Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:1.11.1.9 (glutathione peroxidase)
22,002 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The nonsense codon, UGA, has for the first time recently been shown to encode selenocysteine in two proteins, mouse glutathione peroxidase (GSH-Px) (EC 1.11.1.9) and bacterial formate dehydrogenase. A co-translational rather than post-translational selenium-incorporation mechanism has been implicated. Furthermore, high expression levels of GSH-Px have suggested that suppression of termination is efficient and specific. We have isolated and characterized pituitary, kidney and placenta cDNAs for bovine, human and mouse GSH-Px respectively. It is demonstrated that this novel suppression event occurs in diverse tissues, in at least three mammalian species and at the translational step. Surprisingly, GSH-Px is shown to be extramitochondrially encoded, indicating a cytosolic suppression event rather than one utilizing the mitochondria's well-documented extended codon-reading ability. Sequence analysis reveals that a simple proximal contextual pattern responsible for readthrough does not exist. Analysis of predicted secondary structures of mRNAs, however, has revealed a conformation which may be unique to selenocysteine proteins and may prove useful as a tool for artificial incorporation of selenocysteines. A human intron for GSH-Px from an unspliced mRNA has been isolated whose position indicates an ancient, divergent evolutionary relationship with thioredoxin-S2, rather than an independent convergent one.
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PMID:Selenocysteine's mechanism of incorporation and evolution revealed in cDNAs of three glutathione peroxidases. 297 39

A full-length cDNA from the parasitic nematode Brugia pahangi encoding a secreted homolog of glutathione peroxidase in which the codon for the active site selenocysteine is substituted naturally by a cysteine codon has been expressed in Spodoptera frugiperda (insect) cells via Autographa californica nuclear polyhedrosis virus (baculovirus). The recombinant protein was glycosylated and secreted from the cells in tetrameric form. The purified protein showed glutathione peroxidase activity with a range of organic hydroperoxides, including L-alpha-phosphatidylcholine hydroperoxide, but no significant activity against hydrogen peroxide. Glutathione was the only thiol tested that served as a substrate for the enzyme, which showed no activity with the thioredoxin system (thioredoxin, thioredoxin reductase, and NADPH). No glutathione-conjugating activity was detected against a range of electrophilic compounds that are common substrates for glutathione S-transferases. The apparent (pseudo)m for glutathione was determined as 4.9 mM at a fixed concentration of linolenic acid hydroperoxide (3 microM). The enzyme showed low affinity for hydroperoxide substrates (apparent Km for linolenic acid hydroperoxide and L-alpha-phosphatidylcholine hydroperoxide of 3.8 and 9.7 mM, respectively at a fixed glutathione concentration of 3 mM).
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PMID:Heterologous expression and enzymatic properties of a selenium-independent glutathione peroxidase from the parasitic nematode Brugia pahangi. 762 52

The ability of human cells to regenerate ascorbic acid from dehydroascorbate is partially dependent on the glutathione redox status of the cell and the relative activity of dehydroascorbate reductases. Mammalian dehydroascorbate reductase activity is associated with two proteins known as thioltransferase (glutaredoxin) and protein disulfide isomerase. We compared the specific activity of thioltransferase, protein disulfide isomerase, and other GSH-related enzymes in Adriamycin-resistant human breast tumor cells, MCF-7 ADRR, and Adriamycin-sensitive, MCF-7 WT, tumor cells. MCF-7 ADRR cells had higher activities of glutathione peroxidase (34.7 fold), nonseleno-glutathione peroxidase (glutathione S-transferases; 5.3 fold), thioredoxin (2.3 fold), and thioltransferase (4.0 fold) compared with the WT Adriamycin-sensitive cell line. Thioltransferase was detected in Western blots in extracts of ADRR MCF-7 cells but not in WT MCF-7 cells. alpha-Tocopherol in the membrane and cytosolic fractions was 2.8 and 3.0 fold higher, respectively, in Adriamycin-resistant compared with Adriamycin-sensitive cells. Supplementation of MCF-7 cells with L-ascorbic acid 2-phosphate (2 and 10 mM) had no effect on WT cell viability after 5 days incubation with up to 0.33 microM Adriamycin. In contrast, supplementation of ADRR MCF-7 cells with L-ascorbic acid 2-phosphate resulted in enhanced resistance up to 3.4 microM Adriamycin over a 5-day incubation. Both lines of MCF-7 cells demonstrated the ability to utilize ascorbic acid as the 2-phosphate derivative. After 48 h incubation with 8.6 microM Adriamycin, the resistant cells maintained normal viability and ascorbate-dehydroascorbate levels, whereas drug-sensitive cells had significantly lower ascorbate with a higher percent dehydroascorbate and increased cell death as judged by cell protein levels (52% of controls).
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PMID:Ascorbic acid and cell survival of adriamycin resistant and sensitive MCF-7 breast tumor cells. 775 Jul 94

The possible relationship of selenium to immunological function which has been suggested for decades was investigated in studies on selenium metabolism in human T cells. One of the major 75Se-labeled selenoproteins detected was purified to homogeneity and shown to be a homodimer of 55-kDa subunits. Each subunit contained about 1 FAD and at least 0.74 Se. This protein proved to be thioredoxin reductase (TR) on the basis of its catalytic activities, cross-reactivity with anti-rat liver TR antibodies, and sequence identities of several tryptic peptides with the published deduced sequence of human placental TR. Physicochemical characteristics of T-cell TR were similar to those of a selenocysteine (Secys)-containing TR recently isolated from human lung adenocarcinoma cells. The sequence of a 12-residue 75Se-labeled tryptic peptide from T-cell TR was identical with a C-terminal-deduced sequence of human placental TR except that Secys was present in the position corresponding to TGA, previously thought to be the termination codon, and this was followed by Gly-499, the actual C-terminal amino acid. The presence of the unusual conserved Cys-Secys-Gly sequence at the C terminus of TR in addition to the redox active cysteines of the Cys-Val-Asn-Val-Gly-Cys motif in the FAD-binding region may account for the peroxidase activity and the relatively low substrate specificity of mammalian TRs. The finding that T-cell TR is a selenoenzyme that contains Se in a conserved C-terminal region provides another example of the role of selenium in a major antioxidant enzyme system (i.e., thioredoxin-thioredoxin reductase), in addition to the well-known glutathione peroxidase enzyme system.
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PMID:Selenocysteine, identified as the penultimate C-terminal residue in human T-cell thioredoxin reductase, corresponds to TGA in the human placental gene. 865 Feb 34

Chemotherapy for adult T-cell leukemia (ATL) has been reported to fail to induce complete remission because of drug resistance in most patients. We have examined the expression of an ATL-derived factor (ADF)/thioredoxin in relation to resistance to adriamycin (ADM) in various T-cell leukemia cell lines including ATL cell lines. Immunoblot analysis demonstrated that ATL cell lines expressed ADF/thioredoxin at levels 2.8 to 12 times those of other T-cell acute lymphocytic leukemia (T-ALL) cell lines, and that ATL cell lines were 2 to 15 times more resistant to ADM than other T-ALL cell lines. Therefore, we established ADM-resistant cell lines from three different ATL cell lines, and examined the correlation between ADM resistance and expression of ADF/thioredoxin. ADM-resistant ATL cell lines were also found to be resistant to other drugs such as cisplatin and etoposide, and they expressed ADF/thioredoxin at levels 5 to 10 times those of parent ATL cell lines. Diamide and sodium selenite, which have been reported to inhibit ADF/thioredoxin, restored the sensitivity to ADM in ATL and ADM-resistant ATL cell lines. The MDR-1 gene product, a membrane P-glycoprotein (Pgp), was not expressed on ATL cell lines or ADM-resistant ATL cell lines. Topoisomerase II and glutathione peroxidase activities in T-cell leukemia cell lines were not correlated with ADM resistance. These results suggest that ADF/thioredoxin may play an important role in the drug resistance of ATL cells to ADM.
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PMID:Possible roles of an adult T-cell leukemia (ATL)-derived factor/thioredoxin in the drug resistance of ATL to adriamycin. 911 92

Recycling of ascorbate from its oxidized forms is essential to maintain stores of the vitamin in human cells. Whereas reduction of dehydroascorbate to ascorbate is thought to be largely GSH-dependent, we reconsidered the possibility that the selenium-dependent thioredoxin system might contribute to ascorbate regeneration. We found that purified rat liver thioredoxin reductase functions as an NADPH-dependent dehydroascorbate reductase, with an apparent Km of 2. 5 mM for dehydroascorbate, and a kcat of 90 min-1. Addition of 2.8 microM purified rat liver thioredoxin lowered the apparent Km to 0.7 mM, without affecting the turnover (kcat of 71 min-1). Since thioredoxin reductase requires selenium, we tested the physiologic importance of this enzyme for dehydroascorbate reduction in livers from control and selenium-deficient rats. Selenium deficiency lowered liver thioredoxin reductase activity by 88%, glutathione peroxidase activity by 99%, and ascorbate content by 33%, but did not affect GSH content. NADPH-dependent dehydroascorbate reductase activity due to thioredoxin reductase, on the basis of inhibition by aurothioglucose, was decreased 88% in dialyzed liver cytosolic fractions from selenium-deficient rats. GSH-dependent dehydroascorbate reductase activity in liver cytosol was variable, but typically 2-3-fold that of NADPH-dependent activity. These results show that the thioredoxin system can reduce dehydroascorbate, and that this function is required for maintenance of liver ascorbate content.
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PMID:Reduction of dehydroascorbate to ascorbate by the selenoenzyme thioredoxin reductase. 927 16

Human thioredoxin reductase was recently shown to contain a TGA encoded selenocysteine residue at the penultimate position of its amino acid chain. Depending on the availability of selenium during biosynthesis, an authentic selenocysteine-containing or a selenium-free enzyme truncated at the penultimate position is expected to be formed. Correspondingly, the enzymatic activity should be altered by selenium restriction, if the selenocysteine residue is functionally important. In order to check the catalytic role of the selenocysteine residue, four different human cell lines were grown in selenium deficient media or with adequate selenium supplementation (40 nM sodium selenite) and thioredoxin reductase activity was measured as NADPH-dependent DTNB reduction or thioredoxin-mediated insulin reduction. Thioredoxin reductase activities, like glutathione peroxidase activities, were consistently higher in selenium supplemented cells, whereas glutathione reductase activity was not affected by the selenium. The dose-response was similar for thioredoxin reductase and glutathione peroxidase, but the recovery of glutathione peroxidase activity upon selenium supplementation was faster than with thioredoxin reductase. Also the increase of glutathione peroxidase activities was substantially higher than that of thioredoxin reductase (400-1200% versus a maximum of 250%). These observations clearly indicate a catalytic role of the selenocysteine residue in the thioredoxin reductase, but suggest either the existence of a selenium-unresponsive isoenzyme or a residual disulfide reductase activity in the selenium-free truncated protein made under conditions of selenium deficiency.
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PMID:Evidence for a functional relevance of the selenocysteine residue in mammalian thioredoxin reductase. 928 5

A new type of peroxidase enzyme, named thioredoxin peroxidase (TPx), that reduces H2O2 with the use of electrons from thioredoxin and contains two essential cysteines was recently identified. TPx homologs, termed peroxiredoxin (Prx), have also been identified and include several proteins, designated 1-Cys Prx, that contain only one conserved cysteine. Recombinant human 1-Cys Prx expressed in and purified from Escherichia coli has now been shown to reduce H2O2 with electrons provided by dithiothreitol. Furthermore, human 1-Cys Prx transiently expressed in NIH 3T3 cells was able to remove intracellular H2O2 generated in response either to the addition of exogenous H2O2 or to treatment with platelet-derived growth factor. The conserved Cys47-SH group was shown to be the site of oxidation by H2O2. Thus, mutation of Cys47 to serine abolished peroxidase activity. Moreover, the oxidized intermediate appears to be Cys-SOH. In contrast to TPx, in which one of the two conserved cysteines is oxidized to Cys-SOH and then immediately reacts with the second conserved cysteine of the second subunit of the enzyme homodimer to form an intermolecular disulfide, the Cys-SOH of 1-Cys Prx does not form a disulfide. Neither thioredoxin, which reduces the disulfide of TPx, nor glutathione, which reduces the Cys-SeOH of oxidized glutathione peroxidase, was able to reduce the Cys-SOH of 1-Cys Prx and consequently could not support peroxidase activity. Human 1-Cys Prx was previously shown to exhibit a low level of phospholipase A2 activity at an acidic pH; the enzyme was thus proposed to be lysosomal, and Ser32 was proposed to be critical for lipase function. However, the mutation of Ser32 or Cys47 has now been shown to have no effect on the lipase activity of 1-Cys Prx, which was also shown to be a cytosolic protein. Thus, the primary cellular function of 1-Cys Prx appears to be to reduce peroxides with the use of electrons provided by an as yet unidentified source; the enzyme therefore represents a new type of peroxidase.
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PMID:Characterization of a mammalian peroxiredoxin that contains one conserved cysteine. 949 58

Human thioredoxin reductase is a pyridine nucleotide-disulfide oxidoreductase closely related to glutathione reductase but differing from the latter in having a Cys-SeCys (selenocysteine) sequence as an additional redox center. Because selenoproteins cannot be expressed yet in heterologous systems, we optimized the purification of the protein from placenta with respect to final yield (1-2 mg from one placenta), specific activity (42 units/mg), and selenium content (0.94 +/- 0.03 mol/mol subunit). The steady state kinetics showed that the enzyme operates by a ping-pong mechanism; the value of kcat was 3330 +/- 882 min-1, and the Km values were 18 microM for NADPH and 25 microM for Escherichia coli thioredoxin. The activation energy of the reaction was found to be 53.2 kJ/mol, which allows comparisons of the steady state data with previous pre-steady state measurements. In its physiological, NADPH-reduced form, the enzyme is strongly inhibited by organic gold compounds that are widely used in the treatment of rheumatoid arthritis; for auranofin, the Ki was 4 nM when measured in the presence of 50 microM thioredoxin. At 1000-fold higher concentrations, that is at micromolar levels, the drugs also inhibited human glutathione reductase and the selenoenzyme glutathione peroxidase.
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PMID:Human placenta thioredoxin reductase. Isolation of the selenoenzyme, steady state kinetics, and inhibition by therapeutic gold compounds. 968 51

Peroxiredoxins catalyze reduction of hydrogen peroxide or alkyl peroxide, to water or the corresponding alcohol. Detailed analysis of their sequences indicates that these enzymes possess a thioredoxin (Trx)-like fold and consequently are homologues of both thioredoxin and glutathione peroxidase (GPx). Sequence- and structure-based multiple sequence alignments indicate that the peroxiredoxin active site cysteine and GPx active site selenocysteine are structurally equivalent. Homologous peroxiredoxin and GPx enzymes are predicted to catalyze equivalent reactions via similar reaction intermediates.
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PMID:Evidence that peroxiredoxins are novel members of the thioredoxin fold superfamily. 982 14


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