Strange organelles--Plasmodium mitochondria lack a pyruvate dehydrogenase complex

Our understanding of the Plasmodium mitochondrion and apicoplast has been greatly assisted by the genome sequence project. Sequence data have seeded recent research showing that the apicoplast is the site of several anabolic pathways including fatty acid synthesis. The discovery of an active apicoplast pyruvate dehydrogenase complex implies this enzyme generates the acetyl-CoA needed for fatty acid synthesis. However, the absence of a corresponding mitochondrial complex suggests that energy generation in Plasmodium is considerably different from pathways described in other eukaryotes.

The SR protein SC35 is responsible for aberrant splicing of the E1alpha pyruvate dehydrogenase mRNA in a case of mental retardation with lactic acidosis

Pyruvate dehydrogenase (PDH) complex deficiency is a major cause of lactic acidosis and Leigh's encephalomyelopathies in infancy and childhood, resulting in early death in the majority of patients. Most of the molecular defects have been localized in the coding regions of the E1alpha PDH gene. Recently, we identified a novel mutation of the E1alpha PDH gene in a patient with an encephalopathy and lactic acidosis. This mutation, located downstream of exon 7, activates a cryptic splice donor and leads to the retention of intronic sequences. Here, we demonstrate that the mutation results in an increased binding of the SR protein SC35. Consistently, ectopic overexpression of this splicing factor enhanced the use of the cryptic splice site, whereas small interfering RNA-mediated reduction of the SC35 protein levels in primary fibroblasts from the patient resulted in the almost complete disappearance of the aberrantly spliced E1alpha PDH mRNA. Our findings open the exciting prospect for a novel therapy of an inherited disease by altering the level of a specific splicing factor.

Structure and function of carnitine acyltransferases

Carnitine acyltransferases catalyze the exchange of acyl groups between carnitine and coenzyme A (CoA). These enzymes include carnitine acetyltransferase (CrAT), carnitine octanoyltransferase (CrOT), and carnitine palmitoyltransferases (CPTs). CPT-I and CPT-II are crucial for the beta-oxidation of long-chain fatty acids in the mitochondria by enabling their transport across the mitochondrial membrane. The activity of CPT-I is inhibited by malonyl-CoA, a crucial regulatory mechanism for fatty acid oxidation. Mutation or dysregulation of the CPT enzymes has been linked to many serious, even fatal human diseases, and these enzymes are promising targets for the development of therapeutic agents against type 2 diabetes and obesity. We have determined the crystal structures of murine CrAT, alone and in complex with its substrate carnitine or CoA. The structure contains two domains. Surprisingly, these two domains share the same backbone fold, which is also similar to that of chloramphenicol acetyltransferase and dihydrolipoyl transacetylase. The active site is located at the interface between the two domains, in a tunnel that extends through the center of the enzyme. Carnitine and CoA are bound in this tunnel, on opposite sides of the catalytic His343 residue. The structural information provides a molecular basis for understanding the catalysis by carnitine acyltransferases and for designing their inhibitors. In addition, our structural information suggests that the substrate carnitine may assist the catalysis by stabilizing the oxyanion in the reaction intermediate.

A novel mutation in the dihydrolipoamide dehydrogenase E3 subunit gene (DLD) resulting in an atypical form of α-ketoglutarate dehydrogenase deficiency

The alpha-ketoglutarate dehydrogenase complex (KGDC) catalyses the decarboxylation of alpha-ketoglutarate into succinyl-coenzyme A in the Krebs cycle. This enzymatic complex is made up of three subunits (E1, encoded by PDHA1; E2, encoded by DLST; and E3, encoded by DLD). The E3 subunit is common to two other enzymatic complexes, namely pyruvate dehydrogenase complex (PDC) and branched-chain ketoacid dehydrogenase complex (BCKDC). KGDC deficiency is a rare autosomal recessive disorder, most often presenting with severe encephalopathy and hyperlactatemia with neonatal onset. We found a KGDC deficiency in cultured skin fibroblasts from three siblings born to consanguinous parents. E3 subunit activity was shown to be deficient (20% of control values), despite the absence of usual clinical clues to E3 deficiency, i.e. accumulation of pyruvate and branched-chain amino acids in plasma and branched-chain alpha-ketoacids in urine. RT-PCR of E3 mRNA from the three patients, followed by sequencing, revealed an homozygous c.1444A>G substitution located in E3 exon 13, predictive of a p.R482G (or R447G in the processed gene product) substitution in a highly conserved domain of the protein. Only eleven E3 mutations have been reported so far. The only other case of E3 deficiency without clinical or biochemical evidences of PDC and BCKDC deficiencies has been ascribed to a c.1436A>T (p.D479V; or D444V in the processed gene product) mutation, very close to the mutation reported herein. Since c.1444A>G (p.R482G; or R447G in the processed gene product) and c.1436A>T (p.D479V; or D444V in the processed gene product) lie within the interface domain of E3 with E2 (KGDC and BCKDC) or the E3-binding protein (PDC), our data suggest that interaction of E3 with these other subunits differs in some extent among KGDC, PDC, and BCKDC.

Biochemical and structural brain alterations in female mice with cerebral pyruvate dehydrogenase deficiency

Pyruvate dehydrogenase complex (PDC) deficiency is an inborn metabolic disorder associated with a variety of neurologic abnormalities. This report describes the development and initial characterization of a novel murine model system in which PDC deficiency has been introduced specifically into the developing nervous system. The absence of liveborn male and a roughly 50% reduction in female offspring following induction of the X-linked mutation indicate that extensive deficiency of PDC in the nervous system leads to pre-natal lethality. Brain tissue from surviving females at post-natal days 15 and 35 was shown to have approximately 75% of wild-type PDC activity, suggesting that a threshold of enzyme activity exists for post-natal survival. Detailed histological analyses of brain tissue revealed structural defects such as disordered neuronal cytoarchitecture and neuropil fibers in grey matter, and reduced size of bundles and disorganization of fibers in white matter. Many of the histologic abnormalities resemble those found in human female patients who carry mutations in the X-linked ortholog. These findings demonstrate a requirement for PDC activity within the nervous system for survival in utero and suggest that impaired pyruvate metabolism in the developing brain can affect neuronal migration, axonal growth and cell-cell interactions.

Age-related compensatory activation of pyruvate dehydrogenase complex in rat heart

Mitochondrial uptake and beta-oxidation of long-chain fatty acids are markedly impaired in the aging rat heart. While these alterations would be expected to adversely affect overall pyridine nucleotides, NADH levels do not change significantly with age. This conundrum suggests that specific compensatory mechanisms occur in the aging heart. The comparison of cardiac pyruvate dehydrogenase complex (PDC) kinetics in 4- and 24- to 28-month-old F344 rats revealed a 60% significant increase in V(max) with no change in PDC expression, and a 1.6-fold decrease in the Michaelis constant (K(m)) in old compared to young rats. The observed kinetic adjustments were selective to PDC, as neither the V(max) nor K(m) of citrate synthase changed with age. PDC kinase-4 mRNA levels decreased by 57% in old vs young rat hearts and correlated with a 45% decrease in PDC phosphorylation. We conclude that PDC from old rat hearts catabolizes pyruvate more efficiently due to an adaptive change in phosphorylation.

Disease-specific cross-reactivity between mimicking peptides of heat shock protein of Mycobacterium gordonae and dominant epitope of E2 subunit of pyruvate dehydrogenase is common in Spanish but not British patients with primary biliary cirrhosis

Previous studies on Spanish patients with Primary Biliary Cirrhosis (PBC) have shown extensive, disease-specific cross-reactivity between the 65-kDa heat shock protein (hsp65) of Mycobacterium gordonae and pyruvate dehydrogenase complex-E2 (PDC-E2), the major target of anti-mitochondrial antibody (AMA). Studies on a British population were unable to substantiate these findings. Having found that there is an excellent and almost unique match between the PDC-E2 autoepitope and a sequence in mycobacterial hsp65s, we tested the corresponding peptides by ELISA for cross-reactivity using sera from 90 PBC patients, 40 Spanish and 50 British, and 84 pathological controls. Reactivity to the MYCGO hsp65(90-104)/human PDC-E2(212-226)pair was present in 19 (47.5%) Spanish PBC patients and in 2 (4%) of the 50 British. Reactivity was not seen in any of the controls. Simultaneous reactivity to mimics was due to cross-reactivity as confirmed by inhibition studies. Three dimensional modelling predicts mycobacterial hsp65(90-104)to be exposed on the surface of the protein. The affinity of anti-hsp65(90-104)antibody was higher than that of anti-PDC-E2(212-226). Hsp65(90-104)is a target of disease-specific cross-reactivity to PDC-E2(212-226). The geographical confinement of this phenomenon is probably the result of complex genetic, environmental and immunological interaction.

Pyruvate dehydrogenase kinase isoform 2 activity stimulated by speeding up the rate of dissociation of ADP

Pyruvate dehydrogenase kinase 2 (PDK2) activity is stimulated by NADH and NADH plus acetyl-CoA via the reduction and reductive acetylation of the lipoyl groups of the dihydrolipoyl acetyltransferase (E2) component. Elevated K(+) and Cl(-) were needed for significant stimulation. Stimulation substantially increased both k(cat) and the K(m) for ATP; the fractional stimulation increased with the level of ATP. With an E2 structure lacking the pyruvate dehydrogenase (E1) binding domain, stimulation of PDK2 was retained, the K(m) for E1 decreased, and the equilibrium dissociation constant for ATP increased but remained much lower than the K(m) for ATP. Stimulation of PDK2 activity greatly reduced the fraction of bound ADP. These results fit an ordered reaction mechanism with ATP binding before E1 and stimulation increasing the rate of dissociation of ADP. Conversion of all of the lipoyl groups in the E2 60mer to the oxidized form (E2(ox)) greatly reduced k(cat) and the K(m) of PDK2 for ATP. Retention over an extended period of time of a low portion of reduced lipoyl groups maintains E2 in a state that supported much higher PDK2 activity than short-term (5 min) reduction of a large portion of lipoyl groups of E2(ox), but reduction of E2(ox) produced a larger fold stimulation. Reduction and to a greater extent reductive acetylation increased PDK2 binding to E2; conversion to E2(ox) did not significantly hinder binding. We suggest that passing even limited reducing equivalents among lipoyl groups maintains E2 lipoyl domains in a conformation that aids kinase function.

[Prediction and identification of autoepitopes of PDC-E2 specific CD8+ CTL in primary biliary cirrhosis patients]

OBJECTIVE

To identify autoepitopes of E2 subunit of pyruvate dehydrogenase complex (PDC-E2) specific CD8+ CTL in primary biliary cirrhosis (PBC) patients.

METHODS

An online database SYFPEITHI was applied to predict HLA-A*0201 restricted epitopes which located in PDC-E2 30-50 aa and 150-190 aa where B-cell epitopes clustered with CD4+ T-cell epitopes. T2 cell line reconstitution and stabilization assay, induction of specific CTL lines from peripheral blood mononuclear cells (PBMCs) of patients with PBC and cytotoxicity of peptides-induced CTL were performed to screen the epitopes from those candidates.

RESULTS

Five potential epitopes were predicted by database. Of the 5 candidates, two peptides 159-167 aa and 165-174 aa, with highly binding activity to HLA-A*0201 molecules, could stimulate PBMCs from most HLA-A*0201 positive PBC patients to proliferate and peptide-induced CTL lines showed specific cytotoxicity.

CONCLUSION

Peptides of KLSEGDLLA (159-167 aa) and LLAEIETDKA (165-174 aa) in the inner lipoyl domain of PDC-E2 are HLA-A*0201 restricted CD8+ CTL immunodominant epitopes in PBC.

Manipulating the pyruvate dehydrogenase bypass of a multi-vitamin auxotrophic yeast Torulopsis glabrata enhanced pyruvate production

AIMS

To investigate the relationship between the activity of pyruvate dehydrogenase (PDH) bypass and the production of pyruvate of a multi-vitamin auxotrophic yeast Torulopsis glabrata.

METHODS AND RESULTS

Torulopsis glabrata CCTCC M202019, a multi-vitamin auxotrophic yeast that requires acetate for complete growth on glucose minimum medium, was selected after nitrosoguanidine mutagenesis of the parent strain T. glabrata WSH-IP303 screened in previous study [Li et al. (2001) Appl. Microbiol. Biotechnol. 55, 680-685]. Strain CCTCC M202019 produced 21% higher pyruvate than the parent strain and was genetically stable in flask cultures. The activities of the pyruvate metabolism-related enzymes in parent and mutant strains were measured. Compared with the parent strain, the activity of pyruvate decarboxylase (PDC) of the mutant strain CCTCC M202019 decreased by roughly 40%, while the activity of acetyl-CoA synthetase (ACS) of the mutant increased by 103.5 or 57.4%, respectively, in the presence or absence of acetate. Pyruvate production by the mutant strain CCTCC M202019 reached 68.7 g l(-1) at 62 h (yield on glucose of 0.651 g g(-1)) in a 7-l jar fermentor.

CONCLUSIONS

The increased pyruvate yield in T. glabrata CCTCC M202019 was due to a balanced manipulation of the PDH bypass, where the shortage of cytoplasmic acetyl-CoA caused by the decreased activity of PDC was properly compensated by the increased activity of ACS.

SIGNIFICANCE AND IMPACT OF THE STUDY

Manipulating the PDH bypass may provide an alternative approach to enhance the production of glycolysis-related metabolites.

Ischemic preconditioning exaggerates cardiac damage in PKC-δ null mice

Ischemic preconditioning confers cardiac protection during subsequent ischemia-reperfusion, in which protein kinase C (PKC) is believed to play an essential role, but controversial data exist concerning the PKC-delta isoform. In an accompanying study (26), we described metabolic changes in PKC-delta knockout mice. We now wanted to explore their effect on early preconditioning. Both PKC-delta(-/-) and PKC-delta(+/+) mice underwent three cycles of 5-min left descending artery occlusion/5-min reperfusion, followed by 30-min occlusion and 2-h reperfusion. Unexpectedly, preconditioning exaggerated ischemia-reperfusion injury in PKC-delta(-/-) mice. Whereas ischemic preconditioning increased superoxide anion production in PKC-delta(+/+) hearts, no increase in reactive oxygen species was observed in PKC-delta(-/-) hearts. Proteomic analysis of preconditioned PKC-delta(+/+) hearts revealed profound changes in enzymes related to energy metabolism, e.g., NADH dehydrogenase and ATP synthase, with partial fragmentation of these mitochondrial enzymes and of the E(2) component of the pyruvate dehydrogenase complex. Interestingly, fragmentation of mitochondrial enzymes was not observed in PKC-delta(-/-) hearts. High-resolution NMR analysis of cardiac metabolites demonstrated a similar rise of phosphocreatine in PKC-delta(+/+) and PKC-delta(-/-) hearts, but the preconditioning-induced increase in phosphocholine, alanine, carnitine, and glycine was restricted to PKC-delta(+/+) hearts, whereas lactate concentrations were higher in PKC-delta(-/-) hearts. Taken together, our results suggest that reactive oxygen species generated during ischemic preconditioning might alter mitochondrial metabolism by oxidizing key mitochondrial enzymes and that metabolic adaptation to preconditioning is impaired in PKC-delta(-/-) hearts.

Mutations in the gene for the E1beta subunit: a novel cause of pyruvate dehydrogenase deficiency

We describe two unrelated patients with pyruvate dehydrogenase (PDH) deficiency attributable to mutations in the gene encoding the E1beta subunit of the complex. This is a previously unrecognised form of PDH deficiency, which most commonly results from mutations in the X-linked gene for the E1alpha subunit. Both patients had reduced immunoreactive E1beta protein and both had missense mutations in the E1beta gene. Activity of the PDH complex was restored in cultured fibroblasts from both patients by transfection and expression of the normal E1beta coding sequence.

Tetrahedral Intermediates in Thiamin Diphosphate-Dependent Decarboxylations Exist as a 1‘,4‘-Imino Tautomeric Form of the Coenzyme, Unlike the Michaelis Complex or the Free Coenzyme

Two circular dichroism signals observed on thiamin diphosphate (ThDP)-dependent enzymes, a positive band in the 300-305 nm range and a negative one in the 320-330 nm range, were investigated on yeast pyruvate decarboxylase (YPDC) and on the E1 subunit of the Escherichia coli pyruvate dehydrogenase complex (PDHc-E1). Addition of the tetrahedral ThDP-acetaldehyde adduct, 2-alpha-hydroxyethylThDP, to PDHc-E1 generates the positive band at 300 nm, consistent with the formation of the 1',4'-iminopyrimidine tautomer, as also demonstrated for phosphonolactylthiamin diphosphate, a stable analogue of the tetrahedral ThDP-pyruvate adduct 2-alpha-lactylThDP (Jordan, F. et al. (2003) J. Am. Chem. Soc. 125, 12732-12738). Therefore, we suggest that all tetrahedral ThDP-bound covalent complexes will also prefer this tautomer, and that the 4'-aminopyrimidine of ThDP participates in multiple steps of acid-base catalysis on ThDP enzymes. Studies with YPDC and PDHc-E1, and their active center variants, in conjunction with chemical models, enabled assignment of the negative band at 330 nm to a charge-transfer transition between the 4'-aminopyrimidine tautomer (presumed electron donor) and the thiazolium ring (presumed electron acceptor) of ThDP, with no significant contributions from any amino acid side chain of the proteins. However, in both YPDC and PDHc-E1, the presence of substrate or substrate surrogate was required to enable detection, suggesting that the band at 320-330 nm be used as a reporter for the Michaelis complex, involving the amino tautomer, on both enzymes. As the positive band near 300 nm reports on the 1',4'-imino tautomer of ThDP, methods are now available for kinetic monitoring of both tautomeric forms.

Use of fusion proteins and procaryotic display systems for delivery of HIV-1 antigens: development of novel vaccines for HIV-1 infection

Two non-pathogenic scaffolds (represented by the filamentous bacteriophage fd and the dihydrolipoyl acetyltransferase E2 protein of the Bacillus stearothermophilus pyruvate dehydrogenase (PDH) complex) able to deliver human immunodeficiency virus (HIV)-1 antigenic determinants, were designed in our laboratories and investigated in controlled assay conditions. Based on a modification of the phage display technology, we developed an innovative concept for a safe and inexpensive vaccine in which conserved antigenic determinants of HIV-1 reverse transcriptase (RTase) were inserted into the N-terminal region of the major pVIII coat protein of bacteriophagefd virions. Analogously, we developed another antigen delivery system based on the E2 component from the PDH complex and capable of displaying large intact proteins on the surface of an icosahedral lattice. Our data show that both of these systems can deliver B and T epitopes to their respective presentation compartments in target cells and trigger a humoral response as well as a potent helper and cytolytic response in vitro and in vivo.

Azorhizobium caulinodans electron-transferring flavoprotein N electrochemically couples pyruvate dehydrogenase complex activity to N2 fixation

Azorhizobium caulinodans thermolabile point mutants unable to fix N2 at 42 degrees C were isolated and mapped to three, unlinked loci; from complementation tests, several mutants were assigned to the fixABCX locus. Of these, two independent fixB mutants carried missense substitutions in the product electron-transferring flavoprotein N (ETFN) alpha-subunit. Both thermolabile missense variants Y238H and D229G mapped to the ETFNalpha interdomain linker. Unlinked thermostable suppressors of these two fixB missense mutants were identified and mapped to the lpdA gene, encoding dihydrolipoamide dehydrogenase (LpDH), immediately distal to the pdhABC genes, which collectively encode the pyruvate dehydrogenase (PDH) complex. These two suppressor alleles encoded LpDH NAD-binding domain missense mutants G187S and E210G. Crude cell extracts of these fixB lpdA double mutants showed 60-70% of the wild-type PDH activity; neither fixB lpdA double mutant strain exhibited any growth phenotype at the restrictive or the permissive temperature. The genetic interaction between two combinations of lpdA and fixB missense alleles implies a physical interaction of their respective products, LpDH and ETFN. Presumably, this interaction electrochemically couples LpDH as the electron donor to ETFN as the electron acceptor, allowing PDH complex activity (pyruvate oxidation) to drive soluble electron transport via ETFN to N2, which acts as the terminal electron acceptor. If so, then, the A. caulinodans PDH complex activity sustains N2 fixation both as the driving force for oxidative phosphorylation and as the metabolic electron donor.

A zebrafish model for pyruvate dehydrogenase deficiency: rescue of neurological dysfunction and embryonic lethality using a ketogenic diet

Defects in the pyruvate dehydrogenase (PDH) complex result in severe neurological dysfunction, congenital lactic acidosis, growth retardation, and early death. Current treatments for PDH deficiency are administered postnatally and are generally unsuccessful. Because many patients with this disease are born with irreversible defects, a model system for the development of effective pre- and postnatal therapies would be of great value. In a behavioral genetic screen aimed to identify zebrafish with visual function defects, we previously isolated two alleles of the recessive lethal mutant no optokinetic response a (noa). Here we report that noa is deficient for dihydrolipoamide S-acetyltransferase (Dlat), the PDH E2 subunit, and exhibits phenotypes similar to human patients with PDH deficiency. To rescue the deficiency, we added ketogenic substrates to the water in which the embryos develop. This treatment successfully restored vision, promoted feeding behavior, reduced lactic acidosis, and increased survival. Our study demonstrates an approach for establishing effective therapies for PDH deficiency and other congenital diseases that affect early embryonic development.

Effects of aerobic training on pyruvate dehydrogenase and pyruvate dehydrogenase kinase in human skeletal muscle

This study examined the effects of short- and long-term aerobic training on the stable up-regulation of pyruvate dehydrogenase (PDH) and PDH kinase (PDK) in human skeletal muscle. We hypothesized that 8 weeks, but not 1 week, of aerobic training would increase total PDH (PDHt) and PDK activities compared to pretraining, and this would be detectable at the level of gene transcription (mRNA) and/or gene translation (protein). Resting muscle biopsies were taken before and after 1 and 8 weeks of aerobic cycle exercise training. PDHt and PDK activities, and their respective protein and mRNA expression, did not differ after 1 week of aerobic training. PDHt activity increased 31% after 8 weeks and this may be partially due to a 1.3-fold increase in PDH-E(1)alpha protein expression. PDK activity approximately doubled after 8 weeks of aerobic training and this was attributed to a 1.3-fold increase in PDK2 isoform protein expression. Similar to 1 week, no changes were observed at the mRNA level after 8 weeks of training. These findings suggest that aerobically trained human skeletal muscle has an increased maximal capacity to utilize carbohydrates, evident by increased PDHt, but increased metabolic control sensitivity to pyruvate through increased contribution of PDK2 to total PDK activity.

Interactions of the peripheral subunit-binding domain of the dihydrolipoyl acetyltransferase component in the assembly of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus

The enzymes pyruvate decarboxylase (E1) and dihydrolipoyl dehydrogenase (E3) bind tightly but in a mutually exclusive manner to the peripheral subunit-binding domain (PSBD) of dihydrolipoyl acetyltransferase in the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus. The use of directed mutagenesis, surface plasmon resonance detection and isothermal titration microcalorimetry revealed that several positively charged residues of the PSBD, most notably Arg135, play an important part in the interaction with both E1 and E3, whereas Met131 makes a significant contribution to the binding of E1 only. This indicates that the binding sites for E1 and E3 on the PSBD are overlapping but probably significantly different, and that additional hydrophobic interactions may be involved in binding E1 compared with E3. Arg135 of the PSBD was also replaced with cysteine (R135C), which was then modified chemically by alkylation with increasingly large aliphatic groups (R135C -methyl, -ethyl, -propyl and -butyl). The pattern of changes in the values of DeltaG degrees, DeltaH degrees and TDeltaS degrees that were found to accompany the interaction with the variant PSBDs differed between E1 and E3 despite the similarities in the free energies of their binding to the wild-type. The importance of a positive charge on the side-chain at position 135 for the interaction of the PSBD with E3 and E1 was apparent, although lysine was found to be an imperfect substitute for arginine. The results offer further evidence of entropy-enthalpy compensation ('thermodynamic homeostasis') - a feature of systems involving a multiplicity of weak interactions.

A novel deletion creating a new terminal exon of the dihydrolipoyl transacylase gene is a founder mutation of Filipino maple syrup urine disease

Maple syrup urine disease (MSUD) is a rare, autosomal-recessive disorder of branched-chain amino-acid metabolism. In the Philippines, many MSUD cases have been diagnosed clinically. Here, molecular analysis of the dihydrolipoyl transacylase (E2) gene was done in 13 unrelated families from the Philippines. A novel deletion spanning 4.1 kb of intron 10 and 601 bp of exon 11, caused by non-homologous recombination between an L1 repeat in intron 10 and an Alu repeat in exon 11, was found in 8 out of 13 families, with 5 of them being homozygous for the mutation, implicating it as a founder mutation of Filipino MSUD. The resulting mutant E2 mRNA contains a 239-bp insertion after exon 10, thereby producing a new terminal exon. Large-scale population screening of the deletion revealed that one carrier of the mutation was identified in 100 normal Filipinos. These findings suggest that a limited number of mutations might underlie MSUD in the Filipino population, potentially facilitating prenatal diagnosis and carrier detection of MSUD in this group.

Prediction of the binding site on E1 in the assembly of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus

The beta-subunit (E1beta) of the pyruvate decarboxylase (E1, alpha(2)beta(2)) component of the Bacillus stearothermophilus pyruvate dehydrogenase complex was comparatively modelled based on the crystal structures of the homologous 2-oxoisovalerate decarboxylase of Pseudomonas putida and Homo sapiens. Based on this homology modelling, alanine-scanning mutagenesis studies revealed that the negatively charged side chain of Glu285 and the hydrophobic side chain of Phe324 are of particular importance in the interaction with the peripheral subunit-binding domain of the dihydrolipoyl acetyltransferase component of the complex. These results help to identify the site of interaction on the E1beta subunit and are consistent with thermodynamic evidence of a mixture of electrostatic and hydrophobic interactions being involved.

Lactic acidosis and developmental delay due to deficiency of E3 binding protein (protein X) of the pyruvate dehydrogenase complex

Pyruvate dehydrogenase deficiency is an important cause of primary lactic acidosis. Most cases occur as a result of mutations in the gene for the E1 alpha subunit of the complex, with a small number resulting from mutations in genes for other components, most commonly the E3 and E3-binding protein subunits. We describe pyruvate dehydrogenase E3-binding protein deficiency in two siblings in each of two unrelated families from Kuwait. The index patient in each family had reduced pyruvate dehydrogenase activity in cultured fibroblasts and no detectable immunoreactive E3-binding protein. Both were homozygous for nonsense mutations in the E3-binding protein gene, one involving the codon for glutamine 266, the other the codon for tryptophan 5.

Characterization of the isozymes of pyruvate dehydrogenase phosphatase: implications for the regulation of pyruvate dehydrogenase activity

The activity of mammalian pyruvate dehydrogenase complex (PDC) is regulated by a phosphorylation/dephosphorylation cycle. Dephosphorylation accompanied by activation is carried out by two genetically different isozymes of pyruvate dehydrogenase phosphatase, PDP1c and PDP2c. Here, we report data showing that PDP1c and PDP2c display marked biochemical differences. The activity of PDP1c strongly depends upon the simultaneous presence of calcium ions and the E2 component of PDC. In contrast, the activity of PDP2c displays little, if any, dependence upon either calcium ions or E2. Furthermore, PDP2c does not appreciably bind to PDC under the conditions when PDP1c exists predominantly in the PDC-bound state. The stimulatory effect of E2 on PDP1c can be partially mimicked by a monomeric construct consisting of the inner lipoyl-bearing domain and the E1-binding domain of E2 component. This strongly suggests that the E2-mediated activation of PDP1c largely reflects the effects of co-localization and mutual orientation of PDP1c and E1 component facilitated by their binding to E2. Both PDP1c and PDP2c can efficiently dephosphorylate all three phosphorylation sites located on the alpha chain of the E1 component. For PDC phosphorylated at a single site, the relative rates of dephosphorylation of individual sites are: 2>site 3>site 1. Phosphorylation of sites 2 or 3 in addition to site 1 does not have a significant effect on the rates of dephosphorylation of individual sites by PDP1c, suggesting a random mechanism of dephosphorylation. In contrast, there is a significant decrease in the overall rate of dephosphorylation of pyruvate dehydrogenase by PDP2c under these conditions. This indicates that the mechanism of dephosphorylation of PDC phosphorylated at multiple sites by PDP2c is not purely random. These marked differences in the site-specificity displayed by PDP1c and PDP2c should be particularly important under conditions such as starvation and diabetes, which are associated with a great increase in phosphorylation of sites 2 and 3 of pyruvate dehydrogenase.

Patients with primary biliary cirrhosis react against a ubiquitous xenobiotic-metabolizing bacterium

Infectious and environmental agents have been proposed as immunologic triggers for primary biliary cirrhosis (PBC). Recently, a ubiquitous organism that metabolizes organic compounds and estrogens, Novosphingobium aromaticivorans, has been defined. Importantly, 2 bacterial proteins have homology with the E2 component of the pyruvate dehydrogenase complex (PDC-E2). Sera from 97 patients with PBC, 46 first-degree relatives, 10 spouses, and 195 controls were studied for reactivity against N. aromaticivorans and Escherichia coli. The reactivity was defined by absorption, affinity purification, and using monoclonal antibodies to PDC-E2. Stool samples from 20 patients with PBC and 34 controls were analyzed by polymerase chain reaction (PCR) for the presence of N. aromaticivorans. Sera from 100% of anti-PDC-E2 positive (77/77), 33% of anti-BCOADC E2 positive (1/3), and 12% of antimitochondrial antibody (AMA) negative patients with PBC (2/17) reacted with titers up to 10(-6) against two known lipoylated bacterial proteins (47 and 50 kd) from N. aromaticivorans, including patients with early disease. This titer was approximately 100- to 1,000-fold higher than against E. coli and verified by absorption, use of affinity-purified sera, and monoclonal antibody reagents. Moreover, 78 of 80 AMA-positive and 5 of 17 AMA-negative patients with PBC had antibodies against 3 other N. aromaticivorans proteins. In contrast, 0 of 195 control sera reacted against N. aromaticivorans. Approximately 25% of patients and controls had N. aromaticivorans in their fecal specimens. In conclusion, based on protein homology, capacity to metabolize xenobiotics as well as modulate estrogens, its presence in feces, and specific immunologic response, we propose that N. aromaticivorans is a candidate for the induction of PBC.

The pyruvate dehydrogenase complex of Mycoplasma hyopneumoniae contains a novel lipoyl domain arrangement

The genes encoding the pyruvate dehydrogenase (PDH) complex (pdhA, pdhB, pdhC and pdhD) from Mycoplasma hyopneumoniae have been cloned and sequenced. The genes are arranged into two operons, designated pdhAB and pdhCD, which are not found together in the chromosome. The pdhA, pdhB, pdhC and pdhD genes encode proteins of predicted molecular masses of 44.2 kDa (pyruvate dehydrogenase major subunit; E1alpha), 36.6 kDa (pyruvate dehydrogenase minor subunit; E1beta), 33.1 kDa (dihydrolipoyl acetyltransferase; E2) and 66.3 kDa (dihydrolipoyl dehydrogenase; E3), respectively. Sequence analysis of the pdhCD operon revealed the presence of a lipoyl-binding domain in pdhD but not in pdhC. The lipoyl domain is believed to act as a "swinging arm" that spans the gaps between the catalytic domains of each of the subunits. Portions of the N-terminal regions of pdhA and pdhD were expressed as 6xHis-tag fusion proteins in Escherichia coli and purified by nickel affinity chromatography. The purified proteins were used to raise antibodies in rabbits, and Western blot analysis was performed with the polyclonal rabbit antiserum. Both the pdhA and pdhD genes were expressed among various strains of M. hyopneumoniae as well as the porcine mycoplasmas, Mycoplasma hyorhinis and Mycoplasma flocculare. Southern hybridisation analysis using probes from pdhA and pdhD detected one copy of each gene in the chromosome of M. hyopneumoniae. Since previous studies have shown pyruvate dehydrogenase activity in M. hyopneumoniae [J. Gen. Microbiol. 134 (1988) 791], it appears likely that a functional lipoyl-binding domain in the N terminus of PdhC is not an absolute prerequisite for pyruvate dehydrogenase enzyme activity. We hypothesise that the lipoyl-binding domain of PdhD is performing the enzymatic function normally attributed to the PdhC lipoyl-binding domain in other organisms. Searches of pyruvate dehydrogenase gene sequences derived from other Mycoplasma species showed that a putative lipoyl domain was absent in the pdhC gene from Mycoplasma pulmonis. However, like other bacterial species, pdhC gene sequences from Mycoplasma capricolum, Mycoplasma genitalium and Mycoplasma pneumoniae contain a putative lipoyl domain.

[Cloning and expressing the E2 subunit of pyruvate dehydrogenase complex]

OBJECTIVES

To construct the expression vector of the pyruvate dehydrogenase complex E2 subunit gene (PDC-E2).

METHODS

The PDC-E2 gene was amplified from human lymphocytes with RT-PCR, and was cloned into pExSecI vector to induce the PDC-E2 expression. The products were identified with western blot and ELISA.

RESULTS

The expression vector pExSecI/PDC-E2 was successfully constructed. The products could be identified by the specific self-antibodies in the sera from the primary biliary cirrhosis patients.

CONCLUSION

High efficient expression vector of PDC-E2 lays the foundation for serum assay of primary biliary cirrhosis patients with prokaryotic expressing PDC-E2.