Speciation in Vanadium Bioinorganic Systems. 2. An NMR, ESR, and Potentiometric Study of the Aqueous H(+)-Vanadate-Maltol System

Mechanism of the antimicrobial activity induced by phosphatase inhibitor sodium ortho-vanadate

The present study describes a novel antimicrobial mechanism based on Sodium Orthovanadate (SOV), an alkaline phosphatase inhibitor. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) were employed to examine the surface morphologies of the test organism, Escherichia coli (E. coli), during various antibacterial phases. Our results indicated that SOV kills bacteria by attacking cell wall growth and development, leaving E. coli's outer membrane intact. Our antimicrobial test indicated that the MIC of SOV for both E. coli and Lactococcus lactis (L. lactis) is 40 μM. A combination of quantum mechanical calculations and vibrational spectroscopy revealed that divanadate from SOV strongly coordinates with Ca2+ and Mg2+, which are the activity centers for the phosphatase that regulates bacterial cell wall synthesis. The current study is the first to propose the antibacterial mechanism caused by SOV attacking cell wall.

Computational Prediction of Speciation Diagrams and Nucleation Mechanisms: Molecular Vanadium, Niobium, and Tantalum Oxide Nanoclusters in Solution

Understanding the aqueous speciation of molecular metal-oxo-clusters plays a key role in different fields such as catalysis, electrochemistry, nuclear waste recycling, and biochemistry. To describe the speciation accurately, it is essential to elucidate the underlying self-assembly processes. Herein, we apply a computational method to predict the speciation and formation mechanisms of polyoxovanadates, -niobates, and -tantalates. While polyoxovanadates have been widely studied, polyoxoniobates and -tantalates lack the same level of understanding. First, we propose a pentavanadate cluster ([V₅O₁₄]^3-) as a key intermediate for the formation of the decavanadate. Our computed phase speciation diagram is in particularly good agreement with the experiments. Second, we report the formation constants of the heptaniobate, [Nb₇O₂₂]^9-, decaniobate, [Nb₁₀O₂₈]^6-, and tetracosaniobate [H₉Nb₂₄O₇₂]^15-. Additionally, we compute the speciation and phase diagram of niobium, which so far was restricted to Lindqvist derivates. Finally, we predict the formation constant of the decatantalate ([Ta₁₀O₂₆]^6-) in water, even though it had only been synthesized in toluene. Furthermore, we also calculate the corresponding speciation and phase diagrams for polyoxotantalates. Overall, we show that our method can be successfully applied to different families of molecular metal oxides without any need for readjustments; therefore, it can be regarded as a trustworthy tool for exploring polyoxometalates' chemistry.

Acute Toxicity Evaluation of Non-Innocent Oxidovanadium(V) Schiff Base Complex

The vanadium(V) complexes have been investigated as potential anticancer agents which makes it essential to evaluate their toxicity for safe use in the clinic. The large-scale synthesis and the acute oral toxicity in mice of the oxidovanadium(V) Schiff base catecholate complex, abbreviated as [VO(HSHED)dtb] containing a redox-active ligand with tridentate Schiff base (HSHED = N-(salicylideneaminato)-N’-(2-hydroxyethyl)-1,2-ethylenediamine) and dtb = 3,5-di-(t-butyl)catechol ligands were carried out. The body weight, food consumption, water intake as well biomarkers of liver and kidney toxicity of the [VO(HSHED)dtb] were compared to the precursors, sodium orthovanadate, and free ligand. The 10-fold scale-up synthesis of the oxidovanadium(V) complex resulting in the preparation of material in improved yield leading to 2–3 g (79%) material suitable for investigating the toxicity of vanadium complex. No evidence of toxicity was observed in animals when acutely exposed to a single dose of 300 mg/kg for 14 days. The toxicological results obtained with biochemical and hematological analyses did not show significant changes in kidney and liver parameters when compared with reference values. The low oral acute toxicity of the [VO(HSHED)dtb] is attributed to redox chemistry taking place under biological conditions combined with the hydrolytic stability of the oxidovanadium(V) complex. These results document the design of oxidovanadium(V) complexes that have low toxicity but still are antioxidant and anticancer agents.

Misinterpretations in Evaluating Interactions of Vanadium Complexes with Proteins and Other Biological Targets

In aqueous media, VIV- and VV-ions and compounds undergo chemical changes such as hydrolysis, ligand exchange and redox reactions that depend on pH and concentration of the vanadium species, and on the nature of the several components present. In particular, the behaviour of vanadium compounds in biological fluids depends on their environment and on concentration of the many potential ligands present. However, when reporting the biological action of a particular complex, often the possibility of chemical changes occurring has been neglected, and the modifications of the complex added are not taken into account. In this work, we highlight that as soon as most vanadium(IV) and vanadium(V) compounds are dissolved in a biological media, they undergo several types of chemical transformations, and these changes are particularly extensive at the low concentrations normally used in biological experiments. We also emphasize that in case of a biochemical interaction or effect, to determine binding constants or the active species and/or propose mechanisms of action, it is essential to evaluate its speciation in the media where it is acting. This is because the vanadium complex no longer exists in its initial form.

Integrated experimental/computational approaches to characterize the systems formed by vanadium with proteins and enzymes

An integrated instrumental/computational approach to characterize metallodrug–protein adducts at the molecular level is reviewed. A series of applications are described, focusing on potential vanadium drugs with a generalization to other metals.

Bis-maltol-polyamine family: structural modifications at strategic positions. Synthesis, coordination and antineoplastic activity of two new ligands

Substitution at the maltol C6 position affects the antineoplastic and coordination properties of Malten, acting on degradation time, binding ability and biological activity.

Biospeciation of Potential Vanadium Drugs of Acetylacetonate in the Presence of Proteins

Among vanadium compounds with potential medicinal applications, [V^IVO(acac)₂] is one of the most promising for its antidiabetic and anticancer activity. In the organism, however, interconversion of the oxidation state to +III and +V and binding to proteins are possible. In this report, the transformation of V^III(acac)₃, V^IVO(acac)₂, and V^VO₂(acac)2- after the interaction with two model proteins, lysozyme (Lyz) and ubiquitin (Ub), was studied with ESI-MS (ElectroSpray Ionization-Mass Spectroscopy), EPR (Electron Paramagnetic Resonance), and computational (docking) techniques. It was shown that, in the metal concentration range close to that found in the organism (15–250 μM), V^III(acac)₃ is oxidized to V^IVO(acac)⁺ and V^IVO(acac)₂, which—in their turn—interact with proteins to give n[V^IVO(acac)]–Protein and n[V^IVO(acac)₂]–Protein adducts. Similarly, the complex in the +IV oxidation state, V^IVO(acac)₂, dissociates to the mono-chelated species V^IVO(acac)⁺ which binds to Lyz and Ub. Finally, V^VO₂(acac)2- undergoes complete dissociation to give the 'bare' V^VO2+ ion that forms adducts n[V^VO₂]–Protein with n = 1–3. Docking calculations allowed the prediction of the residues involved in the metal binding. The results suggest that only the V^IVO complex of acetylacetonate survives in the presence of proteins and that its adducts could be the species responsible of the observed pharmacological activity, suggesting that in these systems V^IVO²⁺ ion should be used in the design of potential vanadium drugs. If V^III or V^VO₂ potential active complexes had to be designed, the features of the organic ligand must be adequately modulated to obtain species with high redox and thermodynamic stability to prevent oxidation and dissociation.

Playing with Structural Parameters: Synthesis and Characterization of Two New Maltol-Based Ligands with Binding and Antineoplastic Properties

Two maltol-based ligands, N,N′-bis((3-hydroxy-4-pyron-2-yl)methyl)-1,4-piperazine (L1) and N,N′,N′-tris((3-hydroxy-4-pyron-2-yl)methyl)-N-methylethylendiamine (L2), were synthesized and characterized. L1 and L2, containing, respectively, two and three maltol units spaced by a diamine fragment, were designed to evaluate how biological and binding features are affected by structural modifications of the parent compound malten. The acid-base behavior and the binding properties towards transition, alkaline-earth (AE) and rare-earth (RE) cations in aqueous solution, studied by potentiometric, UV-Vis and NMR analysis, are reported along with biological studies on DNA and leukemia cells. Both ligands form stable complexes with Cu(II), Zn(II) and Co(II) that were studied as metallo-receptors for AE and RE at neutral pH. L1 complexes are more affected than L2 ones by hard cations, the L1-Cu(II) system being deeply affected by RE. The structural modifications altered the mechanism of action: L1 partially maintains the ability to induce structural alterations of DNA, while L2 provokes single strand (nicks) and to a lesser extent double strand breaks of DNA.

Polyoxometalates in solution: speciation under spotlight

The review covers stability and transformations of classical polyoxometalates in aqueous solutions and provides their ion-distribution diagrams over a wide pH range.

Vanadium: History, chemistry, interactions with α-amino acids and potential therapeutic applications

In the last 30 years, since the discovery that vanadium is a cofactor found in certain enzymes of tunicates and possibly in mammals, different vanadium-based drugs have been developed targeting to treat different pathologies. So far, the in vitro studies of the insulin mimetic, antitumor and antiparasitic activity of certain compounds of vanadium have resulted in a great boom of its inorganic and bioinorganic chemistry. Chemical speciation studies of vanadium with amino acids under controlled conditions or, even in blood plasma, are essential for the understanding of the biotransformation of e.g. vanadium antidiabetic complexes at the physiological level, providing clues of their mechanism of action. The present article carries out a bibliographical research emphaticizing the chemical speciation of the vanadium with different amino acids and reviewing also some other important aspects such as its chemistry and therapeutical applications of several vanadium complexes.

Colorimetric detection of hydrogen peroxide by dioxido-vanadium(V) complex containing hydrazone ligand: synthesis and crystal structure

Dioxido-vanadium(V) complex has been synthesized in good yield, the complex was characterized by IR, UV-visible and ¹H NMR spectroscopy. Single crystal X-ray crystallography techniques were used to assign the structure of the complex. Complex crystallized with monoclinic P2₁/c space group with cell parameters a (Å) = 39.516(5), b (Å) = 6.2571(11), c (Å) = 17.424(2), α (°) = 90, β (°) = 102.668(12) and γ (°) = 90. The hydrazone ligand is coordinate to metal ion in tridentate fashion through -ONO- donor atoms forming a distorted square pyramidal geometry around the metal ion.

Vanadium(IV/V) complexes of Triapine and related thiosemicarbazones: Synthesis, solution equilibrium and bioactivity

The stoichiometry and thermodynamic stability of vanadium(IV/V) complexes of Triapine and two related α(N)-heterocyclic thiosemicarbazones (TSCs) with potential antitumor activity have been determined by pH-potentiometry, EPR and (51)V NMR spectroscopy in 30% (w/w) dimethyl sulfoxide/water solvent mixtures. In all cases, mono-ligand complexes in different protonation states were identified. Dimethylation of the terminal amino group resulted in the formation of vanadium(IV/V) complexes with considerably higher stability. Three of the most stable complexes were also synthesized in solid state and comprehensively characterized. The biological evaluation of the synthesized vanadium complexes in comparison to the metal-free ligands in different human cancer cell lines revealed only minimal influence of the metal ion. Thus, in addition the coordination ability of salicylaldehyde thiosemicarbazone (STSC) to vanadium(IV/V) ions was investigated. The exchange of the pyridine nitrogen of the α(N)-heterocyclic TSCs to a phenolate oxygen in STSC significantly increased the stability of the complexes in solution. Finally, this also resulted in increased cytotoxicity activity of a vanadium(V) complex of STSC compared to the metal-free ligand.

The (biological) speciation of vanadate(V) as revealed by (51)V NMR: A tribute on Lage Pettersson and his work

Four decades of research carried out by Lage Pettersson, his group and his coworkers are reviewed, research that has been directed predominantly towards the speciation of vanadate and systems containing, along with vanadate and co-reactants such as phosphate and peroxide, biologically relevant organics. In particular, those organics have been addressed that either are (potential) ligands for vanadate-derived coordination compounds generated at physiological conditions and/or function as constituents in medicinally interesting oxidovanadium compounds. Examples for molecules introduced in the context of the physiological vanadate-ligand interaction include the dipeptides Pro-Ala, Ala-Gly, Ala-His and Ala-Ser, the serum constituents lactate and citrate, and the nucleobases adenosine and uridine. The speciation in the vanadate-picolinate and vanadate-maltol systems is geared towards insulin-enhancing vanadium drugs. The speciation as a function of pH, ionic strength and the concentration of vanadate and the ligand(s) is based on potentiometric and (51)V NMR investigations, a methodical combination that allows reliable access to composition, formation constants and, to some extent, also structural details for the manifold of species present in aqueous media at physiological pH and beyond. The time frame 1971 to 2014 is reviewed, emphasizing the interval 1985 to 2006, and thus focusing on biologically interesting vanadium systems. Figurative representations from the original literature have been included.

Coordination chemistry may explain pharmacokinetics and clinical response of vanadyl sulfate in type 2 diabetic patients

Vanadium, abbreviated V, is an early transition metal that readily forms coordination complexes with a variety of biological products such as proteins, metabolites, membranes and other structures. The formation of coordination complexes stabilizes metal ions, which in turn impacts the biodistribution of the metal. To understand the biodistribution of V, V in oxidation state iv in the form of vanadyl sulfate (25, 50, 100 mg V daily) was given orally for 6 weeks to 16 persons with type 2 diabetes. Elemental V was determined using Graphite Furnas Atomic Absorption Spectrometry against known concentrations of V in serum, blood or urine. Peak serum V levels were 15.4 ± 6.5, 81.7 ± 40 and 319 ± 268 ng ml(-1) respectively, and mean peak serum V was positively correlated with dose administered (r = 0.992, p = 0.079), although large inter-individual variability was found. Total serum V concentration distribution fit a one compartment open model with a first order rate constant for excretion with mean half times of 4.7 ± 1.6 days and 4.6 ± 2.5 days for the 50 and 100 mg V dose groups respectively. At steady state, 24 hour urinary V output was 0.18 ± 0.24 and 0.97 ± 0.84 mg in the 50 and 100 mg V groups respectively, consistent with absorption of 1 percent or less of the administered dose. Peak V in blood and serum were positively correlated (r = 0.971, p < 0.0005). The serum to blood V ratio for the patients receiving 100 mg V was 1.7 ± 0.45. Regression analysis showed that glycohemoglobin was a negative predictor of the natural log(ln) peak serum V (R(2) = 0.40, p = 0.009) and a positive predictor of the euglycemic-hyperinsulinemic clamp results at high insulin values (R(2) = 0.39, p = 0.010). Insulin sensitivity measured by euglycemic-hyperinsulinemic clamp was not significantly correlated with ln peak serum V. Globulin and glycohemoglobin levels taken together were negative predictors of fasting blood glucose (R(2) = 0.49, p = 0.013). Although V accumulation in serum was dose-dependent, no correlation between total serum V concentration and the insulin-like response was found in this first attempt to correlate anti-diabetic activity with total serum V. This study suggests that V pools other than total serum V are likely related to the insulin-like effect of this metal. These results, obtained in diabetic patients, document the need for consideration of the coordination chemistry of metabolites and proteins with vanadium in anti-diabetic vanadium complexes.

Metal speciation in health and medicine represented by iron and vanadium

The influence of metals in biology has become more and more apparent within the past century. Metal ions perform essential roles as critical scaffolds for structure and as catalysts in reactions. Speciation is a key concept that assists researchers in investigating processes that involve metal ions. However, translation of the essential area across scientific fields has been plagued by language discrepancies. To rectify this, the IUPAC Commission provided a framework in which speciation is defined as the distribution of species. Despite these attempts, contributions from inorganic chemists to the area of speciation have not fully materialized in part because the past decade's contributions focused on technological advances, which are not yet to the stage of measuring speciation distribution in biological solutions. In the following, we describe how speciation influences the area of metals in medicine and how speciation distribution has been characterized so far. We provide two case studies as an illustration, namely, vanadium and iron. Vanadium both has therapeutic importance and is known as a cofactor for metalloenzymes. In addition to being a cation, vanadium(V) has analogy with phosphorus and as such is a potent inhibitor for phosphorylases. Because speciation can change the metal's existence in cationic or anionic forms, speciation has profound effects on biological systems. We also highlight how speciation impacts iron metabolism, focusing on the rather low abundance of biologically relevant iron cation that actually exists in biological fluids. fluids. Furthermore, we point to recent investigations into the mechanism of Fenton chemistry, and that the emerging results show pH dependence. The studies suggest formation of Fe(IV)-intermediates and that the generally accepted mechanism may only apply at low pH. With broader recognition toward biological speciation, we are confident that future investigations on metal-based systems will progress faster and with significant results. Studying metal complexes to explore the properties of a potential "active species" and further uncovering the details associated with their specific composition and geometry are likely to be important to the action.

Synthesis, basicity, structural characterization, and biochemical properties of two [(3-hydroxy-4-pyron-2-yl)methyl]amine derivatives showing antineoplastic features

The N,N'-bis[(3-hydroxy-4-pyron-2-yl)methyl]-N,N'-dimethylethylendiamine (malten) and 4,10-bis[(3-hydroxy-4-pyron-2-yl)methyl]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane (maltonis) were synthesized and characterized. The acid-base behavior, structural characterizations, and biochemical studies in aqueous solution were reported. Each compound contains two 3-hydroxy-2-methyl-4-pyrone units (maltol) symmetrically spaced by a polyamine fragment, the 1,4-dimethylethylendiamine (malten), or the 1,7-dimethyl-1,4,7,10-tetraazacyclododecane (maltonis). They are present at physiological pH 7.4 in the form of differently charged species: neutral but in a zwitterion form for malten and monopositive with an internal separation of charges for maltonis. Malten and maltonis are both able to alter the chromatin structure inducing the covalent binding of genomic DNA with proteins, a feature consistent with the known antiproliferative activity exerted by this class of molecules. Solid-state results and MD simulations in water show that malten, because of its molecular topology, should be more prone than maltonis to act as a donor of H-bonds in intermolecular contacts, thus it should give a better noncovalent approach with the negatively charged DNA. Crystal structures of [H(2)malten](2+) and [H(2)maltonis](2+) cations were also reported.

Vanadate complexes in serum: a speciation modeling study

The speciations of two drug candidate ligands, 2-hydroxypyridine-N-oxide (Hhpno) and 2-mercaptopyridine-N-oxide (Hmpno), with vanadate (V(V)) were determined at 25.0 degrees C and 0.20 mol dm(-3) KCl by pH-metric and (51)V-NMR methods. At pH 7.4, the two predominant compounds with both ligands are the VO(2)L(2) and VO(2)L(OH). NH(4)[VO(2)(hpno)(2)] x 3 H(2)O was prepared in solid form, and its crystal structure was determined by X-ray diffraction. The stabilities of the complexes VO(2)L(2) of five drug candidate ligands were compared at pH 7.4. In view of the stability sequence hpno > maltol approximately hdp (Hhdp: 3-hydroxy-1,2-dimethyl-4-pyridinone) >> mpno > picolinic acid, the first two of these ligands were chosen for equilibrium studies with apotransferrin (apoTf) competition. The V(V)-apoTf stability constants (log K(1) = 6.03 +/- 0.10; log K(2) = 5.46 +/- 0.18) determined by (51)V-NMR spectroscopy were confirmed by ultrafiltration. Both methods proved that there seems to be no hydrogencarbonate-vanadate competition for the apoTf anion-binding positions. The other potential high molecular mass V(V) binder in the serum is human serum albumin (HSA). As no interaction was detected by (51)V-NMR spectroscopy or fluorimetry, the binding properties of HSA were quantified on the basis of literature data. As a final conclusion, speciation modeling calculations suggest that, under serum conditions, apoTf is probably the primary metal ion binder, even in the presence of the most stable V(V) carrier ligands hpno and maltol and HSA plays a negligible role in V(V) binding.

Nonapiperidinium monohydrogen deca-vanadate tetra-nitrate

The title compound, (C₅H₁₂N)₉[HV₁₀O₂₈](NO₃)₄, contains a monoprotonated deca­vanadate polyanion which lies on an inversion center. All the piperidinium cations adopt chair conformations. In the crystal structure, inter­molecular N—H⋯O hydrogen bonds form chains along [001]. As well as half of a polyanion, the asymmetric unit contains one full and two half-occupancy nitrate ions and four full occupancy and one half-occupancy piperidinium cations; the half-occupancy piperidinium cation is disordered over two general sites with occupancies of 0.32 and 0.18, and is, in turn, disordered over an inversion center.

Vanadium treatment of type 2 diabetes: a view to the future

3-Hydroxy-2-methyl-4-pyrone and 2-ethyl-3-hydroxy-4-pyrone (maltol and ethyl maltol, respectively) have proven especially suitable as ligands for vanadyl ions, in potential insulin enhancing agents for diabetes mellitus. Both bis(maltolato)oxovanadium(IV) (BMOV), and the ethylmaltol analog, bis(ethylmaltolato)oxovanadium(IV) (BEOV), have the desired intermediate stability for pro-drug use, and have undergone extensive pre-clinical testing for safety and efficacy. Pharmacokinetic evaluation indicates a pattern of biodistribution consistent with fairly rapid dissociation and uptake, binding to serum transferrin for systemic circulation and transport to tissues, with preferential uptake in bone. These bis-ligand oxovanadium(IV) (VOL(2)) compounds have a clear advantage over inorganic vanadyl sulfate in terms of bioavailability and pharmaceutical efficacy. BEOV has now completed Phase I and has advanced to Phase II clinical trials. In the Phase I trial, a range of doses from 10 mg to 90 mg BEOV, given orally to non-diabetic volunteers, resulted in no adverse effects; all biochemical parameters remained within normal limits. In the Phase IIa trial, BEOV (AKP-020), 20 mg, daily for 28 days, per os, in seven type 2 diabetic subjects, was associated with reductions in fasting blood glucose and %HbA1c; improved responses to oral glucose tolerance testing, versus the observed worsening of diabetic symptoms in the two placebo controls.

Calorimetric studies of the interaction between the insulin-enhancing drug candidate bis(maltolato)oxovanadium(IV) (BMOV) and human serum apo-transferrin

Bis(maltolato)oxovanadium(IV) (BMOV), and its ethylmaltol analog, bis(ethylmaltolato)oxovanadium(IV) (BEOV), are candidate insulin-enhancing agents for the treatment of type 2 diabetes mellitus; in mid-2008, BEOV advanced to phase II clinical testing. The interactions of BMOV and its inorganic congener, vanadyl sulfate (VOSO(4)), with human serum apo-transferrin (hTf) were investigated using differential scanning calorimetry (DSC). Addition of BMOV or VOSO(4) to apo-hTf resulted in an increase in thermal stability of both the C- and N-lobes of transferrin as a result of binding to either vanadyl compound. A series of DSC thermograms of hTf solutions containing different molar ratios of BMOV and VOSO(4) were used to determine binding constants; at 25 degrees C the binding constants of BMOV to the C- and N-lobes of apo-hTf were found to be 3 (+/-1)x10(5) and 1.8 (+/-0.7)x10(5)M(-1), respectively. The corresponding values for VOSO(4) were 1.7 (+/-0.3)x10(5) and 7 (+/-2)x10(4)M(-1). The results show that the vanadium species initially presented as either BMOV or VOSO(4) had similar affinities for human serum transferrin due to oxidation of solvated vanadyl(IV) prior to complexation to transferrin. Binding of metavanadate (VO(3)(-)) was confirmed by DSC and isothermal titration calorimetry (ITC) experiments of the interaction between sodium metavanadate (NaVO(3)) and hTf.

Sarcoplasmic reticulum calcium ATPase is inhibited by organic vanadium coordination compounds: pyridine-2,6-dicarboxylatodioxovanadium(V), BMOV, and an amavadine analogue

The general affinity of the sarcoplasmic reticulum (SR) Ca (2+)-ATPase was examined for three different classes of vanadium coordination complexes including a vanadium(V) compound, pyridine-2,6-dicarboxylatodioxovanadium(V) (PDC-V(V)), and two vanadium(IV) compounds, bis(maltolato)oxovanadium(IV) (BMOV), and an analogue of amavadine, bis( N-hydroxylamidoiminodiacetato)vanadium(IV) (HAIDA-V(IV)). The ability of vanadate to act either as a phosphate analogue or as a transition-state analogue with enzymes' catalysis phosphoryl group transfer suggests that vanadium coordination compounds may reveal mechanistic preferences in these classes of enzymes. Two of these compounds investigated, PDC-V(V) and BMOV, were hydrolytically and oxidatively reactive at neutral pH, and one, HAIDA-V(IV), does not hydrolyze, oxidize, or otherwise decompose to a measurable extent during the enzyme assay. The SR Ca (2+)-ATPase was inhibited by all three of these complexes. The relative order of inhibition was PDC-V(V) > BMOV > vanadate > HAIDA-V(IV), and the IC 50 values were 25, 40, 80, and 325 microM, respectively. Because the observed inhibition is more potent for PDC-V(V) and BMOV than that of oxovanadates, the inhibition cannot be explained by oxovanadate formation during enzyme assays. Furthermore, the hydrolytically and redox stable amavadine analogue HAIDA-V(IV) inhibited the Ca (2+)-ATPase less than oxovanadates. To gauge the importance of the lipid environment, studies of oxidized BMOV in microemulsions were performed and showed that this system remained in the aqueous pool even though PDC-V(V) is able to penetrate lipid interfaces. These findings suggest that the hydrolytic properties of these complexes may be important in the inhibition of the calcium pump. Our results show that two simple coordination complexes with known insulin enhancing effects can invoke a response in calcium homeostasis and the regulation of muscle contraction through the SR Ca (2+)-ATPase.

Inhibition of protein tyrosine phosphatase 1B and alkaline phosphatase by bis(maltolato)oxovanadium (IV)

Vanadate has been recognized as a specific and potent phosphatase inhibitor since its structure is similar to that of phosphate. In this study, we measured the inhibition of glutathione S-transferase-tagged protein tyrosine phosphatase 1B (GST-PTP1B) and alkaline phosphatase (ALP) by the insulin enhancing compounds, bis(maltolato)oxovanadium(IV) (BMOV). The results showed that the activity of GST-PTP1B was reversibly inhibited by solutions of BMOV with an IC(50) value of 0.86+/-0.02 microM. Steady state kinetic studies showed that inhibition of GST-PTP1B by BMOV was of a mixed competitive and noncompetitive type. In addition, incubation of GST-PTP1B with BMOV showed a time-dependent biphasic inactivation of the protein. On the other hand, the inhibitory behavior of BMOV on ALP activity was reversible and competitive with an IC(50) value of 32.1+/-0.6 microM. Incubation with BMOV did not show biphasic inactivation of ALP. The reversible inhibition of GST-PTP1B by BMOV is more potent than that of ALP, but solutions of BMOV inhibited both enzymes. This data support the suggestion that mechanisms for the inhibitory effects of BMOV on GST-PTP1B and ALP are very different.

Simple oxovanadates as multiparameter probes of reverse micelles

Using a wide range of different methods, researchers have found that the environment inside reverse micelles differs from bulk aqueous solution in many ways. Here, we present a new tool, a series of aqueous oxovanadium(V) reactions, to probe pH, viscosity, and ionic strength in the aqueous interior of reverse micelles. In addition to their potential as anionic probe analogues to phosphates, simple oxovanadium(V) compounds have equilibrium characteristics in aqueous media exquisitely sensitive to their environment. Therefore, the speciation of vanadate equilibria can be used as a parameter to characterize the intramicellar medium. Vanadate speciation is monitored through 51V NMR spectroscopy, which also yields information through chemical shifts and linewidths of spectral features. The speciation observed suggests that the relative acidity of a basic vanadate stock solution is slightly reduced in large, w0 >or= 12, reverse micelles, but that for smaller reverse micelles, speciation reflects the strong interaction of these negatively charged oxometalates with the reverse micelle and suggest an increased solution viscosity in the reverse micelles. This interpretation is obtained through different responses closely linked to the reverse micellar size and the specific conditions in the stock solutions used to form reverse micelles.