Overcoming Challenges with Biochemical Studies of Selenocysteine and Selenoproteins.

Selenocysteine (Sec) is an essential amino acid that distinguishes itself from cysteine by a selenium atom in place of a sulfur atom. This single change imparts distinct chemical properties to Sec which are crucial for selenoprotein (Sec-containing protein) function. These properties include a lower pKa, enhanced nucleophilicity, and reversible oxidation. However, studying Sec incorporation in proteins is a complex process. While we find Sec in all domains of life, each domain has distinct translation mechanisms. These mechanisms are unique to canonical translation and are composed of Sec-specific enzymes and an mRNA hairpin to drive recoding of the UGA stop codon with Sec. In this review, we highlight the obstacles that arise when investigating Sec insertion, and the role that Sec has in proteins. We discuss the strategic methods implemented in this field to address these challenges. Though the Sec translation system is complex, a remarkable amount of information has been obtained and specialized tools have been developed. Continued studies in this area will provide a deeper understanding on the role of Sec in the context of proteins, and the necessity that we have for maintaining this complex translation machinery to make selenoproteins.

Enzymatic strategies for selenium incorporation into biological molecules.

The trace element selenium (Se) is essential to the physiology of most organisms on the planet. The most well documented of Se's biological forms are selenoproteins, where selenocysteine often serves as the catalytic center for crucial redox processes. Se is also found in several other classes of biological molecules, including nucleic acids, sugars, and modified amino acids, although its role in the function of these metabolites is less understood. Despite its prevalence, only a small number of Se-specific biosynthetic pathways have been discovered. Around half of these were first characterized in the past three years, suggesting that the selenometabolome may be more diverse than previously appreciated. Here, we review the recent advances in our understanding of this intriguing biochemical space, and discuss prospects for future discovery efforts.

PRDX6 augments selenium utilization to limit iron toxicity and ferroptosis.

Ferroptosis is a form of regulated cell death induced by iron-dependent accumulation of lipid hydroperoxides. Selenoprotein glutathione peroxidase 4 (GPX4) suppresses ferroptosis by detoxifying lipid hydroperoxides via a catalytic selenocysteine (Sec) residue. Sec, the genetically encoded 21st amino acid, is biosynthesized from a reactive selenium donor on its cognate tRNA[Ser]Sec. It is thought that intracellular selenium must be delivered 'safely' and 'efficiently' by a carrier protein owing to its high reactivity and very low concentrations. Here, we identified peroxiredoxin 6 (PRDX6) as a novel selenoprotein synthesis factor. Loss of PRDX6 decreases the expression of selenoproteins and induces ferroptosis via a reduction in GPX4. Mechanistically, PRDX6 increases the efficiency of intracellular selenium utilization by transferring selenium between proteins within the selenocysteyl-tRNA[Ser]Sec synthesis machinery, leading to efficient synthesis of selenocysteyl-tRNA[Ser]Sec. These findings highlight previously unidentified selenium metabolic systems and provide new insights into ferroptosis.

Asgard archaeal selenoproteome reveals a roadmap for the archaea-to-eukaryote transition of selenocysteine incorporation machinery.

Selenocysteine (Sec) is encoded by the UGA codon that normally functions as a stop signal and is specifically incorporated into selenoproteins via a unique recoding mechanism. The translational recoding of UGA as Sec is directed by an unusual RNA structure, the SECIS element. Although archaea and eukaryotes adopt similar Sec encoding machinery, the SECIS elements have no similarities to each other with regard to sequence and structure. We analyzed >400 Asgard archaeal genomes to examine the occurrence of both Sec encoding system and selenoproteins in this archaeal superphylum, the closest prokaryotic relatives of eukaryotes. A comprehensive map of Sec utilization trait has been generated, providing the most detailed understanding of the use of this nonstandard amino acid in Asgard archaea so far. By characterizing the selenoproteomes of all organisms, several selenoprotein-rich phyla and species were identified. Most Asgard archaeal selenoprotein genes possess eukaryotic SECIS-like structures with varying degrees of diversity. Moreover, euryarchaeal SECIS elements might originate from Asgard archaeal SECIS elements via lateral gene transfer, indicating a complex and dynamic scenario of the evolution of SECIS element within archaea. Finally, a roadmap for the transition of eukaryotic SECIS elements from archaea was proposed, and selenophosphate synthetase may serve as a potential intermediate for the generation of ancestral eukaryotic SECIS element. Our results offer new insights into a deeper understanding of the evolution of Sec insertion machinery.

Glutathione peroxidase (GPX1) - Selenocysteine metabolism preserves the follicular fluid's (FF) redox homeostasis via IGF-1- NMD cascade in follicular ovarian cysts (FOCs).

Follicular ovarian cysts (FOCs) are characterized by follicles in the ovaries that are >20 mm in diameter and persist for >10 days without the corpus luteum, leading to anovulation, dysregulation of folliculogenesis and subfertility in humans and livestock species. Despite their clinical significance, the precise impact of FOCs on oocyte reserve, maturation, and quality still needs to be explored. While FOCs are observed in both human and livestock populations, they are notably prevalent in livestock species. Consequently, livestock species serve as valuable models for investigating the molecular intricacies of FOCs. Thus, in this study, using goat FOCs, we performed integrated proteomic, metabolomic and functional analyses to demonstrate that oocyte maturation is hampered due to increased reactive oxygen species (ROS) in FOCs follicular fluid (FF) via downregulation of glutathione peroxidase (GPX1), a critical antioxidant seleno enzyme required to negate oxidative stress. Notably, GPX1 reduction was positively correlated with the FF's decline of free selenium and selenocysteine metabolic enzymes, O-phosphoryl-tRNA (Sec) selenium transferase (SEPSECS) and selenocysteine lyase (SCLY) levels. Adding GPX1, selenocysteine, or selenium to the culture media rescued the oocyte maturation abnormalities caused by FOCs FF by down-regulating the ROS. Additionally, we demonstrate that substituting GPX1 regulator, Insulin-like growth factor-I (IGF-1) in the in vitro maturation media improved the oocyte maturation in the cystic FF by down-regulating the ROS activity via suppressing Non-sense-mediated decay (NMD) of GPX1. In contrast, inhibition of IGF-1R and the target of rapamycin complex 1 (mTORC1) hampered the oocyte maturation via NMD up-regulation. These findings imply that the GPX1 regulation via selenocysteine metabolism and the IGF-1-mediated NMD may be critical for the redox homeostasis of FF. We propose that GPX1 enhancers hold promise as therapeutics for enhancing the competence of FOCs oocytes. However, further in vivo studies are necessary to validate these findings observed in vitro.

Replacing a Cysteine Ligand by Selenocysteine in a [NiFe]-Hydrogenase Unlocks Hydrogen Production Activity and Addresses the Role of Concerted Proton-Coupled Electron Transfer in Electrocatalytic Reversibility.

Hydrogenases catalyze hydrogen/proton interconversion that is normally electrochemically reversible (having minimal overpotential requirement), a special property otherwise almost exclusive to platinum metals. The mechanism of [NiFe]-hydrogenases includes a long-range proton-coupled electron-transfer process involving a specific Ni-coordinated cysteine and the carboxylate of a nearby glutamate. A variant in which this cysteine has been exchanged for selenocysteine displays two distinct changes in electrocatalytic properties, as determined by protein film voltammetry. First, proton reduction, even in the presence of H2 (a strong product inhibitor), is greatly enhanced relative to H2 oxidation: this result parallels a characteristic of natural [NiFeSe]-hydrogenases which are superior H2 production catalysts. Second, an inflection (an S-shaped "twist" in the trace) appears around the formal potential, the small overpotentials introduced in each direction (oxidation and reduction) signaling a departure from electrocatalytic reversibility. Concerted proton-electron transfer offers a lower energy pathway compared to stepwise transfers. Given the much lower proton affinity of Se compared to that of S, the inflection provides compelling evidence that concerted proton-electron transfer is important in determining why [NiFe]-hydrogenases are reversible electrocatalysts.

The Selenoproteome as a Dynamic Response Mechanism to Oxidative Stress in Hydrogenotrophic Methanogenic Communities.

Methanogenesis is a critical process in the carbon cycle that is applied industrially in anaerobic digestion and biogas production. While naturally occurring in diverse environments, methanogenesis requires anaerobic and reduced conditions, although varying degrees of oxygen tolerance have been described. Microaeration is suggested as the next step to increase methane production and improve hydrolysis in digestion processes; therefore, a deeper understanding of the methanogenic response to oxygen stress is needed. To explore the drivers of oxygen tolerance in methanogenesis, two parallel enrichments were performed under the addition of H2/CO2 in an environment without reducing agents and in a redox-buffered environment by adding redox mediator 9,10-anthraquinone-2,7-disulfonate disodium. The cellular response to oxidative conditions is mapped using proteomic analysis. The resulting community showed remarkable tolerance to high-redox environments and was unperturbed in its methane production. Next to the expression of pathways to mitigate reactive oxygen species, the higher redox potential environment showed an increased presence of selenocysteine and selenium-associated pathways. By including sulfur-to-selenium mass shifts in a proteomic database search, we provide the first evidence of the dynamic and large-scale incorporation of selenocysteine as a response to oxidative stress in hydrogenotrophic methanogenesis and the presence of a dynamic selenoproteome.

Ancient Loss of Catalytic Selenocysteine Spurred Convergent Adaptation in a Mammalian Oxidoreductase.

Selenocysteine, the 21st amino acid specified by the genetic code, is a rare selenium-containing residue found in the catalytic site of selenoprotein oxidoreductases. Selenocysteine is analogous to the common cysteine amino acid, but its selenium atom offers physical-chemical properties not provided by the corresponding sulfur atom in cysteine. Catalytic sites with selenocysteine in selenoproteins of vertebrates are under strong purifying selection, but one enzyme, glutathione peroxidase 6 (GPX6), independently exchanged selenocysteine for cysteine <100 million years ago in several mammalian lineages. We reconstructed and assayed these ancient enzymes before and after selenocysteine was lost and up to today and found them to have lost their classic ability to reduce hydroperoxides using glutathione. This loss of function, however, was accompanied by additional amino acid changes in the catalytic domain, with protein sites concertedly changing under positive selection across distant lineages abandoning selenocysteine in glutathione peroxidase 6. This demonstrates a narrow evolutionary range in maintaining fitness when sulfur in cysteine impairs the catalytic activity of this protein, with pleiotropy and epistasis likely driving the observed convergent evolution. We propose that the mutations shared across distinct lineages may trigger enzymatic properties beyond those in classic glutathione peroxidases, rather than simply recovering catalytic rate. These findings are an unusual example of adaptive convergence across mammalian selenoproteins, with the evolutionary signatures possibly representing the evolution of novel oxidoreductase functions.

Maternal seafood consumption is associated with improved selenium status: Implications for child health.

Selenium (Se) is required for synthesis of selenocysteine (Sec), an amino acid expressed in the active sites of Se-dependent enzymes (selenoenzymes), including forms with essential functions in fetal development, brain activities, thyroid hormone metabolism, calcium regulation, and to prevent or reverse oxidative damage. Homeostatic mechanisms normally ensure the brain is preferentially supplied with Se to maintain selenoenzymes, but high methylmercury (CH3Hg) exposures irreversibly inhibit their activities and impair Sec synthesis. Due to Hg's high affinity for sulfur, CH3Hg initially binds with the cysteine (Cys) moieties of thiomolecules which are selenoenzyme substrates. These CH3Hg-Cys adducts enter selenoenzyme active sites and transfer CH3Hg to Sec, thus irreversibly inhibiting their activities. High CH3Hg exposures are uniquely able to induce a conditioned Se-deficiency that impairs synthesis of brain selenoenzymes. Since the fetal brain lacks Se reserves, it is far more vulnerable to CH3Hg exposures than adult brains. This prompted concerns that maternal exposures to CH3Hg present in seafood might impair child neurodevelopment. However, typical varieties of ocean fish contain far more Se than CH3Hg. Therefore, eating them should augment Se-status and thus prevent Hg-dependent loss of fetal selenoenzyme activities. To assess this hypothesis, umbilical cord blood and placental tissue samples were collected following delivery of a cohort of 100 babies born on Oahu, Hawaii. Dietary food frequency surveys of the mother's last month of pregnancy identified groups with no (0 g/wk), low (0-12 g/wk), or high (12 + g/wk) levels of ocean fish consumption. Maternal seafood consumption increased Hg contents in fetal tissues and resulted in ∼34% of cord blood samples exceeding the EPA Hg reference level of 5.8 ppb (0.029 µM). However, Se concentrations in these tissues were orders of magnitude higher and ocean fish consumption caused cord blood Se to increase ∼9.4 times faster than Hg. Therefore, this study supports the hypothesis that maternal consumption of typical varieties of ocean fish provides substantial amounts of Se that protect against Hg-dependent losses in Se bioavailability. Recognizing the pivotal nature of the Hg:Se relationship provides a consilient perspective of seafood benefits vs. risks and clarifies the reasons for the contrasting findings of certain early studies.

Guiding bar motif of thioredoxin reductase 1 modulates enzymatic activity and inhibitor binding by communicating with the co-factor FAD and regulating the flexible C-terminal redox motif.

Thioredoxin reductase (TXNRD) is a selenoprotein that plays a crucial role in cellular antioxidant defense. Previously, a distinctive guiding bar motif was identified in TXNRD1, which influences the transfer of electrons. In this study, utilizing single amino acid substitution and Excitation-Emission Matrix (EEM) fluorescence spectrum analysis, we discovered that the guiding bar communicates with the FAD and modulates the electron flow of the enzyme. Differential Scanning Fluorimetry (DSF) analysis demonstrated that the aromatic amino acid in guiding bar is a stabilizer for TXNRD1. Kinetic analysis revealed that the guiding bar is vital for the disulfide reductase activity but hinders the selenocysteine-independent reduction activity of TXNRD1. Meanwhile, the guiding bar shields the selenocysteine residue of TXNRD1 from the attack of electrophilic reagents. We also found that the inhibition of TXNRD1 by caveolin-1 scaffolding domain (CSD) peptides and compound LCS3 did not bind to the guiding bar motif. In summary, the obtained results highlight new aspects of the guiding bar that restrict the flexibility of the C-terminal redox motif and govern the transition from antioxidant to pro-oxidant.

Se-(Methyl)-selenocysteine ameliorates blood-brain barrier disruption of focal cerebral ischemia mice via ferroptosis inhibition and tight junction upregulation in an Akt/GSK3ß-dependent manner.

BACKGROUND: Ischemic stroke still ranks as the most fatal disease worldwide. Blood-brain barrier (BBB) is a promising therapeutic target for protection. Brain microvascular endothelial cell is a core component of BBB, the barrier function maintenance of which can ameliorate ischemic injury and improve neurological deficit. Se-methyl L-selenocysteine (SeMC) has been shown to exert cardiovascular protection. However, the protection of SeMC against ischemic stroke remains to be elucidated. This research was designed to explore the protection of SeMC from the perspective of BBB protection. METHODS: To simulate cerebral ischemic injury, C57BL/6J mice were subjected to middle cerebral artery occlusion/reperfusion (MCAO/R), and bEnd.3 was exposed to oxygen-glucose deprivation/reoxygenation (OGD/R). After the intervention of SeMC, the barrier function and the expression of tight junction and ferroptosis-associated proteins were determined. For mechanism exploration, LY294002 (Akt inhibitor) was introduced both in vivo and in vitro. RESULTS: SeMC lessened the brain infarct volume and attenuated the leakage of BBB in mice. In vitro, SeMC improved cell viability and maintained the barrier function of bEnd.3 cells. The protection of SeMC was accompanied with ferroptosis inhibition and tight junction protein upregulation. Mechanism studies revealed that the effect of SeMC was reversed by LY294002, indicating that the protection of SeMC against ischemic stroke was mediated by the Akt signal pathway. CONCLUSION: These results suggested that SeMC exerted protection against ischemic stroke, which might be attributed to activating the Akt/GSK3ß signaling pathway and increasing the nuclear translocation of Nrf2 and ß-catenin, subsequently maintaining the integrity of BBB.

Discovery of GPX4 inhibitors through FP-based high-throughput screening.

Ferroptosis is a form of non-apoptotic cell death, regulated by phospholipid hydroperoxide glutathione peroxidase 4 (GPX4), a selenoprotein with a selenocysteine residue (sec) in the active site. GPX4 is a promising target for cancer cells in therapy-resistant conditions via ferroptosis, which can reduce the level of lipid reactive oxygen species (ROS). So far, all existing GPX4 inhibitors covalently bind to GPX4 via a reactive alkyl chloride moiety or masked nitrile-oxide electrophiles with poor selectivity and pharmacokinetic properties and most were obtained by cell phenotype-based screening. Lacking of effective high-throughput screening methods for GPX4 protein limits the discovery of GPX4 inhibitors. Here, we report a fluorescence polarization (FP)-based high throughput screening (HTS) assay for GPX4-U46C-C10A-C66A in vitro, and found Metamizole sodium from our in-house compound library inhibits GPX4-U46C-C10A-C66A enzyme activity. Structure-activity relationships (SAR) demonstrated the importance of sulfonyl group on interaction between Metamizole sodium and GPX4-U46C-C10A-C66A. Our FP assay could be an effective tool for discovery of GPX4 inhibitors and Metamizole sodium was a potential inhibitor for GPX4 in vitro.

Effect of graded levels of selenium supplementation as selenite on expression of selenosugars, selenocysteine, and other selenometabolites in rat liver.

Using high pressure liquid chromatography (HPLC) coupled with selenium-specific inductively coupled plasma mass spectrometry (ICP-MS) and molecule specific (Orbitrap MS/MS) detection, we previously found that far more selenium (Se) is present as selenosugar (seleno-N-acetyl galactosamine) in Se-adequate turkey liver than is present as selenocysteine (Sec) in true selenoproteins, and that selenosugars account for half of the Se in high-Se turkey liver. To expand these observations to mammals, we studied Se metabolism in rats fed graded levels of selenite from 0 to 5 µg Se/g for 4 wk. In Se-adequate (0.24 µg Se/g) rats, 43% of liver Se was present as Sec, 32% was present as selenosugars, and 22% as inorganic Se bound to protein. In liver of rats fed 5 µg Se/g as selenite, the quantity of Sec remained at the Se-adequate plateau (11% of total Se), 22% was present as low molecular weight (LMW) selenosugars with substantial additional selenosugars linked to protein, but 64% was present as inorganic Se bound to protein. No selenomethionine was found at any level of selenite supplementation. Below the Se requirement, Se is preferentially incorporated into Sec-selenoproteins. Above the dietary Se requirement, selenosugars become by far the major LMW water soluble Se species in liver, and levels of selenosugar-decorated proteins are far higher than Sec-selenoproteins, making these selenosugar-decorated proteins the major Se-containing protein species in liver with high Se supplementation. This accumulation of selenosugars linked to cysteines on proteins or the build-up of inorganic Se bound to protein may underlie Se toxicity at the molecular level.

Selenomethionine supplementation and expression of selenosugars, selenocysteine, and other selenometabolites in rat liver.

Selenomethionine (SeMet) as a methionine analog can be incorporated into protein. In turkeys, we recently found that selenium (Se) as selenite is not metabolized to SeMet but rather to selenosugars (seleno-N-acetyl galactosamine) bound to protein as well as to selenocysteine (Sec) in selenoproteins. To characterize the metabolism of SeMet, we fed rats graded levels of SeMet from 0 to 5 µg Se/g in a Se-deficient diet for 4 wk, and investigated the fate and accumulation of liver Se using high pressure liquid chromatography (HPLC) coupled with Se-specific inductively coupled plasma mass spectrometry (ICP-MS) and molecule specific (Orbitrap MS/MS) detection. Up to 0.24 µg Se/g (Se requirement for maximal glutathione peroxidase activity), Sec accounted for ∼40% of total liver Se whereas SeMet only accounted for 3-11%. Analysis of water-soluble extracts found negligible low molecular weight (LMW) Se species in rats fed 0 and 0.08 µg Se/g, including no SeMet. At 0.24 µg Se/g and above, SeMet accounted for only 10% of LMW Se species, whereas methyl- and glutathionyl-selenosugars accounted for 70% of LMW Se species. Above the Se requirement, SeMet was ∼30% of the proteinaceous amino acids, whereas Sec levels fell to 5% in rats fed 5 µg Se/g as SeMet. Last, considerably less inorganic Se was bound to liver protein with high SeMet as compared to selenite in a parallel study. SeMet is efficiently metabolized and mixes with the common Se metabolite pool, where Se is preferentially incorporated into Sec and Sec-selenoproteins until selenoproteins plateau; with high SeMet intake, Se is increasingly accumulated as LMW selenosugars and as selenosugar-decorated proteins.

"Alphabet" Selenoproteins: Implications in Pathology.

Selenoproteins are a group of proteins containing selenium in the form of selenocysteine (Sec, U) as the 21st amino acid coded in the genetic code. Their synthesis depends on dietary selenium uptake and a common set of cofactors. Selenoproteins accomplish diverse roles in the body and cell processes by acting, for example, as antioxidants, modulators of the immune function, and detoxification agents for heavy metals, other xenobiotics, and key compounds in thyroid hormone metabolism. Although the functions of all this protein family are still unknown, several disorders in their structure, activity, or expression have been described by researchers. They concluded that selenium or cofactors deficiency, on the one hand, or the polymorphism in selenoproteins genes and synthesis, on the other hand, are involved in a large variety of pathological conditions, including type 2 diabetes, cardiovascular, muscular, oncological, hepatic, endocrine, immuno-inflammatory, and neurodegenerative diseases. This review focuses on the specific roles of selenoproteins named after letters of the alphabet in medicine, which are less known than the rest, regarding their implications in the pathological processes of several prevalent diseases and disease prevention.

Seryl-tRNA synthetase promotes translational readthrough by mRNA binding and involvement of the selenocysteine incorporation machinery.

Translational readthrough of UGA stop codons by selenocysteine-specific tRNA (tRNASec) enables the synthesis of selenoproteins. Seryl-tRNA synthetase (SerRS) charges tRNASec with serine, which is modified into selenocysteine and delivered to the ribosome by a designated elongation factor (eEFSec in eukaryotes). Here we found that components of the human selenocysteine incorporation machinery (SerRS, tRNASec, and eEFSec) also increased translational readthrough of non-selenocysteine genes, including VEGFA, to create C-terminally extended isoforms. SerRS recognizes target mRNAs through a stem-loop structure that resembles the variable loop of its cognate tRNAs. This function of SerRS depends on both its enzymatic activity and a vertebrate-specific domain. Through eCLIP-seq, we identified additional SerRS-interacting mRNAs as potential readthrough genes. Moreover, SerRS overexpression was sufficient to reverse premature termination caused by a pathogenic nonsense mutation. Our findings expand the repertoire of selenoprotein biosynthesis machinery and suggest an avenue for therapeutic targeting of nonsense mutations using endogenous factors.

Selenoproteins and tRNA-Sec: regulators of cancer redox homeostasis.

In the past two decades significant progress has been made in uncovering the biological function of selenium. Selenium, an essential trace element, is required for the biogenesis of selenocysteine which is then incorporated into selenoproteins. These selenoproteins have emerged as central regulators of cellular antioxidant capacity and maintenance of redox homeostasis. This review provides a comprehensive examination of the multifaceted functions of selenoproteins with a particular emphasis on their contributions to cellular antioxidant capacity. Additionally, we highlight the promising potential of targeting selenoproteins and the biogenesis of selenocysteine as avenues for therapeutic intervention in cancer. By understanding the intricate relationship between selenium, selenoproteins, and reactive oxygen species (ROS), insights can be gained to develop therapies that exploit the inherent vulnerabilities of cancer cells.

Nontoxic Levels of Se-Containing Compounds Increase Survival by Blocking Oxidative and Inflammatory Stresses via Signal Pathways Whereas High Levels of Se Induce Apoptosis.

Selenium is a main group element and an essential trace element in human health. It was discovered in selenocysteine (SeC) by Stadtman in 1974. SeC is an encoded natural amino acid hailed as the 21st naturally occurring amino acid (U) present in several enzymes and which exquisitely participates in redox biology. As it turns out, selenium bears a U-shaped toxicity curve wherein too little of the nutrient present in biology leads to disorders; concentrations that are too great, on the other hand, pose toxicity to biological systems. In light of many excellent previous reviews and the corpus of literature, we wanted to offer this current review, in which we present aspects of the clinical and biological literature and justify why we should further investigate Se-containing species in biological and medicinal contexts, especially small molecule-containing species in biomedical research and clinical medicine. Of central interest is how selenium participates in biological signaling pathways. Several clinical medical cases are recounted; these reports are mainly pertinent to human cancer and changes in pathology and cases in which the patients are often terminal. Selenium was an option chosen in light of earlier chemotherapeutic treatment courses which lost their effectiveness. We describe apoptosis, and also ferroptosis, and senescence clearly in the context of selenium. Other contemporary issues in research also compelled us to form this review: issues with CoV-2 SARS infection which abound in the literature, and we described findings with human patients in this context. Laboratory scientific studies and clinical studies dealing with two main divisions of selenium, organic (e.g., methyl selenol) or inorganic selenium (e.g., sodium selenite), are discussed. The future seems bright with the research and clinical possibilities of selenium as a trace element, whose recent experimental clinical treatments have so far involved dosing simply and inexpensively over a set of days, amounts, and time intervals.

An efficient selenium transport pathway of selenoprotein P utilizing a high-affinity ApoER2 receptor variant and being independent of selenocysteine lyase.

Selenoprotein P (SeP, encoded by the SELENOP gene) is a plasma protein that contains selenium in the form of selenocysteine residues (Sec, a cysteine analog containing selenium instead of sulfur). SeP functions for the transport of selenium to specific tissues in a receptor-dependent manner. Apolipoprotein E receptor 2 (ApoER2) has been identified as a SeP receptor. However, diverse variants of ApoER2 have been reported, and the details of its tissue specificity and the molecular mechanism of its efficiency remain unclear. In the present study, we found that human T lymphoma Jurkat cells have a high ability to utilize selenium via SeP, while this ability was low in human rhabdomyosarcoma cells. We identified an ApoER2 variant with a high affinity for SeP in Jurkat cells. This variant had a dissociation constant value of 0.67 nM and a highly glycosylated O-linked sugar domain. Moreover, the acidification of intracellular vesicles was necessary for selenium transport via SeP in both cell types. In rhabdomyosarcoma cells, SeP underwent proteolytic degradation in lysosomes and transported selenium in a Sec lyase-dependent manner. However, in Jurkat cells, SeP transported selenium in Sec lyase-independent manner. These findings indicate a preferential selenium transport pathway involving SeP and high-affinity ApoER2 in a Sec lyase-independent manner. Herein, we provide a novel dynamic transport pathway for selenium via SeP.

Biological and Catalytic Properties of Selenoproteins.

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.