Regulation of mammalian pyruvate dehydrogenase alpha subunit gene expression by glucose in HepG2 cells

We report the effect of glucose on the expression of the gene encoding the pyruvate dehydrogenase (E1) alpha subunit (E1alpha) in human hepatoma (HepG2) cells. Total pyruvate dehydrogenase complex activity as well as the levels of protein and mRNA of the E1alpha subunit were significantly increased in HepG2 cells cultured in medium containing 16.7 mM glucose compared with 1.0 mM glucose for a period of 4 weeks. The level of E1alpha mRNA was elevated approx. 2-fold in HepG2 cells cultured for 24 h in medium containing 16.7 mM glucose compared with 1 mM glucose. This effect was specific to glucose and independent of insulin. Nuclear run-on assays and promoter analysis indicate that the glucose-induced increases in the levels of E1alpha mRNA in HepG2 cells are due to increased transcription of the human E1alpha (PDHA1) gene. Mutational analysis of the E1alpha promoter region has identified two regions, from -78 to -73 bp (CCCCTG) and from -8 to -3 bp (GCGGTG), that are responsible for the effect of glucose on promoter activity; the former exhibits a larger effect. These two sequences represent new variations of the carbohydrate-response element that has been identified in other genes. The stimulation of E1alpha promoter activity by glucose was abolished by okadaic acid at 100 nM but not at 5 nM, suggesting that glucose-mediated regulation of pyruvate dehydrogenase complex E1alpha gene transcription involves a phosphorylation/dephosphorylation mechanism, possibly involving protein phosphatase-1.

The RAG3 gene of Kluyveromyces lactis is involved in the transcriptional regulation of genes coding for enzymes implicated in pyruvate utilization and genes of the biosynthesis of thiamine pyrophosphate

The RAG3 gene of Kluyveromyces lactis, a homolog of PDC2 of Saccharomyces cerevisiae, is known to be a regulator of the pyruvate decarboxylase gene KlPDC1. We have identified new target genes for Rag3p. The RAG3 gene product was found to be required for the transcription of two genes of the biosynthetic pathway of thiamine (a cofactor of pyruvate decarboxylase). Conversely, the RAG3 gene product partially repressed the expression of the pyruvate dehydrogenase gene KlPDA1. Therefore, RAG3 may act as a general regulator in the balance of the two alternative pathways of pyruvate metabolism in yeast.

Mechanisms of expression of pyruvate dehydrogenase deficiency caused by an E1alpha subunit mutation

OBJECTIVE

To characterize the biochemical mechanisms of expression of the pyruvate dehydrogenase (PDH) E1alpha subunit exon 10 R302C missense mutation.

BACKGROUND

Mutations in the X-linked E1alpha subunit gene are responsible for most cases of PDH deficiency, an important cause of neurodevelopmental defects and neurodegeneration with primary lactic acidemia. Although the disease shows extreme allelic heterogeneity, the R302C mutation has been defined in several unrelated cases.

METHODS

Cell lines expressing selectively either the mutant or wild-type E1alpha alleles against identical genetic backgrounds were generated from the fibroblasts of a female heterozygous for the R302C mutation. Enzyme activity, mRNA, polypeptide expression, and turnover were studied in each.

RESULTS

The residual PDH activity was below measurable levels in the cell line (B5) expressing only the mutant allele and normal in the wild-type polypeptide expressing (A10) cell line, confirming that the R302C mutation alone is sufficient to cause a severe PDH deficiency. The mutant polypeptide was less stable than the wild-type polypeptide, but the steady-state level of the mutant E1alpha protein was reduced only two- to threefold.

CONCLUSIONS

The primary mechanism of expression of the R302C mutation must be limitation of catalytic efficiency. We speculate that catalysis may be inhibited in the mutant polypeptide because conformational changes are induced near serine 300, a residue that is particularly important as a regulatory phosphorylation site in the wild-type polypeptide.

Longitudinal study of tissue- and subunit-specific obesity-induced regulation of the pyruvate dehydrogenase complex

The tissue-specific expression of the mitochondrial pyruvate dehydrogenase complex (PDHc) has been studied in an animal model of obesity with hyperinsulinemia, the obese (fa/fa) Zucker rat. Liver and heart were obtained from 4 and 8 week-old obese rats and age-matched lean animals, and in each tissue the following parameters were analyzed: (1) total activity of the mitochondrial PDHc; (2) abundance of the mitochondrial PDHc subunits on Western blots; and (3) abundance of the E1alpha and E1beta subunit mRNAs on Northern blots and semi-quantitative RT-PCR. Regardless of age, obese rats showed an increase in liver total PDHc activity and a coordinate increase in liver E1alpha and E1beta PDHc subunit abundance. At 4 weeks, obese rats also showed an increase in liver PDH E1alpha mRNA level, but regardless of age E1beta mRNA level was unchanged. In contrast, neither total PDHc activity nor the concentration of its protein subunits were increased in heart of obese rats. Thus, obese Zucker rats display a liver-specific early increase in PDHc which results from a selective up-regulation of the E1alpha gene expression.

Heterologously expressed inner lipoyl domain of dihydrolipoyl acetyltransferase inhibits ATP-dependent inactivation of pyruvate dehydrogenase complex. Identification of important amino acid residues

The activity of the pyruvate dehydrogenase multienzyme complex (PDC), which catalyses the oxidation of pyruvate to acetyl-CoA within the mitochondrion, is diminished in animal models of diabetes. Studies with purified PDC components have suggested that the kinases responsible for inactivating the decarboxylase catalytic subunits of the complex are most efficient in their regulatory role when they are bound to dihydrolipoyl acetyltransferase (E2) subunits, which form the structural core of the complex. We report that the addition of an exogenous E2 subdomain (inner lipoyl domain) to an intact PDC inhibits ATP-dependent inactivation of the complex. By combining molecular modelling, site-directed mutagenesis and biophysical characterizations, we have also identified two amino acid residues in this subdomain (Ile229 and Phe231) that largely determine the magnitude of this effect.

Plant mitochondrial pyruvate dehydrogenase complex: purification and identification of catalytic components in potato

The pyruvate dehydrogenase complex (mPDC) from potato (Solanum tuberosum cv. Romano) tuber mitochondria was purified 40-fold to a specific activity of 5.60 micromol/min per mg of protein. The activity of the complex depended on pyruvate, divalent cations, NAD+ and CoA and was competitively inhibited by both NADH and acetyl-CoA. SDS/PAGE revealed the complex consisted of seven polypeptide bands with apparent molecular masses of 78, 60, 58, 55, 43, 41 and 37 kDa. N-terminal sequencing revealed that the 78 kDa protein was dihydrolipoamide transacetylase (E2), the 58 kDa protein was dihydrolipoamide dehydrogenase (E3), the 43 and 41 kDa proteins were alpha subunits of pyruvate dehydrogenase, and the 37 kDa protein was the beta subunit of pyruvate dehydrogenase. N-terminal sequencing of the 55 kDa protein band yielded two protein sequences: one was another E3; the other was similar to the sequence of E2 from plant and yeast sources but was distinctly different from the sequence of the 78 kDa protein. Incubation of the mPDC with [2-14C]pyruvate resulted in the acetylation of both the 78 and 55 kDa proteins.

The pyruvate dehydrogenase multi-enzyme complex from Gram-negative bacteria

Pyruvate dehydrogenase multi-enzyme complexes from Gram-negative bacteria consists of three enzymes, pyruvate dehydrogenase/decarboxylase (E1p), dihydrolipoyl acetyltransferase (E2p) and dihydrolipoyl dehydrogenase (E3). The acetyltransferase harbors all properties required for multi-enzyme catalysis: it forms a large core of 24 subunits, it contains multiple binding sites for the E1p and E3 components, the acetyltransferase catalytic site and mobile substrate carrying lipoyl domains that visit the active sites. Today, the Azotobacter vinelandii complex is the best understood oxo acid dehydrogenase complex with respect to structural details. A description of multi-enzyme catalysis starts with the structural and catalytic properties of the individual components of the complex. Integration of the individual properties is obtained by a description of how the many copies of the individual enzymes are arranged in the complex and how the lipoyl domains couple the activities of the respective active sites by way of flexible linkers. These latter aspects are the most difficult to study and future research need to be aimed at these properties.

Gene and subunit organization of bacterial pyruvate dehydrogenase complexes

Pyruvate dehydrogenase complexes of bacterial origin are compared with respect to subunit composition, organization of the corresponding genes, and the number and location of lipoyl domains. Special attention is given to two unusual examples of pyruvate dehydrogenase complexes, formed by Zymomonas mobilis and Thiobacillus ferrooxidans.

Thiamin metabolism and thiamin diphosphate-dependent enzymes in the yeast Saccharomyces cerevisiae: genetic regulation

The yeast Saccharomyces cerevisiae utilises external thiamin for the production of thiamin diphosphate (ThDP) or can synthesise the cofactor itself. Prior to uptake into the cell thiamin phosphates are first hydrolysed and thiamin is taken up as free vitamin which is then pyrophosphorylated by a pyrophosphokinase. Synthesis of ThDP starts with the production of hydroxyethylthiazole and hydroxymethylpyrimidine. Those are linked to yield thiamin phosphate which is hydrolysed to thiamin and subsequently pyrophosphorylated. The THI genes encoding the enzymes of these final steps of ThDP production and of thiamin utilisation have been identified. Their expression is controlled by the level of thiamin and a number of regulatory proteins involved in regulated expression of the THI genes are known. However, the molecular details of the regulatory circuits need to be deciphered. Since the nucleotide sequence of the entire yeast genome is known we can predict the number of ThDP-dependent enzymes in S. cerevisiae. Eleven such proteins have been found: pyruvate decarboxylase (Pdc, three isoforms), acetolactate synthase, a putative alpha-ketoisocaproate decarboxylase with a regulatory role in ThDP synthesis and two proteins of unknown function form the group of Pdc related enzymes. In addition there are two isoforms for transketolase as well as the E1 subunits of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. Expression of most of these genes is either induced or repressed by glucose. Surprisingly, it has been found recently that expression of one of the genes for Pdc is repressed by thiamin. In addition, the regulatory protein Pdc2p was shown to be required for high level expression of both the THI and the PDC genes. Apparently, the production of ThDP and of the enzymes using this cofactor is coordinately regulated. Future research will focus on the elucidation of the molecular mechanisms of this novel type of regulation.

A novel, rapid in cell RNA amplification technique for the detection of low copy mRNA transcripts

Growing interest now focuses on improvements of in situ polymerase chain reaction (PCR) technology for the detection of DNA and RNA cellular sequences. In this study, reverse transcription PCR in situ hybridisation (RT PCR-ISH) was developed and used to determine gene expression of pyruvate dehydrogenase in a cell model system, using human peripheral blood lymphocytes (PBLs). The success of in cell RNA amplification depends on the type of cell/tissue fixation, cell permeabilisation, and the efficiency of reverse transcription and cDNA amplification. This paper presents new approaches to overcome the critical aspects of fixation, permeabilisation, and reverse transcription when performing in cell RNA amplification. A novel fixative, "Permeafix", possessing fixative and permeabilisation properties, was used for cell fixation procedures. "Permeafix" obviated the need for pre-amplification proteolysis, facilitating entry of PCR reagents to target sequences within the cell. In addition, a simple on step RNA in cell amplification protocol using recombinant Thermus thermophilus (rTth) DNA polymerase, which reverse transcribes mRNA efficiently to cDNA and then catalyses cDNA amplification, was used. The value of a semi-junctional primer system for in cell gene expression studies, without the need to perform DNase digestion, is demonstrated.

Expression of pyruvate dehydrogenase isoforms during the aerobic/anaerobic transition in the development of the parasitic nematode Ascaris suum: altered stoichiometry of phosphorylation/inactivation

The pyruvate dehydrogenase complex (PDC) plays a key role in the anaerobic metabolism of the parasitic nematode Ascaris suum. Two isoforms of the alpha-subunit of pyruvate dehydrogenase (E1) have been identified: alpha I is most abundant in anaerobic adult muscle and alpha II in aerobic larvae. Both isoforms have been expressed as alpha 2 beta 2 tetramers with a muscle-specific beta-subunit, purified to apparent homogeneity, reconstituted with E1-deficient adult A. suum muscle PDC, and assayed for PDC and E1 kinase activity. Recombinant alpha II is a poor substrate for the adult E1 kinase, but its stoichiometry of phosphorylation/inactivation is similar to that reported for the human E1. Initially, inactivation parallels the incorporation of about 1 mol 32P/mol E1 and at maximal phosphorylation about 2.4 32P/mol E1 is incorporated. In contrast, recombinant alpha I (r alpha I) is phosphorylated rapidly, and substantially more phosphorylation accompanies inactivation. To examine this altered pattern of phosphorylation, the two phosphorylation sites in each E1 alpha subunit of the r alpha I (site 1 and site 2) were changed either individually or together from Ser to Ala by site-directed mutagenesis. Site 1 was phosphorylated more rapidly than site 2, but the phosphorylation of either site resulted in inactivation, and the phosphorylation of only a single E1 alpha subunit of the tetramer was necessary for inactivation. However, both E1 alpha subunits of the tetramer were phosphorylated, based on the incorporation of about 3.5 mol 32P/mol E1 at maximal phosphorylation and the altered mobility of most of the E1 alpha subunits during SDS-PAGE. These observations suggest that the regulation of both E1 isoforms is modified to maintain PDC activity during the transition to anaerobiosis.

Global analysis of genomic texts: the distribution of AGCT tetranucleotides in the Escherichia coli and Bacillus subtilis genomes predicts translational frameshifting and ribosomal hopping in several genes

Present availability of the genomic text of bacteria allows assignment of biological known functions to many genes (typically, half of the genome's gene content). It is now time to try and predict new unexpected functions, using inductive procedures that allow correlating the content of the genomic text to possible biological functions. We show here that analysis of the genomes of Escherichia coli and Bacillus subtilis for the distribution of AGCT motifs predicts that genes exist for which the mRNA molecule can be translated as several different proteins synthesized after ribosomal frameshifting or hopping. Among these genes we found that several coded for the same function in E. coli and B. subtilis. We analyzed in depth the situation of the infB gene (experimentally known to specify synthesis of several proteins differing in their translation starts), the aceF/pdhC gene, the eno gene, and the rplI gene. In addition, genes specific to E. coli were also studied: ompA, ompFand tolA (predicting epigenetic variation that could help escape infection by phages or colicins).

Thiamine deficiency decreases steady-state transketolase and pyruvate dehydrogenase but not alpha-ketoglutarate dehydrogenase mRNA levels in three human cell types

Reductions in the levels and activities of enzymes that utilize thiamine diphosphate (ThDP) as a cofactor are thought to be responsible for the tissue damage suffered during thiamine deficiency. Although loss of cofactor can account in part for loss of enzyme activity, thiamine and its phosphorylated derivatives may also regulate the expression of the genes encoding these proteins. To examine this possibility, steady-state mRNA levels for three ThDP-dependent enzymes were measured in human fibroblasts, lymphoblasts and neuroblastoma cells cultured under conditions of thiamine sufficiency and deficiency. In all three cell types, the mRNA levels of transketolase and the E1beta subunit of pyruvate dehydrogenase complex were lower in thiamine-deficient cultures. In contrast, mRNA levels for a ThDP-binding subunit of alpha-ketoglutarate dehydrogenase, the E1 subunit did not differ. These results indicate that thiamine or a thiamine metabolite regulates the expression in humans of some, but not all, genes encoding ThDP-utilizing enzymes.

Purification of the pyruvate dehydrogenase multienzyme complex of Zymomonas mobilis and identification and sequence analysis of the corresponding genes

The pyruvate dehydrogenase (PDH) complex of the gram-negative bacterium Zymomonas mobilis was purified to homogeneity. From 250 g of cells, we isolated 1 mg of PDH complex with a specific activity of 12.6 U/mg of protein. Analysis of subunit composition revealed a PDH (E1) consisting of the two subunits E1alpha (38 kDa) and E1beta (56 kDa), a dihydrolipoamide acetyltransferase (E2) of 48 kDa, and a lipoamide dehydrogenase (E3) of 50 kDa. The E2 core of the complex is arranged to form a pentagonal dodecahedron, as shown by electron microscopic images, resembling the quaternary structures of PDH complexes from gram-positive bacteria and eukaryotes. The PDH complex-encoding genes were identified by hybridization experiments and sequence analysis in two separate gene regions in the genome of Z. mobilis. The genes pdhAalpha (1,065 bp) and pdhAbeta (1,389 bp), encoding the E1alpha and E1beta subunits of the E1 component, were located downstream of the gene encoding enolase. The pdhB (1,323 bp) and lpd (1,401 bp) genes, encoding the E2 and E3 components, were identified in an unrelated gene region together with a 450-bp open reading frame (ORF) of unknown function in the order pdhB-ORF2-lpd. Highest similarities of the gene products of the pdhAalpha, pdhAbeta, and pdhB genes were found with the corresponding enzymes of Saccharomyces cerevisiae and other eukaryotes. Like the dihydrolipoamide acetyltransferases of S. cerevisiae and numerous other organisms, the product of the pdhB gene contains a single lipoyl domain. The E1beta subunit PDH was found to contain an amino-terminal lipoyl domain, a property which is unique among PDHs.

Detection of a homozygous four base pair deletion in the protein X gene in a case of pyruvate dehydrogenase complex deficiency

While the presence of a lipoyl-containing protein (protein X) separate from lipoyl transacetylase in the pyruvate dehydrogenase complex (PDC) has been known for some time, until recently only the cDNA for the yeast enzyme has been cloned. We have cloned, sequenced and characterized the cDNA encoding the human protein X and localized the protein X gene to chromosome 11p13. We also report here a new case of protein X deficiency identified immunologically, with decreased activity of PDC and without mutations in the E1alpha subunit or E1beta subunit. We report that the cDNA and gene of this patient for protein X has a homozygous 4 bp deletion, specifically in the putative mitochondrial targeting signal sequence which results in a premature stop codon. This is the first documented case of a molecular defect in pyruvate dehydrogenase protein X.

Mouse mutants carrying deletions that remove the genes mutated in Coffin-Lowry syndrome and lactic acidosis

The mouse X-linked mutants lined and stripey are associated with lethality of affected males in utero and a striping of the coat in carrier females. We demonstrate that the underlying mutations are nested deletions which lie in the Phex-Amelx chromosomal segment conserved between man and mouse. The lined deletion contains less than approximately 0.7 cM of genetic material and includes the growth factor-regulated protein kinase gene, Rsk2. Stripey carries a larger deletion which removes approximately 2.0 cM of genetic material, including Rsk2 and the pyruvate dehydrogenase E1alpha subunit gene, Pdha1 . Since Coffin-Lowry syndrome and neonatal lactic acidosis are associated with mutations in the human homologues of Rsk2 and Pdha1 respectively, lined and stripey provide models for gene deficiencies in these disorders.

Cooperative folding of a protein mini domain: the peripheral subunit-binding domain of the pyruvate dehydrogenase multienzyme complex

The peripheral subunit-binding domain from the dihydrolipoamide acetyltransferase (E2) component of the pyruvate dehydrogenase multienzyme complex from Bacillus stearothermophilus is stably folded, despite its short sequence of only 43 amino acid residues. A 41 residue peptide derived from this domain, psbd41, undergoes a cooperative thermal unfolding transition with a tm of 54 degrees C. This three-helix protein is monomeric as judged by ultracentrifugation and concentration-dependent CD measurements. Peptides corresponding to the individual helices are largely unstructured both alone and in combination, indicating that the unusual stability of this protein does not arise solely from unusually stable alpha-helices. Chemical denaturation by guanidine hydrochloride is also cooperative with a delta GH2O of 3.1 kcal mol-1 at pH 8.0 and 25 degrees C. The chemical denaturation is broad with an m-value of 760 cal mol-1 M-1. psbd41 contains a buried aspartate residue at position 34 that may provide stability and specificity to the fold. A mutant peptide, psbd41Asn was synthesized in which the buried aspartate residue was mutated to asparagine. This peptide still folds cooperatively and it is monomeric, but is much less thermostable than the wild-type with a tm of only 31 degrees C. Chemical denaturations at 4 degrees C give an m-value of 740 cal mol-1 M-1, similar to the wild-type, but the stability delta GH2O is only 1.4 kcal mol-1. Both the wild-type and the mutant unfold at extremes of pH, but at 4 degrees C psbd41Asn is folded over a narrower pH range than the wild-type. Although the mutant unfolds cooperatively by thermal and by chemical denaturation, its NMR spectrum is significantly broader than that of the wild-type and it binds ANS. These results show that Asp34 is vital for the stability and specificity of this structure, the second smallest natural sequence known to fold in the absence of disulfide bonds or metal or ligand-binding sites.

Environmental control of pyruvate dehydrogenase complex expression in Escherichia coli

The effects of changing environmental conditions on expression of the pdh operon were studied in strains containing pyruvate dehydrogenase (PDH) complexes having either one or three lipoyl domains per lipoate acetyltransferase chain. The expression of the pdh operon was lowered during growth on reduced carbon sources and when the mode of energy generation was changed from aerobic respiration to anaerobic respiration and fermentation. In contrast, growth at non-optimal pH increased expression. Operon expression was generally higher in the 1 lip strain compared to the 3 lip strain. Expression of the pdh operon was shown to be tightly controlled in response to environmental stimuli, consistent with its importance in defining metabolic flux.

Systematic study of the six cysteines of the E1 subunit of the pyruvate dehydrogenase multienzyme complex from Escherichia coli: none is essential for activity

Variants of the Escherichia coli 1-lip pyruvate dehydrogenase multienzyme complex (1-lip PDHc) with the C259N and C259S substitutions in the putative thiamin diphosphate-(ThDP-) binding motif of the pyruvate dehydrogenase component (E1, EC 1.2.4.1) were characterized. Single substitutions were made at the five remaining cysteines of the E1 component, creating the C120A, C575A, C610A, C654A, and C770S variants to test the hypothesis that the activity loss that accompanies exposure of the enzyme to fluoropyruvate, bromopyruvate, and 2-oxo-3-butynoic acid is the result of the modification of approximately one cysteine residue per E1 monomer. Surprisingly, all single cysteine E1 variants could be reconstituted with E2-E3 subcomplex and showed PDHc activity ranging from 74% to 96% that of the parental enzyme. The specific activities of C259N and C259S variants of 1-lip PDHc were 58% and 27% relative to that of the parental 1-lip PDHc. All five single cysteine E1 variants, along with the C259N and C259S variants of 1-lip PDHc, could also (1) be inactivated with fluoropyruvate and 2-oxo-3-butynoic acid, (2) were subject to inactivation by the monoclonal antibody 18A9 reported from one of our laboratories, and (3) were subject to regulation by pyruvate and acetyl-CoA. It was therefore concluded that none of the six cysteine residues is essential for the activity of the E1 component or of the complex. When tested with the putative transition-state analogue, thiamin 2-thiothiazolone diphosphate, all but the C259S and C259N variants were very potently inhibited, the stoichiometry for parental E1 being about 1.6 mol of inhibitor/mol of E1 subunit. The C259S and C259N E1 variants required at least 25-fold greater inhibitor concentration to achieve the same level of inhibition. C259 is located in the putative thiamin diphosphate-binding motif of the enzyme [more exactly, it is adjacent to a ligand to the Mg(II) ion]. It is therefore concluded that thiamin 2-thiothiazolone diphosphate is not a transition-state analogue; rather, it is a potent inhibitor of the complex because of a specific interaction with the C259 residue.

Evidence for existence of tissue-specific regulation of the mammalian pyruvate dehydrogenase complex

Tissue distribution and kinetic parameters for the four isoenzymes of pyruvate dehydrogenase kinase (PDK1, PDK2, PDK3 and PDK4) identified thus far in mammals were analysed. It appeared that expression of these isoenzymes occurs in a tissue-specific manner. The mRNA for isoenzyme PDK1 was found almost exclusively in rat heart. The mRNA for PDK3 was most abundantly expressed in rat testis. The message for PDK2 was present in all tissues tested but the level was low in spleen and lung. The mRNA for PDK4 was predominantly expressed in skeletal muscle and heart. The specific activities of the isoenzymes varied 25-fold, from 50nmol/min per mg for PDK2 to 1250nmol/min per mg for PDK3. Apparent Ki values of the isoenzymes for the synthetic analogue of pyruvate, dichloroacetate, varied 40-fold, from 0.2 mM for PDK2 to 8 mM for PDK3. The isoenzymes were also different with respect to their ability to respond to NADH and NADH plus acetyl-CoA. NADH alone stimulated the activities of PDK1 and PDK2 by 20 and 30% respectively. NADH plus acetyl-CoA activated these isoenzymes nearly 200 and 300%. Under comparable conditions, isoenzyme PDK3 was almost completely unresponsive to NADH, and NADH plus acetyl-CoA caused inhibition rather than activation. Isoenzyme PDK4 was activated almost 2-fold by NADH, but NADH plus acetyl-CoA did not activate above the level seen with NADH alone. These results provide the first evidence that the unique tissue distribution and kinetic characteristics of the isoenzymes of PDK are among the major factors responsible for tissue-specific regulation of the pyruvate dehydrogenase complex activity.

Mutations in PDX1, the human lipoyl-containing component X of the pyruvate dehydrogenase-complex gene on chromosome 11p1, in congenital lactic acidosis

We have identified and sequenced a cDNA that encodes an apparent human orthologue of a yeast protein-X component (ScPDX1) of pyruvate dehydrogenase multienzyme complexes. The new human cDNA that has been referred to as "HsPDX1" cDNA was cloned by use of the "database cloning" strategy and had a 1,506-bp open reading frame. The amino acid sequence of the protein encoded by the cDNA was 20% identical with that encoded by the yeast PDX1 gene and 40% identical with that encoded by the lipoate acetyltransferase component of the pyruvate dehydrogenase and included a lipoyl-bearing domain that is conserved in some dehydrogenase enzyme complexes. Northern blot analysis demonstrated that the major HsPDX1 mRNA was 2.5 kb in length and was expressed mainly in human skeletal and cardiac muscles but was also present, at low levels, in other tissues. FISH analysis performed with a P1-derived artificial chromosome (PAC)-containing HsPDX1 gene sublocalized the gene to 11p1.3. Molecular investigation of PDX1 deficiency in four patients with neonatal lactic acidemias revealed mutations 78del85 and 965del59 in a homozygous state, and one other patient had no PDX1 mRNA expression.

Expression and characterisation of the homodimeric E1 component of the Azotobacter vinelandii pyruvate dehydrogenase complex

We have cloned and sequenced the gene encoding the homodimeric pyruvate dehydrogenase component (E1p) of the pyruvate dehydrogenase complex from Azotobacter vinelandii and expressed and purified the E1p component in Escherichia coli. Cloned E1p can be used to fully reconstitute complex activity. The enzyme was stable in high ionic strength buffers, but was irreversibly inactivated when incubated at high pH, which presumably was caused by its inability to redimerize correctly. This explains the previously found low stability of the wild-type E1p component after resolution from the complex at high pH. Cloned E1p showed a kinetic behaviour exactly like the wild-type complex-bound enzyme with respect to its substrate (pyruvate), its allosteric properties, and its effectors. These experiments show that acetyl coenzyme A acts as a feedback inhibitor by binding to the E1p component. Limited proteolysis experiments showed that the N-terminal region of E1p was easily removed. The resulting protein fragment was still active with artificial electron acceptors but had lost its ability to bind to the core component (E2p) and thus reconstitute complex activity. E1p was protected against proteolysis by E2p. The allosteric effector pyruvate changed E1p into a conformation that is more resistant to proteolysis.