Bipyridylium quaternary salts and related compounds. V. Pulse radiolysis studies of the reaction of paraquat radical with oxygen. Implications for the mode of action of bipyridyl herbicides

Rapid quantitative analysis of paraquat by electron spin resonance spectroscopy

A very simple and sensitive method for quantitative measurement of paraquat in plasma, urine and some drinks is described. The paraquat radical formed by its reduction can be detected by electron spin resonance spectroscopy (ESR) at room temperature without clean-up or concentration process of samples. The sensitivity limit is 0.1 microgram paraquat/ml and the required amount of sample is 100 microliter. The time needed for the measurement is within 10 min.

Isolation and characterization of methyl viologen-sensitive mutants of Escherichia coli K-12

Escherichia coli mutants sensitive to methyl viologen (MV), an active oxygen propagator, were isolated. Among them, the new genes mvrA and mvrB were mapped at 7 and 28 min on the E. coli linkage map, respectively. MV toxicity was exerted only in the presence of oxygen and was suppressed by the radical scavenger uric acid but not by the hydroxyl radical scavenger mannitol. The mvr mutants were sensitive only to MV and had a normal repair capacity for the MV-damaged DNA. From these results, these mutants were assumed to be related to the elimination of MV-specific toxic species. Gene mvrA was cloned into vector pBR322 and its sequence was determined. The mvrA gene, which was predicted to range in size from 600 to 900 base pairs (bp) by transposon Tn1000 insertion analysis, was identified to be 807 bp, with an approximately 60-bp promoter sequence carrying consensus sequences for the -35 region, the -10 region, and a ribosome-binding site. The MvrA protein deduced from the DNA sequence was 29.7 kilodaltons, which was in good agreement with the 29 kilodaltons of the MvrA protein identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after a maxicell labeling experiment.

Free radicals and oxygen toxicity

Most organisms are constantly exposed to molecular oxygen, and this has become a requirement of life for many of them. Oxygen is not totally innocuous, however, and it has long been known to be toxic to many organisms, including humans. The deleterious effects of oxygen are thought to result from its metabolic reduction to highly reactive and toxic species, including superoxide anion radical and hydroxyl radical. Peroxidation of lipids is a major consequence of exposure to these species and the cell possesses various enzymes, including superoxide dismutase and catalase, as well as cellular antioxidants which are able to scavenge oxygen free radicals and repair peroxidized lipids. These aspects of oxygen toxicity are reviewed, as well as the involvement of oxygen free radicals in the toxicity of the herbicide paraquat.

Lipid peroxidation and tissue injury by ferric citrate in paraquat-intoxicated mice

To determine whether iron toxicity is caused by iron-catalyzed radical production, the in vivo effect of ferric citrate was studied in paraquat-intoxicated mice. Intraperitoneally injected Fe3+-citrate complex was distributed mainly in the liver and kidney, and promoted lipid peroxidation, as measured by expiratory ethane in both normal and paraquat-intoxicated mice. Plasma glutamic-oxaloacetic transaminase (L-aspartate: 2-oxoglutarate aminotransferase, EC 2.6.1.1) activity increased significantly only in paraquat and Fe3+-citrate-injected mice (PFe group). The rate of ethane production increased prior to the elevation of plasma glutamic-oxaloacetic transaminase levels, and was greater in the PFe group than in the mice, that were injected Fe3+-citrate alone. Pretreatment of animals with desferrioxamine mesylate inhibited both ethane production and elevation of plasma glutamic-oxaloacetic transaminase levels in the PFe group. Administration of 100% oxygen or glucose, which is expected to increase cellular NADPH, to the PFe group further elevated the plasma glutamic-oxaloacetic transaminase level, but had little effect on ethane production, indicating that tissue injury occurs independently of lipid peroxidation. These results suggest that iron toxicity is due to radical production and that, although iron stimulated lipid peroxidation, it might not be the only cause of tissue injury.

The involvement of superoxide and iron ions in the NADPH-dependent lipid peroxidation in human placental mitochondria

Incubation of human term placental mitochondria with Fe2+ and a NADPH-generating system initiated high levels of lipid peroxidation, as measured by the production of malondialdehyde. Malondialdehyde formation was accompanied by a corresponding decrease of the unsaturated fatty acid content. This NADPH-dependent lipid peroxidation was strongly inhibited by superoxide dismutase and singlet oxygen scavengers, markedly stimulated by paraquat, but was not affected by hydroxyl radical scavengers. Catalase enhanced the production of malondialdehyde by placental mitochondria. The effects of catalase and hydroxyl radical scavengers suggest that the initiation of NADPH-dependent lipid peroxidation is not dependent upon the hydroxyl radical produced via an iron-catalyzed Fenton reaction. These studies provide evidence that hydrogen peroxide strongly inhibits NADPH-dependent mitochondrial lipid peroxidation. The inhibitory effect of superoxide dismutase and stimulatory effect of paraquat, which was abolished by the addition of superoxide dismutase, suggests that superoxide may promote NADPH-dependent lipid peroxidation in human placental mitochondria.

Selective inhibition of bipyridyl-stimulated NADPH oxidation by ascorbic acid

The effect of the bipyridyl herbicides, paraquat and diquat (0.01-1.0 mM), on NADPH oxidation was determined in vitro using rat lung microsomal preparations. Experiments were performed in the absence of mixed function oxidation (MFO) substrates, in the presence of substrates (ethylmorphine or benzphetamine), and also in the presence of ascorbic acid (0.1-10.0 mM). NADPH oxidation was stimulated by both herbicides in the absence or presence of either substrate in a concentration-dependent manner. When ascorbic acid was included in incubations along with either bipyridyl, the stimulated rate of NADPH oxidation decreased in the presence of benzphetamine but the stimulation was unaltered in the presence of ethylmorphine or in the absence of substrate. These studies indicate that ascorbic acid may offer some protection from bipyridyl-mediated NADPH oxidation in rat lung microsomal fractions, but that protection appears to be dependent upon the simultaneous presence of specific MFO substrates.

Model study on the bioreduction of paraquat, MPP+, and analogs. Evidence against a "redox cycling" mechanism in MPTP neurotoxicity

The ability of paraquat, MPP+, and analogs to be reduced by chemical reductants and by NADPH, as catalyzed by liver microsomes or purified NADPH cytochrome P-450 reductase, is reported. The analogs span a range of electrochemical potential, including values in-between that of paraquat and MPP+. Analogs with an Eo below -.55 V (vs. NHE) are not reduced by either the NADPH-microsomes or NADPH-reductase systems. The inability of MPP+ to be bio-reduced or to stimulate the production of superoxide during aerobic reduction is evidence against a redox-cycling (oxidant stress) role of MPP+ in MPTP neurotoxicity.

Contrasting fates of the paraquat monocation radical in Escherichia coli B and in Dunaliella salina

Anaerobic cultures of Escherichia coli exposed to paraquat (PQ2+) accumulated the corresponding monocation radical PQ+., both within the cells and in the suspending medium. The green alga, Dunaliella salina, which is susceptible to a light- and O2-dependent toxicity of PQ2+, was nevertheless unable to cause accumulation of PQ+. when illuminated anaerobically and could, moreover, discharge the ESR signal and the blue color of PQ+. accumulated by E. coli. Spin trapping allowed demonstration of the photoproduction of O2- within D. salina and of the augmentation of that O-2 production by PQ2+. D. salina appears to contain an electron sink and a heat-labile mechanism for transferring electrons from PQ+. to that sink. This mechanism was demonstrable anaerobically but did not prevent PQ+.-mediated O2- production under aerobic conditions.

Ultrastructural alteration in the rat liver produced by benzyl viologen

This report presents qualitative data of the effects of a free radical generator (benzyl viologen) on rat liver. Rats were administered benzyl viologen (0.825 mg per kg of body weight) or saline solution (controls) intraperitoneally, in single daily dosage for 15 days. For microscopic examination animals were killed after 3 or 15 days of treatment. In the control animals, liver ultrastructure remained normal throughout the experiment. But in rats with benzyl viologen it was possible to observe important morphological alterations essentially in mitochondria, frequently forming ring-like bodies. Endoplasmic reticulum was observed as concentric patterns which cover the mitochondria and, on occasion, the lipid droplets within the cell. Increased number of dense granular lysosomes were observed in the apical cytoplasm mainly close to the bile canaliculus. A large quantity of lipid inclusions and cellular degeneration of the liver of treated rats showed a marked dose-response relationship.

Mechanism of paraquat toxicity in lung and its relevance to treatment

The symptoms of paraquat poisoning depend largely on the amount of compound consumed, although in many cases the most characteristic feature of poisoning is lung damage, causing severe anoxia which leads to death. Studies in experimental animals have demonstrated that paraquat produces an acute damaging phase in the lung, followed by a reparative phase dominated by an extensive fibrosis. The latter is a major contributor to the lung lesion that causes anoxia. The specific toxicity in the lung can be explained in part by the selective accumulation of paraquat into this organ in comparison with other tissues. The accumulation is energy-dependent and probably specific to certain lung cells. It is now known that paraquat is accumulated into the lung by a recently described diamine transport process located in the alveolar epithelial cells and the Clara cells of the airways. When accumulated, paraquat undergoes a NADPH-dependent one-electron reduction to for its free radical which almost instantly reacts with molecular oxygen to reform the cation and concomitantly produce superoxide anion. This species of oxygen radical can contribute to the formation of more toxic species of radical which may directly damage vital cellular constituents. Paraquat has been shown to stimulate rapidly the pentose phosphate pathway and inhibit the synthesis of fatty acids in the lung in a dose-dependent manner. In addition there is a rapid increase in the pulmonary levels of mixed disulphides and the eventual reduction of NADPH levels in the lung. These results are consistent with the suggestion that paraquat causes a rapid and pronounced oxidation of NADPH which initiates compensatory biochemical responses in the lung.(ABSTRACT TRUNCATED AT 250 WORDS)

Thiyl and phenoxyl free radicals and NADH. Direct observation of one-electron oxidation

Absolute rate constants for the reaction of NADH with thiyl free radicals derived from various sulphur-containing compounds of biological significance were measured by using the technique of pulse radiolysis. These and related reactions with phenoxyl free radicals are believed to occur through one-electron-transfer processes. Further evidence comes from studies with deuterated NADH. The results support the possibility that, in biochemical systems, thiols may act as catalysts linking hydrogen-atom and electron-transfer reactions.

Mutagenic and cytotoxic effects of oxygen free radicals generated by methylviologen (paraquat) on Escherichia coli with different DNA-repair capacities

Investigations were carried out to examine the mutagenic and cytotoxic effects of oxygen free radicals on E. coli. E. coli B strains with different DNA-repair capacities were exposed to methyl viologen, commonly called paraquat (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV), which has been shown to act as an intracellular generator of superoxide radicals. The results obtained were as follows: The cytotoxicity of MV in E. coli was dioxygen-dependent and due to the extent of intracellular generation of superoxide radicals. Cells containing higher levels of superoxide dismutase were more resistant to the toxic effect of MV. The cytotoxicity of MV was greater in DNA repair-deficient E. coli, Bs-1(exrA uvrB), NG30(recA) and R15(polA), than in DNA-repair-proficient strains (B/r and H/r30) and Hs30 (uvrB). MV was found to be mutagenic to E. coli H/r30 and Hs30 under aerobic conditions. The mutation frequencies to streptomycin resistance and to arginine prototrophy increased with the dose of MV in both strains. However, E. coli NG30 was unmutable by MV. The mutation induction did not occur under anaerobic conditions. The expression of the umu operon in E. coli was induced by MV under aerobic conditions. From these results, it was concluded that superoxide radicals intracellularly generated by MV include DNA damage, which causes cytotoxicity and mutation induction in E. coli, and that DNA damage induced by oxygen radicals is repairable by at least recA, polA and exrA(lexA) gene-controlled mechanisms.

Effects of salts on the lethality of paraquat

Escherichia coli suffered 95 to 100% lethality when exposed to 1.0 mM paraquat for 30 min at 37 degrees C in aerobic nutrient broth medium but did not lose viability when the exposure was done in Vogel Bonner or tryptic soy yeast extract medium. Paraquat was, however, bacteriostatic in all of these media. Salts, added to the nutrient broth medium, protected against the lethality of paraquat, whereas sucrose did not. Salts of divalent cations were much more effective than salts of monovalent cations. Paraquat increases cyanide-resistant respiration by E. coli; salts added before, but not after, the paraquat diminished this effect. 2,4-Dinitrophenol similarly decreased the cyanide-resistant respiration when added before, but not after, the paraquat. The lethality imposed by paraquat correlated with the rate of cyanide-resistant respiration whether this respiration was modulated by varying salt concentration at a fixed concentration of paraquat or by varying paraquat concentration at a fixed concentration of salt. We conclude that salts or 2,4-dinitrophenol interferes with the active uptake of paraquat by E. coli and thus prevents its lethal effect. The salt concentrations found in a number of commonly used microbiological media are sufficient to exert this effect.

Effect of the free radical-generating herbicide paraquat on the expression of the superoxide dismutase (Sod) genes in maize

10-day-old maize leaves were treated with the oxygen free radical-generating herbicide paraquat for 12 h. Paraquat treatments (10(-5) M) resulted in a 40% increase in superoxide dismutase activity and a smaller increase in catalase activity. The increase in total superoxide dismutase (SOD) activity correlates with higher levels of specific isozymes. The chloroplast (SOD-1) and cytosolic (SOD-2 and SOD-4) forms were increased significantly; however, the mitochondrial form (SOD-3) was increased only slightly. Higher levels of SOD-4 and SOD-3 after paraquat exposure were the result of increased synthesis of these proteins, as determined by labeling in vivo with [35S]methionine. Isolation and in vitro translation of polysomes from 10(-5) M paraquat-treated leaves indicated that paraquat increased the amount of polysomal mRNA which codes for SOD-4 and SOD-3. Superoxide dismutase induction does not appear to be a response that is specific to paraquat, since another superoxide-generating compound, juglone, caused a similar increase in total superoxide dismutase activity. Therefore, the effect of these compounds on the expression of the maize Sod genes is exerted via their ability to generate superoxide.

Superoxide mediates the toxicity of paraquat for Chinese hamster ovary cells

The roles of superoxide and H2O2 in the cytotoxicity of paraquat were assessed in Chinese hamster ovary cells. Neither catalase nor superoxide dismutase inhibited the loss of ability to form colonies when added to the medium. When introduced into the cells, superoxide dismutase but not catalase inhibited the toxicity of paraquat. That superoxide dismutase acted by its known catalytic action is shown by the loss of inhibition when the enzyme was inactivated by H2O2 before being introduced into the cells. The lack of inhibition by catalase, by dimethyl sulfoxide, and by desferoxamine suggests that the toxicity is not mediated by a reaction between H2O2 and superoxide to engender the hydroxyl radical. Exposure of Chinese hamster ovary cells to paraquat may be a suitable means to determine the effects of superoxide anion in cultured cells and the ways in which cells can resist this toxic action.

Antioxidant action of natural health products and Chinese herbs

The scavenging effects of reactive oxygen species (ROS) by two natural health products, antioxidant analogs (AOA), and green magma (GM), and 16 medical Chinese herbs were investigated in two in vitro ROS-generating systems, activated neutrophils and xanthine-xanthine oxidase. Native, unheated AOA and GM products significantly reduced ROS levels, while unheated Chinese herbs had a negligible effect on ROS levels. In contrast, heat-extracted Chinese herbs and AOA markedly, and GM mildly, suppressed the levels of ROS in both systems. The ROS scavenging activity of these native, unheated products was unaffected by dialysis, but that of heated products was markedly diminished by dialysis. Further, the incubation of these products with gastric juice obtained by a gastric tube from healthy volunteers revealed results comparable to those induced by heat treatment with or without dialysis. Although the antioxidant activity of these natural products appears to be partly due to enzymes such as superoxide dismutase (SOD), the predominant factor seems to be low-molecular-weight ROS scavengers that are liberated or activated by gastric juice digestion as observed after heat treatment.

Effect of methyl viologen and oxygen concentration on thermophilic bacteria

The level of superoxide dismutase, catalase, and peroxidase was determined in obligately thermophilic bacteria after growth either in the presence of methyl viologen (paraquat) or exposed to an increased atmospheric oxygen concentration. In general, the superoxide dismutase level in these organisms was not altered by paraquat addition and had a varied response to oxygenation. Incorporation of paraquat in the growth medium resulted in minimally three-fold higher levels of catalase. The peroxidase level varied as a response to oxygen stress whereas cellular catalase levels were generally higher.

The effects of paraquat on Escherichia coli: distinction between bacteriostasis and lethality

Paraquat exerted a progressively more pronounced bacteriostatic effect on Escherichia coli as its concentration was raised in the range 0-1.0 microM. In contrast, concentrations of 100 microM or greater were required before significant lethality could be observed. This bacteriostatic effect of paraquat could be eliminated by supplementation of the glucose-plus-salts medium with either yeast extract or a casein hydrolysate. This protection was seen whether the supplement was added a few minutes prior to or following the addition of paraquat and was thus not due to the inhibition of active uptake of paraquat by the cells. The lethal effect of high levels of paraquat was not influenced by supplementation of the medium with yeast extract. It follows that the bacteriostatic and lethal effects of paraquat involve attack upon distinct targets within the cell.

Bipyridylium herbicide toxicity in vitro: comparative study of the cytotoxicity of paraquat and diquat toward the pulmonary alveolar macrophage

In vitro exposure of adult rat alveolar macrophages to either paraquat or diquat resulted in concentration dependent cytotoxicity (cell death). The herbicide paraquat, however was statistically significantly more potent toward these cells than was diquat. The LC50 value for paraquat (8-h exposure, 37 degrees C) was determined to be 0.94 mM [95% confidence interval (C.I.) 0.79-1.12 mM], whereas the corresponding LC50 value for diquat was 1.97 mM (C.I. 1.58-2.51 mM). Interestingly, diquat was shown to enter these cells to a much greater extent than was paraquat. The latter data, while seemingly contradictory to the above findings, is consistent with other reported findings in this study that show that cell respiration, as measured by loss of oxygen consumption, was more sensitive to diquat than it was to paraquat. Also, only paraquat cytotoxicity was found to be dependent on oxygen tension and could be altered by the presence of antioxidant enzymes in the culture medium. Both compounds, however, were found to be equipotent toward purified mitochondria. Both paraquat and diquat were able to uncouple oxidative phosphorylation and induce active oxygen species (superoxide anions and hydrogen peroxide) from this organelle. It is concluded that free-radical pathology is the most likely mechanism of action by which paraquat is cytotoxic toward these cells, but that diquat poisoning probably originates from some other mode of action.

Changes in plant gene expression during stress

Changes in gene expression which occur during periods of environmentally induced stress provide models for the study of gene regulation. Several types of stress have been shown to elicit a specific and reproducible pattern of gene expression in various plant species. These stress factors include heat shock, anaerobiosis, plant pathogens, oxygen free radicals, heavy metals, water stress, and chilling. In some cases, changes in specific genes have been identified, such as increases in the expression of the gene encoding the phytoalexin-synthesizing enzyme in pathogen elicitor-treated cells. However, in most cases, the functional identity of stress-induced genes is unknown. The alterations in gene expression during stress usually are rapid and repeatable, making these genetic systems ideal for examination of factors and mechanisms involved in gene regulation.

Alteration of enzymes in ageing human fibroblasts in culture. IV. Effect of glutathione on the alteration of glucose-6-phosphate dehydrogenase

Alteration and inactivation of glucose-6-phosphate dehydrogenase (G6PD) can be induced in human fibroblasts by incubation of a cell supernatant at 4 degrees C and pH 7.4. When added in such conditions, glutathione (GSH) had a stabilizing effect on the enzyme. On the other hand, substances which are known to deplete the cells of their GSH content, dramatically increase the inactivation rate. When analysed by gel filtration after 24 h of incubation at 4 degrees C, the inactive G6PD appears as a dimeric protein when GSH is present, while as a monomer in the control experiment. Reactivation of the monomers was stimulated with GSH. The heat inactivation of the dimeric fraction first started with a sharp activity increase of 20%. This increase vanished when the enzyme was first reactivated before the thermolability experiment. We propose that what is called altered G6PD is the expression of a quick reactivation of an inactive, labile dimer. Finally, a schematic view of the G6PD alteration is proposed.

[Morphology of a paraquat poisoning]

The predominant pathomorphological findings in acute lethal paraquat intoxication are interstitial and alveolar edema, as well as structural changes of the pulmonary architecture, probably as a result of damage of the surfactant by peroxides. The typical histopathological changes are illustrated in a suicide case in which paraquat was used. An oligodendroglioma of the left frontal lobe was the organic cause of suicide.

Systematically applied chemicals that damage lung tissue

A large, and increasing number of drugs and chemicals have been found which are toxic to lung following systemic administration. These agents damage lung tissue specifically, or in addition to damage to other tissues. Mechanisms explaining the pulmonary damage produced by some lung toxins have been uncovered. These include concentration of the agent within lung, the absence of adequate pulmonary detoxication systems, and bioactivation to a toxic species within specific lung cells or at distant sites followed by transport to the lung. The basic biochemical lesions underlying lung damage, responses of individual lung cells and pulmonary repair processes to the toxic agent, and species and age differences in susceptibility to lung damage have not, however, been well defined for most lung toxins. This review describes the information available on pulmonary biochemical and pathological changes associated with some of these lung-toxic agents. In addition, mechanisms proposed to explain the lung damage are discussed. The agents covered include: paraquat, the thioureas, butylated hydroxytoluene, the trialkylphosphorothioates, various lung-toxic furans and antineoplastic agents, the pyrrolizidine alkaloids, metals and organometallic compounds, amphiphilic agents, hydrocarbons, oleic acid, 3-methylindole, and diabetogenic agents. Detailed reviews on the overall toxicity of many of these agents have been published elsewhere. This review concentrates on their pulmonary toxicity. Information is presented as an overview to illustrate both the extensive literature that is available and the important questions that remain to be answered about systemic chemicals that damage lung tissue.

Failure of desferrioxamine to modify the toxicity of paraquat in rats

The feasibility of using desferrioxamine (DF), an iron chelator, as a therapeutic agent against paraquat (PQ++) toxicity in male Sprague-Dawley rats was explored, based on the rationale of limiting toxic hydroxyl radical production from hydrogen peroxide by removing redox-active iron. Body weights, mortality, and lung histopathology were followed for periods up to 14 days after intraperitoneal injection of PQ++ (20 or 25 mg/kg body weight) with or without concurrent daily subcutaneous injections of DF (300 mg/day). Animals receiving PQ++ showed the expected typical patterns of mortality and of lung histopathology, namely: marked edema, subpleural hemorrhage, acute inflammation, perivascular mononuclear cell infiltrates, sloughing of alveolar and bronchiolar lining cells, and diffuse interstitial fibrosis. Desferrioxamine alone was non-toxic. Surprisingly, results when both PQ++ and DF were administered indicated a failure of DF to ameliorate toxic effects of PQ++ in the lung, and even suggested an accentuation of PQ++-induced damage by DF. Mortality data showed that PQ++/DF animals died in greater numbers (20 mg PQ++/kg) or died earlier (25 mg PQ++/kg) than animals receiving DF alone. Qualitative histopathology in PQ++/DF animals was comparable to PQ++ animals in early stages, but damage was more severe in both incidence and severity of lesions in PQ++/DF animals, particularly at the 25 mg PQ++/kg dose level. After 14 days, surviving animals receiving PQ++ alone showed almost complete resolution of previous inflammation and other acute effects, whereas in the only surviving PQ++/DF animal initial fibrosis had persisted and become more generalized. Thus, chelation therapy with DF may not be straightforward in its effects on PQ++ toxicity.

Transition metals potentiate paraquat toxicity

The involvement of transition metal ions in paraquat toxicity was studied in bacterial model system. We show that the addition of micromolar, or lower, concentrations of copper dramatically enhanced the rate of bacterial inactivation. In contrast, the addition of chelating agents totally eliminated the killing of E. coli. No inactivation was observed under anaerobic exposure to paraquat, both in the absence and presence of copper. However, in the presence of copper, the anaerobic addition of hydrogen peroxide resulted in complete restoration of inactivation as under aerobiosis. Paraquat either produces superoxide ions or directly reduces bound copper ions in a catalytic mode. The reduced cuprous complexes react with hydrogen peroxide to locally form hydroxyl radicals (OH.) which are probably responsible for the deleterious effects. This study indicates the involvement of a site-specific metal-mediated Haber-Weiss mechanism in paraquat toxicity. It is in agreement with earlier observations that copper unusually enhance biological damage induced by either superoxide or ascorbate.

Paraquat toxicity in vitro. I. Pulmonary alveolar macrophages

When the herbicide paraquat (1,1'-dimethyl-4,4'-bipyridylium) was administered to adult rat pulmonary alveolar macrophages (PAM) in primary culture, both a time-dependent and a dose-dependent cytotoxic response (cell death) was observed. An LD50 value of 1 mM was calculated when these cells were exposed to paraquat in vitro for 12 h in Ham's F12 culture medium at 30 degrees C. Cell death was accompanied by the formation of TBA-reactive substances (lipid peroxidation) and was potentiated by hyperoxia (95% O2). In a 95% O2-5% CO2 atmosphere, an LD50 value of 0.1 mM was calculated. In addition, the presence of superoxide dismutase in the culture medium (1700 units/ml) inhibited the cytotoxic response. Since [14C]paraquat was not absorbed into these cells, extracellular superoxide anion radical formation was investigated as the cause of the observed cell death. Paraquat (0.5 mM) was found to stimulate extracellular O-2 generation, from PAM, but only in nonactivated cells. A sevenfold enhancement over the resting rate of radical generation was observed in the presence of paraquat. No increase in the O-2 generation rate of activated macrophages was observed upon the addition of paraquat to the culture medium. These data indicate that paraquat is cytotoxic to the pulmonary alveolar macrophage and further suggest that this cytotoxicity is mediated, at least in part, by an excess, extracellular production of active oxygen species. Implications of these findings with respect to the currently accepted hypothesis of paraquat poisoning in vivo are discussed.

Hydroxyl radical production from hydrogen peroxide and enzymatically generated paraquat radicals: catalytic requirements and oxygen dependence

The ability of paraquat radicals (PQ+.) generated by xanthine oxidase and glutathione reductase to give H2O2-dependent hydroxyl radical production was investigated. Under anaerobic conditions, paraquat radicals from each source caused chain oxidation of formate to CO2, and oxidation of deoxyribose to thiobarbituric acid-reactive products that was inhibited by hydroxyl radical scavengers. This is in accordance with the following mechanism derived for radicals generated by gamma-irradiation [H. C. Sutton and C. C. Winterbourn (1984) Arch. Biochem. Biophys. 235, 106-115] PQ+. + Fe3+ (chelate)----Fe2+ (chelate) + PQ++ H2O2 + Fe2+ (chelate)----Fe3+ (chelate) + OH- + OH.. Iron-(EDTA) and iron-(diethylenetriaminepentaacetic acid) (DTPA) were good catalysts of the reaction; iron complexed with desferrioxamine or transferrin was not. Extremely low concentrations of iron (0.03 microM) gave near-maximum yields of hydroxyl radicals. In the absence of added chelator, no formate oxidation occurred. Paraquat radicals generated from xanthine oxidase (but not by the other methods) caused H2O2-dependent deoxyribose oxidation. However, inhibition by scavengers was much less than expected for a reaction of hydroxyl radicals, and this deoxyribose oxidation with xanthine oxidase does not appear to be mediated by free hydroxyl radicals. With O2 present, no hydroxyl radical production from H2O2 and paraquat radicals generated by radiation was detected. However, with paraquat radicals continuously generated by either enzyme, oxidation of both formate and deoxyribose was measured. Product yields decreased with increasing O2 concentration and increased with increasing iron(DTPA). These results imply a major difference in reactivity between free and enzymatically generated paraquat radicals, and suggest that the latter could react as an enzyme-paraquat radical complex, for which the relative rate of reaction with Fe3+ (chelate) compared with O2 is greater than is the case with free paraquat radicals.

Chelated iron-catalyzed OH. formation from paraquat radicals and H2O2: mechanism of formate oxidation

Traces of iron, when complexed with either EDTA or diethylenetriaminepentaacetic acid (DTPA), catalyze an OH.-producing reaction between H2O2 and paraquat radical (PQ+.): H2O2 + PQ+.----PQ++ + OH. + OH-.[1]. Kinetic studies show that oxidation of formate induced by this reaction occurs by a Fenton-type mechanism, analagous to that assumed in the metal-catalyzed Haber-Weiss reaction, in which the rate determining step is H2O2 + Fe2+ (chelator)----Fe3+(chelator) + OH. + OH-,[7]; with k7 = 7 X 10(3) M-1 s-1 for EDTA and 8 X 10(2) M-1 s-1 for DTPA at pH 7.4. PQ+. rapidly reduces both Fe3+ (EDTA) and Fe3+ (DTPA), and hence allows both agents to catalyze [1] with comparable efficiency, in contrast to the much lower efficiency reported for the latter as a catalyst for the Haber-Weiss reaction. The catalytic properties of these chelating agents is attributed to their lowering of E0 (Fe3+/Fe2+) by 0.65 V, thus making [7] thermodynamically possible at pH 7. Approximately 2.5% of the OH. produced is consumed by internal or "cage" reactions, which decompose the chelator and produce CO2; however, the majority (97%) diffuses into the bulk solution and participates in competitive reactions with OH. scavengers.

Positive correlation between superoxide dismutase and resistance to paraquat toxicity in the green alga Chlorella sorokiniana

Paraquat (10-30 microM) exerted a dose-dependent and light-dependent toxicity on Chlorella sorokiniana. Paraquat was also seen to increase the superoxide dismutase content of these cells and to cause the appearance of a new electrophoretically distinct isozyme. Cells grown in the absence of paraquat contained one manganese-superoxide dismutase and two iron-superoxide dismutases, while the paraquat-grown cells contained an additional manganese-superoxide dismutase. Cells which were grown in the presence of 25 microM Paraquat, and which therefore possessed elevated levels of superoxide dismutase, were resistant to 30 microM Paraquat, whereas control cells were bleached and killed by this level of Paraquat. Electron micrography and chemical analysis revealed that Paraquat decreased the starch content of the cells and caused a failure of dividing cells to separate. It appears that Paraquat increases the photoproduction of O2- in C. sorokiniana and that an increase in the cell content of superoxide dismutase is an adaptive response which provides protection against this herbicide.