Effects of peroxidase substrates on the Amplex red/peroxidase assay: antioxidant properties of anthracyclines

Oxidation of Amplex red (AR) by H(2)O(2) in the presence of horseradish peroxidase (HRP) gives rise to an intensely colored product, resorufin. This reaction has been frequently employed for measurements of low concentrations of H(2)O(2) in biological samples. In the current study, we show that alternative peroxidase substrates, such as p-hydroquinone, acetaminophen, anticancer mitoxantrone, and ametantrone, inhibit AR oxidation by consuming H(2)O(2) in a competitive process. In contrast, the anthracycline agents doxorubicin, daunorubicin, and 5-iminodaunorubicin are markedly less efficient as competitors in these reactions, as is salicylic acid. When [H(2)O(2)]>[AR], the generated resorufin was oxidized by HRP and H(2)O(2). In the presence of anthracyclines, this process was inhibited and occurred with a lag time, the duration of which depended on the concentration of anthracycline. We propose that the mechanism of this inhibition is due to the antioxidant activity of anthracyclines involving the reduction of the resorufin-derived phenoxyl radical by the drugs' hydroquinone moiety back to resorufin. In addition to HRP, lactoperoxidase, myeloperoxidase, and HL-60 cells, naturally rich in myeloperoxidase, also supported these reactions. Results of this study suggest that extra caution is needed when using AR to measure cellular H(2)O(2) in the presence of alternative peroxidase substrates. They also demonstrate that the anticancer anthracyclines may function as antioxidants.     

Direct electron spin resonance detection of free radical intermediates during the peroxidase catalyzed oxidation of phenacetin metabolites

The oxidation of the phenacetin metabolites p-phenetidine and acetaminophen by peroxidases was investigated. Free radical intermediates from both metabolites were detected using fast-flow ESR spectroscopy. Oxidation of acetaminophen with either lactoperoxidase and hydrogen peroxide or horseradish peroxidase and hydrogen peroxide resulted in the formation of the N-acetyl-4-aminophenoxyl free radical. Totally resolved spectra were obtained and completely analyzed. The radical concentration was dependent on the square root of the enzyme concentration, indicating second-order decay of the radical, as is consistent with its dimerization or disproportionation. The horseradish peroxidase/hydrogen peroxide-catalyzed oxidation of p-phenetidine (4-ethoxyaniline) at pH 7.5-8.5 resulted in the one-electron oxidation products, the 4-ethoxyaniline cation free radical. The ESR spectra were well resolved and could be unambiguously assigned. Again, the enzyme dependence of the radical concentration indicated a second-order decay. The ESR spectrum of the conjugate base of the 4-ethoxyaniline cation radical, the neutral 4-ethoxyphenazyl free radical, was obtained at pH 11-12 by the oxidation of p-phenetidine with potassium permanganate.     

Peroxidase- and nitrite-dependent metabolism of the anthracycline anticancer agents daunorubicin and doxorubicin.

Oxidation of the anticancer anthracyclines doxorubicin (DXR) and daunorubicin (DNR) by lactoperoxidase(LPO)/H(2)O(2) and horseradish peroxidase(HRP)/H(2)O(2) systems in the presence and absence of nitrite (NO(2)(-)) has been investigated using spectrophotometric and EPR techniques. We report that LPO/H(2)O(2)/NO(2)(-) causes rapid and irreversible loss of anthracyclines' absorption bands, suggesting oxidative degradation of their chromophores. Both the initial rate and the extent of oxidation are dependent on both NO(2)(-) concentration and pH. The initial rate decreases when the pH is changed from 7 to 5, and the reaction virtually stops at pH 5. Oxidation of a model hydroquinone compound, 2,5-di-tert-butylhydroquinone, by LPO/H(2)O(2) is also dependent on NO(2)(-); however, in contrast to DNR and DXR, this oxidation is most efficient at pH 5, indicating that LPO/H(2)O(2)/NO(2)(-) is capable of efficiently oxidizing simple hydroquinones even in the neutral form. Oxidation of anthracyclines by HRP/H(2)O(2)/NO(2)(-) is substantially less efficient relative to that by LPO/H(2)O(2)/NO(2)(-) at either pH 5 or pH 7, most likely due to the lower rate of NO(2)(-) metabolism by HRP/H(2)O(2). EPR measurements show that interaction of anthracyclines and 2,5-di-tert-butylhydroquinone with LPO/H(2)O(2)/NO(2)(-) generates the corresponding semiquinone radicals presumably via one-electron oxidation of their hydroquinone moieties. The possible role of the (*)NO(2) radical, a putative LPO metabolite of NO(2)(-), in oxidation of these compounds is discussed. Because in vivo the anthracyclines may co-localize with peroxidases, H(2)O(2), and NO(2)(-) in tissues, their oxidation via the proposed mechanism is likely. These observations reveal a novel, peroxidase- and nitrite-dependent mechanism for the oxidative transformation of the anticancer anthracyclines, which may be pertinent to their biological activities in vivo.     

Spin trapping of the azidyl radical in azide/catalase/H2O2 and various azide/peroxidase/H2O2 peroxidizing systems

The azidyl radical is formed during the oxidation of sodium azide by the catalase/hydrogen peroxide system, as detected by the ESR spin-trapping technique. The oxidation of azide by horseradish peroxidase, chloroperoxidase, lactoperoxidase, and myeloperoxidase also forms azidyl radical. It is suggested that the evolution of nitrogen gas and nitrogen oxides reported in the azide/catalase/hydrogen peroxide system results from reactions of the azidyl radical. The azide/horseradish peroxidase/hydrogen peroxide system consumes oxygen, and this oxygen uptake is inhibited by the spin trap 5,5-dimethyl-1-pyrroline-N-oxide, presumably due to the competition with oxygen for the azidyl radical. Although azide is used routinely as an inhibitor of myeloperoxidase and catalase, some consideration should be given to the biochemical consequences of the formation of the highly reactive azidyl radical by the peroxidase activity of these enzymes.     

Photosensitization by selected anticancer agents

Novel anticancer anthrapyrazoles and anthracenediones are available as alternatives to the cardiotoxic clinical agents, doxorubicin and daunorubicin. Certain representatives of these new classes of compounds possess photosensitizing properties. The structural features influencing the photophysical parameters of these agents are discussed. Photosensitizing reactions involving singlet oxygen production, free radical formation, decomposition of hydrogen peroxide and organic hydroperoxides, oxidation of certain biochemical electron donors, DNA damage and killing of human leukemic cells in vitro in the presence of photoactive anthrapyrazoles, anthracenediones and anthracyclines are described.     

Doxorubicin inhibits oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by a lactoperoxidase/H(2)O(2) system by reacting with ABTS-derived radical

The effect of doxorubicin on oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by lactoperoxidase and hydrogen peroxide has been investigated. It was found that: (1) oxidation of ABTS to its radical cation (ABTS*(+)) is inhibited by doxorubicin as evidenced by its induction of a lag period, duration of which depends on doxorubicin concentration; (2) the inhibition is due to doxorubicin hydroquinone reducing the ABTS*(+) radical (stoichiometry 1: 1.8); (3) concomitant with the ABTS*(+) reduction is oxidation of doxorubicin; only when the doxorubicin concentration decreases to a near zero level, net oxidation of ABTS could be detected; (4) oxidation of doxorubicin leads to its degradation to 3-methoxysalicylic acid and 3-methoxyphthalic acid; (5) the efficacy of doxorubicin to quench ABTS*(+) is similar to the efficacy of p-hydroquinone, glutathione and Trolox C. These observations support the assertion that under certain conditions doxorubicin can function as an antioxidant. They also suggest that interaction of doxorubicin with oxidants may lead to its oxidative degradation.     

Formation of aminoxyl radicals in the reaction between penicillins and hydrogen peroxide.

Aminoxyl radicals are formed in high yield in the reaction between penicillins and hydrogen peroxide in water solutions in the pH range between 7 and 8. The nine-line EPR spectrum, 3 x 3 (1:2:1), indicated an interaction of the unpaired electron with one 14N nucleus (aN = 1.44 mT) and two equivalent hydrogen nuclei (aH = 2.00 mT). The reaction involves an oxidative cleavage of the beta-lactam ring of the penicillins with the formation of a cyclic aminoxyl radical, in which the thiazolidine ring carries the nitroxide group (= N-O.). It is suggested that the reaction with the formation of aminoxyl radicals can also take place in vivo in the deactivation of penicillins by metabolically formed hydrogen peroxide.     

Bis(alkylamino)anthracenedione antineoplastic agent metabolic activation by NADPH-cytochrome P-450 reductase and NADH dehydrogenase: diminished activity relative to anthracyclines

Stimulation of the rates of NAD(P)H oxidation, superoxide generation, and hydrogen peroxide formation by three anthracenedione antineoplastic agents in the presence of NADPH-cytochrome P-450 reductase, NADH dehydrogenase, or rabbit hepatic microsomes was studied and the results compared with those obtained for the anthracyclines Adriamycin and daunorubicin. In all cases the anthracenediones, including mitoxantrone and ametantrone, were significantly (5- to 20-fold) less effective than the anthracyclines in stimulating NAD(P)H oxidation, superoxide formation, or hydrogen peroxide production. Of the three anthracenediones studied, the ring-monohydroxylated compound showed the greatest activity followed by the ring-dihydroxylated derivative (mitoxantrone). In contrast, the non-ring-hydroxylated anthracenedione (ametantrone) was a relatively ineffective electron acceptor and inhibited the reduction of more effective acceptors such as Adriamycin. Michaelis-Menten kinetic constants were determined by analysis of the rates of NADPH oxidation. NADP+ and 2'-AMP inhibited the reduction of the ring-hydroxylated anthracenediones and anthracyclines, demonstrating the enzymatic nature of the reaction. The non-ring-hydroxylated anthracenedione inhibited the reduction of Adriamycin by both P-450 reductase and NADH dehydrogenase with 50% inhibition achieved at approximately 300 microM. Thus, there appears to exist a structural relationship between anthracenedione ring hydroxylation and metabolic activation. These results also suggest that the relative inability of the anthracenediones to function as artificial electron acceptors in comparison to the anthracyclines may be correlated with diminished anthracenedione cardiotoxicity.     

Effect of acetaminophen on the myeloperoxidase-hydrogen peroxide-nitrite mediated oxidation of LDL

Myeloperoxidase, in the presence of hydrogen peroxide and nitrite, promotes the lipid peroxidation of low density lipoprotein (LDL); the modified lipoprotein is then capable of being readily endocytosed by macrophages. Since acetaminophen has been shown to inhibit the leukocyte myeloperoxidase antimicrobial system and is, under certain experimental conditions, an antioxidant, the effect of acetaminophen on the myeloperoxidase-hydrogen peroxide-nitrite mediated oxidation of LDL was examined. The content of LDL lipid hydroperoxides after incubation with 50 nM myeloperoxidase, 100 microM nitrite and a hydrogen peroxide generating system for 6 h was reduced by approx. 80% in the presence of 25-250 microM acetaminophen. The production of thiobarbituric acid-reactive substances was also inhibited by acetaminophen to a similar extent. Acetylsalicylic acid (25-100 microM) did not inhibit LDL lipid peroxidation mediated by the myeloperoxidase enzyme system. LDL, treated with myeloperoxidase, hydrogen peroxide and nitrite for 14 h, was metabolized by macrophages to a much greater extent than native LDL. The presence of acetaminophen prevented the modification of LDL; the lipoprotein was metabolized by macrophages to the same extent as was native LDL. These results demonstrate that acetaminophen is a potent inhibitor of the myeloperoxidase-hydrogen peroxide-nitrite mediated modification of LDL.     

Lactoperoxidase-catalyzed oxidation of thiocyanate by hydrogen peroxide: 15N nuclear magnetic resonance and optical spectral studies

To establish the agent(s) responsible for the activity of the lactoperoxidase (LPO)/SCN-/H2O2 system, the oxidation of thiocyanate with hydrogen peroxide, catalyzed by lactoperoxidase, has been studied by 15N NMR and optical spectroscopy at different concentrations of thiocyanate and hydrogen peroxide and at different pHs. The formation of hypothiocyanite ion (OSCN-) as one of the oxidation products correlated well with the activity of the LPO/SCN-/H2O2 system and was maximum when the concentrations of the H2O2 and SCN- were nearly the same and the pH was less than 6.0. At [H2O2]/[SCN-] = 1, OSCN- decomposed very slowly back to thiocyanate. When the ratio [H2O2]/[SCN-] was above 2, formation of CN- was observed, which was confirmed by 15N NMR and also by changes in the optical spectrum of LPO. The oxidation of thiocyanate by H2O2 in the presence of LPO does not take place at pH greater than 8.0. Since thiocyanate does not bind to LPO above this pH, the binding of thiocyanate to LPO is considered to be prerequisite for the oxidation of thiocyanate. Maximum inhibition of oxygen uptake by Streptococcus cremoris 972 bacteria was observed when hydrogen peroxide and thiocyanate were present in equimolar amounts and the pH was below 6.0.     

Redox cycling of anthracyclines by cardiac mitochondria. I. Anthracycline radical formation by NADH dehydrogenase

In the present study we have used beef heart submitochondrial preparations (BH-SMP) to demonstrate that a component of mitochondrial Complex I, probably the NADH dehydrogenase flavin, is the mitochondrial site of anthracycline reduction. During forward electron transport, the anthracyclines doxorubicin (Adriamycin) and daunorubicin acted as one-electron acceptors for BH-SMP (i.e. were reduced to semiquinone radical species) only when NADH was used as substrate; succinate and ascorbate were without effect. Inhibitor experiments (rotenone, amytal, piericidin A) indicated that the anthracycline reduction site lies on the substrate side of ubiquinone. Doxorubicin and daunorubicin semiquinone radicals were readily detected by ESR spectroscopy. Doxorubicin and daunorubicin semiquinone radicals (g congruent to 2.004, signal width congruent to 4.5 G) reacted avidly with molecular oxygen, presumably to produce O2-, to complete the redox cycle. The identification of Complex I as the site of anthracycline reduction was confirmed by studies of ATP-energized reverse electron transport using succinate or ascorbate as substrates, in the presence of antimycin A or KCN respiratory blocks. Doxorubicin and daunorubicin inhibited the reduction of NAD+ to NADH during reverse electron transport. Furthermore, during reverse electron transport in the absence of added NAD+, doxorubicin and daunorubicin addition caused oxygen consumption due to reduction of molecular oxygen (to O2-) by the anthracycline semiquinone radicals. With succinate as electron source both thenoyltrifluoroacetone (an inhibitor of Complex II) and rotenone blocked oxygen consumption, but with ascorbate as electron source only rotenone was an effective inhibitor. NADH oxidation by doxorubicin during BH-SMP forward electron transport had a KM of 99 microM and a Vmax of 30 nmol X min-1 X mg-1 (at pH 7.4 and 23 degrees C); values for daunorubicin were 71 microM and 37 nmol X min-1 X mg-1. Oxygen consumption at pH 7.2 and 37 degrees C exhibited KM values of 65 microM for doxorubicin and 47 microM for daunorubicin, and Vmax values of 116 nmol X min-1 X mg-1 for doxorubicin and 114 nmol X min-1 X mg-1 for daunorubicin. In marked contrast with these results, 5-iminodaunodrubicin (a new anthracycline with diminished cardiotoxic potential) exhibited little or no tendency to undergo reduction, or to redox cycle with BH-SMP. Redox cycling of anthracyclines by mitochondrial NADH dehydrogenase is shown, in the accompanying paper (Doroshow, J. H., and Davies, K. J. A. (1986) J. Biol. Chem. 261, 3068-3074), to generate O2-, H2O2, and OH which may underlie the cardiotoxicity of these antitumor agents.     

Some properties of human eosinophil peroxidase, a comparison with other peroxidases

Eosinophil peroxidase (donor:hydrogen peroxide oxidoreductase, EC 1.11.1.7) was isolated from outdated human white blood cells. The purified enzyme has a molecular weight of 71000 +/- 1000. The enzyme is composed of two subunits, of Mr 58000 and 14000, in a 1:1 stoichiometry. Amino-acid analyses showed that eosinophil peroxidase has a high content of the amino acids arginine, leucine and aspartic acid. The millimolar absorbance coefficient of the Soret band at 412 nm of eosinophil peroxidase was determined. Three independent methods yield a value for epsilon 412nm of 110 +/- 4 mm-1 X cm-1. Purified eosinophil peroxidase showed a homogeneous high-spin EPR signal with rhombic symmetry (gx = 6.50; gy = 5.40; gz = 1.982) for the haem group. EPR spectroscopy of low-spin cyanide and azide derivatives of eosinophil peroxidase, lactoperoxidase, myeloperoxidase and catalase revealed that the haem-ligand structure of eosinophil peroxidase is closely related to lactoperoxidase, whereas that of myeloperoxidase shows great resemblance to catalase.     

Free Radical Formation from the Antineoplastic Agent VP 16-213

Free radical formation from VP 16-213 was studied by ESR spectroscopy. Incubation of VP 16-213 with the one-electron oxidators persulphate-ferrous, myeloperoxidase (MPO)/hydrogen peroxide and horseradish peroxidase (HRP)/hydrogen peroxide readily led to the formation of a free radical. The ESR spectra obtained in the last two cases, were in perfect accord with that of a product obtained by electrochemical oxidation of VP 16-213 at +550 mV. The half-life of the free radical in 1 mM Tris (pH 7.4), 0.1 MNaClat 20°C, was 257 ± 4 s. The signal recorded on incubation with HRP/H₂O₂ or MPO/H₂O₂ did not disappear on addition of 0.3 - 1.2 mg/ml microsomal protein. From incubations with rat liver microsomes in the presence of NADPH, no ESR signals were obtained.     

Redox cycling of anthracyclines by cardiac mitochondria. II. Formation of superoxide anion, hydrogen peroxide, and hydroxyl radical

In the accompanying paper (Davies, K. J. A., and Doroshow, J. A. (1986) J. Biol. Chem. 261, 3060-3067), we have demonstrated that anthracycline antibiotics are reduced to the semiquinone form at Complex I of the mitochondrial electron transport chain. In the experiments presented in this study we examined the effects of doxorubicin (Adriamycin), daunorubicin, and related quinonoid anticancer agents on superoxide, hydrogen peroxide, and hydroxyl radical production by preparations of beef heart submitochondrial particles. Superoxide anion formation was stimulated from (mean +/- S.E.) 1.6 +/- 0.2 to 69.6 +/- 2.7 or 32.1 +/- 1.5 nmol X min-1 X mg-1 by the addition of 90 microM doxorubicin or daunorubicin, respectively. However, the anthracycline 5-iminodaunorubicin, in which an imine group has been substituted in the C ring quinone moiety, did not increase superoxide production over control levels. In the presence of rotenone, initial rates of oxygen consumption and superoxide formation were identical under comparable experimental conditions. Furthermore, H2O2 production increased from undetectable control levels to 2.2 +/- 0.3 nmol X min-1 X mg-1 after treatment of submitochondrial particles with doxorubicin (200 microM). The hydroxyl radical, or a related chemical oxidant, was also detected after the addition of an anthracycline to this system by both ESR spectroscopy using the spin trap 5,5-dimethylpyrroline-N-oxide and by gas chromatographic quantitation of CH4 produced from dimethyl sulfoxide. Hydroxyl radical production, which was iron-dependent in this system, occurred in a nonlinear fashion with an initial lag phase due to a requirement for H2O2 accumulation. We also found that two quinonoid anti-cancer agents which produce less cardiotoxicity than the anthracyclines, mitomycin C, and mitoxantrone, stimulated significantly less or no hydroxyl radical production by submitochondrial particles. These experiments suggest that injury to cardiac mitochondria which is produced by anthracycline antibiotics may result from the generation of the hydroxyl radical during anthracycline metabolism by NADH dehydrogenase.     

Transient and steady-state kinetics of the oxidation of substituted benzoic acid hydrazides by myeloperoxidase

Myeloperoxidase is the most abundant protein in neutrophils and catalyzes the production of hypochlorous acid. This potent oxidant plays a central role in microbial killing and inflammatory tissue damage. 4-Aminobenzoic acid hydrazide (ABAH) is a mechanism-based inhibitor of myeloperoxidase that is oxidized to radical intermediates that cause enzyme inactivation. We have investigated the mechanism by which benzoic acid hydrazides (BAH) are oxidized by myeloperoxidase, and we have determined the features that enable them to inactivate the enzyme. BAHs readily reduced compound I of myeloperoxidase. The rate constants for these reactions ranged from 1 to 3 x 10(6) M-1 s-1 (15 degrees C, pH 7.0) and were relatively insensitive to the substituents on the aromatic ring. Rate constants for reduction of compound II varied between 6.5 x 10(5) M-1 s-1 for ABAH and 1.3 x 10(3) M-1 s-1 for 4-nitrobenzoic acid hydrazide (15 degrees C, pH 7.0). Reduction of both compound I and compound II by BAHs adhered to the Hammett rule, and there were significant correlations with Brown-Okamoto substituent constants. This indicates that the rates of these reactions were simply determined by the ease of oxidation of the substrates and that the incipient free radical carried a positive charge. ABAH was oxidized by myeloperoxidase without added hydrogen peroxide because it underwent auto-oxidation. Although BAHs generally reacted rapidly with compound II, they should be poor peroxidase substrates because the free radicals formed during peroxidation converted myeloperoxidase to compound III. We found that the reduction of ferric myeloperoxidase by BAH radicals was strongly influenced by Hansch's hydrophobicity constants. BAHs containing more hydrophilic substituents were more effective at converting the enzyme to compound III. This implies that BAH radicals must hydrogen bond to residues in the distal heme pocket before they can reduce the ferric enzyme. Inactivation of myeloperoxidase by BAHs was related to how readily they were oxidized, but there was no correlation with their rate constants for reduction of compounds I or II. We propose that BAHs destroy the heme prosthetic groups of the enzyme by reducing a ferrous myeloperoxidase-hydrogen peroxide complex.     

Oxidation of cyanide to the cyanyl radical by peroxidase/H2O2 systems as determined by spin trapping

The cyanyl radical was formed during the oxidation of potassium or sodium cyanide by horseradish peroxidase, lactoperoxidase, chloroperoxidase, NADH peroxidase, or methemoglobin in the presence of hydrogen peroxide. The spin adducts of the cyanyl radical with 5,5-dimethyl-1-pyrroline-N-oxide and N-tert-butyl-alpha-phenylnitrone were quite stable at neutral pH. The identity of these spin adducts could be demonstrated using 13C-labeled cyanide and by comparison with the spin adducts of the formamide radical, a hydrolysis product of the cyanyl radical adduct. The enzymatic conversion of cyanide to cyanyl radical by peroxidases should be considered in addition to its well-known role as a metal ligand. Furthermore, since cyanide is used routinely as an inhibitor of peroxidases, some consideration should be given to the biochemical consequences of this formation of the cyanyl radical by the catalytic activity of these enzymes.     

Albumin oxidation to diverse radicals by the peroxidase activity of Cu,Zn-superoxide dismutase in the presence of bicarbonate or nitrite: diffusible radicals produce cysteinyl and solvent-exposed and -unexposed tyrosyl radicals

The peroxidase activity of Cu,Zn-superoxide dismutase (Cu,Zn-SOD) has been extensively studied in recent years due to its potential relationship to familial amyotrophic lateral sclerosis. The mechanism by which Cu,Zn-SOD/hydrogen peroxide/bicarbonate is able to oxidize substrates has been proposed to be dependent on an oxidant whose nature, diffusible carbonate radical anion or enzyme-bound peroxycarbonate, remains debatable. One possibility to distinguish these species is to examine whether protein targets are oxidized to protein radicals. Here, we used EPR methodologies to study bovine serum albumin (BSA) oxidation by Cu,Zn-SOD/hydrogen peroxide in the absence and presence of bicarbonate or nitrite. The results showed that BSA oxidation in the presence of bicarbonate or nitrite at pH 7.4 produced mainly solvent-exposed and -unexposed BSA-tyrosyl radicals, respectively. Production of the latter was shown to be preceded by BSA-cysteinyl radical formation. The results also showed that hydrogen peroxide/bicarbonate extensively oxidized BSA-cysteine to the corresponding sulfenic acid even in the absence of Cu,Zn-SOD. Thus, our studies support the idea that peroxycarbonate acts as a two-electron oxidant and may be an important biological mediator. Overall, the results prove the diffusible and radical nature of the oxidants produced during the peroxidase activity of Cu,Zn-SOD in the presence of bicarbonate or nitrite.     

Oxidation of biological electron donors and antioxidants by a reactive lactoperoxidase metabolite from nitrite (NO2-): an EPR and spin trapping study

We report that a lactoperoxidase (LPO) metabolite derived from nitrite (NO2-) catalyses one-electron oxidation of biological electron donors and antioxidants such as NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C. The radical products of the reaction have been detected and identified using either direct EPR or EPR combined with spin trapping. While LPO/H2O2 alone generated only minute amounts of radicals from these compounds, the yield of radicals increased sharply when nitrite was also present. In aerated buffer (pH 7) the nitrite-dependent oxidation of NAD(P)H by LPO/H2O2 produced superoxide radical, O2*-, which was detected as a DMPO/*O2H adduct. We propose that in the LPO/H2O2/NO2-/biological electron donor systems the nitrite functions as a catalyst because of its preferential oxidation by LPO to a strongly oxidizing metabolite, most likely a nitrogen dioxide radical *NO2, which then reacts with the biological substrates more efficiently than does LPO/H2O2 alone. Because both nitrite and peroxidase enzymes are ubiquitous our observations point at a possible mechanism through which nitrite might exert its biological and cytotoxic action in vivo, and identify some of the physiological targets which might be affected by the peroxidase/H2O2/nitrite systems.     

Free Radical Formation from the Antineoplastic Agent VP 16-213

Free radical formation from VP 16-213 was studied by ESR spectroscopy. Incubation of VP 16-213 with the one-electron oxidators persulphate-ferrous, myeloperoxidase (MPO)/hydrogen peroxide and horseradish peroxidase (HRP)/hydrogen peroxide readily led to the formation of a free radical. The ESR spectra obtained in the last two cases, were in perfect accord with that of a product obtained by electrochemical oxidation of VP 16-213 at +550 mV. The half-life of the free radical in 1 mM Tris (pH 7.4), 0.1 MNaClat 20°C, was 257 ± 4 s. The signal recorded on incubation with HRP/H₂O₂ or MPO/H₂O₂ did not disappear on addition of 0.3 - 1.2 mg/ml microsomal protein. From incubations with rat liver microsomes in the presence of NADPH, no ESR signals were obtained.     

Acetaminophen and low-flow myocardial ischemia: efficacy and antioxidant mechanisms

In the current study, the cardioprotective efficacy of 0.35 mmol/l acetaminophen administered 10 min after the onset of a 20-min period of global, low-flow myocardial ischemia was investigated. Matched control hearts were administered an equal volume of Krebs-Henseleit physiological buffer solution (vehicle). In separate groups of hearts, the concentration-dependent, negative inotropic properties of hydrogen peroxide and the ability of acetaminophen to attenuate these actions, as well as the effects of acetaminophen on ischemia-reperfusion-mediated protein oxidation, were studied. Acetaminophen-treated hearts regained a significantly greater fraction of baseline, preischemia control function during reperfusion than vehicle-treated hearts. For example, contractility [rate of maximal developed pressure in the left ventricle (+/-dP/dt(max))] after 10 min of reperfusion was 109 +/- 24 and 42 +/- 9 mmHg/s (P < 0.05), respectively, in the two groups. The corresponding pressure-rate products were 1,840 +/- 434 vs. 588 +/- 169 mmHg*beats*min(-1) (P < 0.05). Acetaminophen attenuated peroxynitrite-mediated chemiluminescence in the early minutes of reperfusion (e.g., at 6 min, corresponding values for peak light production were approximately 8 x 10(6) counts/min for vehicle vs. <4 x 10(6) counts/min for acetaminophen, P < 0.05) and the negative inotropic effects of exogenously administered hydrogen peroxide (e.g., at 0.4 mmol/l hydrogen peroxide, pressure-rate products were approximately 1.0 x 10(4) and 3.8 x 10(3) mmHg*beats*min(-1) in acetaminophen- and vehicle-treated hearts, respectively, P < 0.05). Ischemia-mediated protein oxidation was reduced by acetaminophen. The ability of acetaminophen to attenuate the damaging effects of peroxynitrite and hydrogen peroxide and to limit protein oxidation suggest antioxidant mechanisms are responsible for its cardioprotective properties during postischemia-reperfusion.