to a certain degree, similar in terms of structure and thus behaviour. We have, therefore, devised a small library of six tridentate N-heterocyclic coordinating agents (L1-L6), which, upon complexation, form two groups of artificial, monometallic non-heme iron species. Utilization of iron(III) chloride leads to the formation of the 1:1 (Fe:Ln) 'open' complexes, whereas iron(II) trifluoromethanosulfonate allows for the synthesis of 1:2 (M:Ln) 'closed' systems. The structural differences between the individual complexes are a result of the information encoded within the metallic centre and the chosen counterion, whereas the organic scaffold influences the observed properties. Indeed, the number and nature of the external hydrogen bond donors coming from the presence of (benz)imidazole moieties in the ligand framework are responsible for the observed biological behaviour in terms of mimicking phenoxazinone synthase activity and interaction with DNA.

Rational redox tuning of transition metal sites: learning from superoxide reductase.

Using superoxide reductase as a model system, a computational approach reveals how histidine tautomerism tunes the redox properties of metalloenzymes to enable their catalytic function. Inspired by these experimentally inaccessible insights, non-canonical histidine congeners are introduced as new versatile tools for the rational engineering of biological transition metal sites.

Transition metals and host-microbe interactions in the inflamed intestine.

Host-associated microbial communities provide critical functions for their hosts. Transition metals are essential for both the mammalian host and the majority of commensal bacteria. As such, access to transition metals is an important component of host-microbe interactions in the gastrointestinal tract. In mammals, transition metal ions are often sequestered by metal binding proteins to limit microbial access under homeostatic conditions. In response to invading pathogens, the mammalian host further decreases availability of these micronutrients by regulating their trafficking or releasing high-affinity metal chelating proteins, a process termed nutritional immunity. Bacterial pathogens have evolved several mechanisms to subvert nutritional immunity. Here, we provide an overview on how metal ion availability shapes host-microbe interactions in the gut with a particular focus on intestinal inflammatory diseases.

Interaction of amylin species with transition metals and membranes.

Islet Amyloid Polypeptide (IAPP), also known as amylin, is a 37-amino-acid peptide hormone that is secreted by pancreatic islet ß-cells. Amylin is complementary to insulin in regulating and maintaining blood glucose levels in the human body. The misfolding and aggregation of amylin is primarily associated with type 2 diabetes mellitus, which is classified as an amyloid disease. Recently, the interactions between amylin and specific metal ions, e.g., copper(II), zinc(II), and iron(II), were found to impact its performance and aggregation processes. Therefore, the focus in this review will be on how the chemistry and structural properties of amylin are affected by these interactions. In addition, the impact of amylin and other amyloidogenic peptides interacting with metal ions on the cell membranes is discussed. In particular, recent studies on the interactions of amylin with copper, zinc, iron, nickel, gold, ruthenium, and vanadium are discussed.

Molecular dynamics simulation on the effect of transition metal binding to the N-terminal fragment of amyloid-ß.

We report molecular dynamics simulations of three possible adducts of Fe(II) to the N-terminal 1-16 fragments of the amyloid-ß peptide, along with analogous simulations of Cu(II) and Zn(II) adducts. We find that multiple simulations from different starting points reach pseudo-equilibration within 100-300 ns, leading to over 900 ns of equilibrated trajectory data for each system. The specifics of the coordination modes for Fe(II) have only a weak effect on peptide secondary and tertiary structures, and we therefore compare one of these with analogous models of Cu(II) and Zn(II) complexes. All share broadly similar structural features, with mixture of coil, turn and bend in the N-terminal region and helical structure for residues 11-16. Within this overall pattern, subtle effects due to changes in metal are evident: Fe(II) complexes are more compact and are more likely to occupy bridge and ribbon regions of Ramachandran maps, while Cu(II) coordination leads to greater occupancy of the poly-proline region. Analysis of representative clusters in terms of molecular mechanics energy and atoms-in-molecules properties indicates similarity of four-coordinate Cu and Zn complexes, compared to five-coordinate Fe complex that exhibits lower stability and weaker metal-ligand bonding. Communicated by Ramaswamy H. Sarma.

Transition Metal Sequestration by the Host-Defense Protein Calprotectin.

In response to microbial infection, the human host deploys metal-sequestering host-defense proteins, which reduce nutrient availability and thereby inhibit microbial growth and virulence. Calprotectin (CP) is an abundant antimicrobial protein released from neutrophils and epithelial cells at sites of infection. CP sequesters divalent first-row transition metal ions to limit the availability of essential metal nutrients in the extracellular space. While functional and clinical studies of CP have been pursued for decades, advances in our understanding of its biological coordination chemistry, which is central to its role in the host-microbe interaction, have been made in more recent years. In this review, we focus on the coordination chemistry of CP and highlight studies of its metal-binding properties and contributions to the metal-withholding innate immune response. Taken together, these recent studies inform our current model of how CP participates in metal homeostasis and immunity, and they provide a foundation for further investigations of a remarkable metal-chelating protein at the host-microbe interface and beyond.

Assessment of metal concentrations in the SOD1G93A mouse model of amyotrophic lateral sclerosis and its potential role in muscular denervation, with particular focus on muscle tissue.

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is among the most common of the motor neuron diseases, and arguably the most devastating. During the course of this fatal neurodegenerative disorder, motor neurons undergo progressive degeneration. The currently best-understood animal models of ALS are based on the over-expression of mutant isoforms of Cu/Zn superoxide dismutase 1 (SOD1); these indicate that there is a perturbation in metal homeostasis with disease progression. Copper metabolism in particular is affected in the central nervous system (CNS) and muscle tissue. METHODS: This present study assessed previously published and newly gathered concentrations of transition metals (Cu, Zn, Fe and Se) in CNS (brain and spinal cord) and non-CNS (liver, intestine, heart and muscle) tissues from transgenic mice over-expressing the G93A mutant SOD1 isoform (SOD1G93A), transgenic mice over-expressing wildtype SOD1 (SOD1WT) and non-transgenic controls. RESULTS: Cu accumulates in non-CNS tissues at pre-symptomatic stages in SOD1G93A tissues. This accumulation represents a potentially pathological feature that cannot solely be explained by the over-expression of mSOD1. As a result of the lack of Cu uptake into the CNS there may be a deficiency of Cu for the over-expressed mutant SOD1 in these tissues. Elevated Cu concentrations in muscle tissue also preceded the onset of symptoms and were found to be pathological and not be the result of SOD1 over-expression. CONCLUSIONS: It is hypothesized that the observed Cu accumulations may represent a pathologic feature of ALS, which may actively contribute to axonal retraction leading to muscular denervation, and possibly significantly contributing to disease pathology. Therefore, it is proposed that the toxic-gain-of-function and dying-back hypotheses to explain the molecular drivers of ALS may not be separate, individual processes; rather our data suggests that they are parallel processes.

Transition metals and trace elements in the retinal pigment epithelium and choroid: correlative ultrastructural and chemical analysis by analytical electron microscopy and nano-secondary ion mass spectrometry.

Understanding the localisation and abundance of structural elements, trace elements and especially transition metals like Cu and Zn in ocular tissue sections is important for physiology, and also for the characterisation of diseases related to oxidative stress like age-related macular degeneration. Transition metal abundances were investigated in an aged donor eye by nano-secondary ion mass spectrometry (nano-SIMS) elemental mapping using Cs+ and O- primary ions, respectively, and correlated to their respective mole fractions investigated by analytical electron microscopy (AEM). The ultrastructure of the tissue and the elemental composition of melanosomes of the choroid and RPE, and RPE lipofuscin and melanolipofuscin granules can adequately be investigated by nano-SIMS using the secondary ion maps. Melanosomes, 0.5-1 µm in size, yield sulphur maps and maps of stored metals like calcium, sodium and copper. Lipofuscin shows especially high phosphorus signals. Elements with mole fractions of about 0.1 at%, e.g. for P and Cu, as investigated by AEM before, can be validated using simultaneous SIMS maps with an estimated lateral resolution of 66 nm with typical acquisition times of 30 minutes for each area of interest. However, Zn (0.19 at%) was not detected by SIMS. Nano-SIMS imaging of CN-, PO2-, S-, Cu-, Ca+, Fe+ and Na+ ions provides excellent detection limits demonstrating the possibilities for chemical mapping with high-sensitivity trace element detection and reduced acquisition times. Quantification of nano-SIMS data was achieved by correlating mole fractions obtained by AEM to secondary ions per pixel obtained by nano-SIMS. Both methods yield the melanin type in melanosomes and trace metal storage.

Global Structural Flexibility of Metalloproteins Regulates Reactivity of Transition Metal Ion in the Protein Core: An Experimental Study Using Thiol-subtilisin as a Model Protein.

The functions of metal-containing proteins (metalloproteins) are determined by the reactivities of transition metal ions at their active sites. Because protein macromolecular structures have several molecular degrees of freedom, global structural flexibility may also regulate the properties of metalloproteins. However, the influence of this factor has not been fully delineated in mechanistic studies of metalloproteins. Accordingly, we have investigated the relationship between global protein flexibility and the characteristics of a transition metal ion in the protein core using thiol-subtilisin (tSTL) with a Cys-coordinated Cu2+ ion as a model system. Although tSTL has two Ca2+ -binding sites, the Ca2+ -binding status hardly affects its secondary structure. Nevertheless, guanidinium-induced denaturation and amide H/D exchange indicated the increase in the structural flexibility of tSTL by the removal of bound Ca2+ ions. Electron paramagnetic resonance and absorption spectral changes have revealed that the protein flexibility determines the characteristics of a Cu2+ ion in tSTL. Therefore, global protein flexibility should be recognized as an important factor that regulates the properties of metalloproteins.