Regulation of Brain Pyruvate Dehydrogenase Multienzyme Complex

LE JOURNAL CANAD1EN DES SCIENCES NEUROLOGIQUES

T. T. NGO AND A. BARBEAU

SUMMARY: A number of excellent and REACTION MECHANISM distinct enzymes: (1) pyruvate de- comprehensive reviews on various as The oxidative decarboxylation of carboxylaste (Ei) (also called pyru pects of pyruvate dehydrogenase mul pyruvate to form acetyl CoA and vate dehydrogenase); (2) dihyd- tienzyme complex have been written re CO2 at the expense of NAD is rolipoate transacetylase (E2); and (3) cently. The purpose of the present review dihydrolipoate dehydrogenase (E3). is to summarize briefly the reaction catalyzed by pyruvate dehyd mechanism and the regulation of this rogenase (PDH). In all organisms These enzymes catalyze the follow enzyme. Emphasis is put on the most studied, PDH is a multienzyme ing reactions (reactions 1 to 5) in a recent literature not covered by previous complex. In bacteria, it consists of 3 |sequential manner (Table I). reviews. Particular attention is also paid to the regulation of brain pyruvate de TABLE I hydrogenase multienzyme complex, since a number of patients with (1) CH -C-COOH + TPP-E:., > CH,-C-TPP-E + neuromuscular diseases, such as 1 C0„ Friedreich's ataxia, show a decreased 1 ' i \ 1 rate of pyruvate oxidation. OH 0 I II (2) CH3-C-TPP-E1 + (Lip S2)-E2 ^ (O^-C- S-Lip SH)-E2 + TPP-Ej^ RESUME: Un grand nombre de revues H completes sur les differents aspects du complexe multienzymatique de la pyru 0 0 vate deshydrogenase (PDH) ont ete 11 II recemment publies. Le but du present (3) (CH3-C-S-Lip SH)-E2 + CoA SH -} (Lip S2)-E2 + CH3-C-S-CoA travail sera de resumer certains points nouveaux concernant le mecanisme de la reaction el de la regulation de ce complexe enzymatique dont (4) Lip(SH)2-E2 + Ox.(FAD)-E3 (Lip S2)-E2 + red. (FAD)-E I'importance grandissante apparait dans I'etude de plusieurs maladies mus- culaires, dont Vataxie de Friedreich, qui toutes demontrent un ralentissement du taux d'oxydation du pyruvate. (5) Red. (FAD)-E + NAD -^ ox. (FAD)-E3 + NADH + H

0 O II II (6) Net reaction: CH -C-COOH + CoA SH + NAD CH3-C-S CoA

+ C02 + NADH + H Ei catalyzes reactions 1 and 2, E2 pyruvate dehydrogenase multien catalyzes reaction 3, E3 catalyzes zyme complex (Reed, 1960, 1974; reactions 4 and 5. E2 forms the core Hucho, 1975; Denton et al., 1975). of the complex, and Ei and E3 are attached to E2. The lipoyl group is The detailed chemical mechanism bound covalently to E2 via an amide of the pyruvate dehydrogenase linkage to the £ -amino group of a catalyzed reactions having been re lysyl residue in E2. This attachment viewed by Hucho (1975), only a very From the Department of Neurobiology, Clinical Research Institute of Montreal. provides a flexible arm, permitting brief survey will be given here. A the lipoyl group to rotate between schematic representation of the Reprint requests to: Dr. A. Barbeau, Clinical the catalytic centers of the three Research Institute of Montreal, 110 Pine Avenue reaction sequence of PDH is shown West, Montreal, Quebec, Canada H2W 1R7. associated enzymes which form the in Figure 1.

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Ei catalyzes 2 reactions: 1) the decarboxylation of pyruvate which is initiated by its attachment to the 2-position of the thiazole ring of TPP. The loss of CO2 leads to hydroxyethyl-TPP. Magnesium ion is the co-factor of Ei; 2) the oxida tion of the hydroxyethyl group and transfer of the resulting acetyl group to lipoic acid of E2. Thus, in addition to the decarboxylating activity, Ei also possesses dehydrogenase and transferase activities. The reaction catalyzed by Ei is irreversible. The reaction sequence catalyzed by Ei can be summarized as: E2 catalyzes the transfer of the acetyl group of the dihydrolipoic acid to CoA SH. This reaction is reversible. E3 catalyzes the regeneration of active E2 making it possible for the Figure I—Sequence of the reaction catalyzed by pyruvate dehydrogenase multienzyme next catalytic cycle of the complex. m* i—^~R Regeneration of E2 occurs by the complex (PDH). TPP= thiamine pyrophosphate; s-s = oxidized lipoamide: s s = + transfer of two electrons from dihyd H H reduced lipoamide. CoA SH = coenzyme A; FAD=flavin adenine dinucleotide; rolipoic acid to NAD, a reaction in adeninNAD =e nicotinamiddinucleotidee ;adenin Ei = epyruvat dinucleotidee decarboxylase; NADH=; reduceE2 :dihydrolipoamidd form of nicotinamide transe- which two molecules of NAD, one acetylase; E3 = dihydrolipoamide dehydrogenase. molecule of FAD, and one disulfide group of the protein take part. Thus, functionally, E3 can be depicted as: 0 0 II II = CH3- C-C-OH ^l ^S +C02

PYRUVATE HO I

0 11 These chemical evidences of the REACTION A CH -C-S 3 reaction mechanism suggest that the ACETYL- over-all reaction catalyzed by pyru DIHYDROUPOATE vate dehydrogenase proceeds via a three-site "ping-pong" mechanism. Indeed, kinetic studies of Tsai, Burgett and Reed (1973) and of Ngo HS HS and Barbeau (J. Neurochem., in + HSCoA ^=±CH3-C-S-CoA + press, 1978) have provided evidence CH3-C-S HS that both bovine kidney and rat brain pyruvate dehydrogenase multien ACETYL- COENZYME A ACETYL CoA DIHYDRO- zyme complexes catalyze the reac DIHYDROUPOATE LIPOATE tion via a three-site "ping-pong" REACTION B mechanism.

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METABOLIC REGULATION 3. The acetyl CoA formed from ox ganisms differ from that of unicellu Pyruvate occupies a strategic pos idative decarboxylatation of lar organisms. The regulatory ition in the metabolism of both car pyruvate can be used for the start mechanisms that have evolved in bohydrates and lipids. Any enzyme ing material in the biosynthesis of bacteria are geared to achieve a max that catalyzes the metabolism of fatty acids, prostaglandins and imal rate of cell proliferation consis pyruvate is expected to be under steroidal hormones. tent with available nutritional sources. In contrast the individual strict metabolic regulation. The 4. Pyruvate supplies the carbon cell of a multicellular organism more complexity of the control skeleton for the de novo synthesis often divides slowly, and is variable mechanism arises from the position of alanine and cysteine. in degree of organisational com of pyruvate and pyruvate metaboliz plexity and in the type a specific ing enzyme in energy metabolism. The apparent activity of an en function it carries out. Such cells are This enzyme is at the crossroads of zyme can be regulated in many associated with both similar and dis several metabolic routes. Pyruvate ways. Enzyme activity can be mod similar cells into tissues and organs, is formed in glycolysis from glucose, ulated by extracellular agents such functioning as an integrated whole in as hormones or their second mes from lactate, and from amino acids the protection and propagation of sengers, i.e., cyclic nucleotides. In- such as alanine and cysteine. Con specialized cells. In bacteria, the re tracellularly, an enzyme activity can currently, it is the starting point of moval process can involve dilution be controlled by the following pro four important metabolic routes during phases of rapid growth. In cesses: (Figure 2): animal tissues, where little cellular 1. Pyruvate can be oxidatively de A) Coarse control mechanism division takes place, the process of graded to acetyl CoA and NADH A coarse control for enzyme activ protein degradation becomes in which finally, through the trica- ity means a long term regulation of creasingly significant as a means of boxylic acid cycle and via oxida enzyme activity by changing the removing unneeded metabolic tive phosphorylation, can gener concentration of enzyme. The en machinery, and as a means for con ate ATP. zyme level of cells can be increased trolling enzyme levels. 2. Pyruvate can also be carboxy- by induction (derepression) or de The interaction between the lated to form oxolacetate and thus creased by repression. The proces synthesis and degradation of specific can form the starting point of ses of enzyme induction and repres enzymes in controlling enzyme gluconeogenesis. sion in the cells of multicellular or- levels in animal tissues is reflected in a general comparison of differences GLUCOSE in the time course of enzyme induc tion in animal tissues and bacteria (Figure 3). In bacteria the total en * zyme activity increases, following the introduction of an inducer. Simi ->- PHOSPHOENOLPYRUVATE larly, in animal cells the level of enzyme can be increased by indue- \ ALANINE, CYSTEINE PYRUVATE LACTATE -IHDUCER ATP N^C02 >- /, t- 7 kN ACETYL CoA -*- FATTY ACIDS ^~ 1 \ ADP o ! N «* 1 Ul \ \ y // N N>l- // \ Uzl / / \ OXALACETATE *****' CITRATE / / \ / / v~ / / (- / / \y TIME Figure 5—Time courses of enzyme in Figure 2—Pathways of pyruvate metabolism showing the strategic position of pyru duction in bacteria (solid line) and vate at the crossroads of carbohydrate and lipid metabolisms. animal cells (broken line).

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ers such as hormones, substrate, or both pyruvate and oxoglutarate de ATP+0.5 ADP changes in diet. However, in con hydrogenase multienzyme com EC = trast to the sustained level of the plexes (Blass et al., 1976; Barbeau et ATP + ADP+AMP induced enzymes in bacteria, in al., 1976). Furthermore, we have ob The concept of energy charge as a mammalian cells the induced en served that rat brain pyruvate de regulatory parameter was formu zyme activity returns to a basal level hydrogenase multienzyme complex lated by Atkinson (1968). It stated along an exponential time course is inactivated by pyridoxal phos that ATP forming systems, i.e., once the inducer is removed phate, a coenzymatically active form reactions leading to the synthesis of (Schimke, 1969; Changeux, 1965). of Vitamin Be. In addition, we also ATP or ATP utilizing systems, i.e., In E. coli the synthesis of the observed that the putative neuro biosynthetic reactions using ATP as pyruvate dehydrogenase multien transmitter, taurine, strongly acti the energy sources, would respond zyme complex is subject to genetic vates the pyridoxal phosphate inac to energy balance in the cell in terms regulation. It can be induced by tivated enzyme. The mechanisms of of all the adenylate compounds pyruvate. The genetic control of this pyridoxal phosphate inactivation rather than to any single adenylate multienzyme complex in mammalian and taurine reactivation of the inac compound. In the cell, the total aden cells in not clearly understood. Dif tivated enzyme are currently being ylate pool (AMP + ADP + ATP) is ficulties in obtaining appropriate investigated. constant over short time periods. mutants of mammalian cells have Slowing down of pyruvate oxida The amount of energy stored can be hampered our understanding of the tion in patients with neurologic dis considered proportional to the num genetic control of the synthesis of eases will cause an abnormal ber of anhydride phosphate bonds pyruvate dehydrogenase multien metabolism of carbohydrates and per adenosine. zyme complex. Blass et al. (1970) lipids. This in turn leads to impaired For an energy-regenerating en observed a defect in pyruvate decar functioning of the brain, since the zyme system (R), the activity would boxylase in a child with an intermit brain depends on oxidative be high at low charge and then fall tent movement disorder. Pyruvate metabolism for its proper function off steeply at high charge. The oppo decarboxylase is the first enzyme in (Mcllwain and Bachelard, 1971). site would be true for an energy- the coordinated reactions of pyruvate utilizing enzyme system (U). The dehydrogenase multi-enzyme com B) Fine control mechanism activity would be low at low charge plex. Subsequently, Kark et al. Fine control mechanisms refer to and then increase rapidly at high (1974) showed that patients with the direct control of any enzyme charge (Figure 4). spinocerebellar degeneration and catalytic activity by a modifier. Thus There is an important relationship motor neuropathies oxidized pyru the enzyme activity can be switched between the intermediary metabo vate in their muscles significantly on or off almost instantaneously. lites and the adenylate compounds in more slowly than the controls with 1. Product inhibition. The activity energy-regenerating and energy myopathic disease or normal mus of pig and rat brain PDH is competi utilizing metabolic sequences. In the cle. Impaired pyruvate oxidation in tively inhibited by acetyl CoA, a generation of ATP, the pools of the anterior cerebellar vermis by reaction product of PDH (Ngo and metabolic intermediates would be deficiencies of pyruvate dehyd Barbeau, J. Neurochem., in press, diminished. Their replenishment rogenase, which would be too mild 1978; Siess et al., 1971). Further would be linked to regeneration of to affect pyruvate oxidation in other more, we have shown that NADH ATP. This overlap of function be areas of the brain, has been post also competitively inhibits rat brain tween metabolites and adenylates has ulated as a possible mechanism for PDH with respect to NAD. These selective cerebellar damage in par observations suggest that the activ tial pyruvate dehydrogenase defi ity of PDH may be regulated in vivo ciency (Reynolds and Blass, 1976). by acetyl CoA and CoA ratio and the Low activities of the pyruvate and oxidation state of the cell. The sites oxoglutarate dehydrogenase mul of acetyl CoA and NADH inhibition tienzyme complexes in fibroblasts of are the dihydrolipoate trans- patients with Friedreich's ataxia acetylase and the lipoamide dehyd were recently reported by Blass et rogenase components of the com al. (1976). Barbeau (1975) has also plex, respectively. observed a decrease in pyruvate 2. Regulation by Energy Charge. oxidation and an increase in the level Energy charge was defined by of both pyruvate and lactate in the Atkinson (1968) as equal to one half blood of Friedreich's ataxia after the phosphate anhydride bonds per glucose loading. Recent findings in ENEBGV CHARGE adenosine. Since the total anhydride Figure 4—The enzymatic response to dicate that the most likely cause of phosphates would, in terms of con the adenylate energy charge for en this decrease is an impaired func centration, be equal to 2ATP + ADP, zymes involved in regulation of ATP- tioning of lipoamide dehydrogenase, the energy charge (EC) can be writ regenerating (R) and ATP-utilizing (U) the terminal enzyme common to ten mathematically as: sequences.

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been termed amphibolic (Sanwal, mediates occur in cytoplasm takes Mg21" into account, then the 1970). It seems clear that there must whereas the PDH system is located operation of the cycle seems to make be overlapping control of these over in the intramitochondria matrix, sense, since at physiological con lapping functions. being physically separated from centrations only the phosphatase Since oxidation of pyruvate via cytoplasmic compartment. It is not fails to be saturated by magnesium the Krebs cycle leads to the genera surprising that a different ions. Thus, a variation of this con tion of ATP, it is expected that the mechanism of control must have centration in vivo should influence activity of PDH is subject to regula evolved in mammalian cells. the phosphatase activity, which in tion by the energy charge, according 4. Regulation by enzymatic inter turn will regulate the activity of the to Atkinson's hypothesis (R-system) conversion (covalent enzymatic whole pyruvate dehydrogenase mul (Atkinson, 1968). The activity of E. modification). Apart from the regu tienzyme complex. coli PDH decreases with increasing lation by metabolites, the mam Brain pyruvate dehydrogenase energy charge of the adenylate pool, malian PDH is also regulated by two complex was shown to be intercon and this effect is maximal in the enzymes which are both compo vertible through the phosphorylation presence of acetyl CoA (Shen et al., nents of the multienzyme complex. and dephosphorylation cycle by 1968). A kinase inactivates the complex by Siess et al. (1972). Unlike the heart The activity of the mammalian phosphorylation in the presence of and kidney PDH, the brain PDH pyruvate dehydrogenase multien- ATP and low Mg2+ concentration, activity does not fall below 50% of zyme complex in vitro also depends and the reactivation is carried out by the total activity upon fasting, on the level of the energy charge. An a phosphatase that dephosphory- whereas the activity of the kidney increase in the energy charge from lates the phosphorylated complex in and muscle enzymes drops drasti 0.5 to 0.8 can cause a decrease of the the presence of high Mg2+ concent cally to only about 15% of the total activity of the complex by 70% ration. The site of phosphorylation activity (Wieland et al., 1971; Wie- (Hucho, 1974). However, the con by kinase is the pyruvate decarbox land et al., 1972). PDH is an excel trol mechanisms of PDH in mam ylase (Ei) (Reed et al., 1972). The lent example, illustrating an impor malian systems are much more com kinase is strongly bound to the di- tant principle in enzymatic regula plex than what have been observed hydrolipoamide transacetylase (E2) tion, i.e., the same enzyme in differ in E. coli, because mammalian PDH and the phosphatase is much more ent tissues or organs can be regu undergoes phosphorylation in the loosely bound than the kinase (Linn lated in a very different manner. presence of ATP which leads to et al., 1969). The inactivation and Blass and Lewis (1973) showed self-inactivation, and this inactiva- reactivation by the kinase and the that brain PDH complex has a broad tion is competitively inhibited by phosphatase are under metabolic pH optimum around 7.8. Further ADP and AMP. Various aspects of control. Pyruvate was found to pro more, they found that acetyl CoA interconversion of mammalian PDH tect PDH from phosphorylation and does not inhibit the PDH activity, will be discussed later. inactivation by the kinase. This pro which is in contradiction with our 3. Regulation by glycolytic inter tective effect of the pyruvate can be results (Ngo and Barbeau, J. mediates. Shen and Atkinson (1970) of physiological importance, be Neurochem. in press, 1978) and the showed that the activity of E. coli cause the reductive equivalents results of Wieland et al. (1972, 1975). PDH is stimulated by glycolytic in necessary for gluconeogenesis must The rate of decarboxylation of pyru termediates: fructose-diphosphate, be produced by oxidation by pyru vate to CO: and acetyl CoA by PDH fructose-6-phosphate, glucoses- vate in the mitochondria when lac of rat brain homogenate is very low phosphate, 3-phosphoglyceric acid, tate is not the starting material for in newborn rats, however, it in phosphoenolpyruvate, dihydroxy- gluconeogenesis. Gluconeogenesis creases markedly during the first acetone phosphate, and glyceralde- by way of oxalacetate is possible postnatal month (Wilbur and Portel, hyde-3-phosphate. These stimula only if part of the pyruvate is ox 1974). The activity of brain tory effects are exerted on the idized so as to provide the NADH mitochondria PDH from mature rats pyruvate decarboxylase compo necessary for this purpose. Pyruvate is initially high and does not increase 2+ 2 nent (Ei) of the multienzyme com itself makes this possible by protec with the addition of Mg and Ca + plex. Fructose-diphosphate over tion of the pyruvate dehydrogenase or partially purified pyruvate dehyd comes the inhibition resulting from a multienzyme complex against inac rogenase phosphatase or with longer high value of the adenylate energy tivation by the kinase. The incubations. The proportion of charge. However, Ngo and Barbeau Michaelis constant of ATP for kinase pyruvate dehydrogenase in the ac (unpublished results) were unable to is low, so that in most metabolic tive form in brain changes inversely observe any significant stimulation states of the cell ATP concentration with changes in mitochondrial of rat brain PDH by glycolytic in suffices to saturate the kinase. Thus, energy charge, whereas total pyru termediates. This could be explained in the absence of interaction with vate dehydrogenase activity does by the evolutionary consequence of other metabolic parameters the not change (Jope and Blass, 1975). multicellular organisms. Unlike E. phosphorylation-dephosphorylation Jope and Blass (1976) reported that coli, in mammalian cells the glycoly cycle is a futile cycle representing a treatment of mice with amobarbital and pentobarbital decreases the ac- tic reactions and hence their inter waste of ATP. However, when one

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tive form of PDH by 50%. The prop GLUCOSE ortion of PDH in the active form increases during ischemia. Anes thesia with amobarbital slows down the activation during ischemia, but does not alter the total amount of PDH. The concentration and ratios PHOSPHOENOL- of the adenylate nucleotides and the PYRUVATE i it/ energy charge increases as the prop H ortion of PDH in the active form " i' ' ATPVfcV > H * ADP decreases during ischemia. Similar PYRUVATE results have been observed by CoASH' ' ' Ksiezak (1976) in guinea pig brain. After treatment with insulin, the proportion of brain PDH in the ac NAD- tive form increases by 30%, but the . i PDH ' /^\* PDH energy charge does not change. 0 inactive Furthermore, treatment of mice with i i active -^ ether, morphine, ethanol or diazepam does not change the prop ortion of pyruvate dehydrogenase in the active form, although these treatments have been reported to alter pyruvate oxidation in brain in vivo. Thus, one can conclude that treat ments which alter pyruvate oxida tion in the brain do not consistently alter the proportion of PDH in the active form, unless they also simul- taneoulsy alter the energy charge (Jope and Blass, 1976). The activity of the PDH in isolated rat liver mitochondria declines rapidly and irreversibly. This has been attri buted to a limited proteolysis by some lysosomal proteolytic enzymes (Wieland, 1975). Insulin, by lower ing long chain acyl-CoA in adipose tissue, leads to deinhibition of the mitochondria adenine nucleotide - - - ATP translocase system and thereby to a Figure 5—Diagramatic summary of the regulation of pyruvate metabolism by metabo drop in the mitochondrial ATP/ADP lites, neurotransmitter, metal ions and enzymatic interconversion. Solid lines ratio. The relative increase of ADP indicate chemical transformation and dashed-lines indicate regulatory signal which inhibits the PDH kinase com with 0 and ©indicating inhibition and activation, respectively. PLP= pyridoxal petitively with ATP is to shift the phosphate. phosphorylation equilibrium of the PDH system towards the active de- and acetyl CoA/CoASH ratios lactin may act by priming the tissue phosphorylated enzyme (LofFler et (Batenburg and Olson, 1975; Kerbey to respond directly to normal con al, 1975). Isolated beef heart and et al., 1976). Using purified pig centration of circulating isulin and rabbit heart mitochondria have also heart pyruvate dehydrogenase, by this may be responsible for the been shown to be regulated by phos- Cooper et al. (1975) showed that increased activation of the enzyme pho and dephosphorylation (Schus the ratios of acetyl Co A/Co ASH and during the course of normal lactation ter et al., 1975; Chiang and Sacktor, NADH/NAD modulate the kinase (Field and Coore, 1976). Stansbie et 1975). The activity of the regulatory and phosphatase reactions in addi al. (1976) found that, in rat enzymes involved in the inactivation tion to their effects on the dehyd epididymal foot-pads, about 80% of and reactivation of the pyruvate de rogenase reaction. Withdrawal of the pyruvate dehydrogenase phos hydrogenase multienzyme complex prolactin or insulin from the circu phatase was extramitochondrial in from isolated rat liver mitochondria lation of lactating rats leads to in location and therefore probably not may be controlled by both the creased inactivation by phosphory directly concerned with the regula intramitochondrial NADH/NAD lation of mammary gland PDH. Pro tion of PDH activity. Human heart

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PDH was also shown to be regulated mitochondria. J. Bio. Chem., 251, LINN, T. C, PETTIT, F. H., HUCHO, F. by phosphorylation and dephos- 1364-1370. and REED, L. J. (1969). k acid de phorylation (Stansbie, 1976). BLASS, J. P., AVIGAN, J. and UHLEN- hydrogenase complexes, XI. Comparative DORF, B. W. (1970). A defect in pyruvate studies of regulatory properties of the Batenburg and Olson (1976) re decarboxylase in a child with an intermit pyruvate dehydrogenase complexes from cently showed that the pyruvate de tent movement disorder. J. Clin. Invest., kidney, heart and liver mitochondria. Proc. hydrogenase kinase activity of a rat 49, 423-432. Nat. Acad. Sci. (Wash.) 64-227-234. liver mitochondrial extract was BLASS, J. P., KARK, R. A. P. and LOFFLER, G., BARD, S. and WIELAND, stimulated strongly by acetyl CoA MENON, N. K. (1976). Low activities of O. H. (1975). Control of pyruvate dehyd and was inhibited by NAD and CoA the pyruvate and oxoglutarate dehydro rogenase interconversion by palmitoyl- genase complexes in five patients with Coenzyme A as related to adenine nuc SH. In contrast to acetyl CoA, Friedreich's ataxia. New Engl. J. Med., leotide translocation in isolated fat cell octanoyl-CoA inhibited the kinase 295, 62-67. mitochondria. F.E.B.S. Letters, 60, 269-274. activity. Thus, the inactivation of BLASS, J. P. and LEWIS, C. A. (1973). PDH by fatty acid in isolated rat Kinetic properties of the partially purified McILWAIN, H. and BACHELARD, H. S. pyruvate dehydrogenase complex of ox (1971). Biochemistry and the central nerv liver mitochondria may be mediated ous system. 4th Edition, Baltimore, Wil through effects of the NADH/NAD brain. Biochem. J., 131, 31-37 CHANGEUX, J. P. (1965). The control of liams and Wilkins. and acetyl CoA/CoASH ratios on biochemical reactions. Scient. Amer., 212, REED, L. J. (1969). Pyruvate dehydrogenase the interconversion of the active and 36-45. complex. In: Current Topics in Cellular inactive forms of the enzyme com CHAING, P. K. and SACKTOR, B. (1975). Regulation ( B. L. Horecker and E. R. plex catalyzed by PDH kinase and Control of pyruvate dehydrogenase activity stadtman, editors), Volume 1, Academic PDH phosphatase. in intact cardiac mitochondria. J. Biol. Press, New York, pp. 233-251. Chem., 250, 3399-3408. REED, L. J. (1974). Multienzyme complexes. Figure 5 summarizes the known In: Accounts of Chemical Research, Vol. 7, regulatory parameters in mammalian COOPER, R. H., RANDLE, P. J. and DENTON, R. M. (1975). Stimulation of phos pp. 40-46. pyruvate metabolism. It is to be phorylation and inactivation of pyruvate REED, L. J., LINN T. C, HUCHO, F., noted that the products of the PDH dehydrogenase by physiological inhibitors NAMIHIRA, G., BARRERA, B. R., catalyzed reaction are not only capa of the pyruvate dehydrogenase. Nature, ROCHE, T. E., PELLEY, J. W. and ble of inhibiting directly the active 257, 808-809. RANDALL, D. D. (1972). Molecular as form of the enzyme, but may also, DENTON, R. M., RANDLE, P. J., pects of regulation of the mammalian pyru BRIDGES, B. J., COOPER, R. H., KER- vate dehydrogenase complex. In: together with substrates of the en BEY, A. L., PARK, H. T., SEVERSON, Metabolic Interconversion of Enzymes. zyme complex, regulate the PDH D.L., STANBIE, D. and WHITEHOUSE, (Wieland, O., Heimreich, E. and Holzer, complex by covalent modification S. (1975). Regulation of mammalian pyru H., editors). Springer-Verlag, Berlin, pp. (Batenburg and Olson, 1976). vate dehydrogenase. Molecular and Cellu 281-291. lar Biochem., 9, 27-53. REYNOLDS, S. F. and BLASS J. P. (1976). FIELD, B. and COORE, H. G. (1976). 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