Enhanced lung toxicity of paraquat in selenium-deficient rats

Neonatal Selenium Deficiency Decreases Selenoproteins in the Lung and Impairs Pulmonary Alveolar Development

Decreased selenium (Se) levels during childhood and infancy are associated with worse respiratory health. Se is biologically active after incorporation into Se-containing antioxidant enzymes (AOE) and proteins. It is unknown how decreased maternal Se during pregnancy and lactation impacts neonatal pulmonary selenoproteins, growth, and lung development. Using a model of neonatal Se deficiency that limits Se intake to the dam during pregnancy and lactation, we evaluated which neonatal pulmonary selenoproteins are decreased in both the saccular (postnatal day 0, P0) and early alveolar (postnatal day 7, P7) stages of lung development. We found that Se deficient (SeD) pups weigh less and exhibit impaired alveolar development compared to Se sufficient (SeS) pups at P7. The activity levels of glutathione peroxidase (GPx) and thioredoxin reductase (Txnrd) were decreased at P0 and P7 in SeD lungs compared to SeS lungs. Protein content of GPx1, GPx3 and Txnrd1 were decreased in SeD lungs at P0 and P7, whereas Txnrd2 content was unaltered compared to SeS controls. The expression of NRF-2 dependent genes and several non-Se containing AOE were similar between SeS and SeD lungs. SeD lungs exhibited a decrease in selenoprotein N, an endoplasmic reticulum protein implicated in alveolar development, at both time points. We conclude that exposure to Se deficiency during pregnancy and lactation impairs weight gain and lung growth in offspring. Our data identify multiple selenoproteins in the neonatal lung that are vulnerable to decreased Se intake, which may impact oxidative stress and cell signaling under physiologic conditions as well as after oxidative stressors.

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

Biochemical changes in mouse lung after subcutaneous injection of the sulfur mustard 2-chloroethyl 4-chlorobutyl sulfide

Sulfur mustard (HD) is a vesicant-type chemical warfare agent (CWA) introduced in World War I which continues to be produced, stockpiled, and occasionally deployed by some countries, and could be used potentially by terrorists. Exposure to HD can cause erythema, blisters, corneal opacity, and airway damage. We have reported previously that subcutaneous (SC) injection of immunodeficient athymic nude mice with the half mustard butyl 2-chloroethyl sulfide (BCS) causes systemic biochemical changes in several organs distal to the exposure site. In the present study, we examined the response of non-immunodeficient Swiss Webster mice to the mustard, 2-chloroethyl 4-chlorobutyl sulfide (CECBS). In a pilot study, we found that a single SC injection of 20-25 microl/mouse causes death within 24h. Consequently, we used 5 microl/mouse (approx. 0.017 mg/kg body weight) of neat CECBS or an equal volume of saline as control. We examined the lungs after 1, 24, and 48 h for biochemical changes including total and oxidized glutathione, protein, DNA, and lipid peroxidation contents in tissue homogenate, and superoxide dismutase, catalase, glucose-6-phosphate dehydrogenase, and glutathione S-transferases activities in the cytosol. After 1h and/or 24h, we found statistically significant changes that were resolved by 48 h. These changes mimicked those of HD and BCS and were generally consistent with free radical-mediated oxidative stress. The implications of these observations are two-fold. First, dermal exposure to low-dose mustard gas could elicit systemic changes impacting distal organs such as the lungs. It also suggests that antioxidants could potentially modulate the response and reduce the damage. Second, although the use of known CWAs such as HD is prohibited, analogs that are not recognized as agents are as toxic and could be dangerous if acquired and used by potential terrorists.

Lung function and airway inflammation in rats following exposure to combustion products of carbon-graphite/epoxy composite material: comparison to a rodent model of acute lung injury

Pulmonary function and inflammation in the lungs of rodents exposed by inhalation to carbon/graphite/epoxy advanced composite material (ACM) combustion products were compared to that of a rodent model of acute lung injury (ALI) produced by pneumotoxic paraquat dichloride. This investigation was undertaken to determine if short-term exposure to ACM smoke induces ALI; and to determine if smoke-related responses were similar to the pathogenic mechanisms of a model of lung vascular injury. We examined the time-course for mechanical lung function, infiltration of inflammatory cells into the lung, and the expression of three inflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha), macrophage inflammatory protein-2 (MIP-2) and interferon-gamma (IFN-gamma). Male Fischer-344 rats were either exposed to 26.8-29.8 g/m(3) nominal concentrations of smoke or were given i.p. injections of paraquat dichloride. Measurements were determined at 1, 2, 3, and 7 days post exposure. In the smoke-challenged rats, there were no changes in lung function indicative of ALI throughout the 7-day observation period, despite the acute lethality of the smoke atmosphere. However, the animals showed signs of pulmonary inflammation. The expression of TNF-alpha was significantly increased in the lavage fluid 1 day following exposure, which preceded the maximum leukocyte infiltration. MIP-2 levels were significantly increased in lavage fluid at days 2, 3, and 7. This followed the leukocyte infiltration. IFN-gamma was significantly increased in the lung tissue at day 7, which occurred during the resolution of the inflammatory response. The paraquat, which was also lethal to a small percentage of the animals, caused several physiologic changes characteristic of ALI, including significant decreases in lung compliance, lung volumes/capacities, distribution of ventilation, and gas exchange capacity. The expression of TNF-alpha and MIP-2 increased significantly in the lung tissue as well as in the lavage fluid. Increased MIP-2 levels also preceded the maximum neutrophil infiltration. The differences in the time-course and primary site of TNF-alpha, MIP-2, and IFN-gamma expression; and the differences in the temporal relationship between their expression and infiltration of inflammatory cells may have accounted for the differences in lung function between paraquat treated and ACM smoke exposed animals.

Enhanced resistance of adriamycin-treated MCR-5 lung fibroblasts by increased intracellular glutathione peroxidase and extracellular antioxidants

Considerable evidence indicates that reactive oxygen species play an etiological role in both cardiotoxicity and the skin necrosis induced by adriamycin (ADM). An increase in glutathione peroxidase activity on addition of selenium to cultured MCR-5 lung fibroblasts was observed; this increase was accompanied by enhanced cellular resistance to ADM toxicity. Moreover, the presence of exogenous antioxidant systems, such as superoxide dismutase, catalase, vitamin E, dimethylsulfoxide, and desferroxamine, an iron chelating agent, resulted in significant protection from ADM-mediated damage.

Metabolic changes in the mouse kidney after subcutaneous injection of butyl 2-chloroethyl sulfide

Subcutaneous exposure to vesicants such as butyl 2-chloroethyl sulfide (butyl mustard, BCS) produces local tissue injury (vesication) primarily by alkylation and cross-linking of the purine nucleotides and rapidly binding to proteins. We recently reported that administering BCS can cause other biochemical and morphological alterations, not only in tissues at the injection site but in other areas as well. In this study, we have examined the metabolic effects of BCS administration on the mouse kidney. At 1, 24, and 48 h after injection (5 microliters neat, sc), treated mice were terminated along with an untreated control group, and the kidneys were analyzed for metabolic changes. Glutathione (GSH) peroxidase (GPx) activity markedly increased, (+78 and +85%), but NADP-dependent isocitrate dehydrogenase activity decreased (-43 and -37%) at 1 and 24 h, respectively. Glucose-6-phosphate dehydrogenase (G6PD) remained unchanged at 1 and 24 h, but increased 20% (p less than .05) at 48 h after injection. Kidney glutathione S-transferase (GST) was increased at 24 h after injection. Both total and oxidized GSH levels were significantly lower than control values (approximately 30%) at all time points. Lipid peroxidation, as estimated by the thiobarbituric (TBA) acid-reactive products, was 45% lower (p less than .05) after 1 h. Kidney GPx, G6PD, and GT activities and kidney GSH levels were consistent with changes associated with oxidative stress or detoxication mechanism for BCS. The decrease in TBA-reactive products suggests that mouse kidney metabolic response to BCS injection was different from responses observed for other organs (eyes, brain, and lung).

Correlation between lipid peroxidation and morphological manifestation of paraquat-induced lung injury in rats

Biochemical and morphological studies of rat lung were performed to determine the role of lipid peroxidation in the in vivo lung toxicity of paraquat. Two injections of 20 mg/kg paraquate were administered intraperitoneally every other day. While notable epithelial damage in the lungs was observed on the day after the second paraquat injection and progressed through the 5th day, the concentration of lipid peroxides in the rat lungs did not increase by the 3rd day after the injection. The lipid peroxide concentrations increased after the 5th day post-injection, and reached the maximum concentrations on the 7th day, when the damaged alveolar surface had been mostly repaired by regenerative pneumocytes. On the other hand, the delayed increase of lung lipid peroxides in paraquat-treated rats paralleled the increased number of macrophages in the lung, which reached maximum numbers on the 7th day. Glutathione peroxidase activity in the lungs also increased with a similar time course. Macrophages from the lungs contained a large amount of engulfed degradation products and cellular debris, and immunohistochemical study showed high glutathione peroxidase content on the 5th and 7th days. These results suggest that lipid peroxidation is a relatively late event in the in vivo paraquat-treated lung and that the delayed increase of lipid peroxides in the lungs occurs from the phagocytic activities of macrophages rather than from toxic cell injury.

A highly sensitive gas chromatographic method does not detect exhalation of volatile hydrocarbons from isolated ventilated lungs under massive peroxidative stress

Lung lipid peroxidation is thought to be a basic pathophysiological phenomenon responsible for pulmonary damage in different types of oxidant attack. Measurement of volatile hydrocarbons, especially ethane and pentane, produced during peroxidative degradation of polyunsaturated fatty acids and exhaled into an animal housing chamber, has attracted increasing interest for the monitoring of in vivo lipid peroxidation. However, this approach cannot distinguish between pulmonary exhalation of hydrocarbons generated in different organs or even the intestinum and pulmonary generation of these lipid peroxidation markers. In the present study we developed a respiration and hydrocarbon trapping system for isolated, ventilated, and perfused lungs that avoided rebreathing and allowed complete sampling and gas chromatographic separation and quantification of exhaled alkanes and alkenes (C1-C5) in the absence of background levels. Using an "artificial lung," the recoveries of exogenously administered hydrocarbons ranged between 80 and 95% with good reproducibility (SD between 1.7 and 9.6%). The detection limit of the system was approximately 3 fmol of each alkane or alkene/g wet lung weight.min. However, neither under basal conditions nor during massive peroxidative stress by the application of high doses of H2O2, FeCl3/ascorbate, paraquat, or ozone was any material with a retention time similar to that of hydrocarbons exhaled from isolated rabbit lungs. We conclude that under the experimental conditions employed, there is only insignificant generation of hydrocarbons in intact lungs.

Selenium in total parenteral nutrition

In clinical practice, selenium deficiency may arise under conditions of chronic malnutrition and especially after long-term total parenteral nutrition (TPN). In infants receiving long-term TPN, we observed plasma selenium levels as low as those previously reported in Chinese children with Keshan disease. Low plasma selenium levels were also usually associated with very low activities of glutathione peroxidase. Although clinical symptoms of selenium deficiency did not occur in our patients, several cases have been described in the literature, indicating the need for supplementation in TPN. In order to derive at the appropriate dosage, it is proposed to correlate it with the total protein supply. According to our present knowledge, .5-1.0 micrograms selenium/g of protein appears to be adequate to keep patients in Se balance. For Se repletion of body stores, this dosage has been increased up to 3 micrograms of Se/g of protein. Advantages and disadvantages of selenite and of selenomethionine as possible supplemental forms of Se for TPN solutions are discussed.

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.

Selected pulmonary biochemical and hematological changes produced by prolonged hypoxia in the rat

Rats were exposed to 12% O2 (1 atm) for 48 hr, then 10% O2 for the duration of the exposures. Significant elevations in enzymes of the glutathione peroxidase system were found in the lungs of rats killed 3.5, 7.5, and 12.5 days of exposure compared to air-breathing controls. Superoxide dismutase was also elevated after hypoxia but nonsignificantly. Animals killed 12.5 days after exposures exhibited 75% (P less than 0.05) more thiobarbituric acid reactive products in their lungs compared to controls. These results along with significant increases of lung lipid peroxidation and in an augmentation of protective antiperoxidative lung defense capabilities. Hypoxia also resulted in enzyme elevations of lung phosphofructokinase and pyruvate kinase, which may indicate an adaptive increase in lung glycolytic capabilities which would be helpful in maintaining lung tissue energy requirements during hypoxia. In addition, it was confirmed that red blood cell count, hemoglobin and hematocrit values increased after prolonged hypoxia. The results of this study might also reflect enzyme elevation/induction due to cellular reparative-proliferative processes following hypoxia.

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.

Dietary antioxidants and the biochemical response to oxidant inhalation : III. Selenium influence on mouse lung response and tolerance to ozone

We fed female strain A/St mice selenium (Se) test diets containing either no Se (-Se) or 1 ppm Se (+Se) for 11 wk. Both diets contained 55 ppm vitamin E. We then exposed three groups of mice from each dietary regimen to either 0.8 ppm (1568 μg/m(3)) O3 (low-level) continuously for 5 d, 10.0 ppm (19,600 μg/m(3)) O3 (high-level) for 12 h, or filtered room air, where the latter served as a control for both O3 exposures. After O3 exposures we analyzed the lungs for various physical and biochemical parameters, and compared the results to those obtained from the air controls. The results showed that the difference in dietary Se intake produced an eightfold difference in Se content and a three-fold difference in glutathione peroxidase (GP) activity in the lung, but few changes in other lung parameters. With low-level O3 exposure, NADPH production increased significantly in +Se mice, but did not change in -Se mice. With high-level O3 exposure we observed comparable effects for both dietary regimens, including animal mortality, which was 24% for -Se and 14% for +Se mice. Thus, it seems that diminished GP activity resulting from Se deficiency and the ensuing lack of increase in NADPH production were poorly correlated with mouse tolerance to O3. The lung Se content increased in both dietary regimens after O3 exposure, but the increase was greater after high-level O3 exposure. This suggests a "mobilization" of Se to the lung under O3 stress. It is possible that such a mobilization contributes to the lung reserve of antioxidants, and hence the comparable mortality in both dietary Se regimens.

Safety of megavitamin therapy

Carbon compounds that are needed in small amounts in the diet because they are not made in the body of vertebrates are defined as vitamins. Excluded from this definition are vitamins D, K, and niacin which can be synthesized by the organism or, as in the case of vitamin K, by the host's intestinal bacteria. Lack of such vitamins can result in characteristic deficiency diseases. The therapeutic use of such compounds (megavitamin intake) is based on the spectacular effect of vitamins on deficiency diseases; however, evidence that the ingestion of large amounts of vitamins beyond the "Recommended Daily Allowances" (RDA) is beneficial is not within the basic concept of nutrition. Vitamins, like many substances, may be toxic when taken in large quantities, especially the fat-soluble vitamins, and the concept of "more is better" is a common misconception. Vitamin supplements can be suggested only in the unusual cases of patients having inadequate intake, disturbed absorption (genetic or otherwise), or increased tissue requirements. A well-balanced diet that includes a wide variety of foods from each of the four food groups is adequate for the supply of vitamins, as well as other nutrients, in healthy people. This paper will review some of the recent findings regarding vitamin toxicity and the mechanisms of toxicity.

Dietary antioxidants and the biochemical response to oxidant inhalation. II. Influence of dietary selenium on the biochemical effects of ozone exposure in mouse lung

We examined the influence of dietary selenium (Se) on the pulmonary biochemical response to ozone (O3) exposure. For 11 weeks, weanling female strain A/St mice were fed a test diet containing Se either at 0 ppm (-Se) or 1 ppm (+Se). Each diet contained 55 ppm vitamin E (vit E). Mice from each dietary group were exposed to 0.8 +/- 0.05 ppm (1568 +/- 98 micrograms/m3) O3 continuously for 5 days. After O3 exposure, they were killed along with a matched number of unexposed controls, and their lungs were analyzed for various biochemical parameters. The Se contents of lung tissue and whole blood were determined, and the levels were seven- to eightfold higher in +Se mice than in -Se mice, reflecting the Se intake of the animals. In unexposed control mice, Se deficiency caused a decline in glutathione peroxidase (GP) activity relative to +Se group. After O3 exposure, the GP activity in the -Se group was associated with a lack of stimulation of glutathione reductase (GR) activity and the pentose phosphate cycle (PPC) as assessed by measuring glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD) activities. In contrast, the +Se group after O3 exposure exhibited increases in all four enzyme activities. Other parameters, e.g., lung weight, total lung protein, DNA and nonprotein sulfhydryl contents, and O2 consumption, were not affected by dietary Se in the presence or absence of O3 exposure. The data indicate that dietary Se alters the GP activity, which in turn influences the GR and PPC activities in the lung evidently through a reduced demand for NADPH. The level of vit E in the lung was found to be twofold higher in the -Se group than in the +Se group, suggesting a compensatory relationship between Se and vit E in the lung. With O3 exposure, both Se and vit E contents further increased in the lungs of each dietary group. It is plausible that Se and vit E under oxidant stress are "mobilized" to the lung from other body sites.

Effect of acute and repeated selenium treatment on hepatic monooxygenase enzyme activity in male rats

The effect of sodium selenite administered acutely or repeatedly on the biochemical components of the hepatic microsomal monooxygenase enzyme system was examined in male rats. 72 h following acute administration of selenium (2.4 mg Se/kg, i.p.), there was a significant decrease in ethylmorphine-N-demethylase activity and cytochrome P-450 levels but no change in aniline hydroxylase or NADPH cytochrome c reductase activity. Following repeated administration of selenite in the drinking water (1, 2, or 4 ppm Se) for 30 days, there was no alteration in any of the parameters measured. Following the in vitro additions of selenite to microsomes obtained from untreated rats, ethylmorphine-N-demethylase and aniline hydroxylase activities were inhibited at selenium concentrations of 10(-4) M or greater, but the inhibition achieved was less than 50%. No alterations in cytochrome P-450 levels were observed. These results indicate that selenium is a rather weak, indirect, and substrate-specific inhibitor of the hepatic monooxygenase enzyme system.

Toxicology of selenium: a review

The concentration of selenium in soil, water, or minerals is site specific. World or regional averages are of little practical value. In one report from the front range area of Colorado, average selenium concentrations in bodies of standing water were from 0.3 to 15.8 micrograms Se per liter of water. In some aquatic organisms there is a strong correlation between the Se content of the water ant that of the body tissues; in others no such correlation obtains. Some organisms bioaccumulate Se by factors as high as 1300 to 3800. In most fish the amount of Se in the flesh seems to depend on the amount in the food taken in; there are exceptions, however. Aquatic organisms from seleniferous regions bioconcentrate selenium so as to reach total body levels of 60 micrograms Se per gram or up to 100 micrograms Se per gram of liver. There seems to be no evidence for "biomagnification" of selenium by aquatic organisms. Selenium exerts a strong protective action against the poisoning effects of many heavy metals (lead, cadmium, arsenic, and mercury, for example) and of some organic toxicants (paraquat, for example) in birds, mammals, and man. Data on man are sketchy. Selenium is released into the environment from the burning of coal. No identifiable hazard to man or to plants and animals useful to man can, at this time, be attributed to this source. Selenium is poisonous to man and animal in large amounts. It is a necessary micronutrient for many animals in small amounts; it may also be a needed micronutrient for man, but the data are sparse. The usual American diet contains adequate selenium for human health. Occupational selenium poisoning is mostly accidental and rare.

Liver necrosis and lipid peroxidation in the rat as the result of paraquat and diquat administration. Effect of selenium deficiency

Paraquat and diquat facilitate formation of superoxide anion in biological systems, and lipid peroxidation has been postulated to be their mechanism of toxicity. Paraquat has been shown to be more toxic to selenium-deficient mice than to controls, presumably as the result of decreased activity of the selenoenzyme glutathione peroxidase. The present study was designed to measure lipid peroxidation and to assess toxicity in control and selenium-deficient rats given paraquat and diquat. Lipid peroxidation was measured by determining ethane production rates of intact animals; toxicity was assessed by survival and by histological and serum enzyme evidence of liver and kidney necrosis. Paraquat and diquat were both much more toxic to selenium-deficient rats than to control rats. Diquat (19.5 mumol/kg) caused rapid and massive liver and kidney necrosis and very high ethane production rates in selenium-deficient rats. The effect of paraquat (78 mumol/kg) was similar to that of diquat but was not as severe. Acutely lethal doses of paraquat (390 mumol/kg) and diquat (230 mumol/kg) in control rats caused very little ethane production and no evidence of liver necrosis. These findings suggest that paraquat and diquat exert their acute toxicity largely through lipid peroxidation in selenium-deficient rats. Selenium deficiency had no effect on superoxide dismutase activity in erythrocytes or in 105,000 g supernate of liver or kidney. Glutathione peroxidase, which represents the only well-characterized biochemical function of selenium in animals, was dissociated from the protective effect of selenium against diquat-induced lipid peroxidation and toxicity by a time-course study in which selenium-deficient rats were injected with 50 mug of selenium and later given diquat (19.5 mumol/kg). Within 10 h, the selenium injection provided significant protection against diquat-induced lipid peroxidation and mortality even though this treatment resulted in no rise in glutathione peroxidase activity of liver, kidney, lung, or plasma at 10 h. This suggests that a selenium-dependent factor in addition to glutathione peroxidase exists that protects against lipid peroxidation.

Enzymatic defenses of the mouse heart against reactive oxygen metabolites: alterations produced by doxorubicin

The endogenous defenses of the mouse heart against reactive oxygen metabolites were investigated. The activities of three enzymes capable of detoxifying activated oxygen were determined in both the heart and liver; cardiac muscle contains 150 times less catalase and nearly four times less superoxide dismutase than liver. Glutathione peroxidase activities were, however, similar to the two tissues. Assay of glutathione peroxidase in the heart after 6 wk of selenium depletion with both hydrogen peroxide and cumene hydroperoxide as substrates revealed a >80% drop in enzyme activity and gave no indication that murine cardiac tissue contains nonselenium-dependent glutathione peroxidase. The selenium-deficient state, which was characterized by markedly decreased cardiac glutathione peroxidase levels, led to significantly enhanced doxorubicin toxicity at a dose of 15 mg/kg i.p. Doxorubicin administration in selenium-sufficient animals resulted in a dose-dependent decrease in cardiac glutathione peroxidase activity; the decrease in enzyme activity lasted 72 h after 15 mg/kg i.p. In contrast, cardiac superoxide dismutase and hepatic superoxide dismutase and glutathione peroxidase were unaffected by this dose of doxorubicin. These results suggest that the major pathway in cardiac tissue for detoxification of reactive oxygen metabolites is via the concerted action of superoxide dismutase and selenium-dependent glutathione peroxidase. The latter enzyme may be depleted by a selenium-deficient diet or doxorubicin treatment, leaving the heart with limited mechanisms for disposing of hydrogen peroxide or lipid peroxides.