Homeostasis of the protonmotive force in phosphorylating mitochondria

Regulation of oxidative phosphorylation during work transitions results from its kinetic properties

The regulation of oxidative phosphorylation (OXPHOS) during work transitions in skeletal muscle and heart is still not well understood. Different computer models of this process have been developed that are characterized by various kinetic properties. In the present research-polemic theoretical study it is argued that models belonging to one group (Model A), which predict that among OXPHOS complexes complex III keeps almost all of the metabolic control over oxygen consumption (Vo2) and involve a strong complex III activation by inorganic phosphate (Pi), lead to the conclusion that an increase in Pi is the main mechanism responsible for OXPHOS activation (feedback-activation mechanism). Models belonging to another group (Model B), which were developed to take into account an approximately uniform distribution of metabolic control over Vo2 among particular OXPHOS complexes (complex I, complex III, complex IV, ATP synthase, ATP/ADP carrier, phosphate carrier) encountered in experimental studies in isolated mitochondria, predict that all OXPHOS complexes are directly activated in parallel with ATP usage and NADH supply by some external cytosolic factor/mechanism during rest-to-work or low-to-high work transitions in skeletal muscle and heart ("each-step-activation" mechanism). Model B demonstrates that different intensities of each-step activation can account for the very different (slopes of) phenomenological Vo2-ADP relationships observed in various skeletal muscles and heart. Thus they are able to explain the differences in the regulation of OXPHOS during work transitions between skeletal muscle (where moderate changes in ADP take place) and intact heart in vivo (where ADP is essentially constant).

Role of mitochondrial phosphate carrier in metabolism-secretion coupling in rat insulinoma cell line INS-1

In pancreatic β-cells, glucose-induced mitochondrial ATP production plays an important role in insulin secretion. The mitochondrial phosphate carrier PiC is a member of the SLC25 (solute carrier family 25) family and transports Pi from the cytosol into the mitochondrial matrix. Since intramitochondrial Pi is an essential substrate for mitochondrial ATP production by complex V (ATP synthase) and affects the activity of the respiratory chain, Pi transport via PiC may be a rate-limiting step for ATP production. We evaluated the role of PiC in metabolism-secretion coupling in pancreatic β-cells using INS-1 cells manipulated to reduce PiC expression by siRNA (small interfering RNA). Consequent reduction of the PiC protein level decreased glucose (10 mM)-stimulated insulin secretion, the ATP:ADP ratio in the presence of 10 mM glucose and elevation of intracellular calcium concentration in response to 10 mM glucose without affecting the mitochondrial membrane potential (Δψm) in INS-1 cells. In experiments using the mitochondrial fraction of INS-1 cells in the presence of 1 mM succinate, PiC down-regulation decreased ATP production at various Pi concentrations ranging from 0.001 to 10 mM, but did not affect Δψm at 3 mM Pi. In conclusion, the Pi supply to mitochondria via PiC plays a critical role in ATP production and metabolism-secretion coupling in INS-1 cells.

Nitration of tyrosine residues 368 and 345 in the β-subunit elicits FoF1-ATPase activity loss

Tyrosine nitration is a covalent post-translational protein modification associated with various diseases related to oxidative/nitrative stress. A role for nitration of tyrosine in protein inactivation has been proposed; however, few studies have established a direct link between this modification and loss of protein function. In the present study, we determined the effect of nitration of Tyr345 and Tyr368 in the β-subunit of the F1-ATPase using site-directed mutagenesis. Nitration of the β-subunit, achieved by using TNM (tetranitromethane), resulted in 66% ATPase activity loss. This treatment resulted in the modification of several asparagine, methionine and tyrosine residues. However, nitrated tyrosine and ATPase inactivation were decreased in reconstituted F1 with Y368F (54%), Y345F (28%) and Y345,368F (1%) β-subunits, indicating a clear link between nitration at these positions and activity loss, regardless of the presence of other modifications. Kinetic studies indicated that an F1 with one nitrated tyrosine residue (Tyr345 or Tyr368) or two Tyr368 residues was sufficient to grant inactivation. Tyr368 was four times more reactive to nitration due to its lower pKa. Inactivation was attributed mainly to steric hindrance caused by adding a bulky residue more than the presence of a charged group or change in the phenolic pKa due to the introduction of a nitro group. Nitration at this residue would be more relevant under conditions of low nitrative stress. Conversely, at high nitrative stress conditions, both tyrosine residues would contribute equally to ATPase inactivation.

Control over the contribution of the mitochondrial membrane potential (DeltaPsi) and proton gradient (DeltapH) to the protonmotive force (Deltap). In silico studies

The protonmotive force across the inner mitochondrial membrane (Deltap) has two components: membrane potential (DeltaPsi) and the gradient of proton concentration (DeltapH). The computer model of oxidative phosphorylation developed previously by Korzeniewski et al. (Korzeniewski, B., Noma, A., and Matsuoka, S. (2005) Biophys. Chem. 116, 145-157) was modified by including the K+ uniport, K+/H+ exchange across the inner mitochondrial membrane, and membrane capacitance to replace the fixed DeltaPsi/DeltapH ratio used previously with a variable one determined mechanistically. The extended model gave good agreement with experimental results. Computer simulations showed that the contribution of DeltaPsi and DeltapH to Deltap is determined by the ratio of the rate constants of the K+ uniport and K+/H+ exchange and not by the absolute values of these constants. The value of Deltap is mostly controlled by ATP usage. The metabolic control over the DeltaPsi/DeltapH ratio is exerted mostly by K+ uniport and K+/H+ exchange in the presence of these processes, and by the ATP usage, ATP/ADP carrier, and phosphate carrier in the absence of them. The K+ circulation across the inner mitochondrial membrane is controlled mainly by K+ uniport and K+/H+ exchange, whereas H+ circulation by ATP usage. It is demonstrated that the secondary K+ ion transport is not necessary for maintaining the physiological DeltaPsi/DeltapH ratio.

Regulation of oxidative phosphorylation through parallel activation

When the mechanical work intensity in muscle increases, the elevated ATP consumption rate must be matched by the rate of ATP production by oxidative phosphorylation in order to avoid a quick exhaustion of ATP. The traditional mechanism of the regulation of oxidative phosphorylation, namely the negative feedback involving [ADP] and [Pi] as regulatory signals, is not sufficient to account for various kinetic properties of the system in intact skeletal muscle and heart in vivo. Theoretical studies conducted using a dynamic computer model of oxidative phosphorylation developed previously strongly suggest the so-called each-step-activation (or parallel activation) mechanism, due to which all oxidative phosphorylation complexes are directly activated by some cytosolic factor/mechanism related to muscle contraction in parallel with the activation of ATP usage and substrate dehydrogenation by calcium ions. The present polemic article reviews and discusses the growing evidence supporting this mechanism and compares it with alternative mechanisms proposed in the literature. It is concluded that only the each-step-activation mechanism is able to explain the rich set of various experimental results used as a reference for estimating the validity and applicability of particular mechanisms.

Kinetics and control of oxidative phosphorylation in rat liver mitochondria after dexamethasone treatment

The present investigation was undertaken in order to evaluate the contributions of ATP synthesis and proton leak reactions to the rate of active respiration of liver mitochondria, which is altered following dexamethasone treatment (1.5 mg/kg per day for 5 days). We applied top-down metabolic control analysis and its extension, elasticity analysis, to gain insight into the mechanisms of glucocorticoid regulation of mitochondrial bioenergetics. Liver mitochondria were isolated from dexamethasone-treated, pair-fed and control rats when in a fed or overnight fasted state. Injection of dexamethasone for 5 days resulted in an increase in the fraction of the proton cycle of phosphorylating liver mitochondria, which was associated with a decrease in the efficiency of the mitochondrial oxidative phosphorylation process in liver. This increase in proton leak activity occurred with little change in the mitochondrial membrane potential, despite a significant decrease in the rate of oxidative phosphorylation. Regulation analysis indicates that mitochondrial membrane potential homoeostasis is achieved by equal inhibition of the mitochondrial substrate oxidation and phosphorylation reactions in rats given dexamethasone. Our results also suggest that active liver mitochondria from dexamethasone-treated rats are capable of maintaining phosphorylation flux for cellular purposes, despite an increase in the energetic cost of mitochondrial ATP production due to increased basal proton permeability of the inner membrane. They also provide a complete description of the effects of dexamethasone treatment on liver mitochondrial bioenergetics.

Influence of rapid changes in cytosolic pH on oxidative phosphorylation in skeletal muscle: theoretical studies

Cytosolic pH in skeletal muscle may vary significantly because of proton production/consumption by creatine kinase and/or proton production by anaerobic glycolysis. A computer model of oxidative phosphorylation in intact skeletal muscle developed previously was used to study the kinetic effect of these variations on the oxidative phosphorylation system. Two kinds of influence were analysed: (i) via the change in pH across the inner mitochondrial membrane and (ii) via the shift in the equilibrium of the creatine kinase-catalysed reaction. Our simulations suggest that cytosolic pH has essentially no impact on the steady-state fluxes and most metabolite concentrations. On the other hand, rapid acidification/alkalization of cytosol causes a transient decrease/increase in the respiration rate. Furthermore, changes in pH seem to affect significantly the kinetic properties of transition between resting state and active state. An increase in pH brought about by proton consumption by creatine kinase at the onset of exercise lengthens the transition time. At intensive exercise levels this pH increase could lead to loss of the stability of the system, if not compensated by glycolytic H+ production. Thus our theoretical results stress the importance of processes/mechanisms that buffer/compensate for changes in cytosolic proton concentration. In particular, we suggest that the second main role of anaerobic glycolysis, apart from additional ATP supply, may be maintaining the stability of the system at intensive exercise.

Regulation of mitochondrial respiration in heart cells analyzed by reaction-diffusion model of energy transfer

The purpose of this study is to investigate theoretically which intracellular factors may be important for regulation of mitochondrial respiration in working heart cells in vivo. We have developed a model that describes quantitatively the published experimental data on dependence of the rate of oxygen consumption and metabolic state of working isolated perfused rat heart on workload over its physiological range (Williamson JR, Ford G, Illingworth J, Safer B. Circ Res 38, Suppl I, I39-I51, 1976). Analysis of this model shows that for phosphocreatine, creatine, and ATP the equilibrium assumption is an acceptable approximation with respect to their diffusion in the intracellular bulk water phase. However, the ADP concentration changes in the contraction cycle in a nonequilibrium workload-dependent manner, showing the existence of the intracellular concentration gradients. The model shows that workload-dependent alteration of ADP concentration in the compartmentalized creatine kinase system may be taken, together with the changes in P(i) concentration, to be among the major components of the metabolic feedback signal for regulation of respiration in muscle cells.

Regulation of cytochrome oxidase: theoretical studies

Using the dynamic model of oxidative phosphorylation developed previously and tested for its validity under a broad range of conditions some properties of cytochrome oxidase in the whole system considered were simulated. The regulation of this enzyme by oxygen concentration, delta p and reduction level of cytochrome c were studied. Assuming at least qualitative validity of the model, the following conclusions were drawn: (1) Regulation of cytochrome oxidase is different under the same conditions, when changes in the system (oxidative phosphorylation in isolated mitochondria) are imposed by a decrease in oxygen concentration (aerobiosis-->anaerobiosis transition) or by addition of hexokinase (state 4-->state 3 transition). In the former case, cytochrome c and delta p play a very similar role in the compensation for a decrease in the respiration rate caused by lowered oxygen concentration, while in the latter case changes in delta p activate cytochrome oxidase much stronger than changes in the reduction level of cytochrome c. (2) There is no unique thermodynamic flux-force relationship for cytochrome oxidase. This relationship depends on how the thermodynamic span of the reaction catalyzed by this enzyme is changed (aerobiosis-->anaerobiosis transition vs. state 4-->state 3 transition). (3) Under some conditions (aerobiosis-->anaerobiosis transition) the flux-force relationship can be inverse, i.e. increase in a thermodynamic force occurs simultaneously with decrease in a reaction rate.

Simulation of oxidative phosphorylation in hepatocytes

The dynamic mathematical model of oxidative phosphorylation proposed previously was modified, developed and further tested. The description of cytochrome oxidase kinetics was changed to involve dependence on Deltap. Simple, phenomenological descriptions of the kinetics of substrate dehydrogenation and ATP usage, able to reflect experimental data correctly, were found. The kinetic response of the oxidation subsystem (substrate dehydrogenation, respiratory chain), phosphorylation subsystem (ATP synthase, ATP/ADP carrier, phosphate carrier, ATP usage) and proton leak to the changes of Deltap in isolated hepatocytes incubated with different respiratory substrates was simulated. The simulations revealed a good agreement with the experimental results. Simple, intuitive assumptions were able, when introduced into the model, to explain differences in the properties of the oxidative phosphorylation system working with different respiratory substrates. It was proposed, therefore, that our explicit understanding of the oxidative phosphorylation system was good enough to explain many properties of this system correctly, at least in the range of physiological conditions tested.

Simulation of state 4 → state 3 transition in isolated mitochondria

The mathematical dynamic model of oxidative phosphorylation developed previously and in the accompanying paper was modified to involve isolated mitochondria conditions; it was also used to simulate state 4 --> state 3 transition in rat liver mitochondria incubated with succinate as respiratory substrate and glucose-hexokinase as an ADP-regenerating system. Changes in the respiration rate, protonmotive force and reduction level of ubiquinone and cytochrome c as well as the internal (i) and external (e) ATP/ADP ratio between state 4 and state 3 were calculated and compared with the experimental data. Flux control coefficients with respect to oxygen consumption flux for different reactions and processes of oxidative phosphorylation were simulated for different values of the respiration rate (state 4, state 3 and intermediate states). Flux control coefficients for the oxidation, phosphorylation and proton leak subsystems with respect to the oxidation, phosphorylation and proton leak fluxes for different values of the respiration rate were also calculated. These theoretical data were compared with the experimental results obtained in the frame of metabolic control analysis and the 'top-down' approach to this analysis. A good agreement was obtained. Simulated time courses of the respiration rate, the protonmotive force (Deltap) and other parameters after addition of a small amount of ADP to mitochondria in state 4 mimicked at least semiquantitatively the experimentally measured time courses of these parameters. It was concluded, therefore, that in the present stage, the model is able to reflect different properties of the oxidative phosphorylation system in a broad range of conditions fairly well, allows deeper insight into the mechanisms responsible for control and regulation of this process, and can be used for simulation of new experiments, thus inspiring experimental verification of the theoretical predictions.

Effects of cadmium on the control and internal regulation of oxidative phosphorylation in potato tuber mitochondria

The effect of cadmium on the distribution of control over oxidative phosphorylation in potato tuber mitochondria was quantified by measuring control coefficients using top-down metabolic control analysis. Oxidative phosphorylation was divided into three subsystems, namely substrate oxidation, the phosphorylation reactions and the proton leak. The control exerted by each of these subsystems over the system fluxes, the value of the protonmotive force and the effective P/O ratio was quantified in the presence of different concentrations of free cadmium (up to 21 microM). Cadmium is known to stimulate the proton leak and inhibit the substrate oxidation reactions, but it had little effect on the distribution of control over the system variables except to shift the pattern to lower rates. Control exerted by particular subsystems appeared to change or to stay the same as cadmium was varied, depending on whether the control coefficients were presented as a function of respiration rate or protonmotive force. The regulatory strength of protonmotive force on the system variables was also calculated, as partial internal response coefficients. These coefficients changed with ATP turnover rate and with cadmium concentration, showing how the internal regulation of oxidative phosphorylation shifts under different conditions. The values of control coefficients and partial internal response coefficients show where control lies and how intermediates regulate the system variables under different conditions of ATP demand and external effector (i.e. cadmium) concentration. However, they are not useful for identifying the sites of action of external effectors, for which elasticity and regulation analysis must be used.

Quantitative determination of the regulation of oxidative phosphorylation by cadmium in potato tuber mitochondria

The effects of cadmium on respiration rate, phosphorylation rate, proton leak rate, the protonmotive force and the effective P/O ratio were determined over a range of respiratory conditions and cadmium concentrations by applying top-down regulation analysis. To quantify the effects of cadmium, we determined the overall response coefficients of these variables of oxidative phosphorylation to cadmium in different respiratory states between state 4 and state 3 and at different cadmium concentrations. The overall response coefficients to cadmium showed quantitatively how cadmium stimulated substrate oxidation rate at high cadmium concentrations near state 4 but inhibited it to different extents under all other conditions, how cadmium inhibited the rate of proton leak rate at low cadmium concentrations near state 4 but stimulated it under all other conditions, and how cadmium inhibited the rate of phosphorylation and depressed the protonmotive force and the effective P/O ratio to different extents under all conditions. Cadmium is known to stimulate the proton leak and to inhibit the substrate oxidation reactions; we calculated the elasticities of these subsystems to cadmium to quantify its effects. To describe fully how the cadmium effects on different subsystems produce the overall responses of the system to cadmium, we then calculated the partial response coefficients of the system variables to cadmium acting through each subsystem. The partial response coefficients quantify the contribution of each block to the overall effect of cadmium on each variable in any condition, and sum to the overall response coefficient in each condition. Together with the elasticity analysis and the control analysis and internal regulation analysis presented in the preceding papers [Kesseler, A. & Brand, M. D. (1994) Eur. J. Biochem. 225, pp. 897-906; Kesseler, A. & Brand, M. D. (1994) Eur. J. Biochem. 225, pp. 907-922] they completely describe how cadmium exerts its effects on oxidative phosphorylation at the system level.

Control of the effective P/O ratio of oxidative phosphorylation in liver mitochondria and hepatocytes

The control exerted by substrate oxidation reactions, by ATP turnover and by the proton leak over the oxygen consumption rate, the phosphorylation rate, the proton leak rate and the protonmotive force (delta p) in isolated rat liver mitochondria under a range of conditions between non-phosphorylating (State 4) and maximum phosphorylation (State 3) was investigated by using the top-down approach of metabolic control analysis. The experiments were carried out with saturating concentrations of the substrates succinate, glutamate with malate, or pyruvate with malate. The distribution of control was very similar with each of the three substrates. The effective P/O ratio (i.e. not corrected for leak reactions) was also measured; it varied from zero in State 4 to 80-90% of the maximum theoretical P/O ratio in State 3. Under most conditions control over the effective P/O ratio was shared between proton leak (which had negative control) and the phosphorylating subsystem (which had roughly equal positive control); near State 4, substrate oxidation reactions also acquired some control over this ratio. In resting hepatocytes the effective P/O ratio was only 50% of its maximum theoretical value, corresponding to an effective P/O ratio of only 1.3 for complete oxidation of glucose. The effective P/O ratio for intracellular mitochondrial oxygen consumption was 64% of the maximum value. The control coefficient of the mitochondrial proton leak over the effective P/O ratio in cells was -0.34; the control coefficient of phosphorylation reactions over this ratio was 0.31 and the control coefficient of substrate oxidation reactions over the ratio was 0.03, showing how the coupling efficiency in cells can respond sensitively to agents that change the proton leak or the ATP demand, but not to those that change substrate oxidation.

An investigation of the relationships between rate and driving force in simple uncatalysed and enzyme-catalysed reactions with applications of the findings to chemiosmotic reactions

Both the rate and the driving force of a reaction can be expressed in terms of the concentrations of the reactants and products. Consequently, rate and driving force can be expressed as a function of each other. This has been done for a single-reactant, single-product, uncatalysed reaction and its enzyme-catalysed equivalent using the van't Hoff reaction isotherm and Haldane's generalized Michaelis-Menten rate equation, the primary objective being explanation of the exponential and sigmoidal relationships between reaction rate and delta mu H+ commonly observed in studies on chemiosmotic reactions. Acquisition of a purely thermodynamic rate vs. driving-force relationship requires recognition of the intensive and extensive variables and maintenance of the extensive variables constant. This relationship is identical for the two reactions and is hyperbolic or sigmoidal, depending on whether the equilibrium constant is smaller or larger than unity. In the case of the catalysed reaction, acquisition of the purely thermodynamic relationship requires the assumption that the enzyme be equally effective in catalysing the forward and backward reactions. If this condition is not met, the relationship is modified by the enzyme in a manner which can be determined from the ratio of the Michaelis constants of the reactant and product. Under conditions of enzyme saturation in respect to reactant+product, the rate vs. driving-force relationship is determined exclusively by the thermodynamics of the reaction and a single kinetic parameter, the magnitude of which is determined by the relative effectiveness of the enzyme in catalysing the forward and backward reactions. In view of this finding, it is pointed out that, since the catalytic components of chemiosmotic reactions appear to be saturated with respect to the reactant-product pair that is varied in experimental rate vs. delta mu H+ determinations, and that, since many complex enzymic reactions conform to the simple Michaelis-Menten equation with respect to a single reactant-product pair when the concentrations of all other reactants and products are maintained constant, one might expect to be capable of simulating the experimental relationships simply from knowledge of the thermodynamics of the reaction and the relative effectiveness of the catalytic component in catalysing the forward and backward reactions using the simple Michaelis-Menten equation. That this expectation appears to be largely correct is demonstrated with model reactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of mitochondrial ATP synthesis in the heart

Images

An extended dynamic model of oxidative phosphorylation

The presented model based on an earlier one (Korzeniewski, B. and Froncisz, W. (1989) Studia Biophys. 132, 173-187) simulates concentration changes in time of chemical compounds and thermodynamic forces during respiration of cell suspension in a closed chamber. A set of differential equations solved numerically describes the utilization of oxygen up to anaerobiosis and the behaviour of the system after a sudden pulse of oxygen. Flux control coefficients for most important reactions (enzymes) of oxidative phosphorylation were calculated. A good qualitative and (when a direct comparison is possible) quantitative agreement with experimental results can be observed. The following conclusions can be drawn from the simulation: (1) Wilson's steady state model is not in contradiction with sharing of the control over the respiration between some steps and displacement of the ATP/ADP carrier from equilibrium. (2) The overshoot characteristics of the delta microH+ time-course after reoxygenation can be explained without using the lag-phase kinetics of ATP-synthetase. (3) A 'hot region' (sharp changes of many parameters) can be distinguished when the oxygen concentration approaches zero; only cytochrome oxidase is clearly sensitive on oxygen concentration in all its range. (4) Control over oxidative phosphorylation is shared mainly between inputs of the system (ATP utilization and substrate dehydrogenation) and the proton leak.

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.

Analysis of the control of respiration rate, phosphorylation rate, proton leak rate and protonmotive force in isolated mitochondria using the 'top-down' approach of metabolic control theory

The rate of respiration of isolated mitochondria was set at different values by addition of either oligomycin or an ADP-regenerating system (glucose and different amounts of hexokinase). We measured the relationship between respiration rate and membrane potential as respiration was titrated by the addition of malonate under each condition. We used the flux control summation and connectivity theorems and the branching theorem of metabolic control theory to calculate the control over respiration rate exerted by the respiratory chain (and associated reactions), phosphorylating system (and associated reactions) and proton leak at each respiration rate. The analysis also yielded the flux control coefficients of these three reactions over phosphorylation rate and proton leak rate and their concentration control coefficients over protonmotive force. We found that respiration rate was controlled largely by the proton leak under non-phosphorylating conditions, by the phosphorylating system at intermediate rates and by both the phosphorylating system and the respiratory chain in state 3. The rate of phosphorylation was controlled largely by the phosphorylating system itself in state 4 and at intermediate rates, while state 3 control was shared between the phosphorylating system and the respiratory chain; the proton leak had insignificant control. In all states the phosphorylating system had large negative control over the proton leak; the chain and the proton leak both had large positive control coefficients. The protonmotive force was controlled by the chain and by the phosphorylating system; the proton leak had little control.

Resting state respiration of mitochondria: reappraisal of the role of passive ion fluxes

Rat liver mitochondria respiring under resting state conditions in the presence of oligomycin were rapidly blocked with cyanide and the dissipation of the membrane potential, measured with a tetraphenylphosphonium-sensitive electrode, was followed over time. The plot of the rate of membrane potential dissipation versus the actual value of the membrane potential was nonlinear and identical to the plot of resting state respiration (titrated with small amounts of a respiratory inhibitor) versus the membrane potential. The relationship between the respiratory chain activity and the proton-motive force in mitochondria oxidizing succinate with either oxygen or ferricyanide as electron acceptors was also found to be identical. These results are interpreted as an indication that the passive permeability of the inner mitochondrial membrane toward ions is far more significant in maintaining resting state respiration than is the molecular slippage of the pumps in the respiratory chain. These results also confirm the non-ohmic characteristics of the inner mitochondrial membrane.

Mitochondrial membrane potential estimated with the correction of probe binding

Lipophilic ions are widely used as the probe for estimation of the membrane potential. It is suggested that the correction of the probe binding to the membrane and/or intracellular constituents is a problem to be solved in order to evaluate the membrane potential accurately. Previously, we proposed a method for the correction of the probe binding (Demura, M., Kamo, N. and Kobatake, Y. (1985) Biochim. Biophys. Acta 820, 207-215). In this paper, the method was applied to the determination of the membrane potential of intact mitochondria. The probes used constitute a homologous series of (Phe)3-P+-(CH2)n-CH3 (n = 0-4) and tetraphenylphosphonium (TPP+). Binding of these probes to de-energized mitochondria followed the Langmuir isotherm. However, values of parameters determined at high (50-800 microM) and low (under 20 microM) probe concentrations were different, suggesting the existence at least two, high- and low-affinity, binding sites. With extrapolation to the 'state of no binding', the membrane potential of intact mitochondria was estimated to be -147 mV (interior-negative) when they were energized by 5 mM succinate in medium consisting of 125 mM KCl, 10 mM MgCl2, 5 mM phosphate, 0.4 mM EDTA and 50 mM Tris-HCl (pH 7.5) at 25 degrees C. Parameters appearing in the equation for the correction of probe binding were determined with the use of this value of the membrane potential. The validity of the equation and the value of the parameters were revealed by the fact that after the correction, all probes used gave approximately the same value under the same conditions. We expanded the method so as to include the langmuir adsorption isotherm. When the modified equation is used, the estimated membrane potentials were less dependent on a probe concentration less than 10 microM.

The control of electron flux through cytochrome oxidase

1. The electron flux through cytochrome oxidase is a linear function of the net thermodynamic force across the complex over a limited range of conditions. 2. Over a wide range of conditions the electron flux is a complicated function of the percentage reduction of the cytochrome c pool and of delta psi (at low values of delta pH). 3. We have estimated the elasticities of electron flux through cytochrome oxidase to delta Eh of the redox reaction catalysed by cytochrome oxidase (or to cyt c2+/cyt c3+) and to delta psi. The elasticities varied depending on the values of delta psi and of the percentage reduction of the cytochrome c pool. 4. At intermediate rates (which may correspond to those in vivo) the electron flux through cytochrome oxidase is controlled to about the same extent by delta psi and by delta Eh.

Energy-storage capacity of the mitochondrial proton-motive force

Resting state respiration of rat-liver mitochondria in the presence of oligomycin was rapidly blocked with cyanide and the dissipation of the membrane potential was followed with a tetraphenylphosphonium-sensitive electrode. From the rate of this dissipation and the electric capacitance of the mitochondrial membrane the energy stored in form of the membrane potential was calculated as about 7 microJ/mg protein. In the absence of oligomycin, dissipation of the membrane potential was slower, as it was partly compensated by proton ejection by mitochondrial ATPase hydrolyzing endogenous ATP. This allowed to calculate the total energy storage capacity of the proton-motive force. It amounted to the equivalence of 3.3 nmol ATP/mg protein or about 130 microJ/mg protein. The stoichiometry of proton-pumping ATPase utilizing endogenous ATP was estimated as three protons per molecule ATP.

ATP synthase-mediated proton fluxes and phosphorylation in rat liver mitochondria: dependence on delta mu H

The dependence of the proton flux through the ATP synthases of rat liver mitochondria on a driving force composed mainly of a potassium diffusion potential was determined and compared with the relationship between rate of phosphorylation and delta mu H given by titrations with the respiratory inhibitor malonate. The two functions are in good agreement in the lower part of the delta mu H range covered. However, the maximal proton fluxes through the ATP synthases are much lower than needed to account for the rate of State 3 phosphorylation sustained by the same mitochondria oxidizing succinate. Possible reasons for this behavior are discussed.

A mathematical model to study short-term regulation of mitochondrial energy transduction

A mathematical model is presented which includes the following elementary process of mitochondrial energy transduction: hydrogen supply, proton translocation by the respiratory chain, proton-driven ATP synthesis by the F0F1-ATPase, passive back-flow of protons (leak) and carrier-mediated exchange of adenine nucleotides and phosphate. For these processes empirical rate laws are used. The model is applied to calculate time-dependent states of energy transduction in isolated rat liver mitochondria. From the general agreement of the computational results with experimental data (Ogawa, S. and Lee, T.M. (1984) J. Biol. Chem. 259, 10004-10011) the following conclusions can be drawn. (1) The length of the time interval during which mitochondria are able to maintain a relatively high and constant delta pH in the absence of oxygen (anaerobiosis) is limited by the availability of intramitochondrial ATP. (2) The overshoot kinetics of delta pH which appear when reoxigenating mitochondria after a preceeding anaerobiosis might be due to a lag phase kinetics of the F0F1-ATPase. (3) In phosphorylating mitochondria the homeostasis of delta pH is brought about by a high sensitivity of the respiration rate and the rate of the F0F1-ATPase as to changes of delta pH. (4) Analysis of the mean transient times shows that the rate of ATP synthesis in State 3 is controlled to almost the same extent by the hydrogen supply, the respiratory chain, the adenine nucleotide translocator and the proton leak.