Evidence for the regulation of the branched chain alpha-keto acid dehydrogenase multienzyme complex by a phosphorylation/dephosphorylation mechanism

Mitochondrial Alpha-Keto Acid Dehydrogenase Complexes: Recent Developments on Structure and Function in Health and Disease

The present work delves into the enigmatic world of mitochondrial alpha-keto acid dehydrogenase complexes discussing their metabolic significance, enzymatic operation, moonlighting activities, and pathological relevance with links to underlying structural features. This ubiquitous family of related but diverse multienzyme complexes is involved in carbohydrate metabolism (pyruvate dehydrogenase complex), the citric acid cycle (α-ketoglutarate dehydrogenase complex), and amino acid catabolism (branched-chain α-keto acid dehydrogenase complex, α-ketoadipate dehydrogenase complex); the complexes all function at strategic points and also participate in regulation in these metabolic pathways. These systems are among the largest multienzyme complexes with at times more than 100 protein chains and weights ranging up to ~10 million Daltons. Our chapter offers a wealth of up-to-date information on these multienzyme complexes for a comprehensive understanding of their significance in health and disease.

Mitochondrial phosphoproteomics of mammalian tissues

Mitochondria are essential for several biological processes including energy metabolism and cell survival. Accordingly, impaired mitochondrial function is involved in a wide range of human pathologies including diabetes, cancer, cardiovascular, and neurodegenerative diseases. Within the past decade a growing body of evidence indicates that reversible phosphorylation plays an important role in the regulation of a variety of mitochondrial processes as well as tissue-specific mitochondrial functions in mammals. The rapidly increasing number of mitochondrial phosphorylation sites and phosphoproteins identified is largely ascribed to recent advances in phosphoproteomic technologies such as fractionation, phosphopeptide enrichment, and high-sensitivity mass spectrometry. However, the functional importance and the specific kinases and phosphatases involved have yet to be determined for the majority of these mitochondrial phosphorylation sites. This review summarizes the progress in establishing the mammalian mitochondrial phosphoproteome and the technical challenges encountered while characterizing it, with a particular focus on large-scale phosphoproteomic studies of mitochondria from human skeletal muscle.

Characterization of human phosphodiesterase 12 and identification of a novel 2'-5' oligoadenylate nuclease - The ectonucleotide pyrophosphatase/phosphodiesterase 1

The vertebrate 2-5A system is part of the innate immune response and central to cellular antiviral activities. Upon activation by viral double-stranded RNA, 5'-triphosphorylated, 2'-5'-linked oligoadenylate polyribonucleotides (2-5As) are synthesized by one of several 2'-5' oligoadenylate synthetases. The 2-5As bind and activate RNase L, an unspecific endoribonuclease, resulting in viral and cellular RNA decay. Given that most endogenous RNAs are degraded by RNase L, continued enzyme activity will eventually lead to cell growth arrest and cell death. This is averted, when 2-5As and their 5'-dephosphorylated forms, the so-called 2-5A core molecules, are cleaved and thus inactivated by 2'-5'-specific nuclease(s), e.g. phosphodiesterase 12, thereby turning RNase L into its latent form. In this study, we have characterized the human phosphodiesterase 12 in vitro focusing on its ability to degrade 2-5As and 2-5A core molecules. We have found that the enzyme activity is distributive and is influenced by temperature, pH and divalent cations. This allowed us to determine V(max) and K(m) kinetic parameters for the enzyme. We have also identified a novel 2'-5'-oligoadenylate nuclease; the human plasma membrane-bound ectonucleotide pyrophosphatase/phosphodiesterase 1, suggesting that 2-5A catabolism may be a multienzyme-regulated process.

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.

Effects of mitochondrial swelling and calcium on phosphate-activated glutaminase in pig renal mitochondria

The effects of mitochondrial swelling and calcium have been used to study the possible function of the glutamine transporter in regulating glutamine hydrolysis. Salt-induced swelling of pig renal mitochondria and an iso-osmotic mixed salt solution and swelling caused by reducing the osmolarity of the incubation medium, are accompanied by activation of glutamine hydrolysis. Regulation of the glutaminase activity by salt-induced mitochondrial swelling is likely to have physiological importance, similar to the regulation of hepatic glutaminase by changing the matrix volume, that has been described by others. 0.1-1.0 mM calcium stimulates glutamine hydrolysis and the calcium activation curve follows Michaelis-Menten kinetics. The calcium activation is reversible, it is unaffected by phosphate, high glutamine and mitochondrial calcium uptake, as well as by sonication and the activation is calmodulin independent. The calcium activation is additive to that of swelling. Similar to calcium, hypo-osmotic swelling mainly increases the apparent Vmax for glutamine, whereas the apparent Km is little changed, indicating that the effects are primarily on the phosphate-activated glutaminase itself rather than on the glutamine transporter. Furthermore, calcium which activates glutamine hydrolysis, inhibits glutamine uptake into the mitochondria and so does alanine having no effect on glutamine hydrolysis. Therefore, it is indicative that glutamine transport is not rate limiting for glutamine hydrolysis.

Metabolism of branched-chain amino acids in leg muscles from tail-cast suspended intact and adrenalectomized rats

Degradation of branched-chain amino acids was studied in muscles of unloaded hind limbs from rats subjected to six days of tail-cast suspension. The total production of 14CO2 from uniformly labeled 14C-leucine, isoleucine, or valine, and the fluxes through leucine aminotransferase and alpha-ketoisocaproate dehydrogenase, which were measured using L-1-14C-leucine, were generally greater in the soleus and extensor digitorum longus muscles of unloaded than of weight-bearing hind limbs. Adrenalectomy abolished any difference in flux through the aminotransferase, whereas the administration of cortisol to adrenalectomized animals restored the greater flux in the unloaded soleus muscle. Adrenalectomy partially diminished the greater flux through alpha-ketoisocaproate dehydrogenase in the unloaded soleus, whereas cortisol (2 mg/100 g body weight) treatment increased this difference. In the extensor digitorum longus, adrenalectomy abolished the differences in both enzyme fluxes due to hind limb suspension. In this muscle, cortisol treatment increased these fluxes to a similar extent in both weight-bearing and suspended, adrenalectomized animals so that the normal difference was not restored. These results suggest that leucine catabolism in hind limb muscles of suspended rats was influenced primarily by increased circulating glucocorticoid hormones, which are elevated twofold to fourfold in these animals.

Sub-micromolar concentrations of extramitochondrial Ca2+ stimulate the rate of citrulline synthesis by rat liver mitochondria

1. In the presence of physiological concentrations of Na+ and Mg2+, the rate of citrulline synthesis by isolated rat liver mitochondria respiring on a range of substrates was stimulated by up to 60% when the extramitochondrial Ca2+ concentration was raised from 130 pM to 770 nM. 2. Our findings suggest that hormonal stimulation of the urea cycle may be mediated by Ca2+.

Acute regulation of the branched-chain 2-oxo acid dehydrogenase complex by adrenaline and glucagon in the perfused rat heart

Rates of transamination and decarboxylation of [1-14C]leucine at a physiological concentration (0.1 mM) were measured in the perfused rat heart. In hearts from fasted rats, metabolic flux through the branched-chain 2-oxo acid dehydrogenase reaction was low initially, but increased gradually during the perfusion period. The increase in 14CO2 production was accompanied by an increase in the amount of active branched-chain 2-oxo acid dehydrogenase complex present in the tissue. In hearts from rats fed ad libitum, extractable branched-chain dehydrogenase activity was low initially, but increased rapidly during perfusion, and high rates of decarboxylation were attained within the first 10 min. Infusion of glucagon, adrenaline, isoprenaline, or adrenaline in the presence of phentolamine all produced rapid, transient, inhibition (40-50%) of the formation of 4-methyl-2-oxo[1-14C]pentanoate and 14CO2 within 1-2 min, but the specific radioactivity of 4-methyl-2-oxo[14C]pentanoate released into the perfusate remained constant. Glucagon and adrenaline infusion also resulted in transient decreases (16-24%) in the amount of active branched-chain 2-oxo acid dehydrogenase. In hearts from fasted animals, infusion for 10 min of adrenaline, phenylephrine, or adrenaline in the presence of propranolol, but not infusion of glucagon or isoprenaline, stimulated the rate of 14CO2 production 3-fold, and increased 2-fold the extractable branched-chain 2-oxo acid dehydrogenase activity. These results demonstrate that stimulation of glucagon or beta-adrenergic receptors in the perfused rat heart causes a transient inhibition of branched-chain amino acid metabolism, whereas alpha-adrenergic stimulation causes a slower, more sustained, enhancement of branched-chain amino acid metabolism. Both effects reflect interconversion of the branched-chain 2-oxo acid dehydrogenase complex between active and inactive forms. Also, these studies suggest that the concentration of branched-chain 2-oxo acid available for decarboxylation can be regulated by adrenaline and glucagon.

Acidosis, not azotemia, stimulates branched-chain, amino acid catabolism in uremic rats

To investigate branched-chain, amino acid metabolism (BCAA) in muscle in chronic renal failure (CRF), we studied rats with moderately severe uremia (PUN 110 approximately mg/dl) and spontaneous metabolic acidosis (bicarbonate, 19 +/- 1 mEq/liter). Plasma BCAA levels in CRF compared to pair-fed control rats were approximately 15% lower and muscle valine was 93 microM lower (P less than 0.05). BCAA metabolism was measured in incubated epitrochlearis muscles using L-[1-14C]valine or L-[1-14C]leucine in the presence and absence of insulin. BCAA decarboxylation was increased (P less than 0.05) and insulin-stimulated BCAA incorporation into protein was blunted (P less than 0.05) by CRF. Since we have found that metabolic acidosis, by itself, stimulates muscle branched-chain, ketoacid dehydrogenase activity, another group of CRF and control rats was given NaHCO3 which corrected the acidosis, but not the azotemia. BCAA decarboxylation in muscle was reduced in CRF rats given NaHCO3, and this was reflected in increased plasma and muscle BCAA concentrations. We conclude that in CRF, chronic metabolic acidosis stimulates BCAA decarboxylation in skeletal muscle and this could contribute to the reduced intra- and extracellular concentrations of BCAA. Correction of acidosis should be a goal of therapy in CRF, especially when dietary regimens restrict intake of BCAA.

Comparative development of the pyruvate dehydrogenase complex and citrate synthase in rat brain mitochondria

The enzyme activity of the pyruvate dehydrogenase complex (PDHC) was measured in mitochondria prepared from developing rat brain, before and after steady-state dephosphorylation of the E1 alpha subunit. A marked increase in dephosphorylated (fully activated) PDHC activity occurred between days 10 and 15 post partum, which represented approx. 60% of the difference in fully activated PDHC activity measured in foetal and adult rat brain mitochondria. There was no detectable change in the active proportion of the enzyme during mitochondrial preparation nor any qualitative alteration in the detectable catalytic and regulatory components of the complex, which might account for developmental changes in PDHC activity. The PDHC protein content of developing rat brain mitochondria and homogenates was measured by an enzyme-linked immunoadsorbent assay. The development of PDHC protein in both fractions agreed closely with the development of the PDHC activity. The results suggest that the developmental increase in PDHC activity is due to increased synthesis of PDHC protein, which is partly a consequence of an increase in mitochondrial numbers. However, the marked increase in PDHC activity measured between days 10 and 15 post partum is mainly due to an increase in the amount of PDHC per mitochondrion. The development of citrate synthase enzyme activity and protein was measured in rat brain homogenates and mitochondria. As only a small increase in citrate synthase activity and protein was detected in mitochondria between days 10 and 15 post partum, the marked increase in PDHC protein and enzyme activity may represent specific PDHC synthesis. As several indicators of acquired neurological competence become apparent during this period, it is proposed that preferential synthesis of PDHC may be crucial to this process. The results are discussed with respect to the possible roles played by PDHC in changes of respiratory-substrate utilization and the acquisition of neurological competence occurring during the development of the brain of a non-precocial species such as the rat.

Phosphorylation of branched-chain 2-oxo acid dehydrogenase complex in isolated adipocytes. Effects of 2-oxo acids

Isolated adipocytes from rat epididymal fat-pads were incubated with [32P]Pi, and intracellular phosphoproteins were then analysed by SDS/polyacrylamide-gel electrophoresis and autoradiography. A phosphorylated polypeptide of apparent Mr 46,000 was identified as the alpha-subunit of branched-chain 2-oxo acid dehydrogenase complex by immunoprecipitation using antiserum raised against the homogeneous E1 component of branched-chain 2-oxo acid dehydrogenase complex. Immunoprecipitation of this phosphoprotein is blocked in a competitive manner by purified branched-chain 2-oxo acid dehydrogenase complex. Peptide mapping of the isolated phosphoprotein indicates that two sites on the polypeptide are phosphorylated in the intact cells. Addition of branched-chain 2-oxo acids to the incubation medium causes diminution in the extent of labelling of both phosphorylation sites on the alpha-subunit, an effect presumably mediated via their known inhibitory action on branched-chain 2-oxo acid dehydrogenase kinase. These observations provide direct evidence for phosphorylation of branched-chain 2-oxo acid dehydrogenase complex in intact cells.

Regulation of the branched-chain 2-oxo acid dehydrogenase complex in hepatocytes isolated from rats fed on a low-protein diet

Hepatocytes isolated from rats fed on a chow diet or a low-protein (8%) diet were used to study the effects of various factors on flux through the branched-chain 2-oxo acid dehydrogenase complex. The activity of this complex was also determined in cell-free extracts of the hepatocytes. Hepatocytes isolated from chow-fed rats had greater flux rates (decarboxylation rates of 3-methyl-2-oxobutanoate and 4-methyl-2-oxopentanoate) than did hepatocytes isolated from rats fed on the low-protein diet. Oxidizable substrates tended to inhibit flux through the branched-chain 2-oxo acid dehydrogenase, but inhibition was greater with hepatocytes isolated from rats fed on the low-protein diet. 2-Chloro-4-methylpentanoate (inhibitor of branched-chain 2-oxo acid dehydrogenase kinase), dichloroacetate (inhibitor of both pyruvate dehydrogenase kinase and branched-chain 2-oxo acid dehydrogenase kinase) and dibutyryl cyclic AMP (inhibitor of glycolysis) were effective stimulators of branched-chain oxo acid decarboxylation with hepatocytes from rats fed on a low-protein diet, but had little effect with hepatocytes from rats fed on chow diet. Activity measurements indicated that the branched-chain 2-oxo acid dehydrogenase complex was mainly (96%) in the active (dephosphorylated) state in hepatocytes from chow-fed rats, but only partially (50%) in the active state in hepatocytes from rats fed on a low-protein diet. Oxidizable substrates markedly decreased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had much less effect in hepatocytes from chow-fed rats. 2-Chloro-4-methylpentanoate and dichloroacetate increased the activity state of the enzyme in hepatocytes from rats fed on a low-protein diet, but had no effect on the activity state of the enzyme in hepatocytes from chow-fed rats. The results indicate that protein starvation greatly increases the sensitivity of the hepatic branched-chain 2-oxo acid dehydrogenase complex to regulation by covalent modification.

Resolution of branched-chain oxo acid dehydrogenase complex of Pseudomonas aeruginosa PAO

Branched-chain oxo acid dehydrogenase was purified from Pseudomonas aeruginosa strain PAO with the objective of resolving the complex into its subunits. The purified complex consisted of four proteins, of Mr 36,000, 42,000, 49,000 and 50,000. The complex was resolved by heat treatment into the 49,000 and 50,000-Mr proteins, which were separated by chromatography on DEAE-Sepharose. The 49,000-Mr protein was identified as the E2 subunit by its ability to catalyse transacylation with a variety of substrates, with dihydrolipoamide as the acceptor. P. aeruginosa, like P. putida, produces two lipoamide dehydrogenases. One, the 50,000-Mr protein, was identified as the specific E3 subunit of branched-chain oxo acid dehydrogenase and had many properties in common with the lipoamide dehydrogenase LPD-val of P. putida. The second lipoamide dehydrogenase had Mr 54,000 and corresponded to the lipoamide dehydrogenase LPD-glc of P. putida. Fragments of C-terminal CNBr peptides of LPD-val from P. putida and P. aeruginosa corresponded closely, with only two amino acid differences over 31 amino acids. A corresponding fragment at the C-terminal end of lipoamide dehydrogenase from Escherichia coli also showed extensive homology. All three peptides had a common segment of eight amino acids, with the sequence TIHAHPTL. This homology was not evident in any other flavoproteins in the Dayhoff data base which suggests that this sequence might be characteristic of lipoamide dehydrogenase.

Regulation of oxidative decarboxylation of branched-chain 2-oxo acids in rat liver mitochondria

At 0.1 mM 2-oxo[1-14C]isocaproate or 2-oxo[1-14C]isovalerate plots of the reciprocal of the rate of 14CO2 formation by branched-chain 2-oxo acid dehydrogenase complex in mitochondria vs alpha-cyanocinamate concentration were linear up to high inhibitor concentrations, indicating that the monocarboxylate carrier-mediated transport was the rate-limiting step. At low (0.025 mM) concentration of 2-oxo[1-14C]isocaproate or 2-oxo[1-14C]isovalerate the 1/v vs I plots became nonlinear indicating that the branched-chain 2-oxo acid dehydrogenase activity determined the rate of 14CO2 formation. Inhibition of branched-chain 2-oxo acid dehydrogenase complex by clofibric acid or arsenite showed that at 0.1 mM 2-oxoisovalerate the activity of the complex became the rate-limiting step of the pathway. The availability of the 2-oxoisocaproate or 2-oxoisovalerate seems to affect the phosphorylation and the activity of the branched-chain 2-oxo acid dehydrogenase complex only at low, physiological concentrations of these substrates (less than 0.025 mM).

Activation of rat liver branched-chain 2-oxo acid dehydrogenase in vivo by glucagon and adrenaline

The activity of liver branched-chain 2-oxo acid dehydrogenase complex was measured in rats fed on low-protein diets and given adrenaline, glucagon, insulin or dibutyryl cyclic AMP in vivo. Administration of glucagon or adrenaline (200 micrograms/100 g body wt.) resulted in a 4-fold increase in the percentage of active complex. As with glucagon and adrenaline, treatment of rats with cyclic AMP (5 mg/100 g body wt.) resulted in marked activation of branched-chain 2-oxo acid dehydrogenase. Insulin administration (1 unit/100 g body wt.) also resulted in activation of enzyme; however, these effects were less than those observed with glucagon and adrenaline. In contrast with the results obtained with low-protein-fed rats, administration of adrenaline (200 micrograms/100 g body wt.) to rats fed with an adequate amount of protein resulted in only a modest (14%) increase in the activity of the complex. The extent to which these hormones activate branched-chain 2-oxo acid dehydrogenase appears to be correlated with their ability to stimulate amino acid uptake into liver.

Immunochemical comparison of lipoamide dehydrogenases from various sources and reactivity of various lipoamide dehydrogenases with rat heart pyruvate dehydrogenase-subcomplex

Lipoamide dehydrogenases from various sources were purified and their immunochemical properties were compared. Antibody against rat lipoamide dehydrogenase reacted with rat, human, pig, pigeon and frog enzymes, but not with enzymes from E. coli, yeast and Ascaris. Anti-Ascaris enzyme and anti-E. coli enzyme antibodies reacted with Ascaris and E. coli enzymes, respectively. The pyruvate dehydrogenase subcomplex, which consists of pyruvate dehydrogenase and lipoate acetyltransferase, was prepared by releasing the lipoamide dehydrogenase from rat heart pyruvate dehydrogenase complex by anti-lipoamide dehydrogenase antibody. Lipoamide dehydrogenases from various sources were added to rat pyruvate dehydrogenase subcomplex and the complex overall activity was measured. Each lipoamide dehydrogenase effectively recovered the overall activity of rat pyruvate dehydrogenase subcomplex to 80% of the original activity.

Interaction of short-chain and branched-chain fatty acids and their carnitine and CoA esters and of various metabolites and agents with branched-chain 2-oxo acid oxidation in rat muscle and liver mitochondria

Interaction of various compounds with the 14CO2 production from [1-14C]-labelled branched-chain 2-oxo acids was studied in intact rat quadriceps muscle and liver mitochrondria. In the absence of carnitine, CoA esters of short-chain and branched-chain fatty acids, CoA and acetyl-L-carnitine stimulated oxidation of 4-methyl-2-oxopentanoate and 3-methyl-2-oxobutanoate in muscle mitochondria. Octanoyl-L-carnitine inhibited oxidation of the latter, but stimulated that of the former substrate. Isovaleryl-L-carnitine was inhibitory with both substrates. Carnitine stimulates markedly 3-methyl-2-oxobutanoate oxidation in liver mitochondria at substrate concentrations higher than 0.1 mM, in contrast to 4-methyl-2-oxopentanoate oxidation. In the presence of carnitine, 3-methyl-2-oxobutanoate oxidation was inhibited in muscle and liver mitochondria by octanoate, octanoyl-L-carnitine and isovaleryl-L-carnitine. The latter ester and octanoyl-D-carnitine inhibited also 4-methyl-2-oxopentanoate oxidation in muscle mitochondria. Branched-chain 2-oxo acids inhibited mutaly their oxidation, except that 3-methyl-2-oxobutanoate did not inhibit 4-methyl-2-oxopentanoate oxidation in liver mitochondria. Their degradation products, isovalerate, 3-methylcrotonate, isobutyrate and 3-hydroxyisobutyrate inhibited to a different extent 2-oxo acid oxidation in liver mitochondria. The effect of CoA esters was studied in permeabilized and with cofactors reinforced mitochondria. Acetyl-CoA and isovaleryl-CoA inhibited only 3-methyl-2-oxobutanoate oxidation in muscle mitochondria. Octanoyl-CoA inhibited oxidation of both 2-oxo acids in muscle and 4-methyl-2-oxopentanoate oxidation in liver mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory effects of fatty acids on decarboxylation of leucine and 4-methyl-2-oxopentanoate in the perfused rat heart

The regulatory effects of fatty acids on the oxidative decarboxylation of leucine and 4-methyl-2-oxopentanoate were investigated in the isolated rat heart. Infusion of the long-chain fatty acid palmitate resulted in both an inactivation of the branched-chain 2-oxo acid dehydrogenase and an inhibition of the measured metabolic flux through this enzyme complex. Pyruvate addition also caused both an inactivation and an inhibition of the flux through the complex. On the other hand, the medium-chain fatty acid octanoate caused an activation of and a stimulation of flux through the branched-chain 2-oxo acid dehydrogenase when the perfusion conditions before octanoate addition maintained the enzyme complex in its inactive state. When the enzyme complex was activated before octanoate infusion, this fatty acid caused a significant inhibition of the flux through the branched-chain 2-oxo acid dehydrogenase reaction. Inclusion of glucose in the perfusion medium prevented the octanoate-mediated activation of the branched-chain 2-oxo acid dehydrogenase.

Purification and properties of branched-chain alpha-keto acid dehydrogenase phosphatase from bovine kidney

Branched-chain alpha-keto acid dehydrogenase (BCKDH) phosphatase was purified about 8000-fold from extracts of bovine kidney mitochondria. The highly purified phosphatase exhibited a molecular weight of approximately 460,000, as estimated by gel-permeation chromatography. Another form of the phosphatase, with an apparent molecular weight of approximately 230,000, was also detected under conditions of high dilution. In contrast to pyruvate dehydrogenase phosphatase, BCKDH phosphatase was active in the absence of divalent cations. BCKDH phosphatase was inactive toward 32P-labeled phosphorylase a, but exhibited approximately 10% maximal activity with 32P-labeled pyruvate dehydrogenase complex. BCKDH phosphatase activity was inhibited by GTP, GDP, ATP, ADP, UTP, UDP, CTP, and CDP. Half-maximal inhibition occurred at about 60, 200, 200, 400, 100, 250, 250, and 400 microM, respectively. These inhibitions were reversed completely by 2 mM Mg2+. GTP was replaceable by guanosine 5'-(beta, gamma-imido)triphosphate. GMP, AMP, UMP, CMP, NAD, and NADH showed little effect, if any, on BCKDH phosphatase activity at concentrations up to 1 mM. Heparin showed half-maximal inhibition at 2 micrograms/ml. This inhibition was only partially (30%) reversed by 2 mM Mg2+. CoA and various acyl-CoA compounds exhibited half-maximal inhibition at 150-300 microM. These inhibitions were not reversed by 2 mM Mg2+. BCKDH phosphatase activity was stimulated 1.5- to 3-fold by protamine, poly(L-lysine), and poly(L-arginine) at 3.6 micrograms/ml.

Valine metabolism in vivo: effects of high dietary levels of leucine and isoleucine

The short-term effects of feeding rats high levels of L-leucine or L-isoleucine on valine metabolism in vivo have been investigated. Consumption of a low-protein diet containing an additional 5% of leucine resulted in depression within one hour of the plasma concentrations of isoleucine, valine, alpha-keto-beta-methylvalerate, and alpha-ketoisovalerate. Concurrently with these changes in blood branched-chain amino acids and branched-chain ketoacids was a rapid increase (51%) in whole-body L-[1-14C]-valine oxidation. Studies with intragastrically administered leucine solutions indicated that the depressions in blood concentrations of valine occurred over the same time period as the stimulation in valine oxidation. In contrast, consumption of a low-protein diet containing an additional 5% of isoleucine had no significant effect on the plasma concentrations of leucine, valine, and alpha-ketoisocaproate; a significant (P less than 0.01) depression in the plasma concentration of alpha-ketoisovalerate was observed three hours after the diet containing excess isoleucine had been consumed. In contrast to the results obtained with excess leucine, consumption of excess isoleucine had no significant effect on the rate of valine oxidation in vivo. As part of an effort to explain the leucine-induced depletion of plasma valine and stimulation of valine oxidation, liver and muscle branched-chain aminotransferase and liver branched-chain ketoacid dehydrogenase activities were measured. Consumption of excess leucine had no significant effect on either muscle or liver aminotransferase activities, but was associated with a greater than two-fold increase in hepatic dehydrogenase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Multi-site phosphorylation of branched-chain 2-oxoacid dehydrogenase complex within mitochondria isolated from rat liver, kidney and heart

The alpha subunit of the E1 component of branched-chain 2-oxoacid dehydrogenase complex becomes rapidly phosphorylated in rat liver, kidney and heart mitochondria incubated in the presence of succinate and [32P]phosphate. Peptide mapping of tryptic digests of the phosphorylated alpha subunit indicates that 3 distinct sites are phosphorylated, as has been reported previously by us for phosphorylation in vitro of highly purified complex.