Relationship between oxygen uptake of perifused rat-liver cells and the cytosolic phosphorylation state calculated from indicator metabolites and a redetermined equilibrium constant

Magnesium activated adenosine formation in intact perfused heart: predominance of ecto 5'-nucleotidase during hypermagnesemia

Magnesium ion is an allosteric effector of 5'-nucleotidase and thus activates adenosine production from AMP. Two distinct 5'-nucleotidase systems, the membrane-bound ecto and the soluble cytosolic isoforms, exist in mammalian myocardium. The aim of this study was to delineate the contributions of the ecto vs. cytosolic isoforms to Mg2+-stimulated cardiac purine nucleoside formation and release. Isolated guinea pig hearts were retrogradely perfused at their physiological aortic pressure with Krebs-Henseleit bicarbonate buffer fortified with 10 mM glucose. AMP and the adenylate degradatives adenosine and inosine were measured in coronary venous effluent and in epicardial transudate, which was sampled to estimate concentrations of adenylate degradatives in the interstitium. When perfusate Mg2+ was increased from 0.6 to 6 mM, coronary vascular resistance and spontaneous heart rate fell, and steady-state coronary venous release of adenosine + inosine rose severalfold. Cytosolic free magnesium, as estimated by 31P-NMR after 15 min of perfusion with 6 mM Mg2+ or from chemically measured indicator metabolites after 30 min, rose 60 and 144% respectively (P < 0.05). Excess Mg2+ stimulated purine nucleoside release nearly threefold in coronary venous effluent and four- to sevenfold in epicardial transudate. 50 microM, alpha,beta-methylene adenosine 5'-diphosphate (AOPCP), a selective inhibitor of ecto 5'-nucleotidase, elevated interstitial AMP concentration tenfold, did not attenuate basal nucleoside release, but completely inhibited Mg2+-stimulated coronary venous purine nucleoside release and blunted Mg2+-stimulated interstitial purine nucleoside formation by 69%. During perfusion with exogenous 1 microM [8-14C]AMP, excess perfusate MgCl2 increased [14C]adenosine release by 63% in coronary effluent and 133% in epicardial transudate. AOPCP decreased baseline [14C]adenosine release in coronary effluent and epicardial transudate by 85-90%, caused equilibration of arterial and epicardial AMP, and attenuated MgCl2 activation of p[14C]adenosine formation by approx. 75%, in both the vascular and interstitial compartments. Intramyocytic concentrations of allosteric regulators of the cytosolic 5'-nucleotidases were evaluated in stop-frozen myocardium. Excess magnesium did not appreciably alter intracellular pH and ATP concentration, but lowered free cytosolic ADP and AMP concentrations by 50 and 70%, respectively. A simplified model of compartmentalized adenosine metabolism is proposed in which magnesium ion-activated cardiac purine release originates predominantly from the ecto 5'-nucleotidase; magnesium ion stimulation of metabolic flux through the cytosolic isoforms was constrained by concomitant reductions in intracellular AMP substrate and allosteric activator ADP. Magnesium ion-enhanced adenosine formation by 5'-nucleotidase could contribute to the known cardioprotective effects of this clinically used cation.

Regulation of energy metabolism in liver

Energy metabolism in liver has to cope with the special tasks of this organ in intermediary metabolism. Main ATP-generating processes in the liver cell are the respiratory chain and glycolysis, whereas main ATP-consuming processes are gluconeogenesis, urea synthesis, protein synthesis, ATPases and mitochondrial proton leak. Mitochondrial respiratory chain in the intact liver cell is subject to control mainly by substrate (hydrogen donors, ADP, oxygen) transport and supply and proton leak/slip. Whereas hormonal control is mainly on substrate supply to mitochondria, proton leak/slip is supposed to play an important role in the modulation of the efficiency of oxidative phosphorylation.

Alteration of the cytosolic-mitochondrial distribution of high-energy phosphates during global myocardial ischemia may contribute to early contractile failure

Depending on its duration, temporary myocardial ischemia leads to a disturbance of myocardial function before irreversible cellular necrosis is developed. Mechanical, electrical, and metabolic disturbances were suggested to be possible mechanisms accounting for the altered mechanical performance in ischemic hearts. To further investigate the alteration of myocardial energy metabolism on the subcellular level, we determined, by means of nonaqueous fractionation, the cytosolic-mitochondrial distribution of high-energy phosphates and other metabolites (ATP, ADP, phosphocreatine, creatine, and inorganic phosphate) in ischemic (zero-flow) guinea pig hearts after isolated perfused working heart preparation. Additional experiments using 31P nuclear magnetic resonance spectroscopy were performed to determine pHi and [Mg2+]i changes during global ischemia. The total ATP content of myocardial tissue dropped only slowly to 76% of control ATP at 10 minutes and to 51% at 30 minutes and reached almost zero at 60 minutes of ischemia. However, striking differences were observed on the subcellular level: While cytosolic phosphocreatine was almost completely consumed after 3 minutes of ischemia (from 19.1 +/- 1.6 to 3.3 +/- 0.5 mmol/L), ATP concentration in the cytosol decreased within 30 minutes from 8.4 +/- 0.6 to only 5.4 +/- 0.9 mmol/L. Mitochondrial ATP was rapidly and linearly reduced to 60% after 5 minutes of ischemia and was nearly unmeasurable after a further 20 minutes. Thus, in contrast to the breakdown of phosphocreatine in cytosol, the only slight alteration of cytosolic ATP reveals a reduction in cytosolic ATP utilization. Moreover, the unaffected cytosolic-mitochondrial difference in the phosphorylation potential of ATP demonstrates the intact function of the ADP/ATP carrier during early ischemia. These results might indicate a disturbance of the functional coupling between carrier and phosphocreatine kinase (phosphocreatine shuttle), which could be of importance for the early contractile failure in myocardial ischemia.

The multidrug-resistance-reverser verapamil interferes with cellular P-glycoprotein-mediated pumping of daunorubicin as a non-competing substrate

We examined P-glycoprotein-mediated verapamil transport, using two drug-sensitive and multi-drug resistant cell-line couples, i.e. A2780, 2780AD and SW-1573, SW-1573/1R500. The interaction of 3H-labeled verapamil with cells was measured using a flow-through system. The verapamil-containing medium was pumped over the cells and monitored on-line for radioactivity. In the P-glycoprotein-expressing cells, verapamil accumulation was increased by vinblastine and some known multidrug resistant (MDR) modifiers. Subsequent removal of these modifiers caused release of verapamil into the medium against a verapamil concentration gradient. In this manner, we obtained evidence that verapamil is actively transported by the MDR-related P-glycoprotein. Using the flow-through system, we also exposed the cells to flowing culture medium containing daunorubicin, and measured the inhibition of daunorubicin efflux by verapamil. We found that, although the active efflux of daunorubicin was maximally blocked by verapamil short-term, longer-term active efflux of daunorubicin resumed. At a daunorubicin concentration in the flowing medium of 5 microM, increasing the verapamil concentration resulted in the same short-term effects, but in a significantly longer period of a maximal inhibition of daunorubicin efflux from the cells. At a daunorubicin concentration of 20 microM, increasing the verapamil concentration affected neither the short-term nor the long-term effects. These and other observations are in agreement with a model in which daunorubicin and verapamil are non-competing substrates for P-glycoprotein. In conclusion, we obtained evidence that verapamil is actively transported by the MDR-related P-glycoprotein and that verapamil and daunorubicin are non-competing substrates for P-glycoprotein. Consequently, the effectiveness of verapamil as an MDR antagonist may be compromised because it is extruded by P-glycoprotein.

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.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

A computer model of pancreatic islet glycolysis

We have modeled an experiment with perifused pancreatic islet cells using our BIOSSIM language. The experiment and the resulting model are concerned with glucose uptake and glycolysis by the beta-cells of pancreatic islets. Although glycolysis appears to be involved in insulin release, we do not have enough information to represent insulin release in detail. The rapid entry of glucose into the beta-cell is promoted by a carrier having a very high tissue capacity. Phosphorylation of glucose by the low affinity enzyme glucokinase appears to be limiting for glycolysis. The effects of several hexose diphosphate activators of phosphofructokinase are modeled. Model behavior is described. The kinetic parameters of the enzyme submodels are given. Because of the difficulties of preparing large amounts of experimental material, information on pancreatic islet metabolism is limited. This model is a plausible explanation of the experimental results. Recent work on the genetically engineered glucose transporter and glucokinase is discussed.

Influence of NAD-linked dehydrogenase activity on flux through oxidative phosphorylation

1. We have examined systematically the relationship between the percentage reduction of cardiac mitochondrial NAD and the flux through oxidative phosphorylation, as measured by O2 uptake. Reduction of NAD was varied by varying the concentration of palmitoyl-L-carnitine, pyruvate, 2-oxoglutarate or glutamate in the presence of malate as the oxidizable substrate. 2. In the presence of ADP (State 3 respiration) there was a substantially linear positive relationship between O2 uptake and the percentage reduction of NAD. Coupled respiration in the absence of ADP also showed an increase with increasing NADH, with the exact shape of the relationship being variable. 3. When pyruvate and 2-oxoglutarate dehydrogenase activity were increased by increasing medium Ca2+ concentration within the range 5 nM to 1.23 microM, at non-saturating substrate concentrations, there was again a positive relationship between O2 uptake and the reduction of NAD; however, rates of O2 uptake tended to be higher at given values of NAD reduction when the incubation medium contained Ca2+. This is taken to indicate an activation by Ca2+ of the enzymes of phosphorylation or of the respiratory chain, in addition to the dehydrogenase activation. 4. When carboxyatractyloside plus ADP were used to generate 50% State 3 rates of O2 uptake with pyruvate or 2-oxoglutarate, sensitivity to Ca2+ was retained. However, when oligomycin plus 1 mM-ADP and 1 mM-ATP were used to generate 50% State 3, no such dependence was seen. 5. The results are interpreted to indicate a substantial role for substrate dehydrogenation in the overall regulation of oxidative phosphorylation when substrates are available at near-physiological concentrations.

A simple model of aerobic metabolism: applications to work transitions in muscle

Adding kinetics to the model of the phosphate energy system [Connett. Am. J. Physiol. 254 (Regulatory Integrative Comp. Physiol. 23): R949-R959, 1988], we provide a framework for analyzing metabolic transients in muscle tissue. We modify the formalism of the earlier model and introduce a buffering factor, which measures buffering of adenine nucleotides by phosphocreatine. The time course of the phosphate energy state can be calculated given the following: 1) adenosinetriphosphatase (ATPase) rate, 2) pH, and 3) a mitochondrial driving function, i.e., ATP production in terms of the phosphate energy state. We use mitochondrial driving functions derived from steady-state measurements to predict the time courses for rest-work transitions. Predictions for transitions in the rat gastrocnemius muscle agree with published values. The model is used to test different existing hypotheses of oxygen consumption (VO2) regulation. Each hypothesis generates a specific mitochondrial driving function, which in turn generates a specific time course of phosphate energy state during transitions. A mitochondrial driving function based on enzyme kinetics with ADP as a substrate leads to time courses not matching the data. Mitochondrial driving functions that are linear with phosphocreatine, Pi, phosphorylation potential, or the pool of high-energy phosphate bonds (phosphate potential energy) gave good agreement with the data.

On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

Octanoate applied to rat liver mitochondria respiring with glutamate plus malate or succinate (plus rotenone) under resting-state (State 4) conditions stimulates oxygen uptake and decreases the membrane potential, both effects being sensitive to oligomycin but not to carboxyatractyloside. Octanoate also decreases the rate of pyruvate carboxylation under the same conditions, this effect being correlated with the decrease of intramitochondrial content of ATP and increase of AMP. The decrease of pyruvate carboxylation and the change of mitochondrial adenine nucleotides are both reversed by 2-oxoglutarate. Fatty acids of shorter chain length have similar effects, though at higher concentrations. Addition of octanoate in the presence of fluoride (inhibitor of pyrophosphatase) produces intramitochondrial accumulation of pyrophosphate, even under conditions when oxidation of octanoate is prevented by rotenone. In isolated hepatocytes incubated with lactate plus pyruvate, octanoate also increases oxygen uptake and produces a shift in the profile of adenine nucleotides similar to that observed in isolated mitochondria. It decreases the 'efficiency' of gluconeogenesis, as expressed by the ratio between an increase of glucose production and an increase of oxygen uptake upon addition of gluconeogenic substrates (lactate plus pyruvate), and increases the reduction state of mitochondrial NAD. These effects taken together are not compatible with uncoupling, but point to intramitochondrial hydrolysis of octanoyl-CoA and probably also shorter chain-length acyl-CoAs. This mechanism probably functions as a 'safety valve' preventing a drastic decrease of intramitochondrial free CoA under a large supply of medium- and short-chain fatty acids.

A linear model of muscle respiration explains monoexponential phosphocreatine changes

Phosphocreatine (PCr) content was measured by phosphorus nuclear magnetic resonance spectroscopy in the gastrocnemius muscles of pentobarbital-anesthetized rats during and after twitch stimulation at rates up to 0.75 Hz. The monoexponential time constant for PCr changes was similar at the onset of vs. during recovery after stimulation and was not significantly different for different stimulation rates (mean time constant 1.44 min). Steady-state PCr level during stimulation was linearly related to the product of stimulation rate times peak twitch force. These results are shown to be consistent with a simple first-order electrical analog model of oxidative metabolism that is applicable at submaximal oxidative rates. The model assumes equilibrium of the creatine kinase reaction, which is modeled as a chemical capacitor, with capacitance proportional to the total creatine level, and PCr level proportional to the cytosolic free energy of ATP hydrolysis.

Thermodynamics of the control of metabolism

A theory is presented, describing the control analysis of metabolic systems in terms of Gibbs free energies, extending earlier work of Kacser and Burns (25), and Heinrich and Rapoport (29). It is shown that relationships exist between flux control coefficients (the degree to which enzymes control steady-state fluxes) and free-energy elasticity coefficients, defined as the fractional change in the rate of a reaction induced by a standard change in one free-energy difference, while all the other free-energy differences are kept constant. Application of this extended control analysis to some biochemical reactions, including proton translocation, demonstrates that 1. Problems arising in the control analysis because of conservation (sum concentration of substrate and product constant) can be circumvented. 2. Although free-energy elasticity coefficients are maximal when the reaction is close to equilibrium, they can also be significant when the reaction is not close to equilibrium. 3. Problems in the control analysis caused by compartmentation can be resolved by defining control parameters that refer to the organelle as a whole. 4. These latter control parameters obey the above-mentioned relationships.

Control of gluconeogenesis in rat liver cells. Flux control coefficients of the enzymes in the gluconeogenic pathway in the absence and presence of glucagon

We have used control analysis to quantify the distribution of control in the gluconeogenic pathway in liver cells from starved rats. Lactate and pyruvate were used as gluconeogenic substrates. The flux control coefficients of the various enzymes in the gluconeogenic pathway were calculated from the elasticity coefficients of the enzymes towards their substrates and products and the fluxes through the different branches in the pathway. The elasticity coefficients were either calculated from gamma/Keq. ratios (where gamma is the mass-action ratio and Keq. is the equilibrium constant) and enzyme-kinetic data or measured experimentally. It is concluded that the gluconeogenic enzyme pyruvate carboxylase and the glycolytic enzyme pyruvate kinase play a central role in control of gluconeogenesis. If pyruvate kinase is inactive, gluconeogenic flux from lactate is largely controlled by pyruvate carboxylase. The low elasticity coefficient of pyruvate carboxylase towards its product oxaloacetate minimizes control by steps in the gluconeogenic pathway located after pyruvate carboxylase. This situation occurs when maximal gluconeogenic flux is required, i.e. in the presence of glucagon. In the absence of the hormone, when pyruvate kinase is active, control of gluconeogenesis is distributed among many steps, including pyruvate carboxylase, pyruvate kinase, fructose-1,6-bisphosphatase and also steps outside the classic gluconeogenic pathway such as the adenine-nucleotide translocator.

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.

Compartmentation of intracellular nucleotides in mammalian cells

The important role of nucleotides in cellular metabolism requires that serious consideration be given to the question of the homogeneity or inhomogeneity of nucleotide pools in cells. The purpose of this review is to summarize the existing evidence for compartmentation of nucleotide pools, discuss the limitations of this evidence, and to discuss the implications of compartmentation for the interpretation of nucleotide concentration measurements. Evidence for nucleotide compartmentation comes from the following types of evidence: compartmentation of RNA precursors; compartmentation of deoxynucleoside triphosphates; mitochondrial compartmentation; the existence of tightly bound nucleotides; pools derived from alternative synthetic routes; compartmentation in cyclic nucleotide metabolism; channeling in the synthesis of pyrimidine nucleotides; and others. The types of evidence adduced for compartmentation will be considered critically and in detail, and alternative explanations considered, as well. Implications of the data and hypotheses on nucleotide compartmentation for the interpretation of nucleotide pool measurements in various types of experiments will be discussed.

Measurement of cytosolic free calcium in mammalian cells with aequorin

Measurement of cytosolic free calcium (Cai) in small mammalian cells has been achieved by incorporating into cells the photoprotein aequorin by hypoosmotic shock treatment (HOST). The method and the instrumentation necessary for monitoring Cai are described in detail. We present evidence that the aequorin-Ca light signal originates in the cytosol and that the cells subjected to the HOST procedure have a normal viability and functional integrity with respect to growth, respiration, membrane transport calcium metabolism, and hormone responsiveness. From 36 measurements made at 25 degrees C, the cytosolic free calcium of cultured kidney cells is estimated to be 60 nM assuming an intracellular free magnesium of 1 mM. Anoxia and metabolic inhibitors (10 mM cyanide) dramatically increase the cytosolic free calcium, and both effects are fully reversible.

Current-voltage relations of the basolateral membrane in tight amphibian epithelia: use of nystatin to depolarize the apical membrane

Exposure of the mucosal side of toad (Bufo bufo) urinary bladder and frog (Rana ridibunda) skin to the polyene ionophore nystatin, resulted in stable preparations in which the apical resistance was negligible compared to the basolateral resistance. The preparations support passive K currents in both directions and an amiloride-insensitive Na current in the apical-serosal direction which is blocked by ouabain. The nystatin-treated toad bladder was used to study the electrical properties of the basolateral membrane by means of current-voltage curves recorded transepithelially. The K current showed strong rectification at cellular potentials negative with respect to the interstitial space. The ouabain-sensitive current increased with membrane voltage at negative voltages but saturated above +20 mV.

Influence of fatty acids on energy metabolism. 1. Stimulation of oxygen consumption, ketogenesis and CO2 production following addition of octanoate and oleate in perfused rat liver

Changes in metabolic rates (oxygen consumption, ketogenesis, 14CO2 production from labelled fatty acids, glycolysis) following the addition of octanoate or oleate were studied in isolated livers from fed and starved rats perfused with Krebs-Henseleit bicarbonate buffer in a non-recirculating system. The following results were obtained. The infusion of fatty acids caused a large increase in the rate of oxygen consumption. The effect was greater with octanoate than with oleate and was half-maximal with fatty acid concentrations (free plus albumin bound) around 0.1 mM. The effects of oleate were only partially suppressed when the perfusate contained albumin concentrations near the physiological range. When fatty acids were oxidized at high rates, the glycolytic rate was diminished by 50%. The increase in oxygen consumption could not be explained fully by the increased ATP demand for fatty acid metabolism or by a compensation for the diminished extramitochondrial ATP generation. In the presence of phenylalkyl oxirane carboxylic acid, an inhibitor of the transport of long-chain acyl-CoA derivates into the mitochondria, ketogenesis and 14CO2 production from labelled oleate were strongly inhibited, whereas the increase in oxygen consumption was only slightly affected. In the presence of antimycin A, the increase in oxygen consumption due to fatty acids was totally abolished. Following pretreatment of rats with ciprofibrate (induction of enzymes for peroxisomal beta-oxidation of long-chain fatty acids), ketogenesis (but not 14CO2 production) from oleate was enhanced threefold. The increase in oxygen consumption, however, was not affected. In conclusion, the increase in hepatic oxygen consumption due to addition of fatty acids reflects a mitochondrial process; it is, in part, independent of the ATP demand of the cell. An uncoupling-like effect of fatty acids on the respiratory chain and its possible physiological significance in ketogenesis are discussed.

Adenine nucleotide control of the rate of oxygen uptake by rat heart mitochondria over a 15- to 20-fold range

Rat heart mitochondria oxidizing pyruvate (in the presence of 20% as much malate) took up nearly the amount of oxygen required for complete oxidation to CO2. Thus pyruvate, a physiological substrate of the citrate cycle, is oxidized through the entire cycle in these mitochondria, and they seem suitable for study of regulation of integrated mitochondrial energy transduction. By addition of graded amounts of hexokinase or pyruvate kinase to the suspending medium (in the presence of excess glucose or phosphoenolpyruvate), a wide range of steady-state values of the ATP/ADP concentration ratio was obtained. At a constant concentration of phosphate, the steady-state rate of oxygen uptake by rat heart mitochondria oxidizing pyruvate was a function of the adenylate energy charge or of the ATP/ADP ratio, and relatively independent of the absolute concentrations of these nucleotides. The oxygen uptake rates typically spanned a range of about 20-fold. At very high values of the ATP/ADP ratio, the rate of oxygen uptake is much lower than the "state 4" rate seen after added ADP has been phosphorylated. This result suggests that "state 4" respiration, at least in these freshly prepared mitochondria, measures the rate at which ADP is made available by ATPase activity, rather than indicating uncoupling of electron transport from phosphorylation. The concentration of orthophosphate affected the rate of oxygen uptake and the pattern of response to the ATP/ADP ratio or the energy charge, but the effects did not seem interpretable in terms of the mass-action expression for hydrolysis of ATP, (ATP)/ (ADP) (Pi).

Control of heart oxidative phosphorylation by creatine kinase in mitochondrial membranes

Three important points must be emphasized in summary. First is the idea that a cellular microcompartment need not be limited by a semi-permeable membrane. We recognize microcompartments in multi-enzyme complexes where substrates are covalently transported from subunit to subunit. An example of this is the lipoic acid moiety of the pyruvate dehydrogenase complex. However, to act as a kinetic microcompartment, covalent transfer is not an obligatory requirement. Proximity effects may be sufficient for substantial rate enhancement. Our data clearly show that the kinetics of ADP translocation are influenced by the site of ADP formation. We contend that this represents a newly recognized and important form of cellular microcompartmentation. The second point is that we do not want our results misinterpreted as an overextension of the known data concerning tissue respiration. We believe that the primary parameter controlling heart mitochondrial oxygen consumption is the availability of ADP at the adenine nucleotide translocase. Our data show, however, that this is not a simple process. Secondary control is exerted by the localization of ADP formation, i.e. microcompartmentation. As a result of the kinetic data (Table 3), we conclude that the forward rate of mitochondrial creatine kinase is the preferential reaction controlling ADP delivery to the translocase. We are left, nonetheless, with questions concerning the secondary regulation of this enzyme in vivo by substrate (ATP and creatine) and inhibition by product (phosphocreatine). The nature of this control awaits further experimental data. Finally, the results are consistent with the creatine kinase energy transport hypothesis. Overall, the rate of tissue oxygen consumption reflects the metabolic activity of the organ, determined by the rate of ATP utilization (see right side of Figure 1). This results in the cytoplasmic production of ADP. In heart, this is coupled via the bound cytoplasmic isozymes of creatine kinase to the local rephosphorylation of ADP to ATP and the simultaneous production of creatine.(ABSTRACT TRUNCATED AT 400 WORDS)

Fructose 1,6-bisphosphatase in rat liver cytosol: interactions between the effects of K+, Zn2+, Mn2+, and fructose 2,6-bisphosphate as measured in a steady-state assay

Fructose 1,6-bisphosphatase activity was determined in rat liver cytosols using glyceraldehyde 3-phosphate as primary substrate. Fructose 1,6-bisphosphate was formed in situ and steady-state concentrations ranging from 1 to 30 microM were observed depending on the activity of fructose 1,6-bisphosphatase and the concentration of added glyceraldehyde 3-phosphate. The system was free of contaminating low-molecular-weight compounds, divalent cations, and chelators. Under these conditions, fructose 1,6-bisphosphatase was inhibited by K+ (less than or equal to 200 mM). This inhibition was due to a reduction of V and was observed in presence of low (0.4 mM) and high (5 mM) concentrations of Mg2+. In presence of 0.4 mM Mg2+, 1 microM Zn2+ inhibited fructose 1,6-bisphosphatase by 50%; the same effect was obtained with 0.3 microM Zn2+ when the system was supplemented with 100 mM KCl. On the other hand, 0.2 microM Zn2+ enhanced the inhibitory effect of K+ and decreased the concentration of K+ yielding half-maximal inhibition from 175 to 100 mM when measured at 0.4 mM Mg2+. The effect of Zn2+ on the inhibition by K+ could be abolished by Mn2+ (less than 5 microM) or by 5 mM Mg2+. One hundred millimolar K+ enhanced the inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate and changed the type of inhibition from mainly competitive to a mixed-type inhibition (increase of Km, decrease of V). Mn2+ (less than 10 microM) reduced the effect of fructose 2,6-bisphosphate, especially in the presence of K+. It is proposed that K+ and Mn2+ may play a role in the regulation of gluconeogenesis.

Control of mitochondrial respiration. The contribution of the adenine nucleotide translocator depends on the ATP- and ADP-consuming enzymes

The consequence of the complexity of the metabolic network on the amount of control strength of adenine nucleotide translocator was investigated with isolated rat liver mitochondria. Two experimental systems were compared: (i) mitochondria in the presence of yeast hexokinase (hexokinase system) and (ii) the same system plus additional pyruvate kinase (pyruvate kinase system). In both systems the control strength was analysed for the adenine nucleotide translocator by inhibitor titration studies with carboxyatractyloside and for the hexokinase or pyruvate kinase by changing their relative activities. Experimental results were compared with computer simulation of these systems and that of a third one, where the extramitochondrial ATP/ADP ratio was held constant by perifusion (perifusion system). The results demonstrate quite different flux-dependent control strength of the translocator in the three systems. In the hexokinase system the control strength of the translocator on mitochondrial respiration was zero up to respiration rates of about 60 nmol O2/mg protein per min. For higher rates, the control strength increased until the maximum value (0.45) was reached in the fully active state. Here, the same value was also found in the pyruvate kinase system. In all other states of respiration the translocator exerts a higher control strength in the pyruvate kinase system than in the hexokinase system. This different behaviour was attributed to the various changes in the adenine nucleotide pattern caused by partial inhibition of the translocator in the hexokinase and pyruvate kinase system. The data clearly show that the sharing of control strength depends not only on the respiration rate but also on the complexity of the metabolic system.

Mitochondrial function in normal and hypoxic states of the myocardium

The relationships among isometric tension development, the oxidation-reduction states of pyridine nucleotides and cytochrome c, and the oxygenation state of myoglobin have been assessed using the arterially perfused rabbit interventricular septum under different conditions of contraction rate, perfusate [Ca2+] and pH, catecholamine stress, and hypoxia. Hypoxia was produced either by decreasing oxygen availability with maintained flow (high-flow hypoxia) or by decreasing the flow rate (ischemia). Under normoxic conditions, increased work caused a fall of the cytosolic adenine nucleotide phosphorylation potential, delta G(ATP)c, an oxidation of the pyridine nucleotides, and a reduction of cytochrome c; the opposite occurred with decreased work. Thus, the redox potential span from NADH to cytochrome c, delta Eh, varied with the energy demand such that delta Eh and delta G(ATP)c changed in the same direction. Under hypoxic conditions, all respiratory components became more reduced, and myoglobin was partially deoxygenated. The percentage change of developed tension under hypoxic conditions was approximately proportional to the percentage change of oxidized cytochrome c. When high-flow hypoxia and ischemia were compared at the same rates of oxygen delivery, the developed tension at any level of cytochrome c reduction was always lower with ischemia than with high-flow hypoxia. This difference was attributed to the low intracellular pH of ischemic tissue. Myoglobin deoxygenation was linearly related to cytochrome c reduction under all conditions of hypoxia, indicating steep oxygen gradients. The results support the concept of heterogeneous oxygenation of the tissue with mixed populations of aerobic and anaerobic mitochondria in the hypoxic state. In the full aerobic state, the control of mitochondrial respiration in situ appears similar to that of isolated mitochondria.

An evaluation of the metabolite indicator method for determining the cytosolic phosphate potential in rat liver cells

The cytosolic phosphate potential was estimated in isolated rat liver parenchymal cells incubated with various gluconeogenic substrates. The value of the cytosolic [ATP]/[ADP][Pi] ratio was either estimated directly from measurements of ATP, ADP and Pi after digitonin fractionation of the cells, or calculated by the metabolite indicator method. When cells were incubated with lactate, pyruvate or alanine so that net flux through the indicator enzymes was in gluconeogenic direction, there was excellent agreement between the values obtained by the two methods over a wide range of fluxes. However, when the cells were incubated with substrates that could be converted both to glucose and to lactate so that net flux through the indicator enzymes was in the glycolytic direction, a large difference in the values of the cytosolic [ATP]/([ATP][Pi]) ratio as derived by the two methods was observed. It is concluded that the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase plus 3-phosphoglycerate kinase is out of equilibrium when flux through the reaction is the glycolytic direction, and that use of the metabolite indicator method for the calculation of the cytosolic phosphate potential under these conditions leads to erroneous values.

Rate-controlling steps of oxidative phosphorylation in rat liver mitochondria. A synoptic approach of model and experiment

The contribution of different steps to the control of oxidative phosphorylation in isolated rat liver mitochondria was investigated by a combination of experiments and computer simulations. The parameters of the mathematical model of phosphorylating mitochondria were derived from experimental data. The model correctly described the competition between ATP utilization inside and outside mitochondria for the ATP generated in mitochondria. On the basis of the good agreement between experiments and simulations, the contribution of different steps to the control of respiration was estimated by computing their control strengths, i.e., the influence of their activities on the rate of respiration. The rate-controlling influences vary depending on the load of oxidative phosphorylation. The predominant steps are: in the fully active state (State 3)--the hydrogen supply to the respiratory chain; in the resting state (State 4)--the proton leak of the mitochondrial inner membrane; in states of non-maximum ATP export--the adenine nucleotide translocator. Titrations of respiration with phenylsuccinate, antimycin, oligomycin and carboxyatractyloside completely support these conclusions.

Gluconeogenesis in vitro. Formation of glucose 6-phosphate from malate by a cell-free rat-liver system consisting of cytosol and mitochondria

A cell-free system prepared from rat liver containing cytosol and mitochondria as well as a number of cofactors at near physiological concentrations was shown to form glucose 6-phosphate from malate + 3-phosphoglycerate at a rate of 1.11 +/- 0.09 mumol . min-1 . g liver-1 (mean +/- SEM, n = 9, 30 degrees C). At least 70% of glucose 6-phosphate formed was derived from malate as calculated from experiments with [U-14C]malate. The indicated rates were measured between 10 min and 30 min incubation time when the system was near steady state with respect to the lactate/pyruvate ratio and to most of the gluconeogenic intermediates. In the absence of mitochondria, the rate of formation of glucose 6-phosphate from malate was about seven times lower than in their presence. A comparison between incubations carried out in presence or absence of mitochondria revealed that mitochondria decreased the lactate/pyruvate ratio and increased the ratio of (ATP + ITP)/(ADP + IDP). It could be shown that under the present incubation conditions, formation of glucose 6-phosphate was closely linked to the ratio of (ATP + ITP)/(ADP + IDP) whereas changing redox ratios had little influence on the gluconeogenic rate.

Age-linked changes in the activity of enzymes of the tricarboxylate cycle and lipid oxidation, and of carnitine content, in muscles of the rat

The activities of citrate synthase, NAD-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase were measured in homogenates of soleus, diaphragm and heart muscles of the rat, in an attempt to define potential tricarboxylate cycle activity and its response to aging. Activities were significantly decreased in 24-month animals versus 6-month controls in every case (except 2-oxoglutarate dehydrogenase in heart muscle). Age-linked decrements were greatest in the soleus and least in heart. Cytochrome oxidase was measured as an index of total respiratory chain activity and decreased significantly in each case, with the smallest decrease in the heart. Acyl-CoA dehydrogenase and 3-hydroxyacyl-Co-A dehydrogenase were measured as an index of beta-oxidative activity; the former decreased in soleus and diaphragm, the latter in soleus and heart, with the decrease in the soleus being the greater. Carnitine acetyl- and palmitoyltransferases were measured, together with the muscle content of carnitine and acylcarnitine, as determining the potential rate of entry of acyl groups into the mitochondria for oxidation. Carnitine acetyltransferase activity was decreased with age in each of the muscles, but to the greatest extent in the heart. Carnitine palmitoyltransferase was decreased in both soleus and diaphragm. Carnitine content was decreased most in the soleus and the heart and to a lesser extent in the diaphragm. It is concluded that there is a generalized decline in oxidative activity in all of these muscles with age, on the basis of wet weight; this occurs to the greatest extent in the soleus and to the least extent in the heart. There is, in addition, a specific deficiency in the ability to oxidize fatty acids, relative to other substrates, in heart muscle.

The role of the adenine nucleotide translocator in oxidative phosphorylation. A theoretical investigation on the basis of a comprehensive rate law of the translocator

A minimum model of adenine nucleotide exchange through the inner membrane of mitochondria is presented. The model is based on a sequential mechanism, which presumes ternary complexes formed by binding of metabolites from both sides of the membrane. The model explains the asymmetric kinetics of ADP-ATP exchange as a consequence of its electrogenic character. In energized mitochondria, a part of the membrane potential suppresses the binding of extramitochondrial ATP in competition with ADP. The remaining part of the potential difference inhibits the back exchange of internal ADP for external ATP. The assumption of particular energy-dependent conformational states of the translocator is not necessary. The model is not only compatible with the kinetic properties reported in the literature about the adenine nucleotide exchange, but it also correctly describes the response of mitochondrial respiration to the extramitochondrial ATP/ADP ratio under different conditions. The model computations reveal that the translocation step requires some loss of free energy as driving force. The size of the driving force depends depends on the flux rate as well as on the extra- and intramitochondrial ATP/ADP quotients. By both quotients the translocator control the export of ATP formed by oxidative phosphorylation in mitochondria.

Differential effects of the NADPH/NADP+ ratio on the activities of hexose-6-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase

The steady-state kinetics of rat liver hexose-6-phosphate dehydrogenase (beta-D-glucose: NAD(P)+ 1-oxidoreductase, EC 1.1.1.47) using glucose 6-phosphate and NADP+ as substrates is studied. NADPH has been found to inhibit the enzyme noncompetitively with respect to NADP+, and uncompetitively with respect to glucose 6-phosphate. At a given concentration of glucose 6-phosphate, the reaction follows the basic inhibition equation. This suggests the presence of the enzyme-NADP+-NADPH complex, and contrasts with the NADPH inhibition of glucose-6-phosphate dehydrogenase which is competitive with respect to NADP+. An attempt was made to estimate the in vivo activities of the two enzymes in rat liver in the presence of NADPH at various NADPH/NADP+ ratios. The results show that the two enzymes appear to be at about the same level of activity in normal rat liver where the coenzyme redox ratio is 110 and the glucose 6-phosphate concentration is 217 microM. Under the same conditions, but with 50 microM dehydroepiandrosterone, a potent inhibitor of glucose-6-phosphate dehydrogenase, but not of hexose-6-phosphate dehydrogenase, the latter enzyme is estimated to be 1.6-times as active as the former. Such differential effects of NADPH and steroids on the two enzymes may support our notion that hexose-6-phosphate dehydrogenase may have advantages over glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) in steroid-metabolizing tissues (the activity of hexose-6-phosphate dehydrogenase is not, or less, affected by steroids of NADPH).

Influence of different energy drains on the interrelationship between the rate of respiration, proton-motive force and adenine nucleotide patterns in isolated mitochondria

The respiration of rat liver mitochondria was stimulated by three different ways of energy drain: (a) partial uncoupling (equivalent to direct collapse of the proton-motive force), (b) intramitochondrial utilization of ATP for citrulline synthesis, and (c) extramitochondrial utilization of ATP for glucose phosphorylation. At identical rates of respiration, the intramitochondrial ATP : ADP ratios were the same in all three systems. Furthermore, the proton-motive force was the same in partially uncoupled mitochondria and in the presence of hexokinase plus glucose up to a respiration rate amounting to about 60% of that of the fully active state. However, external ATP : ADP ratios were considerably different in various systems at comparable rates of oxygen uptake, being the lowest under conditions when ATP was being utilized externally. On this basis, it is concluded that the respiratory rate is controlled directly by the proton-motive force and the mitochondrial ATP-synthesizing system operates under near-equilibrium conditions with respect to the membrane energy state parameters. However, a disequilibrium exists at the step of the transport of ATP from mitochondria to the external (cytoplasmic) compartment.

The stimulation of glutamine hydrolysis in isolated rat liver mitochondria by Mg2+ depletion and hypo-osmotic incubation conditions

1. In respiring rat liver mitochondria EDTA stimulates glutaminase activity measured in the presence of phosphate and HCO3- ions. The stimulation can be reversed by the addition of low concentrations of MgCl2. EGTA does not stimulate glutamine hydrolysis. 2. Glutaminase activity assayed in disrupted mitochondria is not significantly affected by EDTA or MgCl2. 3. The addition of EDTA results in a decrease in the concentration of phosphate required for half-maximal glutaminase activity. 4. Depletion of mitochondrial Mg2+ by the addition of the ionophore A23187 also stimulates glutamine hydrolysis in both the presence and the absence of EDTA. The effect of the ionophore can be abolished by the addition of MgCl2. 5. Hypo-osmotic incubation conditions increase the rate of mitochondrial glutamine hydrolysis. The effect of hypo-osmoticity on glutaminase is much less when EDTA is present. 6. It is suggested that glutaminase is partially and indirectly inhibited by endogenous mitochondrial Mg2+ and that the inner membrane may play a role in the regulation of glutaminase activity.

Control of energy transformation of mitochondria. Analysis by a quantitative model

A mathematical model of control of energy transformation in mitochondria is presented. The considered processes are: the proton translocation by the respiratory chain, the production of ATP by ATPase, the translocation of adenine nucleotides and of phosphate by their translocators, and a passive backflow of protons through the mitochondrial membrane. The mathematical equations expressing the steady-state kinetics of these processes and the relations between them were derived on the basis of current experimental data. The model predicts fairly well the values of the proton electrochemical gradient, of the ATP/ADP ratios within and outside mitochondria and of the distribution of phosphate between both compartments in different metabolic states of mitochondria. From the general agreement of model computations with experimental data, it is suggested that the electron flux through the respiratory chain is immediately controlled by the energy back-pressure of the proton electrochemical gradient, that the ATPase reaction is near equilibrium in phosphorylating mitochondria but that the adenine nucleotide exchange across the mitochondrial membrane requires some loss of energy. The latter is caused by an inhibition of the translocator by ATP from the outer side or by ADP from the inner side depending on the actual ATP/ADP in both compartments. It explains that no fixed relation exists between the rate of respiration and the phosphorylation state of extramitochondrial adenine nucleotides. The relation is modified by the concentration of phosphate and by intramitochondrial energy utilization.