Ebselen, a selenoorganic compound as glutathione peroxidase mimic

The selenoorganic compound ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)-one, exhibits activity as an enzyme mimic. The reaction catalyzed is that of a glutathione (GSH) peroxidase (i.e., the reduction of a hydroperoxide at the expense of thiol). The specificity for substrates ranges from hydrogen peroxide and smaller organic hydroperoxides to membrane-bound phospholipid and cholesterol hydroperoxides. In addition to glutathione, the thiol reductant cosubstrate can be dithioerythritol, N-acetylcysteine or dihydrolipoate, or other suitable thiol compounds. Ebselen also has properties such as free radical and singlet oxygen quenching. Model experiments in vitro with liposomes, microsomes, isolated cells, and organs show that the protection against oxidative challenge afforded by ebselen can be explained largely by the activity as GSH peroxidase mimic. Whether this also explains the known preliminary results in clinical settings is yet open. The metabolism and disposition of ebselen is presented in this review. The main point is that the selenium is not bioavailable, explaining the extremely low toxicity observed in animal studies. The occurrence of natural GPx mimics, ovothiol and related compounds, is briefly mentioned.

Molecular actions of ebselen--an antiinflammatory antioxidant

1. Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) is a non-toxic seleno-organic drug with antiinflammatory, antiatherosclerotic and cytoprotective properties. 2. Ebselen and some of its metabolites are effective reductants of hydroperoxides including those arising in biomembranes and lipoproteins. 3. By reactions with hydroperoxides and thiols several interconversion cycles are formed which include ebselen metabolites with varying oxidation number of the selenium. 4. In the presence of thiols ebselen mimics the catalytic activities of phospholipid hydroperoxide glutathione peroxidase. 5. Ebselen inhibits at low concentrations a number of enzymes involved in inflammation such as lipoxygenases, NO synthases, NADPH, oxidase, protein kinase C and H+/K(+)-ATPase. The inhibitions are manifested on the cellular level and may contribute to the antiinflammatory potential of ebselen.

Human thioredoxin reductase directly reduces lipid hydroperoxides by NADPH and selenocystine strongly stimulates the reaction via catalytically generated selenols

Human placenta thioredoxin reductase (HP-TR) in the presence of NADPH-catalyzed reduction of (15S)-hydroperoxy-(5Z),(8Z),11(Z),13(E)-eicosatetraenoic acid ((15S)-HPETE) into the corresponding alcohol ((15S)-HETE). Incubation of 50 nM HP-TR and 0.5 mM NADPH with 300 microM 15-HPETE for 5 min resulted in formation of 16.5 microM 15-HETE. After 60 min, 74.7 microM 15-HPETE was reduced. The rate of the reduction of 15-HPETE by the HP-TR/NADPH peroxidase system was increased 8-fold by the presence of 2.5 microM selenocystine, a diselenide amino acid. In this case, 15-HPETE was catalytically reduced by the selenol amino acid, selenocysteine, generated from the diselenide by the HP-TR/NADPH system. To a smaller extent, selenodiglutathione or human thioredoxin also potentiated the reduction of 15-HPETE by HP-TR. Hydrogen peroxide and 15-HPETE were reduced at approximately the same rate by HP-TR, thioredoxin, and selenocystine. In contrast, t-butyl hydroperoxide was reduced at a 10-fold lower rate. Our data suggest two novel pathways for the reduction and detoxification of lipid hydroperoxides, hydrogen peroxide, and organic hydroperoxides, i.e. the human thioredoxin reductase-dependent pathway and a coupled reduction in the presence of selenols or selenide resulting from the reduction of selenocystine or selenodiglutathione.

Generation and function of reactive oxygen species in dendritic cells during antigen presentation

Although reactive oxygen species (ROS) have long been considered to play pathogenic roles in various disorders, this classic view is now being challenged by the recent discovery of their physiological roles in cellular signaling. To determine the immunological consequence of pharmacological disruption of endogenous redox regulation, we used a selenium-containing antioxidant compound ebselen known to modulate both thioredoxin and glutaredoxin pathways. Ebselen at 5-20 micro M inhibited Con A-induced proliferation and cytokine production by the HDK-1 T cell line as well as the LPS-triggered cytokine production by XS52 dendritic cell (DC) line. Working with the in vitro-reconstituted Ag presentation system composed of bone marrow-derived DC, CD4(+) T cells purified from DO11.10 TCR-transgenic mice and OVA peptide (serving as Ag), we observed that 1) both T cells and DC elevate intracellular oxidation states upon Ag-specific interaction; 2) ebselen significantly inhibits ROS production in both populations; and 3) ebselen at 5-20 micro M inhibits DC-induced proliferation and cytokine production by T cells as well as T cell-induced cytokine production by DC. Thus, Ag-specific, bidirectional DC-T cell communication can be blocked by interfering with the redox regulation pathways. Allergic contact hypersensitivity responses in BALB/c mice to oxazolone, but not irritant contact hypersensitivity responses to croton oil, were suppressed significantly by postchallenge treatment with oral administrations of ebselen (100 mg/kg per day). These results provide both conceptual and technical frameworks for studying ROS-dependent regulation of DC-T cell communication during Ag presentation and for testing the potential utility of antioxidants for the treatment of immunological disease.

Diselenides and Allyl Selenides as Glutathione Peroxidase Mimetics. Remarkable Activity of Cyclic Seleninates Produced in Situ by the Oxidation of Allyl ω-Hydroxyalkyl Selenides

A series of aliphatic diselenides and selenides containing coordinating substituents was tested for glutathione peroxidase (GPx)-like catalytic activity in a model system in which the reduction of tert-butyl hydroperoxide with benzyl thiol to afford dibenzyl disulfide and tert-butyl alcohol was performed under standard conditions and monitored by HPLC. Although the diselenides showed generally poor catalytic activity, allyl selenides proved more effective. In particular, allyl 3-hydroxypropyl selenide (25) rapidly generated 1,2-oxaselenolane Se-oxide (31) in situ by a series of oxidation and [2,3]sigmatropic rearrangement steps. The remarkably active cyclic seleninate 31 proved to be the true catalyst, reacting with the thiol via a postulated mechanism in which the thioseleninate 32 is first produced, followed by further thiolysis to selenenic acid 33 and oxidation-dehydration to regenerate 31. In contrast to catalysis with GPx, formation of the corresponding selenenyl sulfide 34 comprises a competing deactivation pathway in the catalytic cycle of 31, as a separate experiment revealed that authentic 34 was a much less effective catalyst than 31. 1,2-Oxaselenane Se-oxide (37), the six-membered homologue of 31, was formed similarly from allyl 4-hydroxybutyl selenide (26), but proved a less effective catalyst than 31. Compounds 31 and 37 are the first examples of unsubstituted monocyclic seleninate esters.

Ebselen: a substrate for human thioredoxin reductase strongly stimulating its hydroperoxide reductase activity and a superfast thioredoxin oxidant

Ebselen [2-phenyl-1,2-benzisoselenazol-3(2H)-one], a seleno-organic compound with glutathione peroxidase-like activity is used in clinical trials against stroke. Human and bovine TrxR catalyzed the reduction of ebselen to ebselen selenol by NADPH with an apparent K(M)-value of 2.5 microM and a kcat of 588 min(-1). The addition of thioredoxin (Trx) stimulated the TrxR-catalyzed reduction of ebselen several-fold. This result was caused by a very fast oxidation of reduced Trx by ebselen with a rate constant in excess of 2 x 10(7) M(-1) s(-1). This rate is orders of magnitude faster than the reaction of dithiol Trx with insulin disulfides. Ebselen competed with disulfide substrates for reduction by Trx and, therefore, acted as an inhibitor of protein disulfide reduction by the Trx system. The inherent H2O2 reductase activity of mammalian TrxR dependent on its active-site selenocysteine residue was stimulated 10-fold by 2 microM ebselen and 25-fold in the additional presence of 5 microM Trx. Furthermore, the apparent K(M)-value of TrxR for H2O2 was lowered 25-fold to about 100 microM. Our results demonstrate that ebselen is a TrxR peroxidase which, in the presence of Trx, acted as a mimic of a peroxiredoxin. The activity with TrxR and oxidation of reduced Trx offer mechanistic explanations for the in vivo effects of ebselen as an antioxidant and anti-inflammatory agent. Our results demonstrate that the mechanism of action of ebselen may be predominantly via the Trx system rather than via glutathione.

Spatial imaging, speciation, and quantification of selenium in the hyperaccumulator plants Astragalus bisulcatus and Stanleya pinnata

Astragalus bisulcatus and Stanleya pinnata hyperaccumulate selenium (Se) up to 1% of plant dry weight. In the field, Se was mostly present in the young leaves and reproductive tissues of both hyperaccumulators. Microfocused scanning x-ray fluorescence mapping revealed that Se was hyperaccumulated in trichomes in young leaves of A. bisulcatus. None of 10 other elements tested were accumulated in trichomes. Micro x-ray absorption spectroscopy and liquid chromatography-mass spectrometry showed that Se in trichomes was present in the organic forms methylselenocysteine (MeSeCys; 53%) and gamma-glutamyl-MeSeCys (47%). In the young leaf itself, there was 30% inorganic Se (selenate and selenite) in addition to 70% MeSeCys. In young S. pinnata leaves, Se was highly concentrated near the leaf edge and surface in globular structures that were shown by energy-dispersive x-ray microanalysis to be mainly in epidermal cells. Liquid chromatography-mass spectrometry revealed both MeSeCys (88%) and selenocystathionine (12%) inside leaf edges. In contrast, both the Se accumulator Brassica juncea and the nonaccumulator Arabidopsis thaliana accumulated Se in their leaf vascular tissues and mesophyll cells. Se in hyperaccumulators appears to be mobile in both the xylem and phloem because Se-treated S. pinnata was found to be highly toxic to phloem-feeding aphids, and MeSeCys was present in the vascular tissues of a S. pinnata young leaf petiole as well as in guttation fluid. The compartmentation of organic selenocompounds in specific storage areas in the plant periphery appears to be a unique property of Se hyperaccumulators. The high concentration of Se in the plant periphery may contribute to Se tolerance and may also serve as an elemental plant defense mechanism.

Dinuclear palladium(II) complexes containing two monofunctional [Pd(en)(pyridine)Cl]+ units bridged by Se or S. Synthesis, characterization, cytotoxicity and kinetic studies of DNA-binding

Two novel dinuclear palladium(II) complexes, ¿[Pd(en)Cl]2(bpse)¿(NO3)2 (1) and ¿[Pd(en)Cl]2 (bpsu)¿(NO3)2 (2), (where en is ethylenediamine; bpse is bis(3-methyl-4-pyridyl) selenide; bpsu is bis(3-methyl-4-pyridyl) sulfide) have been synthesized. The complexes have been characterized by elemental analysis, IR, 1H NMR, and 13C NMR. They have been assayed for antitumor activity in vitro against the mice leukemia L1210 and the human coloadenocarcinoma HCT8 cell lines. The results show that compound 1 has a lower I.D.50 value against the two cancer cell lines as compared to compound 2; the compounds also shows a lower I.D.50 value than cisplatin against the HCT8 cell line, but a higher I.D.50 value than cisplatin against the L1210 cell line. Binding studies indicate that compound 1 possibly interacts with DNA by a nonintercalative mode. Kinetics of binding of the two compounds to DNA are firstly studied using ethidium bromide as a fluorescence probe with stopped-flow spectrophotometer under pseudo-first-order condition. The stronger binding of two steps in the process of the compounds interacting with DNA are observed, and the Kobs and Ea of binding of the two steps (where Kobs is the observed pseudo-first-order rate constant, Ea is the observed energy of activation) are obtained.

Protective role of aryl and alkyl diselenides on lipid peroxidation

The concept that selenium-containing molecules may be better nucleophiles (and therefore antioxidants) than classical antioxidants has led to the design of synthetic organoselenium compounds. In the present study we appraised the antioxidant potential, thiol peroxidase activity, and rate of dithiotreitol and reduced glutathione oxidation of simple organodiselenide compounds in rats and mice. The present results demonstrate that alkyl and aryl diselenides are antioxidant compounds. We verified that the substitution on the aromatic moiety of diphenyl diselenide or the replacement of on aryl group by an alkyl substitute on diselenides changes their antioxidant and thiol peroxidase-like properties. The diaryl diselenides (PhSe)(2) and (p-ClPhSe)(2) presented higher thiol peroxidase activity and demonstrated better antioxidant potential than the other diselenides tested. In fact, the results revealed that alkyl diselenides, at low concentrations, were prooxidants and that aryl diselenides did not present this effect. Alkyl diselenides [(C(2)H(5)Se)(2) and (C(3)H(7)Se)(2)] demonstrated a higher potential for -SH group oxidation than aryl diselenides. In addition, this study demonstrated that diselenide protection against lipid peroxidation was different in mice and rats. The compounds tested acted more as antioxidants in the brains of mice than in the brains of rats.

Molecular biological and biochemical characterization of the human type 2 selenodeiodinase

Type 2 deiodinase (D2) is a low K(m) iodothyronine deiodinase that catalyzes the removal of a single iodine from the phenolic ring of T4 or rT3. We sequenced and subcloned the open reading frame from a partial complementary DNA (cDNA) clone (2.1 kilobases) prepared by Genethon (Z44085) from a human infant brain cDNA library. The open reading frame encodes a putative 273-amino acid protein of 31 kDa with greater than 70% similarity to the Rana catesbeiana D2 protein. Transient expression of the cDNA produces a low K(m) (5 nM for T4; 8 nM for rT3) propylthiouracil- and gold thioglucose-resistant 5'-deiodinase in 293-HEK cells. Human D2, like human type 1 (D1) and type 3 (D3) deiodinases, is a selenoenzyme, as evidenced by 1) the presence of two in-frame UGA codons (positions 133 and 266), 2) the synthesis of a 31-kDa 75Selabeled protein in D2 cDNA-transfected cells, and 3) the requirement for a 3'-selenocysteine incorporation sequence element for its translation. Unlike D1 and D3, we were not able to covalently label overexpressed D2 with N-bromoacetyl [125I]T3 or -T4. We found that the human D2 messenger RNA is 7-8 kilobases and is expressed in brain, placenta, and, surprisingly, cardiac and skeletal muscle. Type 2 deiodinase activity was also present in human skeletal muscle. These results indicate that there are unique features of D2 that distinguish it from the two other selenodeiodinases. The expression of D2 in muscle suggests that it could play a role in peripheral, as well as intracellular, T3 production.

Molecular mechanisms of selenium tolerance and hyperaccumulation in Stanleya pinnata

The molecular mechanisms responsible for selenium (Se) tolerance and hyperaccumulation were studied in the Se hyperaccumulator Stanleya pinnata (Brassicaceae) by comparing it with the related secondary Se accumulator Stanleya albescens using a combination of physiological, structural, genomic, and biochemical approaches. S. pinnata accumulated 3.6-fold more Se and was tolerant to 20 microm selenate, while S. albescens suffered reduced growth, chlorosis and necrosis, impaired photosynthesis, and high levels of reactive oxygen species. Levels of ascorbic acid, glutathione, total sulfur, and nonprotein thiols were higher in S. pinnata, suggesting that Se tolerance may in part be due to increased antioxidants and up-regulated sulfur assimilation. S. pinnata had higher selenocysteine methyltransferase protein levels and, judged from liquid chromatography-mass spectrometry, mainly accumulated the free amino acid methylselenocysteine, while S. albescens accumulated mainly the free amino acid selenocystathionine. S. albescens leaf x-ray absorption near-edge structure scans mainly detected a carbon-Se-carbon compound (presumably selenocystathionine) in addition to some selenocysteine and selenate. Thus, S. albescens may accumulate more toxic forms of Se in its leaves than S. pinnata. The species also showed different leaf Se sequestration patterns: while S. albescens showed a diffuse pattern, S. pinnata sequestered Se in localized epidermal cell clusters along leaf margins and tips, concentrated inside of epidermal cells. Transcript analyses of S. pinnata showed a constitutively higher expression of genes involved in sulfur assimilation, antioxidant activities, defense, and response to (methyl)jasmonic acid, salicylic acid, or ethylene. The levels of some of these hormones were constitutively elevated in S. pinnata compared with S. albescens, and leaf Se accumulation was slightly enhanced in both species when these hormones were supplied. Thus, defense-related phytohormones may play an important signaling role in the Se hyperaccumulation of S. pinnata, perhaps by constitutively up-regulating sulfur/Se assimilation followed by methylation of selenocysteine and the targeted sequestration of methylselenocysteine.

Selenium Metabolites in Urine: A Critical Overview of Past Work and Current Status

Background: Selenium is an essential trace element that also elicits toxic effects at modest intakes. Investigations of selenium metabolites in urine can help our understanding of the transformations taking place in the body that produce these beneficial and detrimental effects. There is, however, considerable discord in the scientific literature regarding the selenium metabolites thought to play important roles in these biotransformation processes.

Approach: We critically assessed the published reports on selenium urinary metabolites, from the first report in 1969 to the present, in terms of the rigor of the data on which structures have been proposed.

Content: We present and discuss data from ∼60 publications reporting a total of 16 identified selenium metabolites in urine of humans or rats, a good model for human selenium metabolism. We assessed the analytical methods used and the validity of the ensuing structural assignments.

Summary: Many of the studies of selenium metabolites in urine appear to have assigned incorrect structures to the compounds. The long-held view that trimethylselenonium ion is a major human urinary metabolite appears unjustified. On the other hand, recent work describing selenosugars as major urinary metabolites looks sound and provides a firm basis for future studies.

Mechanisms of mammary cancer chemoprevention by organoselenium compounds

Searching for optimal diets and for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to finding ways to control and prevent cancer. To date, the use of the micronutrient selenium in human clinical trials is limited but the outcome of these investigations indicates that selenium is one of the most promising agents. Data presented in this mini-review indicate that the dose and the form (structure) in which selenium is used are the most critical determinants of success in future clinical trials. The focus of this mini-review is on the mechanisms of mammary cancer chemoprevention by organoselenium compounds. Among the naturally occurring organoselenium compounds, Se-Methylselenocysteine is more efficacious than the most extensively studied forms, such as selenomethionine. However, we showed that synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal side effects by structural modifications; it is evident that synthetic agents are superior to the inorganic selenite, naturally occurring selenium compounds and their sulfur-containing analogs. We have demonstrated that 1,4-phenylenebis (methylene) selenocyanate (p-XSC) and its putative metabolite glutathione conjugate (p-XSeSG) are highly promising agents in the chemoprevention of mammary carcinogenesis in the 7,12-dimethylbenz[a]anthracene (DMBA)-rat mammary tumor model system. Both compounds inhibit the initiation phase of carcinogenesis by inhibiting DMBA-DNA adduct formation in the target organ in vivo. cDNA microarray analysis indicates that both selenium compounds alter genes in a manner that leads to inhibition of cell proliferation and induction of apoptosis; modulation of apoptosis and cell proliferation can account for chemoprevention during the post-initiation phase of mammary carcinogenesis. Using a rat mammary cancer cell line, we compared p-XSC and p-XSeSG as inhibitors of cell proliferation; depending on the selenium dose and time point selected, p-XSC was comparable to or better than p-XSeSG. Collectively, the results described here, suggest that the molecular targets modulated by organoselenium compounds are highly useful indicators of success in clinical cancer chemoprevention trials.

Kinetic study of the reaction of ebselen with peroxynitrite

The second-order rate constant for the reaction of ebselen with peroxynitrite (ONOO-) is (2.0+/-0.1) X 10(6) M(-1) s(-1) at pH > or = 8 and 25 degrees C, 3-4 orders of magnitude higher than the rate constants observed for cysteine, ascorbate, or methionine. The activation energy is relatively low, 12.8 kJ/mol. This is the fastest reaction of peroxynitrite observed so far. It may allow Se-containing compounds to play a novel role in the defense against peroxynitrite, one of the important reactive species generated during inflammatory processes.

Methioninase and selenomethionine but not Se-methylselenocysteine generate methylselenol and superoxide in an in vitro chemiluminescent assay: implications for the nutritional carcinostatic activity of selenoamino acids

Methylselenol from selenium metabolism is postulated to be and most experimental evidence now indicates that it is the selenium metabolite responsible for the dietary chemoprevention of cancers. Using the recombinant enzyme methioninase, methylselenol-generating chemiluminesence by superoxide (O2*-) is shown to be catalytically produced from L-selenomethionine and D,L-selenoethionine, but not from methionine or L-Se-methylselenocysteine (SeMC). Methylselenol enzymaticaly generated by methioninase activity from the substrate selenomethionine arises from an initial putative selenium radical as measured by chemiluminesence in the absence of glutathione (GSH). In the presence of GSH, superoxide was generated as measured by chemiluminesence and superoxide dismutase inhibition of chemiluminescence. Ascorbic acid also quenched the chemiluminesence from the activity of methioninase with selenomethionine. Methylselenol and other redox cycling selenium compounds are almost assuredly accountable for inducing cell-cycle arrest and apoptosis in cancer cells in vitro and in vivo. Methylselenol generated from selenomethionine by methioninase is catalytic alone in oxidizing thiols, i.e. GSH, generating superoxide and inducing oxidative stress in direct proportion to its concentration. Se-methylselenocysteine in vivo is very likely carcinostatic in like manner to selenomethionine by generating methylselenol from other enzymatic activity, i.e. beta-lyase or amino acid oxidases.

Mutagenesis of structural half-cystine residues in human thioredoxin and effects on the regulation of activity by selenodiglutathione

A human thioredoxin cDNA was modified to optimize Escherichia coli expression and subcloned into the plasmid pACA, a vector for T7 RNA polymerase-directed expression. The substitution of structural (noncatalytic) half-cystines in human thioredoxin (hTrx) was made by site-directed mutagenesis. The recombinant wild-type (wt) hTrx and its mutant C61S, C72S, and C61S/C72S were expressed and purified to homogeneity. Characterization of the wt and mutant hTrx was done with respect to redox activity with thioredoxin reductase (TR), tryptophan fluorescence, and effects of incubation with GS-Se-SG, which is believed to be the major metabolite of inorganic selenium compounds in mammalian tissues. The Km and kcat of wild-type hTrx for human placenta thioredoxin reductase (HP-TR) at pH 7.0 were 2.0 microM and 2800 min-1, respectively. The mutant proteins C61S, C72S, and C61S/C72S had Km and kcat values similar to those of the wt thioredoxin. Tryptophan fluorescence measurements showed that the wt and mutant proteins had similar stability to a denaturing agent. Incubation of fully reduced thioredoxin with 0.1 molar equivalent of GS-Se-SG resulted in continued oxidation of SH groups. After 3.5 h only 0.5 of initially 4.6 SH groups/thioredoxin remained. With the oxidized protein, a pronounced lag phase in thioredoxin reductase-dependent insulin disulfide reduction was present. Disulfide-linked dimers of the protein were present. The results clearly showed that noncatalytic cysteine residues in hTrx were oxidized accompanied by dimerization and inactivation. The activities of the mutant proteins C72S and C61S/C72S were unchanged after 3 h of incubation with GS-Se-SG. No dimer appeared of the C72S thioredoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Selenoglutathione: efficient oxidative protein folding by a diselenide

Diselenide bonds are intrinsically more stable than disulfide bonds. To examine how this stability difference affects reactivity, we synthesized selenoglutathione (GSeSeG), an analogue of the oxidized form of the tripeptide glutathione that contains a diselenide bond in place of the natural disulfide. The reduction potential of this diselenide bond was determined to be -407 +/- 9 mV, a value which is 151 mV lower than that of the disulfide bond in glutathione (GSSG). Thus, the diselenide bond of GSeSeG is 7 kcal/mol more stable than the disulfide bond of GSSG. Nonetheless, we found that GSeSeG can be used to oxidize cysteine residues in unfolded proteins, a process that is driven by the gain in protein conformational stability upon folding. Indeed, the folding of both ribonuclease A (RNase A) and bovine pancreatic trypsin inhibitor (BPTI) proceeded efficiently using GSeSeG as an oxidant, in the former case with a 2-fold rate increase relative to GSSG and in the latter case accelerating conversion of a stable folding intermediate to the native state. In addition, GSeSeG can also oxidize the common biological cofactor NADPH and is a good substrate for the NADPH-dependent enzyme glutathione reductase (kcat = 69 +/- 2 s-1, Km = 54 +/- 7 microM), suggesting that diselenides can efficiently interact with the cellular redox machinery. Surprisingly, the greater thermodynamic stability of diselenide bonds relative to disulfide bonds is not matched by a corresponding decrease in reactivity.

Crystal structure of human type II inosine monophosphate dehydrogenase: implications for ligand binding and drug design

Inosine monophosphate dehydrogenase (IMPDH) controls a key metabolic step in the regulation of cell growth and differentiation. This step is the NAD-dependent oxidation of inosine 5' monophosphate (IMP) to xanthosine 5' monophosphate, the rate-limiting step in the synthesis of the guanine nucleotides. Two isoforms of IMPDH have been identified, one of which (type II) is significantly up- regulated in neoplastic and differentiating cells. As such, it has been identified as a major target in antitumor and immunosuppressive drug design. We present here the 2.9-A structure of a ternary complex of the human type II isoform of IMPDH. The complex contains the substrate analogue 6-chloropurine riboside 5'-monophosphate (6-Cl-IMP) and the NAD analogue selenazole-4-carboxamide adenine dinucleotide, the selenium derivative of the active metabolite of the antitumor drug tiazofurin. The enzyme forms a homotetramer, with the dinucleotide binding at the monomer-monomer interface. The 6 chloro-substituted purine base is dehalogenated, forming a covalent adduct at C6 with Cys-331. The dinucleotide selenazole base is stacked against the 6-Cl-IMP purine ring in an orientation consistent with the B-side stereochemistry of hydride transfer seen with NAD. The adenosine end of the ligand interacts with residues not conserved between the type I and type II isoforms, suggesting strategies for the design of isoform-specific agents.

Synthesis of 5-methylaminomethyl-2-selenouridine in tRNAs: 31P NMR studies show the labile selenium donor synthesized by the selD gene product contains selenium bonded to phosphorus

An enzyme preparation from Salmonella typhimurium catalyzes the conversion of 5-methylaminomethyl-2-thiouridine in tRNAs to 5-methylaminomethyl-2-selenouridine when supplemented with selenide and ATP. Similar preparations from a Salmonella mutant strain carrying a defective selD gene fail to catalyze this selenium substitution reaction. However, supplementation of the deficient enzyme preparation with the purified selD gene product (SELD protein) restored synthesis of seleno-tRNAs. In the absence of the complementary enzyme(s), the SELD protein catalyzes the synthesis of a labile selenium donor compound from selenide and ATP. 31P NMR studies show that among the products of this reaction are AMP and a compound containing selenium bonded to phosphorus. The reaction is completely dependent on the addition of both selenide and magnesium. The dependence of reaction velocity on ATP concentration shows sigmoidal kinetics, whereas dependence on selenide concentration obeys Michaelis-Menten kinetics indicating a Km value of 46 microM for selenide.

Remarkable activity of a novel cyclic seleninate ester as a glutathione peroxidase mimetic and its facile in situ generation from allyl 3-hydroxypropyl selenide

1,2-Oxaselenolane Se-oxide is a novel cyclic seleninate ester that functions as a remarkably efficient glutathione peroxidase mimetic by catalyzing the reduction of tert-butyl hydroperoxide to tert-butyl alcohol in the presence of benzyl thiol. The seleninate ester can be conveniently generated in situ by oxidation of allyl 3-hydroxypropyl selenide with tert-butyl hydroperoxide. Its catalytic activity surpasses that of several other known GPx mimetics containing cyclic selenenamide structures, which were also tested for comparison.

Attenuation of oxidation and nitration reactions of peroxynitrite by selenomethionine, selenocystine and ebselen

The effect of the selenium-containing compounds selenomethionine, selenocystine and ebselen [2-phenyl-1,2-benzisoselenazol-3(2H)-one] on the oxidation of dihydrorhodamine 123 caused by peroxynitrite and on the nitration of 4-hydroxyphenylacetate by peroxynitrite was studied in comparison with their sulphur analogues methionine, cystine and ebsulfur [2-phenyl-1,2-benzisothiazol-3(2H)-one]. The selenocompounds protected dihydrorhodamine 123 from oxidation and 4-hydroxyphenylacetate from nitration more effectively than their sulphur analogues. Sodium selenite exhibited no effect. These observations are corollaries to the recent finding [Roussyn, Briviba, Masumoto and Sies (1996) Arch. Biochem. Biophys. 330, 216-218] that selenium-containing compounds are efficient in protecting against peroxynitrite-induced DNA single-strand breaks.

Synthesis and characterization of a triphenylphosphonium-conjugated peroxidase mimetic. Insights into the interaction of ebselen with mitochondria

Mitochondrial production of peroxides is a critical event in both pathology and redox signaling. Consequently their selective degradation within mitochondria is of considerable interest. Here we have explored the interaction of the peroxidase mimetic ebselen with mitochondria. We were particularly interested in whether ebselen was activated by mitochondrial glutathione (GSH) and thioredoxin, in determining whether an ebselen moiety could be targeted to mitochondria by conjugating it to a lipophilic cation, and in exploring the nature of ebselen binding to mitochondrial proteins. To achieve these goals we synthesized 2-[4-(4-triphenylphosphoniobutoxy) phenyl]-1,2-benzisoselenazol)-3(2H)-one iodide (MitoPeroxidase), which contains an ebselen moiety covalently linked to a triphenylphosphonium (TPP) cation. The fixed positive charge of TPP facilitated mass spectrometric analysis, which showed that the ebselen moiety was reduced by GSH to the selenol form and that subsequent reaction with a peroxide reformed the ebselen moiety. MitoPeroxidase and ebselen were effective antioxidants that degraded phospholipid hydroperoxides, prevented lipid peroxidation, and protected mitochondria from oxidative damage. Both peroxidase mimetics required activation by mitochondrial GSH or thioredoxin to be effective antioxidants. Surprisingly, conjugation to the TPP cation led to only a slight increase in the uptake of ebselen by mitochondria due to covalent binding of the ebselen moiety to proteins. Using antiserum against the TPP moiety we visualized those proteins covalently attached to the ebselen moiety. This analysis indicated that much of the ebselen present within mitochondria is bound to protein thiols through reversible selenenylsulfide bonds. Both MitoPeroxidase and ebselen decreased apoptosis induced by oxidative stress, suggesting that they can decrease mitochondrial oxidative stress. This exploration has led to new insights into the behavior of peroxidase mimetics within mitochondria and to their use in investigating mitochondrial oxidative damage.

Effect of inorganic versus organic selenium on hen production and egg selenium concentration

A 28-d experiment using 288 Hy-Line W-36 laying hens was conducted to compare sodium selenite (SS) with Se-enriched yeast (SY). The Se from SS or SY was supplemented into a corn-soybean meal basal diet at 0, 0.15, 0.30, 0.60, or 3.00 ppm, and the basal diet was formulated to provide 0.82% lysine and 2,950 kcal/kg of ME. Each treatment was replicated 4 times with 2 cages of 4 hens per cage in each replicate. Hen production was assessed daily, and 2 eggs per replicate were collected every 4 d for whole-egg Se analysis. Albumen quality was assessed at 2 egg storage temperatures (7.2 vs. 22.2 degrees C) with the eggs collected on d 24 and 28, respectively. The percentage of dirty and cracked eggs was greater (P < 0.04) in hens fed SY than in those fed SS. Percentage hen-day production was not affected (P > 0.05) by diet. Albumen quality of eggs stored at 22.2 degrees C was improved (P < 0.04) in eggs from hens fed SS, but there was no difference (P > 0.05) in albumen quality of eggs stored at 7.2 degrees C. Egg weight was linearly increased (P < 0.01) by SY. Whole-egg Se levels were linearly increased (P < 0.01) as dietary Se level increased for both sources of Se, but eggs from hens fed SY had higher (P < 0.01) Se concentrations than those fed SS. The results from this experiment indicate that percentage hen-day production is not affected by Se source, and that SY increases egg Se concentrations more than SS.

Interaction of peroxynitrite with selenoproteins and glutathione peroxidase mimics

Peroxynitrite is an oxidant generated under inflammatory conditions, acting in defense against invading microorganisms. There is a need for protection of the organism from damage inflicted by peroxynitrite. Selenium-containing compounds, notably ebselen, have a high second-order reaction rate constant (approx. 2 x 10(6) M(-1) s(-1)), which makes them candidates for efficient protection. This applies also for selenium in proteins, occurring as selenocysteine or selenomethionine residues. Glutathione peroxidases, thioredoxin reductase, and selenoprotein P have been shown to play a potential role in protection against peroxynitrite. Tellurium-containing compounds also react with peroxynitrite.

Aryl Thiol Substrate 3-Carboxy-4-Nitrobenzenethiol Strongly Stimulating Thiol Peroxidase Activity of Glutathione Peroxidase Mimic 2, 2'-Ditellurobis(2-Deoxy-β-Cyclodextrin)

Artificial glutathione peroxidase (GPx) model 2, 2'-ditellurobis(2-deoxy-beta-cyclodextrin) (2-TeCD) which has the desirable properties exhibited high substrate specificity and remarkably catalytic efficiency when 3-carboxy-4-nitrobenzenethiol (ArSH) was used as a preferential thiol substrate. The complexation of ArSH with beta-cyclodextrin was investigated through UV spectral titrations, fluorescence spectroscopy, 1H NMR and molecular simulation, and these results indicated that ArSH fits well to the size of the cavity of beta-cyclodextrin. Furthermore, 2-TeCD was found to catalyze the reduction of cumene peroxide (CuOOH) by ArSH 200,000-fold more efficiently than diphenyl diselenide (PhSeSePh). Its steady-state kinetics was studied and the second rate constant kmax/KArSH was found to be 1.05 x 10(7) M(-1) min(-1) and similar to that of natural GPx. Moreover, the kinetic data revealed that the catalytic efficiency of 2-TeCD depended strongly upon the competitive recognition of both substrates for 2-TeCD. The catalytic mechanism of 2-TeCD catalysis agreed well with a ping-pong mechanism, in analogy with natural GPx, and might exert its thiol peroxidase activity via tellurol, tellurenic acid, and tellurosulfide.