Debbie C. Crans

Chemistry and insulin-like properties of vanadium(IV) and vanadium(V) compounds.

The chemistry of vanadium compounds that can be taken orally is very timely since a vanadium(IV) compound, KP-102, is currently in clinical trials in humans, and the fact that human studies with inorganic salts have recently been reported. VO(acac)2 and VO(Et-acac)2 (where acac is acetylacetonato and Et-acac is 3-ethyl-2,4-pentanedionato) have long-term in vivo insulin mimetic effects in streptozotocin induced diabetic Wistar rats. Structural characterization of VO(acac)2 and two derivatives, VO(Me-acac)2 and VO(Et-acac)2, in the solid state and solution have begun to delineate the size limits of the insulin-like active species. Oral ammonium dipicolinatooxovanadium(V) is a clinically useful hypoglycemic agent in cats with naturally occurring diabetes mellitus. This compound is particularly interesting since it represents the first time that a well-characterized organic vanadium compound with the vanadium in oxidation state five has been found to be an orally effective hypoglycemic agent in animals.

Effect of vanadium(IV) compounds in the treatment of diabetes: in vivo and in vitro studies with vanadyl sulfate and bis(maltolato)oxovandium(IV)

Vanadyl sulfate (VOSO(4)) was given orally to 16 subjects with type 2 diabetes mellitus for 6 weeks at a dose of 25, 50, or 100 mg vanadium (V) daily [Goldfine et al., Metabolism 49 (2000) 1-12]. Elemental V was determined by graphite furnace atomic absorption spectrometry (GFAAS). There was no correlation of V in serum with clinical response, determined by reduction of mean fasting blood glucose or increased insulin sensitivity during euglycemic clamp. To investigate the effect of administering a coordinated V, plasma glucose levels were determined in streptozotocin (STZ)-induced diabetic rats treated with the salt (VOSO(4)) or the coordinated V compound bis(maltolato)oxovandium(IV) (abbreviated as VO(malto)(2)) administered by intraperitoneal (i.p.) injection. There was no relationship of blood V concentration with plasma glucose levels in the animals treated with VOSO(4), similar to our human diabetic patients. However, with VO(malto)(2) treatment, animals with low plasma glucose tended to have high blood V. To determine if V binding to serum proteins could diminish biologically active serum V, binding of both VOSO(4) and VO(malto)(2) to human serum albumin (HSA), human apoTransferrin (apoHTf) and pig immunoglobulin (IgG) was studied with EPR spectroscopy. Both VOSO(4) and VO(malto)(2) bound to HSA and apoHTf forming different V-protein complexes, while neither V compound bound to the IgG. VOSO(4) and VO(malto)(2) showed differences when levels of plasma glucose and blood V in diabetic rodents were compared, and in the formation of V-protein complexes with abundant serum proteins. These data suggest that binding of V compounds to ligands in blood, such as proteins, may affect the available pool of V for biological effects.

Decavanadate (V10O286-) and oxovanadates: Oxometalates with many biological activities

The decameric vanadate species V(10)O(28)(6-), also referred to as decavanadate, impact proteins, lipid structures and cellular function, and show some effects in vivo on oxidative stress processes and other biological properties. The mode of action of decavanadate in many biochemical systems depends, at least in part, on the charge and size of the species and in some cases competes with the simpler oxovanadate species. The orange decavanadate that contains 10 vanadium atoms is a stable species for several days at neutral pH, but at higher pH immediately converts to the structurally and functionally distinct lower oxovanadates such as the monomer, dimer or tetramer. Although the biological effects of vanadium are generally assumed to derive from monomeric vanadate or the vanadyl cation, we show in this review that not all effects can be attributed to these simple oxovanadate forms. This topic has not previously been reviewed although background information is available [D.C. Crans, Comments Inorg. Chem. 16 (1994) 35-76; M. Aureliano (Ed.), Vanadium Biochemistry, Research Signpost Publs., Kerala, India, 2007]. In addition to pumps, channels and metabotropic receptors, lipid structures represent potential biological targets for decavanadate and some examples have been reported. Decavanadate interact with enzymes, polyphosphate, nucleotide and inositol 3-phosphate binding sites in the substrate domain or in an allosteric site, in a complex manner. In mitochondria, where vanadium was shown to accumulate following decavanadate in vivo administration, nM concentration of decavanadate induces membrane depolarization in addition to inhibiting oxygen consumption, suggesting that mitochondria may be potential targets for decameric toxicity. In vivo effects of decavanadate in piscine models demonstrated that antioxidant stress markers, lipid peroxidation and vanadium subcellular distribution is dependent upon whether or not the solutions administered contain decavanadate. The present review summarizes the reports on biological effects of decavanadate and highlights the importance of considering decavanadate in evaluations of the biological effects of vanadium.

Effects of vanadium complexes with organic ligands on glucose metabolism: a comparison study in diabetic rats.

Vanadium compounds can mimic actions of insulin through alternative signalling pathways. The effects of three organic vanadium compounds were studied in non‐ketotic, streptozotocin‐diabetic rats: vanadyl acetylacetonate (VAc), vanadyl 3‐ethylacetylacetonate (VEt), and bis(maltolato)oxovanadium (VM). A simple inorganic vanadium salt, vanadyl sulphate (VS) was also studied. Oral administration of the three organic vanadium compounds (125 mg vanadium element l−1 in drinking fluids) for up to 3 months induced a faster and larger fall in glycemia (VAc being the most potent) than VS. Glucosuria and tolerance to a glucose load were improved accordingly. Activities and mRNA levels of key glycolytic enzymes (glucokinase and L‐type pyruvate kinase) which are suppressed in the diabetic liver, were restored by vanadium treatment. The organic forms showed greater efficacy than VS, especially VAc. VAc rats exhibited the highest levels of plasma or tissue vanadium, most likely due to a greater intestinal absorption. However, VAc retained its potency when given as a single i.p. injection to diabetic rats. Moreover, there was no relationship between plasma or tissue vanadium levels and any parameters of glucose homeostasis and hepatic glucose metabolism. Thus, these data suggest that differences in potency between compounds are due to differences in their insulin‐like properties. There was no marked toxicity observed on hepatic or renal function. However, diarrhoea occurred in 50% of rats chronically treated with VS, but not in those receiving the organic compounds. In conclusion, organic vanadium compounds, in particular VAc, correct the hyperglycemia and impaired hepatic glycolysis of diabetic rats more safely and potently than VS. This is not simply due to improved intestinal absorption, indicating more potent insulin‐like properties.

Aqueous Chemistry of Labile Oxovanadates: Relevance to Biological Studies

Abstract The aqueous chemistry of vanadate (vanadium(V)) is very complex since many protonation equilibria and oligomerization equilibria are occurring simultaneously. Vanadate monomer (V1), dimer (V2), tetramer (V4) and pentamer (V5) are exchanging with each other on a millisecond time scale so that none of these species can be isolated for aqueous biological studies. Measuring the effects of each anion on an enzyme must be carried out in an equilibrium mixture containing the other vanadate oligomers. Defining conditions to measure the effects of oxovanadates is non-trivial, since vanadate interacts with buffers and other assay components. Information concerning the simple aqueous chemistry of vanadate with various ligands is thus necessary to ensure that the vanadate is free to interact with an enzyme or a protein. Experimental approaches, which take into account the aqueous chemistry of labile oxovanadates, to biological studies are described here. For this purpose, the interactions of oxovanadates wit...

The permeability and cytotoxicity of insulin-mimetic vanadium compounds.

AbstractPurpose. The aim of this study was to investigate the mechanism of permeation and cytotoxicity of vanadium compounds, [VO(acac)2], [VO(ma)2], and vanadate. Methods. Absorptive transport were carried out in Caco-2 monolayers grown on transwell inserts. Vanadium was quantified using inductively coupled plasma atomic emission spectrometry (ICP-AES). The change of Caco-2 cells in the microvilli morphology and F-actin structure was visualized by transmission electron microscopy and confocal laser scanning microscopy. Results. The three vanadium compounds were taken up by Caco-2 cells via simple passive diffusion. [VO(acac)2] were mainly transcellularly transported and exhibited the highest apparent permeabilty coefficients (8.2 × 10-6 cm-1). The cell accumulation of [VO(acac)2] was found to be greater than that of [VO(ma)2], and vanadate caused much less accumulation than the other two compounds. Vanadium compounds induced intracellular reactive oxygen species, reduced the transepithelial electric resistance, caused morphological change in microvilli, and led to different perturbation of F-actin structure. Conclusions. The three compounds exhibited different permeability due to different diffusion process and cellular uptake. The toxicity of vanadium complexes on Caco-2 monolayer involved F-actin-related change of tight junction and impairment of microvilli. The toxicity was also related to elevated intracellular reactive oxygen species (ROS) and their cellular accumulation.

Vanadium(IV) and vanadium(V) complexes of dipicolinic acid and derivatives. Synthesis, X-ray structure, solution state properties: and effects in rats with STZ-induced diabetes

Abstract Preparation and characterization of new vanadium compounds with insulin mimetic properties is important for investigations into the mechanism of action of these complexes. In this work, we report the synthesis of three new compounds related to the (dipicolinato)dioxovanadium(V) complex: (4-hydroxy-2,6-pyridinedicarboxylato)dioxovanadium(V), (diaqua)(4-hydroxy-2,6-pyridinedicarboxylato)oxovanadium(IV) and (4-amino-2,6-pyridinedicarboxylato)dioxovanadium(V), and present the first preliminary kinetic study of a vanadium dipicolinate system. One of these complexes, (4-hydroxy-2,6-pyridinedicarboxylato)oxovanadium(V), is investigated in greater detail with regard to chemistry, structure, kinetics and insulin-like properties in rats with streptozotocin-induced diabetes. Oral administration of [VO 2 (dipic-OH)] − was found to be effective in lowering both the hyperglycemia and hyperlipidemia of diabetes. This is the first report of the effective lowering of diabetic hyperlipidemia by a vanadium(V) coordination complex.

Is vanadate reduced by thiols under biological conditions? Changing the redox potential of V(V)/V(IV) by complexation in aqueous solution.

Although dogma states that vanadate is readily reduced by glutathione, cysteine, and other thiols, there are several examples documenting that vanadium(V)-sulfur complexes can form and be observed. This conundrum has impacted life scientists for more than two decades. Investigation of this problem requires an understanding of both the complexes that form from vanadium(IV) and (V) and a representative thiol in aqueous solution. The reactions of vanadate and hydrated vanadyl cation with 2-mercaptoethanol have been investigated using multinuclear NMR, electron paramagnetic resonance (EPR), and UV-vis spectroscopy. Vanadate forms a stable complex of 2:2 stoichiometry with 2-mercaptoethanol at neutral and alkaline pH. In contrast, vanadate can oxidize 2-mercaptoethanol; this process is favored at low pH and high solute concentrations. The complex that forms between aqueous vanadium(IV) and 2-mercaptoethanol has a 1:2 stoichiometry and can be observed at high pH and high 2-mercaptoethanol concentration. The solution structures have been deduced based on coordination induced chemical shifts and speciation diagrams prepared. This work demonstrates that both vanadium(IV) and (V)-thiol complexes form and that redox chemistry also takes place. Whether reduction of vanadate takes place is governed by a combination of parameters: pH, solute- and vanadate-concentrations and the presence of other complexing ligands. On the basis of these results it is now possible to understand the distribution of vanadium in oxidation states (IV) and (V) in the presence of glutathione, cysteine, and other thiols and begin to evaluate the forms of the vanadium compounds that exert a particular biological effect including the insulin-enhancing agents, antiamoebic agents, and interactions with vanadium binding proteins.

Novel Insights into the Mechanism of Inhibition of MmpL3, a Target of Multiple Pharmacophores in Mycobacterium tuberculosis

ABSTRACT MmpL3, a resistance-nodulation-division (RND) superfamily transporter, has been implicated in the formation of the outer membrane of Mycobacterium tuberculosis; specifically, MmpL3 is required for the export of mycolic acids in the form of trehalose monomycolates (TMM) to the periplasmic space or outer membrane of M. tuberculosis. Recently, seven series of inhibitors identified by whole-cell screening against M. tuberculosis, including the antituberculosis drug candidate SQ109, were shown to abolish MmpL3-mediated TMM export. However, this mode of action was brought into question by the broad-spectrum activities of some of these inhibitors against a variety of bacterial and fungal pathogens that do not synthesize mycolic acids. This observation, coupled with the ability of three of these classes of inhibitors to kill nonreplicating M. tuberculosis bacilli, led us to investigate alternative mechanisms of action. Our results indicate that the inhibitory effects of adamantyl ureas, indolecarboxamides, tetrahydropyrazolopyrimidines, and the 1,5-diarylpyrrole BM212 on the transport activity of MmpL3 in actively replicating M. tuberculosis bacilli are, like that of SQ109, most likely due to their ability to dissipate the transmembrane electrochemical proton gradient. In addition to providing novel insights into the modes of action of compounds reported to inhibit MmpL3, our results provide the first explanation for the large number of pharmacophores that apparently target this essential inner membrane transporter.

Membrane transport of vanadium compounds and the interaction with the erythrocyte membrane

In the present work, the membrane transport and the biotransformation of vanadate, bis(maltolato)oxovanadium (VO(ma)2), and vanadyl acetylacetonate (VO(acac)2) were investigated to explore the relationship with their insulin-like activity. Cellular uptake kinetics were performed by ICP-AES and EPR. The uptake of VO(acac)2 and VO(ma)2 by human erythrocytes showed intracellular vanadium level higher than NaVO3 and the membrane transport of these two vanadyl complexes was presumed to be via the passive diffusion mechanism. A fraction of vanadyl was oxidized to anionic vanadium(V) species and also entered the cells by the anion channel. The stability of VO(acac)2 and VO(ma)2 to oxidation in human erythrocyte membrane vesicles was investigated using EPR. VO(ma)2 was found to be more sensitive to oxidation than VO(acac)2 in aqueous buffer solution. However, in the presence of membrane vesicles, the oxidation of VO(ma)2 and VO(acac)2 was retarded and the differences between them became insignificant. Thus, the lifetime of vanadium complexes might be prolonged in physiological fluids. The interaction with membranes appears to be important in the stabilization of vanadyl complexes. Meanwhile, structural changes of membrane proteins were also observed. The higher uptake of the vanadyl complexes and the observed changes of membrane proteins might attribute to their insulin-mimetic mechanisms and toxicities.