Toxicity of various selenium derivatives to chick embryos

An Early-Life Stage Alternative Testing Strategy for Assessing the Impacts of Environmental Chemicals in Birds

Early-life stage (ELS) toxicity tests are recognized as an advancement over current testing methodologies in terms of cost, animal use, and biological relevance. However, standardized ELS tests are not presently available for some vertebrate taxa, including birds. The present study describes a Japanese quail (Coturnix japonica) ELS test that is a promising candidate for standardization and applies it to test 8 environmental chemicals (ethinylestradiol, benzo[a]pyrene, chlorpyrifos, fluoxetine, lead(II)nitrate, trenbolone, seleno-L-methionine, hexabromocyclododecane). Individual chemicals were injected into the air cell of unincubated Japanese quail eggs at 3 concentrations, all predicted to cause ≤20% mortality. Survival to embryonic day 16 was consistently high (>90%) among the vehicle-injected controls. All chemicals, except ethinylestradiol, were detected in liver tissue, most at concentrations suggestive of embryonic clearance. Adverse effects were observed for 5 of the 8 chemicals; chlorpyrifos (41.1 µg/g) significantly increased developmental abnormalities and decreased embryo and gallbladder mass. Ethinylestradiol (54.2 µg/g) and hexabromocyclododecane (0.02 µg/g) decreased embryo mass and tarsus length, respectively. Benzo[a]pyrene (0.83 µg/g) and fluoxetine hydrochloride (32.7 µg/g) exceeded the 20% mortality cutoff. No effects were observed following lead(II)nitrate, seleno-L-methionine, or trenbolone exposure up to 10.7, 0.07, and 4.4 µg/g, respectively. Overall, our ELS approach was time- and cost-effective, caused minimal mortality in controls, effectively delivered diverse chemicals to the embryo, and permitted identification of apical outcomes, all of which provide support toward standardization. Environ Toxicol Chem 2019;39:141-154. © 2019 SETAC.

Pathways of Selenium Metabolism Including Respiratory Excretory Products

Selenium occurs in a variety of different chemical forms in plants and animals. Plants take up inorganic forms of selenium and make numerous selenium analogs of sulfur compounds. Exclusion of selenium from proteins is characteristic of plant species able to tolerate high concentrations of selenium. Animals metabolize both inorganic and organic forms of selenium into biologically active compounds, including selenocysteine in glutathione peroxidase. Intermediary metabolism of selenium in animals characteristically involves reduction and methylation, resulting in the excretion of urinary or respiratory metabolites that include the trimethylselenonium ion and dimethylselenide. Some mechanisms for reduction and methylation of selenium in animals are discussed, as well as the effects of methylation on toxicity.

Selenium Toxicity in Animals with Emphasis on Man

This review was undertaken to establish what might be the maximum safe dose of selenium that could be administered to man in studies on the use of the element in cancer prevention. The early history of selenium poisoning is briefly summarized. The literature on clinical signs and toxicity data for acute and for chronic selenosis in farm and experimental animals is discussed. Several cases of acute selenosis in man are reviewed, and a number of reports on chronic selenosis in man are reviewed and evaluated. Based on these, the maximum safe single oral dose of selenite, selenate, DL-selenocysteine, or DL-selenomethionine is suggested as 0.05 mg Se/kg body weight (milligrams of selenium per kilogram of body weight). The maximum safe multiple oral dose is suggested as 5 μg Se/kg body weight.

Teratogenic effects of diphenyl diselenide in Wistar rats

Diphenyl diselenide is an organoselenium compound with potential therapeutic use. The present study evaluates the effects of single maternal subcutaneous injection of 50 and 100mg/kg diphenyl diselenide [(PhSe)2] at gestational days (GD) 6, 10 or 17 in Wistar rats. The highest dose of (PhSe2 was also administered at GD 7-12. External and internal fetal soft-tissue examination was performed at GD 20. No mortality was observed in fetuses or dams at any (PhSe)2 treatment group. Neither did exposure to (PhSe)2 cause significant changes to fetal body weight, organ weight, or fetal size when administered at GD 6-8, 10-12 or 17. Exposure to 100mg/kg (PhSe)2 at GD 9 produced significant changes in fetal biometry (crown-rump (CR) length) and body weight. No significant increase in the proportion of fetuses with external visible abnormalities was observed in groups exposed to (PhSe)2. Skeletal anomalies were observed in fetuses in the GD 9-11 treatment groups and included incomplete ossification of cranial bones, misshapen and incomplete ossification of sternebrae, reduced sternebrae number, wavy and extra ribs, incomplete ossification of fore and hindpaw bones and incomplete ossification of sacral and caudal bones. We conclude that maternal administration of (PhSe)2 during GD 7-12 led to increased incidences of these skeletal variations or anomalies, but did not cause externally visible malformations in rat fetuses.

Effects of methionine on selenium embryotoxicity in cultured rat embryos

Effects of methionine, an essential amino acid, on the embryotoxicity of selenium (Se) were examined using the rat embryo culture. Rat embryos at day 9.5 of gestation were cultured for 48 h in the presence of sodium selenite at 10 and 20 microM or sodium selenate at 30 and 100 microM with or without the addition of 1 mM DL-methionine. Selenite at 20 microM or selenate at 100 microM alone increased the incidence of embryonic malformation and inhibited the embryonic growth. The addition of methionine increased the incidence of embryonic malformation at 10 microM of selenite but decreased the incidence of embryonic malformation at 100 microM of selenate. On the other hand, the addition of methionine partially restored the inhibited embryonic growth at 20 microM of selenite or at 100 microM of selenate. It was considered from these results that the methionine availability in the embryonic environment and the oxidation state of Se are critical in Se embryotoxicity.

Selenium toxicity: cause and effects in aquatic birds

There are several manners in which selenium may express its toxicity: (1) an important mechanism appears to involve the formation of CH(3)Se(minus sign) which either enters a redox cycle and generates superoxide and oxidative stress, or forms free radicals that bind to and inhibit important enzymes and proteins. (2) Excess selenium as selenocysteine results in inhibition of selenium methylation metabolism. As a consequence, concentrations of hydrogen selenide, an intermediate metabolite, accumulate in animals and are hepatotoxic, possibly causing other selenium-related adverse effects. (3) It is also possible that the presence of excess selenium analogs of sulfur-containing enzymes and structural proteins play a role in avian teratogenesis. L-selenomethionine is the most likely major dietary form of selenium encountered by aquatic birds, with lesser amounts of L-selenocysteine ingested from aquatic animal foods. The literature is suggestive that L-selenomethionine is not any more toxic to adult birds than other animals. L-Selenomethionine accumulates in tissue protein of adult birds and in the protein of egg white as would be expected to occur in animals. There is no suggestion from the literature that the levels of L-selenomethionine that would be expected to accumulate in eggs in the absence of environmental concentration of selenium pose harm to the developing embryo. For several species of aquatic birds, levels of Se as selenomethionine in the egg above 3 ppm on a wet weight basis result in reduced hatchability and deformed embryos. The toxicity of L-selenomethionine injected directly into eggs is greater than that found from the entry of L-selenomethionine into the egg from the normal adult diet. This suggests that there is unusual if not abnormal metabolism of L-selenomethionine in the embryo not seen when L-selenomethionine is present in egg white protein where it likely serves as a source of selenium for glutathione peroxidase synthesis in the developing aquatic chick.

Effects of glutathione depletion on selenite- and selenate-induced embryotoxicity in cultured rat embryos

Effects of depletion of reduced glutathione (GSH) on selenium (Se) embryotoxicity in cultured rat embryos were examined. Rat embryos at day 9.5 of gestation were cultured for 48 h in the presence of Se as either sodium selenite at 10 and 20 microM or sodium selenate at 30 and 100 microM. Embryonic GSH was depleted by the addition of 0.1 mM of L-buthionine-[S,R]-sulfoximine (BSO) without embryotoxicity, i.e., significant growth retardation and malformation of the embryos. Selenite at 10 microM or selenate at 100 microM significantly increased the incidence of malformation of the embryos. The incidence of selenite-induced malformation of the embryos at 20 microM was significantly decreased with BSO. On the contrary, the incidence of selenate-induced malformation at 30 microM was significantly increased with BSO. It was noted that the major malformed regions of the embryos by the embryotoxic concentration of BSO alone were the same to those affected by selenite or selenate. It was considered from these results that embryonic GSH was involved in the embryotoxicity of selenite and selenate. The embryotoxicity of selenate may not be mediated through the reduction to selenite. It was suggested that the formation of selenodiglutathione and the oxidative stress were involved in the embryotoxicity of selenite and selenate, respectively.

Effects of ascorbic acid on selenium teratogenicity in cultured rat embryos

The effects of ascorbic acid, as an exogenous antioxidant, on selenium (Se) teratogenicity were examined using rat embryo culture. Rat embryos at day 9.5 of gestation were cultured for 48 h in the presence of sodium selenite at 10 and 20 microM or sodium selenate at 30 and 100 microM with or without the addition of 1 mM of L-ascorbic acid (AsA). Selenite or selenate alone increased the incidence of embryonic malformation. With AsA, the incidence of selenite-induced embryonic malformation was increased. On the contrary, the incidence of selenate-induced embryonic malformation was decreased with AsA. It was considered from these results that the redox states in the embryonic environment and of Se are critical in Se teratogenicity.

Incorporation of a micronucleus study into a developmental toxicology and pharmacokinetic study of L-selenomethionine in nonhuman primates

Concomitant to a developmental toxicology study of selenium in long-tailed macaques (Macaca fascicularis), a transplacental bone marrow micronucleus assay was conducted in the fetuses of treated animals. Selenium was administered as L-selenomethionine by nasogastric intubation at 0, 150 or 300 micrograms/kg-day to pregnant macaques daily throughout organogenesis (gestation days 20-50). Pregnancy was terminated on gestation day 100 +/- 2 and fetuses were obtained by hysterotomy. Selenium concentrations in maternal blood were monitored throughout pregnancy and selenium concentrations in fetal blood were measured at hysterotomy. Maternal circulating selenium did not exceed 4 ppm in plasma or 3.7 ppm in erythrocytes. Selenium in cord blood was < or = 0.1 ppm in plasma and < or = 1.1 ppm in erythrocytes at 300 micrograms/kg-day. Fetal bone marrow smears were prepared from the humerus and micronucleated polychromatic erythrocytes were scored. No increase of micronucleus frequency was detected in any dose group, although signs of maternal selenosis were obvious. This finding is compared to the previous observation that micronuclei were induced in the bone marrow of adult nonpregnant macaques treated at 600 micrograms/kg-day, a lethal dose yielding blood selenium levels to 7.3 ppm in plasma and 5.7 ppm in erythrocytes after 15 days of daily treatment, when death occurred. These data demonstrate that measurement of circulating xenobiotics can be useful for the interpretation of genetic toxicology results.

Route-dependent pharmacokinetics, distribution, and placental permeability of organic and inorganic selenium in hamsters

Inorganic selenium (Se) salts (selenite and selenate oxyanions) and the organic selenoamino acids (selenomethionine and seleniferous grains) are teratogenic and embryolethal in domestic and wild birds. Selenium bioaccumulation has been held responsible for reproductive failure among waterfowl at the Kesterson Reservoir (California), the Ouray and Stewart Lake Wildlife Refuges (Utah), and the Carson Sink (Nevada). Anecdotal field and controlled laboratory reports have implicated Se exposure in mammalian embryotoxicity (including human), but developmental toxicity studies in hamsters failed to demonstrate an adverse response, except at maternally toxic doses (Ferm et al., Reprod. Toxicol., in press). Uptake, distribution, and elimination of Se after a single bolus equimolar dose (60 mumol/kg) of selenate or selenomethionine by oral or intravenous administration were compared using day 8 pregnant hamsters. Intravenous selenate was eliminated ten times more rapidly from maternal plasma than oral selenate, but concentrated in liver, kidney, and placenta to the same degree. Intravenous (iv) L-selenomethionine achieved lower maximum circulating total [Se], but it was eliminated more slowly than iv selenate. Larger areas under the plasma and peripheral tissue [Se]:time curve (AUC) after oral or parenteral selenomethionine than after equimolar selenate were consistent with previous studies in rodents and in humans. Embryonic [Se] plateaued at 3 nmol/g after selenate, but embryonic [Se] after selenomethionine continued to accumulate (80 nmol/g) as gestation progressed. The lack of a teratogenic response in hamsters at doses of either selenate or selenomethionine less than those associated with maternal intoxication cannot be attributed to lack of Se accumulation in early embryonic and placental tissue.

Embryotoxicity and dose-response relationships of selenium in hamsters

Pregnant hamsters were treated with selenite, selenate, and selenomethionine during the critical stages of embryogenesis. The dosing regimens were oral, intravenous, and osmotic minipump infusion. Malformations, mainly encephaloceles, were noted with oral and intravenous selenite and selenate but were associated with maternal toxicity manifested by inanition and weight loss. Fetal body weights and lengths were reduced in a dose-dependent manner with the inorganic forms. Single oral doses of selenomethionine above 77 mumol/kg induced similar malformations but not when the dose was delivered orally over four days nor by minipump over several days. Fetal body weights and lengths were decreased by selenomethionine in a dose-dependent manner. Maternal toxicity was pronounced with the higher doses of selenomethionine. Assigning a specific teratogenic effect to selenium is confounded by maternal toxicity.

Cellular selenoproteins and the effects of selenite on cell proliferation

The incorporation of radioactive selenium into cellular proteins and the effect of selenite on proliferation were examined in human (HeLa, HT-29, and IMR-90) and mouse (3T3 and CMT-93) cell lines. Proteins incorporating selenium were detected by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Major polypeptide subunits at 60, 23, 21, 19, and 16 kD were detected in the two tumorigenic and one normal human cell lines. The 23 kD polypeptide migrated to the same position on the gel as the major subunit of human erythrocyte glutathione peroxidase. In the mouse cells, the 60 kD polypeptide was almost entirely absent; four other major selenoproteins were detected, with molecular weights similar to those in the human cells. In both mouse and human cells, the same pattern of selenoproteins was observed irrespective of whether the cells were grown in medium containing 10% fetal bovine serum or in defined medium supplemented with 0.1 or 1 microM selenite, or with 1% serum. The effect of selenite on proliferation of HeLa, HT-29, and CMT-93 cells in medium supplemented with 10% fetal bovine serum and in serum-free medium was examined. At concentrations up to about 1 microM, selenite stimulated proliferation of the human cells slightly in serum-free medium but not in serum-supplemented medium. At concentrations of about 5 microM and higher selenite significantly inhibited proliferation of all cells in both types of media. In CMT-93 cells, this inhibition was greater in serum-free medium, but there were no significant differences in this regard in the human cells. These results demonstrate that selenium is stably incorporated into several polypeptides in human and mouse cells, that there are no significant differences in this regard among several cell lines, and slight differences between human and mouse cells. They further confirm that selenium can have a slight stimulatory effect on cell growth, and a much larger inhibitory effect, depending on its concentration.

Embryotoxic and teratogenic effects of selenium in the diet of mallards

Mallards (Anas platyrhynchos) were fed a control diet, diets containing 1, 5, 10, or 25 ppm Se as sodium selenite, or a diet containing 10 ppm Se as seleno-DL-methionine in the first of two experiments. Selenium at 10 ppm as selenomethionine or 25 ppm as sodium selenite caused a 40-44% decrease in the total number of eggs that hatched compared to controls. Selenium at 25 ppm (sodium selenite) resulted in a 19% decrease in mean embryonic weight at 18 d of incubation, accompanied by a 6% decrease in crown-rump length. Ten parts per million Se as selenomethionine was more teratogenic than sodium selenite at 25 ppm. Selenomethionine (10 ppm Se) resulted in an incidence of 13.1% malformations that were often multiple, whereas sodium selenite (10 and 25 ppm Se) resulted in 3.6 and 4.2% malformations. The teratogenicity of selenomethionine was confirmed in a second experiment in which mallards received 1, 2, 4, 8, or 16 ppm Se as selenomethionine, resulting in 0.9, 0.5, 1.4, 6.8, and 67.9% malformations, respectively. These malformations included hydrocephaly, microphthalmia, lower bill defects, and foot defects with ectrodactyly. Both forms of selenium increased the incidence of edema and stunted embryonic growth. Selenomethionine (10 ppm Se) resulted in a significant increase of approximately 40% in plasma glutathione peroxidase activity and a 70% increase in sorbitol dehydrogenase activity (indicative of hepatotoxicity) in hatchlings. Sodium selenite (25 ppm Se) resulted in fourfold elevation in plasma uric acid concentration, indicative of renal alteration. Selenomethionine accumulated much better in eggs than did sodium selenite. These findings indicate that selenomethionine is considerably more teratogenic and generally more embryotoxic than sodium selenite, probably due to higher uptake of selenomethionine.

Selenium toxicosis in wild aquatic birds

Severe gross and microscopic lesions and other changes were found in adult aquatic birds and in embryos from Kesterson Reservoir (a portion of Kesterson National Wildlife Refuge), Merced County, Calif., during 1984. Adult birds from that area were emaciated, had subacute to extensive chronic hepatic lesions, and had excess fluid and fibrin in the peritoneal cavity. Biochemical changes in their livers included elevated glycogen and non-protein-bound sulfhydryl concentrations and glutathione peroxidase activity but lowered protein, total sulfhydryl, and protein-bound sulfhydryl concentrations. Congenital malformations observed grossly in embryos were often multiple and included anophthalmia, microphthalmia, abnormal beaks, amelia, micromelia, ectrodactyly, and hydrocephaly. Mean concentrations of selenium in livers (94.4 ppm, dry weight) and kidneys (96.6 ppm) of birds collected at the Kesterson ponds were about 10 times those found at a nearby control area (8.3 and 12.2 ppm). We conclude that selenium present in the agricultural drainage water supplied to the Kesterson ponds accumulated in the food chain of aquatic birds to toxic concentrations and caused the lesion and other changes observed.

Cytotoxic effects of sodium selenite on tadpoles (Xenopus laevis)

The cytotoxic effects of sodium selenite on developing tadpoles (Xenopus laevis) were examined by scanning, light, and electron microscopy. Selenium exposure resulted in disorganization, vacuolization, and swelling of the outer layer of epithelial cells in the tadpole epidermis. Examination of muscle cells in the somites revealed myofibril disorganization and cell degeneration. Mitochondria in both epithelial and muscle cells were swollen and showed loss of cristae. It is likely that sublethal exposures to selenium compounds result in cellular damage which could affect motility, and thus survival, over longer periods of time.

Effect of selenium and mercury on gross morphology and histopathology of chick embryos

Gross lesions of selenium toxicity in chick embryos included webbed, fused, and curled toes; cracked, crooked, and shortened beaks and gastroschisis. Mercury injection on day 3 of incubation caused leg defects but older embryos were much less susceptible. Histopathological studies showed that injection of selenium as selenite caused dissociation of hepatic cells, particularly around the central veins. Glomeruli of affected kidneys were enlarged, and in each glomerulus the lumen of the capillary tuft was dilated and the space of Bowman widened. Epithelial cells of the proximal convoluted tubules were detached from the basement membranes. Injection of mercury as chloride resulted in liver pathology in the late dead embryos which included dilated central veins and sinusoids. The cytoplasm of many hepatic cells was vacuolated. Foci of hemorrhages were observed in all liver lobes. In the kidneys, the renal corpuscles showed either shrinkage or enlargement. They seemed to be in the process of degeneration and disintegration. Sclerotic glomeruli were characterized by disappearance of Bowman's space and clumping of the glomerular tuft. Combined treatment with selenium and mercury resulted in liver pathology similar to that observed in mercury toxicity. The hepatic cell dissociation seen in selenium treated embryos was not observed following the combined treatment. In the kidneys the combined treatment resulted in typical lesions of both selenium and mercury toxicity.

Effect of selenium and mercury on survival of chick embryos

Factorial experiments were arranged in a completely randomized or randomized block design. The factors included: selenium and day of injection; mercury and day of injection; selenium and mercury; and selenium, mercury and day of injection. Each treatment factor consisted of several levels, selenium ranged from 0.00 p.p.m. to 0.05 p.p.m., mercury from 0.00 p.p.m. to 0.30 p.p.m. and injection was performed on day-3, 9, and 15 of incubation. Babcock-300, and White Leghorn x New Hampshire cross eggs were obtained from 13-15 month old hens. Mercury was injected into the air cell at 4 or 24 hours after selenium injection. Analysis of variance on arcsine transformed data showed that selenium significantly decreased survival at all 3 injection times (P less than 0.01). Survival was significantly greater with increasing age at injection (P less than 0.01). Survival of embryos significantly decreased (P less than 0.01) with increasing levels of mercury from 0.00 p.p.m. to 0.20 p.p.m. injected into eggs on day-3 of incubation. Survival of embryos injected at later stages was less than that of controls but not significantly less. Injection of low levels of selenium, 0.01 p.p.m. or 0.02 p.p.m., to mercury treated eggs tended to improve the survival of embryos as compared to treatment with mercury alone, although individual differences were not significant. At higher levels, selenium accentuated the toxicity of mercury.