Studies on the in vitro interaction of mitomycin C, nitrofurantoin and paraquat with pulmonary microsomes. Stimulation of reactive oxygen-dependent lipid peroxidation

Fanconi anemia (FA) and crosslinker sensitivity: Re-appraising the origins of FA definition

The commonly accepted definition of Fanconi anemia (FA) relying on DNA repair deficiency is submitted to a critical review starting from the early reports pointing to mitomycin C bioactivation and to the toxicity mechanisms of diepoxybutane and a group of nitrogen mustards causing DNA crosslinks in FA cells. A critical analysis of the literature prompts revisiting the FA phenotype and crosslinker sensitivity in terms of an oxidative stress (OS) background, redox-related anomalies of FA (FANC) proteins, and mitochondrial dysfunction. This re-appraisal of FA basic defect might lead to innovative approaches both in elucidating FA phenotypes and in clinical management.

Modulation of bile acids induced by paraquat in rabbits

Rabbit bile was examined for changes in composition induced by paraquat. Paraquat was administered intraperitoneally and changes in bile components were monitored by high performance liquid chromatography. Alterations in the ratios of total glycine/taurine conjugated bile acids (TGC/TTC), cholic acid/deoxycholic acid (CA/DC), cholic acid/chenodeoxycholic acid (CA/CDC) and cholic acid/cholesterol (CA/CH) were measured as an index of paraquat toxicity. A statistically significant increase in the ratio of TGC/TTC was observed, while CA/DC, CA/CDC and CA/CH showed a decrease. Phospholipids, protein, sugar, bilirubin, beta-carotene, vitamin A and vitamin E in the bile and serum of the experimental animals were also monitored. In bile, the levels of cholesterol, phospholipids, protein, sugar, and total bile acids increased while the levels of the antioxidants beta-carotene, vitamin A and vitamin E decreased. A decrease in the bilirubin content of the bile was also observed. These modifications may be useful clinically for assessment of paraquat toxicity.

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.

The cytotoxic effect of paraquat to isolated renal proximal tubular segments from rabbits

Paraquat (PQ) induces lung, liver and kidney damage. Since PQ mainly is eliminated by the kidney, the kidney damage is of particular importance to the outcome of PQ poisoning. The exact toxic mechanism of PQ is still unclear but it is assumed to involve redox cycling and formation of reactive oxygen species. In this study, further investigations on the toxic mechanism and metabolic effects of PQ were performed using isolated renal proximal tubules from rabbits. Proximal tubules were isolated using a combined iron perfusion and collagenase method. Suspended tubules were incubated for varying periods and concentrations of PQ at 25 or 37 degrees C in Krebs-Ringer phosphate buffer or HCO3-/CO2 buffer. The cytotoxic effect of PQ was evaluated by (1) markers of oxidative stress: status of glutathione (GSH/GSSG) and formation of malondialdehyde (MDA); and (2) markers of tubular metabolism: oxygen consumption (QO2), transport of 14C-p-aminohippuric acid (PAH) and 14C-tetraethylammonium (TEA). Using 0.5 and 5 mM PQ, the GSH/GSSG ratio decreased whereas formation of MDA increased indicating oxidative stress. PQ reduced the accumulation of PAH and TEA, the basal QO2 and the ouabain sensitive QO2 indicating inhibition of the Na/K-ATPase. Nystatin-stimulated QO2 was reduced by PQ, excluding inhibition of Na+ entry as a possible cytotoxic mechanism and suggesting mitochondrial injury. This was confirmed by measuring FCCP-uncoupled QO2. Thus high concentrations of PQ appear to disrupt mitochondrial electron chain transfer resulting in reduction of metabolic functions.

Nitrofurantoin-induced hepatic and pulmonary biochemical changes in mice fed different vitamin E doses

The hepatic and pulmonary effects of nitrofurantoin (40 mg/kg, intraperitoneally) were determined at 4 and 24 hr following its administration in mice fed for 10 weeks with a vitamin E sufficient, deficient or enriched diet. Liver glutathione (GSH) was reduced by nitrofurantoin at 4 hr but was unchanged 20 hr later. Nitrofurantoin did not affect liver glutathione peroxidase, glutathione reductase or superoxide dismutase activities. Liver catalase activities were decreased by nitrofurantoin at 4 hr. Lung GSH levels were increased whilst glutathione peroxidase activity was decreased at 4 and 24 hr. Lung glutathione reductase activity was reduced in certain groups. Nitrofurantoin did not affect lung superoxide dismutase, but catalase was decreased at 24 hr. Liver malondialdehyde levels were increased by nitrofurantoin in the vitamin E deficient group whilst lung malondialdehyde levels remained unchanged. Both liver and lung malondialdehyde levels were unaffected by vitamin E supplementation when compared to the vitamin E-sufficient group. These results suggest that nitrofurantoin (40 mg/kg) was deleterious to the liver and lung. Nitrofurantoin-induced lipid peroxidation was seen in vitamin E deficiency but an increase in dietary vitamin E content did not provide additional protection compared to the recommended daily allowance. The antioxidant activities of alpha-tocopherol and gamma-enriched tocotrienol were similar.

Anti-tumour activity of photodynamic therapy in combination with mitomycin C in nude mice with human colon adenocarcinoma

The interaction of photodynamic therapy (PDT) and a chemotherapeutic drug, mitomycin C (MMC), was investigated using WiDr human colon adenocarcinoma tumours implanted on Balb/c athymic nude mice. The WiDr tumours were treated with PDT alone, MMC alone or with both. It was found that the combined treatment produced a greater retardation in the growth of the WiDr tumour than monotherapy with MMC or PDT. The synergistic effect was especially prominent when PDT was used in combination with a low dose of MMC (1 mg kg-1), since treatment of 1 mg kg-1 MMC alone had no effect on the tumour. The anti-tumour activity of PDT was found to be increased with MMC of 5 mg kg-1. The response of normal skin on mice feet to PDT slightly greater when PDT was combined with 5 mg kg-1 MMC than when PDT was applied alone, while no detectable additional effect on skin photosensitivity was observed when PDT was combined with 1 mg kg-1 MMC. An enhanced uptake of Photofrin in tumours was found 12 h and 24 h after administration of MMC. The effect of MMC on the cell cycle distribution of cell dissociated directly from the tumours was studied. The results suggest that the increased susceptibility to photoinactivation of Photofrin-sensitised tumours may be due to MMC-induced accumulation of the tumour cells in S-phase.

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.

Comparative studies of sheep lung and liver nitrofurantoin reductase

1. Nitrofurantoin reductase which catalyzes the bioactivation of nitrofurantoin was purified to electrophoretic homogenity from sheep liver and lung microsomes, with a yield of 15% and 35%, respectively. The specific activity of both reductases was found to be similar (140 nmol/min/mg protein). 2. The effects of nitrofurantoin and NADPH concentrations, pH, ionic strength, amount of enzyme and reaction period, on the enzyme activity were studied and the optimum conditions for maximum activity of purified liver and lung nitrofurantoin reductases were determined. 3. The enzyme concentration was found proportional with the square root of the rate of nitrofuratoin reduction up to approximately 15 micrograms protein/ml and 25 micrograms protein/ml incubation mixture for liver and lung nitrofurantoin reductases, respectively. 4. The plots of inverse of the nitrofurantoin concentration against the inverse of the square root of the velocity for the reduction of nitrofurantoin by liver and lung enzymes gave Km values as 27.78 microM and 32.25 microM, respectively. 5. The purified liver and lung enzymes were also saturated by NADPH at similar concentrations and the Km values were calculated as 29.4 microM and 35.5 microM, respectively. 6. The effects of magnesium, nickel, cadmium and copper ions on the nitrofurantoin reductase activity were examined. Magnesium ion was found to have almost no effect, whereas the other ions inhibited the activity of both liver and lung reductases.

Inhibitory effects of alpha- and beta-carotene on croton oil-induced or enzymatic lipid peroxidation and hydroperoxide production in mouse skin epidermis

1. The effects of carotenes (alpha- and beta-) on edema, MDA contents and peroxidizability of croton oil-treated mouse skin epidermis, hydroperoxide production and enzymatic lipid peroxidation of epidermal homogenates were studied. Edema was determined as ear punch weight and the intensity of lipid peroxidation was measured using malondialdehyde formation. 2. Carotenes (alpha- and beta-) significantly suppressed edema formation, hydroperoxide production, lipid peroxidation caused by croton oil, Fe + 3-ADP/NADPH or paraquat/NADPH in vivo as well as in vitro. 3. These results indicate that both alpha- and beta-carotene have chemopreventive effects on croton oil-induced tumor promotion in skin tumorigenesis by scavenging oxygen free radicals, indirectly determined as carotene inhibition of lipid peroxidation and hydroperoxide formation.

Effect of mitomycin C on the uptake of photofrin II in a human colon adenocarcinoma cell line

Flow cytometry (FCM) was used to investigate the effect of mitomycin C (MC) on the cellular uptake of Photofrin II (PII) in a cultured human colon adenocarcinoma cell line (WiDr). The surface area of the cells increased as they passed through the cell cycle from G0/G1 to G2/M phase. MC retarded the cells in G2/M phase and enhanced the surface area of the cells. A 1.3-2.3-fold increase in the cell surface area and a 1.3-2.7-fold increase in the cellular uptake of PII in the tumor cells was observed after 2 h-8 h incubation with MC. Within each sample, an almost linear relationship between the intensity of PII fluorescence in the cells and the surface area of the cells was found. However, for the cells incubated with MC the surface area was not the only determinant of PII uptake. Effects of MC on the cell cycle, the cell surface area and the permeability of the cell membrane are suggested as possible reasons for the increase of cellular uptake of PII in the tumor cells.

Enzyme mediated superoxide radical formation initiated by exogenous molecules in rat brain preparations

The ability of brain tissue preparation to generate superoxide from xenobiotic interactions has been investigated. We showed that a significant superoxide production occurred with different molecules known to undergo a single electron reductive pathway of metabolism, both in a homogenate derived from neuronal and glial cells and in isolated cerebral microvessels which form the blood-brain barrier. Determination of the nucleotide cofactors requirement and data obtained with different subcellular fractions indicated that this production was largely associated with the microsomal fraction in a NADPH-dependent pathway and was probably mediated by NADPH-cytochrome P450 (c) reductase. A significant xenobiotic-mediated production of superoxide also occurred in mitochondria under in vitro conditions. Thus the evidence of reductive pathways of xenobiotic metabolism and the generation of oxygenated free radicals observed are of neurotoxicological significance.

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.

Mechanism of ochratoxin A stimulated lipid peroxidation

Lipid peroxidation, measured as malondialdehyde formation or by oxygen uptake, was stimulated markedly by the mycotoxin ochratoxin A (OTA) in a reconstituted system consisting of phospholipid vesicles, the flavoprotein NADPH-cytochrome P450 reductase, Fe3+, EDTA and NADPH. Deletion of EDTA lowered the extent of lipid peroxidation but did not eliminate it. Fluorometric and spectrophotometric studies demonstrated the formation of a 1:1 Fe3(+)-OTA complex. The rate of reduction of Fe3+ to Fe2+ was enhanced markedly in the presence of OTA, and there was a further increase in the rate when EDTA was also included. The data indicate that OTA stimulates lipid peroxidation by complexing Fe3+ and facilitating its reduction. Subsequent to oxygen binding, an iron-oxygen complex of undetermined nature initiates lipid peroxidation. Free hydroxyl radicals appear not to participate in lipid peroxidation stimulated by Fe3(+)-OTA.

Metabolism and pulmonary toxicity of cyclophosphamide

Pulmonary toxicity caused by an antineoplastic drug, cyclophosphamide is becoming a more frequently recognized entity. Metabolism of cyclophosphamide in lung to alkylating metabolites and acrolein, a reactive aldehyde are in part responsible for pulmonary toxicity. Alterations in pulmonary mixed-function oxidase activity, glutathione content, and microsomal lipid peroxidation may be caused by the reactive metabolite acrolein. Potentiation of cyclophosphamide-induced pulmonary injury under hyperoxic conditions is caused by depression of pulmonary antioxidant defense mechanisms by cyclophosphamide and its other metabolites but not acrolein. Cyclophosphamide- and acrolein-induced alterations in the physical state of membrane lipid bilayer may be the major cause of inactivation of membrane-bound enzymes. These data suggest that cyclophosphamide and its reactive metabolites initiate peroxidative injury resulting in alterations in the physical state of membrane lipids which may be functionally linked to manifestations of cyclophosphamide-induced pulmonary toxicity.

Pulmonary toxicity of cytotoxic and immunosuppressive agents. A review

Cytotoxic agents may cause interstitial or eosinophilic pneumonitis, alveolar proteinosis, pulmonary venous occlusive disease, pulmonary fibrosis, pneumothorax, or pulmonary oedema. These agents may also potentiate lung injury caused by radiotherapy or high oxygen fractions in inspired air. Clinical and roentgenological features of lung damage induced by cytotoxic drugs are usually non-specific, and differential diagnoses include progression of the malignant disease and a plethora of opportunistic infections. Monitoring of blood gases and carbon monoxide transfer factor may facilitate early detection of drug induced lung injury. Fiberoptic bronchoscopy, bronchoalveolar lavage, transbronchial biopsy, or open lung biopsy may be necessary for reliable diagnosis. Early detection of lung damage and immediate withdrawal of the responsible agent(s) are essential. Steroids may be of therapeutic value in some patients.

Inhibition of rat liver microsomal lipid peroxidation by N-acyldehydroalanines: an in vitro comparative study

Captodative substituted olefins are radical scavengers which react with free radicals to form stabilized radical adducts. One of those compounds, N-(paramethoxyphenylacetyl)dehydroalanine (AD-5), may react and scavenge both superoxide anion (O-2) and alk-oxyl radicals (RO.), and in this way prevent the appearance of their mediated biological effects. Nitrofurantoin and tert-butyl hydroperoxide were used as model compounds to stimulate free radical production and their mediated lipid peroxidation in rat liver microsomes. In addition, lipid peroxidation was also initiated by exposure of rat liver microsomal suspensions to ionizing radiation (gamma rays). The microsomal lipid peroxidation induced by these chemicals and physical agents was inhibited by the addition of AD-5. These effects were dose-dependent in a millimolar range of concentration. In addition, AD-5 has no effect on microsomal electron transport, showing that NADPH-cytochrome P450 reductase activity was not modified. These data, together with the comparisons of the effects of AD-5 and some antioxidant molecules such as superoxide dismutase, uric acid, and mannitol, support the conclusion that inhibition of lipid peroxidation by AD-5 is the result of its free radical scavenger activity. In addition, the inhibitory effect of AD-5 on microsomal lipid peroxidation was dependent of the nature of the free radical species involved in the initiation of the process, suggesting that O-2 is scavenged more efficiently than RO.

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.

Free radical formation by antitumor quinones

Quinones are among the most frequently used drugs to treat human cancer. All of the antitumor quinones can undergo reversible enzymatic reduction and oxidation, and form semiquinone and oxygen radicals. For several antitumor quinones enzymatic reduction also leads to formation of alkylating species but whether this involves reduction to the semiquinone or the hydroquinone is not always clear. The antitumor activity of quinones is frequently linked to DNA damage caused by alkylating species or oxygen radicals. Some other effects of the antitumor quinones, such as cardiotoxicity and skin toxicity, may also be related to oxygen radical formation. The evidence for a relationship between radical formation and the biological activity of the antitumor quinones is evaluated.

Passive sequestration of putrescine, spermidine and spermine by rat lungs

The pulmonary uptake and accumulation of the three polyamines putrescine, spermidine and spermine by isolated ventilated and perfused rat lungs was investigated using 0.1, 1 or 5 mM concentrations of these compounds. The lung uptake of putrescine for all concentrations was greater than that of spermidine and spermine, but all three showed concentration-dependent linear uptake. A significant uptake of all three polyamines was also observed when incubated separately with rat lung slices for 60 min. Harmaline (0.4 mM), ouabain (0.2 mM) and perfusate with decreased Na+ (50 mEq/l) did not affect the uptake of any of the three polyamines by isolated perfused rat lungs or rat lung slice incubations. HPLC analysis of the whole lung or slices and media after perfusion or incubation studies, respectively, with polyamines did not reveal the presence of any metabolites. Likewise, the analysis of the lung homogenate incubated at 37 degrees C for 60 min with polyamines did not show any metabolites, confirming the absence of detectable pulmonary metabolism. These findings indicate a significant accumulation of polyamines in the rat lungs, accumulation predominantly occurring via simple diffusion, at variance with the reported active polyamine uptake process in the lung.

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.

Stimulation of cyclophosphamide-induced pulmonary microsomal lipid peroxidation by oxygen

Cyclophosphamide (CP) causes lung toxicity in a wide variety of animals including humans. Recent reports suggest that CP increases lipid peroxide formation in the lung, and that oxygen (O2) potentiates CP-induced lung toxicity. We hypothesized that CP, or one of its toxic metabolites, acrolein, stimulates lung lipid peroxide formation in the presence of high O2 tensions. To test this, rat lung microsomes were treated in vitro with CP or acrolein in the presence of NADPH and 0-100% O2 with and without superoxide dismutase (SOD), glutathione (GSH), dithiothreitol (DTT), and EDTA (agents which scavenge reactive O2 species and/or detoxify reactive metabolites). Lipid peroxide formation in untreated microsomes was increased 40, 39, and 37% in 60, 80 and 100% O2 respectively (P less than 0.02 vs. 21% O2 air). Lipid peroxide formation in microsomes treated with CP increased 2-3-fold under 21% O2 (P less than 0.05 vs. untreated under 21% O2). However, increases in lipid peroxide formation were 3-4 fold in CP treated microsomes under 40-100% O2 (P less than 0.001 vs. untreated at same % O2). CP and acrolein-stimulated lipid peroxidation with and without O2 exposure was significantly (P less than 0.05) reduced by prior addition of SOD, GSH, DTT, or EDTA to the lung microsomal suspension. These results indicate that lipid peroxide formation increases in CP and acrolein-treated lung microsomes, and high O2 tensions stimulate CP-induced lipid peroxidation. Stimulation of CP-induced microsomal lipid peroxidation appears to be mediated by reactive O2 species or metabolites.

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.

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.

Reductive metabolism of nitrofurantoin by rat lung and liver in vitro

In the present study, the metabolism of NF has been examined in detail in both rat lung and liver 9000 g supernatants using a specific radiometric HPLC assay. Over 92% of the total radioactivity chromatographed with authentic NF after incubations from either organ were carried out under oxygen for 60 min. Under anaerobic conditions, only 19% and 5% of the total unbound radioactivity corresponded to unchanged NF in lung and liver respectively. At least 4 metabolites were evident from the HPLC trace (M1, M2, M3, M4 according to increasing retention times). In the absence of oxygen, liver 9000 g supernatants generated 65% more M1 and 260% more M3 than did lung 9000 g supernatants, but the lung produced significantly more M4. Covalent binding to tissue macromolecules was similar in both tissues under oxygen but was 7 times greater in lung than in liver in the absence of oxygen (compared per unit protein). Neither piperonyl butoxide nor indomethacin affected NF metabolism. However, allopurinol almost completely inhibited the anaerobic and aerobic (superoxide generation measured by the rate of acetylated cytochrome c reduction) metabolism in the lung with little or no effect in the liver. The data indicate a quantitative difference in NF metabolism between the two tissues that may be related to the organ-selective toxicity of the drug.

Enhanced production of ethylene from methional by iron chelates and heme containing proteins in the system consisting of quinone compounds and NADPH-cytochrome P-450 reductase

The addition of iron chelates or heme containing proteins to the systems consisting of NADPH-cytochrome P-450 reductase and quinone compounds, such as vitamin K3 (menadione), adriamycin, tetrahydropyranyladriamycin, daunomycin, aclacinomycin A, carbazilquinone, and mitomycin C, showed the enhanced production of ethylene from methional. In the vitamin K3 system, the effective iron chlates were Fe(II)-EDTA, Fe(II)-ADP, Fe(II)-bleomycin A2, and hemin, and the effective iron containing proteins were methemoglobin, myoglobin, ferredoxin, and partially purified cytochromes P-450, P-420, and b5, and the reversed effects were observed by horse radish peroxidase and sulfite reductase from yeast. In the system consisting of aclacinomycin A and methemoglobin, the ethylene production was potently inhibited by radical scavengers, such as Tiron, Tris, thiourea, and KI, and weakly inhibited by some other scavengers. In the system containing vitamin K3 and methemoglobin, the ethylene production was potently inhibited by catalase, but partially by superoxide dismutase, KCN, and NaN3. In this system, the absorption spectrum of methemoglobin was immediately changed to oxyform and quenched with time, and catalase protected the decrement of the spectrum. The addition of hydrogen peroxide or cumene hydroperoxide to methemoglobin also produced ethylene from methional.

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.

Cyclophosphamide-induced depression of the antioxidant defense mechanisms of the lung

Cyclophosphamide causes lung toxicity in a wide variety of animals, including humans. Recent evidence suggests that oxygen (O2) potentiates cyclophosphamide-induced pulmonary injury. We hypothesized that cyclophosphamide or one of its toxic metabolites, acrolein, may potentiate O2 toxicity by depressing lung antioxidant defense mechanisms. To test this, we gave rats cyclophosphamide (100 mg/kg), acrolein (5 mg/kg), or a vehicle (control) in a single intraperitoneal injection and then killed them during a 5-day study period. Excised lungs were analyzed for reduced glutathione (GSH) content, glucose-6-phosphate dehydrogenase (G6PD), glutathione reductase (GSH-R), glutathione peroxidase (GSH-P), and superoxide dismutase (SOD) activities. In the lungs of cyclophosphamide-treated rats, GSH content was increased 48% (P less than 0.001) on day 2 but progressively decreased to 50% of control values (P less than 0.001) on day 5. Significant reductions (P less than 0.005) in G6PD, GSH-R, and GSH-P activities occurred on days 1-5, and SOD activity was significantly decreased (P less than 0.005) on days 4 and 5 by cyclophosphamide. In acrolein-treated rats, GSH content and GSH-R, GSH-P, and SOD activities were indistinguishable from those in controls. However, G6PD was increased (35-38%) on days 2 and 3 but returned to control values thereafter. To assess whether the cyclophosphamide-induced reduction in lung antioxidant defenses increased susceptibility to acute O2 toxicity, we gave a separate group of rats cyclophosphamide, acrolein, or vehicle, and 4 days later exposed them to 100% O2 or air at 1 atmosphere absolute. All cyclophosphamide-, acrolein-, and vehicle-treated rats survived 60 h air exposure, and all vehicle-treated rats exposed to 100% O2 survived. In contrast, all of the cyclophosphamide-treated rats exposed to 100% O2 died (P less than 0.05) within 40 h. Acrolein had no effect on survival in 100% O2. These results indicate that cyclophosphamide, but not acrolein, depresses lung antioxidant defense mechanisms, which may be responsible for increased mortality from O2 toxicity in cyclophosphamide-treated animals.

Oxidant-mediated electronic excitation of imipramine

The interaction of imipramine with both resting and zymosan-activated human polymorphonuclear leukocytes (PMNs) resulted in the generation of chemiluminescence (CL). This CL was not accompanied, however, by an enhanced release of superoxide anion. CL was also observed following the interaction of imipramine with either a xanthine oxidase or a horseradish peroxidase catalyzed system. Collectively, these observations support the concept that the CL elicited from these interactions is reflective of the electronic excitation of the imipramine molecule. In contrast to the response seen with PMNs, addition of imipramine to resting alveolar macrophages (AMs) failed to yield CL. However, CL from imipramine was observed with resting AMs upon supplementation with exogenous horseradish peroxidase. The lack of response with control AMs and the significant inhibition of the imipramine-PMN CL by the myeloperoxidase inhibitor azide suggests that a peroxidase-derived oxidant facilitated the oxidation of imipramine, yielding a product in an electronically excited state. In addition to PMNs, CL was elicited from imipramine by rat or rabbit liver microsomes, suggesting that PMNs may be a useful model system to predict a xenobiotic effect on the CL response elicited by other cellular oxidant-generating systems. Moreover, these observations underscore the possibility that the metabolic activation of drugs by PMNs may be of pharmacologic and toxicologic importance.

Glutathione status of isolated rabbit lungs. Effects of nitrofurantoin and paraquat perfusion with normoxic and hyperoxic ventilation

Thirty-minute perfusion of isolated rabbit lungs with a Krebs-Ringer bicarbonate buffer containing 420 microM paraquat (PQ) or nitrofurantoin (NF) resulted in increases in lung oxidized glutathione (GSSG) content of 589 and 2656%, respectively, over control levels. The degree of glutathione efflux was also increased with both agents, i.e. 77 and 238% above control leakage for PQ and NF respectively. The pulmonary toxicity of both compounds is known to be heightened by conditions of hyperoxia(O2). Ventilation of lungs with 95% O2-5% CO2 did not, in itself, significantly alter glutathione efflux, GSH or GSSG levels. However, ventilation with 95% O2-5% CO2 increased lung GSSG levels in PQ-perfused lungs 225% over PQ-air-perfused lungs, a combined effect not observed with NF-O2, wherein mean GSSG levels were only 72% of that observed with NF-air. Glutathione efflux in PQ-O2-treated lungs declined somewhat (20%) compared to that observed with PQ-air, but a significant increase in the amount of glutathione efflux was seen with NF-O2-treated lungs, i.e. 120 and 310%, respectively, over that attributable to NF or O2 alone. Although the biochemical mechanisms of toxicity of these compounds are thought to be very similar, the disparate degree of GSH oxidation observed with equimolar levels of PQ and NF may indicate differences in reactivity towards glutathione and other lung sulfhydryl pools. The stimulation of the oxidative effects of PQ and NF on lung GSH due to hyperoxic ventilation may be related to the reported O2 enhancement of their toxicity.

Cocaine-induced hepatotoxicity: lipid peroxidation as a possible mechanism

In vitro experiments with hepatic washed microsomal preparations showed that malondialdehyde (MDA) formation was increased in a time- and concentration-dependent manner using COC or NC as the substrate. Though 1 mM COC or NC inhibited MDA formation, significant elevations were observed for 100, 10 or 1 microM concentrations. NC at 10 microM after a 30 minute incubation produced a 34% decrease in hepatic microsomal cytochrome P450 whereas 1 mM NC had no such effect. MDA formation in vivo, measured as total absorbance at 535 nm per gram liver, was found to be maximal 4 hours after 40 mg/kg NC ip. Elevations of serum transaminase (SGPT) however were not found until 6 hours after NC. We conclude from these studies that COC and NC induce lipid peroxidation in the liver of PB-pretreated Swiss-origin mice and that peroxidative attack may be a mechanism for hepatotoxicity of these compounds.

Studies on the interaction of bleomycin A2 with rat lung microsomes. III. Effect of exogenous iron on bleomycin-mediated DNA chain breakage

The interaction of bleomycin A2 with rat lung microsomes results in bleomycin-mediated DNA chain breakage due to the mixed-function oxidase catalyzed activation of bleomycin. This study demonstrates that the addition of exogenous Fe3+ significantly enhances the bleomycin-mediated cleavage of DNA deoxyribose, that the enhancing effect of Fe3+ is maximum when a 1:1 ratio of bleomycin to Fe3+ is achieved and that either NADPH or NADH can serve as pyridine cofactors for this reaction. Since the activation of bleomycin can be facilitated by iron in the Fe2+ form, these observations support the hypothesis that the mixed-function oxidase system may serve to maintain either adventitious or exogenous iron in the Fe2+ form. In the absence of DNA, the interaction of bleomycin with rat lung microsomes results in the self-inactivation of bleomycin, a reaction which is also enhanced by the addition of exogenous Fe3+. Thus, the microsomal mixed-function oxidase system represents an efficient biological system for the 'activation-inactivation' of bleomycin.

Activities of free radical metabolizing enzymes in tumours

The activity of enzymatic defences against free radical attack including superoxide dismutase (SOD), catalase, glutathione peroxidase and glutathione reductase have been compared in some experimental animal tumours with the corresponding normal mouse tissues. The activity of SOD in PC6 plasmacytoma and P388 lymphocytic leukaemia was lower than in normal lymphocytes and the activity in a mouse bladder carcinoma (MB) was one-half of that of the normal bladder tissue. Similarly PC6, P388, TLX5 lymphoma and MB showed lower catalase activity than the corresponding normal tissues. The activity of glutathione peroxidase in tumours was in general comparable with that of the normal tissues with the exception of MB, while TLX5, PC6 and P388 contained much lower glutathione reductase activity than normal lymphocytes. The results suggest that it may be possible to selectively destroy certain tumours by peroxidative attack, and that P388 leukaemia would be much more sensitive than L1210 leukaemia to free radical production.

In vitro inhibition of misonidazole toxicity and melphalan chemopotentiation by metyrapone

Overnight exposure of Chinese hamster cells, V-79-753B, to 10(-3)M metyrapone protected them against the hypoxiamediated toxicity of 10(-2)M misonidazole. This protection was accompanied by an increase in radiation resistance. There was no appreciable change in the oxygen-enhancement ratio, nor in the amount of sensitisation produced by 10(-3)M misonidazole. Treatment of cells with metyrapone (10(-3)M) or dexamethasone (1 microgram ml-1 [approximately 2 X 10(-6)M]) prior to exposure first to 5 X 10(-3)M misonidazole in hypoxia and then to melphalan in air, substantially decreased the amount of chemopotentiation produced by the sensitiser, although the toxicity of melphalan alone was not affected in cells treated with either compound. Cells pretreated with either metyrapone or dexamethasone had 2-3 times more glutathione than control cells. This increase in GSH could not explain the change in radiation response, since cells pretreated with 5 X 10(-5)M flurbiprofen, a non-steroidal anti-inflammatory agent, had similarly high GSH levels, but their radiation response is similar to that of untreated cells (Millar et al, 1981). Neither dexamethasone nor flurbiprofen affected cell growth, whilst metyrapone markedly decreased the growth of cells. The results are discussed in terms of possible mechanism(s).

Mechanisms of superoxide radical-mediated toxicity

Free radicals generated from metabolism of foreign compounds can have extremely detrimental consequences on cell function and survival. Due to their high reactivity, free radicals may potentially perturb a wide spectrum of important cellular macromolecules such as nucleic acids, proteins, lipids and polysaccharides. Recently, the toxicity of several xenobiotics has been suggested to be mediated by formation of free radicals derived from reduction of molecular oxygen, forming superoxide anion (O(2)) and hydroxyl radical (OH .). For example, the pulmonary toxicity of the bipyridylium herbicide paraquat has been attributed to an enzymatically catalyzed one-electron redox cycling of the parent molecule, resulting in generation of O(2). Examples of other compounds that are subject to redox cycling with associated O(2) formation are those agents containing quinone or aromatic nitro structural elements. An important aspect of free-radical-mediated toxicity is that it is moderated by several cellular defense mechanisms including superoxide dismutase, catalase, glutathione peroxidase, vitamin E and reduced glutathione. Thus, toxicity mediated by free radical generation may not occur unless defense mechanisms are overwhelmed by radical production.