The vanadate-stimulated oxidation of NAD(P)H by biomembranes is a superoxide-initiated free radical chain reaction

Differential in vitro inhibition studies of some cerium vanadate derivatives on xanthine oxidase

In this preliminary study, a new series of some cerium vanadate derivatives have been investigated as new type of inhibitors of xanthine oxidase (XO; E.C 1.17.3.2). XO is a superoxide-producing enzyme found normally in serum and the lungs, and its activity is concerned with several important health problems such as gout, severe liver damage, vascular dysfunction and injury, oxidative eye injury and renal failure. In this study, we present a critical overview of the effects of these novel type agents on XO with comparing the efficacy and safety profiles of allopurinol, the efficient classical inhibitor of XO.

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.

With the help of giants

A childhood fascination with animals, plants, and insects was aided and abetted by many giants, beginning with my parents. The Bronx High School of Science and the City College of New York (CCNY) made a solid and priceless grounding in chemistry and biology available free of charge. Abe Mazur at CCNY revealed the wonders of biochemistry and illustrated that it was possible to pursue these wonders while being paid to do so. He also directed me to Duke University Medical School for PhD work under the tutelage of Phil Handler. With the exception of a sabbatical year at Harvard with Frank Westheimer, my entire career has been spent at Duke serving under three fine and supportive chairmen: Handler, Hill, and Raetz. The premier discoveries to emanate from my laboratory have been the sulfite oxidase, the several superoxide dismutases, the manganese catalase, and the catalase/peroxidase. Many other topics piqued my interest and resulted in ~ 400 publications. Herein I have recounted some of the circumstances surrounding that work and named a few of the people involved. The first 20 years I worked happily at the bench and the next 35 years just as happily facilitating the work of younger people. It has been so rewarding that I wish for nothing more than to be allowed to keep at it.

Melatonin inhibits the contractile effect of vanadate in the isolated pulmonary arterial rings of rats: possible role of hydrogen peroxide

The effect and possible mechanism of action of vanadate on the isolated pulmonary arterial rings of normal rats were studied. Pulmonary arterial rings contracted in response to vanadate (0.1-1 mM) in a concentration-dependent manner. Preincubation of the pulmonary arterial rings with 1 mM melatonin significantly reduced the contractile effect of vanadate by more than 60%. Furthermore, addition of hydrogen peroxide (50 microM) or enzymatic generation of hydrogen peroxide by the addition of glucose oxidase (10 U/mL) to the medium containing glucose produced remarkable increases in the pulmonary arterial tension, 46.2 +/- 7.3 and 78.7 +/- 9.7 g tension/g tissue, respectively. Similarly, incubation of the pulmonary arterial rings with 1 mM melatonin significantly reduced the contractile responses of the arterial rings to hydrogen peroxide and glucose/glucose oxidase to 25.7 +/- 2.9 and 24.7 +/- 4.4 g tension/g tissue, respectively. Vanadate, in vitro, significantly stimulated the oxidation of NADH by xanthine oxidase, and the rate of oxidation was increased by increasing either time or vanadate concentration. Similarly, addition of melatonin to a reaction mixture containing xanthine oxidase and vanadate significantly inhibited the rate of NADH oxidation in a concentration-dependent fashion. The results of the present study indicated that vanadate induced contraction in the isolated pulmonary arterial rings, which was significantly reduced by melatonin. Furthermore, the contractile effect of vanadate on the pulmonary arterial rings may be attributed to the intracellular generation of hydrogen peroxide.

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.

Superoxide-independent reduction of vanadate by rat liver microsomes/NAD(P)H: vanadate reductase activity

It has been reported that vanadate-stimulated oxidation of NAD(P)H by microsomal systems can proceed anaerobically, in contrast to the general notion that the oxidation proceeds exclusively by an O(2-)-dependent free radical chain mechanism. The current study indicates that microsomal systems are endowed with a vanadate-reductase property, involving a NAD(P)H-dependent electron transport cytochrome P450 system. Our ESR measurements demonstrated the formation of a vanadium(IV) species in a mixture containing vanadate, rat liver microsomes, and NAD(P)H. This vanadium(IV) species was identified as the vanadyl ion (VO2+) by comparison with the ESR spectrum of VOSO4. The initial rate of vanadium(IV) formation depends linearly on the concentration of microsomes. The Michaelis-Menten constants were found to be: km = 1.25 mM and Vmax = 0.066 mumol (min)-1 (mg microsomes)-1, respectively. Pretreatment of the microsomes with carbon monoxide or K3Fe(CN)6 reduced vanadium(IV) generation, suggesting that the NAD(P)H-dependent electron transport cytochrome P450 system plays a significant role in the microsomal reduction of vanadate. Measurements under argon or in the presence of superoxide dismutase caused only minor (less than 10%) reductions in vanadium(IV) generation. The VO2+ species was also detected in NAD(P)H oxidation by fructose plus vanadate, a reaction known to proceed via an O(2-)-mediated chain mechanism. However, the amount of vanadium(IV) generated by this reaction was an order of magnitude smaller than that by the microsomal system and was inhibitable by superoxide dismutase, affirming the conclusion that the microsomal/NAD(P)H system is endowed with the (O(2-)-independent) vanadium(V) reductase property.

Superoxide generated by glutathione reductase initiates a vanadate-dependent free radical chain oxidation of NADH

Vanadate V(V) markedly stimulated the oxidation of NADPH by GSSG reductase and this oxidation was accompanied by the consumption of O2 and the accumulation of H2O2. Superoxide dismutases completely eliminated this effect of V(V), whereas catalase was without effect, as was exogenous H2O2 added to 0.1 mM. These effects could be seen equally well in phosphate- or in 4-(2-hydroxyethyl)1-piperazineethanesulfonic acid-buffered solutions. Under anaerobic conditions there was no V(V)-stimulated oxidation of NADPH. Approximately 4% of the electrons flowing from NADPH to O2, through GSSG reductase, resulted in release of O2-. The average length of the free radical chains causing the oxidation of NADPH, initiated by O2- plus V(V), was calculated to be in the range 140-200 NADPH oxidized per O2- introduced. We conclude that GSSG reductase, and by extension other O2(-)-producing flavoprotein dehydrogenases such as lipoyl dehydrogenase and ferredoxin reductase, catalyze V(V)-stimulated oxidation of NAD(P)H because they release O2- and because O2- plus V(V) initiate a free radical chain oxidation of NAD(P)H. There is no reason to suppose that these enzymes can act as NAD(P)H:V(V) oxidoreductases.

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)

Flavoenzymes reduce vanadium(V) and molecular oxygen and generate hydroxyl radical

ESR spectroscopic evidence is presented for the formation of vanadium(IV) in the reduction of vanadium(V) by three typical, NADPH-dependent, flavoenzymes: glutathione reductase, lipoyl dehydrogenase, and ferredoxin-NADP+ oxidoreductase. The vanadium(V)-reduction mechanism appears to be an enzymatic one-electron reduction process. Addition of superoxide dismutase (SOD) showed that the generation of vanadium(IV) does not involve the superoxide (O2-) radical significantly. Measurements under anaerobic atmosphere showed, however, that the enzymes-vanadium-NADPH mixture can cause the reduction of molecular oxygen to generate H2O2. The H2O2 and vanadium(IV) thus formed react to generate hydroxyl (.OH) radical. The .OH formation is inhibited strongly by catalase and to a lesser degree by SOD, but it is enhanced by exogenous H2O2, suggesting the occurrence of a Fenton-like reaction. The inhibition of vanadium(IV) formation by N-ethylmaleimide indicates that the SH group on the flavoenzyme's cystine residue plays an important role in the enzyme's vanadium(V) reductase function. These results thus reveal a new property of the above-mentioned, NADPH-dependent flavoenzymes--their function as vanadium(V) reductases, as well as that as generators of .OH radical in the vanadium(V) reduction mechanism.

Radiolytic reduction of protein and nonprotein disulfides in the presence of formate: a chain reaction

We have reported recently that the disulfide groups in bovine serum albumin can be reduced by a radiolytic chain reaction which occurs in deoxygenated solutions containing formate ions. This reaction, which involves the reduction of disulfide groups by hydrated electrons and carbon dioxide radical anions, has now been studied in greater detail and compared with an analogous reaction in small, disulfide containing molecules over a range of pH values and substrate concentrations. A two-step reaction is proposed to account for the reduction of disulfides in reactions which can have chain lengths of 20 or more. Thiols produced by the disulfide reduction are stable to the conditions of the reaction. For example, a biological assay showed that the integrity of glutathione was maintained even at radiation doses much larger than those required to achieve complete reduction of glutathione disulfide. It was found that the extent of disulfide reduction could easily be controlled by varying the radiation dose delivered to the solutions. Radiolytic reduction is a very useful way of reducing protein and low molecular weight disulfides without the use of excess quantities of reagents such as dithiothreitol. In many cases, the reaction solutions could be used directly for subsequent reactions and this may be of considerable value in modifying the structure of hormones, enzymes, membrane receptors, and other disulfide containing proteins. If ammonium formate is used, freeze drying is an effective way to remove the formate salt, should this be required.

Reaction of vanadium(V) with thiols generates vanadium (IV) and thiyl radicals

The in vivo toxicity of vanadium(V) has been found to correlate with the depletion of cellular glutathione and related non-protein thiols. With a view to understanding the mechanism for this observation, we have investigated the oxidation of glutathione, cysteine N-acetylcysteine and penicillamine by vanadium(V), using electron spin resonance (ESR) and ESR spin trapping methodology. The spin trap used was 5,5-dimethyl-1-pyrroline 1-oxide (DMPO). It is found that the oxidation of these thiols by vanadium(V) generates the corresponding thiyl radicals and vanadium- (IV) complexes. The results suggest that free radical reactions play a significant role in the depletion of cellular thiols by vanadium(V) and hence in vanadium(V) toxicity.

Glutathione reductase functions as vanadate(V) reductase

The oxidation of NADPH by vanadate(V) in the presence of glutathione reductase showed typical enzymatic kinetics. The oxidation was inhibited by N-ethylmaleimide, a glutathione reductase inhibitor. Superoxide dismutase had no significant effect on the oxidation, indicating noninvolvement of the superoxide radical. The vanadate(V) reduction was found to be a one-electron transfer process. These results suggest a new pathway for vanadate(V) metabolism and a new function of glutathione reductase.

Tetravalent vanadium mediated oxidation of low density lipoprotein

1. Tetravalent vanadium causes oxidation of low density lipoprotein (LDL) as manifest by protein degradation and lipid peroxidation. 2. Oxidative modification of the apolipoprotein B-100 is paralleled by the formation of thiobarbituric acid reactive substance and fluorescent chromolipid production. 3. The metal chelators ethylenediamine tetracetic acid and desferrioxamine, and the alcohols, ethanol and isopropanol inhibit the oxidation of LDL by tetravalent vanadium. No inhibition is observed with superoxide dismutase, catalase or mannitol. 4. The data suggest that aldehydes formed during the process of lipid peroxidation induced by tetravalent vanadium react with the proteins in LDL to form fluorescent chromolipids and that the oxidative process originates within the hydrophobic domain of LDL.

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

The vanadate-stimulated oxidation of NADH by an enzymatic flux of O2- is inhibited by superoxide dismutase, but not by catalase. Keller et al. (1989, Free Radical Biol. Med. 6, 15-22) observed inhibition by catalase presumably because they used a commercial preparation contaminated with superoxide dismutase. Their proposal, that H2O2 and hydroxyl radical play significant roles in vanadate-stimulation of NAD(P)H oxidation, may be discounted on the basis of these and of previously reported results.

Vanadate-mediated oxidation of NADH: description of an in vitro system requiring ascorbate and phosphate

Oxidation of NADH has been observed in an in vitro system requiring NADH, vanadate, ascorbate, and phosphate. Similar results were observed with NADPH. Ascorbate provides the reducing equivalents necessary to reduce vanadate to vanadyl. Vanadyl autoxidizes producing superoxide which initiates a free radical chain reaction resulting in oxidation of NADH. Oxidation is inhibited by superoxide dismutase but not by catalase or ethanol. Ascorbate functions to initiate the free radical chain reaction but is not required in stoichiometric concentrations. At higher concentrations, ascorbate inhibits NADH oxidation. Inorganic phosphate was required for NADH oxidation. Dialysis of phosphate buffers against solutions containing apoferritin or conalbumin or addition of transition metal cations or chelators to the reaction medium did not alter dependence on phosphate. Phosphate and vanadate were interchangeable in their effects on kinetic parameters of NADH oxidation except that vanadate was 100 times more potent than phosphate. Vanadate participates directly in the initiating and propagating redox reactions of NADH oxidation. Phosphate may be important in lowering the energy of activation for the necessary transfer of hydronium ion and water in the transition state between vanadate anion and vanadyl cation.

Oxidation of NADH by vanadium compounds in the presence of thiols

The nonenzymatic oxidation of NADH was studied spectrophotometrically in the presence of two vanadium compounds, sodium orthovanadate and vanadyl sulfate. At physiological pH 7.4, in 25 mM sodium phosphate buffer, addition of the synthetic thiol, dithioerythritol (DTE) results in a marked increase of NADH oxidation in the presence of sodium orthovanadate, but not in the presence of vanadyl sulfate. Other reductants, such as dithiothreitol and cysteine, can also increase NADH oxidation, whereas glutathione and ascorbate cannot. In all reactions, superoxide dismutase and catalase completely inhibit the vanadium-stimulated oxidation of NADH. Inhibition occurs in a concentration-dependent manner, and the boiled enzymes do not inhibit the thiol reaction. The hydroxyl radical scavenger, thiourea, inhibits the reaction, whereas urea cannot. ESR studies show that the ability of the thiol to reduce vanadate can be correlated with the degree of NADH oxidation. Using spin trapping techniques, hydroxyl radicals are detected during the course of the reaction. Addition of hydrogen peroxide to vanadyl in the presence of DTE greatly increases NADH oxidation; however, no NADH oxidation occurs when hydrogen peroxide is added to vanadyl and ascorbic acid. These results provide a partial explanation for the ability of vanadium compounds to both decrease cellular reducing equivalents and promote lipid peroxidation.

Vanadate-dependent NAD(P)H oxidation by microsomal enzymes

Vanadate-dependent NAD(P)H oxidation, catalyzed by rat liver microsomes and microsomal NADPH-cytochrome P450 reductase (P450 reductase) and NADH-cytochrome b5 reductase (b5 reductase), was investigated. These enzymes and intact microsomes catalyzed NAD(P)H oxidation in the presence of either ortho- or polyvanadate. Antibody to P450 reductase inhibited orthovanadate-dependent NADPH oxidation catalyzed by either purified P450 reductase or rat liver microsomes and had no effect on the rates of NADH oxidation catalyzed by b5 reductase. NADPH-cytochrome P450 reductase catalyzed orthovanadate-dependent NADPH oxidation five times faster than NADH-cytochrome b5 reductase catalyzed NADH oxidation. Orthovanadate-dependent oxidation of either NADPH or NADH, catalyzed by purified reductases or rat liver microsomes, occurred in an anaerobic system, which indicated that superoxide is not an obligate intermediate in this process. Superoxide dismutase (SOD) inhibited orthovanadate, but not polyvanadate-mediated, enzyme-dependent NAD(P)H oxidation. SOD also inhibited when pyridine nucleotide oxidation was conducted anaerobically, suggesting that SOD inhibits vanadate-dependent NAD(P)H oxidation by a mechanism independent of scavenging of O2-.

Effects of vanadate on the oxidation of NADH by xanthine oxidase

Vanadate (V(V)) stimulates the oxidation of NADH by xanthine oxidase and superoxide dismutase eliminates the effect of V(V). Paraquat stimulates both the oxidation of NADH by xanthine oxidase and the V(V) enhancement of that oxidation. Xanthine, which is a better substrate for xanthine oxidase than is NADH, causes a V(V)-dependent co-oxidation of NADH which is transient and eliminated by SOD. Urate inhibits the V(V)-stimulated oxidation of NADH by xanthine oxidase or by Rose Bengal plus light. Measurement of rates of both O2- production and V(V)-stimulated NADH oxidation showed that many molecules of NADH were oxidized per O2-. These chain lengths were an inverse function of overall reaction rate. Minimum chain lengths, calculated on the basis of 100% univalent reduction of O2 to O2-, were smaller than measured average chain lengths by a factor of five. All of these results are in accord with the view that V(V) does not directly affect the activity of the enzyme, but rather catalyzes the free radical chain oxidation of NADH by O2-. It was further shown that phosphate was not involved and that the active form of V(V) was orthovanadate, rather than decavanadate.

Importance of hydroxyl radical in the vanadium-stimulated oxidation of NADH

Vanadium compounds are known to stimulate the oxidation of NAD(P)H, but the mechanism remains unclear. This reaction was studied spectrophotometrically and by electron spin resonance spectroscopy (ESR) using vanadium in the reduced state (+4, vanadyl) and the oxidized state (+5, vanadate). In 25 mM sodium phosphate buffer at pH 7.4, vanadyl was slightly more effective in stimulating NADH oxidation than was vanadate. Addition of a superoxide generating system, xanthine/xanthine oxidase, resulted in a marked increase in NADH oxidation by vanadyl, and to a lesser extent, by vanadate. Decreasing the pH with superoxide present increased NADH oxidation for both vanadate and vanadyl. Addition of hydrogen peroxide to the reaction mixture did not change the NADH oxidation by vanadate, regardless of concentration or pH. With vanadyl however, addition of hydrogen peroxide greatly enhanced NADH oxidation which further increased with lower pH. Use of the spin trap DMPO in reaction mixtures containing vanadyl and hydrogen peroxide or a superoxide generating system resulted in the detection by ESR of hydroxyl. In each case, the hydroxyl radical signal intensity increased with vanadium concentration. Catalase was able to inhibit the formation of the DMPO--OH adduct formed by vanadate plus superoxide. These results show that the ability of vanadium to act in a Fenton-type reaction is an important process in the vanadium-stimulated oxidation of NADH.

Vanadate-stimulated oxidation of NAD(P)H

Vanadate stimulates the oxidation of NAD(P)H by biological membranes because such membranes contain NAD(P)H oxidases which are capable of reducing dioxygen to O2- and because vanadate catalyzes the oxidation of NAD(P)H by O2-, by a free radical chain mechanism. Dihydropyridines, such as reduced nicotinamide mononucleotide (NMNH), which are not substrates for membrane-associated NAD(P)H oxidases, are not oxidized by membranes plus vanadate unless NAD(P)H is present to serve as a source of O2-. When [NMNH] greatly exceeds [NAD(P)H], in such reaction mixtures, one can observe the oxidation of many molecules of NMNH per NAD(P)H consumed. This reflects the chain length of the free radical chain mechanism. We have discussed the mechanism and significance of this process and have tried to clarify the pertinent but confusing literature.

Opioids stimulate sarcolemmal NAD(P)H-vanadate dehydrogenase activity

The present study demonstrates that the bovine cardiac sarcolemma possesses an NAD(P)H dehydrogenase activity which is able to oxidize both NADH and NAD(P)H in the presence of vanadate as an electron acceptor. The NADH dehydrogenase activity was significantly higher than the NAD(P)H dehydrogenase activity and both of them were almost completely inhibited by superoxide dismutase and atebrin and markedly reduced by the addition of the protonophore 2,4-dinitrophenol. The incubation of the sarcolemma in the presence of 10(-10), 10(-9), 10(-8) M methionine-enkephalin, a prevalent delta-opioid receptor agonist, or dynorphin A (1-17), a prevalent kappa-receptor agonist, produced a dose-dependent increase in the NAD(P)H dehydrogenase activity, with 10(-10) and 10(-9) M dynorphin A (1-17) more effective than the corresponding doses of methionine-enkephalin. The preincubation of the sarcolemma in the presence of superoxide-dismutase, atebrin or 2,4-dinitrophenol strongly inhibited the opioid-stimulated dehydrogenase activity. The stimulatory action elicited by 10(-8) M methionine-enkephalin or dynorphin A (1-17) was completely antagonized by 10(-8) M naloxone or Mr 1452, respectively, whilst 10(-8) M naloxone exerted only a partially antagonistic action against the effect produced by 10(-8) M dynorphin A (1-17), significantly more accentuated than the action of 10(-8) M Mr 1452 versus the same dose of methionine-enkephalin.

Superoxide is responsible for the vanadate stimulation of NAD(P)H oxidation by biological membranes

Vandate augments the oxidation of NAD(P)H, but not of NMNH, by rat liver microsomes. Paraquat increases the vanadate effect on NADPH, but not on NADH, oxidation. Substoichiometric levels of NADPH caused the co-oxidation of NADH or NMNH and SOD inhibited in all cases. The ratio of NADH or NMNH co-oxidized per NADPH added allowed estimation of average chain length, which increased as the pH was lowered from 8.0 to 7.1. The initial rate of this co-oxidation of NMNH was a saturating function of the concentration of microsomes, reflecting a decrease in chain length with an increase in number of concomitant reaction chains, and due to increasing radical-radical termination reactions. Mitochondrial outer membranes behaved like the microsomal membranes, but mitochondrial inner membranes catalyzed a rapid oxidation of NADH which could be augmented by vanadate, whose action was enhanced by paraquat and inhibited by antimycin or rotenone. These and related observations support the view that vanadate stimulates NAD(P)H oxidation by biological membranes, not by virtue of interacting with enzymes, but rather by interacting with O-2.

What is unique about superoxide toxicity as compared to other biological reductants? A hypothesis

Usually the toxicity of superoxide is attributed to its ability to reduce metal ions and subsequently reoxidation of the metal by hydrogen peroxide yields deleterious oxidizing species. As many other nontoxic biological reductants reduce metal compounds, we suggest that part of the mechanism of superoxide toxicity results from its ability to oxidize metal ions bound to biological targets, which subsequently degrade the target via an intramolecular electron transfer reaction.

A study on the mechanism of the vanadate-dependent NADH oxidation

The mechanism of the vanadate (V(V]-dependent oxidation of NADH was different in phosphate buffers and in phosphate-free media. In phosphate-free media (aqueous medium or HEPES buffer) the vanadyl (V(IV] generated by the direct V(V)-dependent oxidation of NADH formed a complex with V(V). In phosphate buffers V(IV) autoxidized instead of forming a complex with V(V). The generated superoxide radical (O2-) initiated, in turn, a high-rate free radical chain oxidation of NADH. Phosphate did not stimulate the V(V)-dependent NADH oxidation catalyzed by O2--generating systems. Monovanadate proved to be a stronger catalyzer of NADH oxidation as compared to polyvanadate.

In vitro effect of mercury and vanadium on superoxide anion production and plasminogen activator activity of mouse peritoneal macrophages

The in vitro effects of mercuric chloride and vanadate were examined on two functions of mouse peritoneal macrophages, i.e., the superoxide anion production and the plasminogen activator (PA) activity. Vanadate, at concentrations which do not affect the viability of the cells, does not seriously alter any of these functions. High concentrations of mercury depress the respiratory burst; this effect results from loss of the reducing properties of cellular NADPH. Low concentrations of mercury stimulate the effect of phorbol 12-myristate 13-acetate on PA activity. The mechanism of this stimulation does not involve the protein kinase C system. It is hypothesized that mercury could enhance the synthesis of PA, its translocation to the cell surface, or its binding to the membrane receptors.

Vanadyl- and vanadate-induced lipid peroxidation in mitochondria and in phosphatidylcholine suspensions

Vanadyl (V(IV] was found to induce rapidly developing lipid peroxidation in intact and sonicated mitochondria as well as in phosphatidylcholine suspension. The ability of vanadate (V(V] to induce lipid peroxidation was much less pronounced compared to that of vanadyl. The peroxidative action of vanadate on phosphatidylcholine much increased in the presence of NADH and ascorbate. Preincubation of vanadate with glucose had the same effect. Vanadyl-induced lipid peroxidation was not essentially influenced by SOD, catalase and ethanol but was completely inhibited by butylated hydroxytoluene. All these effects of vanadyl and vanadate are thought to participate in the insulin-like and other biological actions of vanadium.

A survey of chemicals inducing lipid peroxidation in biological systems

A great number of drugs and chemicals are reviewed which have been shown to stimulate lipid peroxidation in any biological system. The underlying mechanisms, as far as known, are also dealt with. Lipid peroxidation induced by iron ions, organic hydroperoxides, halogenated hydrocarbons, redox cycling drugs, glutathione depleting chemicals, ethanol, heavy metals, ozone, nitrogen dioxide and a number of miscellaneous compounds, e.g. hydrazines, pesticides, antibiotics, are mentioned. It is shown that lipid peroxidation is stimulated by many of these compounds. However, quantitative estimates cannot be given yet and it is still impossible to judge the biological relevance of chemical-induced lipid peroxidation.

Peroxidative membrane damage in human erythrocytes induced by a concerted action of iodoacetate, vanadate and ferricyanide

Human erythrocytes incubated without substrate in the presence of iodoacetate (0.2 mM), vanadate (0.5 mM) and ferricyanide (5 mM) form aqueous membrane leaks of equivalent radii of 0.5-0.8 nm leading to complete colloid-osmotic lysis within 180 min. All three components are indispensable for the effect. Inosine but not glucose markedly enhances the rate of hemolysis. These effects are due to oxidative damage, as indicated by concomitant destruction of polyunsaturated fatty acids and suppression of both effects by radical scavengers. Hemoglobin is not oxidized under these conditions. GSH and membrane SH levels remain almost normal, and no crosslinking or irreversible aggregation of membrane proteins is observed. In the absence of O2 no membrane damage can be observed. It is proposed that radical formation originates from reduction of O2 by NADPH, analogous to processes described in microsomal membranes. NADH seems not to be involved, since leak formation occurs in spite of the blockage of NADH formation by iodoacetate. Vanadate and ferricyanide are probably required to amplify the peroxidative reaction sufficiently to overcome the cellular antioxidative capacity.

Vanadate-dependent oxidation of pyridine nucleotides in rat liver microsomal membranes

An enzymatic Na3VO4-dependent system for the oxidation of reduced pyridine nucleotides in purified rat liver microsomes was characterized. The system has a pH optimum of 6.5, and appears to be specific for vanadate, since activity in the presence of a related transition metal, molybdate, was not detected. Vanadate-dependent oxidation occurred with a concomitant consumption of O2 and, contrary to previous reports, preferred NADPH over NADH. At pH 6.5, the NADPH/NADH oxidase activity ratio was greater than 2:1. Sodium vanadate-dependent oxidation of NADH was inhibited by rotenone, antimycin A, NaN3, and NaCN. Conversely, Na3VO4-dependent NADPH oxidation was slightly affected by rotenone, but was insensitive to antimycin A, NaN3, NaCN, or quinacrine. Vanadate-dependent oxidation of either pyridine nucleotide was inhibited by the addition of either superoxide dismutase or catalase, indicating that both superoxide and hydrogen peroxide may be intermediates in the process. Linear sucrose gradient purification of the microsomes showed that the vanadate-dependent system for NADPH oxidation resides primarily in the endoplasmic reticulum. These studies indicate the existence of separate and distinct enzymatic systems for vanadate-stimulated oxidation of NADPH and NADH in mammalian microsomal membranes, and argue against an exclusive role of endogenous superoxide in the process.

Trypsin- and chymotrypsin-induced chemiluminescence by isolated rat glomeruli

We examined whether the generation of reactive oxygen metabolites (as quantified by measuring luminol-amplified chemiluminescence) by isolated rat glomeruli could be triggered enzymatically. No response was observed with thrombin (1 or 10 U/ml), collagenase (100, 200, or 400 U/ml), or plasmin (0.1 or 1 U/ml). In contrast, chymotrypsin and trypsin caused a dose-dependent (10-200 micrograms/ml) increase in chemiluminescence from glomeruli. The peak response with chymotrypsin (100 micrograms/ml) and trypsin (50 micrograms/ml) was as follows: resting, 16 +/- 2 X 10(3) cpm/mg protein, n = 17; chymotrypsin, 233 +/- 58 X 10(3) cpm/mg protein, n = 17; and trypsin, 221 +/- 38 X 10(3) cpm/mg protein, n = 10. Tubules had only a minor response. Soybean trypsin inhibitor and aprotinin caused marked inhibition, indicating the dependency of the chemiluminescence response on the protease enzyme activity. The chemiluminescence response was by glomeruli rather than by "contaminating" leukocytes, since a similar marked response (n = 6) was observed in glomeruli isolated from cyclophosphamide-treated leukopenic (leukocyte less than 1,000/mm3) rats. Superoxide dismutase, a scavenger of superoxide, and free-radical scavengers benzoate and tryptophan inhibited the glomerular chemiluminescence response to trypsin and chymotrypsin. Neutral proteases from infiltrating leukocytes and/or renal tissue have been shown to be released in glomerular diseases; our results, which show the generation of chemiluminescence in response to neutral proteases, suggest a potential mechanism for the production of reactive oxygen metabolites in glomerular diseases.

The oxidation of NADH by tetravalent vanadium

Vanadyl (V(IV)) salts autoxidize in neutral aqueous solution yielding O2- plus vanadate (V(V)) and these, in turn, cause the oxidation of NADH, by a free radical chain reaction. This oxidation of NADH was inhibited by superoxide dismutase, but not by a scavenger of HO.. When H2O2 was present V(IV) caused rapid oxidation of NADH by a process which was unaffected by superoxide dismutase but was inhibited by a scavenger of HO.. This appeared to be dependent upon reduction of H2O2 to OH- plus HO., by V(IV)), followed by oxidation of NADH by HO.. Since there are reductants, within cells, capable of reducing V(V) to V(IV), these reactions are likely to contribute to the toxicity of vanadate.