Redox control of hydrogenase activity in the green alga Scenedesmus obliquus by thioredoxin and other thiols.

The activity of the NiFe-hydrogenase from the green alga Scenedesmus obliquus is inhibited by both algal thioredoxins f and I+II, and by Escherichia coli thioredoxin. The strongest inhibition was observed with homologous chloroplastic thioredoxin f (I50 = 21 nM) and E. coli thioredoxin (I50 = 83 nM). For the homologous cytoplasmic thioredoxins I+II an I50 of 667 nM was determined. Glutathione shows a similar but much less pronounced inhibitory effect whereas dithiothreitol had no effect. In addition to glucose-6-phosphate dehydrogenase, NiFe-hydrogenase is only the second enzyme known to be inhibited by reduced thioredoxin.

Trypanosoma brucei tryparedoxin, a thioredoxin-like protein in African trypanosomes.

A gene has been cloned from Trypanosoma brucei which encodes a protein of 144 amino acid residues containing the thioredoxin-like motif WCPPCR. Overexpression of the gene in E. coli resulted in 4 mg pure protein from 100 ml bacterial cell culture. Recombinant T. brucei tryparedoxin acts as a thiol-disulfide oxidoreductase. It is spontaneously reduced by trypanothione. This dithiol, exclusively found in parasitic protozoa, also reduces E. coli glutaredoxin but not thioredoxin. The trypanothione/tryparedoxin couple is an effective reductant of T. brucei ribonucleotide reductase. Like thioredoxins it has a poor GSH:disulfide transhydrogenase activity. The catalytic properties of tryparedoxin are intermediate between those of classical thioredoxins and glutaredoxins which indicates that these parasite proteins may form a new class of thiol-disulfide oxidoreductases.

Ribonucleotide reductase in Bacillus subtilis--evidence for a Mn-dependent enzyme.

The reduction of 2'-ribonucleotides to 2'-deoxyribonucleotides, a unique step in DNA formation, is catalyzed by ribonucleotide reductase (RRase), an allosterically regulated, cell cycle-dependent enzyme. This work reports a reversible impairment of DNA formation and ribonucleotide reduction upon manganese depletion in Bacillus subtilis demonstrated through in vivo labeling with necleic acid precursors and enzyme assays with ether-permeabilized cells. No deoxyadenosylcobalamin-dependent reduction of ribonucleotides was detected in the cytosol, and the properties of a partially purified enzyme fraction, i.e., sensitivity towards EDTA and hydroxyurea (HU), indicated a metal-dependent type of RRase. The enzyme was enriched by gel filtration on Superose 12 from glycerol- or fumarate-grown cells and submitted to Q-band electron paramagnetic resonance (EPR) spectroscopy for further characterization of the metal center. A distinct Mn(II) signal was obtained in both preparations characteristic of a protein-bound mangaenese in a mononuclear metal center with axial symmetry. The intensity of this Mn signal was not affected by addition of the radical scavenger HU (10 mM) but reduced in the presence of 2.5 mM EDTA. On the basis of these results, we suggest that Bacillus subtilis has a Mn-dependent ribonucleotide reductase.

Sulfitolysis and thioredoxin-dependent reduction reveal the presence of a structural disulfide bridge in spinach chloroplast fructose-1,6-bisphosphatase.

A significant difference between cytosolic and chloroplastic fructose-1,6-bisphosphatase (FbPase) is an extra peptide in the middle of chloroplast FbPase which contains three additional cysteine residues. Sit-directed mutagenesis experiments have shown that at least two of these cysteine residues are involved in forming the regulatory disulfide bridge [Jacquot, J.-P. et al., FEBS Lett. 401 (1997) 143-147] which is the presupposition for the thioredoxin-dependent control of chloroplast FbPase activity. Here we report that each subunit of the FbPase contains an additional structural disulfide bridge which has been observed by combined application of thioredoxins and sulfitolysis. Observation of the structural disulfide bridges by sulfitolysis was only possible when the FbPase was already specifically reduced by the homologous thioredoxin species TRm. and TRf from spinach chloroplasts. Interestingly, the accessibility of the structural disulfide bridge for sulfite ions depends on the thioredoxin species engaged in the thioredoxin/FbPase complex.

Dehydroascorbate and dehydroascorbate reductase are phantom indicators of oxidative stress in plants.

In many physiological studies dehydroascorbate (DHA) reductase is regarded as one of the chloroplast enzymes involved in the protection against oxidative stress. Here, evidence is presented that plant cells do not possess a specific DHA reductase. The DHA reductase activities measured in plant extracts are due to side reactions of proteins containing redox-active dicysteine sites. Native gel electrophoresis combined with specific activity staining revealed three different proteins with DHA reductase activity in leaf and chloroplast extracts. These proteins have been identified as thioredoxins and trypsin inhibitors (Kunitz type) by Western blot analysis. The essential regulatory functions of thioredoxins in chloroplast metabolism are strongly inhibited in the presence of as little as 50 microM DHA. Thus, the intracellular DHA concentration should be kept below 50 microM but not all proteins with DHA reductase activity are effective enough for this purpose. A specific DHA reductase is frequently demanded as part of the enzymatic equipment to avoid oxidative stress. We argue that this is not necessary because in chloroplasts DHA does not accumulate to any significant extent due to the high activities of monodehydroascorbate reductase and of reduced ferredoxin.

Large-scale production and plaque titration of European Chlorella viruses.

Viruses of the exsymbiotic green freshwater algae Chlorella, family Phycodnaviridae, appear to be distributed worldwide but those found in North American algae have been characterized in detail. The distinct European Chlorella viruses were studied and it was necessary to adapt both large scale purification and the plaque titration assay to the host organisms' different physiology and to our specific laboratory needs. In the virus purification scheme, a precipitation step with polyethylene glycol was introduced which allows high yield recovery of infective particles from large volumes by rapid low-speed centrifugation. In the plaque assay, a standardized algal culture was introduced. The influence of other factors, e.g. circadian rhythm, on plaque growth is also described.

Characterization of thioredoxins by sodium dodecyl sulfate-slab gel electrophoresis and high performance capillary electrophoresis.

Disulfide containing proteins--thioredoxins from E. coli and pig heart mitochondria--were characterized by sodium dodecyl sulfate (SDS)-electrophoresis and high performance capillary electrophoresis (HPCE). Following the mitochondrial thioredoxin samples at different stages of purification, we found that their electrophoretic patterns vary, dependent on the redox condition of isolation, preparation of the samples for SDS-electrophoresis, and sample storage. All these factors influenced the relative intensities of several protein bands with thioredoxin-like mobility, whereas the sample storage also resulted in the appearance of SDS- and dithiothreitol (DTT)-resistant high molecular mass forms, probably thioredoxin dimers. The multiple forms of the thioredoxin from pig heart mitochondria in SDS-electrophoresis might be dependent on the oxidation state of the protein cysteine residues. A commercial preparation of the thioredoxin from E. coli did not exhibit any changes in mobility in SDS gels whether the sample was prepared with or without DTT. After the final purification step no correlation was found between mitochondrial thioredoxin activity, determined in the insulin assay, and its purity in SDS-electrophoresis. A correlation was, however, found when analyzing the thioredoxin by HPCE. The latter approach demonstrated the heterogeneity of the thioredoxin samples homogeneous on SDS electrophoresis, only one of the several HPCE peaks being active in the insulin assay. Also, thioredoxin from E. coli, homogeneous on SDS-electrophoresis, was found heterogeneous on HPCE. The peak corresponding to the insulin-dependent thioredoxin activity was split into two by DTT treatment, suggesting that redox transformations of thioredoxin could be followed by HPCE.

Activation of mitochondrial 2-oxoacid dehydrogenases by thioredoxin.

The regulation of mitochondrial dehydrogenases of 2-oxoacids by thioredoxin is established. It is found that at low NAD+ and saturating concentrations of 2-oxoacids and CoA, inactivation of 2-oxoacid dehydrogenase complexes takes place, preventing NAD+ reduction under such conditions. However, addition of oxidized E. coli thioredoxin to the reaction medium without dithiothreitol allows effective NAD+ reduction at this substrate ratio. Product accumulation curves show that thioredoxin activates the complexes by protecting them from the inactivation observed in the conditions when the complex-bound dihydrolipoate is accumulated. Disappearance of the activatory effect of thioredoxin after its treatment with SH-specific reagents indicates the involvement of the redox-active cysteine couple of thioredoxin in its activation of 2-oxoacid dehydrogenase complexes. The redox-inactive thioredoxin not only shows no activation, but in fact exerts an inhibitory effect. The inhibition manifests the complex formation between SH-modified thioredoxin and dehydrogenase systems, involving amino acid residues of thioredoxin other than cysteine. High efficiency of thioredoxin from E. coli as compared to chloroplast thioredoxin f and glutathione disulfide is revealed. This indicates the importance of specific protein structure also for the influence of the redox-active thioredoxin upon the 2-oxoacid dehydrogenase complexes. The results obtained suggest that these key enzyme systems of mitochondrial metabolism represent previously unidentified targets for the action of mitochondrial thioredoxin, which is known to resemble the E. coli counterpart studies in this work.

Cross-linked iron dextran is an efficient oral phosphate binder in the rat.

BACKGROUND: There is a need for alternative oral phosphate binders. In-vitro studies showed that iron(III)oxide-hydroxide-modified cross-linked dextran is a promising, insoluble phosphate-binding agent. The present study was designed to assess its in-vivo efficacy and safety in the rat. STUDY, DESIGN AND METHODS: Iron(III)oxide-hydroxide modified dextran beads were mixed with normal rat feed in a proportion of 8% by weight. With this formula rats were fed for 4 weeks. A control group received the same diet without added phosphate binder. Samples of blood, urine, and faeces were taken from each animal before the phosphate binder was administered, 2 weeks later, and at the end of the examination period (day 29). Phosphate, calcium, iron were analysed in the blood samples. Calcium and phosphate concentrations were determined in the urine, phosphate, calcium, and iron concentrations in the excrements. Stability of the material in the duodenum was also simulated. RESULTS AND CONCLUSIONS: The results demonstrate an excellent phosphate-binding capacity of the material and a good tolerance during the intestinal passage. No significant chemical or enzymatic degradation, histological alterations, or other treatment-related macroscopic findings were recorded. The present efficacy and toxicity study has shown effective phosphate binding with no toxicity and no iron release after ingestion of this novel phosphate binding agent. We propose clinical evaluation studies to assess whether similar efficacy and safety can be shown in humans.

Using lipoate enantiomers and thioredoxin to study the mechanism of the 2-oxoacid-dependent dihydrolipoate production by the 2-oxoacid dehydrogenase complexes.

The thioredoxin-catalyzed insulin reduction by dihydrolipoate was applied to study the 2-oxoacid: lipoate oxidoreductase activity of 2-oxoacid dehydrogenase complexes. The enzymatic and non-enzymatic mechanisms of the transfer of reducing equivalents from the complexes to free lipoic acid (alpha-lipoic acid, 6,8-thiooctic acid) were distinguished using the high stereoselectivity of the complex enzymes to the R-enantiomer of lipoate. Unlike these enzymes, thioredoxin from E. coli exhibited no stereoselectivity upon reduction with chemically obtained dihydrolipoate. However, coupled to the dihydrolipoate production by the dehydrogenase complexes, the process was essentially sensitive both to the enantiomer used and the dihydrolipoyl dehydrogenase activity of the complexes. These results indicated the involvement of the third complex component, dihydrolipoyl dehydrogenase, in the 2-oxoacid-dependent dihydrolipoate formation. The implication of the investigated reaction for a connection between thioredoxin and the 2-oxoacid dehydrogenase complexes in the mitochondrial metabolism are discussed.

Identification and localization of the first glutaredoxin in leaves of a higher plant.

Glutaredoxin(thioltransferase) has been identified and purified to homogeneity from spinach leaves. Its cytosolic localization was demonstrated by chromatographic and immunological analysis of extracts from isolated spinach chloroplasts and mitochondria, respectively. Spinach glutaredoxin shows a significant crossreactivity with antibodies raised against E. coli glutaredoxin and possesses a specific thioltransferase activity comparable to that of the E. coli protein. Minor thioltransferase activities (less than 10% of total leaf activity) have been observed in spinach chloroplasts which are probably due to the presence of trypsin inhibitor and thioredoxins (TRf and TRm).

Characterization and extracorporeal application of a new phosphate-binding agent.

A new phosphate-binding agent which does not cause any severe side effects in vivo was developed by modifying a crosslinked dextran with polynuclear iron(III)oxide-hydroxide. Its particle size ranges from 150 to 300 microns, and the iron content was about 18% by dry weight. The oxidation state of iron was characterized by ESCA and Mössbauer spectroscopy. The maximum phosphate binding capacity of the iron(III)oxide-hydroxide-modified dextran was determined with respect to aqueous phosphate solutions, human serum and whole blood. The effects on whole blood count, haemolysis, protein concentration and enzyme activities were examined. In addition, the influence of phosphate concentration, pH and temperature on the phosphate uptake of the material was determined. The results show that this new adsorbent might provide an alternative to conventional phosphate-binding agents. This paper also describes the first experiments on the therapeutic application of the material in an extracorporeal blood perfusion system for the treatment of hyperphosphataemia during haemodialysis.

A novel-dehydroascorbate reductase from spinach chloroplasts homologous to plant trypsin inhibitor.

Dehydroascorbate reductase has been isolated from spinach chloroplasts and purified to apparent homogeneity. The N-terminal amino acid sequence of the enzyme is homologous to the Kunitz-type trypsin inhibitors from plant sources. It is shown that spinach DHA reductase and soybean trypsin inhibitor are both capable of reducing dehydroascorbate when in the reduced (thiol) form but acquire trypsin-inhibiting activity in the oxidized (disulfide) state. Reduced chloroplast thioredoxins also reduce dehydroascorbate.

Thioredoxin reduction dependent on alpha-ketoacid oxidation by alpha-ketoacid dehydrogenase complexes.

The pyruvate and alpha-ketoglutarate dehydrogenase complexes isolated from pig heart mitochondria promote the reduction of thioredoxin in the presence of their alpha-ketoacid substrates, coenzyme A, and free lipoate. Substrate-specific generation of reduced thioredoxin was established by two independent methods, viz. reduction of insulin and thioredoxin reductase-catalyzed NADPH formation. Dihydrolipoate accumulating in the absence of NAD+ is the likely intermediate. A redox function in alpha-ketoacid oxidation provides a potential role for the specific thioredoxins previously identified by us in mitochondria.

Facile sulfitolysis of the disulfide bonds in oxidized thioredoxin and glutaredoxin.

Thioredoxins and glutaredoxins, in their oxidized form, possess a single disulfide bridge located on an edge of the small compact molecules. In contrast to most other disulfide-containing proteins, this S-S bridge is cleaved by millimolar concentrations of sulfite in the absence of protein denaturing agents at pH 7-8 and ambient temperature; however, the reaction is not quantitative. Sulfitolysis of Escherichia coli thioredoxin was found to be associated with an increase in fluorescence at 345 nm. A comparative study of sulfitolysis in 12 different thioredoxins and glutaredoxins of bacterial and plant origin has been made. Although they are all thought to be highly conserved in three-dimensional structure, their reactivities towards sulfite and the effects of 6 M guanidinium chloride (not affecting, or enhancing sulfitolysis) vary strongly in the series, with E. coli thioredoxin being less reactive and plant thioredoxins and E. coli glutaredoxin being more susceptible molecules. Contrary to expectation, reaction with sulfite is not generally correlated with the presence of negatively or positively charged amino acid residues near the disulfide loop but is determined by individual sequence and surface features in every single protein. These results confirm our hypothesis that thioredoxin sulfitolysis and inactivation [Würfel, M., Häberlein, I., Follmann, H. (1990) FEBS Lett. 268, 146-148] can occur in plant cells under physiological conditions and provide a biochemical rationale for the phytotoxicity of SO2.

Non-redox protein interactions in the thioredoxin activation of chloroplast enzymes.

Thioredoxin derivatives lacking SH groups such as S,S'-dicarboxymethyl-, dicarboxamidomethyl-thioredoxin and cysteine----serine mutant protein are capable of activating chloroplast NADP malate dehydrogenase and fructose-bisphosphatase when added to enzyme assays together with suboptimal amounts of native thioredoxin. The modified thioredoxins alone are inactive. These findings indicate that protein-protein interactions play a significant role in addition to disulfide/thiol exchange reactions in the light-driven regulation of plant enzymes by the various plant thioredoxins.

A possible new class of ribonucleotide reductase from Methanobacterium thermoautotrophicum.

The ribonucleotide reductase from the strictly anaerobic methanogen Methanobacterium thermoautotrophicum has been partially purified by ion-exchange and gel-filtration chromatography. Its molecular weight is estimated to be 100,000 by the latter step. Unlike all previously studied ribonucleotide reductases, the enzyme does not employ dithiol compounds such as dithiothreitol as artificial electron donors in in vitro assays. Inhibition of the enzyme by S-adenosylmethionine, oxygen, and azide further distinguishes it from the Escherichia coli anaerobic enzyme, the iron- and manganese-containing, and the adenosylcobalamin-dependent enzymes. Our preliminary results suggest that this enzyme has an activation mechanism different from the known classes of ribonucleotide reductases.

Characterization of two thioredoxins in pig heart including a new mitochondrial protein.

Heart tissue contains two different thioredoxins. One is a specific mitochondrial protein and is best prepared from pre-isolated, intact heart mitochondria (mt-thioredoxin) whereas mitochondria-depleted tissue homogenates contain the major cellular thioredoxin of cytoplasmic origin (c-thioredoxin). Both heat-stable proteins are clearly differentiated chromatographically. They exhibit slightly different molecular weights (12,300 vs. 12,000) and isoelectric points (4.7 vs. 4.8) but differ remarkably in their cysteine content: mt-Thioredoxin has two cysteine residues like the bacterial proteins, and c-thioredoxin possesses six cysteines. Heart extracts were also shown to contain a NADPH-specific thioredoxin reductase of the known mammalian type. A specific function or target enzyme of mt-thioredoxin has not as yet been established.