Measurement of the ATP/ADP ratio in mitochondria and in the extramitochondrial compartment by fractionation of freeze-stopped liver tissue in non-aqueous media

Influence of divalent cations on the reconstituted ADP, ATP exchange

1. Divalent cations cause a decrease in the exchange activity of the reconstituted ADP,ATP translocator from beef heart mitochondria. This effect is due to complex formation with the adenine nucleotides. 2. It is confirmed that only the free nucleotides are transported. A possible competition of free adenine nucleotides and the Mg2+-complexes for the binding site at the carrier protein is excluded. 3. The stability constants (Kn) for the cation-nucleotide complexes are derived from these experiments. For Mg2+-ATP, Kn = 0.8 x 10(4) M-1 and for Mg2+-ADP, Kn = 0.8 x 10(3) M-1 is obtained. 4. The carrier system was reconstituted with the neutral phospholipids phosphatidylcholine and phosphatidylethanolamine. Interaction of the divalent cations with these phospholipids seem not to be important for the exchange suppression.

The optimal efficiency and the economic degrees of coupling of oxidative phosphorylation

A phenomenological theory considering the output characteristics of oxidative phosphorylation has been worked out by adopting the formalism of linear nonequilibrium thermodynamics. The linearity of oxidative phosphorylation in the range of the output forces of practical interest has been experimentally verified. the efficiency of oxidative phosphorylation is zero if either a load with a zero conductance (open-circuited situation) or a load with an infinite conductance (short-circuited situation) is attached to oxidative phosphorylation. In between these extreme conductances there exists a finite load conductance permitting oxidative phosphorylation to operate with optimal efficiency. The necessary and sufficient condition for optimal efficiency was found to be L33/L11 = square root 1 - q2 where L11 is the phenomenological conductance of phosphorylation, L33 the phenomenological conductance of the load and q the degree of coupling of oxidative phosphorylation driven by respiration. This condition was called conductance matching. Under the condition of conductance matching, four output functions of oxidative phosphorylation of practical interest were optimized. A maximal net rate of oxidative phosphorylation occurs at a degree of coupling qf = 0.78. A maximal output power of oxidative phosphorylation, i.e. net rate times established phosphate potential, resuls at qp = 0.91. The maximization of the function net rate times efficiency yielded an economic degree of coupling qfec = 0.95 for maximal ATP flow. Finally, maximization of the function output power times efficiency led to a degree of coupling qpec = 0.97. This last function simultaneously maximized net rate of ATP production, developed phosphate potential and efficiency and reflects therefore the most economic solution to the output problem under the condition of conductance matching. In isolated rat livers perfused in a metabolic resting state, the condition of conductance matching is fulfilled. In addition, the degree of coupling oxidative phosphorylation under these conditions corresponds to the economic degree of coupling qpec.

Validity of the digitonin method for metabolite compartmentation in isolated hepatocytes

1. A modification of the digitonin method of Zuurendonk & Tager (1974) (Biochim. Biophys. Acta 333, 393-399) (i.e. the 'convaentional' method) was developed that allows the fractionation of isolated hepatocytes at -5 degrees C (i.e. 'low-temperature' method). 2. With respect to compartmentation of adenine nucleotides, glutamate and citrate, the two methods yielded very similar results. 3. In contrast, the mitochondrial amounts of aspartate and malate, as revealed by the low-temperature method, were about twice as high as those found by the conventional procedure. No change in the total cellular content occurred. 4. With n-butylmalonate and glisoxepid present in the conventional digitonin medium, significantly higher amounts of malate and aspartate respectively were found in the mitochondrial pellets. The results obtained by the low-temperature method, however, were not influenced by the these inhibitors. 5. It is concluded that under the conventional conditions of cell fractionation no appreciable redistribution of adenine nucleotides, glutamate and citrate occurs.

Subcellular distribution of di- and tricarboxylates and pH gradients in perfused rat liver

The subcellular distribution of malate, 2-oxoglutarate, citrate, aspartate and glutamate and the mitochondrial and cytosolic pH values were determined in perfused livers from fed and fasted rats. The method of fractionation of the freeze-dried tissue in non-aqueous solvents was employed. The following results were obtained: 1) Di- and tricarboxylates are not equally distributed between mitochondria and cytosol. Under the two metabolic conditions studied, the concentrations of citrate and 2-oxoglutarate are higher and those of glutamate and aspartate are lower in the mitochondria than in the cytosol. The distribution of malate varies with the metabolic state. 2) From the distribution of 5,5-dimethyl-2,4-oxazolidinedione it was calculated that the mitochondrial matrix has a more alkaline milieu than the cytosol. The cytosol is also more acidic than the perfusate. The pH difference between mitochondria and cytosol is 0.3 in the fed state and 0.6 in the fasted one, whereas the pH difference between perfusate and cytosol is 0.4 in the fed state and 0.2 in the fasted one. 3) In livers from fed rats, citrate, malate and 2-oxoglutarate appear to be distributed according to the pH gradient across the mitochondrial membrane, whereas in the fasted state, the distribution does not correspond to the pH gradient. Glutamate and aspartate do not follow the pH gradient in either metabolic state. 4) The data indicate that the pH gradient across the mitochondrial membrane does not reflect the membrane potential or the energy state of mitochondria in the intact cell.

Computer simulation of the fructose bisphosphatase/phosphofructokinase couple in rat liver

Recycling of fructose 6-phosphate and fructose 1,6-bisphosphate in the rat liver under gluconeogenic and glycolytic conditions was investigated with a computer model containing representations of the kinetic properties of phosphofructokinase and fructose 1,6-bisphosphatase under realistic physiological conditions. The two enzyme submodels were constructed from data for the isolated enzymes in vitro by formal optimization. Tissue metabolite concentrations were corrected for cytosolic/mitochondrial compartmentation and effects of chelation and protonation equilibria. This model, which mostly considers the behavior of livers from starved rats, predicts negligible recycling under physiologically realistic conditions. Metabolic regulation of fructose 6-phosphate, the magnesium ion concentration and the distribution of adenine nucleotides appear to prevent operation of a 'futile cycle' in vivo. Rate-limiting chemical species were identified by sensitivity analysis.

Adenine nucleotides in foetal rat liver cells. Compartmentation and variation with age

The digitonin method for the separation of cytosolic and mitochondrial fractions was applied to liver cells isolated from foetal rats. The cytosolic [ATP]/[ADP] ratio approximately doubles during the last 4 days of gestation, whereas the mitochondrial ratio remains constant. In the presence of oligomycin and added glucose, the cytosolic [ATP]/[ADP] ratio does not increase with age, but is still considerably higher than the mitochondrial ratio. Without added glucose, and when the glycogen content of foetal liver is still very low (more than 3 days before birth), the cytosolic [ATP]/[ADP] ratio in the presence of oligomycin becomes very low and equal to the mitochondrial ratio. It is concluded that the increasein the cytosolic [ATP]/[ADP] ratio during the last 4 days of gestation is solely due to enhanced mitochondrial activity in this period. Atractyloside and bongkrekic acid do not influence the O2 consumption, nor the [ATP]/[ADP] ratios in either compartment of foetal liver cells. Respiration of isolated foetal mitochondria, however, is strongly inhibited by both compounds. The implications of these findings are discussed.

Distribution of metabolites between the cytosolic and mitochondrial compartments of hepatocytes isolated from fed rats

In isolated hepatocytes from normal fed rats, the subcellular distribution of malate, citrate, 2-oxoglutarate, glutamate, aspartate, oxaloacetate, acetyl-CoA and CoASH has been determined by a modified digitonin method. Incubation with various substrates (lactate, pyruvate, alanine, oleate, oleate plus lactate, ethanol and aspartate) markedly changed the total cellular amounts of metabolites, but their distribution between the cytosolic and mitochondrial compartments was kept fairly constant. In the presence of lactate, pyruvate or alanine, about 90% of cellular aspartate, malate and oxaloacetate, and 50% of citrate was located in the cytosol. The changes in acetyl-CoA in the cytosol were opposite to those in the mitochondrial space, the sum of both remaining nearly constant. The mitochondrial acetyl-CoA/CoASH ratio ranged from 0.3-0.9 and was positively correlated with the rate of ketone body formation. The mitochondrial/cytosolic (m/c) concentration gradients for malate, citrate, 2-oxoglutarate, glutamate, aspartate, oxaloacetate, acetyl-CoA and CoASH averaged from hepatocytes under different substrate conditions were determined to be 1.0, 8.8, 1.6, 2.2, 0.5, 0.7, 13 and 40, respectively. From the distribution of citrate, a pH difference of 0.3 across the inner mitochondrial membrane was calculated, yet lower values resulted from the m/c gradients of 2-oxoglutarate, glutamate and malate. The mass action ratios for citrate synthase and mitochondrial aspartate aminotransferase have been calculated from the metabolite concentrations measured in the mitochondrial pellet fraction. A comparison with the respective equilibrium constants indicates that in intact hepatocytes, neither enzyme maintains its reactants at equilibrium. On the assumption that mitochondrial malate dehydrogenase and 3-hydroxybutyrate dehydrogenase operate near equilibrium, the concentration of free oxaloacetate appears to be 0.3-2 micron, depending on the substrate used. Plotting the calculated free mitochondrial oxaloacetate concentration against the citrate concentration measured in the mitochondrial pellet yielded a hyperbolic saturation curve, from which an apparent Km of citrate synthase for oxaloacetate in the intact cells of 2 micron can be derived, which is comparable to the value determined with purified rat liver citrate synthase. The results are discussed with respect to the supply of substrates and effectors of anion carriers and of key enzymes of the tricarboxylic acid cycle and fatty acid biosynthesis.

Subcellular metabolite concentrations. Dependence of mitochondrial and cytosolic ATP systems on the metabolic state of perfused rat liver

Mitochondrial and cytosolic contents of adenine nucleotides and phosphate were measured in perfused rat livers employing a technique of fractionation of freeze-fixated tissue in non-aqueous solvents. From the subcellular contents the mitochondrial and cytosolic concentrations of ATP, ADP, AMP and phosphate and the phosphorylation potentials of the subcellular ATP systems were calculated. An attempt was made to elucidate the relationship between mitochondrial and cytosolic adenine nucleotide systems and the dependency on the metabolic state of the liver. The following results were obtained: 1. Under all metabolic conditions studied the mitochondrial ATP/ADP ratios were considerably lower than the cytosolic ratios (mitochondria: 0.1-0.7; cytosol: 2-11). 2. The ATP/ADP ratios calculated from overall tissue contents reflect mainly the cytosolic ratios. 3. An inverse relationship was found between mitochondrial and cytosolic ATP/ADP ratios, i.e. when the mitochondrial ratios tended to increase, the cytosolic ratios decreased and vice versa. 4. The phosphorylation potentials calculated from the subcellular concentrations were higher in the cytosol than in the mitochondria. The potential difference varied between 11 and 3 kj/mol in livers from fed and starved rats, respectively. 5. In the presence of mitochondrial inhibitors, i.e. amytal, dinitrophenol and carboxyatractyloside, the potential difference between the subcellular ATP systems decreased predominantly due to an increase in the mitochondrial ATP/ADP ratios. 6. A correlation between mitochondrial ATP/ADP ratios and the respiratory rates was not observed, but the subcellular ratios appeared to correlate with the rate of glycolysis. When the rate of lactate + pyruvate production was increased, the cytosolic ATP/ADP ratios were increased, too, whereas the mitochondrial ratios tended to decrease. 7. The adenine nucleotides in the cytosol appear to be in near equilibrium catalysed by the adenylate kinase. In the mitochondria, the AMP concentration is much lower than to be expected under equilibrium conditions. These results were discussed with respect to rate control of processes involved in ATP generation, i.e. oxidative phosphorylation, adenine nucleotide translocation and glycolysis.

Mitochondrial and cytosolic NADPH systems and isocitrate dehydrogenase indicator metabolites during ureogensis from ammonia in isolated rat hepatocytes

1. Citrate isocitrate and 2-oxoglutarate levels were determined in isolated rat hepatocytes and in particulate and soluble fractions, thereof, obtained by the digitonin and silicone oil fractionation technique. 2. Caculated from isocitrate/2-oxoglutarate ratios ("indicator metabolite method"), the redox potential of mitochondrial free NADPH is -402 mV, whereas that of the extramitochondrial (cytosolic) space is about 10 mV more positive, -392 mV. 3; Addition of ammonia (either as ammonium chloride or from urea plus urease) to isolated hepatocytes causes preferential oxidation of mitochondrial NADPH, is demonstrated by spectrophotometry of the dihydro band and by the changes in the isocitrate/2-oxoglutarate ratios. The redox potential difference of free NADPH between mitochondria and cytosol is abolished or even reserved. 4. It is concluded that during urogenesis from ammonia mitochondrial isocitrate oxidation is shifted largely in favor of the NADP-linked as opposed to the NAD-linked enzyme; isocitrate concentration under these conditions is less than 10 muM, below the Km (isocitrate) of the NAD-linked enzyme but in the range of that for the NADP-linked enzyme. 5. Both in the absence and in the presence of ammonia there is a concentration gradient across the mitochondrial inner membrane (from mitochondria to cytosol) for citrate, isocitrate, and also, to a smaller extent, for 2-oxoglutarate. 6. These results and data in the literature on enzyme activity are in agreement with the assumption of near-equilibrium of NADP-dependent isocitrate dehydrogenases in the mitochondrial matrix and cytosolic spaces in the absence of ammonia; accordingly, during urea formation from added ammonia the redox potential of mitochondrial free NADPH is increased to -391 mV or possibly even higher if there exists an indicator error under this condition.

Control of oxidative phosphorylation by the extra-mitochondrial ATP/ADP ratio

The control of mitochondrial ATP synthesis by the extramitochondrial adenine nucleotide pattern was investigated with rat liver mitochondria. It is demonstrated that any stationary state between the two limit states of maximum activity (state 3) and of resting activity (state 4) can be obtained by a hexokinase-glucose trap as an ADP-regenerating system. These intermediate states are characterized by stationary respiratory rates, stationary redox levels of the cytochromes b and c and stationary levels of extramitochondrial ATP and ADP between the rates and levels of the limit states. At a constant concentration of inorganic phosphate the activity of mitochondria between the limit states is controlled by the extramitochondrial ATP/ADP ratio independent of the total concentration of adenine nucleotides present. The control range was found to be between ratios of about 5 and 100 at 10 mM phosphate. At lower ratios the mitochondria are in their maximum phosphorylating state. With succinate+rotenone and glutamate+malate the same control range was observed, indicating that it is independent of the nature of substrate oxidized. The results suggest that in the control range the mitochondrial activity is limited by the competition of ADP and ATP for the adenine nucleotide translocator.

Brown adipose tissue mitochondria: recoupling caused by substrate level phosphorylation and extramitochondrial adenosine phosphates

1. Uncoupled oxidative phosphorylation in isolated guinea pig brown-adipose-tissue mitochondria is reflected by a low phosphorylation state of adenosine phosphates in the mitochondrial matrix and in the extramitochondrial space during oxidation of succinate or glycerol 1-phosphate in the presence of serum albumin and 100 muM ADP. Recoupling of respiration and phosphorylation in the mitochondria is indicatdd by a dramatic increase in the phosphorylation state of adenine nucleotides in both compartments, when substrates inducing substrate level phosphorylation are respired. In this case ATP/ADP ratios in the extramitochondrial compartment are 10-15 times higher than in the mitochondrial matrix. 2. Recoupling mediated by substrate level phosphorylation depends on the presence of extramitochondrial adenosine phosphate and on intact adenine nucleotide translocation. In the presence of substrate level phosphorylation the amount of extramitochondrial ADP required to restore energy coupling can be extremely low (20 muM ADP or 10 nmol ADP/mg mitochondrial protein respectively). If substrate level phosphorylation is prevented by rotenone or in the presence of atractyloside, 20-50 times higher amounts of extramitochondrial adenine nucleotides are necessary to cause coupled oxidative phosphorylation. The recoupling effect of ATP is significantly stronger than that of ADP. 3. GDP (100 muM) causes a rapid increase of the ATP/ADP ratio in both compartments which is independent of substrate level phosphorylation as well as of the extramitochondrial adenosine phosphate concentration and the adenine nucleotide carrier. 4. The amount of extramitochondrial adenosine phosphate in guinea pig brown-adipose-tissue (18 nmol/mg mitochondrial protein or 2.5 mM respectively) would suffice for recoupling of oxidative phosphorylation mediated by substrate level phosphorylation under conditions in vitro; this suggests that substrate level phosphorylation is of essential importance in brown fat in vivo with respect to energy conditions in the tissue during different states of thermogenesis.

Regulation of mammalian pyruvate dehydrogenase

In mammalian tissues, two types of regulation of the pyruvate dehydrogenase complex have been described: end product inhibition by acetyl CoA and NADH: and the interconversion of an inactive phosphorylated form and an active nonphosphorylated form by an ATP requiring kinase and a specific phosphatase. This article is largely concerned with the latter type of regulation of the complex in adipose tissue by insulin (and other hormones) and in heart muscle by lipid fuels. Effectors of the two interconverting enzymes include pyruvate and ADP which inhibit the kinase, acetoin which activates the kinase and Ca2+ and Mg2+ which both activate the phosphatase and inhibit the kinase. Evidence is presented that all components of the pyruvate dehydrogenase complex including the phosphatase and kinase are located within the inner mitochondrial membrane. Direct measurements of the matrix concentration of substrates and effectors is not possible by techniques presently available. This is the key problem in the identification of the mechansims involved in the alterations in pyruvate dehydrogenase activity observed in adipose tissue and muscle. A number of indirect approaches have been used and these are reviewed. Most hopeful is the recent finding in this laboratory that in both adipose tissue and heart muscle, differences in activity of pyruvate dehydrogenase in the intact tissue persist during preparation and subsequent incubation of mitochondria.