Characterization of oxygen free radicals generated during vanadate-stimulated NADH oxidation

Biological Consequences of Vanadium Effects on Formation of Reactive Oxygen Species and Lipid Peroxidation

Lipid peroxidation (LPO), a process that affects human health, can be induced by exposure to vanadium salts and compounds. LPO is often exacerbated by oxidation stress, with some forms of vanadium providing protective effects. The LPO reaction involves the oxidation of the alkene bonds, primarily in polyunsaturated fatty acids, in a chain reaction to form radical and reactive oxygen species (ROS). LPO reactions typically affect cellular membranes through direct effects on membrane structure and function as well as impacting other cellular functions due to increases in ROS. Although LPO effects on mitochondrial function have been studied in detail, other cellular components and organelles are affected. Because vanadium salts and complexes can induce ROS formation both directly and indirectly, the study of LPO arising from increased ROS should include investigations of both processes. This is made more challenging by the range of vanadium species that exist under physiological conditions and the diverse effects of these species. Thus, complex vanadium chemistry requires speciation studies of vanadium to evaluate the direct and indirect effects of the various species that are present during vanadium exposure. Undoubtedly, speciation is important in assessing how vanadium exerts effects in biological systems and is likely the underlying cause for some of the beneficial effects reported in cancerous, diabetic, neurodegenerative conditions and other diseased tissues impacted by LPO processes. Speciation of vanadium, together with investigations of ROS and LPO, should be considered in future biological studies evaluating vanadium effects on the formation of ROS and on LPO in cells, tissues, and organisms as discussed in this review.

Decavanadate in vitro and in vivo effects: facts and opinions

This review covers recent advances in the understanding of the in vitro and in vivo effects of decavanadate, (V10O28)(6-), particularly in mitochondria. In vivo toxicological studies involving vanadium rarely account for the fact that under physiological conditions some vanadium may be present in the form of the decavanadate ion, which may behave differently from ortho- and metavanadates. It has for example been demonstrated that vanadium levels in heart or liver mitochondria are increased upon decavanadate exposure. Additionally, in vitro studies have shown that mitochondrial depolarization (IC50, 40 nM) and oxygen consumption (IC50, 99 nM) are strongly affected by decavanadate, which causes reduction of cytochrome b (complex III). We review these recent findings which together suggest that the observed cellular targets, metabolic pathway and toxicological effects differ according to the species of vanadium present. Finally, the toxicological effects of decavanadate depend on several factors such as the mode of administration, exposure time and type of tissue.

Chemical, biochemical, and biological behaviors of vanadate and its oligomers

Vanadate is widely used as an inhibitor of protein tyrosine phosphatases (PTPase) and is routinely applied in cell lysis buffers or immunoprecipitations of phosphotyrosyl proteins. Additionally, vanadate has been extensively studied for its antidiabetic and anticancer effects. In most studies, orthovanadate or metavanadate was used as the starting compound, whereas these "vanadate" solutions may contain more or less oligomerized species. Whether and how different species of vanadium compounds formed in the biological media exert specific biological effect is still a mystery. In the present commentary, we focus on the chemical, biochemical, and biological behaviors of vanadate. On the basis of species formation of vanadate in chemical and biological systems, we compared the biological effects and working mechanism of monovanadate with that of its oligomers, especially the decamer. We propose that different oligomers may exert a specific biological effect, which depends on their structures and the context of the cell types, by different modes of action.

Decavanadate: a journey in a search of a role

Currently, efforts have been directed towards using decavanadate as a tool for the understanding of several biochemical processes such as muscle contraction, calcium homeostasis, in vivo changes of oxidative stress markers, mitochondrial oxygen consumption, mitochondrial membrane depolarization, actin polymerization and glucose uptake, among others. In addition, studies have been conducted in order to make vanadium available and safe for clinical use, for instance with decavanadate compounds that present interesting pharmacological properties, eventually useful for the treatment of diabetes. Here, recent contributions of decavanadate to the effects of vanadium in biological systems, not only in vitro, but also in vivo, are analysed.

Decavanadate interacts with microsomal NADH oxidation system and enhances cytochrome c reduction

Oxidation of NADH with accompanying oxygen consumption (NADH:O(2) = 1:1) was observed in the combined presence of metavanadate (MV), decavanadate (DV) and microsomes. Oxygen consumption was negligible in the absence of MV, but NADH was oxidized and DV was reduced to a form of vanadyl-V(IV), colored blue like vanadyl sulfate but differed from it in having a 23-fold higher absorbance at 700 nm. DV can interact with the NADH oxidation system of microsomes as an electron acceptor, in addition to the known ferricyanide and cytochrome c. DV enhances rate of cytochrome c reduction significantly at microM concentrations. These studies indicate potential of DV as a redox intermediate.

Mitochondria as a target for decavanadate toxicity in Sparus aurata heart

In a previous in vivo study we have reported that vanadium distribution, antioxidant enzymes activity and lipid peroxidation in Sparus aurata heart are strongly dependent on the oligomeric vanadate species being administered. Moreover, it was suggested that vanadium is accumulated in mitochondria, in particular when V10 was intravenously injected. In this work we have done a comparative study of the effects of V10 and monomeric vanadate (V1) on cardiac mitochondria from S. aurata. V10 inhibits mitochondrial oxygen consumption with an IC(50) of 400 nM, while the IC(50) for V1 is 23 microM. V10 also induced mitochondrial depolarization at very low concentrations, with an IC(50) of 196 nM, and 55 microM of V1 was required to induce the same effect. Additionally, up to 5 microM V10 did inhibit neither F(0)F(1)-ATPase activity nor NADH levels and it did not affect respiratory complexes I and II, but it induced changes in the redox steady-state of complex III. It is concluded that V10 inhibits mitochondrial oxygen consumption and induces membrane depolarization more strongly than V1, pointing out that mitochondria is a toxicological target for V10 and the importance to take into account the contribution of V10 to the vanadate toxic effects.

Decavanadate induces mitochondrial membrane depolarization and inhibits oxygen consumption

Decavanadate induced rat liver mitochondrial depolarization at very low concentrations, half-depolarization with 39 nM decavanadate, while it was needed a 130-fold higher concentration of monomeric vanadate (5 microM) to induce the same effect. Decavanadate also inhibits mitochondrial repolarization induced by reduced glutathione in vitro, with an inhibition constant of 1 microM, whereas no effect was observed up to 100 microM of monomeric vanadate. The oxygen consumption by mitochondria is also inhibited by lower decavanadate than monomeric vanadate concentrations, i.e. 50% inhibition is attained with 99 M decavanadate and 10 microM monomeric vanadate. Thus, decavanadate is stronger as mitochondrial depolarization agent than as inhibitor of mitochondrial oxygen consumption. Up to 5 microM, decavanadate does not alter mitochondrial NADH levels nor inhibit neither F(O)F(1)-ATPase nor cytochrome c oxidase activity, but it induces changes in the redox steady-state of mitochondrial b-type cytochromes (complex III). NMR spectra showed that decameric vanadate is the predominant vanadate species in decavanadate solutions. It is concluded that decavanadate is much more potent mitochondrial depolarization agent and a more potent inhibitor of mitochondrial oxygen consumption than monomeric vanadate, pointing out the importance to take into account the contribution of higher oligomeric species of vanadium for the biological effects of vanadate solutions.

Modification of alterations in cardiac function and sarcoplasmic reticulum by vanadate in ischemic-reperfused rat hearts

To study the cardioprotective effects of vanadate on ischemia-reperfusion (I/R) injury, isolated rat hearts perfused at constant flow were subjected to global ischemia for 30 min followed by reperfusion for 30 min. In this experimental model, I/R markedly decreased ventricular developed pressure and increased end-diastolic pressure. Pretreatment of hearts with 4 microM vanadate attenuated I/R-induced cardiac dysfunction. The reduction in sarcoplasmic reticulum (SR) Ca2+ uptake and Ca2+ release, as well as SR protein contents for Ca2+-pump ATPase and Ca2+-release channel, was also prevented by vanadate. Pretreatment of hearts with an antioxidant mixture containing superoxide dismutase + catalase exerted effects similar to those of vanadate in I/R hearts. Postischemic treatment of hearts with vanadate or superoxide dismutase + catalase also had beneficial effects on I/R-induced changes in cardiac performance and SR function. Alterations in cardiac function and SR Ca2+ transport due to an oxyradical-generating system (xanthine + xanthine oxidase) or an oxidant (H2O2) were attenuated by treatment with vanadate. These results suggest that vanadate may exert beneficial effects on cardiac performance and SR function in I/R hearts because of its antioxidant action.

Vanadate oligomers: in vivo effects in hepatic vanadium accumulation and stress markers

The formation of vanadate oligomeric species is often disregarded in studies on vanadate effects in biological systems, particularly in vivo, even though they may interact with high affinity with many proteins. We report the effects in fish hepatic tissue of an acute intravenous exposure (12, 24 h and 7 days) to two vanadium(V) solutions, metavanadate and decavanadate, containing different vanadate oligomers administered at sub-lethal concentration (5 mM; 1 mg/kg). Decavanadate solution promotes a 5-fold increase (0.135 +/- 0.053 microg V(-1) dry tissues) in the vanadium content of the mitochondrial fraction 7 days after exposition, whereas no effects were observed after metavanadate solution administration. Reduced glutathione (GSH) levels did not change and the overall reactive oxygen species (ROS) production was decreased by 30% 24 h after decavanadate administration, while for metavanadate, GSH levels increased 35%, the overall ROS production was depressed by 40% and mitochondrial superoxide anion production decreased 45%. Decavanadate intoxication did not induce changes in the rate of lipid peroxidation till 12 h, but later increased 80%, which is similar to the increase observed for metavanadate after 24 h. Decameric vanadate administration clearly induces different effects than the other vanadate oligomeric species, pointing out the importance of taking into account the different vanadate oligomers in the evaluation of vanadium(V) effects in biological systems.

Formation of an oxo-radical of peroxovanadate during reduction of diperoxovanadate with vanadyl sulfate or ferrous sulfate

Formation of oxygen radicals during reduction of H(2)O(2) or diperoxovanadate with vanadyl sulfate or ferrous sulfate was indicated by the 1:2:2:1 electron spin resonance (ESR) signals of the DMPO adduct typical of standard ()OH radical. Signals derived from diperoxovanadate remained unchanged in the presence of ethanol in contrast to those from H(2)O(2). This gave the clue that they represent a different radical, possibly (*)OV(O(2))(2+), formed on breaking a peroxo-bridge of diperoxovanadate complex. The above reaction mixtures evolved dioxygen or, when NADH was present, oxidized it rapidly which was accompanied by consumption of dioxygen. Operation of a cycle of peroxovanadates including this new radical is suggested to explain these redox activities both with vanadyl and ferrous sulfates. It can be triggered by ferrous ions released from cellular stores in the presence of catalytic amounts of peroxovanadates.

Inactivation of glucose oxidase by diperoxovanadate-derived oxidants

Inactivation of glucose oxidase occurred in the presence of bromide, vanadate, H(2)O(2), and phosphate (the bromide system), and this was prevented by NADH or phenol red, a bromine acceptor. Glucose oxidase present during the reaction between diperoxovanadate and a reduced form of vanadate, vanadyl (the vanadyl system), but not added after mixing the reactants, was inactivated, and this was accompanied by a loss of binding of the dye, Coomassie blue, to the protein. The transient intermediate of the type OVOOV(O(2)), known to form in these reactions and used in the oxidation of bromide ion and NADH, appears to be responsible for inactivating glucose oxidase. In both systems, the inactivation of the enzyme was prevented by histidine and DTT, known to quench singlet-oxygen. By direct measurement of 1270-nm emission of singlet-oxygen, its generation was demonstrated in the bromide system, and in the reaction of hypohalous acids with diperoxovanadate, but not in the vanadyl system. By themselves both hypohalous acids, HOCl and HOBr inactivated glucose oxidase, and their prior reaction with H(2)O(2) during which singlet-oxygen was released, protected the enzyme. The results provide support for possible oxidative inactivation of glucose oxidase by diperoxovanadate-derived oxidants.

Ethanol-dependent oxygen consumption and acetaldehyde formation during vanadyl oxidation by H2O2

Sequential addition of vanadyl sulfate to a phosphate-buffered solution of H2O2 released oxygen only after the second batch of vanadyl. Ethanol added to such reaction mixtures progressively decreased oxygen release and increased oxygen consumption during oxidation of vanadyl by H2O2. Inclusion of ethanol after any of the three batches of vanadyl resulted in varying amounts of oxygen consumption, a property also shared by other alcohols (methanol, propanol and octanol). On increasing the concentration of ethanol, vanadyl sulfate or H2O2, both oxygen consumption and acetaldehyde formation increased progressively. Formation of acetaldehyde decreased with increase in the ratio of vanadyl:H2O2 above 2:1 and was undetectable with ethanol at 0.1 mM. The reaction mixture which was acidic in the absence of phosphate buffer (pH 7.0), released oxygen immediately after the first addition of vanadyl and also in presence of ethanol soon after initial rapid consumption of oxygen, with no accompanying acetaldehyde formation. The results underscore the importance of some vanadium complexes formed during vanadyl oxidation in the accompanying oxygen-transfer reactions.

Protective effect of vanadate on oxyradical-induced changes in isolated perfused heart

In order to examine the mechanisms of the beneficial effects of vanadate on cardiac dysfunction in chronic diabetes, rat hearts were perfused with xanthine plus xanthine oxidase, an oxyradical generating system in the absence or presence of vanadate. The heart failed to generate contractile force and increased the resting tension markedly within 5 min of perfusion with xanthine plus xanthine oxidase. These changes were prevented by the addition of 4 microM vanadate in the perfusion medium. The protective effects of vanadate on the loss of developed tension and increased resting tension due to xanthine plus xanthine oxidase were dose-dependent (0.1-5 microM). Perfusion of the hearts with glucose-free medium did not abolish the protective actions of vanadate. The sarcolemmal Ca(2+)-pump (ATP-dependent Ca2+ uptake and Ca(2+)-stimulated ATPase) and Na(+)-dependent Ca2+ uptake activities were decreased upon perfusing the hearts with a medium containing xanthine plus xanthine oxidase for 5 min; these effects were prevented by the addition of 2-4 microM vanadate in the perfusion medium. The signals for superoxide radicals produced by xanthine plus xanthine oxidase, as detected by electron paramagnetic resonance spectroscopic technique, were inhibited by 5-100 microM vanadate. These results suggest that vanadate is an oxyradical scavenger and thus may prevent heart dysfunction under some pathological conditions by its antioxidant action.

Multiple reactions in vanadyl-V(IV) oxidation by H2O2

Oxidation of vanadyl sulfate by H2O2 involves multiple reactions at neutral pH conditions. The primary reaction was found to be oxidation of V(IV) to V(V) using 0.5 equivalent of H2O2, based on the loss of blue color and the visible spectrum. The loss of V(IV) and formation V(V) compounds were confirmed by ESR and 51V-NMR spectra, respectively. In the presence of excess H2O2 (more than two equivalents), the V(V) was converted into diperoxovanadate, the major end-product of these reactions, identified by changes in absorbance in ultraviolet region and by the specific chemical shift in NMR spectrum. The stoichiometric studies on the H2O2 consumed in this reaction support the occurrence of reactions of two-electron oxidation followed by complexing two molecules of H2O2. Addition of a variety of compounds--Tris, ethanol, mannitol, benzoate, formate (hydroxyl radical quenching), histidine, imidazole (singlet oxygen-consumption that also used V(IV) as the reducing source. This reaction requires concomitant oxidation of vanadyl by H2O2, favoured at low H2O2:V(IV) ratio. Another secondary reaction of oxygen release was found to occur during vanadyl oxidation by H2O2 in acidic medium in which the end-product was not diperoxovanadate but appears to be a mixture of VO3+ (-546 ppm), VO3+ (-531 ppm) and VO2+ (-512 ppm), as shown by the 51V-NMR spectrum. This reaction also occurred in phosphate-buffered medium but only on second addition of vanadyl. The compounds that stimulated the oxygen-consumption reaction were found to inhibit the oxygen-release reaction. A combination of these reactions occur depending on the proportion of the reactants (vanadyl and H2O2), the pH of the medium and the presence of some compounds that affect the secondary reactions.

Polyvanadate-stimulated NADH oxidation by plasma membranes--the need for a mixture of deca and meta forms of vanadate

Polyvanadate solutions obtained by extracting vanadium pentoxide with dilute alkali over a period of several hours contained increasing amounts of decavanadate as characterized by NMR and ir spectra. Those solutions having a metavanadate:decavanadate ratio in the range of 1-5 showed maximum stimulation of NADH oxidation by rat liver plasma membranes. Reduction of decavanadate, but not metavanadate, was obtained only in the presence of the plasma membrane enzyme system. High simulation of activity of NADH oxidation was obtained with a mixture of the two forms of vanadate and this further increased on lowering the pH. Addition of increasing concentrations of decavanadate to metavanadate and vice versa increased the stimulatory activity, reaching a maximum when the metavanadate:decavanadate ratio was in the range of 1-5. Increased stimulatory activity can also be obtained by reaching these ratios by conversion of decavanadate to metavanadate by alkaline phosphate degradation, and of metavanadate to decavanadate by acidification. These studies show for the first time that both deca and meta forms of vanadate present in polyvanadate solutions are needed for maximum activity of NADH oxidation.

Stimulation of NADH oxidation by xanthine oxidase and polyvanadate in presence of some dehydrogenases and flavin compounds

The rates of NADH oxidation in presence of xanthine oxidase increase to a small and variable extent on addition of high concentrations of lactate dehydrogenase and other dehydrogenases. This heat stable activity is similar to polyvanadate-stimulation with respect to pH profile and SOD sensitivity. Isocitric dehydrogenase (NADP-specific) showed heat labile, SOD-sensitive polyvanadate-stimulated NADH oxidation activity. Polyvanadate-stimulated SOD-sensitive NADH oxidation was also found to occur with riboflavin, FMN and FAD in presence of a non-specific protein, BSA, suggesting that some flavoproteins may possess this activity.