Compartmentation of citrate in relation to the regulation of glycolysis and the mitochondrial transmembrane proton electrochemical potential gradient in isolated perfused rat heart

Optical Imaging Opportunities to Inspect the Nature of Cytosolic Iron Pools

The chemical nature of intracellular labile iron pools (LIPs) is described. By virtue of the kinetic lability of these pools, it is suggested that the isolation of such species by chromatography methods will not be possible, but rather mass spectrometric techniques should be adopted. Iron-sensitive fluorescent probes, which have been developed for the detection and quantification of LIP, are described, including those specifically designed to monitor cytosolic, mitochondrial, and lysosomal LIPs. The potential of near-infrared (NIR) probes for in vivo monitoring of LIP is discussed.

Recovery of ΔF508-CFTR Function by Citrate

Treatment of cystic fibrosis relies so far on expensive and sophisticated drugs. A logical approach to rescuing the defective ΔF508-CFTR protein has not yet been published. Therefore, virtual docking of ATP and CFTR activators to the open conformation of the CFTR protein was performed. A new ATP binding site outside of the two known locations was identified. It was located in the cleft between the nucleotide binding domains NBD1 and NBD2 and comprised six basic amino acids in close proximity. Citrate and isocitrate were also bound to this site. Citrate was evaluated for its action on epithelial cells with intact CFTR and defective ΔF508-CFTR. It activated hyaluronan export from human breast carcinoma cells and iodide efflux, and recovered ΔF508-CFTR from premature intracellular degradation. In conclusion, citrate is an activator for ΔF508-CFTR and increases export by defective ΔF508-CFTR into the extracellular matrix of epithelial cells.

The Role of GSH in Intracellular Iron Trafficking

Evidence is reviewed for the role of glutathione in providing a ligand for the cytosolic iron pool. The possibility of histidine and carnosine forming ternary complexes with iron(II)glutathione is discussed and the physiological significance of these interactions considered. The role of carnosine in muscle, brain, and kidney physiology is far from established and evidence is presented that the iron(II)-binding capability of carnosine relates to this role.

Optical Recording of Zn2+ Dynamics in the Mitochondrial Matrix and Intermembrane Space with the GZnP2 Sensor

The zinc ion (Zn²⁺) is emerging as an important signaling molecule. Here, we engineered an improved Zn²⁺ probe GZnP2 based on a previously developed fluorescent sensor GZnP1 to provide a higher fluorescent readout (2-fold higher) that is proportional to cellular labile Zn²⁺ concentrations. We further developed a set of GZnP2 derived imaging tools to determine the labile Zn²⁺ concentrations in the mitochondrial matrix, mitochondrial intermembrane space (IMS), and cytosol in four different cell lines (HeLa, Cos-7, HEK293, and INS-1). The labile Zn²⁺ concentration in the matrix was less than 1 pM, while the labile Zn²⁺ concentration in the IMS was comparable to the cytosol (∼100 pM). With these sensors, we showed that upon exposure to high Zn²⁺, only the cytosol and the IMS were overloaded with Zn²⁺, while the mitochondrial matrix was unable to sequester excess labile Zn²⁺ in depolarized INS-1 cells. This work highlighted the importance of distinguishing the labile Zn²⁺ concentrations and dynamics between the mitochondrial matrix and IMS.

Oncometabolite d-2-hydroxyglutarate impairs α-ketoglutarate dehydrogenase and contractile function in rodent heart

Hematologic malignancies are frequently associated with cardiac pathologies. Mutations of isocitrate dehydrogenase 1 and 2 (IDH1/2) occur in a subset of acute myeloid leukemia patients, causing metabolic and epigenetic derangements. We have now discovered that altered metabolism in leukemic cells has a profound effect on cardiac metabolism. Combining mathematical modeling and in vivo as well as ex vivo studies, we found that increased amounts of the oncometabolite d-2-hydroxyglutarate (D2-HG), produced by IDH2 mutant leukemic cells, cause contractile dysfunction in the heart. This contractile dysfunction is associated with impaired oxidative decarboxylation of α-ketoglutarate, a redirection of Krebs cycle intermediates, and increased ATP citrate lyase (ACL) activity. Increased availability of D2-HG also leads to altered histone methylation and acetylation in the heart. We propose that D2-HG promotes cardiac dysfunction by impairing α-ketoglutarate dehydrogenase and induces histone modifications in an ACL-dependent manner. Collectively, our results highlight the impact of cancer cell metabolism on function and metabolism of the heart.

Sites of superoxide and hydrogen peroxide production by muscle mitochondria assessed ex vivo under conditions mimicking rest and exercise

The sites and rates of mitochondrial production of superoxide and H2O2 in vivo are not yet defined. At least 10 different mitochondrial sites can generate these species. Each site has a different maximum capacity (e.g. the outer quinol site in complex III (site IIIQo) has a very high capacity in rat skeletal muscle mitochondria, whereas the flavin site in complex I (site IF) has a very low capacity). The maximum capacities can greatly exceed the actual rates observed in the absence of electron transport chain inhibitors, so maximum capacities are a poor guide to actual rates. Here, we use new approaches to measure the rates at which different mitochondrial sites produce superoxide/H2O2 using isolated muscle mitochondria incubated in media mimicking the cytoplasmic substrate and effector mix of skeletal muscle during rest and exercise. We find that four or five sites dominate during rest in this ex vivo system. Remarkably, the quinol site in complex I (site IQ) and the flavin site in complex II (site IIF) each account for about a quarter of the total measured rate of H2O2 production. Site IF, site IIIQo, and perhaps site EF in the β-oxidation pathway account for most of the remainder. Under conditions mimicking mild and intense aerobic exercise, total production is much less, and the low capacity site IF dominates. These results give novel insights into which mitochondrial sites may produce superoxide/H2O2 in vivo.

Imaging mitochondrial flux in single cells with a FRET sensor for pyruvate

Mitochondrial flux is currently accessible at low resolution. Here we introduce a genetically-encoded FRET sensor for pyruvate, and methods for quantitative measurement of pyruvate transport, pyruvate production and mitochondrial pyruvate consumption in intact individual cells at high temporal resolution. In HEK293 cells, neurons and astrocytes, mitochondrial pyruvate uptake was saturated at physiological levels, showing that the metabolic rate is determined by intrinsic properties of the organelle and not by substrate availability. The potential of the sensor was further demonstrated in neurons, where mitochondrial flux was found to rise by 300% within seconds of a calcium transient triggered by a short theta burst, while glucose levels remained unaltered. In contrast, astrocytic mitochondria were insensitive to a similar calcium transient elicited by extracellular ATP. We expect the improved resolution provided by the pyruvate sensor will be of practical interest for basic and applied researchers interested in mitochondrial function.

Iron speciation in the cytosol: an overview

The nature of the cytosolic iron pool remains largely uncharacterized, although a range of candidate ligands and chaperones have been proposed. Herein an overview is presented of cytosolic non heme and non iron-sulphur cluster protein iron binding sites and the influence of ligands on the redox activity of iron. This analysis leads to the concept of iron(II) glutathione functioning as the labile cytosolic iron pool and offers a means for the selection of iron over manganese in subsequent incorporation into a wide range of iron-dependent enzymes and electron transfer proteins. Glutathione and glutathione-binding glutaredoxins play a critical role in iron sulfur cluster synthesis and Fe(II)GS (iron(II) coordinated by the thiol function of glutathione) is a suitable iron donor for this biosynthetic route. Significantly, both glutathione and glutaredoxins are universally distributed and thus a controlling influence of glutathione on intracellular iron trafficking is likely to be a common feature of the majority of living organisms.

A genetically encoded FRET lactate sensor and its use to detect the Warburg effect in single cancer cells

Lactate is shuttled between and inside cells, playing metabolic and signaling roles in healthy tissues. Lactate is also a harbinger of altered metabolism and participates in the pathogenesis of inflammation, hypoxia/ischemia, neurodegeneration and cancer. Many tumor cells show high rates of lactate production in the presence of oxygen, a phenomenon known as the Warburg effect, which has diagnostic and possibly therapeutic implications. In this article we introduce Laconic, a genetically-encoded Forster Resonance Energy Transfer (FRET)-based lactate sensor designed on the bacterial transcription factor LldR. Laconic quantified lactate from 1 µM to 10 mM and was not affected by glucose, pyruvate, acetate, betahydroxybutyrate, glutamate, citrate, α-ketoglutarate, succinate, malate or oxalacetate at concentrations found in mammalian cytosol. Expressed in astrocytes, HEK cells and T98G glioma cells, the sensor allowed dynamic estimation of lactate levels in single cells. Used in combination with a blocker of the monocarboxylate transporter MCT, the sensor was capable of discriminating whether a cell is a net lactate producer or a net lactate consumer. Application of the MCT-block protocol showed that the basal rate of lactate production is 3–5 fold higher in T98G glioma cells than in normal astrocytes. In contrast, the rate of lactate accumulation in response to mitochondrial inhibition with sodium azide was 10 times lower in glioma than in astrocytes, consistent with defective tumor metabolism. A ratio between the rate of lactate production and the rate of azide-induced lactate accumulation, which can be estimated reversibly and in single cells, was identified as a highly sensitive parameter of the Warburg effect, with values of 4.1 ± 0.5 for T98G glioma cells and 0.07 ± 0.007 for astrocytes. In summary, this article describes a genetically-encoded sensor for lactate and its use to measure lactate concentration, lactate flux, and the Warburg effect in single mammalian cells.

Fructose-2,6-bisphosphate is lower in copper deficient rat cerebellum despite higher content of phosphorylated AMP-activated protein kinase

Limitation in copper (Cu) leads to pathophysiology in developing brain. Cu deficiency impairs brain mitochondria and results in high brain lactate suggesting augmented anaerobic glycolysis. AMP activated protein kinase (AMPK) is a cellular energy "master-switch" that is thought to augment glycolysis through phosphorylation and activation phosphofructokinase 2 (PFK2) resulting in increases of the glycolytic stimulator fructose-2,6-bisphosphate (F2,6BP). Previously, Cu deficiency has been shown to augment cerebellar AMPK activation. Cerebella of Cu-adequate (Cu+) and Cu-deficient (Cu-) rat pups were assessed to evaluate if AMPK activation in Cu- cerebella functioned to enhance PFK2 activation and increase F2,BP concentration. Higher levels of pAMPK were detected in Cu- cerebella. However, PFK2 activity, mRNA, and protein abundance were not affected by Cu deficiency. Surprisingly, F2,6BP levels were markedly lower in Cu- cerebella. Lower F2,6BP may be due to inhibition of PFK2 by citrate, as citrate concentration was significantly higher in Cu- cerebella. Data suggest AMPK activation in Cu- cerebellum does not augment glycolysis through a PFK2 mechanism. Furthermore, other metabolite data suggest that glycolysis may actually be blunted, since levels of glucose and glucose-6-phosphate were higher in Cu- cerebella than controls.

Modulation of Mg2+ efflux from rat ventricular myocytes studied with the fluorescent indicator furaptra

The fluorescent Mg(2+) indicator furaptra (mag-fura-2) was introduced into single ventricular myocytes by incubation with its acetoxy-methyl ester form. The ratio of furaptra's fluorescence intensity at 382 and 350 nm was used to estimate the apparent cytoplasmic [Mg(2+)] ([Mg(2+)](i)). In Ca(2+)-free extracellular conditions (0.1 mM EGTA) at 25 degrees C, [Mg(2+)](i) averaged 0.842 +/- 0.019 mM. After the cells were loaded with Mg(2+) by exposure to high extracellular [Mg(2+)] ([Mg(2+)](o)), reduction of [Mg(2+)](o) to 1 mM (in the presence of extracellular Na(+)) induced a decrease in [Mg(2+)](i). The rate of decrease in [Mg(2+)](i) was higher at higher [Mg(2+)](i), whereas raising [Mg(2+)](o) slowed the decrease in [Mg(2+)](i) with 50% reduction of the rate at approximately 10 mM [Mg(2+)](o). Because a part of the furaptra molecules were likely trapped inside intracellular organelles, we assessed possible contribution of the indicator fluorescence emitted from the organelles. When the cell membranes of furaptra-loaded myocytes were permeabilized with saponin (25 microg/ml for 5 min), furaptra fluorescence intensity at 350-nm excitation decreased to 22%; thus approximately 78% of furaptra fluorescence appeared to represent cytoplasmic [Mg(2+)] ([Mg(2+)](c)), whereas the residual 22% likely represented [Mg(2+)] in organelles (primarily mitochondria as revealed by fluorescence imaging). [Mg(2+)] calibrated from the residual furaptra fluorescence ([Mg(2+)](r)) was 0.6-0.7 mM in bathing solution [Mg(2+)] (i.e., [Mg(2+)](c) of the skinned myocytes) of either 0.8 mM or 4.0 mM, suggesting that [Mg(2+)](r) was lower than and virtually insensitive to [Mg(2+)](c). We therefore corrected furaptra fluorescence signals measured in intact myocytes for this insensitive fraction of fluorescence to estimate [Mg(2+)](c). In addition, by utilizing concentration and dissociation constant values of known cytoplasmic Mg(2+) buffers, we calculated changes in total Mg concentration to obtain quantitative information on Mg(2+) flux across the cell membrane. The calculations indicate that, in the presence of extracellular Na(+), Mg(2+) efflux is markedly activated by [Mg(2+)](c) above the normal basal level (approximately 0.9 mM), with a half-maximal activation of approximately 1.9 mM [Mg(2+)](c). We conclude that [Mg(2+)](c) is tightly regulated by an Mg(2+) efflux that is dependent on extracellular [Na(+)].

Citrate release by perfused rat hearts: a window on mitochondrial cataplerosis

Cytosolic citrate is proposed to play a crucial role in substrate fuel selection in the heart. However, little is known about factors regulating the transfer of citrate from the mitochondria, where it is synthesized, to the cytosol. Further to our observation that rat hearts perfused under normoxia release citrate whose (13)C labeling pattern reflects that of mitochondrial citrate (B. Comte, G. Vincent, B. Bouchard, and C. Des Rosiers. J. Biol. Chem. 272: 26117-26124, 1997), we report here data indicating that this citrate release is a specific process reflecting the mitochondrial efflux of citrate, a process referred to as cataplerosis. Indeed, measured rates of citrate release, which vary between 2 and 21 nmol/min, are modulated by the nature and concentration of exogenous substrates feeding acetyl-CoA (fatty acid) and oxaloacetate (lactate plus pyruvate) for the mitochondrial citrate synthase reaction. Such release rates that represent at most 2% of the citric acid cycle flux are in agreement with the activity of the mitochondrial tricarboxylate transporter whose participation is also substantiated by 1) parallel variations in citrate release rates and tissue levels of citrate plus malate, the antiporter, and 2) a lowering of the citrate release rate by 1,2, 3-benzenetricarboxylic acid, a specific inhibitor of the transporter. Taken together, the results from the present study indicate that citrate cataplerosis is modulated by substrate supply, in agreement with the role of cytosolic citrate in fuel partitioning, and occurs, at least in part, through the mitochondrial tricarboxylate transporter.

Cardiac accumulation of citrate during brief myocardial ischaemia and reperfusion in the pig in vivo

Citrate is a key intermediate in energy metabolism and an inhibitor of phosphofructokinase of the glycolytic pathway. During myocardial ischaemia glycolysis is the main source of cardiac ATP. The aim of the present study was to determine if myocardial ischaemia and reperfusion alter cardiac tissue levels of citrate. Open-chest, anaesthetized pigs were subjected to 10 min of regional myocardial ischaemia by occlusion of the left anterior descending coronary artery, with and without reperfusion, and to 10 min of global ischaemia by circulatory arrest. Citrate, amino acids, glucose and NH3 were measured in biopsies. Ischaemia, whether regional or global, caused a 60-70% increase in tissue levels of citrate. During 1 min of reperfusion following regional ischaemia the level of citrate increased 460%, to approximately 600 nmol g-1 wet weight. The level of glutamate decreased by 20-33% (corresponding to 1300-2200 nmol g-1 wet weight), indicating net consumption of this amino acid during ischaemia. The level of aspartate decreased 50% indicating conversion of aspartate to oxaloacetate for the synthesis of citrate. Theoretically, the accumulation of myocardial citrate during brief ischaemia and early reperfusion is large enough to significantly inhibit phosphofructokinase activity and could therefore affect the ability of the myocardium to increase the glycolytic rate in response to ischaemia. This could, however, be partly compensated by the metabolism of myocardial glutamate.

Glucose transport and glucose transporter GLUT4 are regulated by product(s) of intermediary metabolism in cardiomyocytes

Alternative substrates of energy metabolism are thought to contribute to the impairment of heart and muscle glucose utilization in insulin-resistant states. We have investigated the acute effects of substrates in isolated rat cardiomyocytes. Exposure to lactate, pyruvate, propionate, acetate, palmitate, beta-hydroxybutyrate or alpha-oxoglutarate led to the depression of glucose transport by up to 50%, with lactate, pyruvate and propionate being the most potent agents. The percentage inhibition was greater in cardiomyocytes in which glucose transport was stimulated with the alpha-adrenergic agonist phenylephrine or with a submaximal insulin concentration than in basal or fully insulin-stimulated cells. Cardiomyocytes from fasted or diabetic rats displayed a similar sensitivity to substrates as did cells from control animals. On the other hand, the amination product of pyruvate (alanine), as well as valine and the aminotransferase inhibitors cycloserine and amino-oxyacetate, stimulated glucose transport about 2-fold. In addition, the effect of pyruvate was counteracted by cycloserine. Since reversible transamination reactions are known to affect the pool size of the citrate cycle, the influence of substrates, amino acids and aminotransferase inhibitors on citrate, malate and glutamate content was examined. A significant negative correlation was found between alterations in glucose transport and the levels of citrate (P < 0.01) or malate (P < 0.01), and there was a positive correlation between glucose transport and glutamate levels (P < 0.05). In contrast, there was no correlation with changes in [1-(14)C]pyruvate oxidation or in glucose-6-phosphate levels. Finally, pyruvate decreased the abundance of GLUT4 glucose transporters at the surface of phenylephrine- or insulin-stimulated cells by 34% and 27 % respectively, as determined by using the selective photoaffinity label [3H]ATB-BMPA [[3H]2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis-(D-man nos-4-yloxy)propyl-2-amine]. In conclusion, cardiomyocyte glucose transport is subject to counter-regulation by alternative substrates. The glucose transport system appears to be controlled by (a) compound(s) of intermediary metabolism (other than glucose 6-phosphate), but in a different way than pyruvate dehydrogenase. Transport inhibition eventually occurs via a decrease in the amount of glucose transporters in the plasma membrane.

Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium

Cytosolic free magnesium (Mgf) is considered relatively constant. To test this concept, Mgf was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mgf estimates utilized a new sliding scale derived from the Mg(2+)-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant K'GAPDH.K'PGK was measured as ([creatine phosphate][3-phosphoglycerate][lactate]KLDH)/([creatine][Pi] [glyceraldehyde 3-phosphate][pyruvate]KCK), which varied with magnesium due to KCK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants KGAPDH.KPGK and KCK were taken from the literature. The [Mg2+] at which pseudo K'GAPDH.K'PGK equalled true KGAPDH.KPGK was the best estimate of Mgf.Mgf fell to approximately 0.13 mM in hyperkalemic arrest from a control of approximately 0.6 mM, rising to approximately 0.85 mM only during maximum adrenergic stress. Mgf increased further to approximately 1.3 mM during sulfate loading which induced ATP catabolism. Mgf and ATP were reciprocally related. Thus; (1) myocardial free [Mg2+] judged from GADPH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg2+ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mgf.

Combined glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase in catecholamine-stimulated guinea-pig cardiac muscle. Comparison with mass-action ratio of creatine kinase

The steady-state reactant levels of triose-phosphate isomerase and the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system were examined in guinea-pig cardiac muscle. Key glycolytic intermediates, including glyceraldehyde 3-phosphate were directly measured and compared with those of creatine kinase. Non-working Langendorff hearts as well as isolated working hearts were perfused with 5 mM glucose (plus insulin) under normoxia conditions to maintain lactate dehydrogenase near-equilibrium. The cytosolic phosphorylation potential ([ATP]/([ADP].[Pi])) was derived from creatine kinase and the free [NAD+]/([NADH].[H+]) ratio from lactate dehydrogenase. In Langendorff hearts glycolysis was varied from near-zero flux (hyperkalemic cardiac arrest) to higher than normal flux (normal and maximum catecholamine stimulation). The triose-phosphate isomerase was near-equilibrium only in control or potassium-arrested Langendorff hearts as well as in postischemic 'stunned' hearts. However, when glycolytic flux increased due to norepinephrine or due to physiological pressure-volume work the enzyme was displaced from equilibrium. The alternative phosphorylation ratio [ATP]'/([ADP]).[Pi]) was derived from the magnesium-dependent glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system assigning free magnesium different values in the physiological range (0.1-2.0 mM). As predicted, [ATP]/([ADP].[Pi]) and [ATP]'/([ADP]'.[Pi]') were in excellent agreement when glycolysis was virtually halted by hyperkalemic arrest (flux approximately 0.2 mumol C3.min-1.g dry mass-1). However, the equality between the two phosphorylation ratios was not abolished upon resumption of spontaneous beating and also not during adrenergic stimulation (flux approximately 5-14 mumol C3.min-1.g dry mass-1). In contrast, when flux increased due to transition from no-work to physiological pressure-volume work (rate increase from approximately 3 to 11 mumol C3.min-1.g dry mass-1), the two ratios were markedly different indicating disequilibrium of the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase. Only during adrenergic stimulation or postischemic myocardial 'stunning', not due to hydraulic work load per se, glyceraldehyde-3-phosphate levels increased from about 4 microM to greater than or equal to 16 microM. Thus the guinea-pig cardiac glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system can realize the potential for near-equilibrium catalysis at significant flux provided glyceraldehyde-3-phosphate levels rise, e.g., due to 'stunning' or adrenergic hormones.

Subcellular distribution of malate-aspartate cycle intermediates during normoxia and anoxia in the heart

The subcellular distribution of adenine nucleotides, phosphocreatine and intermediates of the malate-aspartate cycle was investigated in adult rat heart myocytes under normoxia and anoxia. Cytosolic and mitochondrial concentrations of metabolites were determined by a fractionation method using digitonin. Under normoxia, cytosolic/mitochondrial gradients were found for ATP (c/m = 4), AMP (c/m less than 0.01), citrate (c/m = 0.5), aspartate (c/m = 3), glutamate (c/m = 2), while phosphocreatine and glutamine were confined to the cytosolic space. No gradients were found for malate and 2-oxoglutarate. The results show that the transport of electrons from the cytosol into the mitochondria is supported by the glutamate gradient and by a high glutamate/aspartate ratio inside the mitochondria (Glu/Asp = 15) which is maintained by the energy-dependent Glu-Asp exchange across the mitochondrial membrane. Under anoxia, cytosolic glutamate is transaminated with pyruvate, yielding alanine and 2-oxoglutarate, which is oxidized to succinate inside the mitochondria and leaves the cell. The data indicate that stimulation of transamination is caused by a mass action effect following a decrease in cytosolic 2-oxoglutarate which may be due to succinate-2-oxoglutarate exchange across the mitochondrial membrane. Inhibition of the energy-dependent inward transport of glutamate may support this process.

Aminooxyacetic acid inhibits the malate-aspartate shuttle in isolated nerve terminals and prevents the mitochondria from utilizing glycolytic substrates

Aminooxyacetate, an inhibitor of pyridoxal-dependent enzymes, is routinely used to inhibit gamma-aminobutyrate metabolism. The bioenergetic effects of the inhibitor on guinea-pig cerebral cortical synaptosomes are investigated. It prevents the reoxidation of cytosolic NADH by the mitochondria by inhibiting the malate-aspartate shuttle, causing a 26 mV negative shift in the cytosolic NAD+/NADH redox potential, an increase in the lactate/pyruvate ratio and an inhibition of the ability of the mitochondria to utilize glycolytic pyruvate. The 3-hydroxybutyrate/acetoacetate ratio decreased significantly, indicating oxidation of the mitochondrial NAD+/NADH couple. The results are consistent with a predominant role of the malate-aspartate shuttle in the reoxidation of cytosolic NADH in isolated nerve terminals. Aminooxyacetate limits respiratory capacity and lowers mitochondrial membrane potential and synaptosomal ATP/ADP ratios to an extent similar to glucose deprivation. Thus, the inhibitor induces a functional 'hypoglycaemia' in nerve terminals and should be used with caution.

Role of NADP+ (corrected)-linked malic enzymes as regulators of the pool size of tricarboxylic acid-cycle intermediates in the perfused rat heart

Cytosolic and mitochondrial concentrations of malate, 2-oxoglutarate, isocitrate and pyruvate in the isolated perfused rat heart were measured by non-aqueous tissue fractionation, taking the NADP-linked isocitrate dehydrogenase as indicator reactions for the free [NADPH]/[NADP+] ratios. The mass-action ratios of NADP-linked malic enzymes (EC 1.1.1.40) were found to be on the side of pyruvate carboxylation by more than one order of magnitude in both the cytosolic and the mitochondrial spaces in hearts perfused with glucose, whereas during propionate perfusion this ratio approached the equilibrium constant (Keq.) of malic enzyme. The results consequently indicate that the NADP-linked malic enzymes cannot be responsible for the feed-out (cataplerotic) reactions from the tricarboxylic acid cycle which occur during glucose perfusion. Only when other anaplerotic fluxes into the cycle are high, as during propionate oxidation, which results in accumulation of tricarboxylic acid-cycle intermediates, is a steady state reached which allows efflux via the malic enzyme.

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Mitochondrial membrane potential, transmembrane difference in the NAD+ redox potential and the equilibrium of the glutamate-aspartate translocase in the isolated perfused rat heart

The distribution of glutamate and aspartate and the mitochondrial membrane potential (delta psi) were studied in isolated rat heart mitochondria and in the intact perfused rat heart. The diffusion potential imposed by the glutamate-aspartate exchange through mediation of the electrogenic glutamate-aspartate translocator attained a value close to the mitochondrial delta psi measured from the distribution of triphenylmethylphosphonium ion (TPMP+) both in isolated mitochondria and in intact myocardium. Distributions of the delta psi probe and metabolites were determined by subcellular fractionation of the heart muscle in a non-aqueous medium. The results indicate that the glutamate-aspartate translocator is in near equilibrium in the myocardium. The diffusion potential of the glutamate-aspartate exchange, and the mitochondrial/cytosolic difference in the redox potentials of the free NAD+/NADH pools are equal allowing for experimental error. These data obtained from intact tissue can therefore be interpreted as supporting the notion of the transmembrane uphill transport of reducing equivalent from the cytosolic free NAD+/NADH pool being driven by the malate-aspartate cycle energized by the mitochondrial delta psi.

Proton electrochemical potential of the inner mitochondrial membrane in isolated perfused rat hearts, as measured by exogenous probes

The membrane potential (delta psi) and delta pH of the inner mitochondrial membrane were studied in isolated perfused rat hearts using exogenous labelled probes and tissue fractionation in non-aqueous media. The mitochondrial delta psi, measured by means of the subcellular distribution of [3H]triphenylmethylphosphonium (TPMP+), was 125 +/- 7 mV (negative inside) in hearts beating at 5 Hz and 150 +/- 3 mV (negative inside) in hearts beating at 1.5 Hz. The mitochondrial membrane delta pH, measured by means of the subcellular distribution of low concentrations of [1-14C]propionate, was 0.63 +/- 0.06 pH units (alkaline inside) in hearts beating at 5 Hz and 0.53 +/- 0.12 pH units (alkaline inside) in hearts beating at 1.5 Hz. The implication of proton and electron gradients in the regulation of cellular respiration is discussed. In combination with previous evidence on adenylate distribution in the isolated perfused rat heart, the results indicate that the mitochondrial electrogenic adenylate translocator is in near equilibrium with delta psi.

Regulation of fatty acid oxidation in heart muscle. Effects of pyruvate and dichloroacetate

The possibility of a mutual regulation between carbohydrate and fatty acid oxidation was studied in isolated perfused rat hearts. Infusions of pyruvate and/or dichloroacetate were employed to convert fully the pyruvate dehydrogenase complex into its active form during [1-14C]octanoate oxidation, the rate of which was measured by the production of 14CO2. It was found that 5 mM dichloroacetate suppressed the oxidation of 0.1 mM octanoate by 58%, but 10 mM external pyruvate was without effect. Dichloroacetate reduced the tissue malate concentration by 41% and the citrate concentration by 76% at a 0.1 mM octanoate concentration, but had no effect on the metabolite concentrations or fatty acid oxidation rate in perfusions with 1 mM octanoate. Metabolite depletion was probably partly due to inhibition of the carboxylation of pyruvate, as verified by determination of the metabolite labelling kinetics from [1-14C]pyruvate. It is, therefore, possible that the dichloroacetate-induced inhibition of octanoate oxidation could also be partly due to inhibition of tricarboxylic acid cycle secondary to metabolite depletion. Since both dichloroacetate and pyruvate converted pyruvate dehydrogenase to its active form, but only dichloroacetate inhibited fatty acid oxidation, the latter effect could not be due to oxidation of a competing substrate, but instead may result from an inhibition of fatty acid uptake, activation or transport, as also indicated by the observed decrease in the acid-soluble acyl-CoA concentration. This interpretation is also supported by the mitochondrial redox effects of dichloroacetate observed during octanoate oxidation in the perfused heart.

Metabolic compartmentation of pyruvate in the isolated perfused rat heart

1. Prompted by the finding of markedly differing specific radioactivities of tissue alanine and lactate in isolated rat hearts perfused with [1-14C]pyruvate, a more detailed study on the cytosolic subcompartmentalization of pyruvate was undertaken. Isolated rat hearts were perfused by the once-through Langendorff technique under metabolic and isotopic steady-state conditions but with various routes of radioactive label influx, and the specific radioactivities of pyruvate, lactate and alanine were determined. An enzymic method was devised to determine the specific radioactivity of C-1 of pyruvate. 2. Label introduction as [1-14C]pyruvate resulted in a higher specific radioactivity of tissue alanine and mitochondrial pyruvate than of lactate, and a higher specific radioactivity of perfusate lactate than of tissue lactate. Label introduction as [1-14C]lactate resulted in a roughly similar isotope dilution into the tissue and perfusate pyruvate and the tissue alanine. Label introduction as [3,4-14C]glucose resulted in the same specific radioactivity of tissue lactate and alanine and a roughly similar specific radioactivity of mitochondrial pyruvate. 3. The results can be reconciled with a metabolic model containing two cytosolic functional pyruvate pools. One pool (I) communicates more closely with the glycolytic system, whereas the other (II) communicates with extracellular pyruvate and intracellular alanine. Pool II is in close connection with intramitochondrial pyruvate. The physical identity of the cytosolic subcompartments of pyruvate is discussed.