Hydroxyl radicals is not a significant intermediate in the vanadate-stimulated oxidation of NAD(P)H by O2

Xanthine oxidase-induced neuronal death via the oxidation of NADH: prevention by micromolar EDTA

The oxidation of xanthine by xanthine oxidase (XO) or xanthine dehydrogenase represents an important source of reactive oxygen species (ROS), which contribute to the damaging consequences of cerebral ischemia, inflammation, and neurodegenerative disorders. However, both enzymes are also able to act on reduced nicotinamide adenine dinucleotide (NADH). The FAD binding site to which NADH binds is distinct from that of the xanthine binding site. We report that the combination of xanthine oxidase and NADH is toxic to cultures of cerebellar granule neurons. Protection by superoxide dismutase (Cu,Zn-SOD or Mn-SOD) or catalase indicates mediation of the toxicity by superoxide and hydrogen peroxide. In addition, pre-incubating XO with EDTA at concentrations as low as 2 microM, prevented the toxicity, indicating that a metal contaminating XO is involved in producing the toxic effects of XO/NADH. It is possible that such a metal might play a role in the toxicity of XO in vivo.

Vanadium promotes hydroxyl radical formation by activated human neutrophils

This study was undertaken to investigate the effects of vanadium in the +2, +3, +4, and +5 valence states on superoxide generation, myeloperoxidase (MPO) activity, and hydroxyl radical formation by activated human neutrophils in vitro, using lucigenin-enhanced chemiluminescence (LECL), autoiodination, and electron spin resonance with 5,5-dimethyl-l-pyrroline N-oxide as the spin trap, respectively. At concentrations of up to 25 microM, vanadium, in the four different valence states used, did not affect the LECL responses of neutrophils activated with either the chemoattractant, N-formyl-l-methionyl-l-leucyl-l-phenylalanine (1 microM), or the phorbol ester, phorbol 12-myristate 12-acetate (25 ng/ml). However, exposure to vanadium in the +2, +3, and +4, but not the +5, valence states was accompanied by significant augmentation of hydroxyl radical formation by activated neutrophils and attenuation of MPO-mediated iodination. With respect to hydroxyl radical formation, similar effects were observed using cell-free systems containing either hydrogen peroxide (100 microM) or xanthine/xanthine oxidase together with vanadium (+2, +3, +4), while the activity of purified MPO was inhibited by the metal in these valence states. These results demonstrate that vanadium in the +2, +3, and +4 valence states interacts prooxidatively with human neutrophils, competing effectively with MPO for hydrogen peroxide to promote formation of the highly toxic hydroxyl radical.

Vanadium redox cycling, lipid peroxidation and co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol

Mechanism of lipid peroxidation triggered by vanadium in human term placental microsomes was reinvestigated in vitro. Production of lipid peroxyl radicals was estimated from co-oxygenation of benzo(a)pyrene and benzo(a)pyrene-7,8-dihydrodiol. Vanadyl(IV), but not vanadate(V) caused a dose-dependent co-oxygenation. Vanadate(V) required the presence of reduced nicotinamide adenine dinucleotide phosphate to trigger co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol. To determine the role of pre-formed lipid hydroperoxides, the results obtained with partially peroxidized linoleic acid were compared with those of fresh linoleate. Superoxide dismutase inhibited the co-oxygenation of reaction when fresh linoleic acid was used. To further characterize the role of superoxide anion-radical in the vanadium redox cycling, the increase of optical density of vanadate(V) dissolved in Tris buffer was measured at 328 nm during the addition of KO2. The rate of this reaction producing peroxy-vanadyl complex was decreased by superoxide dismutase, especially, in the presence of catalase. It is suggested that vanadium catalyzes two separate processes, both leading to enhanced lipid peroxidation: (i) initiation, dependent on superoxide and triggered by peroxy-vanadyl; (ii) propagation, dependent on pre-formed lipid hydroperoxide not sensitive to superoxide dismutase. It is postulated that the vanadium-triggered initiation of lipid peroxidation may be crucial for toxicity in organs with limited endogenous lipid peroxidation.

Desferrioxamine enhances the reactivity of vanadium (IV) and vanadium (V) toward ferri- and ferrocytochrome c

Ligands, especially desferrioxamine, affect the rate at which vanadium reduces or oxidizes cytochrome c. Whether reduction or oxidation occurs, and how fast, depends on the nature of the ligand, the state of reduction of the vanadium, the pH (6.0, 7.0, or 7.4), and the availability of oxygen. In general, oxidation of ferrocytochrome c was favored by (1) low pH, (2) an oxidized state of the vanadium, (3) the presence of oxygen, and (4) more strongly binding ligands (desferrioxamine much greater than histidine = ATP greater than EDTA greater than albumin greater than aquo). Thus, at pH 6.0, desferrioxamine accelerated the V(V)-catalyzed ferrocytochrome c oxidation 160-fold aerobically, and 3500-fold anaerobically. In general, strongly binding ligands slowed oxidations, especially at higher pH. Desferrioxamine was unique among the five ligands in that it not only accelerated oxidation of ferrocytochrome c at pH 6.0, but at pH 7.4 the redox balance shifted to the point where it paradoxically reduced ferricytochrome c. V(V) is an improbable electron donor, but desferrioxamine will reduce cytochrome c, and V(V) accelerates this process. Oxidation of cytochrome c by V(V):desferrioxamine was faster anaerobically, and reduction by V(IV):desferrioxamine was faster aerobically. Although V(V) did not oxidize ferrocytochrome c at pH 7.4, V(IV) did, provided oxygen and desferrioxamine were both present. V(IV):desferrioxamine almost completely reduced ferricytochrome c, and this reduction was followed by a slow, progressive oxidation. This latter oxidation of cytochrome c is mediated by active species generated in the reaction between V(IV):desferrioxamine and oxygen, because none of these reagents alone can induce oxidation at a comparable rate. The mediating species were transient, and generated in reactions with oxygen.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidation of NADH by vanadium: kinetics, effects of ligands and role of H2O2 or O2

The mechanism of oxidation of NADH by either vanadium(V) or vanadium(IV) was examined in the presence of reducing agents, complexing agents, and hydrogen peroxide. Reducing agents that stimulate the oxidation of NADH by V(V) include: a variety of cysteine analogues, glutathione, beta-mercaptoethanol, dithiothreitol, and ascorbate. Complexing agents which stimulate NADH oxidation by V(V) include cystine, glutathione disulfide, and dehydroascorbate. Vanadium(IV)-dependent systems which oxidize NADH include combinations of V(IV) with cysteine or air alone. Combination of either V(V) or V(IV) with hydrogen peroxide leads to NADH oxidation. Based on kinetic analysis and the use of the diagnostic inhibitors--superoxide dismutase, catalase, albumin, mannitol, ethanol, and anaerobic conditions--we have assigned two major mechanisms of NADH oxidation. One is the previously reported mechanism which involves V(V)-superoxide as the NADH oxidant. This reaction is inhibited by superoxide dismutase and anaerobic conditions but not by catalase or ethanol. This reaction is observed for V(V) in the presence of reducing agents and complexing agents. The second reaction mechanism operates when V(IV) comes in contact with hydrogen peroxide and involves V(III)-superoxide as the NADH oxidant. This reaction is inhibited by catalase (if unligated hydrogen peroxide is an intermediate) and superoxide dismutase but not anaerobic conditions or ethanol. This mechanism is observed for reactions of V(IV) with air or hydrogen peroxide.

Vanadate-induced toxicity towards isolated perfused rat livers: the role of lipid peroxidation

The toxic potential of sodium orthovanadate towards isolated perfused rat livers was investigated at a dose of 2 mmol/l. In livers from fasted rats, vanadate led to a release of cytosolic (glutamate-pyruvate-transaminase (GPT) and lactate dehydrogenase (LDH] and mitochondrial (glutamate dehydrogenase (GLDH] enzymes, an accumulation of calcium in the liver, a marked depletion of hepatic glutathione and an enhanced release of it into the perfusate, as well as an augmented formation and release of thiobarbituric acid-reactive material by the liver. Furthermore, a marked inhibition of oxygen consumption was observed. Vanadate-induced vasoconstriction resulted in a progressive decrease in perfusate flow rate. Control experiments with similarly reduced flow rates led to a comparable reduction in oxygen consumption. GPT and LDH release and hepatic glutathione depletion were also evident, though to a lesser extent than in the presence of vanadate, but no increase in GLDH release, in tissue calcium content or TBA-reactive material in the liver or the perfusate were observed. Thus, indirect toxic effects due to a reduced flow rate contribute only partly to vanadate hepatotoxicity and do not affect mitochondrial integrity. Omission of calcium from the perfusate did not prevent hepatotoxic responses to vanadate, although less calcium was present in the treated livers than in the control organs, indicating that calcium influx is not involved in vanadate-induced hepatotoxicity in the intact organ, in contrast to isolated hepatocytes. Feeding the animals, resulting in an activation of anaerobic energy conservation reactions, strongly attenuated vanadate hepatotoxicity indicating that the energetic status of the liver is the main target of vanadate. Superoxide dismutase did not affect the hepatotoxic responses of livers from fasted rats towards vanadate, while allopurinol and deferrioxamine inhibited lipid peroxidation and hepatotoxicity due to vanadate. The strong correlation between induction of lipid peroxidation and hepatotoxicity and the inhibition of both processes in parallel by antioxidants are suggestive of a causative role for lipid peroxidation in vanadate-induced hepatotoxicity.

Effect of antioxidants on vanadate-induced toxicity towards isolated perfused rat livers

The effect of trolox C, a water soluble vitamin E analogue, propyl gallate and ascorbate on vanadate hepatotoxicity was investigated in vitro. In isolated perfused livers from fasted rats, sodium orthovanadate (2 mmol/l) led to toxic responses including reduction of oxygen consumption, release of cytosolic (glutamate-pyruvate-transaminase (GPT) and lactate dehydrogenase (LDH)) and mitochondrial (glutamate-dehydrogenase (GLDH)) enzymes, intracellular accumulation of calcium, a marked depletion of glutathione (GSH) and an enhanced formation and release of thiobarbituric acid- (TBA) reactive material. Trolox C and propyl gallate inhibited the release of GPT and LDH partially and that of GLDH totally, but had no influence on vanadate-induced calcium accumulation or on the reduction of oxygen consumption. Both agents suppressed vanadate-induced lipid peroxidation (LPO) and partially prevented GSH depletion. Ascorbate failed to provide any protection probably due to the interference of its pro-oxidant potential with its antioxidant activity. The protection, mainly of mitochondria, afforded by those agents which also inhibited LPO substantiates our previous findings that the pro-oxidant activity of vanadate is mainly responsible for its direct hepatotoxic actions [2]. Besides, reduction of organ perfusion rate due to vasoconstriction also contributes to vanadate toxicity, but oxidative stress is not involved in this indirect toxic activity.

Vanadyl causes hydroxyl radical mediated degradation of deoxyribose

Vanadyl caused a time- and dose-dependent degradation of deoxyribose to carbonyl products detectable with thiobarbituric acid. This process was inhibited by catalase, ethanol or HEPES; whereas superoxide dismutase was without effect. Vanadate did not substitute for vanadyl even in the presence of a source of O2- plus H2O2; but it did so in the presence of reductants such as thiols or NADH. It appears that hydrogen peroxide, generated by the autoxidation of vanadyl, is reduced by vanadyl to the hydroxyl radical; which, in turn, was responsible for the degradation of deoxyribose. A similar process might contribute to the toxic and pharmacological effects of vanadium salts.