Control of oxidative phosphorylation by vitamin A illuminates a fundamental role in mitochondrial energy homoeostasis

The physiology of two metabolites of vitamin A is understood in substantial detail: retinaldehyde functions as the universal chromophore in the vertebrate and invertebrate eye; retinoic acid regulates a set of vertebrate transcription factors, the retinoic acid receptor superfamily. The third member of this retinoid triumvirate is retinol. While functioning as the precursor of retinaldehyde and retinoic acid, a growing body of evidence suggests a far more fundamental role for retinol in signal transduction. Here we show that retinol is essential for the metabolic fitness of mitochondria. When cells were deprived of retinol, respiration and ATP synthesis defaulted to basal levels. They recovered to significantly higher energy output as soon as retinol was restored to physiological concentration, without the need for metabolic conversion to other retinoids. Retinol emerged as an essential cofactor of protein kinase Cdelta (PKCdelta), without which this enzyme failed to be activated in mitochondria. Furthermore, retinol needed to physically bind PKCdelta, because mutation of the retinol binding site rendered PKCdelta unresponsive to Rol, while retaining responsiveness to phorbol ester. The PKCdelta/retinol complex signaled the pyruvate dehydrogenase complex for enhanced flux of pyruvate into the Krebs cycle. The baseline response was reduced in vitamin A-deficient lecithin:retinol acyl transferase-knockout mice, but this was corrected within 3 h by intraperitoneal injection of vitamin A; this suggests that vitamin A is physiologically important. These results illuminate a hitherto unsuspected role of vitamin A in mitochondrial bioenergetics of mammals, acting as a nutritional sensor. As such, retinol is of fundamental importance for energy homeostasis. The data provide a mechanistic explanation to the nearly 100-yr-old question of why vitamin A deficiency causes so many pathologies that are independent of retinoic acid action.

Effects of zinc on SN56 cholinergic neuroblastoma cells

Zinc is a trace element necessary for proper development and function of brain cells. However, excessive accumulation of zinc exerts several cytotoxic effects in the brain. The aim of this work was to see whether cytotoxic effects of zinc are quantitatively correlated with changes in acetyl-CoA metabolism. The zinc levels up to 0.20 mmol/L caused concentration-dependent inhibition of pyruvate dehydrogenase (PDH) activity that correlated with the increase in trypan blue-positive fraction and the decrease in cultured cell number (r = 0.96, p = 0.0001). Chronic exposure of cells to 0.15 mmol/L zinc decreased choline acetyltransferase and aconitase activities, cytoplasmic acetyl-CoA and whole cell ATP level by 38%, 57%, 35%, and 62%, respectively but caused no change in mitochondrial acetyl-CoA level and activities of other enzymes of glycolytic and tricarboxylic acid cycle. dl-alpha-lipoamide when added simultaneously with zinc to cultured cells or their homogenates attenuated its chronic or acute suppressive effects. In homogenates of chronically Zn-treated cells, lipoamide overcame PDH but not aconitase inhibition. Presented data indicate that acute-transient elevation of zinc caused reversible inhibition of PDH, aconitase activities and acetyl-CoA metabolism, which when prolonged could lead to irreversible enzyme inactivation yielding decrease in cell viability and secondary suppression of their cholinergic phenotype.

Modification of thiamine pyrophosphate dependent enzyme activity by oxythiamine inSaccharomyces cerevisiaecells

Oxythiamine is an antivitamin derivative of thiamine that after phosphorylation to oxythiamine pyro phos phate can bind to the active centres of thiamine-dependent enzymes. In the present study, the effect of oxythiamine on the viability of Saccharomyces cerevisiae and the activity of thiamine pyrophosphate dependent enzymes in yeast cells has been investigated. We observed a decrease in pyruvate decarboxylase specific activity on both a control and an oxythiamine medium after the first 6 h of culture. The cytosolic enzymes transketolase and pyruvate decarboxylase decreased their specific activity in the presence of oxythiamine but only during the beginning of the cultivation. However, after 12 h of cultivation, oxythiamine-treated cells showed higher specific activity of cytosolic enzymes. More over, it was established by SDS–PAGE that the high specific activity of pyruvate decarboxylase was followed by an increase in the amount of the enzyme protein. In contrast, the mitochondrial enzymes, pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase complexes, were inhibited by oxythiamine during the entire experiment. Our results suggest that the observed strong decrease in growth rate and viability of yeast on medium with oxythiamine may be due to stronger in hibition of mitochondrial pyruvate dehydrogenase than of cytosolic enzymes.Key words: pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, transketolase, pyruvate decarboxylase, activity, oxythiamine, inhibition.

Effects of NGF on acetylcholine, acetyl-CoA metabolism, and viability of differentiated and non-differentiated cholinergic neuroblastoma cells

Nerve growth factor (NGF) is a peptide displaying multiple cholinotropic activities. The aim of this work was to explain mechanisms of the positive and negative effects of NGF on phenotypic properties and viability of cholinergic cells. To discriminate these effects we used two p75NTR receptor-positive lines of cholinergic neuroblastoma cells, SN56 and T17 that are devoid of or express high affinity NGF (TrkA) receptors, respectively. cAMP and retinoic acid caused differentiation of both cell lines. In addition to the morphologic maturation, the increase of choline acetyltransferase activity, acetylcholine, Ca and cytoplasmic acetyl-CoA levels and decrease of mitochondrial acetyl-CoA and cell viability were observed. NGF caused similar effects in non-differentiated T17 cells but had no influence on non-differentiated SN56 cells. On the contrary, in both cAMP/all-trans-retinoic acid (RA) differentiated cell lines, NGF resulted in a similar suppression of cholinergic phenotype along with an increase of mitochondrial acetyl-CoA and cell susceptibility to nitric oxide and amyloid-beta25-35. These effects of NGF were prevented by an antibody against the p75NTR receptor. Data indicate that: (i) positive cholinotrophic effects of NGF required activation of both TrkA and p75NTR receptors; (ii) cAMP/RA-evoked differentiation inhibited NGF effects mediated by TrkA receptors and activated its p75NTR-dependent suppressing influences and (iii) a differentiation-evoked decrease of mitochondrial acetyl-CoA and an elevation of mitochondrial Ca could augment impairment of cholinergic neurons by neurotoxic signals.

Mice deficient in dihydrolipoamide dehydrogenase show increased vulnerability to MPTP, malonate and 3-nitropropionic acid neurotoxicity

Altered energy metabolism, including reductions in activities of the key mitochondrial enzymes alpha-ketoglutarate dehydrogenase complex (KGDHC) and pyruvate dehydrogenase complex (PDHC), are characteristic of many neurodegenerative disorders including Alzheimer's Disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Dihydrolipoamide dehydrogenase is a critical subunit of KGDHC and PDHC. We tested whether mice that are deficient in dihydrolipoamide dehydrogenase (Dld+/-) show increased vulnerability to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), malonate and 3-nitropropionic acid (3-NP), which have been proposed for use in models of PD and HD. Administration of MPTP resulted in significantly greater depletion of tyrosine hydroxylase-positive neurons in the substantia nigra of Dld+/- mice than that seen in wild-type littermate controls. Striatal lesion volumes produced by malonate and 3-NP were significantly increased in Dld+/- mice. Studies of isolated brain mitochondria treated with 3-NP showed that both succinate-supported respiration and membrane potential were suppressed to a greater extent in Dld+/- mice. KGDHC activity was also found to be reduced in putamen from patients with HD. These findings provide further evidence that mitochondrial defects may contribute to the pathogenesis of neurodegenerative diseases.

Mitochondrial complex I mutations in Caenorhabditis elegans produce cytochrome c oxidase deficiency, oxidative stress and vitamin-responsive lactic acidosis

Mitochondrial dysfunction, with an estimated incidence of 1 in 10 000 live births, is among the most common genetically determined conditions. Missense mutations in the human NDUFV1 gene, which encodes the 51 kDa active site subunit of the NADH-ubiquinone oxidoreductase or complex I, can lead to severe neurological disorders. Owing to the rare and often sporadic nature of mitochondrial disorders, the mechanisms of pathogenesis of most mutations remain poorly understood. We have generated transgenic strains of Caenorhabditis elegans that express disease-causing mutations in the nuo-1 gene, the C. elegans homolog of the NDUFV1 gene. The transgenic strains demonstrate hallmark features of complex I dysfunction such as lactic acidosis and decreased NADH-dependent mitochondrial respiration. They are also hypersensitive to exogenous oxidative stress, suggesting that cellular defense mechanisms against reactive oxygen species are already taxed by an endogenous stress. The lactic acidosis induced by the NDUFV1 mutations could be partially corrected with the vitamins riboflavin and thiamine or with sodium dichloroacetate, an activator of the pyruvate dehydrogenase complex, resulting in significant increases in animal fitness. Surprisingly, cytochrome c oxidase activity and protein levels were reduced, establishing a connection between complexes I and IV. Our results indicate that complex I mutations exert their pathogenic effects in multiple ways: by impeding the metabolism of NADH, by increasing the production of reactive oxygen species, and by interfering with the function or assembly of other mitochondrial respiratory chain components.

Effect of dietary fish oil on insulin sensitivity and metabolic fate of glucose in the skeletal muscle of normal rats

The aim of this work was to study the effect of the administration of cod liver oil on the non-oxidative and oxidative fate of glucose metabolism in the skeletal muscle of normal rats. To achieve this goal, the gastrocnemius was examined regarding glucose oxidation, glycogen synthase activity and glycogen storage both at baseline and during euglycemic hyperinsulinemic clamping. The results show that dietary fish oil decreases plasma insulin levels without alteration in glucose homeostasis (at baseline). In addition, the observed enhancement in whole body glucose utilization during clamping suggests an increased peripheral insulin sensitivity. Furthermore, under insulin-stimulated glucose disposal, an enhancement in the glycolytic pathway (increased levels of muscle glucose-6-phosphate and plasma lactate) rather than changes in the oxidation (pyruvate dehydrogenase complex) and storage components of glucose metabolism was observed in the skeletal muscle of rats fed dietary fish oil. These results coupled with the hypolipidemic effects of fish oil may have implications for the prevention and/or management of some pathological states manifested by insulin resistance with or without dyslipidemia.

Effect of ethanol on the metabolism of primary astrocytes studied by (13)C- and (31)P-NMR spectroscopy

Nuclear magnetic resonance was used as the primary technique to investigate the effect of ethanol (40, 80, and 160 mM) on the levels of high-energy phosphates, glycolytic flux, anaplerotic and oxidative fluxes to the tricarboxylic acid (TCA) cycle, the contribution of the pentose phosphate pathway (PPP), and the uptake and release of amino acids on primary cultures of rat astrocytes. On line (31)P-NMR spectroscopy showed that long-term exposure to ethanol caused a drop in the levels of ATP and phosphocreatine. The ratio between the fluxes through the pyruvate dehydrogenase and pyruvate carboxylase reactions also decreased, whereas the glycolytic flux and the ratio between formation of lactate and glucose consumption increased when cells were exposed to acute doses of ethanol. Flux through the pentose phosphate pathway was not affected. The uptake of cysteine and the release of glutamine were stimulated by ethanol, whereas the release of methionine was inhibited. Moreover, the fractional enrichment in serine was enhanced. The changes in the amino acid metabolism are interpreted as a response to oxidative stress induced by ethanol.

Dichloroacetate improves postischemic function of hypertrophied rat hearts

OBJECTIVES

We sought to determine whether improving coupling between glucose oxidation and glycolysis by stimulating glucose oxidation during reperfusion enhances postischemic recovery of hypertrophied hearts.

BACKGROUND

Low rates of glucose oxidation and high glycolytic rates are associated with greater postischemic dysfunction of hypertrophied as compared with nonhypertrophied hearts.

METHODS

Heart function, glycolysis and glucose oxidation were measured in isolated working control and hypertrophied rat hearts for 30 min before 20 min of global, no-flow ischemia and during 60 min of reperfusion. Selected control and hypertrophied hearts received 1.0 mmol/liter dichloroacetate (DCA), an activator of pyruvate dehydrogenase, at the time of reperfusion to stimulate glucose oxidation.

RESULTS

In the absence of DCA, glycolysis was higher and glucose oxidation and recovery of function were lower in hypertrophied hearts than in control hearts during reperfusion. Dichloroacetate stimulated glucose oxidation during reperfusion approximately twofold in both groups, while significantly reducing glycolysis in hypertrophied hearts. It also improved function of both hypertrophied and control hearts. In the presence of DCA, recovery of function of hypertrophied hearts was comparable to or better than that of untreated control hearts.

CONCLUSIONS

Dichloroacetate, given at the time of reperfusion, normalizes postischemic function of hypertrophied rat hearts and improves coupling between glucose oxidation and glycolysis by increasing glucose oxidation and decreasing glycolysis. These findings support the hypothesis that low glucose oxidation rates and high glycolytic rates contribute to the exaggerated postischemic dysfunction of hypertrophied hearts.

A novel mutation in the thiamine responsive megaloblastic anaemia gene SLC19A2 in a patient with deficiency of respiratory chain complex I

The thiamine transporter gene SLC19A2 was recently found to be mutated in thiamine responsive megaloblastic anaemia with diabetes and deafness (TRMA, Rogers syndrome), an early onset autosomal recessive disorder. We now report a novel G1074A transition mutation in exon 4 of the SLC19A2 gene, predicting a Trp358 to ter change, in a girl with consanguineous parents. In addition to the typical triad of Rogers syndrome, the girl presented with short stature, hepatosplenomegaly, retinal degeneration, and a brain MRI lesion. Both muscle and skin biopsies were obtained before high dose thiamine supplementation. While no mitochondrial abnormalities were seen on morphological examination of muscle, biochemical analysis showed a severe deficiency of pyruvate dehydrogenase and complex I of the respiratory chain. In the patient's fibroblasts, the supplementation with high doses of thiamine resulted in restoration of complex I activity. In conclusion, we provide evidence that thiamine deficiency affects complex I activity. The clinical features of TRMA, resembling in part those found in typical mitochondrial disorders with complex I deficiency, may be caused by a secondary defect in mitochondrial energy production.

Lactate improves cardiac efficiency after hemorrhagic shock

This study was undertaken to examine the role of lactate on cardiac function and metabolism after severe acute hemorrhagic shock. Anesthetized, nonheparinized rats were bled to a mean arterial pressure of 25-30 mm Hg for 1 h; controls were not bled. Their hearts were removed, and cardiac work and efficiency (work/oxygen consumption) were measured in the isolated working heart mode for 60 min. The hearts were perfused with one of five substrate combinations: 1) glucose (11 mM), 2) glucose + 0.4 mM palmitate, 3) glucose + 0.4 mM palmitate + 8.0 mM lactate, 4) glucose + 1.2 mM palmitate, or 5) glucose + 1.2 mM palmitate + 8.0 mM lactate. After perfusion, hearts were freeze-clamped, and tissue contents of free coenzyme-A (CoA), acetyl CoA, and succinyl CoA were measured, as was myocardial pyruvate dehydrogenase (PDH) activity. The addition of 8.0 mM lactate significantly improved cardiac work in shocked hearts perfused with 0.4 mM palmitate and increased cardiac efficiency in the presence of either 0.4 mM or 1.2 mM palmitate. Compared to control hearts, shocked hearts exhibited a 20-30% decrease in PDH activity. Shocked hearts perfused with lactate demonstrated no increase in acetyl CoA content but did have a significant increase in tissue succinyl CoA compared to control hearts perfused with lactate or shocked hearts perfused without lactate. In the heart recovering from severe hemorrhagic shock, lactate improves cardiac efficiency in the presence of free fatty acids, possibly by a anaplerosis of the tricarboxylic acid cycle.

Oxidative metabolites of 5-S-cysteinylnorepinephrine are irreversible inhibitors of mitochondrial complex I and the alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase complexes: possible implications for neurodegenerative brain disorders

The major initial product of the oxidation of norepinephrine (NE) in the presence of L-cysteine is 5-S-cysteinylnorepinephrine which is then further easily oxidized to the dihydrobenzothiazine (DHBT) 7-(1-hydroxy-2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1, 4-benzothiazine-3-carboxylic acid (DHBT-NE-1). When incubated with intact rat brain mitochondria, DHBT-NE-1 evokes rapid inhibition of complex I respiration without affecting complex II respiration. DHBT-NE-1 also evokes time- and concentration-dependent irreversible inhibition of NADH-coenzyme Q(1) (CoQ(1)) reductase, the pyruvate dehydrogenase complex (PDHC), and alpha-ketoglutarate dehydrogenase (alpha-KGDH) when incubated with frozen and thawed rat brain mitochondria (mitochondrial membranes). The time dependence of the inhibition of NADH-CoQ(1) reductase, PDHC, and alpha-KGDH by DHBT-NE-1 appears to be related to its oxidation, catalyzed by an unknown component of the inner mitochondrial membrane, to electrophilic intermediates which bind covalently to active site cysteinyl residues of these enzyme complexes. The latter conclusion is based on the ability of glutathione to block inhibition of NADH-CoQ(1) reductase, PDHC, and alpha-KGDH by scavenging electrophilic intermediates, generated by the mitochondrial membrane-catalyzed oxidation of DHBT-NE-1, forming glutathionyl conjugates, several of which have been isolated and spectroscopically identified. The possible implications of these results to the degeneration of neuromelanin-pigmented noradrenergic neurons in the locus ceruleus in Parkinson's disease are discussed.

Ranolazine attenuates palmitoyl-L-carnitine-induced mechanical and metabolic derangement in the isolated, perfused rat heart

The effect of ranolazine, a novel anti-ischaemic drug that stimulates the activity of pyruvate dehydrogenase, on palmitoyl-L-carnitine-induced mechanical dysfunction and metabolic derangement in isolated perfused rat hearts has been studied and compared with the effect of dichloroacetate, an activator of pyruvate dehydrogenase. Rat hearts paced electrically were perfused aerobically at constant flow by the Langendorff technique. Palmitoyl-L-carnitine (4 microM) increased left ventricular end-diastolic pressure and reduced left ventricular developed pressure (i.e. induced mechanical dysfunction); it also reduced tissue levels of adenosine triphosphate and increased tissue levels of adenosine monophosphate (i.e. induced metabolic derangement). These functional and metabolic alterations induced by palmitoyl-L-carnitine were attenuated by ranolazine (5, 10, and 20 microM) in a concentration-dependent manner. In contrast, dichloroacetate (1 and 10 mM) did not attenuate palmitoyl-L-carnitine-induced mechanical and metabolic derangement. In the normal (palmitoyl-L-carnitine-untreated) heart, however, ranolazine did not modify mechanical function and energy metabolism. These results suggest that ranolazine attenuates palmitoyl-L-carnitine-induced mechanical and metabolic derangement in the rat heart, and that the beneficial action of ranolazine is not because of the energy-sparing effect or activation of pyruvate dehydrogenase.

Stabilization of the pyruvate dehydrogenase E1alpha subunit by dichloroacetate

OBJECTIVE

To test the effects of dichloroacetate (DCA) treatment on the rate of turnover of pyruvate dehydrogenase (PDH) subunits.

BACKGROUND

PDH deficiency is a nuclear-encoded mitochondrial disorder and a major recognized cause of neonatal encephalomyopathies associated with primary lactic acidosis. DCA has been used for its treatment. The primary mechanism of action of DCA has been thought to increase the proportion of enzyme in the activated, dephosphorylated state. However, this mechanism does not readily account for responses to treatment with mutations that do not obviously affect regulation of the enzyme complex.

METHODS

PDH subunit turnover rates were measured using pulse-chase methods in a normal fibroblastic cell line before and after chronic (5-day) treatment with 5 mM DCA.

RESULTS

Chronic DCA treatment causes a more than twofold decrease in the apparent first-order rate constant for degradation of the PDH E1alpha subunit (kE1alpha(pre-DCA) = 0.025 +/- 0.006 hr(-1), n = 6; kE1alpha(post-DCA) = 0.011 /- 0.002 hr(-1), n = 3; p < 0.01) and a selective, progressive increase in the total cell PDH activity by 150 +/- 5% (p < 0.0005).

CONCLUSION

These results suggest an additional novel mechanism of action for the chronic DCA treatment of lactic acidemia; namely, inhibition of mitochondrial E1alpha subunit degradation leading to an increase in maximal PDH complex activity.

Clinical pharmacology and toxicology of dichloroacetate

Dichloroacetate (DCA) is a xenobiotic of interest to both environmental toxicologists and clinicians. The chemical is a product of water chlorination and of the metabolism of various drugs and industrial chemicals. Its accumulation in groundwater and at certain Superfund sites is considered a potential health hazard. However, concern about DCA toxicity is predicated mainly on data obtained in inbred rodent strains administered DCA at doses thousands of times higher than those to which humans are usually exposed. In these animals, chronic administration of DCA induces hepatotoxicity and neoplasia. Ironically, the DCA doses used in animal toxicology experiments are very similar to those used clinically for the chronic or acute treatment of several acquired or hereditary metabolic or cardiovascular diseases. As a medicinal, DCA is generally well tolerated and stimulates the activity of the mitochondrial pyruvate dehydrogenase enzyme complex, resulting in increased oxidation of glucose and lactate and an amelioration of lactic acidosis. By this mechanism, the drug may also enhance cellular energy metabolism. DCA is dehalogenated in vivo to monochloroacetate and glyoxylate, from which it can be further catabolized to glycolate, glycine, oxalate, and carbon dioxide. It remains to be determined whether important differences in its metabolism and toxicology exist in humans between environmentally and clinically relevant doses.

Effect of nicardipine, a Ca2+ channel blocker, on pyruvate dehydrogenase activity and energy metabolites during cerebral ischemia and reperfusion in gerbil brain

The purpose of this study was to determine if nicardipine, a calcium ion channel blocker, affects pyruvate dehydrogenase (PDH) activity and improves energy metabolism during cerebral ischemia and reperfusion. Cerebral ischemia was induced, using the bilateral carotid artery occlusion method, for 60 min followed by reperfusion up to 120 min in gerbils. Nicardipine (1 mg/kg) or saline (vehicle-treated) was given to gerbils 30 min prior to the occlusion of the common carotid arteries. PDH activity and metabolites (ATP, PCr, and lactate) were measured in cortex prior to ischemia, immediately following ischemia, and after each reperfusion period. After 60 min ischemia, PDH activity increased in both groups, and was significantly higher in the nicardipine-treated group. After 20 min reperfusion, PDH activity in the nicardipine-treated group recovered to control levels, whereas, the PDH activity in the vehicle-treated group remained elevated, and was higher than the nicardipine-treated animals. At 60 and 120 min reperfusion, the activities in the vehicle-treated group were significantly below control levels, there were no differences, however, between the two groups. ATP and PCr concentrations were markedly depleted immediately after ischemia in both groups. ATP levels at 20 min reperfusion and PCr levels at 60 min reperfusion were significantly higher in the nicardipine-treated group. Lactate concentrations in both groups increased 7-8 fold, similarly, immediately after ischemia. During reperfusion, the lactate remained elevated in both groups, though the levels in the nicardipine-treated group were lower than those in the vehicle-treated group, but not significantly. Nicardipine treatment normalized PDH activity quickly and improved energy metabolism after reperfusion.

Activation of pyruvate dehydrogenase improves heart function and metabolism after hemorrhagic shock

This study was designed to test the hypothesis that activation of myocardial pyruvate dehydrogenase (PDH) would improve recovery of heart function after brief, severe hemorrhagic shock. Pentobarbital-anesthetized rats were instrumented to monitor arterial blood pressure and right ventricular pressures. Rats were hemorrhaged via femoral artery to 25-30 mmHg mean arterial pressure (MAP) for 60 min, followed by retransfusion of shed blood with either 1.0 cc saline with no dichloroacetate (-DCA) or 1.0 cc saline containing 150 mg/kg sodium dichloroacetate (+DCA). Rats were observed for 3 h after retransfusion. Hearts were freeze-clamped in situ for analysis of adenosine triphosphate (ATP), creatine phosphate (CrP), lactate and pyruvate content as well as PDH activity (PDHa) and total PDH activity (PDHt). Three h after retransfusion, the rate pressure product (RPP=HRxPSP) was 23 000+/-2733 with no DCA treatment v 36 2769 mmHg/min with DCA treatment (P<0.05, ANOVA). Treatment with DCA also increased myocardial tissue content of high energy phosphates (ATP=10.1+/-1.1 and CrP=5.8+/-1.0 micromol/g weight-DCA, v 15.1+/-0.9 and 14.7+/-1.0 micromol/g dry weight+DCA, P<0.05, both measurements). DCA administration also significantly reduced myocardial lactate contents (14.6+/-2.7 micromol/g dry weight-DCA v 5.9+/-1.0+DCA). Hemorrhagic shock did not change PDHa or PDHt compared to hearts obtained during the pre-hemorrhage period. Retransfusion with DCA significantly increased PDHa activity (6.8+/-1.1 micromol/g dry weight/min-DCA v 29.7+/-2.0 micromol/g dry weight/min+DCA). PDHt was not different between controls and DCA-treated groups. These data indicate that activation of myocardial PDH by adding DCA to retransfused blood improved heart function and metabolism after severe hemorrhagic shock.

Pyruvate dehydrogenase inactivity is not responsible for sepsis-induced insulin resistance

OBJECTIVE

To determine whether activation of pyruvate dehydrogenase with dichloroacetate can reverse sepsis-induced insulin resistance in humans or rats.

DESIGN

Prospective, controlled study.

SETTING

Intensive care unit (ICU) and laboratory at a university medical center.

SUBJECTS

Nine patients were admitted to the ICU with Gram-negative sepsis, confirmed by cultures. In addition, chronically instrumented, Sprague-Dawley rats, either controls or with live Escherichia coli-induced sepsis.

INTERVENTIONS

Hyperinsulinemic euglycemic clamp, with or without coadministration of dichloroacetate.

MEASUREMENTS AND MAIN RESULTS

In humans, a primed, constant infusion of [6,6-2H2]glucose was used to determine endogenous glucose production and whole-body glucose disposal. Septic humans exhibited impaired maximal insulin-stimulated glucose utilization (39.5 +/- 2.7 mumol/kg/min), despite complete suppression of endogenous glucose production. In rats, a primed, constant infusion of [3-3H]glucose was used to determine endogenous glucose production and whole-body glucose disposal. Tissue glucose uptake in vivo was determined by [14C]-2-deoxyglucose uptake. Maximal, whole-body, insulin-stimulated glucose utilization was 205 +/- 11 and 146 +/- 9 mumol/kg/min in control and septic rats, respectively. The defect was specific to skeletal muscle and heart. Stimulation of pyruvate dehydrogenase with dichloroacetate caused a 50% decrease in plasma lactate concentration but failed to improve whole-body insulin-stimulated glucose utilization in either the septic human or rat. Dichloroacetate reversed the impairment of insulin-stimulated myocardial glucose uptake in septic rats, but did not influence skeletal muscle glucose uptake either under basal conditions or during insulin stimulation.

CONCLUSIONS

Activation of pyruvate dehydrogenase with dichloroacetate does not ameliorate the impairment of whole-body, insulin-stimulated glucose uptake in septic humans or rats, or reverse the specific defect in insulin-mediated skeletal muscle glucose uptake by septic rats. Therefore, the decreased pyruvate dehydrogenase activity associated with sepsis does not appear to mediate sepsis-induced insulin resistance during insulin-stimulated glucose uptake at either the whole-body or tissue level.

Counter modulation of adipocyte mitochondrial processes by insulin and S-oxalylglutathione

Oxalyl thiolesters, a group of putative intracellular regulators, have been shown to be in vitro inhibitors of some cytosolic enzymes which are stimulated by insulin. In this study, the effects of insulin and oxalyl thiolesters on pyruvate dehydrogenase, beta-oxidation, and acyl-CoA hydrolase activities in mitochondria from rat epididymal adipocytes are compared. Using glutathione, CoASH, cysteine, and cysteamine as thiol sources, oxalyl thioesters were synthesized, purified, and quantitated. Mitochondria were isolated from rat epididymal adipocytes, some of which were incubated with or without insulin. Mitochondrial activities were determined by radioisotopic assay subsequent to control, insulin, or oxalyl thiolester incubation. Under the conditions used in this study, pyruvate dehydrogenase activity was increased 28% subsequent to 10-min incubation of adipocytes with 400 microU/ml insulin; in contrast, preincubation of adipocyte mitochondria with S-oxalylglutathione resulted in a dose-dependent 11-19% inhibition of pyruvate dehydrogenase. S-oxalylglutathione also attenuated the spermine-induced activation of pyruvate dehydrogenase. Insulin treatment resulted in a small but significant increase in beta-oxidation of palmitic acid while 100 microM S-oxalylglutathione mediated a 40% decrease in palmitate oxidation. Palmitoyl-CoA hydrolase activity was decreased 14% by insulin treatment; however, S-oxalylglutathione caused a 14-50% increase in hydrolase activity. The other oxalyl thiolesters were not as effective or as consistent as S-oxalylglutathione in modulation of the mitochondrial activities; free thiols and oxalic acid did not modulate the activities. In summary, pyruvate dehydrogenase, palmitate beta-oxidation, and palmitoyl-CoA hydrolase activities in adipocyte mitochondria were modulated in approximately equal but opposite directions by insulin and S-oxalylglutathione. These findings support the suggestion that oxalyl thiolesters may function as an intracellular signal recruited to return insulin to normal levels.

Increased acetyl group availability enhances contractile function of canine skeletal muscle during ischemia

Skeletal muscle contractile function is impaired during acute ischemia such as that experienced by peripheral vascular disease patients. We therefore, examined the effects of dichloroacetate, which can alter resting metabolism, on canine gracilis muscle contractile function during constant flow ischemia. Pretreatment with dichloroacetate increased resting pyruvate dehydrogenase complex activity and resting acetylcarnitine concentration by approximately 4- and approximately 10-fold, respectively. After 20-min contraction the control group had demonstrated an approximately 40% reduction in isomeric tension whereas the dichloroacetate group had fatigued by approximately 25% (P < 0.05). Dichloroacetate resulted in less lactate accumulation (10.3 +/- 3.0 vs 58.9 +/- 10.5 mmol.kg-1 dry muscle [dm], P < 0.05) and phosphocreatine hydrolysis (15.6 +/- 6.3 vs 33.8 +/- 9.0 mmol.kg-1 dm, P < 0.05) during contraction. Acetylcarnitine concentration fell during contraction by 5.4 +/- 1.8 mmol.kg-1 dm in the dichloroacetate group but increased by 10.0 +/- 1.9 mmol.kg-1 dm in the control group. In conclusion, dichloroacetate enhanced contractile function during ischemia, independently of blood flow, such that it appears oxidative ATP regeneration is limited by pyruvate dehydrogenase complex activity and acetyl group availability.

The hormonal regulation of pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex has a central role in the regulation of mammalian metabolism as it represents the point-of-no-return in the utilization of carbohydrate. This article summarizes our studies into how signalling systems initiated by hormones binding to cell surface receptors can reach the pyruvate dehydrogenase system which is located within the inner mitochondrial membrane. One class of hormones which activate pyruvate dehydrogenase are those that increase cytoplasmic Ca2+. A wide range of studies on isolated enzymes, separated mitochondria and intact cell preparations have shown that the activation is due to the stimulation of pyruvate dehydrogenase phosphatase. Two other intramitochondrial dehydrogenases which regulate the citrate acid cycle are activated in parallel and this is an important means of balancing the supply of ATP to increasing cell demand. Insulin is also able to activate pyruvate dehydrogenase, but this is restricted to fat and other cells capable of lipogenesis. Insulin acts by stimulating pyruvate dehydrogenase phosphatase, but the activation does not involve alterations in Ca2+. The signalling pathway involved has not been established, but it appears to be quite distinct from those involved in many other actions of insulin.

Application of anti-E1 monoclonal antibodies to the study of the pyruvate dehydrogenase complex

It has been shown that monoclonal antibody (mAb) F7F10 raised against pyruvate dehydrogenase component (E1) of pigeon breast muscle pyruvate dehydrogenase complex (PDC) has no influence on the E1 activity, measured in the system with artificial oxidants. However it inhibited the full NAD+ and coenzyme A dependent activity of PDC. The competition of the F7F10 antibody with the E2 component of PDC for the binding with E1 was revealed by immunoenzymatic and kinetic analysis. It is suggested that F7F10 mAb interacts with an antigenic determinant, located in the immediate vicinity of or overlapping with the E1 region, responsible for the interaction with the E2 component of PDC.

Trimetazidine effects on the damage to mitochondrial functions caused by ischemia and reperfusion

Trimetazidine (TMZ) is an anti-ischemic compound whose precise mode of action is unknown, although several studies have suggested a metabolic effect, and there have been reports of protection of mitochondria against oxidative stress damage. Using a Langendorff rat heart model, we examined the effects of TMZ on the mitochondrial damage following 30 minutes of ischemia and 5 minutes of reperfusion. Mitochondrial respiration with succinate, glutamate-malate and ascorbate-N,N,N',N'-tetramethylphenylenediamine (TMPD) as substrates was significantly decreased following ischemia-reperfusion. Preperfusion with 10(-5) M TMZ had no effect on these rates in normoxic or ischemic hearts. However, 10(-3) M TMZ significantly decreased the glutamate-malate rate in mitochondria from normoxic hearts, and this rate was not further decreased following ischemia-reperfusion, and 10(-3) M TMZ also partially protected ascorbate-TMPD activity. The effect on glutamate-malate was probably due to an inhibition of complex I by TMZ, which specifically inhibited reduced nicotinamide-adenine-dinucleotide-cytochrome c reductase and complex I in lysed mitochondria. We also studied the effects of TMZ on the activity of pyruvate dehydrogenase (PDH) in normoxic and ischemic hearts perfused with 0.5 mM palmitate, which caused the enzyme to be almost completely inactivated. After short periods of ischemia (10-20 minutes) the PDH inactivation by palmitate was progressively lost. Preperfusion with 10(-5) M TMZ had a tendency to decrease lactate dehydrogenase release, accompanied by a maintenance of the inhibition of PDH by palmitate. This may allow the heart to oxidize fatty acids preferentially during reperfusion, hence removing possible toxic acyl esters.

Dissociation and unfolding of the pyruvate dehydrogenase complex by guanidinium chloride

The effect of guanidinium chloride (GdnHCl) on the pyruvate dehydrogenase complex (PDC) from bovine heart and its constituent enzymes has been studied. The overall activity of the complex is lost reversibly at low levels of GdnHCl (0.2 M) which cause 40-50% inactivation but no loss of overall secondary or tertiary structures of the individual enzymes; the inactivation of the complex is shown to be caused by dissociation of the E1 and E3 components from the E2/X core assembly. This provides an improved procedure for controlled dissociation of the complex and efficient recovery of its component enzymes in their native states. Higher concentrations of GdnHCl (up to 4 M) lead to the unfolding and irreversible inactivation of the separate enzymes of the complex with the E2/X core proving the most resistant to GdnHCl-induced unfolding. Neither the 60-meric E2/X core assembly nor the dimeric E3 component are dissociated into monomers in the presence of 6 M GdnHCl; the latter enzyme forms higher-M(r) aggregates under these conditions.

Changes in the structure of pyruvate dehydrogenase complex induced by mono- and divalent ions

The activity of pyruvate dehydrogenase complex purified from pig kidney cortex was affected by various mono- and di-valent ions and changes in ionic strength. The fluorescence emission spectrum of PDC exposed to 0.04 M ionic strength and excited at 280 nm exhibited a maximum at 334 nm; the fluorescence intensity of PDC appeared to depend upon the ionic strength and the K+ and Na+ content of the incubation buffer. Alteration of ionic strength to which the enzyme complex was exposed produced a change in the absorption of the complex at 230 nm. The presence of HPO4(2-) ions prevented changes in the UV absorption spectrum of PDC induced by the variation in ionic strength. The K+ and Na+ ions alone had no effect on the UV spectrum of PDC. Upon increasing the ionic strength to which the enzyme complex was exposed, dramatic changes in the circular dichroism (CD) pattern were observed. At 0.04 M ionic strength PDC exhibited a CD spectrum with minima at 216, 218 and 222 nm and a cross-over point at 215 nm. At 0.15 M ionic strength the CD spectrum of PDC exhibited minima at 223, 226, 228 nm and a cross-over point at 221 nm. The presence of HPO4(2-) ions prevented alterations in the CD spectrum of PDC induced by variations in ionic strength. The K+ and Na+ ions had no effect on the CD spectrum of PDC.

The effects of dichloroacetate on lactate accumulation and endurance in an exercising rat model

Severe lactic acidosis usually accompanies intense endurance exercise. It has been postulated that glycogen depletion working in concert with elevated muscle and plasma lactate levels lead to a concomitant reduction in pH. Their cumulative effect during prolonged physical exertion now leads to muscular fatigue and eventually limit endurance capacity. Therefore in the present study, dichloroacetate (DCA), a compound which enhances the rate of pyruvate oxidation thus reducing lactate formation, has been evaluated in a validated rat model of sub-maximal exercise performance. Male rats (350 g) were divided into two groups (control-saline, i.v. and DCA 5 mg/kg, i.v.) and were exercised to exhaustion in a chamber (26 degrees C) on a treadmill (11 m/min, 6 degrees incline). When compared to controls, the DCA-treated rats had longer run times (169 vs 101 min) and a decreased heating rate (0.020 vs 0.029 degrees C/min). In addition, DCA attenuated the increase in plasma lactate (28 vs 40 mg/dl) and significantly reduced both the rate and absolute amount of depletion of muscle glycogen stores. These results suggest that the activation of pyruvate dehydrogenase activity by DCA resulted in a reduction in the rate of glycogenolysis in addition to decreasing lactate accumulation by presumably limiting the availability of pyruvate for conversion to lactate, therefore increasing muscle carbohydrate oxidation via the TCA cycle. Thus DCA effected a significant delay in muscle fatigue.

Recombinant expression and evaluation of the lipoyl domains of the dihydrolipoyl acetyltransferase component of the human pyruvate dehydrogenase complex

The subunits of the dihydrolipoyl acetyltransferase (E2) component of mammalian pyruvate dehydrogenase complex (PDC) associate to form a large inner core with a protruding structure composed of three globular domains connected by mobile linker regions. This exterior region of E2 includes two lipoyl domains which engage not only in the intermediate reactions of the complex but also have integral roles in the kinase-phosphatase regulatory interconversion of the pyruvate dehydrogenase (E1) component. To facilitate understanding of these roles, lipoyl domain constructs of the E2 component of human PDC were expressed as glutathione S-transferase (GST)-linked fusion proteins from plasmid inserts prepared by polymerase chain reaction procedures. The NH2-terminal lipoyl domain, E2L1, and the interior lipoyl domain, E2L2, are connected by a 30-amino-acid hinge region, H1. Constructs designed and expressed were E2L1(1-98), E2L1.H1(1-128), E2L2(120-233), E2H1.L2(98-233), and E2L1.H1.L2(1-233), where numbers in parentheses give the amino acid sequence for the portions of the E2 component incorporated into a construct. The domains were expressed in Escherichia coli with and without lipoate supplementation. GST constructs were purified to homogeneity by affinity chromatography and selectively released by thrombin treatment. Sequencing of insert DNAs and NH2-terminal sequencing confirmed that domains were produced as designed. Measurement of masses by electrospray mass spectrometry indicated that constructs with lipoylated, nonlipoylated, and octanoylated forms were produced when expression was with E. coli grown without lipoate supplementation and that fully lipoylated forms were produced upon lipoate supplementation. The lipoylation status was confirmed, following delipoylation with Enterococcus faecalis lipoamidase, by the expected decrease in mass and by the observation in native gel electrophoresis of a shift to a slower mobility (possibly less compact) form. Constructs were used in E1-catalyzed reductive-acetylation reaction in proportion to their degree of lipoylation and were effective substrates in a NADH-dependent dihydrolipoyl dehydrogenase reduction reaction. Thus, we have produced lipoyl domain constructs that can be employed in sorting the specific roles of E2L1 and E2L2 in facilitating catalytic and regulatory processes.

Evidence for a role of protein kinase C in the activation of the pyruvate dehydrogenase complex by insulin in Zajdela hepatoma cells

The signal transduction pathway involved in the activation of pyruvate dehydrogenase (PDH) by insulin is still unknown. In this study, we have examined the possible involvement of protein kinase C (PKC) in the process. In addressing this question, we examined (1) the insulin-like effects of the PKC activator 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) on the PDH complex, (2) the effects of various PKC inhibitors on the PDH activation by insulin, and (3) the response of PKC-depleted cells to insulin. We used as an experimental model Zajdela hepatoma cultured (ZHC) cells, which have been demonstrated to be responsive to physiological doses of insulin. Half-maximal and maximal stimulations of the PDH complex by insulin were observed at 0.05 and 5 nmol/L, respectively. Stimulation of PDH activity by insulin (5 nmol/L) occurred within 5 minutes of incubation and was maximal (+70%) at 7.5 minutes. In the presence of PMA (162 nmol/L), enzyme activity increased within 30 seconds, was maximal (+90%) at 5 minutes, and was no longer detectable after 10 minutes. Total PDH activity was unchanged by insulin or PMA treatment. The effects of PMA and insulin on basal PDH activity were not additive. Moreover, various inhibitors of PKC--staurosporine, sphingosine, acridine orange--completely blocked the stimulation of PDH activity induced by insulin or PMA. A 17-hour treatment of ZHC cells with 500 nmol/L PMA efficiently downregulated PKC, as attested by the marked decrease in the enzyme activity and the loss of phorbol 12,13-dibutyrate binding to intact cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of L-carnitine on the pyruvate dehydrogenase complex and carnitine palmitoyl transferase activities in muscle of endurance athletes

The effects of L-carnitine on the pyruvate dehydrogenase (PDH) complex and carnitine palmitoyl transferase (CPT) were studied in muscle of 16 long-distance runners (LDR). These subjects received placebo or L-carnitine (2 g orally) during a 4-week period of training. Athletes receiving L-carnitine showed a dramatic increase (P < 0.001) in the PDH complex activities. By contrast, the levels of CPT, both 1 and 2, were unchanged. No significant changes were observed after placebo administration. We previously reported [Huertas R. et al., Biochem. Biophys. Res. Commun. 188 (1992) 102-107] that L-carnitine induces an increase in the activities of complexes I, III and IV of the respiratory chain in muscle of LDR. Taken together, our data suggest that the improvement in (maximal oxygen consumption) VO2max observed in LDR after L-carnitine administration is based on these biochemical findings.

[Effect of thiamine and its metabolites on the activity of tissue and purified pyruvate dehydrogenase]

Thiochrome is able to suppress pyruvate dehydrogenase activity. This effect is observed after injection of thiochrome to an organism and in experiments on a purified enzyme.

Enhanced post-receptor insulin effects in women following dehydroepiandrosterone infusion

OBJECTIVE

We hypothesized that intravenous dehydroepiandrosterone (DHEA) would decrease insulin resistance in normal and insulin-resistant women.

METHODS

Five insulin-resistant women diagnosed as having polycystic ovaries (PCO) with elevated testosterone and normal dehydroepiandrosterone sulfate (DHEAS) with amenorrhea were recruited. Obese controls (OC) with normal menses and normal testosterone and DHEAS were recruited and matched to each PCO woman for age and weight. The PCO women had a mean testosterone of 3.2 +/- 0.4 nmol/L, fasting serum insulin level of 330 +/- 55 pmol/L, and DHEAS level of 3.4 +/- 1.3 mumol/L. An oral glucose tolerance test (OGTT) was performed at 8 AM after an overnight fast. A DHEA infusion (1 mg/hour for 17 hours) was begun at 6 PM and continued until the completion of the second OGTT performed the following morning at 8 AM. T-lymphocytes were drawn at 8 AM each morning.

RESULTS

The DHEA infusion had no significant effect on any of the in vivo indices of insulin sensitivity, ie, basal and OGTT insulin, C-peptide, and ratios of insulin/glucose. In vitro, DHEA significantly increased insulin binding to T-lymphocytes of PCO women but caused no significant change in OC women. There was, however, marked enhancement of T-lymphocyte pyruvate dehydrogenase (PDH) activities in both groups of study subjects following DHEA.

CONCLUSION

We conclude that a 17-hour infusion of DHEA enhanced T-lymphocyte insulin binding and PDH activity while producing no detectable improvements in in vivo indices of insulin sensitivity.

Long-term regulation of pyruvate dehydrogenase kinase by high-fat feeding. Experiments in vivo and in cultured cardiomyocytes

The provision of the high-fat diet (47% of calories as fat) for 28 days evoked a significant decline in cardiac PDHa activity, together with marked increases in the activity of PDH kinase measured in isolated mitochondria and freshly-prepared cardiomyocytes from adult rats. Plasma insulin concentrations in fat-fed rats were not significantly different from control, but plasma NEFA concentrations were elevated. PDH kinase activity in cardiomyocytes from fat-fed rats fell substantially in culture (25 h). This decline was prevented by the inclusion of n-octanoate and DBcAMP in combination, but not individually, in the culture medium. The results are discussed in relation to the role for fatty acids and insulin in the long-term modulation of cardiac PDH kinase activity by high-fat feeding.

Differences in sensitivity to NADH of purified pyruvate dehydrogenase complexes of Enterococcus faecalis, Lactococcus lactis, Azotobacter vinelandii and Escherichia coli: implications for their activity in vivo

The effect of NADH on the activity of the purified pyruvate dehydrogenase complexes (PDHc) of Enterococcus (Ec.) faecalis, Lactococcus lactis, Azotobacter vinelandii and Escherichia coli was determined in vitro. It was found that the PDHc of E. coli and L. lactis was active only at relatively low NADH/NAD ratios, whereas the PDHc of Ec. faecalis was inhibited only at high NADH/NAD ratios. The PDHc of Azotobacter vinelandii showed an intermediate sensitivity. The organisms were grown in chemostat culture under conditions that led to different intracellular NADH/NAD ratios and the PDHc activities in vivo could be calculated from the specific rates of product formation. Under anaerobic growth conditions, only Ec. faecalis expressed PDHc activity in vivo. The activities in vivo of the complexes of the different organisms were in good agreement with their properties determined in vitro. The physiological consequences of these results are discussed.

[Effect of pyrazole on the activity of acetaldehyde-producing enzymes in the liver]

Influence of pyrazole on the endogenous ethanol level and activities of acetaldehyde-producing enzymes was investigated. Drastic enhancement of the endogenous ethanol level in the blood and tissues was accompanied by an insignificant increase of phosphoethanolamine lyase activity, while activity of threonine aldolase and pyruvate dehydrogenase was unchanged.

Regulation of pyruvate dehydrogenase activity in rat fat pads and isolated hepatocytes by levamisole

The effect of levamisole on the catalytic activity of the pyruvate dehydrogenase (PDH) complex in rat adipose tissue segments and isolated hepatocytes was studied. Levamisole, an anthelmintic and immunotherapeutic agent with concentration-dependent oxidant/antioxidant properties, increased the catalytic activity of the pyruvate dehydrogenase complex in these systems. The activity of this enzyme complex may be regulated in part by protein-thiol modification. The treatment of rat adipose tissue segments and isolated hepatocytes with and without levamisole (0.1-3.0 mM) for 30 min resulted in a reversible increase in the catalytic activity of this enzyme complex. Levamisole at 1.5 mM and 2.0 mM were as potent as insulin (1.0 mU/ml) in adipose tissue and as dichloroacetate (2.0 mM) in isolated hepatocytes, respectively, in increasing the catalytic activity of the PDH complex. The stimulatory effects seen with levamisole were not accompanied by decreases in adenosine triphosphate (ATP) levels. The results of the present investigation suggest that the pyruvate dehydrogenase complex in adipose tissue and liver may be useful models for studying the mechanism of action of levamisole.

Effects of recombinant monokines on hepatic pyruvate dehydrogenase, pyruvate dehydrogenase kinase, lipogenesis de novo and plasma triacylglycerols. Abolition by prior fasting

1. The effects of recombinant human tumour necrosis factor alpha (TNF) and murine interleukin-1 alpha (IL-1) on the activation state of the hepatic pyruvate dehydrogenase complex (PDHa), the activity of mitochondrial PDH kinase, hepatic lipogenesis de novo and plasma triacylglycerol (TG) concentrations were studied. 2. Monokine effects depended upon prior nutritional state. In rats fasted for 20 h or 45 h before monokine administration and refeeding (orally or with intravenous glucose), PDHa, TG and hepatic lipogenesis were not increased. In rats fed ad libitum, treatment with TNF plus IL-1 increased the contribution of hepatic lipogenesis to circulating TG to 550% of control values (P = 0.03) and plasma TG concentrations to 159% (P = 0.02), whereas PDHa increased slightly to 120% (P = 0.02) and liver glycogen content fell to 45.8% (P = 0.05) of control values. 3. Intrinsic hepatic PDH kinase activity was not changed by monokine treatment in rats fed ad libitum. 4. The increased lipogenesis de novo showed no correlation (r2 = 0.05, not significant) with hepatic PDHa in individual animals fed ad libitum. 5. In conclusion, these results suggest that monokines increase pyruvate flux through hepatic PDH in vivo in rats fed ad libitum primarily by mechanisms other than covalent modification of PDH. Prior nutritional status exerts a permissive effect for monokine stimulation of PDHa and lipogenesis, consistent with a substrate-mediated action, but the mechanism of this permissive effect remains uncertain.

Proteinase-catalyzed activation of porcine heart muscle pyruvate dehydrogenase and identification of its cleavage site

Porcine heart muscle pyruvate dehydrogenase (PDH, EC 1.2.4.1) with subunit composition alpha 2 beta 2 catalyzes the initial decarboxylation step of an oxidative decarboxylation sequence of pyruvate. Highly purified PDH, was further activated several-fold by limited digestion with trypsin, Staphylococcus aureus V8 proteinase (V8) or papain. The activation with these proteinases required about 10 min to attain a maximal level, lasted 1/2-2 h and thereafter decreased gradually. Addition of an inhibitor of each proteinase resulted in an immediate cessation of any further changes in the enzymatic activity. The optimal pH of the proteinase-activated PDH was not affected. Proteinases increased the maximum velocity and the apparent Km values for pyruvate, but the Hill coefficients for pyruvate were unchanged. Proteinase-activated PDH was capable of associating two other component enzymes to produce large unit resembling the native complex. The Coomassie brilliant blue stained gels after SDS-PAGE showed that the PDH alpha subunit (41 kDa) was cleaved by trypsin or V8 into two major fragments (31 and 10 kDa), whereas PDH beta was unaffected. By amino-terminal sequence analyses of these fragments the trypsin cleavage sites were identified as Arg-273 and Arg-282 and the V8 cleavage sites were Glu-277 and Glu-280.