The relationship of the redox potentials of thioredoxin and thioredoxin reductase from Drosophila melanogaster to the enzymatic mechanism: reduced thioredoxin is the reductant of glutathione in Drosophila

Thioredoxin reductase from Drosophila melanogaster (DmTrxR) catalyzes the reversible transfer of reducing equivalents between NADPH and thioredoxin (Trx), a small protein that is involved in a wide variety of biological redox processes. The catalysis involves three essential redox states of the enzyme: the oxidized form of DmTrxR (Eox), the 2-electron-reduced forms (EH2), and the 4-electron-reduced forms (EH4). In the present work, the macroscopic redox potentials of Eox/EH2 and EH2/EH4 couples were determined to be -272 +/- 5 mV for Em(Eox/EH2) and -298 +/- 11 mV for Em(EH2/EH4) on the basis of redox equilibria between DmTrxR and NADH. The value for Em(EH2/EH4) obtained from the steady-state kinetics of the TrxR-catalyzed reaction between NADPH and D. melanogaster Trx-2 (DmTrx-2) was reasonably consistent with that based on redox equilibria. The redox potential of the Trx-(S)2/Trx-(SH)2 couple from D. melanogaster Trx-2 (DmTrx-2) was calculated to be -275.4 +/- 0.3 mV by using the Nernst equation and the Keq for the equilibrium of the reaction involving NADP/NADPH and Trx-(S)2/Trx-(SH)2. For the accurate determination of the Keq, an improved protocol has been developed to minimize errors that can be introduced by using starting concentrations far from equilibrium of the TrxR-catalyzed reaction between NADPH and Trx. This improved approach gives an Em of -284.2 +/- 1.0 mV for Escherichia coli Trx and -271.9 +/- 0.4 mV for Plasmodium falciparum Trx, which agree well with published values (-283 or -285 mV and -270 mV, respectively). The redox potentials determined herein provide further direct evidence for the proposed catalytic mechanism of DmTrxR, and cast new light on the essential role of the DmTrx system in cycling GSSG/GSH and maintaining the intracellular redox homeostasis in D. melanogaster where glutathione reductase is absent.     

A novel antioxidant mechanism of ebselen involving ebselen diselenide,a substrate of mammalian thioredoxin and thioredoxin reductase

The antioxidant mechanism of ebselen involves recently discovered reductions by mammalian thioredoxin reductase (TrxR) and thioredoxin (Trx) forming ebselen selenol. Here we describe a previously unknown reaction; ebselen reacts with its selenol forming an ebselen diselenide with a rate constant of 372 m(-1)s(-1). The diselenide also was a substrate of TrxR forming the selenol with K(m) of 40 microm and k(cat) of 79 min(-1) (k(cat)/K(m) of 3.3 x 10(4) m(-1)s(-1)). Trx increased the reduction because of its fast reaction with diselenide (rate constant 1.7 x 10(3) m(-1)s(-1)). Diselenide stimulated the H2O2 reductase activity of TrxR, even more efficiently with Trx present. Because the mechanism of ebselen as an antioxidant has been assumed to involve glutathione peroxidase-like activity, we compared the H2O2 reductase activity of ebselen with the GSH and Trx systems. TrxR at 50 nm, far below the estimated physiological level, gave 8-fold higher activity compared with 1 mm GSH; addition of 5 microm Trx increased this difference to 13-fold. The rate constant of ebselen selenol reacting with H2O2 was estimated to be faster than 350 m(-1)s(-1). We propose novel mechanisms for ebselen antioxidant action involving ebselen selenol and diselenide formation, with the thioredoxin system rather than glutathione as the predominant effector and target.     

Methylseleninate is a substrate rather than an inhibitor of mammalian thioredoxin reductase. Implications for the antitumor effects of selenium.

Biochemical and clinical evidence indicates that monomethylated selenium compounds are crucial for the tumor preventive effects of the trace element selenium and that methylselenol (CH(3)SeH) is a key metabolite. As suggested by Ganther (Ganther, H. E. (1999) Carcinogenesis 20, 1657-1666), methylselenol and its precursor methylseleninate might exert their effects by inhibition of the selenoenzyme thioredoxin reductase via the irreversible formation of a diselenide bridge. Here we report that methylseleninate does not act as an inhibitor of mammalian thioredoxin reductase but is in fact an excellent substrate (K(m) of 18 microm, k(cat) of 23 s(-1)), which is reduced by the enzyme according to the equation 2 NADPH + 2 H(+) + CH(3)SeO(2)H --> 2 NADP(+) + 2 H(2)O + CH(3)SeH. The selenium-containing product of this reaction was identified by mass spectrometry. Nascent methylselenol was found to efficiently reduce both H(2)O(2) and glutathione disulfide. The implications of these findings for the antitumor activity of selenium are discussed. Methylseleninate was a poor substrate not only for human glutathione reductase but also for the non-selenium thioredoxin reductases enzymes from Drosophila melanogaster and Plasmodium falciparum. This suggests that the catalytic selenocysteine residue of mammalian thioredoxin reductase is essential for methylseleninate reduction.     

The thioredoxin system of the malaria parasite Plasmodium falciparum. Glutathione reduction revisited

In most living cells, redox homeostasis is based both on the glutathione and the thioredoxin system. In the malaria parasite Plasmodium falciparum antioxidative proteins represent promising targets for the development of antiparasitic drugs. We cloned and expressed a thioredoxin of P. falciparum (pftrx), and we improved the stable expression of the thioredoxin reductase (PfTrxR) of the parasite by multiple silent mutagenesis. Both proteins were biochemically characterized and compared with the human host thioredoxin system. Intriguingly, the 13-kDa protein PfTrx is a better substrate for human TrxR (K(m) = 2 microm, k(cat) = 3300 min(-)(1)) than for P. falciparum TrxR (K(m) = 10.4 microm, k(cat) = 3100 min(-)(1)). Possessing a midpoint potential of -270 mV, PfTrx was found to reduce the disease-related metabolites S-nitrosoglutathione and GSSG. The rate constant k(2) for the reaction between reduced P. falciparum thioredoxin and GSSG was determined to be 0.039 microm(-)(1) min(-)(1) at 25 degrees C and pH 7.4. The k(2) for thioredoxins from man, Drosophila melanogaster, and Escherichia coli was approximately 5 times lower. Our data suggest that GSSG reduction can be supported at a high rate by the TrxR/Trx system in glutathione reductase-deficient cells; this may be relevant for certain stages of the malarial parasite but also for cells containing high [GSSG] of other organisms like dormant forms of Neurospora, glutathione reductase-deficient yeast mutants, or CD4(+) lymphocytes of AIDS patients.     

Function of Glu-469' in the acid-base catalysis of thioredoxin reductase from Drosophila melanogaster

Thioredoxin reductase (TrxR) catalyzes the reduction of thioredoxin (Trx) by NADPH. Because dipteran insects such as Drosophila melanogaster lack glutathione reductase, their TrxRs are particularly important for antioxidant protection; reduced Trx reacts nonenzymatically with oxidized glutathione to maintain a high glutathione/glutathione disulfide ratio. Like other members of the pyridine nucleotide-disulfide oxidoreductase family, TrxR is a homodimer; in the enzyme from D. melanogaster (DmTrxR), each catalytically active unit consists of three redox centers: FAD and an N-terminal Cys-57-Cys-62 redox-active disulfide from one monomer and a Cys-489'-Cys-490' C-terminal redox-active disulfide from the second monomer. A dyad of His-464' and Glu-469' in TrxR acts as the acid-base catalyst of the dithiol-disulfide interchange reactions required in catalysis [Huang, H.-H., et al. (2008) Biochemistry 47, 1721-1731]. In this investigation, the role of Glu-469' in catalysis by DmTrxR has been studied. The E469'A and E469'Q DmTrxR variants retain 28 and 35% of the wild-type activity, respectively, indicating that this glutamate residue is important but not critical to catalysis. The pH dependence of V(max) for both glutamate variants yields pK(a) values of 6.0 and 8.7, compared to those in the wild-type enzyme of 6.4 and 9.3, respectively, indicating that the basicity of His-464' in TrxR in complex with its substrate, DmTrx-2, is significantly lower in the glutamate variants than in wild-type enzyme. The rates of some steps in the reductive half-reactions in both glutamate variants are much slower than those of the wild-type enzyme. On the basis of our observations, it is proposed that the function of Glu-469' is to facilitate the positioning of His-464' toward the interchange thiol, Cys-57, as suggested for the analogous residue in glutathione reductase.     

Purification and characterization of delta3Trx-1, a splicing variant of human thioredoxin-1 lacking exon 3

Thioredoxins comprise a growing family of proteins that function as general protein-disulfide reductases and are maintained in their reduced active form by the flavoenzyme thioredoxin reductase. Human Trx-1 is mainly a cytosolic protein, although it has been shown to translocate into the nucleus upon certain stimuli and can also be secreted. We report here the expression and characterization of delta3Trx-1, a splicing variant of human Trx-1, lacking exon 3, which spans from residues 44 to 63 in the wild-type protein. Structure-based prediction of this splicing form indicates that delta3Trx-1 lacks helix alpha2 and strand beta3, which are implicated in substrate positioning and three-dimensional stabilization of the active site residues. Recombinant human delta3Trx-1 is recognized by polyclonal antibodies raised against full-length human Trx-1. However, delta3Trx-1 retains no enzymatic activity either with DTT or thioredoxin reductase and NADPH as reducing systems. Delta3Trx-1 competes with full-length Trx-1 for the interaction with thioredoxin reductase. The absence of helix alpha2 and strand beta3 in delta3Trx-1 is consistent with the lack of enzymatic activity and its potential dominant negative properties.     

The thioredoxin specificity of Drosophila GPx: a paradigm for a peroxiredoxin-like mechanism of many glutathione peroxidases

Some members of the glutathione peroxidase (GPx) family have been reported to accept thioredoxin as reducing substrate. However, the selenocysteine-containing ones oxidise thioredoxin (Trx), if at all, at extremely slow rates. In contrast, the Cys homolog of Drosophila melanogaster exhibits a clear preference for Trx, the net forward rate constant, k'(+2), for reduction by Trx being 1.5x10(6) M(-1) s(-1), but only 5.4 M(-1) s(-1) for glutathione. Like other CysGPxs with thioredoxin peroxidase activity, Drosophila melanogaster (Dm)GPx oxidized by H(2)O(2) contained an intra-molecular disulfide bridge between the active-site cysteine (C45; C(P)) and C91. Site-directed mutagenesis of C91 in DmGPx abrogated Trx peroxidase activity, but increased the rate constant for glutathione by two orders of magnitude. In contrast, a replacement of C74 by Ser or Ala only marginally affected activity and specificity of DmGPx. Furthermore, LC-MS/MS analysis of oxidized DmGPx exposed to a reduced Trx C35S mutant yielded a dead-end intermediate containing a disulfide between Trx C32 and DmGPx C91. Thus, the catalytic mechanism of DmGPx, unlike that of selenocysteine (Sec)GPxs, involves formation of an internal disulfide that is pivotal to the interaction with Trx. Hereby C91, like the analogous second cysteine in 2-cysteine peroxiredoxins, adopts the role of a "resolving" cysteine (C(R)). Molecular modeling and homology considerations based on 450 GPxs suggest peculiar features to determine Trx specificity: (i) a non-aligned second Cys within the fourth helix that acts as C(R); (ii) deletions of the subunit interfaces typical of tetrameric GPxs leading to flexibility of the C(R)-containing loop. Based of these characteristics, most of the non-mammalian CysGPxs, in functional terms, are thioredoxin peroxidases.     

The mechanism of high Mr thioredoxin reductase from Drosophila melanogaster

Drosophila melanogaster thioredoxin reductase-1 (DmTrxR-1) is a key flavoenzyme in dipteran insects, where it substitutes for glutathione reductase. DmTrxR-1 belongs to the family of dimeric, high Mr thioredoxin reductases, which catalyze reduction of thioredoxin by NADPH. Thioredoxin reductase has an N-terminal redox-active disulfide (Cys57-Cys62) adjacent to the flavin and a redox-active C-terminal cysteine pair (Cys489'-Cys490' in the other subunit) that transfer electrons from Cys57-Cys62 to the substrate thioredoxin. Cys489'-Cys490' functions similarly to Cys495-Sec496 (Sec = selenocysteine) and Cys535-XXXX-Cys540 in human and parasite Plasmodium falciparum enzymes, but a catalytic redox center formed by adjacent Cys residues, as observed in DmTrxR-1, is unprecedented. Our data show, for the first time in a high Mr TrxR, that DmTrxR-1 oscillates between the 2-electron reduced state, EH2, and the 4-electron state, EH4, in catalysis, after the initial priming reduction of the oxidized enzyme (Eox) to EH2. The reductive half-reaction consumes 2 eq of NADPH in two observable steps to produce EH4. The first equivalent yields a FADH--NADP+ charge-transfer complex that reduces the adjacent disulfide to form a thiolate-flavin charge-transfer complex. EH4 reacts with thioredoxin rapidly to produce EH2. In contrast, Eox formation is slow and incomplete; thus, EH2 of wild-type cannot reduce thioredoxin at catalytically competent rates. Mutants lacking the C-terminal redox center, C489S, C490S, and C489S/C490S, are incapable of reducing thioredoxin and can only be reduced to EH2 forms. Additional data suggest that Cys57 attacks Cys490' in the interchange reaction between the N-terminal dithiol and the C-terminal disulfide.     

Identification and characterization of thioredoxin and thioredoxin reductase from Aeropyrum pernix K1.

We have identified and characterized a thermostable thioredoxin system in the aerobic hyperthermophilic archaeon Aeropyrum pernix K1. The gene (Accession no. APE0641) of A. pernix encoding a 37 kDa protein contains a redox active site motif (CPHC) but its N-terminal extension region (about 200 residues) shows no homology within the genome database. A second gene (Accession no. APE1061) has high homology to thioredoxin reductase and encodes a 37 kDa protein with the active site motif (CSVC), and binding sites for FAD and NADPH. We cloned the two genes and expressed both proteins in E. coli. It was observed that the recombinant proteins could act as an NADPH-dependent protein disulfide reductase system in the insulin reduction. In addition, the APE0641 protein and thioredoxin reductase from E. coli could also catalyze the disulfide reduction. These indicated that APE1061 and APE0641 express thioredoxin (ApTrx) and thioredoxin reductase (ApTR) of A. pernix, respectively. ApTR is expressed as an active homodimeric flavoprotein in the E. coli system. The optimum temperature was above 90 degrees C, and the half-life of heat inactivation was about 4 min at 110 degrees C. The heat stability of ApTR was enhanced in the presence of excess FAD. ApTR could reduce both thioredoxins from A. pernix and E. coli and showed a similar molar specific activity for both proteins. The standard state redox potential of ApTrx was about -262 mV, which was slightly higher than that of Trx from E. coli (-270 mV). These results indicate that a lower redox potential of thioredoxin is not necessary for keeping catalytic disulfide bonds reduced and thereby coping with oxidative stress in an aerobic hyperthermophilic archaea. Furthermore, the thioredoxin system of aerobic hyperthermophilic archaea is biochemically close to that of the bacteria.     

GPX2, encoding a phospholipid hydroperoxide glutathione peroxidase homologue, codes for an atypical 2-Cys peroxiredoxin in Saccharomyces cerevisiae

We have previously reported that Saccharomyces cerevisiae has three glutathione peroxidase homologues (GPX1, GPX2, and GPX3) (Inoue, Y., Matsuda, T., Sugiyama, K., Izawa, S., and Kimura, A. (1999) J. Biol. Chem. 274, 27002-27009). Of these, the GPX2 gene product (Gpx2) shows the greatest similarity to phospholipid hydroperoxide glutathione peroxidase. Here we show that GPX2 encodes an atypical 2-Cys peroxiredoxin which uses thioredoxin as an electron donor. Gpx2 was essentially in a reduced form even in mutants defective in glutathione reductase or glutaredoxin under oxidative stressed conditions. On the other hand, Gpx2 was partially oxidized in a mutant defective in cytosolic thioredoxin (trx1Deltatrx2Delta) under non-stressed conditions and completely oxidized in tert-butyl hydroperoxide-treated cells of trx1Deltatrx2Delta and thioredoxin reductase-deficient mutant cells. Alanine scanning of cysteine residues of Gpx2 revealed that an intramolecular disulfide bond was formed between Cys37 and Cys83 in vivo. Gpx2 was purified to determine whether it functions as a peroxidase that uses thioredoxin as an electron donor in vitro. Gpx2 reduced H2O2 and tert-butyl hydroperoxide in the presence of thioredoxin, thioredoxin reductase, and NADPH (for H2O2, Km= 20 microm, kcat = 9.57 x 10(2) s(-1); for tert-butyl hydroperoxide, Km= 62.5 microm, kcat = 3.68 x 10(2) s(-1)); however, it showed remarkably less activity toward these peroxides in the presence of glutathione, glutathione reductase, and NADPH. The sensitivity of yeast cells to tert-butyl hydroperoxide was found to be exacerbated by the co-existence of Ca2+, a tendency that was most obvious in gpx2Delta cells. Although the redox state of Gpx2 was not affected by Ca2+, the Gpx2 level was markedly increased in the presence of both tert-butyl hydroperoxide and Ca2+. Gpx2 is likely to play an important role in the protection of cells from oxidative stress in the presence of Ca2+.     

The putative glutathione peroxidase gene of Plasmodium falciparum codes for a thioredoxin peroxidase

A putative glutathione peroxidase gene (Swiss-Prot accession number Z 68200) of Plasmodium falciparum, the causative agent of tropical malaria, was expressed in Escherichia coli and purified to electrophoretic homogeneity. Like phospholipid hydroperoxide glutathione peroxidase of mammals, it proved to be monomeric. It was active with H(2)O(2) and organic hydroperoxides but, unlike phospholipid hydroperoxide glutathione peroxidase, not with phosphatidylcholine hydroperoxide. With glutathione peroxidases it shares the ping-pong mechanism with infinite V(max) and K(m) when analyzed with GSH as substrate. As a homologue with selenocysteine replaced by cysteine, its reactions with hydroperoxides and GSH are 3 orders of magnitude slower than those of the selenoperoxidases. Unexpectedly, the plasmodial enzyme proved to react faster with thioredoxins than with GSH and most efficiently with thioredoxin of P. falciparum (Swiss-Prot accession number 202664). It is therefore reclassified as thioredoxin peroxidase. With plasmodial thioredoxin, the enzyme also displays ping-pong kinetics, yet with a limiting K(m) of 10 microm and a k(1)' of 0.55 s(-)1. The apparent k(1)' for oxidation with cumene, t-butyl, and hydrogen peroxides are 2.0 x 10(4) m(-1) s(-1), 3.3 x 10(3) m(-1) s(-1), and 2.5 x 10(3) m (-1) s(-1), respectively. k(2)' for reduction by autologous thioredoxin is 5.4 x 10(4) m(-1) s(-1) (21.2 m(-1) s(-1) for GSH). The newly discovered enzymatic function of the plasmodial gene product suggests a reconsideration of its presumed role in parasitic antioxidant defense.     

Activation of extracellular transglutaminase 2 by thioredoxin

The mechanism of activation of transglutaminase 2 (TG2) in the extracellular matrix remains a fundamental mystery in our understanding of the biology of this multifunctional mammalian enzyme. Earlier investigations have highlighted the role of a disulfide bond formed by vicinal Cys residues in maintaining calcium-bound TG2 in an inactive state. Here, we have shown that the redox potential of this disulfide bond is approximately -190 mV, a high value for a disulfide bond in proteins. Consistent with this observation, TG2 activity in a freshly wounded fibroblast culture depends upon the redox potential of the environment. We sought to identify a physiological mechanism for the activation of oxidized TG2. With a k(cat)/K(m) of 1.6 μm(-1) min(-1), human thioredoxin (Trx) was a highly specific activator of oxidized human TG2. Trx-mediated activation of TG2 was blocked by PX-12, a small molecule Trx inhibitor that is undergoing clinical trials as a cancer chemotherapeutic agent. In a mixed culture containing fibroblasts and monocytic cells, interferon-γ stimulated Trx release from monocytes, which in turn activated TG2 around the fibroblasts. Recombinant human Trx could also activate extracellular TG2 in cryosections of human and mouse small intestinal biopsies. In addition to explaining how TG2 can be activated by dietary gluten in the small intestinal mucosa of celiac sprue patients, our findings reveal a new strategy for inhibiting the undesirable consequences of TG2 activity in this widespread, lifelong disease.     

Purification and characterization of a chimeric enzyme from Haemophilus influenzae Rd that exhibits glutathione-dependent peroxidase activity

While belonging to the same family of antioxidant enzymes, members of the peroxiredoxins do not necessarily employ one and the same method for their reduction. Most representatives become reduced with the aid of thioredoxin, whereas some members use AhpF, tryparedoxin, or cyclophilin A. Recent research on a new peroxiredoxin isoform (type C) from Populus trichocarpa has shown that these particular types may also use glutaredoxin instead of thioredoxin. This finding is supported by the occurrence of chimeric proteins composed of a peroxiredoxin and glutaredoxin region. A gene encoding such a fusion protein is enclosed in the Haemophilus influenzae Rd genome. We expressed the H. influenzae protein, denoted here as PGdx, in Escherichia coli and purified the recombinant enzyme. In vitro assays demonstrate that PGdx, in the presence of dithiothreitol or glutathione, is able to protect supercoiled DNA against the metal ion-catalyzed oxidation-system. Enzymatic assays did, indeed, characterize PGdx as a peroxidase, requiring the glutathione redox cycle for the reduction of hydrogen peroxide (k(cat)/K(m) 5.01 x 10(6) s(-1) m(-1)) as well as the small organic hydroperoxide tert-butylhydroperoxide (k(cat)/K(m) 5.67 x 10(4) s(-1) m(-1)). Enzymatic activity as function of the glutathione concentration deviated from normal Michaelis-Menten kinetics, giving a sigmoidal pattern with an apparent Hill coefficient of 2.9. Besides the formation of a disulfide-linked PGdx dimer, it was also shown by mass spectrometric analysis that cysteine 49, which is equivalent to the active site cysteine of the peroxiredoxins, undergoes glutathionylation during purification under nonreducing conditions. Based on these results, we propose a model for the catalytic mechanism.     

Arabidopsis cotyledon chloroplast biogenesis factor CYO1 uses glutathione as an electron donor and interacts with PSI (A1 and A2) and PSII (CP43 and CP47) subunits

CYO1 is required for thylakoid biogenesis in cotyledons of Arabidopsis thaliana. To elucidate the enzymatic characteristics of CYO1, we analyzed the protein disulfide isomerase (PDI) activity of CYO1 using dieosin glutathione disulfide (Di-E-GSSG) as a substrate. The reductase activity of CYO1 increased as a function of Di-E-GSSG, with an apparent K(m) of 824nM and K(cat) of 0.53min(-1). PDI catalyzes dithiol/disulfide interchange reactions, and the cysteine residues in PDI proteins are very important. To analyze the significance of the cysteine residues for the PDI activity of CYO1, we estimated the kinetic parameters of point-mutated CYO1 proteins. C117S, C124S, C135S, and C156S had higher values for K(m) than did wild-type CYO1. C158S had a similar K(m) but a higher K(cat), and C138S and C161S had similar K(m) values but lower K(cat) values than did wild-type CYO1. These results suggested that the cysteine residues at positions 138 and 161 were important for PDI activity. Low PDI activity of CYO1 was observed when NADPH or NADH was used as an electron donor. However, PDI activity was observed with CYO1 and glutathione, suggesting that glutathione may serve as a reducing agent for CYO1 in vivo. Based on analysis with the split-ubiquitin system, CYO1 interacted with the A1 and A2 subunits of PSI and the CP43 and CP47 subunits of PSII. Thus, CYO1 may accelerate the folding of cysteine residue--containing PSI and PSII subunits by repeatedly breaking and creating disulfide bonds.     

Reduction of castor-seed 2S albumin protein by thioredoxin

The 2S albumin from the endosperm of castor seed (Ricinus communis L.) seed was reduced by thioredoxin from either wheat germ or Escherichia coli. The 2S protein is made up of a large (approx. 7 kDa) subunit that contains two intramolecular disulfides and a small (approx. 4 kDa) subunit that lacks intramolecular disulfides. The two subunits are joined by at least one intermolecular disulfide bond. Thioredoxin could be reduced either enzymically with NADPH and NADP-thioredoxin reductase or chemically with dithiothreitol. Reduced glutathione and glutaredoxin (from E. coli) were without effect. The ability of the 2S protein to undergo reduction by thioredoxin was demonstrated by a direct reduction procedure based on the fluorescent probe, monobromobimane, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and by an enzymatic procedure in which reduction is linked to activation of chloroplast NADP-malate dehydrogenase. Analyses indicated that thioredoxin actively reduced the intramolecular disulfides of the 2S large subunit, but was ineffective in reducing the intermolecular disulfide(s) that connect the large to the small subunit. These findings extend the role of thioredoxin to the reduction of a seed protein that is widely distributed in oil producing plants.Abbreviations DDT dithiothreitol - mBBr monobromobimane - NTR NADP-thioredoxin reductase - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis This work was supported by a grant from the National Science Foundation.     

Attachment of the N-terminal domain of Salmonella typhimurium AhpF to Escherichia coli thioredoxin reductase confers AhpC reductase activity but does not affect thioredoxin reductase activity

AhpF of Salmonella typhimurium, the flavoprotein reductase required for catalytic turnover of AhpC with hydroperoxide substrates in the alkyl hydroperoxide reductase system, is a 57 kDa protein with homology to thioredoxin reductase (TrR) from Escherichia coli. Like TrR, AhpF employs tightly bound FAD and redox-active disulfide center(s) in catalyzing electron transfer from reduced pyridine nucleotides to the disulfide bond of its protein substrate. Homology of AhpF to the smaller (35 kDa) TrR protein occurs in the C-terminal part of AhpF; a stretch of about 200 amino acids at the N-terminus of AhpF contains an additional redox-active disulfide center and is required for catalysis of AhpC reduction. We have demonstrated that fusion of the N-terminal 207 amino acids of AhpF to full-length TrR results in a chimeric protein (Nt-TrR) with essentially the same catalytic efficiency (k(cat)/K(m)) as AhpF in AhpC reductase assays; both k(cat) and the K(m) for AhpC are decreased about 3-4-fold for Nt-TrR compared with AhpF. In addition, Nt-TrR retains essentially full TrR activity. Based on results from two mutants of Nt-TrR (C129, 132S and C342,345S), AhpC reductase activity requires both centers while TrR activity requires only the C-terminal-most disulfide center in Nt-TrR. The high catalytic efficiency with which Nt-TrR can reduce thioredoxin implies that the attached N-terminal domain does not block access of thioredoxin to the TrR-derived Cys342-Cys345 center of Nt-TrR nor does it impede the putative conformational changes that this part of Nt-TrR is proposed to undergo during catalysis. These studies indicate that the C-terminal part of AhpF and bacterial TrR have very similar mechanistic properties. These findings also confirm that the N-terminal domain of AhpF plays a direct role in AhpC reduction.     

Identification and functional characterization of thioredoxin from Trypanosoma brucei brucei

Trypanosomes and Leishmania, the causative agents of several tropical diseases, lack the glutathione/glutathione reductase system but have trypanothione/trypanothione reductase instead. The uniqueness of this thiol metabolism and the failure to detect thioredoxin reductases in these parasites have led to the suggestion that these protozoa lack a thioredoxin system. As presented here, this is not the case. A gene encoding thioredoxin has been cloned from Trypanosoma brucei, the causative agent of African sleeping sickness. The single copy gene, which encodes a protein of 107 amino acid residues, is expressed in all developmental stages of the parasite. The deduced protein sequence is 56% identical with a putative thioredoxin revealed by the genome project of Leishmania major. The relationship to other thioredoxins is low. T. brucei thioredoxin is unusual in having a calculated pI value of 8.5. The gene has been overexpressed in Escherichia coli. The recombinant protein is a substrate of human thioredoxin reductase with a K(m) value of 6 microM but is not reduced by trypanothione reductase. T. brucei thioredoxin catalyzes the reduction of insulin by dithioerythritol, and functions as an electron donor for T. brucei ribonucleotide reductase. The parasite protein is the first classical thioredoxin of the order Kinetoplastida characterized so far.     

Thioredoxin peroxidase-1 (peroxiredoxin-1) is increased in thioredoxin-1 transfected cells and results in enhanced protection against apoptosis caused by hydrogen peroxide but not by other agents including dexamethasone, etoposide, and doxorubicin

Thioredoxin-1 (Trx-1) is a small redox oncoprotein whose expression is increased in a number of human primary cancers where it is associated with aggressive tumor growth, inhibition of apoptosis and decreased patient survival. We report that Trx-1-transfected MCF-7 human breast cancer cells have increased expression of thioredoxin peroxidase-1 (TrxP-1) a peroxiredoxin family member that scavenges H(2)O(2) using Trx-1 as a source of reducing equivalents. Our work shows that TrxP-1 is more effective than selenium-dependent glutathione peroxidase in protecting cells against H(2)O(2) damage. Transfection of mouse WEHI7.2 lymphoma cells with human TrxP-1 or TrxP-2, but not TrxP-4, protects the cells against H(2)O(2) induced apoptosis but does not protect against apoptosis induced by dexamethasone, etoposide, or doxorubicin. The results show that an increase in TrxP-1 expression contributes to the protection against H(2)O(2) induced apoptosis caused by Trx-1, but does not protect against apoptosis induced by other agents.