Selenite Toxicity Research Articles

Selenite inhibition of Coxsackie virus B5 replication: implications on the etiology of Keshan disease

Keshan disease is a cardiomyopathy of unknown origin reported in some areas of China. Because of epidemiologic features, this disease was ascribed to an infectious agent, likely a Coxsackie virus, but it has also been thought to depend on selenium deficiency, mainly because selenite is effective in its prophylaxis. We examined the hypothesis that pharmacological activity of selenite on Coxsackie virus growth was associated with prevention of Keshan disease. We studied the antiviral effects of three selenium compounds on Coxsackie virus B5 replication: five μM selenite reduced viral replication, whilst 10 μM selenate and selenomethionine did not exhibit any antiviral activity. The inhibitory activity of selenite on viral replication was due to its toxicity following its interaction with thiols, as that activity could be blocked by dithiothreitol, a sulfhydryl-protecting agent known to reverse several toxic effect of selenite. Zinc, another inhibitor of selenite toxicity, also counteracted the antiviral effect of selenite. The selenium compounds showed only limited activity against herpes simplex 1 virus and IHD strain of vaccinia virus. A direct inhibitory effect of selenite on Coxsackie virus replication might explain the efficacy demonstrated by this compound in the prophylaxis of Keshan disease.

Identification of genes affecting selenite toxicity and resistance in Saccharomyces cerevisiae.

Recent studies associating dietary selenium with reduced cancer susceptibility have aroused interest in this substance. In the millimolar range, selenite is toxic and slightly mutagenic for yeast. We show that selenite-treated yeast cells tend to arrest as large budded cells and that this arrest is abolished in a rad9 mutant that is significantly sensitive to selenite. Interestingly, a rev3 mutant affected in the error-prone repair pathway is also sensitive to selenite, whereas mutations in the other DNA repair pathways do not strongly affect resistance to selenite. We propose that selenite treatment leads to DNA damage inducing the RAD9-dependent cell cycle arrest. Selenite-induced DNA damage could be converted to mutations by the Rev3p-dependent lesion bypass system, thus allowing the cell cycle to progress. We have also investigated the selenite detoxification mechanisms and identified three genes involved in this process. In the present study, we show that lack of the cadmium glutathione-conjugate vacuolar pump Ycf1p or overexpression of the sulphite resistance membrane protein Ssu1p enhance the capacity of yeast cells to resist selenite treatment. Finally, we show that overexpression of the glutathione reductase Glr1p increases resistance to selenite, suggesting that selenite toxicity in yeast is closely linked to its oxidative capacity.

Metabolism of Selenoamino Acids and Contribution of Selenium Methylation to Their Toxicity.

Selenium (Se) is an essential trace element and a toxicant for animals. Selenocystine (CySeSeCy) and selenomethionine (SeMet), selenoamino acids, are one of the chemical forms in which selenium exists in foods. This review summarized recent studies on the relation of toxicity and metabolism of selenite, CySeSeCy and SeMet in experimental animals. Hepatotoxicity is caused by repeated oral administration of CySeSeCy. CySeSeCy is metabolized by reduced glutathione (GSH) and/or glutathione reductase to hydrogen selenide (H2Se) via selenocysteine-glutathione selenenyl sulfide (CySeSG). The H2Se is a key intermediate in the methylation process of inorganic and organic selenium compounds. Accumulation of H2Se resulting from inhibition of the Se-methylation metabolism, the detoxification pathway of selenium, is found in animals following repeated oral administration of a toxic dose of CySeSeCy. The Se-methylation inhibition is caused by a reduction in the Sadenosylmethionine (SAM) level due to the repression of methionine adenosyltransferase activity. The excess of H2Se produced by inhibition of methionine adenosyltransferase contributes to the hepatotoxicity caused by CySeSeCy. Moreover, SeMet is now known to be directly metabolized to monomethylselenol (MMSe) as selenide by γ-elimination enzyme in mouse liver. The disturbances in detoxification pathway of Se compounds such as methylation process may be involved in the development of selenosis.

Acute toxicity of sodium selenite to juvenile walleye.

Selenium is a trace element normally found in surface waters at concentrations of approximately 0.1-0.3 μg/L (Lemly 1985). At slightly elevated concentrations (1-5 μg/L), it can bioaccumulate in the aquatic food chain, becoming highly toxic to fish and wildlife (Lemly and Smith 1987). Sandholm et al. (1973) found that fish accumulate selenium primarily through the food chain, although uptake across gills is very efficient at low waterborne concentrations (Hodson and Hilton 1983). Selenium can also be passed to offspring from maternal transfer to the eggs where it is possible to cause complete reproductive failure (Gillespie and Baumann 1986; Coyle et al. 1993; Lemly 1993).

Inhibition of Chlorophyll Synthesis and Enzymes of Nitrogen Assimilation by Selenite in Excised Maize Leaf Sefments during Greening

Supply of 0.1 mM sodium selenite (Na2SeO3 to excised maize leaf segments during greening reduced total chorophylls and inhibited in vivo nitrate reductase activity (NRA). Inhibition of chlorophyll formation by selenite was affected by ammonium but remained unaltered by nitrate. Selenite also inhibited NADH-glutamate dehydrogenase (NADH-GDH) and NADH-glutamate synthase (NADH-GOGAT) activities. Inclusion of ammonium enhanced the inhibition of NADH-GDH activity by selenite but decreased the inhibition of NADH-GOGAT activity by selenite. The results suggest the involvement of the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway of ammonia assimilation in selenite toxicity during chlorophyll formation in greening maize leaf segments.

Growth and biochemical alterations in coffee due to selenite toxicity

Two experiments were conducted to investigate selenite toxicity in coffee (Coffea arabica cv. Catuai). In the first aqueous selenite solution (10 µM Na2SeO3) was used to infiltrate leaves of an adult coffee plant. The infiltrated leaves and fruits adjacent to them showed enhanced contents of caffeine and soluble sugars. Amino acid contents were not affected, whereas pigments (chlorophylls, carotenoids and xanthophylls) exhibited a significant decrease. In the second experiment, coffee seedlings were irrigated with aqueous selenite solutions (10,100 and 1000 µM Na2SeO3) and the first and third pairs of leaves were analyzed. Control plants did not receive selenium. The plants were not different in height, but at the highest selenium concentration showed lower dry matter accumulation in roots and leaves, lower leaf area and thicker leaves. Increases in caffeine and soluble sugars were observed in the first pair of leaves at the highest selenium concentration, although selenium content itself increased steadily with increasing solution concentration. Phenols increased in both leaf pairs and pigments decreased in the third pair. Nitrate reductase activity, measured in the second leaf pair, was much lower at all selenium levels. The profile of free amino acid was altered in leaves of plants treated with selenium.

Toxicity of oxyanions of selenium and of a proposed bioremediation intermediate, dimethyl selenone

AbstractThe relative toxicity of selenate, selenite, and dimethyl selenone were assessed by determining growth inhibition and growth rate inhibition of Pseudomonas fluorescens K27 and using the Microtox® bioassay method at varying concentrations of these three selenium species. The results of EC50 vary depending on the organism tested and the method used; for P. fluorescens K27, EC50 increased in the order of dimethyl selenone < selenate < selenite. However, for Vibrio fisheri, EC50 increased in the order of selenite < dimethyl selenone < selenate. The possibility that different reduction mechanisms may dominate the remediation of the two selenium oxyanions with different oxidation states by P. fluorescens K27 is discussed, as is whether dimethyl selenone is a possible intermediate in selenium reduction and methylation.

TOXICITY OF OXYANIONS OF SELENIUM AND OF A PROPOSED BIOREMEDIATION INTERMEDIATE, DIMETHYL SELENONE

The relative toxicity of selenate, selenite, and dimethyl selenone were assessed by determining growth inhibition and growth rate inhibition of Pseudomonas fluorescens K27 and using the Microtox® bioassay method at varying concentrations of these three selenium species. The results of EC50 vary depending on the organism tested and the method used; for P. fluorescens K27, EC50 increased in the order of dimethyl selenone < selenate < selenite. However, for Vibrio fisheri, EC50 increased in the order of selenite < dimethyl selenone < selenate. The possibility that different reduction mechanisms may dominate the remediation of the two selenium oxyanions with different oxidation states by P. fluorescens K27 is discussed, as is whether dimethyl selenone is a possible intermediate in selenium reduction and methylation.

Influences of fulvic acid on bioavailability and toxicity of selenite for wheat seedling and growth.

The influences of fulvic acid (FA) on bioavailability and toxicity of selenite for wheat seedling and growth were studied by greenhouse hydroponic experiment. The results showed that seed germination, embryo development, and growth were stimulated by selenite in the concentration range of 0.1-1.0 mg/L. In the presence of FA, the stimulation effects were more obviously observed. However, when the concentration of selenite exceeded 1.0 mg/L, toxic effects were observed for most of the measured indicators. The presence of FA could reduce the bioavailability of Se and could antagonize the toxic effects of Se. The reasons for the antagonism were caused by the inhibitory effects of FA on uptake of Se by plants and by the stimulating effects of FA on plant growth.

Intracellular distribution of selenium and the growth of mammary cells in culture.

Retention of Se in CMT-13 cells increased with an increase in the concentration of selenite in the incubation medium, the duration of exposure, and the density of the culture. The enhanced toxicity of selenite coincided with a proportional increase in Se in both the cytoplasm and nucleus. About 90% of the accumulated Se was isolated with cytoplasmic macromolecules. Increased nuclear Se retention correlated with increased cytoplasmic Se retention. Greater quantities of cytosolic Se-containing proteins (74, 55, 41, 34, and 28 kDa) and a nuclear Se-containing protein (56 kDa) were detected as the quantity of Se within CMT-13 cells increased. These findings suggest that cellular retention and distribution of Se are determinants of the degree of cellular growth inhibition caused by this trace element.

Toxicity, metabolism and absorption of selenite by isolated rat hepatocytes

The uptake of various levels of selenite by isolated rat hepatocytes was investigated. The LD 50 value of selenite was about 500 μM. The activity of lactic dehydrogenase in the medium was correlated with cell viability as determined by trypan blue exclusion. After incubation of selenite with hepatocytes, protein-bound Se was the predominant form (80–90% of the cellular Se) present. Subcellular fractionation indicated that most of the radioactivity was present in the cytosol when hepatocytes were incubated with 75Se-selenite. The uptake of 75Se by isolated rat hepatocytes was linear with selenite concentration up to the highest amount tested, 200 μM. Sulfite inhibited the uptake of selenite by hepatocytes.

Toxicity and accumulation of selenite and selenate in the unicellular marine alga Cricosphaera elongata

The toxicity and bioaccumulation of selenium in the 4+ and 6+ oxidation states were investigated in a marine unicellular alga Cricosphaera elongata in culture. Selenite was more toxic than selenate. Exponentially growing cells and cells in the stationary phase of C. elongata rapidly accumulated selenite (0.1 and 1.0 mg/L Na2SeO3) and selenate (0.1 and 1.0 mg/L Na2SeO4). Within the first two hours of contact, the amount of selenium taken up decreased sharply in exponentially growing cells, while cells in the stationary phase continued accumulating selenium until a plateau was reached. The presence of metabolic inhibitors such as KCN (potassium cyanide) or DCMU (3-(3,4-dichloro-phenyl)-1,1 dimethylurea) or glutaraldehyde did not modify the first phase of accumulation of selenite by C. elongata in the stationary phase, whereas further accumulation was inhibited. Possible mechanims of accumulation of selenium are discussed. In a series of long term experiments (14 or 31 d), intracellular partitioning of Se in C. elongata cells, exposed to selenite, was analysed; total, protein-bound and free cytosolic selenium concentrations increased with selenium concentration added to the culture medium (0.1 or 1 mg/L Na2SeO3) and with exposure time (at 0.1 mg/L Na2SeO3) from 14 d or 31 d. Most of the selenium was associated with proteins; these proteins may represent a form of storage or detoxication of selenium.

The selenium metabolite selenodiglutathione induces cell death by a mechanism distinct from H2O2 toxicity.

Our previous studies have implicated the selenium metabolite selenodiglutathione (SDG) in the growth inhibitory effects of selenite in vitro. Other work has suggested that reactive oxygen species, the superoxide anion and hydrogen peroxide, may be implicated in selenite toxicity. In this study the mechanism of growth inhibition by SDG and H2O2 has been compared in a mammary cell line, C57. Both SDG and H2O2 had a rapid effect on C57 cells and markedly reduced cloning efficiency within 1 h. However, the mechanisms involved seem to be different, as judged by the following observations: (i) An SDG-resistant cell line (B19) derived from C57 cells is cross-resistant to selenite, but not H2O2; (ii) SDG reduces the levels of the mRNAs for phospholipid hydroperoxide glutathione peroxidase and cytosolic glutathione peroxidase, whereas H2O2 has no effect; (iii) SDG induces both 560 kb and 50 kb DNA fragments, whereas H2O2 only induces 560 kb DNA fragments. This is of interest, since formation of high molecular weight DNA fragments has been recognized as a characteristic of apoptosis.

Possible involvement of active oxygen species in selenite toxicity in isolated rat hepatocytes.

Mechanisms of selenite cytotoxicity were examined using isolated rat hepatocytes. When selenite was added to a suspension of rat hepatocytes, intracellular reduced glutathione (GSH) was decreased and the oxygen consumption rate was increased. Subsequently, thiobarbituric acid-reactive substances (TBA-RS) and lactate dehydrogenase (LDH) leakage were increased. A ferric iron chelator, desferrioxamine (DF), and a synthetic superoxide dismutase (SOD) mimic, desferrioxamine manganese (DFMn), reduced the selenite toxicity. These results suggest that superoxide anion and its reactive metabolites such as the hydroxyl radical may be involved in the cytotoxicity of selenite.

Comparative toxicity of selenite and selenate to the amphipodHyalella azteca

Selenium contamination of aquatic ecosystems is rapidly becoming a concern throughout the United States, leading to a lowering of the freshwater selenium criteria by the U.S. Environmental Protection Agency from 35 μg/L to 5 μg/L measured as total selenium. However, considerable information indicates important differences in bioaccumulation and toxicity between the various selenium forms. We conducted a series of experiments to determine the comparative toxicity of selenite and selenate to the amphipodHyallela azteca. In 48 h, 96 h, and 240 h (10 d) LC50 tests, selenite was 2 to 4 times more toxic than selenate, the difference decreasing as exposure time increased. This difference in toxicity was even more pronounced in the 24 d reproductive bioassay, where selenite caused a significant decrease in number of young per female at 200 μg/L while selenate had no effect up through 700 μg/L (the highest concentration tested).

Generation of reactive oxygen species from the reaction of selenium compounds with thiols and mammary tumor cells

Sodium selenite, sodium selenate, selenocystine and selenomethionine were tested for their abilities to generate Superoxide by the oxidation of glutathione and other thiols in the absence and presence of cells of the human mammary tumor cell line HTB123/DU4475. Free radical generation was measured by lucigenin- or luminol-amplified chemiluminescence. In the absence of tumor cells, lucigenin-dependent chemiluminescence was observed from the reaction of selenite with the thiols glutathione, 2-mercaptoethanol and l-cysteine, but not with oxidized glutathione. Superoxide dismutase, catalase, and glutathione peroxidase all suppressed the observed chemiluminescence; but when these enzymes were heat inactivated they had little suppressive inhibition on chemiluminescence. Luminoldependent chemiluminescence from the reaction of selenite with glutathione was much less than that observed by lucigenin-amplified chemiluminescence. In the presence of the HTB123/DU4475 mammary tumor cells, lucigenin-dependent chemiluminescence was observed from the reactions of selenite and selenocystine with glutathione which were 5 and 23 times greater than their respective reactions with glutathione in the absence of tumor cells. The enhanced chemiluminescence generated by selenite and selenocystine in the presence of the tumor cells was also suppressed by Superoxide dismutase, catalase and glutathione peroxidase. These data suggest that a free radical, the Superoxide anion (O 2 −, and H 2O 2; are produced from the reaction of selenite and selenocystine with glutathione. These free radical reactions may account for the toxicity of selenite and selenocystine in vitro in comparison to a near absence of acute tumor cell toxicity and Superoxide generation by selenate and selenomethionine with thiols. Enhanced chemiluminescence in the presence of tumor cells may be an expression of cellular selenium metabolism and the capability of cells to form selenium metabolites that more easily oxidize glutathione and other thiols producing reactive free radicals and peroxides.

Effect of Vitamin B 12 Status on Selenium Methylation and Toxicity in Rats: In Vivo and in Vitro Studies

Animals are known to convert inorganic selenium to less toxic methylated compounds such as dimethylselenide (DMSe) and trimethylselenonium (TMSe). This study investigated the role of vitamin B 12, a cofactor of methionine synthetase, in selenium methylation in the rat. Vitamin B 12-depleted rats expired 16% of dosed 75Se-selenite as DMSe compared to 45% for control rats and excreted less TMSe in the urine (6.1% of dose) than control (9% of dose) rats. At the same time, higher ( p < 0.05) tissue (liver, kidney, muscle) selenium levels and lower ( p <0.05) blood selenium levels were found in vitamin B 12-deficient rats. Primary hepatocytes from vitamin B 12-deficient rats volatilized 15% of selenite in incubation medium in 5 hr as compared to 49% in hepatocytes from control rats. Hepatocytes from vitamin B 12-deficient rats were less resistant to selenite toxicity. In vitro methylation of selenium with liver extract from vitamin B 12-deficient rats showed one-third to one-half the rate of volatilization of selenium as compared to control rats. Sadenosylmethionine was required for this reaction. These results show that vitamin B 12 deficiency significantly decreases the ability of rats to methylate selenium.

Effect of dietary fluoride on selenite toxicity in the rat

Three factorial experiments were conducted to determine if high dietary fluoride (F) would inhibit selenite toxicity in rats. Initially, three levels of selenite (0.05, 3, and 5 mg/kg diet) were matched against three levels of F (2, 75, and 150 mg/kg diet). Fluoride failed to prevent the depressive effect of selenite on 8-wk food intake and body wt gain. Selenium (Se) concentration of plasma and kidney and enzymatic activity of whole blood glutathione peroxidase (GSH-Px) were also unaffected by F. Liver Se concentration, however, was slightly (12%) but significantly (p < 0.025) reduced when the highest F and Se levels were combined. Fluoride (150 mg/kg) appeared to reduce liver selenite toxicity (5 mg/kg). Therefore, further study focused on liver histology with treatments that eliminated the middle levels of selenite and F. Fluoride prevented the hepatic necrosis seen in selenite-toxic rats. Similar histological lesions were not observed for kidney or heart. Fluoride partially (26%) but significantly (p < 0.025) reduced thiobarbituric-reactive substances in selenite-toxic rats, but there was no F effect on intracellular distribution of liver Se, glutathione levels in liver and kidney, or on liver xanthine oxidase activity. Overall, the protective effect of F on selenite toxicity appears to be confined to liver pathology. The exact mechanism for this effect, however, remains unclear.

Effect of selenium compounds and thiols on human mammary tumor cells

The effect on cell viability and growth rate of sodium selenite, selenocystine, sodium selenate, and selenomethionine at selenium concentrations of 6.25 and 12.5 uM was studied in vitro on cells of the human mammary tumor cell line HTB123/DU4475. Selenite and selenocystine affected both cell viability and growth rate of the tumor cells at these selenium concentrations. Selenite and selenocystine decreased intracellular glutathione concentrations, but did not affect tumor cell glutathione peroxidase activity. After six days of exposure to either selenate or selenomethionine, the viability of tumor cells remained stable, but cell growth, as measured by numbers of cells, was retarded. Neither selenate nor selenomethionine produced changes in concentrations of intracellular glutathione. The toxic effect of selenite on tumor cells was enhanced by addition of 0.25 mM glutathione to the growth medium. Preincubation of the tumor cells with 62.5 uM buthionine sulfoximine decreased cellular glutathione to 15% of controls at 24 h and enhanced the toxicity of selenite toward the tumor cells. Glutathione, 2-mercaptoethanol, and L-cysteine were all toxic to the tumor cells in a dose-dependent manner.

Toxicity modification of sodium selenite by a brief exposure to heat or cold in mice

The effect of a brief exposure to heat/cold on the subsequent development of toxicity of sodium selenite (SS) was evaluated in male ICR mice. Mice were exposed to one of three ambient temperatures (Ta; approx. 8°, 22° and 33°C). One and a half hours after the beginning of the exposure, they were injected with 45 μmol/kg of SS subcutaneously. The exposure was terminated 3 h after injection and the mice were returned to Ta of 22°C. Heat-induced enhancement of toxicity was recognized in some plasma enzyme activities 3 days after injection and in the suppression of body weight for up to 3 weeks. On the other hand, cold exposure alleviated SS toxicity in terms of these indices. Thus, the Ta during this short period was recognized to be important in determining subsequent development of SS toxicity. At the end of the thermal exposure, heat increased renal concentration of the injected Se. On the other hand, in the liver and the other organs examined, the highest Se concentration was found in the cold-exposed group, followed by the control (room temperature) and the heat-exposed. The relation between the modification of toxicity and the altered distribution was not clear. Neither glutathione level in the liver nor that in the kidney at the time of SS injection could explain the observed modification of the toxicity.