Inhibitory effect of ebselen on lactate dehydrogenase activity from mammals: a comparative study with diphenyl diselenide and diphenyl ditelluride

Does Methionine Status Influence the Outcome of Selenomethinione Supplementation? A Comparative Study of Metabolic and Selenium Levels in HepG2 Cells

Methionine restriction and selenium supplementation are recommended because of their health benefits. As a major nutrient form in selenium supplementation, selenomethionine shares a similar biological process to its analog methionine. However, the outcome of selenomethionine supplementation under different methionine statuses and the interplay between these two nutrients remain unclear. Therefore, this study explored the metabolic effects and selenium utilization in HepG2 cells supplemented with selenomethionine under deprived, adequate, and abundant methionine supply conditions by using nuclear magnetic resonance-based metabolomic and molecular biological approaches. Results revealed that selenomethionine promoted the proliferation of HepG2 cells, the transcription of selenoproteins, and the production of most amino acids while decreasing the levels of creatine, aspartate, and nucleoside diphosphate sugar regardless of methionine supply. Selenomethionine substantially disturbed the tricarboxylic acid cycle and choline metabolism in cells under a methionine shortage. With increasing methionine supply, the metabolic disturbance was alleviated, except for changes in lactate, glycine, citrate, and hypoxanthine. The markable selenium accumulation and choline decrease in the cells under methionine shortage imply the potential risk of selenomethionine supplementation. This work revealed the biological effects of selenomethionine under different methionine supply conditions. This study may serve as a guide for controlling methionine and selenomethionine levels in dietary intake.

Toxicology and pharmacology of synthetic organoselenium compounds: an update

Here, we addressed the pharmacology and toxicology of synthetic organoselenium compounds and some naturally occurring organoselenium amino acids. The use of selenium as a tool in organic synthesis and as a pharmacological agent goes back to the middle of the nineteenth and the beginning of the twentieth centuries. The rediscovery of ebselen and its investigation in clinical trials have motivated the search for new organoselenium molecules with pharmacological properties. Although ebselen and diselenides have some overlapping pharmacological properties, their molecular targets are not identical. However, they have similar anti-inflammatory and antioxidant activities, possibly, via activation of transcription factors, regulating the expression of antioxidant genes. In short, our knowledge about the pharmacological properties of simple organoselenium compounds is still elusive. However, contrary to our early expectations that they could imitate selenoproteins, organoselenium compounds seem to have non-specific modulatory activation of antioxidant pathways and specific inhibitory effects in some thiol-containing proteins. The thiol-oxidizing properties of organoselenium compounds are considered the molecular basis of their chronic toxicity; however, the acute use of organoselenium compounds as inhibitors of specific thiol-containing enzymes can be of therapeutic significance. In summary, the outcomes of the clinical trials of ebselen as a mimetic of lithium or as an inhibitor of SARS-CoV-2 proteases will be important to the field of organoselenium synthesis. The development of computational techniques that could predict rational modifications in the structure of organoselenium compounds to increase their specificity is required to construct a library of thiol-modifying agents with selectivity toward specific target proteins.

Telluric acid ameliorates hepatic ischemia reperfusion-induced injury in rats: Involvement of TLR4, Nrf2, and PI3K/Akt signaling pathways

In past tellurium-based compounds had limited use, however, their therapeutic potential have been target of interest recently due to antioxidant and anti-inflammatory capabilities in experimental endotoxemia. Nevertheless, their potential hepatoprotective effect against ischemia reperfusion (IR) injury is still obscure. This study examined the possible hepatoprotective effect of telluric acid (TELL), one of tellurium-based compound, against the deteriorating effect hepatic IR injury in rats through directing toll like receptor-4 (TLR4) cascade, phosphoinositide 3-kinase(PI3K)/Akt axis, and nuclear erythroid-related factor-2 (Nrf-2) pathway as possible mechanisms contributed to TELL's effect. Indeed, male Wistar rats were randomized into 3 groups: sham-operated, control IR and TELL (50 µg/kg). TELL was administrated once daily for seven consecutive days prior to the IR induction. Pretreatment with TELL attenuated hepatic IR injury as manifested by hampered plasma aminotransaminases and lactate dehydrogenase activities. Also, TELL opposed IR induced elevation in tissue expression/activity of high-mobility group box protein-1 (HMGB1), TLR4, myeloid differentiation primary-response protein 88 (MyD88), phospho-nuclear factor-kappa B p65 (p-NF-κB p65), phospho-mitogen activated protein kinasep38 (p-MAPKp38) and tumor necrosis factor-alpha (TNF-α). Moreover, TELL reduced the elevated thiobarbituric acid reactive substances along with increased both Nrf-2 and endothelial nitric oxide synthase (eNOS) protein expression, beside replenishment of hepatic reduced glutathione. In addition, TELL induced obvious upregulation of p-PI3K and p-Akt protein expressions together with restoration of histopathological changes in IR injury. In conclusion, TELL purveyed conceivable novel hepatoprotective mechanisms and attenuated events associated with acute hepatic injury via inhibition of TLR4 downstream axis and activation of Nrf-2 and PI3K/Akt signaling cascades. Thus, TELL may provide a novel therapeutic potential for complications of hepatic IR injury.

Implications of the mitochondrial interactome of mammalian thioredoxin 2 for normal cellular function and disease

Multiple thioredoxin isoforms exist in all living cells. To explore the possible functions of mammalian mitochondrial thioredoxin 2 (Trx2), an interactome of mouse Trx2 was initially created using (i) a monothiol mouse Trx2 species for capturing protein partners from different organs and (ii) yeast two hybrid screens on human liver and rat brain cDNA libraries. The resulting interactome consisted of 195 proteins (Trx2 included) plus the mitochondrial 16S RNA. 48 of these proteins were classified as mitochondrial (MitoCarta2.0 human inventory). In a second step, the mouse interactome was combined with the current four-membered mitochondrial sub-network of human Trx2 (BioGRID) to give a 53-membered human Trx2 mitochondrial interactome (52 interactor proteins plus the mitochondrial 16S RNA). Although thioredoxins are thiol-employing disulfide oxidoreductases, approximately half of the detected interactions were not due to covalent disulfide bonds. This finding reinstates the extended role of thioredoxins as moderators of protein function by specific non-covalent, protein-protein interactions. Analysis of the mitochondrial interactome suggested that human Trx2 was involved potentially in mitochondrial integrity, formation of iron sulfur clusters, detoxification of aldehydes, mitoribosome assembly and protein synthesis, protein folding, ADP ribosylation, amino acid and lipid metabolism, glycolysis, the TCA cycle and the electron transport chain. The oxidoreductase functions of Trx2 were verified by its detected interactions with mitochondrial peroxiredoxins and methionine sulfoxide reductase. Parkinson's disease, triosephosphate isomerase deficiency, combined oxidative phosphorylation deficiency, and lactate dehydrogenase b deficiency are some of the diseases where the proposed mitochondrial network of Trx2 may be implicated.

Diselenoamino acid derivatives as GPx mimics and as substrates of TrxR: in vitro and in silico studies

Excessive production of reactive species in living cells usually has pathological effects. Consequently, the synthesis of compounds which can mimic the activity of antioxidant enzymes has inspired great interest. In this study, a variety of diselenoamino acid derivatives from phenylalanine and valine were tested to determine whether they could be functional mimics of glutathione peroxidase (GPx) and substrates for liver thioredoxin reductase (TrxR). Diselenides C and D showed the best GPx mimicking properties when compared with A and B. We suppose that the catalytic activity of diselenide GPx mimics depends on the steric effects, which can be influenced by the number of carbon atoms between the selenium atom and the amino acid residue and/or by the amino acid lateral residue. Compounds C and D stimulated NADPH oxidation in the presence of partially purified hepatic mammalian TrxR, indicating that they are substrates for TrxR. Our study indicates a possible dissociation between the two pathways for peroxide degradation (i.e., via a substrate for TrxR or via mimicry of GPx) for compounds tested in this study, except for PhSeSePh, and the antioxidant activity of diselenoamino acids can also be attributed to their capacity to mimic GPx and to be a substrate for mammalian TrxR.

Target discovery of ebselen with a biotinylated probe

Despite numerous studies on ebselen over the past decade, its cellular targets remain obscure. Here we synthesized a biotinylated ebselen probe (biotin-ebselen) and characterized ebselen-binding proteins via an efficient activity-based protein profiling (ABPP) method, which allowed for the robust identification of 462 targeted proteins in HeLa cells. This first work of global target profiling of ebselen will be helpful to re-design ebselen-based therapy appropriately in clinical trials.

Organoselenium compounds as mimics of selenoproteins and thiol modifier agents

Selenium is an essential trace element for animals and its role in the chemistry of life relies on a unique functional group: the selenol (-SeH) group. The selenol group participates in critical redox reactions. The antioxidant enzymes glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) exemplify important selenoproteins. The selenol group shares several chemical properties with the thiol group (-SH), but it is much more reactive than the sulfur analogue. The substitution of S by Se has been exploited in organic synthesis for a long time, but in the last 4 decades the re-discovery of ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) and the demonstration that it has antioxidant and therapeutic properties has renovated interest in the field. The ability of ebselen to mimic the reaction catalyzed by GPx has been viewed as the most important molecular mechanism of action of this class of compound. The term GPx-like or thiol peroxidase-like reaction was previously coined in the field and it is now accepted as the most important chemical attribute of organoselenium compounds. Here, we will critically review the literature on the capacity of organoselenium compounds to mimic selenoproteins (particularly GPx) and discuss some of the bottlenecks in the field. Although the GPx-like activity of organoselenium compounds contributes to their pharmacological effects, the superestimation of the GPx-like activity has to be questioned. The ability of these compounds to oxidize the thiol groups of proteins (the thiol modifier effects of organoselenium compounds) and to spare selenoproteins from inactivation by soft-electrophiles (MeHg⁺, Hg²⁺, Cd²⁺, etc.) might be more relevant for the explanation of their pharmacological effects than their GPx-like activity. In our view, the exploitation of the thiol modifier properties of organoselenium compounds can be harnessed more rationally than the use of low mass molecular structures to mimic the activity of high mass macromolecules that have been shaped by millions to billions of years of evolution.

Alkyl chain modulated cytotoxicity and antioxidant activity of bioinspired amphiphilic selenolanes

A series of amphiphilic conjugates of dihydroxy selenolane (DHS) and monoamine selenolane (MAS), which we had previously reported to inhibit lipid peroxidation and assist the oxidative protein folding reaction respectively in cell free systems, were evaluated for cytotoxicity, associated mechanisms and antioxidant effects in cells. Our results indicated that a fatty acid/alkyl group of variable chain lengths (C6-14) as a lipophilic moiety of the DHS/MAS conjugates not only improved their ability to incorporate within the plasma membrane of cells but also modulated their cytotoxicity. In the concentration range of 1-50 μM, C6 conjugates were non-toxic whereas the long chain (≥C8) conjugates showed significant cytotoxicity. The induction of toxicity investigated by the changes in membrane leakage, fluidity, mitochondrial membrane potential and annexin-V-propidium iodide (PI) staining by using flow cytometry revealed plasma membrane disintegration and subsequent induction of necrosis as the major mechanism. Further, the conjugates of DHS and MAS also showed differential as well as nonlinear tendency in cytotoxicity with respect to chain lengths and this effect was attributed to their self-aggregation properties. Compared with the parent compounds, C6 conjugates not only exhibited better antioxidant activity in terms of the induction of selenoproteins such as glutathione peroxidase 1 (GPx1), GPx4 and thioredoxin reductase 1 (TrxR1) but also protected cells from the AAPH induced oxidative stress. In conclusion, the present study suggests the importance of hydrophilic-lipophilic balance (HLB) in fine tuning the toxicity and activity of bioinspired amphiphilic antioxidants.

Synthesis, isomerization and biological activity of novel 2-selenohydantoin derivatives

A set of novel selenohydantoins were synthesized via a convenient and versatile approach involving the reaction of isoselenocyanates with various amines. We also revealed an unexpected Z→E isomerization of pyridin-2-yl-substituted selenohydantoins in the presence of Cu(2+) cations. The detailed mechanism of this transformation was suggested on the basis of quantum-chemical calculations, and the key role of Cu(2+) was elucidated. The obtained compounds were subsequently evaluated against a panel of different cancer cell lines. As a result, several molecules were identified as promising micromolar hits with good selectivity index. Instead of analogous thiohydantoins, which have been synthesized previously, selenohydantoins demonstrated a relatively high antioxidant activity comparable (or greater) to the reference molecule, Ebselen, a clinically approved drug candidate. The most active compounds have been selected for further biological trials.

Direct synthesis of 4-organylsulfenyl-7-chloro quinolines and their toxicological and pharmacological activities in Caenorhabditis elegans

We describe herein our results on the synthesis and biological properties in Caenorhabditis elegans of a range of 4-organylsulfenyl-7-chloroquinolines. This class of compounds have been easily synthesized in high yields by direct reaction of 4,7-dichloroquinoline with organylthiols using DMSO as solvent at room temperature under air atmosphere and tolerates a range of substituents in the organylsulfenyl moiety. We have performed a toxicological and pharmacological screening of the synthesized 4-organylsulfenyl-7-chloroquinolines in vivo in C. elegans acutely exposed to these compounds, under per se and stress conditions. Hence, we determined the lethal dose 50% (LD50), in order to choose a nonlethal concentration (10 μM) and verified that at that concentration some of the compounds depicted protective action against the induced damage inflicted by paraquat, a superoxide generator. Two compounds (3c and 3h) reduced the toxicity inflicted by paraquat above survival, reproduction and longevity of the worms, at least in part, by reducing the reactive oxygen species (ROS) generated by the toxicant exposure. Besides, these compounds increased the quantities of superoxide dismutase (SOD-3::GFP) and catalase (CTL-1,2,3::GFP), antioxidant enzymes. We concluded that the protective effects of the compounds observed in this study might have been a hormetic response dependent of the transcriptional factor DAF-16/FOXO, causing a non-lethal oxidative stress that protects against the subsequently damage induced by paraquat.

Glutathione peroxidase mimic ebselen improves glucose-stimulated insulin secretion in murine islets

AIMS

Glutathione peroxidase (GPX) mimic ebselen and superoxide dismutase (SOD) mimic copper diisopropylsalicylate (CuDIPs) were used to rescue impaired glucose-stimulated insulin secretion (GSIS) in islets of GPX1 and(or) SOD1-knockout mice.

RESULTS

Ebselen improved GSIS in islets of all four tested genotypes. The rescue in the GPX1 knockout resulted from a coordinated transcriptional regulation of four key GSIS regulators and was mediated by the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α)-mediated signaling pathways. In contrast, CuDIPs improved GSIS only in the SOD1 knockout and suppressed gene expression of the PGC-1α pathway.

INNOVATION

Islets from the GPX1 and(or) SOD1 knockout mice provided metabolically controlled intracellular hydrogen peroxide (H2O2) and superoxide conditions for the present study to avoid confounding effects. Bioinformatics analyses of gene promoters and expression profiles guided the search for upstream signaling pathways to link the ebselen-initiated H2O2 scavenging to downstream key events of GSIS. The RNA interference was applied to prove PGC-1α as the main mediator for that link.

CONCLUSION

Our study revealed a novel metabolic use and clinical potential of ebselen in rescuing GSIS in the GPX1-deficient islets and mice, along with distinct differences between the GPX and SOD mimics in this regard. These findings highlight the necessities and opportunities of discretional applications of various antioxidant enzyme mimics in treating insulin secretion disorders. REBOUND TRACK: This work was rejected during standard peer review and rescued by Rebound Peer Review (Antioxid Redox Signal 16: 293-296, 2012) with the following serving as open reviewers: Regina Brigelius-Flohe, Vadim Gladyshev, Dexing Hou, and Holger Steinbrenner.

Ebselen induces reactive oxygen species (ROS)-mediated cytotoxicity in Saccharomyces cerevisiae with inhibition of glutamate dehydrogenase being a target

Ebselen is a synthetic, lipid-soluble seleno-organic compound. The high electrophilicity of ebselen enables it to react with multiple cysteine residues of various proteins. Despite extensive research on ebselen, its target molecules and mechanism of action remains less understood. We performed biochemical as well as in vivo experiments employing budding yeast as a model organism to understand the mode of action of ebselen. The growth curve analysis and FACS (florescence activated cell sorting) assays revealed that ebselen exerts growth inhibitory effects on yeast cells by causing a delay in cell cycle progression. We observed that ebselen exposure causes an increase in intracellular ROS levels and mitochondrial membrane potential, and that these effects were reversed by addition of antioxidants such as reduced glutathione (GSH) or N-acetyl-l-cysteine (NAC). Interestingly, a significant increase in ROS levels was noticed in gdh3-deleted cells compared to wild-type cells. Furthermore, we showed that ebselen inhibits GDH function by interacting with its cysteine residues, leading to the formation of inactive hexameric GDH. Two-dimensional gel electrophoresis revealed protein targets of ebselen including CPR1, the yeast homolog of Cyclophilin A. Additionally, ebselen treatment leads to the inhibition of yeast sporulation. These results indicate a novel direct connection between ebselen and redox homeostasis.

Organochalcogen peroxidase mimetics as potential drugs: a long story of a promise still unfulfilled

Organochalcogen compounds have attracted the interest of a multitude of studies to design potential therapeutic agents mimicking the peroxidase activity of selenium-based glutathione peroxidases (GPx's). Starting from the pioneering ebselen, various compounds have been synthesized over the years, which may be traced in three major classes of molecules: cyclic selenenyl amides, diaryl diselenides, and aromatic or aliphatic monoselenides. These compounds share common features and determinants needed to exert an efficient GPx-like activity, such as polarizing groups in close proximity to selenium and steric effects. Nonetheless, the reactivity of selenium, and tellurium as well, poses serious problems for the predictability of the biological effects of these compounds in vivo when used as potential drugs. These molecules, indeed, interfere with thiols of redox-regulated proteins and enzymes, leading to unexpected biological effects. The various chemical aspects of the reaction mechanism of peroxidase mimetics are surveyed here, focusing on experimental evidence and quantum mechanics calculations of organochalcogen representatives of the various classes.

Cytotoxicity and genotoxicity evaluation of organochalcogens in human leucocytes: a comparative study between ebselen, diphenyl diselenide, and diphenyl ditelluride

Organochalcogens, particularly ebselen, have been used in experimental and clinical trials with borderline efficacy. (PhSe)₂ and (PhTe)₂ are the simplest of the diaryl dichalcogenides and share with ebselen pharmacological properties. In view of the concerns with the use of mammals in studies and the great number of new organochalcogens with potential pharmacological properties that have been synthesized, it becomes important to develop screening protocols to select compounds that are worth to be tested in vivo. This study investigated the possible use of isolated human white cells as a preliminary model to test organochalcogen toxicity. Human leucocytes were exposed to 5–50 μM of ebselen, (PhSe)₂, or (PhTe)₂. All compounds were cytotoxic (Trypan's Blue exclusion) at the highest concentration tested, and Ebselen was the most toxic. Ebselen and (PhSe)₂ were genotoxic (Comet Assay) only at 50 μM, and (PhTe)₂ at 5–50 μM. Here, the acute cytotoxicity did not correspond with in vivo toxicity of the compounds. But the genotoxicity was in the same order of the in vivo toxicity to mice. These results indicate that in vitro genotoxicity in white blood cells should be considered as an early step in the investigation of potential toxicity of organochalcogens.

Phenylethynyl-butyltellurium inhibits the sulfhydryl enzyme Na+, K+ -ATPase: an effect dependent on the tellurium atom

Organotellurium compounds are known for their toxicological effects. These effects may be associated with the chemical structure of these compounds and the oxidation state of the tellurium atom. In this context, 2-phenylethynyl-butyltellurium (PEBT) inhibits the activity of the sulfhydryl enzyme, δ-aminolevulinate dehydratase. The present study investigated on the importance of the tellurium atom in the PEBT ability to oxidize mono- and dithiols of low molecular weight and sulfhydryl enzymes in vitro. PEBT, at high micromolar concentrations, oxidized dithiothreitol (DTT) and inhibited cerebral Na(+), K(+)-ATPase activity, but did not alter the lactate dehydrogenase activity. The inhibition of cerebral Na(+), K(+)-ATPase activity was completely restored by DTT. By contrast, 2-phenylethynyl-butyl, a molecule without the tellurium atom, neither oxidized DTT nor altered the Na(+), K(+)-ATPase activity. In conclusion, the tellurium atom of PEBT is crucial for the catalytic oxidation of sulfhydryl groups from thiols of low molecular weight and from Na(+), K(+)-ATPase.

Organochalcogens inhibit mitochondrial complexes I and II in rat brain: possible implications for neurotoxicity

Organochalcogens, such as organoselenium and organotellurium compounds, can be neurotoxic to rodents. Since mitochondrial dysfunction plays a pivotal role in neurological disorders, the present study was designed to test the hypothesis that rat brain mitochondrial complexes (I, II, I-III, II-III and IV) could be molecular targets of organochalcogens. The results show that organochalcogens caused statistically significant inhibition of mitochondrial complex I activity, which was prevented by preincubation with NADH and fully blunted by reduced glutathione (GSH). Mitochondrial complex II activity remained unchanged in response to (PhSe)₂ treatment. Ebs and (PhTe)₂ caused a significant concentration-dependent inhibition of complex II that was also blunted by GSH. Mitochondrial complex IV activity was not modified by organochalcogens. Collectively, Ebs, (PhSe)₂ and (PhTe)₂ were more effective inhibitors of brain mitochondrial complex I than of complex II, whereas they did not affect complex IV. These observations are consistent with organochalcogens inducing mitochondrial complex I and II inhibition via their thiol-oxidase-like activity, with Ebs, (PhSe)₂ and (PhTe)₂ effectively oxidising critical thiol groups of these complexes.

Molecular docking studies of disubstituted diaryl diselenides as mammalian δ-aminolevulinic acid dehydratase enzyme inhibitors

δ-Aminolevulinic acid dehydratase (δ-ALAD) is a metalloprotein that catalyzes porphobilinogen formation. This enzyme is sensitive to pro-oxidants and classically used as a biomarker of lead (Pb) intoxication. Diphenyl diselenide [(PhSe)₂] and analogs bis(4-chlorophenyl) diselenide [(pCl₃PhSe)₂], bis(4-methoxyphenyl)diselenide [(pCH₃OPhSe)₂], and bis[3-(trifluoromethy)phenyl] diselenide [(mCF₃PhSe)₂] inhibit mammalian δ-ALAD by oxidizing enzyme cysteinyl residues, which are involved in diselenide-induced toxicity. 2-Cysteinyl residues from δ-ALAD are believed to sequentially interact with (PhSe)₂. Thus this study utilized protein-ligand docking analyses to determine which cysteinyl residues might be involved in the inhibitory effect of (PhSe)₂ and analogs toward δ-ALAD. All diselenides that interact in a similar manner with the active site of δ-ALAD were examined. Docking simulations indicated an important role for π-π interactions involving Phe208 and cation-π interactions involving Lys199 and Arg209 residues with the aromatic ring of (PhSe)₂ and analogs. Based upon these interactions an approximation between Se atoms and -SH of Cys124, with distances ranging between 3.3 Å and 3.5 Å, was obtained. These data support our previous postulations regarding the mechanism underlying δ-ALAD oxidation mediated by (PhSe)₂ and analogs. Based on protein-ligand docking analyses, data indicated that -SH of Cys124 attacks one of the Se atoms of -SH of (PhSe)₂ releasing one PhSeH (selenophenol). Subsequently, the -SH of Cys132 attacks the sulfur atom of Cys124 (from the bond of E-S-Se-Ph indermediate), generating the second PhSe⁻, and the oxidized and inhibited δ-ALAD. In conclusion, AutoDock Vina 1.1.1 was a useful tool to search for diselenides inhibitors of δ-ALAD, and, most importantly, it provided insight into molecular mechanisms involved in enzyme inhibition.