Selenium metabolism and bioavailability

Selenium-enriched sprouts. A raw material for fortified cereal-based diets

The selenium supply in almost all European countries, including Austria and Germany, is below the recommended daily intake. In these countries, selenium fortification of foods and the use of selenium supplements are quite popular to compensate for low Se intake from diets. In general, wheat (Triticum aestivum) is known to be a good source for bioavailable selenium, and many studies have been performed to enrich selenium in wheat by selenium fertilization of the soil. In the present work, the process of sprouting was investigated as an alternative to enrich selenium in wheat. Sprouting was chosen because it additionally improves the nutritional value of seeds, for example, by a higher vitamin content, a better quality of protein, and some other parameters. Wheat, alfalfa (Medicago sativa), and sunflower (Helianthus annuus) seeds were germinated for 5 and 7 days in solutions containing selenate. The selenium sensitivity of the sprouts was tested by measuring visible germination levels and seedling development. Uptake rates were studied by determination of total selenium using inductively coupled plasma mass spectrometry (ICP-MS). Metabolism of the absorbed selenium was analyzed by determination of selenium species in extracts of the sprouts using anion exchange HPLC coupled to ICP-MS. It was shown that sunflower sprouts were the most resistant and had the highest uptake rates (up to 900 mg/kg), but almost 100% of the selenium was extracted with water and found to be nonmetabolized selenate. Wheat and alfalfa were less resistant and enriched selenium up to concentrations of 100 and 150 mg of Se/kg of dry mass, respectively. The metabolism of the selenate was inversely related to the total uptake rates. At low Se enrichment (approximately 1-2 mg of Se/kg), <20% of the total selenium content within the sprouts remained as inorganic selenium, indicating a high metabolism rate. With increasing uptake the amount of selenate increased to approximately 40-50%. However, with the method used it is possible to produce sprouts containing certain amounts of selenium, which might provide substantial proportions of bioavailable selenium. In combination with the generally high nutritional value of sprouts, they might serve for production of improved cereal-based diets.

Fate of selenate and selenite metabolized by Rhodobacter sphaeroides

Cultures of a purple nonsulfur bacterium, Rhodobacter sphaeroides, amended with approximately 1 or approximately 100 ppm selenate or selenite, were grown phototrophically to stationary phase. Analyses of culture headspace, separated cells, and filtered culture supernatant were carried out using gas chromatography, X-ray absorption spectroscopy, and inductively coupled plasma spectroscopy-mass spectrometry, respectively. While selenium-amended cultures showed much higher amounts of SeO(3)(2-) bioconversion than did analogous selenate experiments (94% uptake for SeO(3)(2-) as compared to 9.6% for SeO(4)(2-)-amended cultures from 100-ppm solutions), the chemical forms of selenium in the microbial cells were not very different except at exposure to high concentrations of selenite. Volatilization accounted for only a very small portion of the accumulated selenium; most was present in organic forms and the red elemental form.

Indicators of selenium status in Australian infants

OBJECTIVE

To provide the first Australian data for the commonly used indicators of selenium (Se) status including plasma and erythrocyte Se and erythrocyte glutathione peroxidase (GSHPx) activity in newborn term and preterm infants from Adelaide, South Australia. Plasma Se levels from adults from Adelaide and Tasmania are also reported.

METHODOLOGY

Descriptive, cross-sectional study of the indicators of Se status of newborn Australian preterm and term infants.

RESULTS

Mean (+/- SD) plasma Se concentrations in the first 5 days of life for term (n = 48) and preterm (n = 90) infants were similar at 33 +/- 11 and 29 +/- 14 microg/L, respectively. Glutathione peroxidase activity was lower in preterm than term infants (P < 0.001). Erythrocyte Se was lower in male (n = 74) compared with female (n = 57) infants (491 +/- 98 vs 561 +/- 134 ng/g Hb; P < 0.001). Adelaide adult plasma Se levels were 88 +/- 20 microg/L and higher than those of adult Tasmanian blood donors (77 +/- 13 microg/l, P < 0.05).

CONCLUSIONS

These data indicate that values for indicators of Se status of Australian infants and adults are at the lower end of the international range. This article highlights the paucity of Australian data on the Se content in the food supply and the Se status of the Australian population. Given the increasing range and complexity of the biological roles of Se and the emerging evidence that Se may have important roles in prevention of cancer and other disorders, further data are required to clarify the Se status of Australians.

Reproductive outcomes in a population exposed long-term to inorganic selenium via drinking water

Despite being an essential element in mammals and lower animals at very low doses, selenium is recognized as a teratogen and a growth-inhibiting substance in several animal species. Some inorganic selenium compounds have also been shown to be mutagenic and pro-oxidant: however, very little is known about their effects on human reproduction. Between 1972 and 1988, a few thousand residents in the municipality of Reggio Emilia, northern Italy, were accidentally exposed to drinking water with unusually high levels of inorganic selenium (selenate), range 7-9 microg/l, through a local public water supply system. We found no deleterious effect on overall body weight and length of newborns and stillborns delivered by 18 women previously exposed to high-selenium tapwater. Rates of spontaneous abortions, however, were increased slightly (RR = 1.73; 95% CI = 0.62-4.80), compared with rates among unexposed women from the same municipality. From 1980 through 1988 the prevalence of congenital malformations at birth among the exposed population was similar to that expected, but estimates were imprecise owing to small numbers. Results do not suggest marked effects on human reproduction from chronic exposure to selenate in drinking water at < 10 microg/l.

One-electron reduction of selenomethionine oxide

Experimental evidence is provided that selenomethionine oxide (MetSeO) is more readily reducible than its sulfur analogue, methionine sulfoxide (MetSO). Pulse radiolysis experiments reveal an efficient reaction of MetSeO with one-electron reductants, such as e(aq)-, (k = 1.2x10(10) M(-1) s(-1)), CO2*- (k = 5.9x10(8) M(-1) s(-1)) and (CH3)2 C*OH (k = 3.5x10(7) M(-1) s(-1)), forming an intermediate selenium-nitrogen coupled zwitterionic radical with the positive charge at an intramolecularly formed Se(three-electron bond)N 2sigma/1sigma* three-electron bond, which is characterized by an optical absorption with lambda(max) at 375 nm, and a half-life of about 70 micros. The same transient is generated upon HO* radical-induced one-electron oxidation of selenomethionine (MetSe). This radical thus constitutes the redox intermediate between the two oxidation states, MetSeO and MetSe. Time-resolved optical data further indicate sulfur-selenium interactions between the Se(three-electron bond)N transient and GSH. The Se(three-electron bond)N transient appears to play a key role in the reduction of selenomethionine oxide by glutathione.

Feeding preferences of spodoptera exigua in response to form and concentration of selenium

Minimal information is available on the impact of various organic and inorganic forms of the ecologically and agriculturally important pollutant, selenium (Se), on insect herbivores. We conducted bioassays with artificial diet to examine the feeding responses of a generalist herbivore, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae), to various forms and concentrations of Se. Two different-aged cohorts of larvae were examined in choice tests with control diets vs. test diets incorporating lethal concentrations (LC(10), LC(30), LC(50), and LC(70)) of sodium selenate, sodium selenite, seleno-DL-cystine, and seleno-DL-methionine. Tests initiated with neonates showed larvae significantly preferred control diet over diet with sodium selenate, sodium selenite, or selenocystine, but at most concentrations showed no preference between selenomethionine and control diet. Choice tests initiated with third instars demonstrated a preference for control diet over sodium selenate treatments, and sodium selenite treatments. In contrast, no significant responses were found in tests initiated with third instars offered the choice between selenocystine or selenomethionine and untreated controls. Additionally, comparisons of consumption demonstrated that inorganic selenium compounds were antifeedants whereas the organic selenium compounds tested have little antifeedant activity. The toxicity of all of the tested forms of selenium, in combination with the lack of antifeedant activity of some compounds, has the potential to affect both the distribution and diversity of terrestrial herbivores in both agricultural and natural systems. Arch. Copyright 1999 Wiley-Liss, Inc.

Determination of copper, zinc, and selenium in human plasma and urine samples by potentiometric stripping analysis and constant current stripping analysis

Potentiometric stripping analysis and constant current stripping analysis are proposed as routine methods for analysis of copper, zinc and selenium in plasma and urine samples. The analytical performance of these methods is comparable with that reported for atomic absorption spectrometry. However the low cost, greater simplicity of the apparatus, and the facility of execution make this methodology a valid candidate for routine application in Clinical Chemistry laboratories.

Selenium

The 4 natural oxidation states of selenium are elemental selenium (0), selenide (-2), selenite (+4), and selenate (+6). Inorganic selenate and selenite predominate in water whereas organic selenium compounds (selenomethionine, selenocysteine) are the major selenium species in cereal and in vegetables. The principal applications of selenium include the manufacture of ceramics, glass, photoelectric cells, pigments, rectifiers, semiconductors, and steel as well as use in photography, pharmaceutical production, and rubber vulcanizing. High concentrations of selenium in surface and in ground water usually occur in farm areas where irrigation water drains from soils with high selenium content (Kesterson Reservoir, California) or in lakes receiving condenser cooling water from coal-fired electric power plants (Belews Lake, North Carolina). For the general population, the primary pathway of exposure to selenium is food, followed by water and air. Both selenite and selenate possess substantial bioavailability. However, plants preferentially absorb selenates and convert them to organic compounds. Aquatic organisms (e.g., bivalves) can accumulate and magnify selenium in the food chain. Selenium is an essential component of glutathione peroxidase, which is an important enzyme for processes that protect lipids in polyunsaturated membranes from oxidative degradation. Inadequate concentrations of selenium in the Chinese diet account, at least in part, for the illness called Keshan disease. Selenium deficiency occurs in the geographic areas where Balkan nephropathy appears, but there is no direct evidence that selenium deficiency contributes to the development of this chronic, progressive kidney disease. Several lines of scientific inquiry suggest that an increased risk of cancer occurs as a result of low concentrations of selenium in the diet; however, insufficient evidence exists at the present time to recommend the use of selenium supplements for the prevention of cancer. The toxicity of most forms of selenium is low and the toxicity depends on the chemical form of selenium. The acute ingestion of selenious acid is almost invariably fatal, preceded by stupor, hypotension, and respiratory depression. Chronic selenium poisoning has been reported in China where changes in the hair and nails resulted from excessive environmental exposures to selenium. Garlic odor on the breath is an indication of excessive selenium exposure as a result of the expiration of dimethyl selenide. The US National Toxicology Program lists selenium sulfide as an animal carcinogen, but there is no evidence that other selenium compounds are carcinogens.

Subcellular distribution of selenium and Se-containing proteins in human liver

Selenium is an essential trace element in many living organisms. In the present paper, the subcellular distribution of selenium and Se-containing proteins in human liver samples, which were obtained from normal subjects who had an accidental death, was investigated by differential centrifugation and column chromatography. Selenium was mainly enriched in nuclei, mitochondria and cytosol. Almost half of Se existed in the nuclei due to their large amount in liver and high Se concentration. 15-30% of Se was found in small compounds with Mr<2000 in the liver components separated by dialysis. The average abundance of Se in small molecular mass species of whole-liver was 23.6%, which suggested most of Se associated with biological macromolecules. Eight kinds of Se-containing proteins with molecular mass of 335+/-20, 249+/-15, 106+/-11, 84.6+/-5.8, 70. 5+/-5.4, 45.6+/-1.5, 14.8+/-2.6, 8.5+/-1.2 kDa were found in the subcellular fractions of human liver. Among them the 335, 84.6 and 8. 5 kDa proteins were individually present in one subcellular fraction, whereas the others coexisted in two, three or four subcellular fractions. The most abundant Se-containing proteins, 70.5 and 14.8 kDa, accounted for 33.6% and 48.5% in the whole-liver soluble Se-containing protein, respectively. The former was enriched in cytosol and the latter was mainly present in nuclei and mitochondria.

Separation of selenium-containing proteins in human and mouse plasma using tandem high-performance liquid chromatography columns coupled with inductively coupled plasma-mass spectrometry

An analytical method that uses two different high-performance liquid chromatography (HPLC) columns in tandem has been developed that separates three major selenium-containing proteins (albumin, glutathione peroxidase, and selenoprotein P) found in human blood plasma. The first column was a heparin affinity column and the second was a gel filtration column whose outlet was directly connected to an inductively coupled plasma-mass spectrometer. The method successfully separated plasma selenium into the three selenium-containing proteins and revealed the preferential retention of selenium in the form of selenoprotein P in a selenium-deficient human and in selenium-deficient mice. Our results also confirm the results of previous studies that showed a preference for supplemented selenium to be taken up as selenoprotein P in rats. Advantages of the tandem column method are that it allows rapid and convenient analyses of the distribution of plasma selenium, and that it is suitable for stable isotope tracer studies and metal interaction studies.

Differential expression of selenoproteins by human skin cells and protection by selenium from UVB-radiation-induced cell death

The generation of reactive oxygen species has been implicated as part of the mechanism responsible for UVB-radiation-induced skin damage. In mice, evidence suggests that increased dietary selenium intake may protect skin from many of the harmful effects of UVB radiation. We sought to determine the selenoprotein profile of cultured human skin cells and whether selenium supplementation could protect keratinocytes and melanocytes from the lethal effects of UVB radiation. Labelling experiments using [75Se]selenite showed qualitative and quantitative differences in selenoprotein expression by human fibroblasts, keratinocytes and melanocytes. This was most noticeable for thioredoxin reductase (60 kDa) and phospholipid glutathione peroxidase (21 kDa); these proteins were identified by Western blotting. Despite these differences, we found that a 24 h preincubation with sodium selenite or selenomethionine protected both cultured human keratinocytes and melanocytes from UVB-induced cell death. With primary keratinocytes, the greatest reduction in cell death was found with 10 nM sodium selenite (79% cell death reduced to 21.7%; P<0.01) and with 50 nM selenomethionine (79% cell death reduced to 13.2%; P<0.01). Protection could be obtained with concentrations as low as 1 nM with sodium selenite and 10 nM with selenomethionine. When selenium was added after UVB radiation, little protection could be achieved, with cell death only being reduced from 88.5% to about 50% with both compounds. In all of the experiments sodium selenite was more potent than selenomethionine at providing protection from UVB radiation.

Influence of form and quantity of selenium on the development and survival of an insect herbivore

Even plants classified as 'nonaccumulators' can sequester concentrations of sodium selenate, sodium selenite, selenocystine and selenomethionine that can strongly influence insect development and survival. These forms of selenium (Se), tested in diet-incorporation bioassays, proved toxic to larvae of a generalist insect herbivore at relatively low levels. Sodium selenite was the most toxic form tested against Spodoptera exigua (Hübner), with an LC(50) of 9.14 microg g(-1) wet wt (21.11 microg g(-1) dry wt). Selenocystine was intermediate with an LC(50) of 15.2 microg g(-1) wet wt. The least toxic forms, sodium selenate and selenomethionine, had LC(50)s below 50 microg g(-1) dry wt, the upper level for tissues of plants classified as nonaccumulators. Ingestion of some forms of Se also affected growth and development. Increasing concentrations of sodium selenate and sodium selenite decreased pupal weight and added significantly to the time needed for development to the pupal and adult stages. The time required to complete the larval stage increased by over 25% and the time from egg to adult emergence was extended by 22% to nearly 30%. Selenocystine and selenomethionine did not significantly increase developmental times, even at concentrations that killed 90% or more of the test populations. Analyses of relative growth rate, relative growth index, and an analysis of covariance technique for measuring growth indicated that the form of Se affected growth rates, growth inhibition responses of the larvae, and toxicological effects. Thus, quantity and the form of Se accumulating in plants grown on Se-contaminated sites are likely to influence the population dynamics of insect herbivores. The implications of these results for the ecology of contaminated sites are discussed.

Selenium and zinc levels in surgical patients receiving total parenteral nutrition

The zinc and selenium levels of 40 surgical patients were monitored pre- and post-TPN. The initial selenium level was low normal, and the initial zinc level was also low. Both selenium and zinc are potent antioxidants involved in cellular defense against free radicals. Surgical patients are at risk for selenium and zinc deficiencies secondary to both increased needs and losses. TPN blood work protocols should include monitoring of selenium and zinc with supplementation of the nutrient solutions, as required.

Microcalorimetric study of the toxic effect of selenium on the mitochondrial metabolism of Cyprinus carpio liver

The metabolic thermograms and heat output of mitochondria isolated from carp liver have been determined by using an LKB bioactivity monitor. The thermogram can be divided into four parts: the lag phase, active recovery phase, stationary phase, and decline phase. The thermokinetic equation was established for the active recovery and decline phase of metabolism as follows: dP/dt = k(m)P (1-SP). The rate constants k1 and k2 of two phases of active recovery and decline phase have been also calculated. The metabolism activity of mitochondrial inhibited by a high concentration of trace element selenium has been studied. The metabolic heat released, time of each phase, and rate constants can be significantly influenced by excess of selenite added. These results suggested that a high concentration of selenium can damage the structure and function of mitochondria, and thus influence their metabolism.