Proton electrochemical gradient and phosphate potential in submitochondrial particles

Cardiolipin Alterations during Obesity: Exploring Therapeutic Opportunities

Cardiolipin is a specific phospholipid of the mitochondrial inner membrane that participates in many aspects of its organization and function, hence promoting proper mitochondrial ATP production. Here, we review recent data that have investigated alterations of cardiolipin in different tissues in the context of obesity and the related metabolic syndrome. Data relating perturbations of cardiolipin content or composition are accumulating and suggest their involvement in mitochondrial dysfunction in tissues from obese patients. Conversely, cardiolipin modulation is a promising field of investigation in a search for strategies for obesity management. Several ways to restore cardiolipin content, composition or integrity are emerging and may contribute to the improvement of mitochondrial function in tissues facing excessive fat storage. Inversely, reduction of mitochondrial efficiency in a controlled way may increase energy expenditure and help fight against obesity and in this perspective, several options aim at targeting cardiolipin to achieve a mild reduction of mitochondrial coupling. Far from being just a victim of the deleterious consequences of obesity, cardiolipin may ultimately prove to be a possible weapon to fight against obesity in the future.

Cardiolipin content controls mitochondrial coupling and energetic efficiency in muscle

Unbalanced energy partitioning participates in the rise of obesity, a major public health concern in many countries. Increasing basal energy expenditure has been proposed as a strategy to fight obesity yet raises efficiency and safety concerns. Here, we show that mice deficient for a muscle-specific enzyme of very-long-chain fatty acid synthesis display increased basal energy expenditure and protection against high-fat diet-induced obesity. Mechanistically, muscle-specific modulation of the very-long-chain fatty acid pathway was associated with a reduced content of the inner mitochondrial membrane phospholipid cardiolipin and a blunted coupling efficiency between the respiratory chain and adenosine 5'-triphosphate (ATP) synthase, which was restored by cardiolipin enrichment. Our study reveals that selective increase of lipid oxidative capacities in skeletal muscle, through the cardiolipin-dependent lowering of mitochondrial ATP production, provides an effective option against obesity at the whole-body level.

Stoichiometry of energy coupling by proton-translocating ATPases: a history of variability

One of the central energy-coupling reactions in living systems is the intraconversion of ATP with a transmembrane proton gradient, carried out by proton-translocating F- and V-type ATPases/synthases. These reversible enzymes can hydrolyze ATP and pump protons, or can use the energy of a transmembrane proton gradient to synthesize ATP from ADP and inorganic phosphate. The stoichiometry of these processes (H(+)/ATP, or coupling ratio) has been studied in many systems for many years, with no universally agreed upon solution. Recent discoveries concerning the structure of the ATPases, their assembly and the stoichiometry of their numerous subunits, particularly the proton-carrying proteolipid (subunit c) of the F(O) and V(0) sectors, have shed new light on this question and raise the possibility of variable coupling ratios modulated by variable proteolipid stoichiometries.

Free energy coupling between H+-generating and H+-consuming pumps. Ratio between output and input forces

The delta Gp/delta mu H ratio has been measured in mitochondria close to state 4 in the presence of various uncoupler or K+/valinomycin concentrations in media containing either 1 mM or 50 mM Pi. Care has been taken to control the factors affecting delta Gp and delta mu H which could lead to an artefactual increase of the delta Gp/delta mu H ratio above the highest accepted value for the H+/ATP stoichiometry (n = 4, synthesis + transport). In particular, to avoid overestimation of delta Gp due to inactivation of the ATPases at low delta mu H or to the presence of adenylate kinase, the static head state was approached from the side of net ATP synthesis and delta Gp was measured in a state close to static head but still maintaining a residual rate of aerobic phosphorylation. For each concentration of uncoupler or K+, the Pi concentration and/or the adenylate energy charge (EC) as a function of time have been measured as indicators of net ATP synthesis. Only the values of delta Gp measured during a decrease in Pi concentration and/or an increase in EC have been considered to be meaningful for calculations of delta Gp/delta mu H ratios. Both uncouplers and K+ transport cause a marked depression of delta mu H and a parallel depression of the rate of ATP synthesis. However the low rate of ATP synthesis taking place under conditions of low delta mu H eventually results, especially at high Pi concentrations, in a relatively large delta Gp. The delta Gp/delta mu H ratios obtained at the lower delta mu H values exceed 4 and approach 6. Although slightly higher delta Gp/delta mu H ratios are obtained with valinomycin-treated than with uncoupler-treated mitochondria, the pattern of the rise of the force ratio as delta mu H decreases is similar in both cases. An increase of the delta Gp/delta mu H ratio above 4, the maximal accepted H+/ATP stoichiometry is thermodynamically incompatible with the delocalized protonic coupling model.

An assessment of the chemiosmotic hypothesis of mitochondrial energy transduction

It is argued that a proton concentration difference and/or a membrane potential is not the form into which the free energy of the oxidation-reduction reactions of the mitochondrial respiratory chain is first transduced. It is suggested that the search for a chemical intermediate should be continued in spite of the conclusion by some investigators that the chemical hypothesis is untenable. It is asked whether pH changes when measured in solutions containing mitochondria can be interpreted as evidence for H+ movements, also, whether there is a continuous, renewable and stable electrochemical proton concentration difference (delta mu H+) across the mitochondrial membrane, and whether in fact the delta mu H+ is a necessary intermediate in the synthesis of ATP. The four postulates of Mitchell's chemiosmotic hypothesis of energy transduction are discussed point by point. It is agreed that "The systems are plugged through a topologically closed insulating membrane," which probably is not "a nonaqueous osmotic barrier," and which probably does not have an unusually "low permeability to solutes and to H+ and OH- in particular" when compared with other membranes. There is disagreement with the statement that "Respiratory and photoredox systems are chemiosmotic membrane-located protonmotive chains" in that it is suggested by others that chemiosmosis is chemically nonexistent and that thermodynamically it would lack control. The subsequent statement, "having a characteristic----H+/2 epsilon- stoichiometry," is rendered uncertain by the experimental findings of values greater than 2H+/2 epsilon-/site and probably as large as 4H+/2 epsilon-/site. The proposal that "The synthetase is a chemiosmotic membrane-located reversible motive ATPase" requires the assumption that the ATP synthetase is the same enzyme as the ATPase, but functioning in the reverse direction. It is considered possible that there are two enzymes in the multi-subunit ATPase complex: one the hydrolase, and the other the synthetase. The further proposal, "having characteristic----H+/P stoichiometry" requires that the ratio be 2 according to Mitchell. However, values of 3, as well as larger values, have been reported by others, which introduces a large element of uncertainty. There is no disagreement with the statement that "There are proton-linked (or hydroxyl ion-linked) solute porter systems for osmotic stabilization and metabolite transport." In fact, this may be the principal reason for having proton efflux or "proton-pumping.''(ABSTRACT TRUNCATED AT 400 WORDS)

A minimal hypothesis for membrane-linked free-energy transduction. The role of independent, small coupling units

Experimental data are reviewed that are not in keeping with the scheme of 'delocalized' protonic coupling in membrane-linked free-energy transduction. It turns out that there are three main types of anomalies: (i) rates of electron transfer and of ATP synthesis do not solely depend on their own driving force and on delta mu H, (ii) the ('static head') ratio of delta Gp to delta mu H varies with delta mu H and (iii) inhibition of either some of the electron-transfer chains or some of the H+-ATPases, does not cause an overcapacity in the other, non-inhibited proton pumps. None of the earlier free-energy coupling schemes, alternative to delocalized protonic coupling, can account for these three anomalies. We propose to add a fifth postulate, namely that of the coupling unit, to the four existing postulates of 'delocalized protonic coupling' and show that, with this postulate, protonic coupling can again account for most experimental observations. We also discuss: (i) how experimental data that might seem to be at odds with the 'coupling unit' hypothesis can be accounted for and (ii) the problem of the spatial arrangement of the electrical field in the different free-energy coupling schemes.

Relationships of respiratory chain and ATP-synthetase in energized mitochondria

The present study reveals that the previously described effect of ATP-synthetase inhibition concomitant with inhibition of respiratory chain functioning may be observed at different absolute values of delta psi on the mitochondrial membrane. This fact points out that the membrane potential is not a unique regulator in coupling of ATP-synthetase and respiratory chain activities. We found, using the double-inhibitor titration technique, that ATP-synthetase inhibition induces proportional inhibition of respiratory chain enzymes and vice versa respiratory chain inhibition induces proportional inhibition of ATP-synthetase. This effect is shown to exist only when osmolarity is close to 150-300 (mosM) (in the physiological range). The coupling effectivity (ADP/O) of mitochondria under these conditions is maximal. Under conditions of high osmolarity (400-600 mosM) the respiratory chain and ATP-synthetase behave as if they were coupled by bulk phase delta -mu H+, from the kinetic point of view.

The elucidation of the effect of ammonium chloride on pyruvate distribution and pyruvate dehydrogenase interconversion in isolated rat hepatocytes

The distribution of pyruvate between cell compartments measured in isolated hepatocytes in the presence of lactate was in agreement with delta pH across plasma and mitochondrial membranes. In isolated liver mitochondria NH4Cl decreased the transmembrane potential (delta psi) by about 14 mV, whereas no change of delta pH was observed. In the presence of lactate or alanine NH4Cl increased the mitochondrial pyruvate concentration presumably due to the inhibition of the flux through pyruvate carboxylase. In the presence of lactate or alanine changes in the amount of the active form of pyruvate dehydrogenase (PDHa) were correlated with the mitochondrial pyruvate concentration, NH4Cl increased the amount of PDHa by lowering the mitochondrial ATP/ADP and NADH/NAD+ ratios.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Uptake of thallous ions by mitochondria is stimulated by nonactin but not by respiration alone

Nonrespiring rat-liver mitochondria swell in media containing high concentrations of thallous nitrate, indicating passive penetration of Tl+. This swelling could be further stimulated by 10 nM or more nonactin while even 1 microM valinomycin was without effect. Nonactin was also much more potent than valinomycin in stimulating swelling of respiring mitochondria in the presence of thallous acetate. It is evident that nonactin acts as a potent ionophore of Tl+ able to promote both the passive and energized uptake of Tl+ in mitochondria. The distribution of Tl+, present in trace concentrations below 1 mM, was measured during energisation by respiration both in the presence and absence of ionophores. Respiration induced net uptake of Tl+ only in the presence of ionophores, though Tl+ as a permeant cation was expected to sense respiration-induced changes in the membrane potential. The data may be interpreted as indicating that no transmembrane potential is formed upon energisation, but localized fields, which are able to interact with the lipophilic ionophore complexes of Tl+, but not with the hydrophilic cation Tl+. This interpretation is valid only if thermodynamic equilibrium has been reached.

Clarification of factors influencing the nature and magnitude of the protonmotive force in bovine heart submitochondrial particles

The magnitude of the protonmotive force, and its division between pH gradient and membrane potential components has been further characterised in submitochondrial particles. In a reaction medium containing sucrose for osmotic support and 4-(2-hydroxyethyl)-1-piperazineethanesulfonate (Hepes) as buffer, with succinate as substrate, the total protonmotive force reached a maximum value of 245 mV. The presence of Cl- enhanced the pH gradient with a partial but not fully compensating decrease in the membrane potential. When submitochondrial particles were suspended in a medium of low osmolarity consisting of phosphoric acid neutralised with Tris, again with succinate as substrate, the protonmotive force was lower and did not exceed 185 mV, and the pH gradient component was equivalent to 25 mV or less. The final phosphorylation potential, delta Gp, (formula: see text); maintained by the particles was higher in the phosphate/Tris medium (46--47.7 kJ mol-1) than in the sucrose/Hepes/KCl medium (43.7 kJ mol-1). Thus, comparison of the phosphorylation potential with the protonmotive force would suggest that the mechanistic stoichiometry H+/ATP (H+ translocated per molecule of ATP synthesied) for the ATPase enzyme is 3 in the former medium and 2 in the latter, which might be taken to indicate two different types of mechanism required for ATP synthesis. However it is questioned whether a comparison of the protonmotive force with delta Gp in terms of equilibrium thermodynamics ought not to be complemented by analysis in terms of linear non-equilibrium thermodynamics. The latter treatment shows that it is possible to estimate only a value for the product of a phenomenological stoichiometry and the degree of coupling, which can be variable, but not the mechanistic stoichiometry. This treatment can also rationalise the observation of the higher delta Gp in reaction conditions where the lower values for delta p are estimated. Irrespective of possible explanations, the data show how an unprejudiced choice of reaction conditions can lead to different conclusions about the relationship between the phosphorylation potential and the protonmotive force.

Relationship of transmembrane pH and electrical gradients with respiration and adenosine 5'-triphosphate synthesis in mitochondria

The mechanism of mitochondrial oxidative phosphorylation and its regulation have been studied by using suspensions of isolated rat liver mitochondria. Parallel measurements were made of mitochondrial volume, respiration, transmembrane pH and electrical gradients, and adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and inorganic phosphate (Pi) concentrations under various experimental conditions. The transmembrane electrical gradients were calculated from the equilibrium distributions of [3H]-triphenymethylphosphonium (TPMP+), [3H]tribenzylmethylammonium (TBMA+), and K+ (plus valinomycin). The transmembrane distributions of labeled acetate, methylamine, and 5,5-dimethyloxazolidine-2,4-dione were used for the calculation of pH gradients. Evaluation of the data shows that the respiratory rate is strictly correlated with [ATP]/([ADP][Pi]) (free energy of ATP synthesis), whereas there is no consistent correlation between the transmembrane electrical potential, the pH gradient, or the total "protonmotive force" (delta muH+) and the respiratory rate. Thermodynamic analysis indicates that, in order for the proton electrochemical gradient to serve as an intermediate in ATP synthesis, from three to seven H+ would have to be transported per each ATP synthesized, depending on the experimental conditions. These results suggest that the proton electrochemical gradient may not serve as a primary intermediate in oxidative phosphorylation.

Formulation and some biological uses of a buffer mixture whose buffering capacity is relatively independent of pH in the range pH 4-9

A mixture is described which has a buffering capacity which is essentially independent of pH in the range pH 4.0-9.0. It is shown how this buffer mixture may be used to determine the force-flux relationship of proton transfer between two aqueous phases separated by a phospholipid bilayer in vesicular systems and so demonstrate that this relationship is linear over a wide range of delta mu approximately H+. The buffer mixture can, furthermore, be employed to determine the volume enclosed within a vesicular preparation.

The induction kinetics of bacterial photophosphorylation. Threshold effects by the phosphate potential and correlation with the amplitude of the carotenoid absorption band shift

1. ATP synthesis (monitored by luciferin-luciferase) can be elicited by a single turnover flash of saturating intensity in chromatophores from Rhodopseudomonas capsulata, Kb1. The ATP yield from the first to the fourth turnover is strongly influenced by the phosphate potential: at high phosphate potential (-11.5 kcal/mol) no ATP is formed in the first three turnovers while at lower phosphate potential (-8.2 kcal/mol) and the yield in the first flash is already one half of the maximum, which is reached after 2-3 turnovers. 2. The response to ionophores indicates that the driving force for ATP synthesis in the first 20 turnovers is mainly given by a membrane potential. The amplitude of the carotenoid band shift shows that during a train of flashes an increasing delta psi is built up, which reaches a stationary level after a few turnovers; at high phosphate potential, therefore, more turnovers of the same photosynthetic unit are required to overcome an energetic threshold. 3. After several (six to seven) flashes the ATP yield becomes constant, independently from the phosphate potential; the yield varies, however, as a function of dark time (td) between flashes, with an optimum for td = 160-320 ms. 4. The decay kinetics of the high energy state generated by a long (125 ms) flash have been studied directly measuring the ATP yield produced in post-illumination by one single turnover flash, under conditions of phosphate potential (-10 kcal/mol), which will not allow ATP formation by one single turnover. The high energy state decays within 20 s after the illumination. The decay rate is strongly accelerated by 10(-8) M valinomycin. 5. Under all the experimental conditions described, the amplitude of the carotenoid signal correlates univocally with the ATP yield per flash, demonstrating that this signal monitores accurately an energetic state of the membrane directly involved in ATP synthesis. 6. Although values of the carotenoid signal much larger than the minimal threshold are present, relax slowly, and contribute to the energy input for phosphorylation, no ATP is formed unless electron flow is induced by a single turnover flash. 7. The conclusions drawn are independent from the assumption that a delta psi between bulk phases is evaluable from the carotenoid signal.

The ATPase inhibitor protein in oxidative phosphorylation. The rate-limiting factor to phosphorylation in submitochondrial particles

1. Purified luciferase and luciferin were used to study the time course of phosphorylation in submitochondrial particles. The light emitted was detected by a single-photon counter, using a multichannel analyser, and the results were analysed by an 'on-line' digital computer. 2. Using NADH as substrate, phosphorylation showed, in general, four phases. These were (i) a period of increasing rate ('lag'); (ii) a period of constant (positive) rate; (iii) a period of zero net rate (plateau), when the phosphorylation potential was maintained at its equilibrium value, and (iv) a period of negative rate (atp hydrolysis) after all the oxygen has been consumed. 3. The lag phase, several seconds in length, was a function of the inhibitor protein content of the particles. It was decreased in particles treated to remove the inhibitor protein, either by prior energisation of the particles with NADH, or by addition of aurovertin, which competes with the inhibitor protein for the ATPase. It was concluded that the ATPase inhibitor inhibits both ATP synthesis and hydrolysis by the ATPase. 4. The rate constant for the release of the inhibitor protein from the energised membrane was determined from the time course of ATP production during the lag phase. The activation energy of this process was measured from the temperature dependence of the lag, and was shown to be 13.3 kcal/mol, lower than the activation energy of ATP synthesis or NADH oxidation. 5. The rate constant for inhibitor release was dependent on 'energisation' of the membrane, being lower in the presence of uncouplers. However, it was possible to decrease the rate constant considerably with agents that collapsed the membrane potential without uncoupling the membrane. It was concluded that the inhibitor protein responded to the membrane potential component of the energisation. 6. A kinetic model for energy-dependent dissociation of the ATPase-inhibitor complex is proposed.