Regional assignment of two genes of the human branched-chain alpha-keto acid dehydrogenase complex: the E1 beta gene (BCKDHB) to chromosome 6p21-22 and the E2 gene (DBT) to chromosome 1p31

Maple syrup urine disease (MSUD) is caused by the deficiency of the mitochondrial branched-chain alpha-keto acid dehydrogenase complex. The multienzyme complex is a macromolecule (Mr 4 X 10(6] consisting of at least six distinct subunits. In this study, the human E1 beta gene (BCKDHB) has been localized to human chromosome 6 by hybrid somatic cell analysis, and regionally assigned to chromosome bands 6p21-22 by in situ hybridization. The E2 gene (DBT), which was previously localized to chromosome 1, is regionally assigned to the chromosome band 1p31 also by in situ hybridization. Localization of the E1 beta gene to chromosome 6p21-22 assigns another major human disease locus to a region that contains several important genes, including the major histocompatability complex, tumor necrosis factor, and heat-shock protein HSP70. Mapping of the E1 beta and the E2 genes may provide information for the linkage analysis of MSUD families with mutations in these two loci.

Premature termination of transcription and alternative splicing in the human transacylase (E2) gene of the branched-chain alpha-ketoacid dehydrogenase complex

We have isolated a human genomic clone hgE2-14 containing exons 5, 6, 7 and 8 of the branched-chain alpha-ketoacid dehydrogenase E2 transacylase gene. Sequencing of exon 8 and its surrounding intronic sequences reveals complete identity with the previously reported truncated E2 cDNA (hE2-1) sequence between nucleotides 938 and 1521. We have identified consensus splice site junctions flanking exon 8 and also a cryptic 3' splice site 370 bases upstream from the start of exon 8 in the gene. In addition, two polyadenylation signals located in the hE2-1 cDNA are also present in the intronic sequence downstream of exon 8 which promote termination of transcription. The data indicate that shortened human liver E2 transcripts undergo alternative splicing to yield mRNA of the hE2-1 type.

Maple syrup urine disease: domain structure, mutations and exon skipping in the dihydrolipoyl transacylase (E2) component of the branched-chain alpha-keto acid dehydrogenase complex

Maple syrup urine disease (MSUD) is an autosomal recessive disorder in the oxidative decarboxylation of the branched-chain alpha-keto acids derived from leucine, isoleucine and valine. The enzyme deficient in MSUD, the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex, is a mitochondrial multienzyme complex consisting of at least six distinct subunits. MSUD is genetically heterogeneous as manifested by lesions in different subunits of the BCKAD complex among unrelated patients. To approach the biochemical basis of MSUD involving the dihydrolipoyl transacylase (E2) subunit, the domain structure of this polypeptide from human and bovine livers has been defined by limited proteolysis and cDNA cloning. The assembly of 24 E2 subunits into a cubic structure, forming the core of the mammalian BCKAD complex, was established by electron microscopy and sedimentation equilibrium analysis. Highly assembled bovine E2 devoid of prosthetic lipoic acid has been overexpressed in Escherichia coli. Studies carried out with this bacterial expression system have provided insights into the lipoylation process of E2, and the involvement of the His391 residue in the transacylation reaction. At the genetic level, the human E2 gene (DBT) has been regionally assigned to chromosome 1p31, and a related E2 pseudogene to chromosome 3q24 by in situ hybridization. Genomic cloning has shown that the human E2 gene undergoes premature transcriptional termination and alternate splicing as normal events, although its functional significance is unknown. Through the use of the polymerase chain reaction and other recombinant DNA methods, several compound heterozygous mutations at the E2 locus have been identified in classical as well as thiamine-responsive MSUD patients. These mutations would appear to be useful genetic models, which will facilitate investigations into macromolecular organization and protein-protein interactions. Moreover, an array of precise single and multiple exon deletions has been observed in the amplified mutant E2 transcripts. The results represent unexpected secondary effects that are apparently associated with the above primary mutations in the E2 gene.

A 17-bp insertion and a Phe215----Cys missense mutation in the dihydrolipoyl transacylase (E2) mRNA from a thiamine-responsive maple syrup urine disease patient WG-34

We have amplified the cDNA for the transacylase (E2) subunit of the branched-chain alpha-ketoacid dehydrogenase (BCKAD) complex from a thiamine-responsive MSUD cell line (WG-34) by the polymerase chain reaction. Sequencing of the amplified WG-34 cDNA showed a 17-bp insertion (AAATACCTTGTTACCAG) apparently resulting from an aberrant splicing of the E2 gene, and a missense (T----G) mutation that changes Phe215 to Cys in the E2 subunit. The existence of these two mutations was confirmed by probing the amplified E2 cDNA or genomic DNA with allele-specific oligonucleotides. The above results support the thesis that the thiamine-responsive MSUD patient (WG-34) is a compound heterozygote at the E2 locus. The implication of the E2 mutations for the thiamine-responsiveness observed in this patient is discussed.

Localization of the dihydrolipoamide branched-chain transacylase gene (DBT) of the human branched-chain keto acid dehydrogenase complex to chromosome 1

The gene coding for the transacylase subunit (DBT) of the human branched-chain keto acid dehydrogenase complex was localized to chromosome 1 by probing panels of human x mouse chromosome hybrids with an E2 cDNA amplified by the polymerase chain reaction. Additional data with two hybrids containing chromosome 1 fragments suggest that the DBT gene is located on the short arm (1pter----p21) of the chromosome.

Interleukin-1 beta modulates thyrotropin-induced thyroglobulin mRNA transcription through 3',5'-cyclic adenosine monophosphate

It has been reported that interleukin-1 possesses not only immunoregulatory action, but also multiple hormone like effects. We studied the action of recombinant interleukin-1 beta (IL-1 beta) on the synthesis of thyroglobulin (TG), the precursor of thyroid hormone. Human thyrocytes dispersed from normal and Graves' thyroid tissues were incubated with TSH with or without IL-1 beta. TSH stimulated TG release from cultured human thyrocytes after 4 days. This effect was mimicked by dibutyryl cAMP (dBcAMP). IL-1 beta had no effect on basal TG release but modulates the TSH-stimulated TG secretion. At low dose (10(-5) to 10(-3) U/ml) IL-1 beta increased the TSH-stimulated TG secretion while higher doses (1-100 U/ml) of IL-1 beta had an inhibitory effect. Similar responses were observed with both normal and Graves' thyrocytes. The effect of IL-1 beta occurred at the gene transcription level as reflected by changes in TG mRNA level. At 100 U/ml, IL-1 beta suppressed TG mRNA level to near unmeasurable level. This biphasic effect of IL-1 beta on TG synthesis is paralleled by a similar change in TSH-stimulated cAMP secretion. We conclude that IL-1 beta possesses hormonal action on TG gene transcription and its effect is mediated via cAMP.

Interferon-gamma inhibits thyrotropin-induced thyroglobulin gene transcription in cultured human thyrocytes

It has been shown that interferon-gamma (IFN gamma) can induce HLA class II antigen expression by thyroid epithelial cells and may play a role in the pathogenesis of autoimmune thyroid disease. We have examined the metabolic effect of recombinant human IFN gamma (0.001-500 U/mL) on thyroglobulin (TG) synthesis and secretion in cultured human thyrocytes. Thyrocytes dispersed from human Graves' thyroid tissues were cultured in the presence of TSH with or without IFN gamma. IFN gamma alone had not effect on either basal TG secretion or de novo TG synthesis, as measured by immunoprecipitation. At 100 U/mL and above, IFN gamma inhibited TSH-induced TG secretion into the medium. At 500 U/mL, IFN gamma inhibited TSH- or dibutyryl cAMP-induced TG synthesis at the gene transcription level, as evidenced by the decrease in steady state TG mRNA. IFN gamma had no effect on either basal or TSH-induced cAMP release by the thyrocytes, suggesting that the inhibitory effect occurs at a site distal to cAMP formation. These data demonstrate that IFN gamma directly inhibits TSH-stimulated TG gene expression and TG secretion. This provides further evidence that IFN gamma has a metabolic effect on thyroid hormone synthesis.

Inhibition of the bovine branched-chain 2-oxo acid dehydrogenase complex and its kinase by arylidenepyruvates

A novel class of inhibitors for the branched-chain 2-oxo acid dehydrogenase (BCOAD) complex has been synthesized and studied. The sodium salts of arylidenepyruvates: e.g., furfurylidenepyruvate (compound I), 4-(3-thienyl)-2-oxo-3-butenoate (compound II), cinnamalpyruvate (compound III) and 4-(2-thienyl)-2-oxo-3-butenoate (compound IV) inhibit the overall and kinase reactions of the BCOAD complex from bovine liver. Inhibitions of the overall reaction occur at the decarboxylase (E1) step as determined by a spectrophotometric assay with 2,6-dichlorophenolindophenol as an electron acceptor. Inhibition of the E1 reaction by compound I (Ki = 0.5 microM) is competitive, whereas inhibitions by compounds II (Ki = 150 microM) and III (Ki = 500 microM) are non-competitive with respect to the substrate 2-oxoisovalerate. The Km value for 2-oxoisovalerate is 6.7 microM as measured by the E1 assay. Inhibition of the E1 step by compounds I, II and III are reversible at low inhibitor concentrations based on the Michaelis-Menten kinetics observed. By comparison, compound I does not significantly inhibit pyruvate and 2-oxoglutarate dehydrogenase complexes. The arylidenepyruvates (compounds I, II and IV) inhibit the BCOAD kinase reaction in a manner similar to the substrate 2-oxo acids. The inhibition of the kinase reaction by compound I is non-competitive with respect to ATP, with an apparent Ki value of 4.5 mM. The results suggest that arylidenepyruvates may be useful probes for elucidating the reaction mechanisms of the BCOAD complex and its kinase.

Molecular phenotypes in cultured maple syrup urine disease cells. Complete E1 alpha cDNA sequence and mRNA and subunit contents of the human branched chain alpha-keto acid dehydrogenase complex

The activity of the branched-chain alpha-keto acid dehydrogenase complex is deficient in patients with the inherited maple syrup urine disease (MSUD). To elucidate the molecular basis of this metabolic disorder, we have isolated three overlapping cDNA clones encoding the E1 alpha subunit of the human enzyme complex. The composite human E1 alpha cDNA consists of 1783 base pairs encoding the entire human E1 alpha subunit of 400 amino acids with calculated Mr = 45,552. The human E1 alpha and the previously isolated human E2 cDNAs were used as probes in Northern blot analysis with cultured fibroblasts and lymphoblasts from seven unrelated MSUD patients. The results along with those of Western blotting have revealed five distinct molecular phenotypes according to mRNA and protein-subunit contents. These consist of type I, where the levels of E1 alpha mRNA and E1 alpha and E1 beta subunits are normal in cells, but E1 activity is deficient; Type II, where the E1 alpha mRNA is present in normal quantity, whereas the contents of E1 alpha and E1 beta subunits are reduced; Type III, where the level of E1 alpha mRNA is markedly reduced with a concomitant loss of E1 alpha and E1 beta subunits; Type IV, where the contents of both E2 mRNA and E2 subunits are markedly reduced; and Type V, where the E2 mRNA is normally expressed, but the E2 subunit is markedly reduced or completely absent. Type V includes thiamin-responsive (WG-34) and certain classical MSUD cells. These molecular phenotypes have demonstrated the complexity of MSUD and identified the affected gene in different patients for further characterization.

Characterization and conservation of the inner E2 core domain structure of branched-chain alpha-keto acid dehydrogenase complex from bovine liver. Construction of a cDNA encoding the entire transacylase (E2b) precursor

A cDNA clone encoding the entire transacylase (E2b) precursor of the bovine branched-chain alpha-keto acid dehydrogenase complex has been constructed from two overlapping incomplete cDNA clones which were isolated from a lambda ZAP library prepared from bovine liver poly(A)+ RNA. Nucleotide sequencing indicates that this bovine E2b cDNA insert (bE2-11) is 2701 base pairs in length with an open reading frame of 1446 base pairs. The bE2-11 cDNA insert encodes a leader peptide of 61 residues and a mature E2b polypeptide of 421 amino acid residues with a calculated monomeric molecular mass of 46,518 daltons. The molecular mass of the native E2b component isolated from bovine liver is 1,110,000 daltons as determined by sedimentation equilibrium. This value establishes the 24-subunit octahedral model for the quaternary structure of bovine E2b. The amino-terminal sequences of two tryptic fragments (A and B) of the E2b protein have been determined. Fragment A comprises residues 175 to 421 of the E2b protein and is the inner E2 core domain which contains the transacylase active site. Fragment B, produced by further tryptic cleavage of fragment, comprises residues 205 to 421, but does not have transacylase activity. Both fragments A and B confer the highly assembled 24-mer structure. The primary structure of the inner E2 core domain of bovine E2b (fragment A) is very similar to those of three other E2 proteins (human E2p, Escherichia coli E2p, and E. coli E2k). These similarities suggest that these E2 proteins are structurally and evolutionarily related.

Isolation and sequencing of a cDNA encoding the decarboxylase (E1)alpha precursor of bovine branched-chain alpha-keto acid dehydrogenase complex. Expression of E1 alpha mRNA and subunit in maple-syrup-urine-disease and 3T3-L1 cells

A cDNA clone encoding the entire decarboxylase (E1)alpha precursor of the bovine branched-chain alpha-keto acid dehydrogenase complex has been isolated from a lambda ZAP library prepared from bovine liver poly(A)+ RNA. Nucleotide sequencing indicates that this E1 alpha cDNA clone is 1821 base pairs (bp) in length with an open reading frame of 1365 bp and a 3'-untranslated region of 356 bp. A polyadenylation signal of the type AATAAA is located 27 bp upstream of the start of a poly(A)+ tail. There is a pair of identical 32-bp direct repeats of unknown function at the 5'-end of the cDNA. The bovine E1 alpha cDNA encodes a leader peptide of 55 residues including three candidate initiation methionines, and a mature E1 alpha of 400 amino acids with a calculated Mr of 45,385. The deduced primary structure shows the published peptide sequences flanking the two phosphorylation sites and the amino-terminal sequence (residues 1-32) of bovine E1 alpha determined in this study. The phosphoserine-bearing regions appear to be homologous between bovine E1 alpha and human pyruvate decarboxylase-alpha subunits, with respect to both amino acid identity and the position in each polypeptide chain. Northern blot analysis using the bovine E1 alpha cDNA as probe shows the presence of a single species of E1 alpha mRNA (2 kilobase pairs) in bovine liver, human placenta, and skin fibroblasts. Moreover, the E1 alpha mRNA exists in normal size and quantity in cultured fibroblasts derived from a maple-syrup-urine-disease homozygote deficient in E1 activity. The results preclude a defect in the transcription and processing of E1 alpha mRNA in these maple-syrup-urine-disease cells. Studies with 3T3-L1 cells show that a single species of E1 alpha mRNA (2 kilobase pairs) is expressed in the cells and that contents of the murine E1 alpha mRNA and subunit are markedly elevated during the differentiation of 3T3-L1 preadipocytes into adipocytes. The results indicate that the induction of murine E1 activity during adipocyte differentiation occurs at the pretranslational level.

Conservation of primary structure in the lipoyl-bearing and dihydrolipoyl dehydrogenase binding domains of mammalian branched-chain alpha-keto acid dehydrogenase complex: molecular cloning of human and bovine transacylase (E2) cDNAs

The subunit structures and conservation of the dihydrolipoyl transacylase (E2) components of bovine and human branched-chain alpha-keto acid dehydrogenase complexes were investigated by Western blotting, peptide sequencing, and cDNA cloning methods. Rabbit antiserum prepared against the sodium dodecyl sulfate (SDS) denaturated bovine E2 subunit recognized the inner E2 core, and the first hinge region of the E2 chain, but failed to react with the lipoyl-bearing domain as determined by Western blot analysis. The lack of antigenicity in the lipoyl-bearing domain was confirmed with antibodies directed against the native E2 component. A human E2 cDNA (1.6 kb) was isolated from a human liver cDNA library in lambda gt11 with a combination of the above anti-native and anti-SDS-denatured E2 immunoglobulin G's as a probe. The fidelity of the human E2 cDNA was established by nucleotide sequencing which showed the determined peptide sequences of the amino terminus and tryptic fragments of bovine E2. A bovine E2 cDNA (0.7 kb) was also isolated from a bovine liver cDNA library in lambda ZAP with the human E2 cDNA as a probe. Northern blot analysis using the human E2 cDNA probe showed that E2 mRNAs in bovine liver and human kidney mesangial cells are 3.3 and 4.6 kb in size, respectively. Primary structures derived from human and bovine E2 cDNAs show leader sequences including the initiator methionine and the homologous mature peptides consisting of complete lipoyl-bearing and dihydrolipoyl dehydrogenase (E3) binding domains and two hinge regions. In addition, the human E2 cDNA contains a portion of the inner E2 core sequence, a 3'-untranslated region, and a poly(A+) tail. Deduced amino acid sequences of the mammalian E2's were compared with those of Escherichia coli transacetylase and transsuccinylase and bovine kidney transacetylase. The results indicate a high degree of conservation in the sequence flanking the lipoyl-attachment site and in the E3-binding domain. Models are presented to discuss implications for the conserved structure-function relationship in the lipoyl-bearing and E3-binding domains of alpha-keto acid dehydrogenase complexes.

Subunit structure of the dihydrolipoyl transacylase component of branched-chain alpha-keto acid dehydrogenase complex from bovine liver. Mapping of the lipoyl-bearing domain by limited proteolysis

To characterize the lipoyl-bearing domain of the dihydrolipoyl transacylase (E2) component, purified branched-chain alpha-keto acid dehydrogenase complex from bovine liver was reductively acylated with [U-14C] alpha-ketoisovalerate in the presence of thiamin pyrophosphate and N-ethylmaleimide. Digestion of the modified complex with increasing concentrations of trypsin sequentially cleaved the E2 polypeptide chain (Mr = 52,000) into five radiolabeled lipoyl-containing fragments in the order of L1 (Mr = 28,000), L2 (Mr = 24,500), L3 (Mr = 21,000), L4 (Mr = 15,000) to L5 (Mr = 14,000) as determined by the autoradiography of sodium dodecyl sulfate-polyacrylamide gel. In addition, a lipoate-free inner E2 core consisting of fragment A (Mr = 26,000) and fragment B (Mr = 22,000) was produced. Fragment A contains the active site for transacylation reaction and fragment B is the subunit-binding domain. Fragment L5 and fragment B were stable and resistant to further tryptic digestion. Mouse antiserum against E2 reacted only with fragments L1, L2, and L3, and did not bind fragments L4, L5, A, and B as judged by immunoblotting analysis. The anti-E2 serum strongly inhibited the overall reaction catalyzed by the complex, but was without effect on the transacylation activity of E2. Measurement of incorporation of [1-14C]isobutyryl groups into the E2 subunit indicated the presence of 1 lipoyl residue/E2 chain. Based on the above data, a model is proposed in which the lipoyl-bearing domain is connected to the inner E2 core via a trypsin-sensitive hinge. The lipoyl-bearing domain contains five consecutive tryptic sites (L1 to L5), with the L1 site in the hinge region, and the L5 site next to the terminal lipoyl-binding sequence. An exposed and antigenic region is located between L1 and L4 tryptic sites of the lipoyl-bearing domain. The region accounts for about 24% of the E2 chain length. Binding of antibodies to this region probably impairs the mobility of the lipoyl-containing polypeptide, resulting in an interruption of the active-site interactions that are necessary for the overall reaction. The lack of antigenicity and resistance to tryptic digestion indicate a highly folded conformation for fragment L5, the limit polypeptide carrying the single lipoyl residue.

Purification and properties of a protein activator of phosphorylated branched-chain 2-oxo acid dehydrogenase complex

The protein activator of phosphorylated branched-chain 2-oxo acid dehydrogenase complex was purified greater than 1000-fold from extracts of rat liver mitochondria; the specific activity was greater than 1000 units/mg of protein (1 unit gives half-maximum re-activation of 10 munits of phosphorylated complex). Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis gave two bands (Mr 47700 and 35300) indistinguishable from the alpha- and beta-subunits of the branched-chain dehydrogenase component of the complex. On gel filtration (Sephacryl S-300), apparent Mr was 190000. This and other evidence suggests that activator protein is free branched-chain dehydrogenase; this conclusion is provisional until identical amino acid composition of the subunits has been demonstrated. Activator protein (i.e. free branched-chain dehydrogenase) was inhibited (up to 30%) by NaF, whereas branched-chain complex was not inhibited. There was no convincing evidence for interconvertible active and inactive forms of activator protein in rat liver mitochondria. Activator protein was detected in mitochondria from liver (ox, rabbit and rat) and kidney (ox and rat), but not in rat heart or skeletal-muscle mitochondria. In rat liver mitochondrial extracts, branched-chain complex sedimented with the mitochondrial membranes, whereas activator protein remained in the supernatant. Activator protein re-activated phosphorylated (inactive) particulate complex from rat liver mitochondria, but it did not activate dephosphorylated complex. Liver and kidney, but not muscle, mitochondria apparently contain surplus free branched-chain dehydrogenase, which is bound by the complex with lower affinity than is the branched-chain dehydrogenase intrinsic to the complex. It is suggested that this functions as a buffering mechanism to maintain branched-chain complex activity in liver and kidney mitochondria.

Multi-site phosphorylation in ox-kidney branched-chain 2-oxoacid dehydrogenase complex

Tryptic [32P]phosphopeptides were prepared from [32P]phosphorylated ox-kidney branched-chain complex and analysed by high-voltage paper electrophoresis at pH 1.9. In the maximally phosphorylated complex 3 tryptic [32P]phosphopeptides were identified (TA, TB, TC). RF-values relative to N6-dinitrophenyllysine were (mean +/- SEM for 25 obs.): TA, 1.53 +/- 0.03; TB, 1.07 +/- 0.02; TC, 0.65 +/- 0.01. Relative rates of phosphorylation were TA greater than TB greater than TC. Phosphorylation of TA reached a maximum when about 66% of the complex was inactivated. Phosphorylation of TB and TC was associated mainly with 66-95% inactivation of the complex.

The Australasian Reference Thromboplastin: II. Are corrected prothrombin ratios valid?

A comparative study was made of 2 thromboplastins, the Australasian Reference Thromboplastin, and Simplastin, in a large group of patients on long-term Warfarin therapy. 373 individual samples were obtained. Calibration constants were obtained for those patients with prothrombin ratios within the therapeutic range, and for those well outside the therapeutic range, and found to be different. Study of the relationship between the 2 thromboplastins indicates that comparability is linear only within a specified limited range of prothrombin ratios. At the two extreme ends the relationship is curved, suggesting a logarithmic relationship. Attention is drawn to the need of caution in interpretation of corrected ratios calculated on a linear relationship especially when the ratio is above 4.0 as this may have clinical implications.

The Australasian Reference Thromboplastin: I. A study in bleeding patients

The prothrombin time, activated partial thromboplastin time, thrombin time, and skin bleeding time, with assays of factors II, VII, IX, and X, platelet count, and liver function tests were performed on a group of patients receiving long term warfarin therapy. There were 17 bleeding patients and 13 non-bleeding patients. A study was made, using the Australasian Reference Thromboplastin and 2 other thromboplastic reagents in common use. The Australasian Reference Thromboplastin was shown to be more sensitive to the coumarin induced coagulation defect than rabbit brain thromboplastin, and hence of more value in preventing haemorrhagic complications. The level of factor II assayed by the one stage method was a useful independent indicator of the intensity of oral anticoagulation, and correlated well with the development of bleeding.