XANES spectroscopy: a promising tool for toxicology: a tutorial

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases

Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-Ia RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Ct R2, two different Mn(IV)Fe(III) site configurations were assigned either L(4)Mn(IV)(μO)(2)Fe(III)L(4) (metal-metal distance of ~2.75Å, L = ligand) prevailing in metal-grown R2, or L(4)Mn(IV)(μO)(μOH)Fe(III)L(4) (~2.90Å) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (~2.55Å) with a L(4)Fe(IV)(μO)(2)Fe(III)L(3) core structure. Several Mn,Fe(III)Fe(III) (~2.9-3.1Å) and Mn,Fe(III)Fe(II) species (~3.3-3.4Å) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit R1. In Ct R2, the protonation of a bridging oxide in the Mn(IV)(μO)(μOH)Fe(III) core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.

Metal-based anti-diabetic drugs: advances and challenges

The current status and likely future directions of complexes of V(V/IV), Cr(III), Mo(VI), W(VI), Zn(II), Cu(II), and Mn(III) as potential oral drugs against type 2 diabetes are reviewed. We propose a unified model of extra- and intracellular mechanisms of anti-diabetic efficacies of V(V/IV), Mo(VI), W(VI), and Cr(III), centred on high-oxidation-state oxido/peroxido species that inhibit protein tyrosine phosphatases (PTPs) involved in insulin signalling. The postulated oxidative mechanism of anti-diabetic activity of Cr(III) via carcinogenic Cr(VI/V) (which adds to safety concerns) is consistent with recent clinical trials on Cr(III) picolinate, where activity was apparent only in patients with poorly controlled diabetes (high oxidative stress), and the correlation between the anti-diabetic activities and ease of oxidation of Cr(III) supplements and their metabolites in vivo. Zn(II) and Cu(II) anti-diabetics act via different mechanisms and are unlikely to be used as specific anti-diabetics due to their diverse and unpredictable biological activities. Hence, future research directions are likely to centre on enhancing the bioavailability and selectivity of V(V/IV), Mo(VI), or W(VI) drugs. The strategy of potentiating circulating insulin with metal ions has distinct therapeutic advantages over interventions that stimulate the release of more insulin, or use insulin mimetics, because of many adverse side-effects of increased levels of insulin, including increased risks of cancer and cardiovascular diseases.

Actinide uptake by transferrin and ferritin metalloproteins

In order to better understand the mechanisms of actinide uptake by specific biomolecules, it is essential to explore the intramolecular interactions between the cation and the protein binding site. Although this has long been done for widely investigated transition metals, very few studies have been devoted to complexation mechanisms of actinides by active chelation sites of metalloproteins. In this field, X-ray absorption spectroscopy has been extensively used as a structural and electronic metal cation probe. The two examples that are presented here are related to two metalloproteins in charge of iron transport and storage in eukaryote cells: transferrin and ferritin. U(VI)O2 2+, Np(IV) and Pu(IV) have been selected because of their possible role as contaminant from the geosphere.

Bio-metals imaging and speciation in cells using proton and synchrotron radiation X-ray microspectroscopy

The direct detection of biologically relevant metals in single cells and of their speciation is a challenging task that requires sophisticated analytical developments. The aim of this article is to present the recent achievements in the field of cellular chemical element imaging, and direct speciation analysis, using proton and synchrotron radiation X-ray micro- and nano-analysis. The recent improvements in focusing optics for MeV-accelerated particles and keV X-rays allow application to chemical element analysis in subcellular compartments. The imaging and quantification of trace elements in single cells can be obtained using particle-induced X-ray emission (PIXE). The combination of PIXE with backscattering spectrometry and scanning transmission ion microscopy provides a high accuracy in elemental quantification of cellular organelles. On the other hand, synchrotron radiation X-ray fluorescence provides chemical element imaging with less than 100 nm spatial resolution. Moreover, synchrotron radiation offers the unique capability of spatially resolved chemical speciation using micro-X-ray absorption spectroscopy. The potential of these methods in biomedical investigations will be illustrated with examples of application in the fields of cellular toxicology, and pharmacology, bio-metals and metal-based nano-particles.

Chemical properties and toxicity of chromium(III) nutritional supplements

The status of Cr(III) as an essential micronutrient for humans is currently under question. No functional Cr(III)-containing biomolecules have been definitively described as yet, and accumulated experience in the use of Cr(III) nutritional supplements (such as [Cr(pic) 3], where pic = 2-pyridinecarboxylato) has shown no measurable benefits for nondiabetic people. Although the use of large doses of Cr(III) supplements may lead to improvements in glucose metabolism for type 2 diabetics, there is a growing concern over the possible genotoxicity of these compounds, particularly of [Cr(pic) 3]. The current perspective discusses chemical transformations of Cr(III) nutritional supplements in biological media, with implications for both beneficial and toxic actions of Cr(III) complexes, which are likely to arise from the same biochemical mechanisms, dependent on concentrations of the reactive species. These species include: (i) partial hydrolysis products of Cr(III) nutritional supplements, which are capable of binding to biological macromolecules and altering their functions; and (ii) highly reactive Cr(VI/V/IV) species and organic radicals, formed in reactions of Cr(III) with biological oxidants. Low concentrations of these species are likely to cause alterations in cell signaling (including enhancement of insulin signaling) through interactions with the active centers of regulatory enzymes in the cell membrane or in the cytoplasm, while higher concentrations are likely to produce genotoxic DNA lesions in the cell nucleus. These data suggest that the potential for genotoxic side-effects of Cr(III) complexes may outweigh their possible benefits as insulin enhancers, and that recommendations for their use as either nutritional supplements or antidiabetic drugs need to be reconsidered in light of these recent findings.

Subcellular Speciation Analysis of Trace Element Oxidation States Using Synchrotron Radiation Micro-X-ray Absorption Near-Edge Structure

Identification of chemical species at a subcellular level is a key to understand the mechanisms involved in the biology of chemical elements. When performed with a microbeam, X-ray absorption near-edge structure (micro-XANES) enables the direct speciation analysis of oxidation states in subcellular compartments avoiding cell fractionation and other preparation steps that might modify the chemical species. Here we report the principal characteristics in terms of spatial resolution, detection limit, reproducibility, and repeatability of a micro-XANES experimental setup based on Kirkpatrick-Baez X-ray focusing optics that maintains high flux of incoming radiation (>10(11) photons/s) at micrometric spatial resolution (1.5 x 4.0 microm2). Applications and limitations of this setup are illustrated by examples of iron and arsenic absorption spectra obtained from the cytosol, nucleus, and mitochondrial network of cultured cells. A better repeatability and sensitivity with no oxidation state modification and minimal beam damage is achieved when cells are analyzed in a frozen hydrated state, as compared to freeze-dried cells. This original experimental setup can now be applied for the direct speciation analysis of most trace elements at the subcellular level.

X-ray absorption and EPR spectroscopic studies of the biotransformations of chromium(VI) in mammalian cells. Is chromodulin an artifact of isolation methods?

Very different biological activities are usually ascribed to Cr(VI) (a toxin and carcinogen) and Cr(III) (an antidiabetic agent), although recent evidence suggests that both these types of actions are likely to arise from cellular uptake of varying concentrations of Cr(VI). The first systematic study of XANES spectra of Cr(III) complexes formed in Cr(VI)-treated mammalian cells (A549, HepG2, V79, and C2C12 cell lines), and in subcellular fractions of A549 cells, has been performed using a library of XANES spectra of model Cr(III) complexes. The results of multiple linear regression analyses of XANES spectra, in combination with multiple-scattering fits of XAFS spectra, indicate that Cr(III) formed in Cr(VI)-treated cells is most likely to bind to carboxylato, amine, and imidazole residues of amino acids, and to a lesser extent to hydroxo or aqua ligands. A combination of XANES and EPR spectroscopic data for Cr(VI)-treated cells indicates that the main component of Cr(III) formed in such cells is bound to high-molecular-mass ligands (>30 kDa, probably proteins), but significant redistribution of Cr(III) occurs during the cell lysis, which leads to the formation of a low-molecular-mass (<30 kDa) Cr(III)-containing fraction. The spectroscopic (XANES, XAFS, and EPR) properties of this fraction were strikingly similar to those of the purported natural Cr(III)-containing factor, chromodulin, that was reported to be isolated from the reaction of Cr(VI) with liver. These data support the hypothesis that a chromodulin-like species, which is formed from such a reaction, is an artifact of the reported isolation procedure.

Manganese neurotoxicity: a focus on the neonate

Manganese (Mn) is an essential trace metal found in all tissues, and it is required for normal amino acid, lipid, protein, and carbohydrate metabolism. While Mn deficiency is extremely rare in humans, toxicity due to overexposure of Mn is more prevalent. The brain appears to be especially vulnerable. Mn neurotoxicity is most commonly associated with occupational exposure to aerosols or dusts that contain extremely high levels (>1-5 mg Mn/m(3)) of Mn, consumption of contaminated well water, or parenteral nutrition therapy in patients with liver disease or immature hepatic functioning such as the neonate. This review will focus primarily on the neurotoxicity of Mn in the neonate. We will discuss putative transporters of the metal in the neonatal brain and then focus on the implications of high Mn exposure to the neonate focusing on typical exposure modes (e.g., dietary and parenteral). Although Mn exposure via parenteral nutrition is uncommon in adults, in premature infants, it is more prevalent, so this mode of exposure becomes salient in this population. We will briefly review some of the mechanisms of Mn neurotoxicity and conclude with a discussion of ripe areas for research in this underreported area of neurotoxicity.

Investigations of differences in iron oxidation state inside single neurons from substantia nigra of Parkinson’s disease and control patients using the micro-XANES technique

X-ray absorption near edge structure spectroscopy was applied in order to investigate differences in iron chemical state between the nerve cells of substantia nigra (SN) representing Parkinson's disease (PD) and those of control cases. Autopsy samples were cut using a cryotome, and were not fixed and not embedded in paraffin. The comparison of the absorption spectra near the iron K-edge measured in melanized neurons from SN of PD and control samples did not show significant differences in iron oxidation state. Measurements of inorganic reference materials containing iron in the second and third oxidation states indicate that most of the iron in all the nerve cell bodies examined was oxidized and occurred as trivalent ferric iron (Fe(3+)).

Speciation of manganese in cells and mitochondria: a search for the proximal cause of manganese neurotoxicity

Recent studies of speciation of manganese (Mn) in brain mitochondria, neuron-like cells, and astrocytes are reviewed. No evidence is found for oxidation of Mn(2+) complexes to a Mn(3+) complex. The only evidence for any Mn(3+) complex is found in a spectrum essentially identical to that of mitochondrial manganese superoxide dismutase (MnSOD). While this does not prove that no Mn(3+) is produced in these tissues by oxidation of Mn(2+), it does suggest that formation of an active Mn(3+) complex by oxidation of Mn(2+) probably does not play as important a role in Mn toxicity as has been suggested earlier. Since these results suggest that we should look elsewhere for the proximal causes of Mn neurotoxicity, we consider the possibilities that Mn(3+) may be transported into the cell via transferrin and that Mn(2+) may inhibit Ca(2+)-activation and control of the rate of ATP production by oxidative phosphorylation.

The transport of manganese across the blood-brain barrier

The mammalian central nervous system (CNS) possesses a unique and specialized capillary adaptation, referred to as the blood-brain barrier (BBB). The BBB maintains an optimal neuronal microenvironment, regulating blood-tissue exchange of macromolecules and nutrients. The BBB is characterized by individual endothelial cells that are continuously linked by tight junctions, inhibiting the diffusion of macromolecules and solutes between adjacent endothelial cells. This review will focus on pertinent issues to BBB maintenance, and survey recent dogmas on the transport mechanisms for the essential metal, manganese, across this barrier. Specifically, putative carriers for manganese into and out of the brain will be discussed.

Determining the oxidation states of manganese in NT2 cells and cultured astrocytes

Excessive brain manganese (Mn) can produce a syndrome called "manganism", which correlates with loss of striatal dopamine and cell death in the striatum and globus pallidus. The prevalent hypothesis for the cause of this syndrome has been oxidation of cell components by the strong oxidizing agent, Mn(3+), either formed by oxidation of intracellular Mn(2+) or transported into the cell as Mn(3+). We have recently used X-ray absorption near edge structure spectroscopy (XANES) to determine the oxidation states of manganese complexes in brain and liver mitochondria and in nerve growth factor (NGF)-induced and non-induced PC12 cells. No evidence was found for stabilization or accumulation of Mn(3+) complexes because of oxidation of Mn(2+) by reactive oxygen species in these tissues. Here we extend these studies of manganese oxidation state to cells of brain origin, human neuroteratocarcinoma (NT2) cells and primary cultures of rat astrocytes. Again we find no evidence for stabilization or accumulation of any Mn(3+) complex derived from oxidation of Mn(2+) under a range of conditions.

Determining the oxidation states of manganese in PC12 and nerve growth factor-induced PC12 cells

Excessive brain Mn can produce toxicity with symptoms resembling parkinsonism. This syndrome, called "manganism," correlates with loss of dopamine in the striatum and cell death in the striatum and globus pallidus. A common hypothesis is that cell damage in Mn toxicity is caused by oxidation of important cell components by Mn3+. Determination of the amount of Mn3+ present, under a range of conditions, in neuronal cells and brain mitochondria represents an important step in evaluating the "damage through oxidation by Mn3+ hypothesis." In an earlier paper we used X-ray absorption near-edge structure (XANES) spectroscopy to determine the amount of Mn2+ and Mn3+ in brain mitochondria under a range of conditions. Here we extend the study to investigate the evidence for formation of Mn3+ through oxidation of Mn2+ by ROS in PC12 cells and in PC12 cells induced with nerve growth factor (NGF) to display a phenotype more like that of neurons. Although the results suggest that very small amounts of Mn3+ might be present at low Mn levels, probably in Mn superoxide dismutase, Mn3+ is not stabilized by complex formation in these cells and therefore does not accumulate to detectable amounts.

Determination of the oxidation states of manganese in brain, liver, and heart mitochondria

Excess brain manganese can produce toxicity with symptoms that resemble those of Parkinsonism and causes that remain elusive. Manganese accumulates in mitochondria, a major source of superoxide, which can oxidize Mn2+ to the powerful oxidizing agent Mn3+. Oxidation of important cell components by Mn3+ has been suggested as a cause of the toxic effects of manganese. Determining the oxidation states of intramitochondrial manganese could help to identify the dominant mechanism of manganese toxicity. Using X-ray absorbance near edge structure (XANES) spectroscopy, we have characterized the oxidation state of manganese in mitochondria isolated from brain, liver, and heart over concentrations ranging from physiological to pathological. Results showed that (i) spectra from different model manganese complexes of the same oxidation state were similar to each other and different from those of other oxidation states and that the position of the absorption edge increases with oxidation state; (ii) spectra from intramitochondrial manganese in isolated brain, heart and liver mitochondria were virtually identical; and (iii) under these conditions intramitochondrial manganese exists primarily as a combination of Mn2+ complexes. No evidence for Mn3+ was detected in samples containing more than endogenous manganese levels, even after incubation under conditions promoting reactive oxygen species (ROS) production. While the presence of Mn3+ complexes cannot be proven in the spectrum of endogenous mitochondrial manganese, the shape of this spectrum could suggest the presence of Mn3+ near the limit of detection, probably as MnSOD.