Phosphorylation sites and inactivation of branched-chain alpha-ketoacid dehydrogenase isolated from rat heart, bovine kidney, and rabbit liver, kidney, heart, brain, and skeletal muscle

Regulation of Mitochondrial Function by Epac2 Contributes to Acute Inflammatory Hyperalgesia

G_αs-coupled receptors signaling through cAMP provide a key mechanism for the sensitization of nociceptive sensory neurons, and the cAMP effector Epac has been implicated in the transition from acute to chronic pain. Epac exerts its effects through Rap1 and protein kinase C (PKC). To identify targets of Epac-PKC signaling in sensory neurons of the mouse dorsal root ganglion (DRG), we profiled PKC substrate proteins phosphorylated in response to the activation of Epac with the proinflammatory prostaglandin E2 (PGE2). A prominent Epac-dependent phospho-protein band induced by PGE2 was identified by mass spectrometry as the mitochondrial enzyme pyruvate dehydrogenase (Pdha1). In dissociated DRG from both males and females, the recruitment of Pdha1 to phospho-protein fractions was rapidly induced by PGE2 and prevented by selective inhibition of Epac2. Epac activation increased mitochondrial respiration, consistent with an increase in Pdha1 function mediated by Epac2. Hindpaw injection of PGE2 induced heat hyperalgesia in males and females, but Pdha1 phosphorylation occurred only in males. Hyperalgesia was attenuated in males but not in females by systemic inhibition of Epac2, and also by a mitochondrial membrane potential uncoupler, dinitrophenol, supporting a role for mitochondrial regulation in acute hyperalgesia. These findings identify a mechanism for the regulation of mitochondrial function by Epac2 that contributes to acute inflammatory hyperalgesia in male mice. Systemic administration of the cyclooxygenase 2 inhibitor celecoxib suppressed both PGE2-induced heat hyperalgesia and Pdha1 phosphorylation in DRG of males but not females, suggesting that prostaglandin synthesis within the DRG mediates the phosphorylation of Pdha1 in response to hindpaw insult.SIGNIFICANCE STATEMENT There has been extensive investigation of mitochondrial dysfunction as a causative factor in neuropathic pain disorders. In contrast, results reported here implicate enhanced mitochondrial function as a contributing factor in the development of acute inflammatory hyperalgesia. We describe a mechanism in which Epac2 activation by prostaglandin receptors leads to phosphorylation of pyruvate dehydrogenase and an increase in mitochondrial respiration in peripheral sensory neurons. Although Epac2 activation leads to Pdha1 (pyruvate dehydrogenase) phosphorylation in dissociated neurons from mice of both sexes, induction of this pathway in vivo by hindpaw insult is restricted to males and appears to require intraganglionic prostaglandin synthesis. These findings support a model in which G_s-coupled receptor modulation of mitochondrial function promotes acute nociceptive signaling and inflammatory hyperalgesia.

An Increase in Liver Polyamine Concentration Contributes to the Tryptophan-Induced Acute Stimulation of Rat Hepatic Protein Synthesis

Tryptophan has a unique role as a nutritional signaling molecule that regulates protein synthesis in mouse and rat liver. However, the mechanism underlying the stimulating actions of tryptophan on hepatic protein synthesis remains unclear. Proteomic and metabolomic analyses were performed to identify candidate proteins and metabolites likely to play a role in the stimulation of protein synthesis by tryptophan. Overnight-fasted rats were orally administered L-tryptophan and then sacrificed 1 or 3 h after administration. Four differentially expressed protein spots were detected in rat liver at 3 h after tryptophan administration, of which one was identified as an ornithine aminotransferase (OAT) precursor. OAT is the main catabolic enzyme for ornithine, and its expression was significantly decreased by tryptophan administration. The concentration of ornithine was increased in the liver at 3 h after tryptophan administration. Ornithine is a precursor for polyamine biosynthesis. Significantly increased concentrations of polyamines were found in the liver at 3 h after administration of tryptophan. Additionally, enhanced hepatic protein synthesis was demonstrated by oral administration of putrescine. We speculate that the increase in ornithine level through suppression of OAT expression by tryptophan administration may lead to accelerated polyamine synthesis, thereby promoting protein synthesis in the liver.

Adipose transplant for inborn errors of branched chain amino acid metabolism in mice

Liver transplantation appears to be quite beneficial for treatment of maple syrup urine disease (MSUD, an inherited disorder of branched chain amino acid metabolism); however, there is a limited availability of donor livers worldwide and the first year costs of liver transplants are quite high. Recent studies have suggested that intact adipose tissue, already widely used in reconstructive surgery, may have an underappreciated high capacity for branched chain amino acid (BCAA) metabolism. Here we examined the potential for adipose tissue transplant to lower circulating BCAAs in two models of defective BCAA metabolism, BCATm and PP2Cm [branched chain keto acid dehydrogenase complex (BCKDC) phosphatase] knockout (KO) mice. After 1-2g fat transplant, BCATm and PP2Cm KO mice gained or maintained body weight 3weeks after surgery and consumed similar or more food/BCAAs the week before phlebotomy. Transplant of fat into the abdominal cavity led to a sterile inflammatory response and nonviable transplanted tissue. However when 1-2g of fat was transplanted subcutaneously into the back, either as small (0.1-0.3g) or finely minced pieces introduced with an 18-ga. needle, plasma BCAAs decreased compared to Sham operated mice. In two studies on BCATm KO mice and one study on PP2Cm KO mice, fat transplant led to 52-81% reductions in plasma BCAAs compared to baseline plasma BCAA concentrations of untreated WT type siblings. In PP2Cm KO mice, individual BCAAs in plasma were also significantly reduced by fat transplant, as were the alloisoleucine/Phe ratios. Therefore, subcutaneous fat transplantation may have merit as an adjunct to dietary treatment of MSUD. Additional studies are needed to further refine this approach.

Quantitative mitochondrial phosphoproteomics using iTRAQ on an LTQ-Orbitrap with high energy collision dissociation

With the use of iTRAQ labeling and mass spectrometry on an LTQ-Orbitrap with HCD capability, we assessed relative changes in protein phosphorylation in the mitochondria upon physiological perturbation. As a reference reaction, we monitored the well-characterized regulation of pyruvate dehydrogenase (PDH) activity via phosphorylation/dephosphorylation by pyruvate dehydrogenase kinase/pyruvate dehydrogenase phosphatase in response to dichloroacetate, de-energization and Ca2+. Relative quantification of phosphopeptides of PDH-E1alpha subunit from porcine heart revealed dephosphorylation at three serine sites (Ser231, Ser292 and Ser299). Dephosphorylation at Ser292 (i.e., the inhibitory site) with DCA correlated with an activation of PDH activity as previously reported, consistent with our de-energization data. Calcium also dephosphorylated (i.e., activated) PDH, thus, confirming calcium activation of PDP. With this approach, we successfully monitored other phosphorylation sites of mitochondrial proteins including adenine nucleotide translocase, malate dehydrogenase and mitochondrial creatine kinase. Among them, four proteins exhibited phosphorylation changes with these physiological stimuli: (1) BCKDH-E1alpha subunit increased phosphorylation at Ser337 with DCA and de-energization; (2) apoptosis-inducing factor phosphorylation was elevated at Ser345 with calcium; (3) ATP synthase F1 complex alpha subunit and (4) mitofilin dephosphorylated at Ser65 and Ser264 upon de-energization. This screening validated the iTRAQ/HCD technology as a method for functional quantitation of mitochondrial protein phosphorylation as well as providing insight into the regulation of mitochondria via phosphorylation.

Animal models of maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited aminoacidopathy resulting from dysfunction of the branched-chain keto acid dehydrogenase (BCKDH) complex. This disease is currently treated primarily by dietary restriction of branched-chain amino acids (BCAAs). However, dietary compliance is often challenging. Conversely, liver transplantation significantly improves outcomes, but donor organs are scarce and there are high costs and potential risks associated with this invasive procedure. Therefore, improved treatment options for MSUD are needed. Development of novel treatments could be facilitated by animal models that accurately mimic the human disease. Animal models provide a useful system in which to explore disease mechanisms and new preclinical therapies. Here we review MSUD and currently available animal models and their corresponding relevance to the human disorder. Using animal models to gain a more complete understanding of the pathophysiology behind the human disease may lead to new or improved therapies to treat or potentially cure the disorder.

Obesity-related elevations in plasma leucine are associated with alterations in enzymes involved in branched-chain amino acid metabolism

Elevations in branched-chain amino acids (BCAAs) in human obesity were first reported in the 1960s. Such reports are of interest because of the emerging role of BCAAs as potential regulators of satiety, leptin, glucose, cell signaling, adiposity, and body weight (mTOR and PKC). To explore loss of catabolic capacity as a potential contributor to the obesity-related rises in BCAAs, we assessed the first two enzymatic steps, catalyzed by mitochondrial branched chain amino acid aminotransferase (BCATm) or the branched chain alpha-keto acid dehydrogenase (BCKD E1alpha subunit) complex, in two rodent models of obesity (ob/ob mice and Zucker rats) and after surgical weight loss intervention in humans. Obese rodents exhibited hyperaminoacidemia including BCAAs. Whereas no obesity-related changes were observed in rodent skeletal muscle BCATm, pS293, or total BCKD E1alpha or BCKD kinase, in liver BCKD E1alpha was either unaltered or diminished by obesity, and pS293 (associated with the inactive state of BCKD) increased, along with BCKD kinase. In epididymal fat, obesity-related declines were observed in BCATm and BCKD E1alpha. Plasma BCAAs were diminished by an overnight fast coinciding with dissipation of the changes in adipose tissue but not in liver. BCAAs also were reduced by surgical weight loss intervention (Roux-en-Y gastric bypass) in human subjects studied longitudinally. These changes coincided with increased BCATm and BCKD E1alpha in omental and subcutaneous fat. Our results are consistent with the idea that tissue-specific alterations in BCAA metabolism, in liver and adipose tissue but not in muscle, may contribute to the rise in plasma BCAAs in obesity.

Production and characterization of murine models of classic and intermediate maple syrup urine disease

BACKGROUND

Maple Syrup Urine Disease (MSUD) is an inborn error of metabolism caused by a deficiency of branched-chain keto acid dehydrogenase. MSUD has several clinical phenotypes depending on the degree of enzyme deficiency. Current treatments are not satisfactory and require new approaches to combat this disease. A major hurdle in developing new treatments has been the lack of a suitable animal model.

METHODS

To create a murine model of classic MSUD, we used gene targeting and embryonic stem cell technologies to create a mouse line that lacked a functional E2 subunit gene of branched-chain keto acid dehydrogenase. To create a murine model of intermediate MSUD, we used transgenic technology to express a human E2 cDNA on the knockout background. Mice of both models were characterized at the molecular, biochemical, and whole animal levels.

RESULTS

By disrupting the E2 subunit gene of branched-chain keto acid dehydrogenase, we created a gene knockout mouse model of classic MSUD. The homozygous knockout mice lacked branched-chain keto acid dehydrogenase activity, E2 immunoreactivity, and had a 3-fold increase in circulating branched-chain amino acids. These metabolic derangements resulted in neonatal lethality. Transgenic expression of a human E2 cDNA in the liver of the E2 knockout animals produced a model of intermediate MSUD. Branched-chain keto acid dehydrogenase activity was 5-6% of normal and was sufficient to allow survival, but was insufficient to normalize circulating branched-chain amino acids levels, which were intermediate between wildtype and the classic MSUD mouse model.

CONCLUSION

These mice represent important animal models that closely approximate the phenotype of humans with the classic and intermediate forms of MSUD. These animals provide useful models to further characterize the pathogenesis of MSUD, as well as models to test novel therapeutic strategies, such as gene and cellular therapies, to treat this devastating metabolic disease.

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Leucine has been shown to stimulate adipose tissue protein synthesis in vivo as well as leptin secretion, protein synthesis, hyper-plastic growth, and tissue morphogenesis in in vitro experiments using freshly isolated adipocytes. Recently, others have proposed that leucine oxidation in the mitochondria may be required to activate the mammalian target of rapamycin (mTOR), the cytosolic Ser/Thr protein kinase that appears to mediate some of these effects. The first irreversible and rate-limiting step in leucine oxidation is catalyzed by the branched-chain alpha-keto acid dehydrogenase (BCKD) complex. The activity of this complex is regulated acutely by phosphorylation of the E1alpha-subunit at Ser293 (S293), which inactivates the complex. Because the alpha-keto acid of leucine regulates the activity of BCKD kinase, it has been suggested as a potential target for leucine regulation of mTOR. To study the regulation of BCKD phosphorylation and its potential link to mTOR activation, a phosphopeptide-specific antibody recognizing this site was developed and characterized. Phospho-S293 (pS293) immunoreactivity in liver corresponded closely to diet-induced changes in BCKD activity state. Immunoreactivity was also increased in TREMK-4 cells after the induction of BCKD kinase by a drug-inducible promoter. BCKD S293 phosphorylations in adipose tissue and gastrocnemius (which is mostly inactive in vivo) were similar. This suggests that BCKD complex in epididymal adipose tissue from food-deprived rats is mostly inactive (unable to oxidize leucine), as is the case in muscle. To begin to test the leucine oxidation hypothesis of mTOR activation, the dose-dependent effects of orally administered leucine on acute activation of S6K1 (an mTOR substrate) and BCKD were compared using the pS293 antibodies. Increasing doses of leucine directly correlated with increases in plasma leucine concentration. Phosphorylation of S6K1 (Thr389, the phosphorylation site leading to activation) in adipose tissue was maximal at a dose of leucine that increased plasma leucine approximately threefold. Changes in BCKD phosphorylation state required higher plasma leucine concentrations. The results seem more consistent with a role for BCKD and BCKD kinase in the activation of leucine metabolism/oxidation than in the activation of the leucine signal to mTOR.

Induction of expression of branched-chain aminotransferase and alpha-keto acid dehydrogenase in rat tissues during lactation

This study was designed to determine the effect of lactation and weaning on the gene expression of branched-chain aminotransaminase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD) in different tissues of the lactating rat. BCAT activity increased in mammary tissue during lactation and was 6-fold higher than in virgin rats. This increase was associated with an increase in protein levels measured by immunoblot analysis, and with an increase in BCAT mitochondrial (BCATm) mRNA concentration. Twenty-four hours after weaning, BCAT activity, protein concentration, and mRNA levels in the dam decreased. BCAT activity, protein enzyme levels, and BCATm mRNA concentration in muscle were higher in weaning rats than in lactating rats. BCAT cytosolic (BCATc) mRNA was not expressed in mammary tissue, and there was no BCATc enzyme detected by Western blot in any physiological state. Mammary tissue BCKD activity increased and was active (dephosphorylated) during the lactation period. The level of enzyme also increased and the mRNA level for the E2 subunit in mammary tissue was 10-fold higher than the virgin values. Hepatic enzyme activity increased during weaning, and this was associated with the protein level and with the mRNA level of the E2 subunit. Muscle BCKD activity and protein content were the lowest of all tissues, and the E2 subunit mRNA level was barely detected by Northern blot analysis. The results suggest gene regulation of the two main catabolic enzymes of the branched-chain amino acid metabolism during lactation.

Mechanism of activation of branched-chain alpha-keto acid dehydrogenase complex by exercise

Branched-chain alpha-keto acid dehydrogenase (BCKDH) complex catalyzes the committed step of branched-chain amino acid catabolism, and its activity is regulated by the phosphorylation-dephosphorylation cycle. BCKDH kinase is responsible for inactivation of the complex by phosphorylation. In the present study, we examined acute exercise on the activity state of the complex as well as the amounts of bound and free forms of the kinase in rat liver and skeletal muscle. Acute exercise activated the complex in association with a decrease in the bound form of kinase in both liver and muscle. The free form of kinase in both tissues was slightly increased but the total amount of the kinase was not affected by acute exercise. The protein amount ratio of bound kinase to E1beta component of the complex was much higher in muscle than in the liver of rats, reflecting the low activity state of the complex in muscle. These results suggest that the amount of the bound kinase plays an important role in regulation of the activity state of the complex. We propose that the alteration in the amount of bound BCKDH kinase is a short-term regulatory mechanism for determining the activity of BCKDH complex.

Controlled overexpression of BCKD kinase expression: metabolic engineering applied to BCAA metabolism in a mammalian system

A common metabolic complication of human disease is uncontrolled muscle protein breakdown or cachexia, which occurs in patients with chronic diseases such as cancer, AIDS, renal failure, and diabetes. Increased branched-chain amino acid catabolism is implicated as causal and has stimulated the investigation of methods to regulate the metabolism of these amino acids. Here we demonstrate doxycycline-controlled overexpression of a branched-chain alpha-ketoacid dehydrogenase (BCKD) kinase transgene in mammalian cell culture. This kinase functions to inactivate the BCKD complex by phosphorylation, thus preventing the catabolism of these essential, regulatory metabolites. In this study, doxycycline treatment leads to a 10-fold increase in BCKD kinase protein. The transgene-generated kinase is rapidly incorporated within mitochondria and functions correctly to inactivate the BCKD complex. The maximum reduction in basal BCKD activity achieved was 94%. Unexpectedly, total BCKD activity was also decreased by kinase overexpression despite no observable change in expression of the BCKD catalytic proteins. These results demonstrate that artificial regulation of branched-chain amino acid metabolism is possible through the controlled overexpression of a single endogenous enzyme and suggest the feasibility of clinical applications.

Studies on the regulation of the mitochondrial alpha-ketoacid dehydrogenase complexes and their kinases

Five mitochondrial protein kinases, all members of a new family of protein kinases, have now been identified, cloned, expressed as recombinant proteins, and partially characterized with respect to catalytic and regulatory properties. Four members of this unique family of eukaryotic protein kinases correspond to pyruvate dehydrogenase kinase isozymes which regulate the activity of the pyruvate dehydrogenase complex, an important regulatory enzyme at the interface between glycolysis and the citric acid cycle. The fifth member of this family corresponds to the branched-chain alpha-ketoacid dehydrogenase kinase, an enzyme responsible for phosphorylation and inactivation of the branched-chain alpha-ketoacid dehydrogenase complex, the most important regulatory enzyme in the pathway for the disposal of branched-chain amino acids. At least three long-term control mechanisms have evolved to conserve branched chain amino acids for protein synthesis during periods of dietary protein insufficiency. Increased expression of the branched-chain alpha-ketoacid dehydrogenase kinase is perhaps the most important because this leads to phosphorylation and nearly complete inactivation of the liver branched-chain alpha-ketoacid dehydrogenase complex. Decreased amounts of the liver branched-chain alpha-ketoacid dehydrogenase complex secondary to a decrease in liver mitochondria also decrease the liver's capacity for branched-chain keto acid oxidation. Finally, the number of E1 subunits of the branched-chain alpha-ketoacid dehydrogenase complex is reduced to less than a full complement of 12 heterotetramers per complex in the liver of protein-starved rats. Since the E1 component is rate-limiting for activity and also the component of the complex inhibited by phosphorylation, this decrease in number further limits overall enzyme activity and makes the complex more sensitive to regulation by phosphorylation in this nutritional state. The branched-chain alpha-ketoacid dehydrogenase kinase phosphorylates serine 293 of the E1 alpha subunit of the branched-chain alpha-ketoacid dehydrogenase complex. Site-directed mutagenesis of amino acid residues surrounding serine 293 reveals that arginine 288, histidine 292 and aspartate 296 are critical to dehydrogenase activity, that histidine 292 is critical to binding the coenzyme thiamine pyrophosphate, and that serine 293 exists at or in close proximity to the active site of the dehydrogenase. Alanine scanning mutagenesis of residues in the immediate vicinity of the phosphorylation site (serine 293) indicates that only arginine 288 is required for recognition of serine 293 as a phosphorylation site by the branched-chain alpha-ketoacid dehydrogenase kinase. Phosphorylation appears to inhibit dehydrogenase activity by introducing a negative charge directly into the active site pocket of the E1 dehydrogenase component of the branched-chain alpha-ketoacid dehydrogenase complex. A model based on the X-ray crystal structure of transketolase is being used to predict residues involved in thiamine pyrophosphate binding and to help visualize how phosphorylation within the channel leading to the reactive carbon of thiamine pyrophosphate inhibits catalytic activity. The isoenzymes of pyruvate dehydrogenase kinase differ greatly in terms of their specific activities, kinetic parameters and regulatory properties. Chemically-induced diabetes in the rat induces significant changes in the pyruvate dehydrogenase kinase isoenzyme 2 in liver. Preliminary findings suggest hormonal control of the activity state of the pyruvate dehydrogenase complex may involves tissue specific induced changes in expression of the pyruvate dehydrogenase kinase isoenzymes.

Regulation of branched-chain ketoacid dehydrogenase flux by extracellular pH and glucocorticoids

In muscles of rats with metabolic acidosis, branched-chain alpha-ketoacid dehydrogenase (BCKAD) activity is increased. Potential stimulatory signals include acidemia and/or glucocorticoids. It is unclear whether the signal(s) increases BCKAD activity by changing the activation state of the enzyme or by increasing the amount of enzyme. To separate the influences of extracellular pH and glucocorticoids on leucine catabolism, maximal BCKAD flux and the activation state (the ratio of basal to total flux) were measured in two cell types: 1) cells that do not express glucocorticoid receptors and 2) cells stably transfected to express glucocorticoid receptors. Acidification (pH 6.95) increased 1) the activation state from 67.2% at pH 7.4 to 82.8% at pH 6.95, 2) maximal BCKAD flux by 50%, and 3) the BCKAD subunit contents in both cell types (57, 410, and 270% for E2, E1 alpha, and E1 beta, respectively). Dexamethasone increased the BCKAD activation state from 67.2 to 82.3% in cells expressing glucocorticoid receptors, whereas dexamethasone plus acidification increased the activation state to 98%. The time course of stimulation by dexamethasone was slower than that by acidification. These results demonstrate that BCKAD is differentially regulated by extracellular pH and glucocorticoids.

Effects of insulin on the regulation of branched-chain alpha-keto acid dehydrogenase E1 alpha subunit gene expression

Alterations in dietary intake, especially of protein, may produce changes in the hepatic levels of the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex. The possible role of insulin in the regulation of these observed changes in hepatic capacity for BCKAD expression was therefore examined. Steady-state RNA levels encoding three of the subunits, E1 alpha, E1 beta and E2, increased by 2-4-fold in the livers of mice starved for 3 days, a known hypoinsulinaemic state. In contrast, the levels of E1 beta and E2, but not E1 alpha, RNA were decreased when mice were fed 0% protein diets compared with the levels observed in mice fed standard (23%) or higher protein isocaloric diets. BCKAD subunit protein levels under these conditions changed co-ordinately even though the changes in RNA were not co-ordinate. The effects of hormonal changes that might be associated with these dietary changes were examined, using the rodent hepatoma cell line H4IIEC3. In these cells, the levels of E1 alpha protein and mRNA were significantly depressed in the presence of insulin. In contrast, the levels of E1 beta and E2 RNAs were not decreased by insulin. The half-lives of the E1 alpha and E2 RNAs were determined to be quite long, from 13 to 18 h, with insulin having no dramatic overall effect on the half-lives determined over 24 h. Therefore, it is likely that insulin directly affects the transcription of the E1 alpha gene rather than RNA stability in exerting its negative regulatory effect. This effect is specific to the E1 alpha subunit. The differences in BCKAD subunit RNA levels observed under various nutritional and developmental conditions may therefore be the result of the differential effects of insulin and other hormones on the multiple regulatory mechanisms modulating BCKAD subunit expression.

Influence of subunit transcript and protein levels on formation of a mitochondrial multienzyme complex

Constitutive expression of nuclear genes encoding mitochondrial proteins raises the question of whether these proteins are present in similar amounts in mitochondria of different tissues. We report that amounts of a single multienzyme complex can vary on a per mitochondrion basis depending on the number of mitochondria per cell. Human branched-chain alpha-keto acid dehydrogenase (BCKD) expression is used as a paradigm in these studies. Expression is compared and contrasted in HepG2 and DG75 cells in which mitochondrial content is twofold higher in the hepatocarcinoma line than in the lymphoblastoid line. Per cell, BCKD activity is equal in the two cells types, but BCKD protein concentration per mitochondrion is twofold higher in DG75 cells. Steady-state mRNA levels do not appear to be directly related to amounts of protein in the two cell lines. To test whether one subunit is limiting in formation of complex, overexpression of each BCKD subunit was elicited by plasmid transfection of the DG75 cells. Only overexpression of the beta-subunit of the decarboxylase component induced more BCKD activity without apparent increase in mRNA for the other endogenously expressed subunits. This implies that free BCKD subunits exist in a cell and can be recruited into an active complex when the limiting subunit becomes available.

Roles of amino acid residues surrounding phosphorylation site 1 of branched-chain alpha-ketoacid dehydrogenase (BCKDH) in catalysis and phosphorylation site recognition by BCKDH kinase

Branched-chain alpha-ketoacid dehydrogenase is regulated by reversible phosphorylation of serine 293 (site 1) on the E1 alpha subunit. Alanine-scanning mutagenesis was used to examine the roles of residues surrounding serine 293 in catalysis by the dehydrogenase and in substrate recognition by branched-chain alpha-ketoacid dehydrogenase kinase. Alanine substitution of serine 293 resulted in a 10-fold increased Km for alpha-ketoisovalerate, a less increased (2.8-fold) Km for alpha-ketoisocaproate, but no change in Vmax or the Km for thiamine pyrophosphate. Alanine substitutions of arginine 288, histidine 292, and aspartate 296, residues highly conserved among alpha-ketoacid dehydrogenases, resulted in inactive enzymes. Each of the inactive E1 mutants bound to the E2 core subunit with equal affinity as wild-type E1, and each produced circular dichroism spectra identical to that of wild-type E1. Two mutations, H292A and S293E, abolished the ability of E1 apoenzyme to reconstitute with thiamine pyrophosphate. Each alanine-substituted E1 was phosphorylated at site 1 by branched-chain alpha-ketoacid dehydrogenase kinase with similar rates, with the exception of the R288A mutant, which displayed no detectable phosphorylation. Thiamine pyrophosphate inhibited the phosphorylation of all mutant enzymes with the exception of H292A, the mutant E1 that did not bind thiamine pyrophosphate.

Purification of active E1 alpha 2 beta 2 of Pseudomonas putida branched-chain-oxoacid dehydrogenase

Active E1 component of Pseudomonas putida branched-chain-oxoacid dehydrogenase was purified from P. putida strains carrying pJRS84 which contains bkdR (encoding the transcriptional activator) and bkdA1 and bkdA2 (encoding the alpha and beta subunits). Expression was inducible, however, 45-, 39- and 37-kDa proteins were produced instead of the expected 45-kDa and 37-kDa proteins. The 45-kDa protein was identified as E1 alpha and the 37-kDa and 39-kDa proteins were identified as separate translational products of bkdA2 by their N-terminal sequences. The N-terminal amino acid of the 39-kDa protein was leucine instead of methionine. The 45-, 39- and 37-kDa proteins were also produced in wild-type P.putida. Translation of bkdA1 and bkdA2 from an Escherichia coli expression plasmid produced only 45-kDa and 39-kDa proteins, with N-terminal methionine on the 39-kDa protein. The insertion of guanine residues 5' to the first ATG of bkdA2 did not affect expression of E1 beta in P. putida including the N-terminal leucine which appears to eliminate the possibility of ribosome jumping. The Z-average molecular mass of the E1 component was determined by sedimentation equilibrium to be 172 +/- 9 kDa compared to a calculated value of 166 kDa for the heterotetramer and a Stokes radius of 5.1 nm. E1 alpha Ser313, which is homologous to the phosphorylated residue of rat liver E1 alpha, was converted to alanine resulting in about a twofold increase in Km, but no change in Kcat. S315A and S319A mutations had no effect on Km or Kcat indicating that these residues do not play a major part in catalysis of E1 alpha beta 2.

Cloning and sequencing of a cluster of genes encoding branched-chain alpha-keto acid dehydrogenase from Streptomyces avermitilis and the production of a functional E1 [alpha beta] component in Escherichia coli

A cluster of genes encoding the E1 alpha, E1 beta, and E2 subunits of branched-chain alpha-keto acid dehydrogenase (BCDH) of Streptomyces avermitilis has been cloned and sequenced. Open reading frame 1 (ORF1) (E1 alpha), 1,146 nucleotides long, would encode a polypeptide of 40,969 Da (381 amino acids). ORF2 (E1 beta), 1,005 nucleotides long, would encode a polypeptide of 35,577 Da (334 amino acids). The intergenic distance between ORF1 and ORF2 is 73 bp. The putative ATG start codon of the incomplete ORF3 (E2) overlaps the stop codon of ORF2. Computer-aided searches showed that the deduced products of ORF1 and ORF2 resembled the corresponding E1 subunit (alpha or beta) of several prokaryotic and eukaryotic BCDH complexes. When these ORFs were overexpressed in Escherichia coli, proteins of about 41 and 34 kDa, which are the approximate masses of the predicted S. avermitilis ORF1 and ORF2 products, respectively, were detected. In addition, specific E1 [alpha beta] BCDH activity was detected in E. coli cells carrying the S. avermitilis ORF1 (E1 alpha) and ORF2 (E1 beta) coexpressed under the control of the T7 promoter.

Eukaryotes have "two-component" signal transducers

The eukaryotic proteins discussed above share a number of amino acid sequence features with prokaryotic members of the two-component signal transducer family. ETR1 and Sln1 each contain putative HPK and receivers, while Skn7 has similarity to receivers only. The other proteins--phytochromes, BCKDH kinase and the dr6 gene product--appear to be distant relatives of the two-component family because they have less-conserved motifs, although even biochemically defined prokaryotic family members can lack conserved motifs (Parkinson and Kofoid, 1992). The eukaryotic and prokaryotic proteins also show similar diversity with respect to the arrangements of the components; transmitters and receivers can lie within the same protein (for example, ETR1 and Sln1 in eukaryotes and BarA and LemA in prokaryotes) or receivers can lie on a separate protein (for example, Skn7 in yeast and CheY in bacteria (Stock et al., 1985)). The new eukaryotic members of the two-component family raise a number of questions, especially since their functions are not yet clearly defined. The identities of the signals transduced are unknown, except for the red light signal for phytochrome. Ethylene is presumably the signal for ETR1, but this has not been demonstrated directly. It also remains to be seen whether eukaryotic two-component modules are functionally homologous to the prokaryotic two-component systems; eukaryotic modules could be utilized in uncharacterized ways, possibly in combination with different signalling components. Among the eukaryotic proteins discussed here, BCKDH kinase has the best-understood function, yet its substrate is not an orthodox response regulator, and moreover, the substrate is phosphorylated at serine residues.

Deficiency of the E1 beta subunit in the branched-chain alpha-keto acid dehydrogenase complex due to a single base substitution of the intron 5, resulting in two alternatively spliced mRNAs in a patient with maple syrup urine disease

A patient with maple syrup urine disease (MSUD) associated with a E1 beta subunit deficiency of the branched-chain alpha-keto acid dehydrogenase (BCKDH) complex was investigated at the molecular level. The defect responsible for the deficiency of the E1 beta subunit protein was identified by analysis of cDNA and genomic DNA by polymerase chain reaction. Total RNA isolated from lymphoblastoid cells was transcribed into cDNA and amplified using a set of primers located within exon 3 and exon 9 of the E1 beta gene. Agarose gel electrophoresis of cDNA amplification products revealed two shortened bands as well as a faint band of normal size. Nucleotide sequencing of the shortened cDNA amplification products showed that sequences corresponding to exon 5 and both exons 5 and 6 were absent. Nucleotide sequencing of the proband's amplified genomic DNA corresponding to this region of the E1 beta gene revealed a single base substitution from G to T of the invariant GT dinucleotides at 5' splice site of the intron 5. Analysis of family members using primer-specified restriction map modification showed that the patient is homozygous for this mutation. We postulate that this mutation leads to the skipping of either exon 5 or both exons 5 and 6, thus producing two shortened E1 beta mRNA. The percentage of normal and two shortened transcripts was estimated to be 9, 71 and 20%, respectively. To our best knowledge, this is the first documented example of exon skipping in the E1 beta gene as the cause of MSUD and the novel mutation of the invariant G at the 5' splice site which results in two alternatively spliced mRNA due to the skipping of the preceding exon as well as both preceding and following exon.

Heterogeneity of mutations in maple syrup urine disease (MSUD): screening and identification of affected E1 alpha and E1 beta subunits of the branched-chain alpha-keto-acid dehydrogenase multienzyme complex

Maple syrup urine disease (MSUD) is an autosomal recessive disease caused by a deficiency in subunits of the branched-chain alpha-keto-acid dehydrogenase complex (BCKDH). To characterize the mutations present in five patients with MSUD (four classic and one intermediate), three-step analyses were established: (1), identification of the involved subunit by complementation analysis using three different cell lines derived from homozygotes having E1 alpha, E1 beta or the E2 mutant gene; (2), screening for a mutation site in cDNA of the corresponding subunit by RT-PCR-SSCP and (3), mutant analysis by sequencing the amplified cDNA fragment. Four single-base missense mutations, R115W, Q146K [corrected], A209T and I282T, were detected in the E1 alpha subunit. A single-base missense mutation H156R and three frame-shift mutations to generate stop codons downstream, including an 11-bp deletion of the tandem repeat in exon 1, a single-base (T) deletion and a single-base (G) insertion, were identified in the E1 beta subunit gene. All except one (11-bp deletion in E1 beta (Nobukuni, Y., Mitsubuchi, H., Akaboshi, I., Indo, Y., Endo, F., Yoshioka, A. and Matsuda, I. (1991) J. Clin. Invest. 87, 1862-1866)) were novel mutations. The sites of amino-acid substitution were all conserved in other species. Thus, mutations causing MSUD are heterogenous.

Branched-chain 2-oxo acid dehydrogenase complex activation by tetanic contractions in rat skeletal muscle

Branched-chain 2-oxo acid dehydrogenase complex in rat skeletal muscle was activated by muscle contractions elicited by electrical stimulation. This activation was attributed to dephosphorylation of the phosphorylated enzyme complex, and the total enzyme activity was not altered by muscle contractions. The activation of the enzyme complex occurred in the muscle of the electrically stimulated leg, but not in the muscle of the non-stimulated (control) leg, indicating that blood components are not involved in the mechanism of the enzyme activation in the muscle. Adenine nucleotides, branched-chain amino and 2-oxo acids and lactate in the muscle were determined as possible factors modulating the enzyme complex activity through inhibition of branched-chain 2-oxo acid dehydrogenase kinase activity. The profile of enzyme activation induced by muscle contractions was different from the alteration of the adenine nucleotide concentrations but was similar to the alteration of the concentrations of branched-chain amino and 2-oxo acids in the muscle. The lactate concentration in the stimulated muscle was elevated 3-5-fold during the contractions, indicating intracellular acidification. Previous studies have shown that the 2-oxo acid derived from leucine is a potent inhibitor of the kinase. These results suggest that intracellular branched-chain 2-oxo acids increased by muscle contractions accumulate in the mitochondria due to exercise-induced acidification of the muscle cell, resulting in activation of branched-chain 2-oxo acid dehydrogenase complex by inhibition of the kinase.

Molecular cloning of the branched-chain alpha-keto acid dehydrogenase kinase and the CoA-dependent methylmalonate semialdehyde dehydrogenase

The complete amino acid sequence of rat liver CoA-dependent methylmalonate semialdehyde dehydrogenase, the enzyme responsible for the oxidative decarboxylation of malonate- and methylmalonate semialdehydes to acetyl- and propionyl-CoA in the distal portions of the valine and pyrimidine catabolic pathways, has been deduced from overlapping cDNAs obtained by screening a lambda gt11 library with nondegenerate oligonucleotide probes synthesized according to PCR-amplified portions coding for the N-terminal amino acid sequence of the enzyme. Although unique because of its requirement for coenzyme A, the methylmalonate semialdehyde dehydrogenase clearly belongs to the aldehyde dehydrogenase superfamily of enzymes. Quantitation of mRNA and protein levels indicates tissue-specific expression of methylmalonate semialdehyde dehydrogenase. A large increase in expression of methylmalonate semialdehyde dehydrogenase occurs during 3T3-L1 preadipocyte differentiation into adipocytes. The complete amino acid sequence of rat liver branched-chain alpha-ketoacid dehydrogenase kinase, the enzyme responsible for phosphorylation and inactivation of the branched-chain alpha-ketoacid dehydrogenase complex, was deduced from a cDNA cloned by a procedure similar to that described above for the methylmalonate semialdehyde dehydrogenase. Expression of the cDNA in E. coli yielded a protein that phosphorylated and inactivated the branched-chain alpha-ketoacid dehydrogenase complex. Very little sequence similarity between branched-chain alpha-ketoacid dehydrogenase kinase and other eukaryotic protein kinases could be identified. However, a high degree of similarity within subdomains characteristic of prokaryotic histidine protein kinases was apparent. Thus, this first mitochondrial protein kinase to be cloned appears closer, evolutionarily, to the prokaryotic histidine protein kinases than eukaryotic ser/thr protein kinases.

Possible relationship between coding recognition amino acid sequence motif or residue(s) and post-translational chemical modification of proteins

1. The "code-sequence" of N-glycosylation site(s), the amino acids located around O-glycosylation site(s), the sequence motifs of several kinases, the sequence motifs of--sulfation, amidation, isoprenylation, myristoylation, palmitoylation and N-acetylation, Aspartic and Asparagine hydroxylation-site, gamma-carboxyglutamate domain, phosphopantetheine attachment site etc. are extensively listed, compared to those reported by "PROSITE" Computer Screen Center and discussed. 2. The structural aspects of protein-DNA recognition are quoted as discussion and conclusion.

Oxidation of leucine by Leishmania donovani

The metabolism of leucine by Leishmania donovani was investigated. Washed promastigotes were incubated with [1-14C]- or [U-14C]leucine or [1-14C]alpha-ketoisocaproate (KIC) and 14CO2 release was measured. The amount of KIC-derived acetyl-CoA oxidized in the citric acid cycle was computed. Promastigotes from mid-stationary phase cultures oxidized each of these labeled substrates less rapidly than cells from late log phase cultures, and significantly less acetyl-CoA derived from KIC oxidation was oxidized in the citric acid cycle. Glucose was a stronger inhibitor than was acetate of CO2 formation in the citric acid cycle in log phase promastigotes, but the reverse was observed in cells from mid-stationary phase. Alanine also inhibited leucine catabolism, but glutamate had little effect. Acute hypo-osmotic stress did not affect leucine catabolism, but hyper-osmotic stress caused appreciable inhibition of leucine oxidation. Cells grown under hypo- or hyper-osmotic conditions showed no changes in the effects of hypo- or hyper-osmotic stress on leucine catabolism, i.e. L. donovani is not an osmoconformer with respect to leucine metabolism. Leucine utilization in L. donovani was insensitive to a number of drugs that affect leucine metabolism in mammalian cells, indicating that the leucine pathway in L. donovani is not regulated in the same manner as in mammalian cells.

Purification and comparative study of the kinases specific for branched chain alpha-ketoacid dehydrogenase and pyruvate dehydrogenase

Rat heart branched chain alpha-ketoacid dehydrogenase kinase (BCKDH kinase) and pyruvate dehydrogenase kinase (PDH kinase) were purified from their respective complexes to apparent homogeneity. BCKDH kinase consisted of one subunit with molecular weight 44,000-45,000 Da, whereas PDH kinase consisted of two subunits with molecular weight 48,000 Da (alpha) and 45,000 Da (beta) as previously shown for the bovine kidney enzyme (Stepp et al., 1983, J. Biol. Chem. 258, 9454-9458). Proteolysis maps of BCKDH kinase and the two subunits of PDH kinase were different, suggesting that all subunits are different entities. The alpha subunit of the rat heart PDH kinase could be cleaved selectively by chymotrypsin with concomitant loss of kinase activity, as previously shown for the bovine kidney enzyme, suggesting that the catalytic activity of PDH kinase resides in the alpha subunit. The beta subunit appeared to be a different entity unique to the PDH kinase. Both kinases exhibited marked substrate specificity toward their respective complexes and would not inactivate heterologous complexes. The kinases possessed slightly different substrate specificity toward histones. BCKDH kinase preferentially phosphorylated histones in the order f1 greater than f2B much greater than f2A much greater than f3. The relative order for PDH kinase was the same, but f2A and f3 were considerably better substrates than they were for BCKDH kinase. These observations suggest that the kinases have different requirements for the structure of the protein at their phosphorylation sites.

Decarboxylation of branched-chain alpha-ketoacids in hepatocytes from alloxan-diabetic rats. The effect of insulin

The flux through branched-chain alpha-ketoacid dehydrogenase and the activity of the branched-chain alpha-ketoacid dehydrogenase complex were measured in hepatocytes isolated from fed, starved and alloxan diabetic rats. The highest rate of branched-chain alpha-ketoacid oxidation was found in hepatocytes isolated from starved rats, slightly lower in those from fed rats, and significantly lower in diabetic hepatocytes. The amount of the active form of branched-chain alpha-ketoacid dehydrogenase was only slightly diminished in diabetic hepatocytes, whereas the flux through the dehydrogenase was inversely correlated with the rate of endogenous ketogenesis. The same was observed in hepatocytes isolated from starved rats when branched-chain alpha-ketoacid oxidation was measured in the presence of added oleate. In both cases the diminished flux through the dehydrogenase, restored by a short preincubation of hepatocytes with insulin, was paralleled by a decrease of fatty acid-derived ketogenesis. The significance of these findings is discussed in relation to the role of insulin in branched-chain alpha-ketoacid oxidation in liver of diabetic rats.

Purification and partial characterization of branched-chain alpha-ketoacid dehydrogenase kinase from rat liver and rat heart

Branched-chain alpha-ketoacid dehydrogenase kinase was purified to homogeneity from rat liver and rat heart. The initial step was the purification of rat liver and heart branched-chain alpha-ketoacid dehydrogenase complex with high kinase activity by a modification of a method described previously. Preservation of high kinase activity during purification of the complex required the presence of fresh dithiothreitol throughout the procedure. The kinase was released from the complex by oxidation of dithiothreitol with potassium ferricyanide and purified by high-speed centrifugation, immunoadsorption chromatography, and DEAE-Sephacel chromatography. Both kinase preparations gave only one polypeptide band with a molecular weight of 44,000 on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Phosphorylation and inactivation of the branched-chain alpha-ketoacid dehydrogenase complex by the purified kinase was inhibited by alpha-chloroisocaproate and dichloroacetate, established inhibitors of the phosphorylation of the branched-chain alpha-ketoacid dehydrogenase complex. The kinase did not exhibit autophosphorylation and does not correspond to the same protein as pyruvate dehydrogenase kinase. The kinase phosphorylated histone (type II-S), but this reaction was slow relative to the phosphorylation of the branched-chain alpha-ketoacid dehydrogenase complex and was not inhibited by alpha-chloroisocaproate.

Protein kinase recognition sequence motifs

Protein kinases play a crucial role in the regulation of many cellular processes. They alter the functions of their target proteins by phosphorylating specific serine, threonine and tyrosine residues. Identification of phosphorylation site sequences and studies with corresponding model peptides have provided clues to how these important enzymes recognize their substrate proteins. This knowledge has made it possible to identify potential sites of phosphorylation in newly sequenced proteins as well as to construct specific model substrates and inhibitors.

Cloning and sequence analysis of the genes encoding the alpha and beta subunits of the E1 component of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus

A 4175-bp EcoRI fragment of DNA that encodes the alpha and beta chains of the pyruvate dehydrogenase (lipoamide) component (E1) of the pyruvate dehydrogenase multienzyme complex of Bacillus stearothermophilus has been cloned in Escherichia coli. Its nucleotide sequence was determined. Open reading frames (pdhA, pdhB) corresponding to the E1 alpha subunit (368 amino acids, Mr 41,312, without the initiating methionine residue) and E1 beta subunit (324 amino acids, Mr 35,306, without the initiating methionine residue) were identified and confirmed with the aid of amino acid sequences determined directly from the purified polypeptide chains. The E1 beta gene begins just 3 bp downstream from the E1 alpha stop codon. It is followed, after a longer gap of 73 bp, by the start of another but incomplete open reading frame that, on the basis of its known amino acid sequence, encodes the dihydrolipoyl acetyltransferase (E2) component of the complex. All three genes are preceded by potential ribosome-binding sites and the gene cluster is located immediately downstream from a region of DNA showing numerous possible promoter sequences. The E1 alpha and E1 beta subunits of the B. stearothermophilus pyruvate dehydrogenase complex exhibit substantial sequence similarity with the E1 alpha and E1 beta subunits of pyruvate and branched-chain 2-oxo-acid dehydrogenase complexes from mammalian mitochondria and Pseudomonas putida. In particular, the E1 alpha chain contains the highly conserved sequence motif that has been found in all enzymes utilizing thiamin diphosphate as cofactor.

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.

Regulation of the branched-chain alpha-ketoacid dehydrogenase and elucidation of a molecular basis for maple syrup urine disease

The hepatic branched-chain alpha-ketoacid dehydrogenase complex plays an important role in regulating branched-chain amino acid levels. These compounds are essential for protein synthesis but toxic if present in excess. When dietary protein is deficient, the hepatic enzyme is converted to the inactive, phosphorylated state to conserve branched-chain amino acids for protein synthesis. When dietary protein is excessive, the enzyme is in the active, dephosphorylated state to commit the excess branched-chain amino acids to degradation. Inhibition of protein synthesis by cycloheximide, even when the animal is starving for dietary protein, results in activation of the hepatic branched-chain alpha-ketoacid dehydrogenase complex to prevent accumulation of branched-chain amino acids. Likewise, the increase in branched-chain amino acids caused by body wasting during starvation and uncontrolled diabetes is blunted by activation of the hepatic branched-chain alpha-ketoacid dehydrogenase complex. The activity state of the complex is regulated in the short term by the concentration of branched-chain alpha-ketoacids (inhibitors of branched-chain alpha-ketoacid dehydrogenase kinase) and in the long term by alteration in total branched-chain alpha-ketoacid dehydrogenase kinase activity. cDNAs have been cloned and the primary structure of the mature proteins deduced for the E1 alpha subunit of the human and rat liver branched-chain alpha-ketoacid dehydrogenase complex. The cDNA and protein sequences are highly conserved for the two species. Considerable sequence similarity is also apparent between the E1 alpha subunits of the human branched-chain alpha-ketoacid dehydrogenase complex and the pyruvate dehydrogenase complex. Maple syrup urine disease is caused by an inherited deficiency in the branched-chain alpha-ketoacid dehydrogenase complex. The molecular basis of one maple syrup urine disease family has been determined for the first time. The patient was found to be a compound heterozygote, inheriting an allele encoding an abnormal E1 alpha from the father, and an allele which is not expressed from the mother. The only known animal model for the disease (Polled Hereford cattle) has also been characterized. The mutation in these animals introduces a stop codon in the leader peptide of the E1 alpha subunit, resulting in premature termination of translation. Two thiamine responsive patients have been studied. The deduced amino acid sequences of the mature E1 alpha subunit and its leader sequence were normal, suggesting that the defect in these patients must exist in some other subunit of the complex. 3-Hydroxyisobutyrate dehydrogenase and methylmalonate-semialdehyde dehydrogenase, two enzymes of the valine catabolic pathway, were purified from liver tissue and characterized.(ABSTRACT TRUNCATED AT 400 WORDS)

Phylogenetic comparisons of the branched-chain alpha-ketoacid dehydrogenase complex

1. Antibodies against the E1b and E2b components of bovine branched-chain alpha-ketoacid (BCKA) dehydrogenase (BCKAD) complex completely inhibited BCKA oxidation in mammalian and avian mitochondria. BCKA oxidation by salmonid mitochondria was less affected and the enzyme from Pseudomonas putida was unaffected. 2. In rodents, anti-E1b E2b IgG inhibited oxidation of all three BCKA in a similar dose-dependent manner: oxidation of alpha-ketobutyrate and alpha-keto-y-methiolbutyrate was also partially inhibited. 3. Except for the salmonid BCKAD, a similar Mr for the E2b and E1b alpha proteins was observed in these species. 4. After digestion with V-8 protease similar immunoreactive peptides were observed for the human and rodent complex.

cDNA cloning of the E1 alpha subunit of the branched-chain alpha-keto acid dehydrogenase and elucidation of a molecular basis for maple syrup urine disease

We have cloned cDNAs encoding human and rat liver BCKDH E1 alpha subunits and deduced the primary structure of the mature protein. The sequences of the cDNA and protein are highly conserved between the two species. Significant sequence similarity has also been found between human BCKDH and PDH E1 alpha subunits. We have studied the molecular basis of MSUD by determining the enzyme activity and levels of BCKDH protein and mRNA, and by enzymatic amplification and sequencing of BCKDH E1 alpha-specific mRNA, from an MSUD patient and his parents. Different mutant alleles were identified in the two parents. The patient was a compound heterozygote, inheriting an allele encoding an abnormal E1 alpha from the father and an allele containing a defect in regulation from the mother. Our results demonstrate that a case of MSUD was caused by structural and regulatory mutations involving the E1 alpha subunit.

Evidence for both a regulatory mutation and a structural mutation in a family with maple syrup urine disease

Maple syrup urine disease (MSUD) results from a deficiency of branched chain alpha-ketoacid dehydrogenase (BCKDH). We have studied the etiology of MSUD by determining the enzyme activity, protein, and mRNA levels of BCKDH in fibroblasts from a classic MSUD patient and his parents. By enzymatic amplification of the patient's mRNA followed by cloning and DNA sequencing, we have identified a T to A transversion that alters a tyrosine to an asparagine at residue 394 of the E1 alpha subunit. Amplification of both mRNA and genomic DNA, in combination with allele-specific oligonucleotide hybridization, demonstrated that the father was heterozygous for this mutant allele. The mother was homozygous for the allele encoding the normal Tyr394, but expressed only about half of the normal level of mRNA and protein. The patient was genetically heterozygous for this altered allele, although only the abnormal allele was expressed as mRNA. We conclude that the patient was a compound heterozygote, inheriting an allele encoding an abnormal E1 alpha from the father, and an allele from the mother containing a cis-acting defect in regulation which abolished the expression of one of the E1 alpha alleles. Our results revealed for the first time that a case of MSUD was caused by structural and regulatory mutations involving the E1 alpha subunit.