The Therapeutic Potential of Vitamins B1, B3 and B6 in Charcot-Marie-Tooth Disease with the Compromised Status of Vitamin-Dependent Processes

Understanding the molecular mechanisms of neurological disorders is necessary for the development of personalized medicine. When the diagnosis considers not only the disease symptoms, but also their molecular basis, treatments tailored to individual patients may be suggested. Vitamin-responsive neurological disorders are induced by deficiencies in vitamin-dependent processes. These deficiencies may occur due to genetic impairments of proteins whose functions are involved with the vitamins. This review considers the enzymes encoded by the DHTKD1, PDK3 and PDXK genes, whose mutations are observed in patients with Charcot-Marie-Tooth (CMT) disease. The enzymes bind or produce the coenzyme forms of vitamins B1 (thiamine diphosphate, ThDP) and B6 (pyridoxal-5'-phosphate, PLP). Alleviation of such disorders through administration of the lacking vitamin or its derivative calls for a better introduction of mechanistic knowledge to medical diagnostics and therapies. Recent data on lower levels of the vitamin B3 derivative, NAD+, in the blood of patients with CMT disease vs. control subjects are also considered in view of the NAD-dependent mechanisms of pathological axonal degeneration, suggesting the therapeutic potential of vitamin B3 in these patients. Thus, improved diagnostics of the underlying causes of CMT disease may allow patients with vitamin-responsive disease forms to benefit from the administration of the vitamins B1, B3, B6, their natural derivatives, or their pharmacological forms.

Regulation of p53 Function by Formation of Non-Nuclear Heterologous Protein Complexes

A transcription factor p53 is activated upon cellular exposure to endogenous and exogenous stresses, triggering either homeostatic correction or cell death. Depending on the stress level, often measurable as DNA damage, the dual outcome is supported by p53 binding to a number of regulatory and metabolic proteins. Apart from the nucleus, p53 localizes to mitochondria, endoplasmic reticulum and cytosol. We consider non-nuclear heterologous protein complexes of p53, their structural determinants, regulatory post-translational modifications and the role in intricate p53 functions. The p53 heterologous complexes regulate the folding, trafficking and/or action of interacting partners in cellular compartments. Some of them mainly sequester p53 (HSP proteins, G6PD, LONP1) or its partners (RRM2B, PRKN) in specific locations. Formation of other complexes (with ATP2A2, ATP5PO, BAX, BCL2L1, CHCHD4, PPIF, POLG, SOD2, SSBP1, TFAM) depends on p53 upregulation according to the stress level. The p53 complexes with SIRT2, MUL1, USP7, TXN, PIN1 and PPIF control regulation of p53 function through post-translational modifications, such as lysine acetylation or ubiquitination, cysteine/cystine redox transformation and peptidyl-prolyl cis-trans isomerization. Redox sensitivity of p53 functions is supported by (i) thioredoxin-dependent reduction of p53 disulfides, (ii) inhibition of the thioredoxin-dependent deoxyribonucleotide synthesis by p53 binding to RRM2B and (iii) changed intracellular distribution of p53 through its oxidation by CHCHD4 in the mitochondrial intermembrane space. Increasing knowledge on the structure, function and (patho)physiological significance of the p53 heterologous complexes will enable a fine tuning of the settings-dependent p53 programs, using small molecule regulators of specific protein–protein interactions of p53.

Inhibition of 2-Oxoglutarate Dehydrogenase as a Chemical Model of Acute Hypobaric Hypoxia

Both hypoxia and inhibition of 2-oxoglutarate dehydrogenase complex (OGDHC) are known to change cellular amino acid pools, but the quantitative comparison of the metabolic and physiological outcomes has not been done. We hypothesize that OGDHC inhibition models metabolic changes caused by hypoxia, as both perturb the respiratory chain function, limiting either the NADH (OGDHC inhibition) or oxygen (hypoxia) supply. In the current study, we quantify the changes in the amino acid metabolism after OGDHC inhibition in the highly sensitive to hypoxia cerebellum and compare them to the earlier characterized changes after acute hypobaric hypoxia. In addition, the associated physiological effects are characterized and compared. A specific OGDHC inhibitor succinyl phosphonate (SP) is shown to act similar to hypoxia, increasing levels of many amino acids in the cerebellum of non-pregnant rats, without affecting those in the pregnant rats. Compared with hypoxia, stronger effects of SP in non-pregnant rats are observed on the levels of cerebellar amino acids, electrocardiography (ECG), and freezing time. In pregnant rats, hypoxia affects ECG and behavior more than SP, although none of the stressors significantly change the levels of cerebellar amino acids. The biochemical differences underlying the different physiological actions of SP and hypoxia are revealed by correlation analysis of the studied parameters. The negative correlations of cerebellar amino acids with OGDHC and/or tryptophan, shown to arise after the action of SP and hypoxia, discriminate the overall metabolic action of the stressors. More negative correlations are induced in the non-pregnant rats by hypoxia, and in the pregnant rats by SP. Thus, our findings indicate that the OGDHC inhibition mimics the action of acute hypobaric hypoxia on the cerebellar amino acid levels, but a better prediction of the physiological outcomes requires assessment of integral network changes, such as increases in the negative correlations among the amino acids, OGDHC, and/or tryptophan.

Severe Spinal Cord Injury in Rats Induces Chronic Changes in the Spinal Cord and Cerebral Cortex Metabolism, Adjusted by Thiamine That Improves Locomotor Performance

Our study aims at developing knowledge-based strategies minimizing chronic changes in the brain after severe spinal cord injury (SCI). The SCI-induced long-term metabolic alterations and their reactivity to treatments shortly after the injury are characterized in rats. Eight weeks after severe SCI, significant mitochondrial lesions outside the injured area are demonstrated in the spinal cord and cerebral cortex. Among the six tested enzymes essential for the TCA cycle and amino acid metabolism, mitochondrial 2-oxoglutarate dehydrogenase complex (OGDHC) is the most affected one. SCI downregulates this complex by 90% in the spinal cord and 30% in the cerebral cortex. This is associated with the tissue-specific changes in other enzymes of the OGDHC network. Single administrations of a pro-activator (thiamine, or vitamin B1, 1.2 mmol/kg) or a synthetic pro-inhibitor (triethyl glutaryl phosphonate, TEGP, 0.02 mmol/kg) of OGDHC within 15–20 h after SCI are tested as protective strategies. The biochemical and physiological assessments 8 weeks after SCI reveal that thiamine, but not TEGP, alleviates the SCI-induced perturbations in the rat brain metabolism, accompanied by the decreased expression of (acetyl)p53, increased expression of sirtuin 5 and an 18% improvement in the locomotor recovery. Treatment of the non-operated rats with the OGDHC pro-inhibitor TEGP increases the p53 acetylation in the brain, approaching the brain metabolic profiles to those after SCI. Our data testify to an important contribution of the OGDHC regulation to the chronic consequences of SCI and their control by p53 and sirtuin 5.

Suppressing endoplasmic reticulum stress-related autophagy attenuates retinal light injury

Excessive light exposure is a principal environmental factor, which can cause damage to photoreceptors and retinal pigment epithelium (RPE) cells and may accelerate the progression of age-related macular degeneration (AMD). In this study, oxidative stress, endoplasmic reticulum (ER) stress and autophagy caused by light exposure were evaluated in vitro and in vivo. Light exposure caused severe photo-oxidative stress and ER stress in photoreceptors (661W cells) and RPE cells (ARPE-19 cells). Suppressing either oxidative stress or ER stress was protective against light damage in 661W and ARPE-19 cells and N-acetyl-L-cysteine treatment markedly inhibited the activation of ER stress caused by light exposure. Moreover, suppressing autophagy with 3-methyladenine significantly attenuated light-induced cell death. Additionally, inhibiting ER stress either by knocking down PERK signals or with GSK2606414 treatment remarkably suppressed prolonged autophagy and protected the cells against light injury. In vivo experiments verified neuroprotection via inhibiting ER stress-related autophagy in light-damaged retinas of mice. In conclusion, the above results suggest that light-induced photo-oxidative stress may trigger subsequent activation of ER stress and prolonged autophagy in photoreceptors and RPE cells. Suppressing ER stress may abrogate over-activated autophagy and protect the retina against light injury.

Molecular characterization of pyridoxine 5'-phosphate oxidase and its pathogenic forms associated with neonatal epileptic encephalopathy

Defects of vitamin B₆ metabolism are responsible for severe neurological disorders, such as pyridoxamine 5'-phosphate oxidase deficiency (PNPOD; OMIM: 610090), an autosomal recessive inborn error of metabolism that usually manifests with neonatal-onset severe seizures and subsequent encephalopathy. At present, 27 pathogenic mutations of the gene encoding human PNPO are known, 13 of which are homozygous missense mutations; however, only 3 of them have been characterised with respect to the molecular and functional properties of the variant enzyme forms. Moreover, studies on wild type and variant human PNPOs have so far largely ignored the regulation properties of this enzyme. Here, we present a detailed characterisation of the inhibition mechanism of PNPO by pyridoxal 5'-phosphate (PLP), the reaction product of the enzyme. Our study reveals that human PNPO has an allosteric PLP binding site that plays a crucial role in the enzyme regulation and therefore in the regulation of vitamin B₆ metabolism in humans. Furthermore, we have produced, recombinantly expressed and characterised several PNPO pathogenic variants responsible for PNPOD (G118R, R141C, R225H, R116Q/R225H, and X262Q). Such replacements mainly affect the catalytic activity of PNPO and binding of the enzyme substrate and FMN cofactor, leaving the allosteric properties unaltered.

The multifaceted role of vitamin B6 in cancer: Drosophila as a model system to investigate DNA damage

A perturbed uptake of micronutrients, such as minerals and vitamins, impacts on different human diseases, including cancer and neurological disorders. Several data converge towards a crucial role played by many micronutrients in genome integrity maintenance and in the establishment of a correct DNA methylation pattern. Failure in the proper accomplishment of these processes accelerates senescence and increases the risk of developing cancer, by promoting the formation of chromosome aberrations and deregulating the expression of oncogenes. Here, the main recent evidence regarding the impact of some B vitamins on DNA damage and cancer is summarized, providing an integrated and updated analysis, mainly centred on vitamin B₆. In many cases, it is difficult to finely predict the optimal vitamin rate that is able to protect against DNA damage, as this can be influenced by a given individual's genotype. For this purpose, a precious resort is represented by model organisms which allow limitations imposed by more complex systems to be overcome. In this review, we show that Drosophila can be a useful model to deeply understand mechanisms underlying the relationship between vitamin B₆ and genome integrity.

Hypoxic Adaptation of Mitochondrial Metabolism in Rat Cerebellum Decreases in Pregnancy

Function of brain amino acids as neurotransmitters or their precursors implies changes in the amino acid levels and/or metabolism in response to physiological and environmental challenges. Modelling such challenges by pregnancy and/or hypoxia, we characterize the amino acid pool in the rat cerebellum, quantifying the levels and correlations of 15 amino acids and activity of 2-oxoglutarate dehydrogenase complex (OGDHC). The parameters are systemic indicators of metabolism because OGDHC limits the flux through mitochondrial TCA cycle, where amino acids are degraded and their precursors synthesized. Compared to non-pregnant state, pregnancy increases the cerebellar content of glutamate and tryptophan, decreasing interdependence between the quantified components of amino acid metabolism. In response to hypoxia, the dependence of cerebellar amino acid pool on OGDHC and the average levels of arginine, glutamate, lysine, methionine, serine, phenylalanine, and tryptophan increase in non-pregnant rats only. This is accompanied by a higher hypoxic resistance of the non-pregnant vs. pregnant rats, pointing to adaptive significance of the hypoxia-induced changes in the cerebellar amino acid metabolism. These adaptive mechanisms are not effective in the pregnancy-changed metabolic network. Thus, the cerebellar amino acid levels and OGDHC activity provide sensitive markers of the physiology-dependent organization of metabolic network and its stress adaptations.

Delayed Influence of Spinal Cord Injury on the Amino Acids of NO• Metabolism in Rat Cerebral Cortex Is Attenuated by Thiamine

Severe spinal cord injuries (SCIs) result in chronic neuroinflammation in the brain, associated with the development of cognitive and behavioral impairments. Nitric oxide (NO•) is a gaseous messenger involved in neuronal signaling and inflammation, contributing to nitrosative stress under dysregulated production of reactive nitrogen species. In this work, biochemical changes induced in the cerebral cortex of rats 8 weeks after SCI are assessed by quantification of the levels of amino acids participating in the NO• and glutathione metabolism. The contribution of the injury-induced neurodegeneration is revealed by comparison of the SCI- and laminectomy (LE)-subjected animals. Effects of the operative interventions are assessed by comparison of the operated (LE/SCI) and non-operated animals. Lower ratios of citrulline (Cit) to arginine (Arg) or Cit to ornithine and a more profound decrease in the ratio of lysine to glycine distinguish SCI animals from those after LE. The data suggest decreased NO• production from both Arg and homoarginine in the cortex 8 weeks after SCI. Both LE and SCI groups show a strong decrease in the level of cortex glutathione. The neurotropic, anti-inflammatory, and antioxidant actions of thiamine (vitamin B1) prompted us to study the thiamine effects on the SCI-induced changes in the NO• and glutathione metabolism. A thiamine injection (400 mg/kg intraperitoneally) within 24 h after SCI abrogates the changes in the cerebral cortex amino acids related to NO•. Thiamine-induced normalization of the brain glutathione levels after LE and SCI may involve increased supply of glutamate for glutathione biosynthesis. Thus, thiamine protects from sequelae of SCI on NO•-related amino acids and glutathione in cerebral cortex.

Behavioral impact of the regulation of the brain 2-oxoglutarate dehydrogenase complex by synthetic phosphonate analog of 2-oxoglutarate: implications into the role of the complex in neurodegenerative diseases

Decreased activity of the mitochondrial 2-oxoglutarate dehydrogenase complex (OGDHC) in brain accompanies neurodegenerative diseases. To reveal molecular mechanisms of this association, we treated rats with a specific inhibitor of OGDHC, succinyl phosphonate, or exposed them to hypoxic stress. In males treated with succinyl phosphonate and in pregnancy-sensitized females experiencing acute hypobaric hypoxia, we revealed upregulation of brain OGDHC (within 24 hours), with the activity increase presumably representing the compensatory response of brain to the OGDHC inhibition. This up-regulation of brain OGDHC was accompanied by an increase in exploratory activity and a decrease in anxiety of the experimental animals. Remarkably, the hypoxia-induced elevation of brain OGDHC and most of the associated behavioral changes were abrogated by succinyl phosphonate. The antagonistic action of hypoxia and succinyl phosphonate demonstrates potential therapeutic significance of the OGDHC regulation by the phosphonate analogs of 2-oxoglutarate.

Novel isoenzyme of 2-oxoglutarate dehydrogenase is identified in brain, but not in heart

2-Oxoglutarate dehydrogenase (OGDH) is the first and rate-limiting component of the multienzyme OGDH complex (OGDHC) whose malfunction is associated with neurodegeneration. The essential role of this complex in the degradation of glucose and glutamate, which have specific significance in brain, raises questions about the existence of brain-specific OGDHC isoenzyme(s). We purified OGDHC from extracts of brain or heart mitochondria using the same procedure of poly(ethylene glycol) fractionation, followed by size-exclusion chromatography. Chromatographic behavior and the insufficiency of mitochondrial disruption to solubilize OGDHC revealed functionally significant binding of the complex to membrane. Components of OGDHC from brain and heart were identified using nano-high performance liquid chromatography electrospray tandem mass spectrometry after trypsinolysis of the electrophoretically separated proteins. In contrast to the heart complex, where only the known OGDH was determined, the band corresponding to the brain OGDH component was found to also include the novel 2-oxoglutarate dehydrogenase-like (OGDHL) protein. The ratio of identified peptides characteristic of OGDH and OGDHL was preserved during purification and indicated comparable quantities of the two proteins in brain. Brain OGDHC also differed from the heart complex in the abundance of the components, lower apparent molecular mass and decreased stability upon size-exclusion chromatography. The functional competence of the novel brain isoenzyme and different regulation of OGDH and OGDHL by 2-oxoglutarate are inferred from the biphasic dependence of the overall reaction rate versus 2-oxoglutarate concentration. OGDHL may thus participate in brain-specific control of 2-oxoglutarate distribution between energy production and synthesis of the neurotransmitter glutamate.

The α-Ketoglutarate–Dehydrogenase Complex: A Mediator Between Mitochondria and Oxidative Stress in Neurodegeneration

Damage from oxidative stress and mitochondrial dysfunction occur together in many common neurodegenerative diseases. The enzymes that form the mitochondrial alpha-ketoglutarate- dehydrogenase complex (KGDHC), a key and arguably rate-limiting enzyme system of the tricarboxylic acid cycle, might mediate the interaction of these processes. KGDHC activity is reduced in numerous age-related neurodegenerative diseases and is diminished by oxidative stress. In Alzheimer's disease (AD), the reduction correlates highly to diminished mental performance. Thus, research has focused on the mechanisms by which select oxidants reduce KGDHC and the consequences of such a reduction. Diminished KGDHC in cells is associated with apoptosis without changes in the mitochondrial membrane potential. Studies of isolated mitochondria and of animal models suggest that a reduction in KGDHC can predispose to damage by other toxins that promote neurodegeneration. Diminished oxidative metabolism can be plausibly linked to pathological features of neurodegenerative diseases (e.g., reduced mental function, the plaques and tangles in AD). Thus, reductions in KGDHC might be central to the pathophysiology of these diseases. Studies of proteins, cells, animal models, and humans suggest that treatments to diminish, or bypass, the reduction in KGDHC might be beneficial in age-related neurodegenerative disorders.

Kinetic properties of the 2-oxoglutarate dehydrogenase complex from Azotobacter vinelandii evidence for the formation of a precatalytic complex with 2-oxoglutarate

The 2-oxoglutarate dehydrogenase complex was purified from Azotobacter vinelandii. The complex consists of three components, 2-oxoglutarate dehydrogenase/decarboxylase (E1o), lipoate succinyltransferase (E2o) and lipoamide dehydrogenase (E3). Upon purification, the E3 component dissociates partially from the complex. From reconstitution experiments, the Kd for E3 was found to be 26 nM, about 30 times higher than that for the pyruvate dehydrogenase complex. The Km values for the substrates 2-oxoglutarate, CoA and NAD+ were found to be 0.15, 0.014 and 0.17 mM, respectively. The system has a high specificity for 2-oxoglutarate, which is determined by the action of both E1o and E2o. Above 4 mM substrate inhibition is observed. From steady-state inhibition experiments with substrate analogs, two substrate-binding modes are revealed at different degrees of saturation of the enzyme with 2-oxoglutarate. At low substrate concentrations (10(-6) to 10(-5) M), the binding mainly depends on the interaction of the enzyme with the substrate carboxyl groups. At a higher degree of substrate saturation (10(-4) to 10(-3) M) the relative contribution of the 2-oxo group in the binding increases. A kinetic analysis points to a single binding site for a substrate analog under steady state conditions. Saturation of this site with an analog indicates that two kinetically different complexes are formed with 2-oxoglutarate in the course of catalysis. From competition studies with analogs it is concluded that one of these complexes is formed at the site that is sterically identical to the substrate inhibition site. The data obtained are represented by a minimal scheme that considers formation of a precatalytic complex SE between the substrate and E1o before the catalytic complex ES, in which the substrate is added to the thiamin diphosphate cofactor, is formed. The incorrect orientation of the substrate molecule in SE or the occupation of this site by analogs is supposed to cause substrate or analog inhibition, respectively.

Inhibition of select mitochondrial enzymes in PC12 cells exposed to S-(1,1,2,2-tetrafluoroethyl)-L-cysteine

Many halogenated foreign compounds are detoxified by conversion to the corresponding cysteine S-conjugate, which is N-acetylated and excreted. However, several halogenated cysteine S-conjugates [e.g. S-(1,1,2,2-tetrafluoroethy)-L-cysteine (TFEC)] are converted to mitochondrial toxicants by cysteine S-conjugate beta-lyases. In the present work, we showed that TFEC appreciably inactivated highly purified alpha-ketoglutarate dehydrogenase complex (KGDHC) in the presence of a cysteine S-conjugate beta-lyase. Incubation of PC12 cells (which contain endogenous cysteine S-conjugate beta-lyase activity) with TFEC led to a concentration- and time-dependent loss of endogenous KGDHC activity. A 24-hr exposure to 1 mM TFEC decreased KGDHC activity in the cells by 90%. Although treatment with TFEC did not inhibit intrinsic pyruvate dehydrogenase complex (PDHC) activity, it inhibited dichloroacetate/Mg2+-mediated activation/dephosphorylation of PDHC in the PC12 cells by 90%. To determine the selectivity of enzymes targeted by TFEC, several cytosolic and mitochondrial enzymes involved in energy metabolism [malate dehydrogenase, glyceraldehyde 3-phosphate dehydrogenase, glutamate dehydrogenase, lactate dehydrogenase, cytosolic and mitochondrial aspartate aminotransferases (AspAT)] were also assayed in the PC12 cells exposed to 1 mM TFEC for 24 hr. Of these enzymes, only mitochondrial AspAT, a key enzyme of the malate-aspartate shuttle, was inhibited. The present results demonstrate a selective vulnerability of mitochondrial enzymes to toxic cysteine S-conjugates. The data indicate that TFEC may be a useful cellular/mitochondrial toxicant for elucidating the consequences of the diminished mitochondrial function that accompanies numerous neurodegenerative diseases.

Interaction of thioredoxins with target proteins: role of particular structural elements and electrostatic properties of thioredoxins in their interplay with 2-oxoacid dehydrogenase complexes

The thioredoxin action upon the 2-oxoacid dehydrogenase complexes is investigated by using different thioredoxins, both wild-type and mutated. The attacking cysteine residue of thioredoxin is established to be essential for the thioredoxin-dependent activation of the complexes. Mutation of the buried cysteine residue to serine is not crucial for the activation, but prevents inhibition of the complexes, exhibited by the Clamydomonas reinhardtii thioredoxin m disulfide. Site-directed mutagenesis of D26, W31, F/W12, and Y/A70 (the Escherichia coli thioredoxin numbering is employed for all the thioredoxins studied) indicates that both the active site and remote residues of thioredoxin are involved in its interplay with the 2-oxoacid dehydrogenase complexes. Sequences of 11 thioredoxin species tested biochemically are aligned. The thioredoxin residues at the contact between the alpha3/3(10) and alpha1 helices, the length of the alpha1 helix and the charges in the alpha2-beta3 and beta4-beta5 linkers are found to correlate with the protein influence on the 2-oxoacid dehydrogenase complexes (the secondary structural elements of thioredoxin are defined according to Eklund H et al., 1991, Proteins 11:13-28). The distribution of the charges on the surface of the thioredoxin molecules is analyzed. The analysis reveals the species specific polarization of the thioredoxin active site surroundings, which corresponds to the efficiency of the thioredoxin interplay with the 2-oxoacid dehydrogenase systems. The most effective mitochondrial thioredoxin is characterized by the strongest polarization of this area and the highest value of the electrostatic dipole vector of the molecule. Not only the magnitude, but also the orientation of the dipole vector show correlation with the thioredoxin action. The dipole direction is found to be significantly influenced by the charges of the residues 13/14, 51, and 83/85, which distinguish the activating and inhibiting thioredoxin disulfides.

Subunit interactions in the mammalian alpha-ketoglutarate dehydrogenase complex. Evidence for direct association of the alpha-ketoglutarate dehydrogenase and dihydrolipoamide dehydrogenase components

Selective tryptic proteolysis of the mammalian alpha-ketoglutarate dehydrogenase complex (OGDC) leads to its rapid inactivation as a result of a single cleavage within the N-terminal region of its alpha-ketoglutarate dehydrogenase (E1) component, which promotes the dissociation of the dihydrolipoamide dehydrogenase (E3) enzyme and also a fully active E1' fragment. Similarities between the N-terminal region of E1 and the dihydrolipoamide acetyltransferase (E2) and E3-binding components (E3BP) of the pyruvate dehydrogenase complex are highlighted by the specific cross-reactivities of subunit-specific antisera. Analysis of the pattern of release of E1 and E1' polypeptides from the OGDC during tryptic inactivation suggests that both polypeptide chains of individual E1 homodimers must be cleaved to permit the dissociation of the E1 and E3 components. A new protocol has been devised that promotes E1 dissociation from the oligomeric dihydrolipoamide succinyltransferase (E2) core in an active state. Significant levels of overall OGDC reconstitution could also be achieved by re-mixing the constituent enzymes in stoichiometric amounts. Moreover, a high affinity interaction has been demonstrated between the homodimeric E1 and E3 components, which form a stable subcomplex comprising single copies of these two enzymes.

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.

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.

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.

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.