Anti-diabetic effects of vanadium(III, IV, V)–chlorodipicolinate complexes in streptozotocin-induced diabetic rats

Predicting 51V nuclear magnetic resonance observables in molecular crystals

Solid-state nuclear magnetic resonance (NMR) spectroscopy and quantum chemical density functional theory (DFT) calculations are widely used to characterize vanadium centers in biological and pharmaceutically relevant compounds. Several techniques have been recently developed to improve the accuracy of predicted NMR parameters obtained from DFT. Fragment-based and planewave-corrected methods employing hybrid density functionals are particularly effective tools for solid-state applications. A recent benchmark study involving molecular crystal compounds found that fragment-based NMR calculations using hybrid density functionals improve the accuracy of predicted 51V chemical shieldings by 20% relative to traditional planewave methods. This work extends the previous study, including a careful analysis of 51V chemical shift anisotropy, electric field gradient calculations, and a more extensive test set. The accuracy of planewave-corrected techniques and recently developed fragment-based methods using electrostatic embedding based on the polarized continuum model (PCM) are found to be highly competitive with previous methods. Planewave-corrected methods achieve a 34% improvement in the errors of predicted 51V chemical shieldings relative to planewave. Additionally, planewave-corrected and fragment-based calculations were performed using PCM embedding, improving the accuracy of predicted 51V chemical shielding (CS) tensor principal values by 30% andC q values by 15% relative to traditional planewave methods. The performance of these methods is further examined using a redox-active oxovandium complex and a common 51V NMR reference compound.

Testicular protective effects of hesperidin against chemical and biological toxicants

Toxic agents can adversely impact the male reproductive system mainly via activating oxidative stress affecting the seminiferous epithelia, spermatogenesis, sperms, and the testis. Toxic agents lead to the excessive generation of reactive oxygen species (ROS), such as hydroxyl radicals, hydrogen peroxide, and superoxide anions. ROS exert a cytotoxic effect and oxidative damage to nucleic acids, proteins, and membrane lipids. Hesperidin is a pharmacologically active phytoflavone abundantly occurring in citrus fruits, such as oranges and lemons. It has shown various pharmacological properties such as antioxidant, anti-inflammatory, anti-carcinogenic, analgesic, antiviral, anti-coagulant, hypolipidemic, and hypoglycemic effects. Hesperidin has been found to exert protective effects against natural and chemical toxins-induced organ toxicity. Considerable evidence has implicated the testicular protective effects of hesperidin against the toxicological properties of pharmaceutical drugs as well as biological and chemical agents, and in the present review, we discussed, for the first time, the reported studies. The resultant data indicate that hesperidin can exert testicular protective effects through antioxidant properties.

A Non-Toxic Binuclear Vanadium(IV) Complex as Insulin Adjuvant Improves the Glycemic Control in Streptozotocin-Induced Diabetic Rats

Diabetes mellitus (DM) complications are a burden to health care systems due to the associated consequences of poor glycemic control and the side effects of insulin therapy. Recently. adjuvant therapies, such as vanadium compounds, have gained attention due to their potential to improve glucose homeostasis in patients with diabetes. In order to determine the anti-diabetic and antioxidant effects of the oxidovanadium(IV) complex (Et3NH)2[{VO(OH}2)(ox)2(µ-ox)] or Vox2), rats with streptozotocin (STZ)-induced diabetes were treated with 30 and 100 mg/kg of Vox2, orally administered for 12 days. Vox2 at 100 mg/kg in association with insulin caused a 3.4 times decrease in blood glucose in STZ rats (424 mg/dL), reaching concentrations similar to those in the normoglycemic animals (126 mg/dL). Compared to insulin alone, the association with Vox2 caused an additional decrease in blood glucose of 39% and 65% at 30 and 100 mg/kg, respectively, and an increased pancreatic GSH levels 2.5 times. Vox2 alone did not cause gastrointestinal discomfort, diarrhea, and hepatic or renal toxicity and was not associated with changes in blood glucose level, lipid profile, or kidney or liver function. Our results highlight the potential of Vox2 in association with insulin in treating diabetes.

Medicinal applications of vanadium complexes with Schiff bases

Many transition metal complexes have been explored for their therapeutic properties after the discovery of cisplatin. Schiff bases have an efficient complexation tendency with the transition metals and several medicinal properties have been reported. However, fewer studies have reported the medicinal utility of vanadium and its Schiff base complexes. This paper provides a comprehensive overview of vanadium complexes with Schiff bases along with their mechanistic insight. Vanadium complexes in + 4 and + 5 oxidation states have exhibited well-defined geometry and found to be thermodynamically stable. The studies have reported the G0/G1 phase cell cycle arrest and decreased delta psi m, inducing mitochondrial membrane depolarization in cancer cell lines along with the alterations in the metabolism of the cancer cells upon dosing with the vanadium complexes. Cancer cell invasion and growth are also found to be markedly reduced by peroxo complexes of vanadium. The studies included in the review paper have been taken from leading indexing databases and focus was laid on recent reports in literature. The biological potential of vanadium complexes of Schiff bases opens new horizons for future interdisciplinary studies and investigation focussed on understanding the biochemistry of these complexes, along with designing new complexes which have better bioavailability, solubility and low or non-toxicity.

The antihyperglycemic and hypolipidemic activities of a sulfur-oxidovanadium(IV) complex

This study describes the synthesis, characterization, and biological activity of a new class of antidiabetic oxidovanadium(IV)-complexes with S₂O₂ coordination mode. The target complex 3,6-dithio-1,8-octanediolatooxidovanadium(IV), abbreviated as ([V^IVO(octd)]), where octd = 3,6-dithio-1,8-octanediol, is formed from the reaction between the 3,6-dithio-1,8-octanediol and vanadyl sulfate (V^IVOSO₄). The effects of treatment with ([V^IVO(octd)] on blood glucose, lipidic profile, body weight, food intake, water intake, urinary volume, glycogen levels, and biomarkers for liver toxicity were investigated using a streptozotocin (STZ)-induced diabetic Wistar rats model. The results have shown that the [V^IVO(octd)] complex caused a significant decrease in blood glucose (247.6 ± 19.3 mg/dL vs 430.1 ± 37.6 mg/dL diabetic group, p < 0.05), triglycerides (TG, 50%) and very low-density cholesterol (VLDL-C, 50%) levels in STZ-diabetic rats after 3 weeks of treatment. The [V^IVO(octd)] has shown antihyperglycemic activity in diabetic rats as well as a reduction in elevated lipid levels. Time-dependent studies using EPR and ⁵¹V NMR spectroscopy of [V^IVO(octd)] were done in aqueous solutions to determine the complex stability and species present in the oral gavage solution used for complex administration. The spectroscopic studies have shown that the antidiabetic/hypolipidemic activity could be attributed to [V^IVO(octd)], vanadium species resulting from redox processes, the hydrolysis of [V^IVO(octd)] and its decomposition products, or some combination of these factors. In summary, the oxidovanadium(IV) complex containing the S₂O₂ donor ligand has desirable antidiabetic properties eliminating the symptoms of Diabetes mellitus and its comorbidities.

Beneficial effects of Salvia officinalis essential oil on vanadium-induced testicular injury, DNA damage and histological alterations in Wistar rats

Vanadium has been shown to catalyze the generation of reactive oxygen species. Since free radical production and lipid peroxidation are potentially important mediators in testicular physiology and pathophysiology, the present study was conducted to elucidate vanadium-induced oxidative damage in rat testis and the ameliorative role of Salvia officinalis essential oil (SEO) against the adverse effects of this heavy metal. Adult male Wistar rats were treated daily during 10 days either with ammonium metavanadate (5 mg/kg bw, intraperitoneally), SEO (15 mg/kg bw, orally) or their combination. A group of rats receiving daily a saline solution served as a negative control. Vanadium treatment induced a significant decrease in body and reproductive organ weights, serum testosterone level and sperm number and motility. An increase in lipid peroxidation and protein oxidation as well as a marked inhibition in the activities of antioxidant enzymes in the testes and seminal vesicles indicated the occurrence of oxidative stress after vanadium toxicity. Histopathological changes in testis and seminal vesicles were also observed following vanadium administration. However, co-administration of SEO to vanadium-treated rats resulted in an appreciable improvement of these parameters, emphasizing the therapeutic effects of SEO. It can be suggested that SEO mitigates vanadium-induced reproductive damage due to its antioxidant capacity. Thus, we can hypothesize that SEO supplementation could protect against vanadium poisoning.

Vanadium(IV)-Chlorodipicolinate Protects against Hepatic Steatosis by Ameliorating Lipid Peroxidation, Endoplasmic Reticulum Stress, and Inflammation

Non-alcoholic fatty liver disease (NAFLD) is increasingly prevalent and represents a growing challenge in terms of prevention and treatment. The aim of this study is to investigate the protective effects and the underlying mechanisms of vanadium(IV)-chlorodipicolinate ([VIVO(dipic-Cl)(H2O)2, VOdipic-Cl]) in a mouse model of NAFLD induced by a high-fat diet (HFD). VOdipic-Cl (10 mg/kg/day body weight) treatment for 4 weeks significantly controlled body weight gain, and effectively reduced the increase in serum and hepatic triglyceride (TG) and total cholesterol (TC) levels, mitigated pathological injury, decreased malondialdehyde (MDA) level, and inhibited endoplasmic reticulum (ER) stress and inflammatory response in the livers of C57BL/6 obese mice. Moreover, RNA-sequencing analysis revealed distinct transcriptional profiles with differentially expressed genes (DEGs) in livers. We found that VOdipic-Cl effectively down-regulated genes related to lipid synthesis and up-regulated genes related to fatty acid transport and lipolysis, and down-regulated the expression of genes related to ER stress and immune response in the livers of obese mice. In conclusion, VOdipic-Cl effectively prevented hepatic steatosis by controlling body weight, mitigating oxidative stress, and regulating the expression of genes related to lipid metabolism, ER stress and immune response, which provides new insights into the molecular mechanism of the protective effect of VOdipic-Cl against hepatic steatosis.

Vanadium and diabetic dyslipidemia: A systematic review of animal studies

BACKGROUND

Diabetic dyslipidemia is caused by hyperglycemia and excessive mobilization of storage lipids, leading to increasing concentrations of triglycerides and total cholesterol. Due to the insulin-mimetic or insulin-enhancer features of vanadium, it has been recognized as a regulator of cell metabolism with hypoglycemic and hypolipidemic properties. The purpose of the current animal systematic review was to evaluate the effect of vanadium administration on diabetic dyslipidemia in diabetic animals.

METHODS

This is, to our knowledge, the first systematic review with the aim of investigating the relationship between vanadium and diabetic dyslipidemia among diabetes induced animals. Searches were performed in PubMed, Scopus, and web of science databases for animal studies examining the effect of vanadium on diabetic dyslipidemia in diabetic animals.

RESULTS

Of 124 full-text articles assessed, 48 animal studies were included in the present study with minor risk of bias. The majority of the studies confirmed the beneficial effects of different vanadium compounds in at least one of the parameters of lipid profile, especially regarding triglyceride and total cholesterol.

CONCLUSION

Current findings lend support to assess the long-term effects of different forms and doses of vanadium on lipid profile through well-designed clinical trials.

The effects of a new antidiabetic glycinium [(pyridine-2, 6-dicarboxylato) oxovanadate (V)] complex in high-fat diet of streptozotocin-induced diabetic rats

OBJECTIVE

The present study aimed to evaluate the antidiabetic effects of glycinium [(pyridine-2, 6-dicarboxylato) oxovanadate (V)] complex in type 2 diabetes rat model.

MATERIALS AND METHODS

Rats were allocated into 6 groups. Group I, nondiabetic rats; Group II, diabetic rats; Group III, diabetic rats receiving an intraperitoneal (i.p.) injection of metformin (45 mg/kg); Groups IV, V and VI were diabetic rats receiving i.p. injection of 5, 10, and 20 mg/kg of the complex for 3 weeks, respectively. Fasting blood glucose (FBG), insulin, liver enzymes, malondialdehyde (MDA), total antioxidant capacity (TAC), lipid profile, and HbA1c were measured.

RESULTS

AST, ALT and GGT activities and MDA levels were increased, while TAC was decreased in diabetic animals. Treatment of diabetic rats improved the HOMA-IR and returned HbA1c level to the normal value as well as elevated TAC and reduced MDA level.

CONCLUSION

We found that the complex possesses antidiabetic properties in experimental diabetes.

A Dioxidovanadium Complex cis-[VO2 (obz) py] Attenuates Hyperglycemia in Streptozotocin (STZ)-Induced Diabetic Male Sprague-Dawley Rats via Increased GLUT4 and Glycogen Synthase Expression in the Skeletal Muscle

Vanadium has demonstrated antihyperglycemic effects in diabetes mellitus (DM) but is, however, associated with toxicity. Therefore, new vanadium complexes envisaged to possess heightened therapeutic potency while rendering less toxicity are being explored. Accordingly, the aim of the study was to investigate the effects of a dioxidovanadium (V) complex, cis-[VO2 (obz) py], on selected glucose metabolism markers in streptozotocin (STZ)-induced diabetic rats. STZ-induced diabetic rats were treated orally with cis-[VO2 (obz) py] (10, 20, and 40 mg/kg) twice every 3rd day for 5 weeks. Blood glucose concentrations, body weight, and food and water intake were monitored weekly, for 5 weeks. Rats were then euthanized after which blood, liver, and muscle tissues were collected for biochemical analysis. The administration of dioxidovanadium complex significantly decreased blood glucose concentrations throughout the 5-week period in comparison with the diabetic control (DC). The attenuation of hyperglycemia was accompanied by an increased glycogen concentration in both liver and muscle tissues in the treated groups. Furthermore, a significant increase was observed in the expression of glucose transporter type 4 (GLUT4) in the skeletal muscle tissues and glycogen synthase in the liver tissues. These findings indicate that our vanadium complex cis-[VO2 (obz) py] may exert antihyperglycemic effects through increased glucose uptake, glycogen synthesis, and increased GLUT4 and glycogen synthase expression.

Preparation of Allyl Alcohol Oligomers Using Dipicolinate Oxovanadium(IV) Coordination Compound

Currently, new precatalysts for olefin oligomerization are being sought in the group of vanadium(IV) complexes. Thus, the aim of our research was to examine the catalytic activity of the oxovanadium(IV) dipicolinate complex [VO(dipic)(H₂O)₂] 2 H₂O (dipic = pyridine-2,6-dicarboxylate anion) in 2-propen-1-ol oligomerization as well as to characterize oligomerization products using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF-MS), infrared spectroscopy (IR) and nuclear magnetic resonance (NMR). The oligomerization process took place at room temperature, under atmospheric pressure and under nitrogen atmosphere to prevent oxidation of the activator MMAO-12-the modified methylaluminoxane (7 wt.%) aluminum in toluene. The last point was to determine the catalytic activity of the complex in the oligomerization reaction of 2-propen-1-ol. The aspect that enriches this work is the proposed mechanism of oligomerization of allyl alcohol based on the literature.

Relationship between Antioxidant Activity and Ligand Basicity in the Dipicolinate Series of Oxovanadium(IV) and Dioxovanadium(V) Complexes

Oxidative stress plays an important role in the pathogenesis of many serious diseases, including cancer, atherosclerosis, coronary artery disease, Parkinson’s disease, Alzheimer’s disease, stroke and myocardial infarction. In the body’s natural biochemical processes, harmful free radicals are formed, which can be removed with the help of appropriate enzymes, a balanced diet or the supply of synthetic antioxidant substances such as flavonoids, vitamins or anthocyanins to the body. Due to the growing demand for antioxidant substances, new complex compounds of transition metal ions with potential antioxidant activity are constantly being sought. In this study, four oxovanadium(IV) and dioxovanadium(V) dipicolinate (dipic) complexes with 1,10-phenanthroline (phen), 2,2′-bipyridyl (bipy) and the protonated form of 2-phenylpyridine (2-phephyH): (1) [VO(dipic)(H₂O)₂]·2 H₂O, (2) [VO(dipic)(phen)]·3 H₂O, (3) [VO(dipic)(bipy)]·H₂O and (4) [VOO(dipic)](2-phepyH)·H₂O were synthesized including one new complex, so far unknown and not described in the literature, i.e., [VOO(dipic)](2-phepyH)·H₂O. The oxovanadium(IV) dipicolinate complexes with 1,10-phenanthroline and 2,2′-bipyridyl have been characterized by several physicochemical methods: NMR, MALDI-TOF-MS, IR, but new complex [VOO(dipic)](2-phepyH)·H₂O has been examined by XRD to confirm its structure. The antioxidant activities of four complexes have been examined by the nitrotetrazolium blue (NBT) method towards superoxide anion. All complexes exhibit high reactivity with superoxide anion and [VOO(dipic)](2-phepyH)·H₂O has higher antioxidant activity than L-ascorbic acid. Our studies confirmed that high basicity of the auxiliary ligand increases the reactivity of the complex with the superoxide radical.

Accurate fragment-based 51-V chemical shift predictions in molecular crystals

Nuclear magnetic resonance (NMR) spectroscopy plays a crucial role in determining molecular structure for complex biological and pharmaceutical compounds. NMR investigations are increasingly reliant on computation for mapping spectral features to chemical structures. Here we benchmark the accuracy of fragment-based ⁵¹V chemical shielding tensor calculations using a training set comprised of 10 biologically and pharmaceutically relevant oxovanadium complexes. Using our self-consistent reproduction of the Madelung potential (SCRMP) electrostatic embedding model, we demonstrate comparable performance between fragment methods and computationally demanding cluster-based techniques. Specifically, fragment methods employing hybrid density functionals are capable of reproducing the experimental ⁵¹V isotropic chemical shifts with a training set rms error of ~9 ​ppm, representing a 20% improvement over traditional plane wave techniques. We provide training set-derived linear regression models for mapping the absolute shieldings obtained from computation to the experimentally determined chemical shifts using four common density functionals; PBE0, B3LYP, PBE, and BLYP. Finally, we establish the utility of fragment methods and the reported regression parameters examining four oxovanadium structures excluded from the training set including the tetracoordinate oxovanadium silicate [Formula: see text] , VO¹⁵NGlySalbz which contains redox-active ligands, and the solid-state form of the common ⁵¹V NMR reference compound VOCl₃.

Anticancer effect of sodium metavanadate on murine breast cancer both in vitro and in vivo

Sodium metavanadate (NaVO₃) exhibits important physiological effects including insulin-like, chemoprevention and anticancer activity. However, the effects of NaVO₃ on breast cancer and underlying mechanisms are still unclear. In this study, our results revealed that NaVO₃ was able to inhibit proliferation of murine breast cancer cells 4T1 with IC₅₀ value of 8.19 μM and 1.92 μM at 24 h and 48 h, respectively. The mechanisms underlying the inhibition activity were that NaVO₃ could increase reactive oxygen species (ROS) level in a concentration-dependent way, arrest cells at G2/M phase, diminish the mitochondrial membrane potential (MMP), finally promote the progress of apoptosis. Furthermore, NaVO₃ also exhibited a dose-dependent anticancer activity in breast cancer-bearing mice that led to the shrinkage of tumor volume (about 50%), lower microvessel density, less propagating cells and more apoptotic cells in vivo, as compared to the saline group. Therefore, NaVO₃ may act as a potential chemotherapeutic agent in breast cancer treatment.

Decavanadate Inhibits Mycobacterial Growth More Potently Than Other Oxovanadates

51V NMR spectroscopy is used to document, using speciation analysis, that one oxometalate is a more potent growth inhibitor of two Mycobacterial strains than other oxovanadates, thus demonstrating selectivity in its interaction with cells. Historically, oxometalates have had many applications in biological and medical studies, including study of the phase-problem in X-ray crystallography of the ribosome. The effect of different vanadate salts on the growth of Mycobacterium smegmatis (M. smeg) and Mycobacterium tuberculosis (M. tb) was investigated, and speciation was found to be critical for the observed growth inhibition. Specifically, the large orange-colored sodium decavanadate (V10O 28 6 - ) anion was found to be a stronger inhibitor of growth of two mycobacterial species than the colorless oxovanadate prepared from sodium metavanadate. The vanadium(V) speciation in the growth media and conversion among species under growth conditions was monitored using 51V NMR spectroscopy and speciation calculations. The findings presented in this work is particularly important in considering the many applications of polyoxometalates in biological and medical studies, such as the investigation of the phase-problem in X-ray crystallography for the ribosome. The findings presented in this work investigate the interactions of oxometalates with other biological systems.

Vanadium(IV)-chlorodipicolinate alleviates hepatic lipid accumulation by inducing autophagy via the LKB1/AMPK signaling pathway in vitro and in vivo

Numerous studies have demonstrated that vanadium compounds are able to improve lipemia and triglyceridemia in both humans and animals. However, the molecular mechanism remains elusive. The present study was conducted to investigate the anti-hyperlipidemic effect of vanadium(IV) complex with 4-chlorodipicolinic acid (VOdipic-Cl)-induced autophagy on hepatic lipid accumulation. To explore the possible underlying mechanisms, primary rat hepatocytes, human hepatoma cell line HepG2, and liver tissue from C57BL/6 mice fed a high-fat diet (HFD) were used. In vitro, cultured primary rat hepatocytes were treated with palmitate (0.25, 0.5 and 0.75 mM) prior to VOdipic-Cl (50, 100, and 200 μM) for 24 h, respectively. In vivo, C57BL/6 mice were fed with high-fat diet for 16 weeks. VOdipic-Cl (10 mg V/kg body weight) was given by daily gavage for 4 weeks. In vitro results showed that VOdipic-Cl significantly inhibited lipid droplet formation by increasing the level of conversion and punctuation of microtubule-associated proteins light chain 3 (LC3) in a dose-dependent manner, and activated liver kinase B-1 (LKB1) and adenosine monophosphate-activated protein kinase (AMPK) phosphorylation. Confocal microscopy images also showed that VOdipic-Cl induced sequestration of lipid droplets (LDs) by autophagy. In vivo, VOdipic-Cl attenuated the increase in serum and liver triglyceride levels in the mice fed with high-fat diet, while significantly increased autophagy induction and activated LKB1 and AMPK phosphorylation in the liver. Taken together, these results suggest that VOdipic-Cl reduces hepatic lipid accumulation by inducing autophagy via the activation of LKB1/AMPK-dependent signaling pathway.

Vanadium(V) complexes with hydrazides and their spectroscopic and biological properties

The present study explores the synthesis and inhibitory potential of vanadium(V) complexes of hydrazides (1c-12c) against oxidative enzymes including xanthine oxidase and lipoxygenase (LOX). In addition, non-enzymatic radical scavenging activities of these complexes were also determined. On the basis of spectral, elemental and physical data, synthesized vanadium(V) complexes are tentatively assigned to have an octahedral geometry with two hydrazide ligands and two oxo groups forming a negatively charged sphere complex with ammonium as counter ion. This is further verified by the conductivity studies of the complexes. Results show that hydrazide ligands (1-12) and their respective vanadium(V) complexes (1c-12c) posses scavenging and inhibition potential against DPPH and LOX, respectively. However, contrary to that uncoordinated ligands showed no activity against nitric oxide, superoxide and xanthine oxidase whereas their complexes showed varying degree of activity. These studies indicate that geometry of complex, nature and position of substituent groups play a vital role in scavenging and inhibition potential of these compounds.

Selective speciation improves efficacy and lowers toxicity of platinum anticancer and vanadium antidiabetic drugs

Improving efficacy and lowering resistance to metal-based drugs can be addressed by consideration of the coordination complex speciation and key reactions important to vanadium antidiabetic drugs or platinum anticancer drugs under biological conditions. The methods of analyses vary depending on the specific metal ion chemistry. The vanadium compounds interconvert readily, whereas the reactions of the platinum compounds are much slower and thus much easier to study. However, the vanadium species are readily differentiated due to vanadium complexes differing in color. For both vanadium and platinum systems, understanding the processes as the compounds, Lipoplatin and Satraplatin, enter cells is needed to better combat the disease; there are many cellular metabolites, which may affect processing and thus the efficacy of the drugs. Examples of two formulations of platinum compounds illustrate how changing the chemistry of the platinum will result in less toxic and better tolerated drugs. The consequence of the much lower toxicity of the drug, can be readily realized because cisplatin administration requires hospital stay whereas Lipoplatin can be done in an outpatient manner. Similarly, the properties of Satraplatin allow for development of an oral drug. These forms of platinum demonstrate that the direct consequence of more selective speciation is lower side effects and cheaper administration of the anticancer agent. Therefore we urge that as the community goes forward in development of new drugs, control of speciation chemistry will be considered as one of the key strategies in the future development of anticancer drugs.

Vanadium(IV)-chlorodipicolinate inhibits 3T3-L1 preadipocyte adipogenesis by activating LKB1/AMPK signaling pathway

Our previous studies demonstrated that vanadium(IV) complex with 4-chlorodipicolinic acid (VOdipic-Cl) alleviates lipid abnormalities in streptozotocin (STZ)-induced diabetic rats. However, the molecular mechanisms are not fully understood. In the present study, the effect of VOdipic-Cl on adipogenesis and mechanisms of action in 3T3-L1 preadipocytes were investigated. The 3T3-L1 preadipocytes were induced to differentiate in the presence or absence of VOdipic-Cl for 8days. The cells were determined for proliferation, differentiation, lipid accumulation as well as the protein expressions of molecular targets that are involved in fatty acid synthesis. The results demonstrated that VOdipic-Cl at concentrations ranging from 2.5μM to 10μM reduced the intracellular lipid content by 10%, 22% and 30% compared to control. VOdipic-Cl down-regulated the expression of peroxisome proliferator-activated receptor (PPARγ), CCAAT element binding protein a (C/EBPα), sterol regulatory element binding protein 1c (SREBP-1c), fatty acid synthase (FAS) and fatty acid-binding protein 4 (FABP4) and activated the phosphorylation of acetyl coenzyme A carboxylase (ACC), adenosine monophosphate-activated protein kinase (AMPK) and liver kinase B1 (LKB1) in a dose-dependent manner. Further studies showed that AMPK small interfering RNA (siRNA) markedly up-regulated PPARγ, C/EBPα, FAS and FABP4 expression in the presence of VOdipic-Cl, respectively. When LKB1 was silenced with siRNA, the effect of VOdipic-Cl on AMPK phosphorylation was diminished. Taken together, these results suggested that VOdipic-Cl can inhibit 3T3-L1 preadipocyte differentiation and adipogenesis through activating the LKB1/AMPK-dependent signaling pathway. These findings raise the possibility that VOdipic-Cl may be a promising therapy in treatment of obesity.

Why Antidiabetic Vanadium Complexes are Not in the Pipeline of "Big Pharma" Drug Research? A Critical Review

Public academic research sites, private institutions as well as small companies have made substantial contributions to the ongoing development of antidiabetic vanadium compounds. But why is this endeavor not echoed by the globally operating pharmaceutical companies, also known as "Big Pharma"? Intriguingly, today's clinical practice is in great need to improve or replace insulin treatment against Diabetes Mellitus (DM). Insulin is the mainstay therapeutically and economically. So, why do those companies develop potential antidiabetic drug candidates without vanadium (vanadium- free)? We gathered information about physicochemical and pharmacological properties of known vanadium-containing antidiabetic compounds from the specialized literature, and converted the data into explanations (arguments, the "pros and cons") about the underpinnings of antidiabetic vanadium. Some discoveries were embedded in chronological order while seminal reviews of the last decade about the Medicinal chemistry of vanadium and its history were also listed for further understanding. In particular, the concepts of so-called "noncomplexed or free" vanadium species (i.e. inorganic oxido-coordinated species) and "biogenic speciation" of antidiabetic vanadium complexes were found critical and subsequently documented in more details to answer the question.

Antidiabetic, Chemical, and Physical Properties of Organic Vanadates as Presumed Transition-State Inhibitors for Phosphatases

Studies of antidiabetic vanadium compounds, specifically the organic vanadate esters, are reviewed with regard to their chemistry and biological properties. The compounds are described from the perspective of how the fundamental chemistry and properties of organic vanadate esters impact their effects as inhibitors for phosphatases based on the structural information obtained from vanadium-phosphatase complexes. Vanadium compounds have been reported to have antidiabetic properties for more than a century. The structures and properties of organic vanadate complexes are reviewed, and the potency of such vanadium coordination complexes as antidiabetic agents is described. Because such compounds form spontaneously in aqueous environments, the reactions with most components in any assay or cellular environment has potential to be important and should be considered. Generally, the active form of vanadium remains elusive, although studies have been reported of a number of promising vanadium compounds. The description of the antidiabetic properties of vanadium compounds is described here in the context of recent characterization of vanadate-phosphatase protein structures by data mining. Organic vanadate ester compounds are generally four coordinate or five coordinate with the former being substrate analogues and the latter being transition-state analogue inhibitors. These studies demonstrated a framework for characterization of five-coordinate trigonal bipyramidal vanadium inhibitors by comparison with the reported vanadium-protein phosphatase complexes. The binding of the vanadium to the phosphatases is either as a five-coordinate exploded transition-state analogue or as a high energy intermediate, respectively. Even if potency as an inhibitor requires trigonal bipyramidal geometry of the vanadium when bound to the protein, such geometry can be achieved upon binding from compounds with other geometries. Desirable properties of ligands are identified and analyzed. Ligand interactions, as reported in one peptidic substrate, are favorable so that complementarity between phosphatase and coordinating ligand to the vanadium can be established resulting in a dramatic enhancement of the inhibitory potency. These considerations point to a frameshift in ligand design for vanadium complexes as phosphatase inhibitors and are consistent with other small molecule having much lower affinities. Combined, these studies do suggest that if effective delivery of potentially active antidiabetic compound such a the organic vanadate peptidic substrate was possible the toxicity problems currently reported for the salts and some of the complexes may be alleviated and dramatic enhancement of antidiabetic vanadium compounds may result.

Oxidovanadium(IV) sulfate-induced glucose uptake in HepG2 cells through IR/Akt pathway and hydroxyl radicals

The insulin-mimetic and anti-diabetic properties of vanadium and related compounds have been well documented both in vitro and in vivo. However, the molecular basis of the link between vanadium and the insulin signaling pathway in diabetes mellitus is not fully described. We investigated the effects of reactive oxygen species (ROS) induced by oxidovanadium(IV) sulfate (VOSO4) on glucose uptake and the insulin signaling pathway in human hepatoma cell line HepG2. Exposure of cells to VOSO4 (5-50 μM) resulted in an increase in glucose uptake, insulin receptor (IR) and protein kinase B (Akt) phosphorylation and intracellular ROS generation. Using Western blot, we found that catalase and sodium formate, but not superoxide dismutase, prevented the increase of hydroxyl radical (·OH) generation and significantly decreased VOSO4-induced IR and Akt phosphorylation. These results suggest that VOSO4-induced ·OH radical, which is a signaling species, promotes glucose uptake via the IR/Akt signaling pathway.

Methionine oxidation of amyloid peptides by peroxovanadium complexes: inhibition of fibril formation through a distinct mechanism

Peroxovanadium complexes inhibit the fibril formation of neurodegenerative amyloid peptides by oxidizing methionine residues.

Vanadium methyl-bipyridine organoligand and its influence on energy balance and organs mass

In the treatment of lifestyle diseases, including metabolic syndrome and type 2 diabetes, it is important to lower body mass and fat tissue, and consequently, to increase insulin-sensitivity. Unfortunately, it often happens that low-energy diet which would lower overweight is not observed and, thus, it does not bring the expected effects. This paper discusses the influence of three diets-control, high-fructose, and high-fatty diet-on absorption of energy from food in order to transform it into body mass. The kJ/g ratio which describes this process has been calculated. In the tested diets, the addition of fructose (79.13 ± 2.47 kJ/g) or fat (82.48 ± 2.28 kJ/g) results in higher transformation of energy into body mass than in the case of control diet (89.60 ± 1.86 kJ/g). The addition of Na[VO(O2)2(4,4′-Me2-2,2′-bpy)]•8H2O (where 4,4′-Me2-2,2′-bpy = 4,4′-dimethyl-2,2′-bipyridine) results in statistical increase of that ratio: fructose diet (86.88 ± 0.44 kJ/g), fat diet (104.68 ± 3.01 kJ/g), and control diet (115.98 ± 0.56 kJ/g), respectively. Fat diet statistically influences the decrease of kidney mass in comparison to the other diets. The application of the tested vanadium compound results also in the statistical decrease of the fatty liver caused by fructose and fat diet.

Design, synthesis and pharmacological evaluation of novel vanadium-containing complexes as antidiabetic agents

Based on the data about structure and antidiabetic activity of twenty seven vanadium and zinc coordination complexes collected from literature we developed QSAR models using the GUSAR program. These QSAR models were applied to 10 novel vanadium coordination complexes designed in silico in order to predict their hypoglycemic action. The five most promising substances with predicted potent hypoglycemic action were selected for chemical synthesis and pharmacological evaluation. The selected coordination vanadium complexes were synthesized and tested in vitro and in vivo for their hypoglycemic activities and acute rat toxicity. Estimation of acute rat toxicity of these five vanadium complexes was performed using a freely available web-resource (http://way2drug.com/GUSAR/acutoxpredict.html). It has shown that the selected compounds belong to the class of moderate toxic pharmaceutical agents, according to the scale of Hodge and Sterner. Comparison with the predicted data has demonstrated a reasonable correspondence between the experimental and predicted values of hypoglycemic activity and toxicity. Bis{tert-butyl[amino(imino)methyl]carbamato}oxovanadium (IV) and sodium(2,2'-Bipyridyl)oxo-diperoxovanadate(V) octahydrate were identified as the most potent hypoglycemic agents among the synthesized compounds.

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.

Structural and redox requirements for the action of anti-diabetic vanadium compounds

This study presents the first systematic investigation of the anti-diabetic properties of non-oxido V(IV) complexes. In particular, the insulin-mimetic activity of [V(IV)(taci)2](4+), [V(IV)(inoH-3)2](2-), [V(IV)(dhab)2], [V(IV)(hyph(Ph))2], [V(IV)(cat)3](2-) and [V(IV)(pdbh)2]--where taci is 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, ino is cis-inositol, H2dhab is 2,2'-dihydroxyazobenzene, H2hyph(Ph) is 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole, H2cat is catechol and H2pdbh is pentan-2,4-dione benzoylhydrazone--was evaluated in terms of free fatty acid (FFA) release. Among the six compounds examined, only [V(IV)(pdbh)2], [V(IV)(cat)3](2-) and [V(IV)(hyph(Ph))2], which at the physiological pH convert to the corresponding V(IV)O complexes, were found to exhibit a significant insulin-mimetic activity compared to VOSO4. In contrast, [V(taci)2](4+), [V(inoH-3)2](2-) and [V(dhab)2], which at pH 7.4 keep their 'bare' non-oxido structure, did not cause any inhibition of FFA. The results, therefore, suggest that a V(IV)O functionality is necessary for vanadium complexes to exhibit anti-diabetic effects. This agrees with the notion that the biotransformations of V compounds in the organism are more important than the nature of the species.

Correlation of insulin-enhancing properties of vanadium-dipicolinate complexes in model membrane systems: phospholipid langmuir monolayers and AOT reverse micelles

We explore the interactions of V(III) -, V(IV) -, and V(V) -2,6-pyridinedicarboxylic acid (dipic) complexes with model membrane systems and whether these interactions correlate with the blood-glucose-lowering effects of these compounds on STZ-induced diabetic rats. Two model systems, dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers and AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse micelles present controlled environments for the systematic study of these vanadium complexes interacting with self-assembled lipids. Results from the Langmuir monolayer studies show that vanadium complexes in all three oxidation states interact with the DPPC monolayer; the V(III) -phospholipid interactions result in a slight decrease in DPPC molecular area, whereas V(IV) and V(V) -phospholipid interactions appear to increase the DPPC molecular area, an observation consistent with penetration into the interface of this complex. Investigations also examined the interactions of V(III) - and V(IV) -dipic complexes with polar interfaces in AOT reverse micelles. Electron paramagnetic resonance spectroscopic studies of V(IV) complexes in reverse micelles indicate that the neutral and smaller 1:1 V(IV) -dipic complex penetrates the interface, whereas the larger 1:2 V(IV) complex does not. UV/Vis spectroscopy studies of the anionic V(III) -dipic complex show only minor interactions. These results are in contrast to behavior of the V(V) -dipic complex, [VO2 (dipic)](-) , which penetrates the AOT/isooctane reverse micellar interface. These model membrane studies indicate that V(III) -, V(IV) -, and V(V) -dipic complexes interact with and penetrate the lipid interfaces differently, an effect that agrees with the compounds' efficacy at lowering elevated blood glucose levels in diabetic rats.

Vanadium compounds modulate PPARγ activity primarily by increasing PPARγ protein levels in mouse insulinoma NIT-1 cells

Vanadium compounds are promising agents in the therapeutic treatment of diabetes; however, their mechanism of action has not been clearly elucidated. The current study investigated the effects of vanadium compounds, vanadyl acetylacetonate [V(IV)O(acac)2] and sodium metavanadate (NaV(V)O3), on peroxisome proliferator-activated receptors (PPARs), especially PPARγ, which are important targets of anti-diabetic drugs. Our experimental results revealed that treatment of NIT-1 β-pancreas cells with vanadium compounds resulted in PPARγ activation and elevation of PPARγ protein levels. Vanadium compounds did not increase PPARγ transcription but ameliorated PPARγ degradation induced by inflammatory stimulators TNF-α/IL-6. Vanadium compounds induced binding of PPARγ to heat shock protein (Hsp60). This PPARγ-Hsp60 interaction might cause inhibition of PPARγ degradation, thus elevating the PPARγ level. In addition, modulation of PPARγ phosphorylation was also observed upon vanadium treatment. The present work demonstrated for the first time that vanadium compounds are novel PPARγ modulators. The results may provide new insights for the mechanism of anti-diabetic action of vanadium compounds.

Inhibition of human amylin fibril formation by insulin-mimetic vanadium complexes

Inhibition of human amylin fibril formation by insulin-mimetic vanadium complexes.

Influence of fructose and fatty-rich diet combined with vanadium on bone marrow cells

The aim of the study is to investigate the influence of diet treatment on bone marrow cells. Normal male Wistar rats were divided into six groups (n = 6 per group): control with normal diet (C), increased fructose (31 % w/w in fodder) (Fr) and high fatty (30 % w/w of animal fat in fodder) diet (Fa), and the same diets with vanadium complex ([VO(4,4' Me2-2,2' Bpy)2]SO4) · H2O (CV, FrV and FaV). During 5 weeks, the animals had unlimited access to food and water. Immediately after anaesthetizing and sacrificing the animals, bone marrow smears were prepared from the femurs. Different types of cell lines in the animal smears were examined under the microscope: erythroid line, myeloid line, monocytic line, megakariocytic line and lymphoid line. Addition of fructose or animal fat had evident influence on the proportional composition of the bone marrow cells. In erythroid precursors, addition of both investigated products resulted in a statistically significant increase of percentage of this type of cells. A reverse effect was observed for the lymphoid cell line where addition of both tested diets decreased quantity of these cells in comparison to the control diet. In the same lines, addition of vanadium intensified the observed changes. In the case of other types of cell lines, statistically significant changes were not observed.

Preliminary anti-cancer photodynamic therapeutic in vitro studies with mixed-metal binuclear ruthenium(II)-vanadium(IV) complexes

We report the synthesis and characterisation of mixed-metal binuclear ruthenium(II)-vanadium(IV) complexes, which were used as potential photodynamic therapeutic agents for melanoma cell growth inhibition. The novel complexes, [Ru(pbt)2(phen2DTT)](PF6)2·1.5H2O 1 (where phen2DTT = 1,4-bis(1,10-phenanthrolin-5-ylsulfanyl)butane-2,3-diol and pbt = 2-(2'-pyridyl)benzothiazole) and [Ru(pbt)2(tpphz)](PF6)2·3H2O 2 (where tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine) were synthesised and characterised. Compound 1 was reacted with [VO(sal-L-tryp)(H2O)] (where sal-L-tryp = N-salicylidene-L-tryptophanate) to produce [Ru(pbt)2(phen2DTT)VO(sal-L-tryp)](PF6)2·5H2O 4; while [VO(sal-L-tryp)(H2O)] was reacted with compound 2 to produce [Ru(pbt)2(tpphz)VO(sal-L-tryp)](PF6)2·6H2O 3. All complexes were characterised by elemental analysis, HRMS, ESI MS, UV-visible absorption, ESR spectroscopy, and cyclic voltammetry, where appropriate. In vitro cell toxicity studies (with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay) via dark and light reaction conditions were carried out with sodium diaqua-4,4',4'',4''' tetrasulfophthalocyaninecobaltate(II) (Na4[Co(tspc)(H2O)2]), [VO(sal-L-tryp)(phen)]·H2O, and the chloride salts of complexes 3 and 4. Such studies involved A431, human epidermoid carcinoma cells; human amelanotic malignant melanoma cells; and HFF, non-cancerous human skin fibroblast cells. Both chloride salts of complexes 3 and 4 were found to be more toxic to melanoma cells than to non-cancerous fibroblast cells, and preferentially led to apoptosis of the melanoma cells over non-cancerous skin cells. The anti-cancer property of the chloride salts of complexes 3 and 4 was further enhanced when treated cells were exposed to light, while no such effect was observed on non-cancerous skin fibroblast cells. ESR and (51)V NMR spectroscopic studies were also used to assess the stability of the chloride salts of complexes 3 and 4 in aqueous media at pH 7.19. This research illustrates the potential for using mixed-metal binuclear ruthenium(II)-vanadium(IV) complexes to fight skin cancer.

Effect of V(IV)O(dipic-Cl)(H2O)2 on Lipid Metabolism Disorders in the Liver of STZ-Induced Diabetic Rats

Vanadium complexes are potent antidiabetic agents for therapeutical treatment of diabetes. In the present study, we investigated the hypolipidemic effect of V(IV)O(dipic-Cl)(H2O)2 (V4dipic-Cl) in liver of streptozotocin- (STZ-)-induced diabetic rats. We found that diabetic animals exhibited hepatic inflammatory infiltration and impaired liver function along with triglyceride (TG) accumulation in the liver. V4dipic-Cl treatment not only ameliorated liver pathological state but also reduced hepatic TG level. Moreover, the upregulation of fatty acid translocase (FAT/CD36) mRNA (4.0-fold) and protein (8.2-fold) levels in the liver of diabetic rats were significantly reversed after V4dipic-Cl treatment. However, no significant effects of V4dipic-Cl on the mRNA expression of fatty acid metabolism-related fatty acid bounding protein 1 (FABP1) and fatty acid transporter 5 (FATP5) were observed. These results suggest that the modification of lipid metabolism-related FAT/CD36 in the liver of diabetic rats is likely involved in the hypolipidemic effects of V4dipic-Cl.

Effect of Ancillary Ligand on Electronic Structure as Probed by 51V Solid-State NMR Spectroscopy for Vanadium-o-Dioxolene Complexes

A series of vanadium(V) complexes with o-dioxolene (catecholato) ligands and an ancillary ligand, (N-(salicylideneaminato)ethylenediamine) (hensal), were investigated using ⁵¹V solid-state magic angle spinning NMR spectroscopy (⁵¹V MAS NMR) to assess the local environment of the vanadium(V). The solid-state ⁵¹V NMR parameters of vanadium(V) complexes with a related potentially tetradentate ancillary ligand (N-salicylidene-N'-(2-hydroxyethyl)ethylenediamine) (h₂shed) were previously shown to be associated with the size of the HOMO-LUMO gap in the complex, and as such provide insights on the interaction between metal ion and ligand (P. B. Chatterjee, et al., Inorg. Chem 50 (2011) 9794). Our results show that the modification of the ancillary ligand does not impact the observed trend between complexes ranging from catechols with electron rich to electron poor substituents. However, the ancillary ligand does impact the size of the HOMO-LUMO separation in the parent complex and thus the solid-state vanadium NMR chemical shift of the unsubstituted vanadium complex. For these complexes significant changes observed in the isotropic shifts and more modest changes detected in the C_Q reflect the electronic changes in the complex as the catechol is varied. However, no obvious trend was observed in the chemical shift anisotropies (δ_σ and η_σ) with the variation in the catechol. The electronic changes in the coordination environment of the vanadium can be described using solid-state ⁵¹V NMR spectroscopy.

VO(dmpp)2 normalizes pre-diabetic parameters as assessed by in vivo magnetic resonance imaging and spectroscopy

Type 2 diabetes mellitus has been associated with obesity, metabolic syndrome, cardiovascular diseases and cancer. Attempts have been made for early diagnosis and finding effective drugs to prevent severe consequences and ameliorate the symptoms of this disorder. In this work, the pharmacological properties of VO(dmpp)(2), [bis(1,2-dimethyl-3-hydroxy-4-pyridinonato)oxovanadium(IV)], were in vivo evaluated. For 4 weeks fatty Zucker rats were subjected to a daily dose of VO(dmpp)(2) (44 μmol/kg) and their metabolic profile was followed by assessing different biological parameters at established time points: body weight, subcutaneous fat width and hepatic triglyceride content determined by magnetic resonance imaging and spectroscopy, respectively. A glucose tolerance test was performed at the end of the experiment. After treatment, treated obese rats presented a weight significantly lower than the non-treated obese animals (359.0±11.1 vs. 433.5±6.2g, P<0.05), a thinner subcutaneous fat width, and a statistically significant decrease in hepatic triglyceride content (5.41±0.59 vs. 21.03±1.40%, P<0.0005). Additionally, the glucose intolerant profile of fatty Zucker rats was completely reversed in treated animals (102.3±2.1 vs. 172.4±1.3 mg/100 mL; P<0.0005). These results reinforce the therapeutic action of VO(dmpp)(2) which shows particular effects on lipid metabolism.

Sarcoplasmic reticulum calcium ATPase interactions with decaniobate, decavanadate, vanadate, tungstate and molybdate

Over the last few decades there has been increasing interest in oxometalate and polyoxometalate applications to medicine and pharmacology. This interest arose, at least in part, due to the properties of these classes of compounds as anti-cancer, anti-diabetic agents, and also for treatment of neurodegenerative diseases, among others. However, our understanding of the mechanism of action would be improved if biological models could be used to clarify potential toxicological effects in main cellular processes. Sarcoplasmic reticulum (SR) vesicles, containing a large amount of Ca(2+)-ATPase, an enzyme that accumulates calcium by active transport using ATP, have been suggested as a useful model to study the effects of oxometalates on calcium homeostasis. In the present article, it is shown that decavanadate, decaniobate, vanadate, tungstate and molybdate, all inhibited SR Ca(2+)-ATPase, with the following IC(50) values: 15, 35, 50, 400 μM and 45 mM, respectively. Decaniobate (Nb(10)), is the strongest P-type enzyme inhibitor, after decavanadate (V(10)). Atomic-absorption spectroscopy (AAS) analysis, indicates that decavanadate binds to the protein with a 1:1 decavanadate:Ca(2+)-ATPase stoichiometry. Furthermore, V(10) binds with similar extension to all the protein conformations, which occur during calcium translocation by active transport, namely E1, E1P, E2 and E2P, as analysed by AAS. In contrast, it was confirmed that the binding of monomeric vanadate (H(2)VO(4)(2-); V(1)) to the calcium pump is favoured only for the E2 and E2P conformations of the ATPase, whereas no significant amount of vanadate is bound to the E1 and E1P conformations. Scatchard plot analysis, confirmed a 1:1 ratio for decavanadate-Ca(2+)-ATPase, with a dissociation constant, k(d) of 1 μM(-1). The interaction of decavanadate V(10)O(28)(6-) (V(10)) with Ca(2+)-ATPase is prevented by the isostructural and isoelectronic decaniobate Nb(10)O(28)(6-) (Nb(10)), whereas no significant effects were detected with ATP or with heparin, a known competitive ATP binding molecule, suggesting that V(10) binds non-competitively, with respect to ATP, to the protein. Finally, it was shown that decaniobate inhibits SR Ca(2+)-ATPase activity in a non competitive type of inhibition, with respect to ATP. Taken together, these data demonstrate that decameric niobate and vanadate species are stronger inhibitors of the SR calcium ATPase than simple monomeric vanadate, tungstate and molybdate oxometalates, thus affecting calcium homeostasis, cell signalling and cell bioenergetics, as well many other cellular processes. The ability of these oxometalates to act either as phosphate analogues, as a transition-state analogue in enzyme-catalysed phosphoryl group transfer processes and as potentially nucleotide-dependent enzymes modulators or inhibitors, suggests that different oxometalates may reveal different mechanistic preferences in these classes of enzymes.