The effect of paraquat on microsomal lipid peroxidation in vitro and in vivo

Pharmacological inhibition of CXCR2 chemokine receptors modulates paraquat-induced intoxication in rats

Paraquat (PQ) is an agrochemical agent commonly used worldwide, which is allied to potential risks of intoxication. This herbicide induces the formation of reactive oxygen species (ROS) that ends up compromising various organs, particularly the lungs and the brain. This study evaluated the deleterious effects of paraquat on the central nervous system (CNS) and peripherally, with special attempts to assess the putative protective effects of the selective CXCR2 receptor antagonist SB225002 on these parameters. PQ-toxicity was induced in male Wistar rats, in a total dose of 50 mg/kg, and control animals received saline solution at the same schedule of administration. Separate groups of animals were treated with the selective CXCR2 antagonist SB225002 (1 or 3 mg/kg), administered 30 min before each paraquat injection. The major changes found in paraquat-treated animals were: decreased body weight and hypothermia, nociception behavior, impairment of locomotor and gait capabilities, enhanced TNF-α and IL-1β expression in the striatum, and cell migration to the lungs and blood. Some of these parameters were reversed when the antagonist SB225002 was administered, including recovery of physiological parameters, decreased nociception, improvement of gait abnormalities, modulation of striatal TNF-α and IL-1β expression, and decrease of neutrophil migration to the lungs and blood. Taken together, our results demonstrate that damage to the central and peripheral systems elicited by paraquat can be prevented by the pharmacological inhibition of CXCR2 chemokine receptors. The experimental evidence presented herein extends the comprehension on the toxicodynamic aspects of paraquat, and opens new avenues to treat intoxication induced by this herbicide.

In vitro antioxidant and cytotoxic activities of Arnebia benthamii (Wall ex. G. Don): a critically endangered medicinal plant of Kashmir Valley

Arnebia benthamii is a major ingredient of the commercial drug available under the name Gaozaban, which has antibacterial, antifungal, anti-inflammatory, and wound-healing properties. In the present study, in vitro antioxidant and anticancer activity of different extracts of Arnebia benthamii were investigated. Antioxidant potential of plant extracts was evaluated by means of total phenolics, DPPH, reducing power, microsomal lipid peroxidation, and hydroxyl radical scavenging activity. The highest phenolic content (TPC) of 780 mg GAE/g was observed in ethyl acetate, while the lowest TPC of 462 mg GAE/g was achieved in aqueous extract. At concentration of 700 µg/mL, DPPH radical scavenging activity was found to be highest in ethyl acetate extract (87.99%) and lowest in aqueous extract (73%). The reducing power of extracts increased in a concentration dependent manner. We also observed its inhibition on Fe²⁺/ascorbic acid-induced lipid peroxidation (LPO) on rat liver microsomes in vitro. In addition, Arnebia benthamii extracts exhibited antioxidant effects on Calf thymus DNA damage induced by Fenton reaction. Cytotoxicity of the extracts (10–100 µg/mL) was tested on five human cancer cell lines (lung, prostate, leukemia, colon, and pancreatic cell lines) using the Sulphorhodamine B assay.

Reactive oxygen species formation and cell death in catalase-deficient tobacco leaf discs exposed to paraquat

In the present work, the response of tobacco (Nicotiana tabaccum L.) wild-type SR1 and transgenic CAT1AS plants (with a basal reduced CAT activity) was evaluated after exposure to the herbicide paraquat (PQ). Superoxide anion (O (2) (.-) ) formation was inhibited at 3 or 21 h of exposure, but H(2)O(2) production and ion leakage increased significantly, both in SR1 or CAT1AS leaf discs. NADPH oxidase activity was constitutively 57% lower in non-treated transgenic leaves than in SR1 leaves and was greatly reduced both at 3 or 21 h of PQ treatment. Superoxide dismutase (SOD) activity was significantly reduced by PQ after 21 h, showing a decrease from 70% to 55%, whereas catalase (CAT) activity decreased an average of 50% after 3 h of treatment, and of 90% after 21 h, in SR1 and CAT1AS, respectively. Concomitantly, total CAT protein content was shown to be reduced in non-treated CAT1AS plants compared to control SR1 leaf discs at both exposure times. PQ decreased CAT expression in SR1 or CAT1AS plants at 3 and 21 h of treatment. The mechanisms underlying PQ-induced cell death were possibly not related exclusively to ROS formation and oxidative stress in tobacco wild-type or transgenic plants.

Antioxidant Activity of Extracts of Peh-Hue-Juwa-Chi-Cao in a Cell Free System

In traditional Chinese medicine preparations, Hedyotis diffusa (HD), Hedyotis corymbosa (HC) and Mollugo pentaphylla (MP) are often used interchangeably under the name of "Peh-Hue-Juwa-Chi-Cao (PHJCC)." Although studies have been conducted to characterize the therapeutic activities of these different plant species, their antioxidant activity has never been investigated. In this study, our aim was to evaluate the antioxidant and free radical scavenging activities of these three different plant materials. At a concentration of 10 mg/ml, results showed that HD possesses the strongest inhibition on the FeCl2-ascorbic acid induced lipid peroxidation in rat liver homogenate, followed by HC and MP. MP showed a weak anti-lipid peroxidation activity at 1 and 3 mg/ml. Using Electron Spin Resonance (ESR) analysis, the order of superoxide anion scavenging activity was HC>HD>MP. However, MP was found to have the greatest hydroxyl radical scavenging activity compared to HC and HD. In conclusion, all three species used as PHJCC in Taiwan exhibited different antioxidant and free radical scavenging activities; these differences could explain, at least in part, the variation in therapeutic properties of PHJCC products in the market.

Comparative effects of the herbicides dicamba, 2,4-D and paraquat on non-green potato tuber calli

The effects of the herbicides 1,1'-dimethyl-4,4'-bipyridylium dichloride (paraquat), 3,6-dichloro-2-metoxybenzoic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-D) on cell growth of non-green potato tuber calli are described. We attempted to relate the effects with toxicity, in particular the enzymes committed to the cellular antioxidant system. Cell cultures were exposed to the herbicides for a period of 4 weeks. Cellular integrity on the basis of fluorescein release was strongly affected by 2,4-D, followed by dicamba, and was not affected by paraquat. However, the three herbicides decreased the energy charge, with paraquat and 2,4-D being very efficient. Paraquat induced catalase (CAT) activity at low concentrations (1 microM), whereas at higher concentrations, inhibition was observed. Dicamba and 2,4-D stimulated CAT as a function of concentration. Superoxide dismutase (SOD) activity was strongly stimulated by paraquat, whereas dicamba and 2,4-D were efficient only at higher concentrations. Glutathione reductase (GR) activity was induced by all the herbicides, suggesting that glutathione and glutathione-dependent enzymes are putatively involved in the detoxification of these herbicides. Paraquat slightly inhibited glutathione S-transferase (GST), whereas 2,4-D and dicamba promoted significant activation. These results indicate that the detoxifying mechanisms for 2,4-D and dicamba may be different from the mechanisms of paraquat detoxification. However, the main cause of cell death induced by paraquat and 2,4-D is putatively related with the cell energy charge decrease.

Antioxidants as potential therapeutics for lung fibrosis

Interstitial lung disease encompasses a large group of chronic lung disorders associated with excessive tissue remodeling, scarring, and fibrosis. The evidence of a redox imbalance in lung fibrosis is substantial, and the rationale for testing antioxidants as potential new therapeutics for lung fibrosis is appealing. Current animal models of lung fibrosis have clear involvement of ROS in their pathogenesis. New classes of antioxidant agents divided into catalytic antioxidant mimetics and antioxidant scavengers are being developed. The catalytic antioxidant class is based on endogenous antioxidant enzymes and includes the manganese-containing macrocyclics, porphyrins, salens, and the non-metal-containing nitroxides. The antioxidant scavenging class is based on endogenous antioxidant molecules and includes the vitamin E analogues, thiols, lazaroids, and polyphenolic agents. Numerous studies have shown oxidative stress to be associated with many interstitial lung diseases and that these agents are effective in attenuating fibroproliferative responses in the lung of animals and humans.

Effect of garlic on isoniazid and rifampicin-induced hepatic injury in rats

AIM: To evaluate the hepatoprotective effect of garlic on liver injury induced by isoniazid (INH) and rifampicin (RIF).

METHODS: Wistar rats weighing 150-200 g were treated orally with 50 mg/kg of INH and RIF daily each for 28 d. For hepatoprotective studies, 0.25 g/kg per day of freshly prepared garlic homogenate was administered orally half an hour before the INH+RIF doses. Serum alanine aminotransferase (ALT), aspartate aminotransferase (AST) and bilirubin were estimated on d 0, 14, 21, and 28 in all the rats. Histological analysis was carried out to assess the injury to the liver. Lipid peroxidation (LPO) as a marker of oxidative stress and non-protein thiols (glutathione) for antioxidant levels were measured in liver homogenate.

RESULTS: The treatment of rats with INH+RIF (50 mg/kg per day each) induced hepatotoxicity in all the treated animals as judged by elevated serum ALT, AST, and bilirubin levels, presence of focal hepatocytic necrosis (6/8) and portal triaditis (8/8). Garlic simultaneously administered at a dose of 0.25 g/kg per day prevented the induction of histopathological injuries in INH+RIF co-treated animals, except in 4 animals, which showed only moderate portal triaditis. The histological changes correlated with oxidative stress in INH+RIF treated animals. The group which received 0.25 g/kg per day garlic homogenate along with INH+RIF showed higher levels of glutathione (P < 0.05) and low levels of LPO (P < 0.05) as compared to INH+RIF treated group.

CONCLUSION: Freshly prepared garlic homogenate protects against INH+RIF-induced liver injury in experimental animal model.

A comparative study of plant and animal mitochondria exposed to paraquat reveals that hydrogen peroxide is not related to the observed toxicity

Rat liver mitochondria are much more susceptible to protein oxidation induced by paraquat than plant mitochondria. The unsaturated index and the peroxidizability index are higher in rat than in potato tuber. The levels of superoxide dismutase and glutathione reductase are concurrent with the different sensitivities to paraquat, with higher activities in plant mitochondria. However, glutathione peroxidase and catalase activities are higher in rat mitochondria. Paraquat (10 mM) inhibited all the enzymatic activities; excluding catalase all the other activities were inhibited to a similar degree. The differential sensitivities of plant and animal mitochondria to paraquat correlate with fatty acid composition of mitochondrial lipids and a similar correlation was also established for some antioxidant enzymes. At the mitochondrial level, H(2)O(2) is not a major factor of paraquat toxicity since rat liver mitochondria which exhibit higher activities of glutathione peroxidase and catalase are however more susceptible to paraquat.

Paraquat detoxicative system in the mouse liver postmitochondrial fraction

We examined the paraquat detoxicative system in mouse livers. The survival rate of mice receiving 50 mg/kg paraquat was 41% at 7 days and significantly rose to 88, 64, 69% with pretreatment with phenytoin, phenobarbital, and rifampicin, respectively. Phenytoin induced activity in NADPH-cytochrome P450 reductase, CYP3A, CYP2B, and CYP2C that was 3 to 4 times higher than that of the controls. Phenobarbital induced CYP2B and rifampicin induced CYP3A, respectively, in addition to NADPH-cytochrome P450 reductase. 3-Methylcholanthrene did not induce these enzymes and did not alter the survival rate. All the mice pretreated with CoCl(2) (a CYP synthesis inhibitor) or SKF 525-A (a CYP inhibitor) were dead after 5 days, and troleandomycin (a CYP3A-specific inhibitor) also reduced the survival rate. When cell homogenates were incubated with paraquat and NADPH, paraquat decreased and its metabolic intermediate paraquat-monopyridone was formed. Troleandomycin inhibited the decrease in paraquat and increased the monopyridone. After making a subfraction of the homogenate, monopyridone was produced in the postmicrosomal 105,000g supernatant, but not in the microsomes. The pretreatment of mice with phenytoin decreased the monopyridone in the postmitochondrial fraction, but did not affect the supernatant. These results indicated that paraquat was first metabolized in the postmicrosomal supernatant into monopyridone, and that may have been subsequently hydroxylated by the microsomes. Repeated intravenous injections of alpha-tocopherol to paraquat-loaded mice significantly reduced the paraquat mortality and when these mice were pretreated with rifampicin, 100% of them survived. These studies demonstrate that postmitochondrial fractions play an important role in paraquat detoxication metabolism, and that the combination of CYP induction and alpha-tocopherol administration is highly useful for the survival of paraquat-exposed mice.

Differential sensitivities of plant and animal mitochondria to the herbicide paraquat

Paraquat herbicide is toxic to animals, including humans, via putative toxicity mechanisms associated to microsomal and mitochondrial redox systems. It is also believed to act in plants by generating highly reactive oxygen free radicals from electrons of photosystem I on exposure to light. Paraquat also acts on non-chlorophyllous plant tissues, where mitochondria are candidate targets, as in animal tissues. Therefore, we compared the interaction of paraquat with the mitochondrial bioenergetics of potato tuber, using rat liver mitochondria as a reference. Paraquat depressed succinate-dependent mitochondrial Delta(psi), with simultaneous stimulation of state 4 O2 consumption. It also induced a slow time-dependent effect for respiration of succinate, exogenous NADH, and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD)/ascorbate, which was more pronounced in rat than in potato mitochondria. However, with potato tuber mitochondria, the Delta(psi) promoted by complex-I-dependent respiration is insensitive to this effect, indicating a protection against paraquat radical afforded by complex I redox activity, which was just the reverse of to the findings for rat liver mitochondria. The experimental set up with the tetraphenyl phosphonium (TPP+)-electrode also indicated production of the paraquat radical in mitochondria, also suggesting its accessibility to the outside space. The different activities of protective antioxidant agents can contribute to explain the different sensitivities of both kinds of mitochondria. Values of SOD activity and alpha-tocopherol detected in potato mitochondria were significantly higher than in rat mitochondria, which, in turn, revealed higher values of lipid peroxidation induced by paraquat.

Mitochondrial NADH-quinone oxidoreductase of the outer membrane is responsible for paraquat cytotoxicity in rat livers

We investigated the existence of an NADH-dependent paraquat (PQ) reduction system in rat liver mitochondria (Mt) in respect to the cytotoxic mechanisms of PQ. The outer membrane fractions, free from the contamination of inner membranes but with a few microsomes, catalyzed rotenone-insensitive NADH, but not NADPH, oxidation by menadione or PQ. Anti-NADH-cytochrome b5 reductase antibody and its inhibitor p-hydroxymercuribenzonate did not inhibit the NADH-PQ reduction activity. Therefore, the respiratory systems of the inner membranes and microsomal cytochrome P450 systems could not have been responsible for the reaction. Dicoumarol, an inhibitor of NAD(P)H-quinone oxidoreductase (NQO), dose dependently suppressed the NADH oxidation in the outer membrane via PQ as well as menadione, with I50 values of 190 (for menadione) and 150 microM (for PQ). Because of a lower sensitivity to NADPH and the higher doses of dicoumarol required for its inhibition, the activity in the outer membrane may be an "NADH-quinone oxidoreductase" which partly differs from the NQO previously reported. This outer membrane enzyme produced superoxide anions in the presence of both NADH and PQ and was too tightly membrane-bound to be extracted by Triton X-100 and deoxycholate. From these results, we concluded that the free radical-producing mitochondrial NADH-quinone oxidoreductase is a novel oxidation-reduction system participating in PQ toxicity. This is in good agreement with our previous results showing that PQ selectively damaged Mt in vivo and in vitro, resulting in cell death (K.-I. Hirai et al., 1992, Toxicology 72, 1-16).

Paraquat-induced free radical reaction in mouse brain microsomes

Paraquat has been implicated as an environmental toxin which may induce the syndrome of Parkinson's disease after exposure to this agent. However, the biochemical mechanism by which paraquat causes cell death and neurodegeneration has not been extensively studied. Paraquat was rapidly taken up by nerve terminals isolated from mouse cerebral cortices. It induced lipid peroxidation in a concentration dependent manner in the presence of NADPH and ferrous ion. The maximal stimulation effect was obtained at a paraquat concentration around 100 microM and the Km value for paraquat was 46.7 microM. The lipid peroxidation required microsomal enzymes. Antioxidants, such as superoxide dismutase, catalase and promethazine significantly inhibited paraquat-induced lipid peroxidation. Due to its structural similarity to the pyridinium compound MPP+ (N-methyl-4-phenyl pyridium ion), it may be taken up by dopamine neurons and cause lipid peroxidation and cell death resulting in the manifestation of Parkinsonian syndrome.

Biochemical studies on the toxicity of 1, 1'-dimethyl-4, 4'-bipyridylium dichloride in the rat

The effect of intraperitoneal administration of lethal dose (50 mg/kg) of paraquat on the microsomal cysteine levels in the plasma, liver and lung of adult male Wistar rats has been investigated using Rank Chromaspek amino acid analyzer. The microsomal alanine levels were also determined to help in assessing the extent of paraquat interference with cellular protein. DL-Buthionine-[S,R]-Sulfoximine (BSO) and Diethyl maleate (DEM) were used to potentiate the toxic effect of the bipyridyl. The microsomal cysteine levels were significantly (P < or = 0.05) depressed in the plasma, liver and lung of the paraquat-treated rats compared with the saline-injected group but the alanine levels were not similarly affected. Probably, paraquat poisoning interferes specifically with the cellular cysteine content in the rat. These findings could provide a valuable information on the biochemical mechanism of paraquat intoxication.

Susceptibility of metallothionein-null mice to paraquat

Using transgenic mice in which metallothionein (MT)-I and MT-II genes, we have studied a putative role of MT as a free radical scavenger against paraquat, a free radical generator. Male mice were injected s.c. with paraquat (PQ) at a single dose of 40 or 60 mg/kg of body weight (b.w.). Two of the six MT-null mice died within 16 h at the dose of 60 mg PQ/kg. b. w. PQ administration increased hepatic MT concentration in the normal mice (C57BL/6J), but not in the MT-null mice. The lipid peroxidation (LP) determined by thiobarbituric acid-reactive substance formation was increased by PQ in the liver of normal and MT-null mice, and the enhanced level was greater in the MT-null mice than in the C57BL/6J mice. Administration of PQ significantly increased blood urea nitrogen only in the MT-null mice, indicating renal damage. Without paraquat administration, the hepatic concentration of non-protein sulphydryl compounds was less in the MT-null mice than in the C57BL/6J mice, and the basal level of LP was higher in the MT-null mice than in the C57BL/6J mice. The present results support the notion that MT plays an antioxidative role against paraquat insult under physiological conditions.

Mitochondrial bioenergetics is affected by the herbicide paraquat

The potential toxicity of the herbicide paraquat (1,1-dimethyl-4,4'-bipyridylium dichloride) was tested in bioenergetic functions of isolated rat liver mitochondria. Paraquat increases the rate of State 4 respiration, doubling at 10 mM, indicating uncoupling effects. Additionally, State 3 respiration is depressed by about 15%, at 10 mM paraquat, whereas uncoupled respiration in the presence of CCCP is depressed by about 30%. Furthermore, paraquat partially inhibits the ATPase activity through a direct effect on this enzyme complex. However, at high concentrations (5-10 mM), the ATPase activity is stimulated, probably as consequence of the described uncoupling effect. Depression of respiratory activity is mediated through partial inhibitions of mitochondrial complexes III and IV. Paraquat depresses delta psi as a function of herbicide concentration. In addition, the depolarization induced by ADP is decreased and repolarization is biphasic suggesting a double effect. Repolarization resumes at a level consistently higher than the initial level before ADP addition, for paraquat concentrations up to 10 mM. This particular effect is clear at 1 mM paraquat and tends to fade out with increasing concentrations of the herbicide.

Microsomal membrane peroxidation by an Fe3+/paraquat system. Consequences of phenobarbital induction

Descriptions of the effects of paraquat (P2+) on the peroxidation of liver microsomes are very divergent. Therefore, the presence of ferric iron in the medium and the activity of microsomal mixed-function oxidase system are two factors that we have taken into consideration to explain the discrepancies. The results showed that 100 microM P2+ potentializes the slight production of MDA induced by low concentrations of Fe3+ (< or = 15 microM). In these conditions, P+., arising from the one-step reduction of P2+ by NADPH-cytochrome C reductase, could reduce Fe3+ and cause the formation of species that initiate peroxidation. However, unlike the results obtained with CBrCl3, for animals induced by phenobarbital (Ph), the production of MDA in the presence of FeCl3 and of P2+ was weaker than for the controls. The establishment of a new Fe3+/Fe2+ equilibrium owing to increased production of P+. could be responsible.

Effect of paraquat on the malondialdehyde level in rat liver microsomes (in vitro)

Toxicosis due to paraquat, a redox cycling xenobiotic, is still a subject of much debate. In the present study on lipid peroxidation, paraquat had a biphasic effect on the malondialdehyde (MDA) level in rat liver microsomes; stimulation at the initial stage (within 10 min) and depression at the later stage. Although paraquat increased the initial rate of NADPH oxidation dose-dependently, the rate was not necessarily parallel with the increase in the MDA level. The MDA level increased linearly up to 0.1 mM paraquat added, but then it attained a plateau. The stimulation obtained by paraquat within 10 min was absolutely dependent on exogenous Fe2+ ion and NADPH, and the stimulation was entirely SOD sensitive, while the iron-driven increase in MDA was 20% sensitive. Thus, there were different mechanisms between iron-driven lipid peroxidation and paraquat-modified peroxidation. Catalase increased the level, but mannitol, a scavenger of OH, had no effect. EPR spectra showed that superoxide was formed dose-dependently up to 0.1 mM paraquat and that it attained a plateau at the same as MDA level described above. From these results, we concluded that paraquat stimulates lipid peroxidation through a mechanism dependent on the superoxide complex involving Fe2+ ion.

Stimulation by paraquat of microsomal and cytochrome P-450-dependent oxidation of glycerol to formaldehyde

Glycerol can be oxidized to formaldehyde by microsomes in a reaction that is dependent on cytochrome P-450. An oxidant derived from the interaction of H2O2 with iron was responsible for oxidizing the glycerol, with P-450 suggested to be necessary to produce H2O2 and reduce non-haem iron. The effect of paraquat on formaldehyde production from glycerol and whether paraquat could replace P-450 in supporting this reaction were studied. Paraquat increased NADPH-dependent microsomal oxidation of glycerol; the stimulation was inhibited by glutathione, catalase, EDTA and desferrioxamine, but not by superoxide dismutase or hydroxyl-radical scavengers. The paraquat stimulation was also inhibited by inhibitors, substrate and ligand for P-4502E1 (pyrazole-induced P-450 isozyme), as well as by anti-(P-4502E1) IgG. These results suggest that P-450 still played an important role in glycerol oxidation, even in the presence of paraquat. Purified NADPH-cytochrome P-450 reductase did not oxidize glycerol to formaldehyde; some oxidation, however, did occur in the presence of paraquat. Reductase plus P-4502E1 oxidized glycerol, and a large stimulation was observed in the presence of paraquat. Rates in the presence of P-450, reductase and paraquat were more than additive than the sums from the reductase plus P-450 and reductase plus paraquat rates, suggesting synergistic interactions between paraquat and P-450. These results indicate that paraquat increases oxidation of glycerol to formaldehyde by microsomes and reconstituted systems, that H2O2 and iron play a role in the overall reaction, and that paraquat can substitute, in part, for P-450 in supporting oxidation of glycerol. However, cytochrome P-450 is required for elevated rates of formaldehyde production even in the presence of paraquat.

Stimulation of microsomal chemiluminescence by ferritin

The ability of ferritin to catalyze rat liver microsomal chemiluminescence was determined in the absence and presence of the redox cycling agent paraquat, and with either NADPH or NADH as reductant. Microsomal chemiluminescence was used as a index of lipid peroxidation. In the absence of added ferritin, NADPH-dependent microsomal light emission was 4-fold greater than the NADH-dependent reaction, and was not sensitive to superoxide dismutase, catalase or DMSO. Ferritin stimulated NADPH-, but not NADH-dependent chemiluminescence in a time- and concentration-dependent manner. The stimulation by ferritin was completely sensitive to superoxide dismutase, but not to catalase or DMSO, suggesting the requirement for superoxide to mobilize iron from ferritin. An iron ligand was not required for the stimulation by ferritin; the addition of certain ligands such as EDTA, DETAPAC or desferrioxamine resulted in inhibition of the stimulation by ferritin. Paraquat potentiated the effect of ferritin on microsomal chemiluminescence with NADPH as cofactor and was weakly stimulatory with NADH. The potentiation by paraquat plus ferritin was prevented by superoxide dismutase and was further elevated by ligands such as ATP. Chemiluminescence proved to be a more sensitive parameter than production of thiobarbituric acid-reactive components to evaluate the stimulation of oxygen radical production by iron released from ferritin, in the absence or in the presence of paraquat.

Effects of paraquat on Mg(2+)-ATPase activity in rat liver

Intraperitoneal injection of paraquat (70 mg/kg) elicited a significant decrease of 20% in activity of Mg(2+)-ATPase in hepatic mitochondria, which is always surrounded by a high concentration of oxygen, in rats. The decrease of mitochondrial Mg(2+)-ATPase activity was completely abolished by pretreatment with vitamin E, which is a scavenger of oxygen radicals. On the other hand, paraquat administration did not change the Mg(2+)-ATPase activity in endoplasmic reticulum, which exists in an anaerobic condition in living cells. When liver microsomes were incubated with 1 mM paraquat under aerobic conditions, the Mg(2+)-ATPase activity was decreased by 42%. The decrease of Mg(2+)-ATPase activity was completely eliminated by pretreatment with vitamin E. Furthermore, lipid peroxidation in microsomes was tremendously increased by the addition of 1 mM paraquat under aerobic conditions. The increase of lipid peroxidation was completely abolished by preadministration of vitamin E in rats. The results suggest that the inhibition of Mg(2+)-ATPase activity induced by paraquat may be mediated by active oxygen, which is produced by the reaction of paraquat radicals; molecular oxygen may be involved in the induction of hepatic cell injury.

Changes in lipid peroxidation levels and lipid composition in the lungs, livers, kidneys and brains of mice treated with paraquat

We examined lipid peroxide levels and the lipid composition of homogenates prepared from the lungs, livers, kidneys and brains of 48 male ICR mice treated with 30 mg kg-1 paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride). The mice were divided into eight groups, in which they were killed 0, 1.5, 3, 6, 12, 24, 48 and 120 h after the administration of paraquat. A significant increase in the lipid peroxide level was identified only in the liver. Change in lipid composition was identified in all the examined organs. However, the change was not a characteristic one in which there is a selective decrease of polyunsaturated fatty acids which become degraded in a lipid peroxidation reaction. It is possible that the mechanism of paraquat toxicity may differ in different organs.

Role of lipid peroxidation and DNA damage in paraquat toxicity and the interaction of paraquat with ionizing radiation

Since the introduction of paraquat (PQ) as a herbicide in 1963, there have been many speculations concerning the critical lesion in PQ toxicity. Damage to membrane lipids might be an initial event leading to PQ-induced cell killing. The ability of PQ to induce lipid peroxidation was tested in liver homogenates of the mouse. Lipid peroxidation was indeed induced by PQ and shown to be dose dependent, starting to be significant at 2.5 mM. Subsequently, a possible correlation between lipid peroxidation and PQ-induced cell death was investigated in mouse fibroblasts (LM) and Ehrlich ascites tumour (EAT) cells using a clonogenic assay. It was found that in order to be cytotoxic PQ needs enzymatic activation (incubation at 37 degrees). In both cell lines, PQ-induced cell killing appeared to be dose dependent, starting at a dose of 0.5 mM. Supplementation of LM cells with the antioxidant vitamin E had no effect on PQ-induced cell killing and modification of the membranes of LM cells by incorporation of the polyunsaturated fatty acid 20:4 (arachidonic acid) did not sensitize the cells to PQ toxicity. PQ had no effect on the glutathione (GSH) level in EAT cells and complete GSH depletion by DL-buthionine-(SR)-sulfoximine could not sensitize the cells to PQ toxicity. In LM cells PQ-induced cell killing was enhanced after complete GSH depletion by DEM. This sensitization might, however, be attributed to the binding of DEM to proteins. From these results it seems unlikely that lipid peroxidation is the primary cause for PQ-induced cell killing. Another critical target in PQ toxicity is DNA. This possibility was investigated in EAT cells. PQ was found to induce DNA damage (detected by the alkaline unwinding assay) in the same dose range that caused cell death. A good correlation was obtained for cell killing after PQ treatment and DNA damage measured 2 hr after 37 degrees post-incubation. A proposed possible interaction between PQ and X-rays was also investigated. In EAT cells, X-ray-induced cell death was significantly enhanced by pre-incubation with PQ at doses of 0.5 mM and above. At the level of 10% survival an enhancement factor of 1.6 could be observed by treatment with 1 mM PQ when cell killing by PQ is not taken into account. Induction as well as processing of radiation-induced DNA damage seems to be unaffected by pre-incubation with PQ. The mechanism of radiosensitization by PQ is yet unclear.

NADH-dependent generation of reactive oxygen species by microsomes in the presence of iron and redox cycling agents

NADH was found previously to catalyze the reduction of various ferric complexes and to promote the generation of reactive oxygen species by rat liver microsomes. Experiments were conducted to evaluate the ability of NADH to interact with ferric complexes and redox cycling agents to catalyze microsomal generation of potent oxidizing species. In the presence of iron, the addition of menadione increased NADPH- and NADH-dependent oxidation of hydroxyl radical (.OH) scavenging agents; effective iron complexes included ferric-EDTA, -diethylenetriamine pentaacetic acid, -ATP, -citrate, and ferric ammonium sulfate. The stimulation produced by menadione was sensitive to catalase and to competitive .OH scavengers but not to superoxide dismutase. Paraquat, irrespective of the iron catalyst, did not increase significantly the NADH-dependent oxidation of .OH scavengers under conditions in which the NADPH-dependent reaction was increased. Menadione promoted H2O2 production with either NADH or NADPH; paraquat was stimulatory only with NADPH. Stimulation of H2O2 generation appears to play a major role in the increased production of .OH-like species. Menadione inhibited NADH-dependent microsomal lipid peroxidation, whereas paraquat produced a 2-fold increase. Neither the control nor the paraquat-enhanced rates of lipid peroxidation were sensitive to catalase, superoxide dismutase, or dimethyl sulfoxide. Although the NADPH-dependent microsomal system shows greater reactivity and affinity for interacting with redox cycling agents, the capability of NADH to promote menadione-catalyzed generation of .OH-like species and H2O2 or paraquat-mediated lipid peroxidation may also contribute to the overall toxicity of these agents in biological systems. This may be especially significant under conditions in which the production of NADH is increased, e.g. during ethanol oxidation by the liver.

Comparison of one-electron reduction activity against the bipyridylium herbicides, paraquat and diquat, in microsomal and mitochondrial fractions of liver, lung and kidney (in vitro)

The first one-electron reduction steps of paraquat and diquat were compared using microsomal and mitochondrial fractions of rat liver, lung and kidney. Both fractions reduced each herbicide effectively, with the order of the Vmax values in microsomes and mitochondria being liver greater than lung greater than kidney and kidney greater than liver greater than lung, respectively. Although similar Vmax values were obtained from the liver and lung with the two subcellular fractions, the affinity of mitochondrial enzymes was lower, suggesting that the reduction of both herbicides in a microsomal site would be dominant in these two organs. The Vmax values for radical formation of paraquat were higher than those of diquat in all the endogenous one-electron reducing systems. The apparent Km values for diquat, however, were lower than those for paraquat in both subcellular fractions from the three tissues, indicating the superiority of the reduction for diquat to that for paraquat at low concentrations. This difference in the Km values supported the finding that the reduction velocity for diquat was significantly higher than that for paraquat at 1 mM concentration. Thus, at low concentrations, diquat would be reduced more easily than paraquat. In addition, tissue enzymatic specificity for paraquat was not obtained. From these data, it seems reasonable to conclude that the tissue-selective accumulation of paraquat previously proposed determines its toxicity.

Different effects of paraquat on microsomal lipid peroxidation in mouse brain, lung and liver

Paraquat stimulates NADPH-Fe(2+)-dependent microsomal lipid peroxidation in mouse brain and strongly inhibits it in the liver. In lung microsomes, the lipid peroxidation was stimulated by paraquat at 10(-4) M, but not at higher doses. An antioxidant action of paraquat seemed to account, at least in part, for the lack of stimulation in lung microsomes, but it was inappropriate to explain the result in hepatic microsomes. There was no apparent correlation between the effects of paraquat on the lipid peroxidation and on the activity of NADPH-cytochrome P-450 reductase, the enzyme which initiates redox cycling of paraquat, resulting in generation of active oxygen species. In fact, the effect of paraquat on the lipid peroxidation was independent of paraquat radical production, an intermediate in the cycle. However, the inhibitory potency of N-ethylmaleimide on NADPH-cytochrome P-450 reductase activity paralleled that on the lipid peroxidation stimulated by paraquat in brain and lung. These findings indicate that the effect of paraquat on microsomal lipid peroxidation differs among the organs and that other factors, besides NADPH-cytochrome P-450 reductase, might be involved in the stimulation of lipid peroxidation by paraquat.

NADPH- and NADH-dependent oxygen radical generation by rat liver nuclei in the presence of redox cycling agents and iron

Redox cycling agents such as paraquat and menadione increase the generation of reactive oxygen species in biological systems. The ability of NADPH and NADH to catalyze the generation of oxygen radicals from the metabolism of these redox cycling agents by rat liver nuclei was determined. The oxidation of hydroxyl radical scavenging agents by the nuclei was increased in the presence of menadione or paraquat, especially with NADPH as the reductant. Paraquat, even at high concentrations, was relatively ineffective with NADH. The highest rates of generation of .OH-like species occurred with ferric-EDTA as the iron catalyst. Certain ferric complexes such as ferric-ATP, ferric-citrate, or ferric ammonium sulfate, which were ineffective catalysts for .OH generation in the absence of paraquat or menadione, were reactive in the presence of the redox cycling agents. Oxidation of .OH scavengers was sensitive to catalase and competitive .OH-scavenging agents under all conditions. The redox cycling agents increased NADPH-dependent nuclear generation of H2O2; stimulation of H2O2 production may play a role in the increase in .OH generation by menadione and paraquat. Menadione inhibited nuclear lipid peroxidation, whereas paraquat and adriamycin were stimulatory. The nuclear lipid peroxidation with either NADPH or NADH plus the redox cycling agents was not sensitive to catalase or .OH scavengers. These results indicate that the interaction of rat liver nuclei with redox cycling agents and iron leads to the production of potent oxidants which initiate lipid peroxidation or oxidize .OH scavengers. Although NADPH is more effective, NADH can also participate in catalyzing the production of reactive oxygen intermediates from the interaction of quinone redox cycling agents with nuclei. The ability of redox cycling agents to interact with various ferric complexes to catalyze nuclear generation of potent oxidizing species with either NADPH or NADH as reductants may contribute to the oxidative stress, toxicity, and mutagenicity of these agents in biological systems.

Free malondialdehyde levels in the urine of rats intoxicated with paraquat

We examined the excretion of free malondialdehyde (MDA) in the urine of rats to which a herbicide, Gramoxone, had been orally administered. The herbicide was administered for 2 days at a dose of 60 mg paraquat/kg body weight/day. As a result, the concentration of free MDA decreased following the intake of Gramoxone. The total amount of free MDA increased temporarily, but then it decreased significantly to below normal values. Rats that died during this experimental period did not excrete any free MDA. In the surviving animals, the MDA concentration in serum and lung microsomes decreased, while that in liver microsomes increased slightly after intake of the poison. Although the cause of the decrease in the urinary free MDA level remains unclear, the marked changes may provide valuable information regarding a toxic mechanism of paraquat intake.

Oxygen radicals in lung pathology

Pulmonary tissue can be damaged in different ways, for instance by xenobiotics (paraquat, butylated hydroxytoluene, bleomycin), during inflammation, ischemia reperfusion, or exposure to mineral dust or to normobaric pure oxygen levels. Reactive oxygen species are partly responsible for the observed pulmonary tissue damage. Several mechanisms leading to toxicity are described in this review. The reactive oxygen species induce bronchoconstriction, elevate mucus secretion, and cause microvascular leakage, which leads to edema formation. Reactive oxygen species even induce an autonomic imbalance between muscarinic receptor-mediated contraction and the beta-adrenergic-mediated relaxation of the pulmonary smooth muscle. Vitamin E and selenium have a regulatory role in this balance between these two receptor responses. The autonomic imbalance might be involved in the development of bronchial hyperresponsiveness, occurring in lung inflammation. Finally, several antioxidants are discussed which may be beneficial as therapeutics in several lung diseases.

Interactions between paraquat and ferric complexes in the microsomal generation of oxygen radicals

Transition metals may play a central role in the toxicity associated with paraquat. Studies were carried out to evaluate the interaction of paraquat with several ferric complexes in the promotion of oxygen radical generation by rat liver microsomes. In the absence of added iron, paraquat produced some increase in low level chemiluminescence by microsomes; there was a synergistic increase in light emission in the presence of paraquat plus ferric-ATP or ferric-citrate, but not paraquat plus either ferric-EDTA or ferric-diethylenetriamine pentaacetic acid (ferric-DETAPAC). Synergistic interactions could be observed at a paraquat concentration of 100 microM and a ferric-ATP concentration of 3 microM. In the absence or presence of paraquat, microsomal light emission was not affected by catalase or dimethyl sulfoxide (DMSO), indicating no significant role for hydroxyl radicals. Superoxide dismutase (SOD) did not affect chemiluminescence in the absence of paraquat but produced some inhibition in the presence of paraquat; this inhibition by SOD was most prominent in the absence of added iron and less pronounced in the presence of ferric-ATP or ferric-citrate. Although microsomal chemiluminescence is closely associated with lipid peroxidation, paraquat did not increase malondialdehyde production as reflected by production of thiobarbituric acid-reactive components. However, lipid peroxidation was sensitive to inhibition by SOD in the presence, but not in the absence, of paraquat, analogous to results with chemiluminescence. Paraquat synergistically increased microsomal hydroxyl radical production as measured by the production of ethylene from 2-keto-4-thiomethylbutyrate in the presence of ferric-EDTA or ferric-citrate. The interaction of paraquat with microsomes and ferric complexes resulted in an increase in oxygen radical generation. Various ferric complexes can increase the catalytic effectiveness of paraquat in promoting microsomal generation of oxygen radicals, although, depending on the reaction being investigated, the nature of the ferric complex is important.

Effects of MPTP, MPP+ and paraquat on mitochondrial potential and oxidative stress

The effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-methyl-4-phenylpyridinium (MPP+) and 1,1-dimethyl-4,4-bipyridinium (paraquat) upon the electrical potential across the plasma and mitochondrial membranes within synaptosomes has been investigated. MPTP selectively depressed plasma membrane potential while MPP+ specifically reduced mitochondrial potential. The structurally similar compound paraquat had no effect on either membrane potential. Enhancement of the lipid peroxidative activity with an Fe-ADP complex depressed both potentials. Paraquat effected increased peroxidative activity in brain homogenates that was less pronounced than that due to Fe-ADP. MPTP reduced basal but stimulated Fe-ADP enhanced peroxidation. The mechanisms underlying the toxicity of MPP+ are likely to differ from those of paraquat, primarily involving impaired mitochondrial function rather than increased oxidative stress.

Three models of free radical-induced cell injury

Three models of free radical-induced cell injury are presented in this review. Each model is described by the mechanism of action of few prototype toxic molecules. Carbon tetrachloride and monobromotrichloromethane were selected as model molecules for alkylating agents that do not induce GSH depletion. Bromobenzene and allyl alcohol were selected as prototypes of GSH depleting agents. Paraquat and menadione were presented as prototypes of redox cycling compounds. All these groups of toxins are converted, during their intracellular metabolism, to active species which can be radical species or electrophilic intermediates. In most cases the activation is catalyzed by the microsomal mixed function oxidase system, while in other cases (e.g. allyl alcohol) cytosolic enzymes are responsible for the activation. Radical species can bind covalently to cellular macromolecules and can promote lipid peroxidation in cellular membranes. Of course both phenomena produce cell damage as in the case of CCl4 or BrCCl3 intoxication. However, the covalent binding is likely to produce damage at the molecular site where it occurs; lipid peroxidation, on the other hand, besides causing loss of membrane structure, also gives rise to toxic products such as 4-hydroxyalkenals and other aldehydes which in principle can move from the site of origin and produce effects at distant sites. Electrophilic intermediates readily reacts with cellular nucleophiles, primarily with GSH. The result is a severe GSH depletion as in the case of bromobenzene or allyl alcohol intoxication. When the depletion reaches some threshold values lipid peroxidation develops abruptly and in an extensive way. This event is accompanied by cellular death. The reason for which lipid peroxidation develops in a cell severely depleted of GSH remains to be clarified. Probably the loss of the defense systems against a constitutive oxidative stress is not compatible with cellular life. Some free radicals generated by one-electron reduction can react with oxygen to give superoxide anions which can be converted to other more dangerous reactive oxygen species. This is the case of paraquat and menadione. Damage to cellular macromolecules is due to the direct action of these oxygen radicals and, at least in the menadione-induced cytotoxicity, lipid peroxidation is not involved. All these initial events affect the protein sulfhydryl groups in the membranes. Since some protein thiols are essential components of the molecular arrangement responsible for the Ca2+ transport across cellular membranes, loss of such thiols can affect the calcium sequestration activity of subcellular compartments, that is the capacity of mitochondria and microsomes to regulate the cytosolic calcium level.(ABSTRACT TRUNCATED AT 400 WORDS)

Superoxide dismutase and alpha-tocopherol suppress the paraquat-induced elevation of N1-acetylspermidine and putrescine in primary culture of adult rat hepatocytes

A transient increase in N1-acetylpolyamines and putrescine (PUT) was observed in hepatocytes at the early stage of primary culture of rat hepatocytes. After pre-culture for 36 hr when the polyamine content returned to constant levels, we tested the effects of superoxide dismutase (SOD) and alpha-tocopherol on the paraquat-induced increase in N1-acetylspermidine (N1-acetyl-SPD) and PUT in the culture. Paraquat increased (N1-acetyl-SPD in a dose-dependent manner. It also increased PUT at doses between 0.1 and 1.3 mM. Both SOD and alpha-tocopherol suppressed the increase in N1-acetyl-SPD and PUT induced by paraquat. These results suggested that superoxide anion is one of the factors which increase N1-acetyl-SPD and PUT in hepatocytes. Lipopolysaccharide (LPS) little affected the polyamine concentration in the cultured hepatocytes, though it increases polyamine in mouse liver when given in vivo. These findings suggested that the formation of superoxide anion after administration of LPS in vivo is mediated by Kupffer cells.

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.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Human placental lipid peroxidation--II. NADPH and iron dependent stimulation of microsomal lipid peroxidation by paraquat

Paraquat, a widely used herbicide, was found to cause a marked stimulation of lipid peroxidation in the human placental microsomes in vitro. Both NADPH and chelated iron were necessary to observe paraquat-stimulated lipid peroxidation. The malondialdehyde accumulation in the incubation medium increased with increase in time, protein and paraquat concentration. The reaction did not exhibit the initial lag phase suggesting that endogenous membrane-bound antioxidants in human placental microsomes are either absent or present in extremely small quantities.

Alteration of the redox state of NADPH and glutathione in perfused rabbit lung by paraquat

The glutathione and NADP+ status of perfused rabbit lung was determined both before and after perfusion with 1 mM paraquat. The pulmonary glutathione redox state was similar to that of liver, having a glutathione/glutathione disulfide ratio of about 240. This ratio was lower in lungs perfused with glucose-free medium, a condition in which NADPH probably becomes limiting. Perfusion with paraquat significantly increased the pulmonary glutathione disulfide content, particularly in the absence of glucose, resulting in a glutathione/glutathione disulfide ratio of less than 100. NADP+ levels in rabbit lung were increased approximately two-fold by perfusion with paraquat in medium containing glucose and three-fold in the absence of glucose.

Toxicity of paraquat to microorganisms

The biochemical response of the microorganisms Lipomyces starkeyi (Lod & Rij), Escherichia coli K-12 W3110, Bacillus subtilis 168 (Marburg) and Pseudomonas sp. strain TTO1 to the presence of growth-inhibitory concentrations of paraquat was studied. Paraquat was added to each culture at a concentration previously determined to reduce the culture growth rate by up to 50%. The changes in activity of a number of enzymes previously shown to be associated with the defense of the mammalian system against the action of paraquat were studied. While the response of E. coli was in agreement with that found in other studies of this microorganism and supports a commonly accepted mechanism for paraquat toxicity, the results obtained with L. starkeyi, B. subtilis, and Pseudomonas sp. strain TTO1 suggest that other mechanisms exist for protection against the toxicity of paraquat.

Cytotoxic effects of paraquat and inhibition of them by vitamin E

Paraquat causes failure of multiple organs including the liver in humans. The kinetics and mechanism of paraquat intoxication were studied using cultured rat hepatocytes. Paraquat induced time- and dose-dependent lactate dehydrogenase release, lipid peroxidation, and cell death, estimated as decrease in protein in cells attached to culture dishes. However, the increase in lipid peroxidation occurred after lactate dehydrogenase release had reached a plateau. Vitamin E inhibited the inductions of all these cytotoxic effects of paraquat. Kinetic studies showed that lipid peroxidation was a better indicator of cell death than lactate dehydrogenase release, because vitamin E inhibited the induction of cell death even when added 6 h after paraquat, when lactate dehydrogenase release had reached a plateau but lipid peroxidation had not. The present results strongly suggest that paraquat exerts its cytotoxicity by a mechanism involving oxidation reactions.

Influence of lipid peroxidation on beta-adrenoceptors

The peroxidation of lipids in biological membranes is a destructive phenomenon that can be elicited in various ways. Surface receptor molecules that allow cells to respond to hormones are possibly inactivated during lipid peroxidation. Effects of lipid peroxidation on receptors have not been extensively examined thus far. This investigation shows that there is a decrease in beta-adrenoceptor density (measured as specific (-)-[125I]iodocyanopindolol binding) during lipid peroxidation, in both lungs and erythrocytes of the rat. To this end, lung membranes (containing both beta 1- and beta 2-adrenoceptors) and intact erythrocytes (containing a homogeneous beta 2-adrenoceptor population) were pretreated with cumene hydroperoxide (lung membranes with 0.1 mM and erythrocytes with 1 mM) and Fe2+ (1 X 10(-5) M) for 60 min which resulted in extensive lipid peroxidation measured as malondialdehyde formation. The ration beta 1-:beta 2-adrenoceptor density in lung membranes after treatment with cumene hydroperoxide did not change and remained at 30%:70%. A single injection (i.p.) with the herbicide paraquat (50 mg/kg, 24 h), which is known to cause lung damage via lipid peroxidation, resulted in similar alterations in receptor density to those caused by cumene hydroperoxide in the in vitro experiments.

Mitochondrial injury of pulmonary alveolar epithelial cells in acute paraquat intoxication

The primary ultrastructural changes in pulmonary alveolar epithelial cells are described in paraquat-injected rats. Within 6-12 hr a single intravenous injection of 40 mg/kg paraquat dichloride caused selective mitochondrial swelling and loss of intramitochondrial granules within 24 hr in alveolar Type II cells. As the mitochondrial injury advanced, microvilli disappeared from apical plasma membrane followed by a cell destruction and detachment from basement membrane. This was accompanied by secondary damage to Type II cells and interstitial cells. These results indicate that paraquat may affect primarily the Type II cells and the first lesion produced occurs in mitochondria.

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.

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.

Effect of silica and volcanic ash on the content of lung alveolar and tissue phospholipids

Silica or volcanic ash (VA) was administered to rats via intratracheal instillation and the changes in extracellular (i.e., lavage fluid) and tissue phospholipids, as well as various biochemical parameters, were monitored over a 6-month period. VA produced relatively minor (up to 2.8-fold) increases in lung tissue or lavage fluid phospholipids that were maximal at 1 month postinstillation. These increases were quantitatively similar to the increases in protein and DNA content of lung tissue and lavage fluid induced by VA and, thus, may be attributable to hypercellularity and accumulation of cellular breakdown products in the alveolar lumen. Instillation of silica produced a much greater (up to 11-fold) increase than VA in total phospholipid over time, primarily due to a 14-fold increase in phosphatidylcholine (PC). The accumulation of PC was more pronounced in the lavage fluid during the first month following silica instillation, but thereafter progressed more rapidly in the lung tissue. The relatively small increases (1.3- to 3.5-fold) in other phospholipids induced by silica appeared to be nonspecific, since they did not differ greatly from the increases in lung weight, DNA, and protein. Collectively, these results indicate that intratracheal instillation of silica induces selective accumulation of lung PC, implying enhanced synthesis and secretion of pulmonary surfactant from alveolar epithelial Type II cells into the lumen.

Effect of paraquat on cytochrome P-450-dependent lipid peroxidation in bovine adrenal cortex mitochondria

We have investigated the effect of paraquat (methyl viologen) on lipid peroxidation in bovine adrenal cortex mitochondria. Incubation of a buffered aerobic mixture of mitochondria in the presence of Fe2+ or NADPH resulted in the formation of lipid peroxides whose accumulation could be followed at 532 nm as malondialdehyde. Fe2+ stimulates lipid peroxidation in normal mitochondria and those in which enzymes have been inactivated with heat. In contrast, NADPH has a stimulatory effect only in normal mitochondria, but not in heat-treated mitochondria. These results indicate that NADPH-dependent lipid peroxidation is an enzymatic process. Paraquat strongly inhibits this enzymatic lipid peroxidation, but has no effect on the non-enzymatic Fe2+-dependent process. The chemiluminescence that accompanies the NADPH-dependent lipid peroxidation is also markedly decreased in the presence of paraquat. Superoxide dismutase, which removes superoxide anion efficiently, does not inhibit malondialdehyde production. The mechanism of the inhibition of the lipid peroxidation by paraquat has been examined. Paraquat has no effect on NADPH-2,6-dichlorophenolindophenol reductase and on NADPH-cytochrome c reductase activities in bovine adrenal cortex mitochondria. However, paraquat strongly inhibits the NADPH-dependent reduction of cytochrome P-450. These results suggest that the inhibitory effect of paraquat on NADPH-dependent lipid peroxidation in adrenal cortex mitochondria is due to a decrease in the level of reduced cytochrome P-450 probably by diverting electrons from cytochrome P-450. Cytochrome c, which can compete with P-450 for available electrons from adrenodoxin, like paraquat had an inhibitory effect on NADPH-dependent lipid peroxidation. Lipid peroxidation was also strongly inhibited by steroid hydroxylase inhibitors, e.g., amphenone B, aminoglutethimide and metyrapone.