Utilization of selenocysteine as a source of selenium for selenophosphate biosynthesis

Combinatorial accumulation, stress response, detoxification and synaptic transmission effects of cadmium and selenium in clams Ruditapes philippinarum

This study investigated the toxicological effects and mechanisms of cadmium (Cd) (5 and 50 μg/L) and selenium (Se) (3 and 30 μg/L) at environmentally relevant concentrations on the gills and digestive glands of clams Ruditapes philippinarum. Results indicated that Cd and Se could tissue-specifically impact osmoregulation, energy metabolism, and synaptic transmission in the gills and digestive glands of clams. After exposure to 50 μg/L Cd, the digestive glands of clams up-regulated the expression of methionine-gamma-lyase and metallothionein for detoxification. Clam digestive glands exposed to 3 μg/L Se up-regulated the expression of catalase and glutathione peroxidase to alleviate oxidative stress, and down-regulated the expression of selenide-water dikinase to reduce the conversion of inorganic Se. Additionally, the interaction mode between Cd and Se largely depended on their molar ratio, with a ratio of 11.71 (50 μg/L Cd + 3 μg/L Se) demonstrated to be particularly harmful, as manifested by significantly more lesions, oxidative stress, and detoxification demand in clams than those exposed to Cd or Se alone. Collectively, this study revealed the complex interaction patterns and mechanisms of Cd and Se on clams, providing a reference for exploring their single and combined toxicity.

In vitro digestion and fecal fermentation of selenocompounds: impact on gut microbiota, antioxidant activity, and short-chain fatty acids

Selenium bioavailability is critically influenced by gut microbiota, yet the interaction dynamics with selenocompounds remain unexplored. Our study found that L-Selenomethionine (SeMet) and Se-(Methyl)seleno-L-cysteine (MeSeCys) maintained stability during in vitro gastrointestinal digestion. In contrast, Selenite and L-Selenocystine (SeCys2) were degraded by approximately 13% and 35%. Intriguingly, gut microflora transformed MeSeCys, SeCys2, and Selenite into SeMet. Moreover, when SeCys2 and Selenite incubated with gut microbiota, they produced red selenium nanoparticles with diameters ranging between 100 and 400 nm and boosted glutathione peroxidase activity. These changes were positively associated with an increased relative abundance of unclassified_g__Blautia (Family Lachnospiraceae), Erysipelotrichaceae_UCG-003 (Family Erysipelatoclostridiaceae), and uncultured_bacterium_g__Subdoligranulum (Family Ruminococcaceae). Our findings implied that differential microbial sensitivities to selenocompounds, potentially attributable to their distinct mechanisms governing selenium uptake, storage, utilization, and excretion.

Selenium-Dependent Antioxidant Enzymes: Actions and Properties of Selenoproteins

Unlike other essential trace elements that interact with proteins in the form of cofactors, selenium (Se) becomes co-translationally incorporated into the polypeptide chain as part of 21st naturally occurring amino acid, selenocysteine (Sec), encoded by the UGA codon. Any protein that includes Sec in its polypeptide chain is defined as selenoprotein. Members of the selenoproteins family exert various functions and their synthesis depends on specific cofactors and on dietary Se. The Se intake in productive animals such as chickens affect nutrient utilization, production performances, antioxidative status and responses of the immune system. Although several functions of selenoproteins are unknown, many disorders are related to alterations in selenoprotein expression or activity. Selenium insufficiency and polymorphisms or mutations in selenoproteins' genes and synthesis cofactors are involved in the pathophysiology of many diseases, including cardiovascular disorders, immune dysfunctions, cancer, muscle and bone disorders, endocrine functions and neurological disorders. Finally, heavy metal poisoning decreases mRNA levels of selenoproteins and increases mRNA levels of inflammatory factors, underlying the antagonistic effect of Se. This review is an update on Se dependent antioxidant enzymes, presenting the current state of the art and is focusing on results obtained mainly in chicken.

Biological Chemistry of Hydrogen Selenide

There are no two main-group elements that exhibit more similar physical and chemical properties than sulfur and selenium. Nonetheless, Nature has deemed both essential for life and has found a way to exploit the subtle unique properties of selenium to include it in biochemistry despite its congener sulfur being 10,000 times more abundant. Selenium is more easily oxidized and it is kinetically more labile, so all selenium compounds could be considered to be "Reactive Selenium Compounds" relative to their sulfur analogues. What is furthermore remarkable is that one of the most reactive forms of selenium, hydrogen selenide (HSe- at physiologic pH), is proposed to be the starting point for the biosynthesis of selenium-containing molecules. This review contrasts the chemical properties of sulfur and selenium and critically assesses the role of hydrogen selenide in biological chemistry.

Why Nature Chose Selenium

The authors were asked by the Editors of ACS Chemical Biology to write an article titled "Why Nature Chose Selenium" for the occasion of the upcoming bicentennial of the discovery of selenium by the Swedish chemist Jöns Jacob Berzelius in 1817 and styled after the famous work of Frank Westheimer on the biological chemistry of phosphate [Westheimer, F. H. (1987) Why Nature Chose Phosphates, Science 235, 1173-1178]. This work gives a history of the important discoveries of the biological processes that selenium participates in, and a point-by-point comparison of the chemistry of selenium with the atom it replaces in biology, sulfur. This analysis shows that redox chemistry is the largest chemical difference between the two chalcogens. This difference is very large for both one-electron and two-electron redox reactions. Much of this difference is due to the inability of selenium to form π bonds of all types. The outer valence electrons of selenium are also more loosely held than those of sulfur. As a result, selenium is a better nucleophile and will react with reactive oxygen species faster than sulfur, but the resulting lack of π-bond character in the Se-O bond means that the Se-oxide can be much more readily reduced in comparison to S-oxides. The combination of these properties means that replacement of sulfur with selenium in nature results in a selenium-containing biomolecule that resists permanent oxidation. Multiple examples of this gain of function behavior from the literature are discussed.

Sulfur and selenium antioxidants: challenging radical scavenging mechanisms and developing structure-activity relationships based on metal binding

Because sulfur and selenium antioxidants can prevent oxidative damage, numerous animal and clinical trials have investigated the ability of these compounds to prevent the oxidative stress that is an underlying cause of cardiovascular disease, Alzheimer's disease, and cancer, among others. One of the most common sources of oxidative damage is metal-generated hydroxyl radical; however, very little research has focused on determining the metal-binding abilities and structural attributes that affect oxidative damage prevention by sulfur and selenium compounds. In this review, we describe our ongoing investigations into sulfur and selenium antioxidant prevention of iron- and copper-mediated oxidative DNA damage. We determined that many sulfur and selenium compounds inhibit Cu(I)-mediated DNA damage and that DNA damage prevention varies dramatically when Fe(II) is used in place of Cu(I) to generate hydroxyl radical. Oxidation potentials of the sulfur or selenium compounds do not correlate with their ability to prevent DNA damage, highlighting the importance of metal coordination rather than reactive oxygen species scavenging as an antioxidant mechanism. Additional gel electrophoresis, mass spectrometry, and UV-visible studies confirmed sulfur and selenium antioxidant binding to Cu(I) and Fe(II). Ultimately, our studies established that both the hydroxyl-radical-generating metal ion and the chemical environment of the sulfur or selenium significantly affect DNA damage prevention and that metal coordination is an essential mechanism for these antioxidants.

Cysteine transport through excitatory amino acid transporter 3 (EAAT3)

Excitatory amino acid transporters (EAATs) limit glutamatergic signaling and maintain extracellular glutamate concentrations below neurotoxic levels. Of the five known EAAT isoforms (EAATs 1–5), only the neuronal isoform, EAAT3 (EAAC1), can efficiently transport the uncharged amino acid L-cysteine. EAAT3-mediated cysteine transport has been proposed to be a primary mechanism used by neurons to obtain cysteine for the synthesis of glutathione, a key molecule in preventing oxidative stress and neuronal toxicity. The molecular mechanisms underlying the selective transport of cysteine by EAAT3 have not been elucidated. Here we propose that the transport of cysteine through EAAT3 requires formation of the thiolate form of cysteine in the binding site. Using Xenopus oocytes and HEK293 cells expressing EAAT2 and EAAT3, we assessed the transport kinetics of different substrates and measured transporter-associated currents electrophysiologically. Our results show that L-selenocysteine, a cysteine analog that forms a negatively-charged selenolate ion at physiological pH, is efficiently transported by EAATs 1–3 and has a much higher apparent affinity for transport when compared to cysteine. Using a membrane tethered GFP variant to monitor intracellular pH changes associated with transport activity, we observed that transport of either L-glutamate or L-selenocysteine by EAAT3 decreased intracellular pH, whereas transport of cysteine resulted in cytoplasmic alkalinization. No change in pH was observed when cysteine was applied to cells expressing EAAT2, which displays negligible transport of cysteine. Under conditions that favor release of intracellular substrates through EAAT3 we observed release of labeled intracellular glutamate but did not detect cysteine release. Our results support a model whereby cysteine transport through EAAT3 is facilitated through cysteine de-protonation and that once inside, the thiolate is rapidly re-protonated. Moreover, these findings suggest that cysteine transport is predominantly unidirectional and that reverse transport does not contribute to depletion of intracellular cysteine pools.

Bioavailability of selenium from the selenotrisulphide derivative of lipoic acid

BACKGROUND/PURPOSE

Selenium is a required micronutrient in mammals, needed for the activity of enzymes that contain selenocysteine at their active site. Several isoenzymes of glutathione peroxidase and thioredoxin reductase contain selenocysteine and thus the nutritional status of selenium in tissues can have significant impact on the steady state level of reactive oxygen species. The aims of this study were to evaluate the bioavailability of selenium derived from the selenotrisulfide derivative of lipoic acid (LASe) and determine the ability of this compound to be absorbed into skin.

METHODS

Bioavailability of selenium derived from LASe was determined using a keratinocyte cell model (HaCat). Efficiency of utilization of selenium was assessed by following the decrease in the incorporation of radiolabeled selenite (75Se) in the presence of increasing concentration of selenium compounds. Percutaneous absorption of LASe was measured by determining selenium levels in full thickness biopsy of skin using a Yorkshire pig model.

RESULTS

LASe was efficiently absorbed topically into pig skin, a good model of human skin. In a keratinocyte cell line LASe was an efficient source of selenium for selenoprotein synthesis, demonstrating that LASe is a good candidate as a topical selenium micronutrient. Both L-selenomethionine and selenate were found to be poor sources of selenium for selenoprotein synthesis in the skin cell model and L-selenomethionine was poorly absorbed into pig skin.

CONCLUSION

These results indicate that stable selenotrisulfides, such as LASe, are good candidates for testing as topical selenium supplements.

Selenium uptake, translocation, assimilation and metabolic fate in plants

The chemical and physical resemblance between selenium (Se) and sulfur (S) establishes that both these elements share common metabolic pathways in plants. The presence of isologous Se and S compounds indicates that these elements compete in biochemical processes that affect uptake, translocation and assimilation throughout plant development. Yet, minor but crucial differences in reactivity and other metabolic interactions infer that some biochemical processes involving Se may be excluded from those relating to S. This review examines the current understanding of physiological and biochemical relationships between S and Se metabolism by highlighting their similarities and differences in relation to uptake, transport and assimilation pathways as observed in Se hyperaccumulator and non-accumulator plant species. The exploitation of genetic resources used in bioengineering strategies of plants is illuminating the function of sulfate transporters and key enzymes of the S assimilatory pathway in relation to Se accumulation and final metabolic fate. These strategies are providing the basic framework by which to resolve questions relating to the essentiality of Se in plants and the mechanisms utilized by Se hyperaccumulators to circumvent toxicity. In addition, such approaches may assist in the future application of genetically engineered Se accumulating plants for environmental renewal and human health objectives.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Selenocysteine in proteins-properties and biotechnological use

Selenocysteine (Sec), the 21st amino acid, exists naturally in all kingdoms of life as the defining entity of selenoproteins. Sec is a cysteine (Cys) residue analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group in Cys. The selenium atom gives Sec quite different properties from Cys. The most obvious difference is the lower pK(a) of Sec, and Sec is also a stronger nucleophile than Cys. Proteins naturally containing Sec are often enzymes, employing the reactivity of the Sec residue during the catalytic cycle and therefore Sec is normally essential for their catalytic efficiencies. Other unique features of Sec, not shared by any of the other 20 common amino acids, derive from the atomic weight and chemical properties of selenium and the particular occurrence and properties of its stable and radioactive isotopes. Sec is, moreover, incorporated into proteins by an expansion of the genetic code as the translation of selenoproteins involves the decoding of a UGA codon, otherwise being a termination codon. In this review, we will describe the different unique properties of Sec and we will discuss the prerequisites for selenoprotein production as well as the possible use of Sec introduction into proteins for biotechnological applications. These include residue-specific radiolabeling with gamma or positron emitters, the use of Sec as a reactive handle for electophilic probes introducing fluorescence or other peptide conjugates, as the basis for affinity purification of recombinant proteins, the trapping of folding intermediates, improved phasing in X-ray crystallography, introduction of 77Se for NMR spectroscopy, or, finally, the analysis or tailoring of enzymatic reactions involving thiol or oxidoreductase (redox) selenolate chemistry.

Toxicity of selenite in the unicellular green alga Chlamydomonas reinhardtii: comparison between effects at the population and sub-cellular level

The toxicity of selenium in aquatic ecosystems is mainly linked to its uptake and biotransformation by micro-organisms, and its subsequent transfer upwards into the food chain. Thus, organisms at low trophic level, such as algae, play a crucial role. The aim of our study was to investigate the biological effects of selenite on Chlamydomonas reinhardtii, both at the sub-cellular level (effect on ultrastructure) and at the population level (effect on growth). The cells were grown under batch culture conditions in well-defined media and exposed to waterborne selenite at concentrations up to 500 microM; i.e. up to lethal conditions. Based on the relationship between Se concentration and cell density achieved after a 96 h exposure period, an EC(50) of 80 microM with a 95% confidence interval ranging between 64 and 98 microM was derived. No adaptation mechanisms were observed: the same toxicity was quantified for algae pre-contaminated with Se. The inhibition of growth was linked to impairments observed at the sub-cellular level. The intensity of the ultrastructural damages caused by selenite exposure depended on the level and duration of exposure. Observations by TEM suggested chloroplasts as the first target of selenite cytotoxicity, with effects on the stroma, thylakoids and pyrenoids. At higher concentrations, we could observe an increase in the number and volume of starch grains. For cells collected at 96 h, electron-dense granules were observed. Energy-dispersive X-ray microanalysis revealed that these granules contained selenium and were also rich in calcium and phosphorus. This study confirms that the direct toxicity of selenite on the phytoplankton biomass is not likely to take place at concentrations found in the environment. At higher concentrations, the link between effects at the sub-cellular and population levels, the over-accumulation of starch, and the formation of dense granules containing selenium are reported for the first time in the literature for a phytoplankton species after exposure to selenite.