The interaction of glucagon treatment and uncoupler concentration on ATPase activity of rat liver mitochondria

Mitochondria isolated from livers of rats treated briefly with glucagon show an increased ATPase activity in the presence of appropriate concentrations of protonophoric uncouplers (Yamazaki, R. K., Sax, R.D., and Hauser, M.A. (1977) FEBS Lett. 75, 295-299). With the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) the effect of glucagon treatment was most evident at concentrations of uncoupler higher than required for maximal stimulation of ATPase in control mitochondria. In this range of FCCP concentrations that produced the greatest contrast in ATPase activity of control and hormone-stimulated mitochondria, there were no significant differences in delta pH, delta psi, or delta p between the two groups. The presence of added succinate in the ATPase assay system mimicked the effect of glucagon treatment, permitting greater activity at high concentrations of uncoupler without significantly affecting delta p. No significant effect of glucagon treatment or uncoupler concentrations on mitochondrial volumes was observed. Following treatment with glucagon, the mitochondria retained a greater content of Mg+ and K+ throughout the range of FCCP concentrations tested. In the upper range of FCCP concentrations there was appreciable loss of K+ from the mitochondria which was greater in control mitochondria than in mitochondria from glucagon-treated rats or in mitochondria assayed in the presence of succinate. The activity of the uncoupler-dependent ATPase was greatly stimulated by increased concentrations of potassium chloride in the assay medium without significantly diminishing the hormone effect. It is proposed that the intrinsic peptide inhibitor of ATPase is dissociated from the enzyme to an increased degree following glucagon treatment and that high levels of uncoupler inhibit by causing an increased association of the enzyme and its inhibitor.     

Multiple relationships between rate of oxidative phosphorylation and delta microH in rat liver mitochondria

The relationship between rate of ATP synthesis and transmembrane electrochemical proton gradient has been determined in rat liver mitochondria oxidizing succinate, using the respiratory inhibitor malonate or the uncoupler FCCP to decrease delta microH progressively. As previously reported [(1982) Eur. J. Biochem. 126, 443-451] two different relationships are obtained depending on the method used. Evidence is presented that this result is not due to underestimation of the delta microH maintained by fast-respiring mitochondria, as recently suggested [(1985) FEBS Lett. 181, 323-327].     

Energy-dependent transformation of the catalytic activities of the mitochondrial F0 x F1-ATP synthase

The ADP(Mg2+)-deactivated, azide-trapped F0 x F1-ATPase of coupled submitochondrial particles is capable of ATP synthesis being incapable of ATP hydrolysis and ATP-dependent delta muH+ generation [FEBS Lett. (1995) 366, 29-32]. This puzzling phenomenon was studied further. No ATPase activity of the submitochondrial particles catalyzing succinate-supported oxidative phosphorylation in the presence of azide was observed when ATP was added to the assay mixture after an uncoupler. Rapid ATP hydrolysis was detected in the same system when ATP followed by an uncoupler was added. Less than 5% of the original ATPase activity was seen when the reaction (assayed with ATP-regenerating system) was initiated by the addition of ATP to the azide-trapped coupled particles oxidizing succinate either in the presence or in the absence of the uncoupler. High ATP hydrolytic activity was revealed when the reaction was started by the simultaneous addition of the ATP plus uncoupler to the particles generating delta muH+. The energy-dependent conversion of the enzyme into latent uncoupler-activated ATPase was prevented by free ADP (Ki approximately 20 microM) and was greatly enhanced after multiple turnovers in oxidative phosphorylation. The results suggest that the catalytic properties of F0 x F1 are delta muH+-dependent which is in accord with our hypothesis on different conformational states of the enzyme participating in ATP synthesis or hydrolysis.     

The effect of delta mu H+ on the interaction of rotenone with complex I of submitochondrial particles.

The inhibition by rotenone of the forward (NADH-oxidase) and reverse (delta mu H(+)-dependent succinate-NAD+ reductase activities of submitochondrial vesicles was measured. The inhibition of NADH-oxidase, measured in the presence of uncoupler, followed a monophasic inhibition curve with Ki < or = 2 nM. The reverse electron flow was only partially (40%) inhibited at these rotenone concentrations. The rest of the activity was less sensitive to the inhibitor (Ki approximately 30 nM). The lower affinity for the inhibitor of the reverse electron flow is a consequence of enhanced rate of rotenone dissociation caused by the high delta mu H+ value required for this reaction. The analysis of the results indicates that the AS-SMP preparation consists of two subpopulations: one with a relatively low degree of coupling, which exhibits high sensitivity to rotenone and the other which is highly coupled with lower affinity to the inhibitor.     

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.     

Control processes in oxidative phosphorylation: kinetic constraints and stoichiometry

Control processes in oxidative phosphorylation have been studied in three experimental models. (1) In isolated yeast mitochondria, external ATP is a regulatory effector of cytochrome-c oxidase activity. In phosphorylating or uncoupling states, the relationships between respiratory rate and delta mu H+, and the respiratory rate and cytochrome-c oxidase reduction level are dependent on this kinetic regulation. (2) In rat liver mitochondria, the response of the respiratory rate to uncoupler addition is age-dependent: liver mitochondria isolated from young rats maintain a greater delta mu H+ than liver mitochondria isolated from adults, with the same respiratory rate obtained with the same concentration of uncoupler. This behaviour is linked to redox proton pump properties, i.e., to the degree of intrinsic uncoupling induced by uncoupler addition. (3) The effect of almitrine, a new kind of ATPase/ATPsynthase inhibitor, was studied in mammalian mitochondria. (i) Almitrine inhibits oligomycin-sensitive ATPase - it decreases the ATPase/O value without any change in delta mu H+; (ii) almitrine increased the mechanistic H+/ATP stoichiometry of ATPase/ATPsynthase; (iii) almitrine-induced changes in H+/ATPase stoichiometry depend on the flux magnitude through ATPase. These results are discussed in terms of the following interdependent parameters; flux value, force, pump efficiency and control coefficient.     

Homeostasis of the protonmotive force in phosphorylating mitochondria

The relationship between the respiration rate and the magnitude of the electrochemical proton potential (delta mu H+) in rat liver mitochondria was investigated. (1) Under the active-state conditions, the action of inhibitors of either phosphorylation (oligomycin) or respiration (rotenone, malonate) on the respiration and delta mu H+ was measured. Both inhibitors diminished the respiration, whereas rotenone resulted in a decrease of delta mu H+, and oligomycin produced an increase of this potential. The effect of the inhibitors was much more pronounced on the respiration rate than on delta mu H+; for example, the excess of oligomycin produced a 90% inhibition of the respiration while delta mu H+ was changed only by 9%. (2) Under the resting-state conditions, small concentrations of the uncoupler stimulated the respiration while changing delta mu H+ to a relatively small extent. The uncoupler concentrations which doubled and tripled the respiration rate produced only 5 and 9% decrease of delta mu H+, respectively. (3) The present results enabled us to propose a model describing the interrelationship between respiration and delta mu H+.     

Unique relationships between the rates of oxidation and phosphorylation and the protonmotive force in rat-liver mitochondria

The rate of ATP synthesis (JP) in isolated rat-liver mitochondria was strongly dependent on the magnitude of the protonmotive force (delta mu H+) across the mitochondrial inner membrane. Addition of different concentrations of various uncouplers or malonate to mitochondrial incubations in State 3 led to a depression of delta mu H+ and a concomitant decrease in JP. A unique relationship between JP and delta mu H+ was obtained, which was independent of the way in which delta mu H+ was varied. This unique relationship was observed when K+ (in the presence of valinomycin) was used as a probe for delta psi. Different relationships between JP and delta mu H+ were observed when K+ was used as a probe for delta psi and when K+ was measured after separation of the mitochondria by centrifugation without silicone oil. This led to a serious underestimation of delta psi, specifically when uncouplers were present, and non-unique flow-force relationships were thus obtained. Anomalous relationships between JP and delta mu H+ were also found when TPMP+ was used as a probe for delta psi. However, in uncoupler incubations the presence of TBP- strongly affected the TPMP+ accumulation ratio without any effect on the K+ accumulation or on JP and in the presence of TBP- unique relationships between JP and delta mu H+ were again obtained. This indicates that the accumulation of TPMP+ inside the mitochondria is not a straightforward function of delta psi but also depends on conditions like the presence of TBP- or uncouplers. We conclude that there is a unique relationship between the rate of phosphorylation and the protonmotive force in mitochondria and that under some conditions the behaviour of TPMP+ is anomalous.     

Is there Ca2+(Sr2+)-3-hydroxybutyrate symport in rat-liver mitochondria? A reappraisal

The observation in this laboratory that respiration and Sr2+ import were stimulated by the addition of 3-hydroxybutyrate to suspensions of N-ethylmaleimide-treated mitochondria respiring in state 6, after the addition of Sr2+, in a sucrose medium containing choline as substrate, led to the proposal by Moyle and Mitchell [(1977) FEBS Lett. 84, 135-140] that there is a Ca2+(Sr2+)-3-hydroxybutyrate symporter in rat liver mitochondria. However, experiments described in the present paper support a different interpretation. Under the conditions of the experiments by Moyle and Mitchell, the rate of respiration and the poise of Sr2+ accumulation are mainly limited, not by delta mu H+, but by lack of respiratory substrate. Even though N-ethylmaleimide is a potent inhibitor of 3-hydroxybutyrate dehydrogenase, we have found that, somewhat surprisingly, under the special conditions of these experiments, sufficient 3-hydroxybutyrate dehydrogenase activity remains available to account for the 3-hydroxybutyrate-dependent respiratory stimulation and Sr2+ import.     

THe proton-per-electron stoicheiometry of ''site 1'' of oxidative phosphorylation at high protonmotive force is close to 1.5.

The maximum redox potential difference between the NAD+/NADH couple and the succinate/fumarate couple generated during ATP-energized reduction of NAD+ by succinate in submitochondrial particles was measured, together with the electrochemical potential difference for protons (delta mu approximately H+). The presence of cyanide, the time-independence of the redox potential difference and the irrelevance of the initial redox state of the NAD+/NADH couple ensured that the experimental situation corresponded to a 'static-head condition' with delta mu approximately H+ as the input force and the redox potential difference as the output force, the flow of electrons having reached dynamic equilibrium. Consequently, the observed value of 1.6 for the ratio delta Ge/delta mu approximately H+ is interpreted as indicating that the leads to H+/e- stoicheiometry at 'site 1' is 1.5 and that therefore the mechanism of the proton pump at 'site 1' is not of the group-translocation type (no direct leads to e - leads to H+ coupling).     

The proton motive force lowers the level of ATP required for the in vitro translocation of a secretory protein in Escherichia coli

The role of the electrochemical potential difference of proton (delta mu H+) in protein translocation across the membrane of Escherichia coli was examined in detail using an efficient in vitro assay system (Yamada, H., Tokuda, H., and Mizushima, S. (1989) J. Biol. Chem. 264, 1723-1728). Delta mu H+ reduced the level of ATP necessary for the efficient translocation of OmpF-Lpp, a chimeric model secretory protein. The apparent Km value of the translocation reaction for ATP was lower by 2 orders of magnitude in the presence of delta mu H+ than in its absence. The membrane potential and delta pH, both of which are components of delta mu H+, independently lowered the apparent Km value of the translocation reaction for ATP. An ATP-generating system also lowered the level of ATP required for translocation in the absence of delta mu H+ but not in its presence. It is proposed that ADP formed during protein translocation lowers the affinity of the putative translocation machinery for ATP and that the removal of ADP from the secretory machinery, a possible critical step in the translocation reaction, is stimulated in the presence of either delta mu H+, an ATP-generating system, or a higher concentration of ATP.     

Comparison of the energetics of lactose active transport: artificial versus enzyme-associated energy source

To further consider the thermochemical method as a useful approach for active transport research and to investigate the characteristic of a proton electrochemical potential (delta mu H+) across the membrane, the energetics of lactose active transport across Escherichia coli membrane vesicles coupled with an artificial electron donor (phenazine methosulfate-ascorbate) has been investigated. The results were compared with those obtained with an enzyme-associated electron donor (lactate dehydrogenase-D-lactate). The oxidation of an electron donor provided the energy necessary for the transport process. The observed higher heat of ascorbate oxidation reaction in the presence of a proton ionophore (carbonyl cyanide m-chlorophenylhydrazone) further confirmed the formation of delta mu H+ across the membrane. Part of the oxidation energy was utilized to form delta mu H+. Comparison of the energetics revealed that the magnitudes of delta Hox (the enthalpy of the oxidation reaction) and delta Hm (the enthalpy of the formation of delta mu H+) in the two energy sources were comparable (-46 kcal/mol of ascorbate to -40 kcal/mol of D-lactate for delta Hox and 9.6 kcal/mol of ascorbate to 14 kcal/mol of D-lactate for delta Hm). Comparable and low value (about 1%) was also found in the free energy transfer (defined by delta Gm/delta Gox) from the oxidation reaction to the formation of delta mu H+. These results, in combination with the close values of delta mu H+ observed in the two systems, suggested that the characteristic of the created delta mu H+ was independent of the energy source. Examination of delta Hm might provide the information on the ratio of the number of protons produced, as 1 mol of two different electron donors was oxidized. The oxidation reaction in the presence of membrane vesicles was discussed.     

Beta-oxidation of polyunsaturated fatty acids in peroxisomes. Subcellular distribution of delta 3,delta 2-enoyl-CoA isomerase activity in rat liver

The metabolism of the double bonds at the delta 3 position in fatty acids was studied in rat liver. Infusion of delta 3-trans-dodecenoic acid into isolated perfused liver and subcellular fractionation studies showed the presence of both peroxisomal and mitochondrial delta 3,delta 2-enoyl-CoA isomerase activity (EC 5.3.3.8). These findings together with the previous demonstration of peroxisomal 2,4-dienoyl-CoA reductase (EC 1.3.1.34) [(1981) J. Biol. Chem. 256, 8259-8262] and D-3-OH-acyl-CoA epimerase (EC 5.1.2.3) [(1985) FEBS Lett. 185, 129-134] activities show that peroxisomes possess all the auxiliary enzymes required for the beta-oxidation of unsaturated fatty acids.     

Methanogenesis and ATP synthesis in methanogenic bacteria at low electrochemical proton potentials. An explanation for the apparent uncoupler insensitivity of ATP synthesis

The rate of methane formation from H2 and CO2, the intracellular ATP content and the electrochemical proton potential (delta mu H+) were determined in cell suspensions of Methanobacterium thermoautotrophicum, which were permeabilized for K+ with valinomycin (1.2 mumol/mg protein). In the absence of extracellular K+ the cells formed methane at a rate of 4 mumol min-1 (mg protein)-1, the intracellular ATP content was 20 nmol/mg protein and the delta mu H+ was 200 mV (inside negative). When K+ was added to the suspensions the measured delta mu H+ decreased to the value calculated from the [K+]in/[K+]out ratio. Using this method of delta mu H+ adjustment, it was found that lowering delta mu H+ from 200 mV ([K+]in/[K+]out = 1000) to 100 mV ([K+]in/[K+]out = 40) had no effect on the rate of methane formation and on the intracellular ATP content. At delta mu H+ values below 100 mV ([K+]in/[K+]out less than 40) both the rate of methanogenesis and the ATP content decreased. Methanogenesis completely ceased and the ATP content was 2 nmol/mg when delta mu H+ was adjusted to values lower 50 mV ([K+]in/[K+]out less than 7). The data show that methanogenesis from H2 and CO2 and ATP synthesis in M. thermoautotrophicum are possible at relatively low electrochemical proton potentials. Similar results were obtained with Methanosarcina barkeri. Protonophoric uncouplers like 3,5,3',4'-tetrachlorosalicylanilide (TCS) or 3,5-di-tert-butyl-4-hydroxy-benzylidenemalononitrile (SF 6847) were found not to dissipate delta mu H+ below 100 mV in M. thermoautotrophicum even when used at high concentrations (400 nmol/mg protein). This finding explains the observed uncoupler insensitivity of methanogenesis and ATP synthesis in this organism.     

Activation of the H(+)-ATP synthase in the photosynthetic bacterium Rhodobacter capsulatus.

The regulation of the membrane-bound H(+)-ATPase from the photosynthetic bacterium Rhodobacter capsulatus was investigated. In the presence of uncouplers the rate of ATP hydrolysis was about 40 mM ATP/M bacteriochlorophyll (Bchl)/s. Without uncouplers this rate increased and if, additionally, the chromatophores were illuminated, it was almost doubled. If uncouplers were added shortly after illumination, the rate increased to 300-350 mM ATP/M Bchl/s. Obviously, energization of the membrane leads to the formation of a metastable, active state of the H(+)-ATPase. The maximal rate of ATP hydrolysis can be measured only when first all H(+)-ATPases are activated by delta mu H+ and when the delta mu H+ is abolished in order to release its back pressure on the hydrolysis rate. The half-life time of the metastable state in the absence of delta mu H+ is about 30 s. It is increased by 3 mM Pi to about 80 s and it is decreased by 1 mM ADP to about 15 s. Quantitatively, the fraction of active H(+)-ATPases shows a sigmoidal dependence on pHin (at constant pHout) and the magnitude of delta psi determines the maximal fraction of enzymes which can be activated: delta pH and delta psi are not equivalent for the activation process.     

In vitro analysis of the process of translocation of OmpA across the Escherichia coli cytoplasmic membrane. A translocation intermediate accumulates transiently in the absence of the proton motive force

The proton motive force (delta mu H+) plays an important role, although it is not absolutely essential, in the in vitro translocation of secretory proteins, such as OmpA, across the cytoplasmic membrane of Escherichia coli (Yamada, H., Tokuda, H., and Mizushima, S. (1989) J. Biol. Chem. 264, 1723-1728). The transient accumulation in membrane vesicles of a possible translocation intermediate of OmpA was observed in the absence of delta mu H+. The intermediate was detected on a polyacrylamide gel as a proteinase K-resistant band corresponding to a molecular weight of 26,000. The intermediate did not possess the signal peptide. The appearance of this band was inhibited in the absence of ATP or the presence of adenosine 5'-(beta,gamma-imino)triphosphate (AMP-PNP) and enhanced upon the addition of SecA. Upon the addition of NADH that energizes the membrane, the intermediate was converted to the translocated form of OmpA, even in the presence of AMP-PNP. These results suggest different requirements of ATP and delta mu H+ for the early and late stages of the translocation reaction. The SecA requirement for the early stage of the translocation has also been suggested. In addition to this band, two other bands were observed at higher positions on the gel, when the translocation reaction was performed in the absence of delta mu H+. Although these two bands also represented the mature form of OmpA, which was partly protected from the proteinase K treatment by the membrane vesicles, the accumulation was not transient. These bands did not appear when the translocation reaction was performed in the presence of dithiothreitol. Together with other evidence, the above observations suggest that OmpA, which has an intramolecular disulfide bridge, cannot undergo the translocation unless delta mu H+ is imposed.     

Characterization of a H+-ATPase in rat brain synaptic vesicles. Coupling to L-glutamate transport

Synaptic vesicles contain a H+-ATPase that generates a proton electrochemical gradient (delta mu H+) required for the uptake of neurotransmitters into the organelles. In this study, the synaptic vesicle H+-ATPase was examined for structural and functional similarities with other identified ATPases that generate a delta mu H+ across membranes. The synaptic vesicle H+-ATPase displayed immunological similarity with the 115-, 72-, and 39-kDa subunits of a vacuolar-type H+-ATPase purified from chromaffin granules. Functionally, the ATP-dependent H+ pumping across synaptic vesicles and ATP hydrolysis were sensitive to the sulfhydryl-modifying reagents, N-ethylmaleimide and 4-chloro-7-nitrobenz-2-oxa-1,3-diazole, at concentrations known to affect vacuolar-type H+-ATPases. In addition, as with vacuolar-type H+-ATPases, the presence of NO3-, SO4(2-), or F- inhibited the generation of a delta mu H+, but addition of vanadate or oligomycin had no effect. The delta mu H+ is a function of the pH gradient (delta pH) and membrane potential (delta psi sv) across the synaptic vesicle. Acidification (delta pH) of the synaptic vesicle interior was enhanced in the presence of permeant anions, such as Cl-, or the K+ ionophore, valinomycin. In the absence of permeant anions, the H+-ATPase generated a delta psi sv that effected the transport of L-glutamate into the synaptic vesicles. Dissipation of delta psi sv by incubation with increased external Cl- or nigericin resulted in the abolition of glutamate uptake, despite the continued maintenance of a delta mu H+ across the synaptic vesicle as a substantial delta pH. The results suggest that the synaptic vesicle H+-ATPase is of a vacuolar type and energizes the uptake of anionic glutamate by virtue of the delta psi sv component of the delta mu H+ it generates.     

Redox-dependent change of nucleotide affinity to the active site of the mammalian complex I

A very potent and specific inhibitor of mitochondrial NADH:ubiquinone oxidoreductase (complex I), a derivative of NADH (NADH-OH) has recently been discovered (Kotlyar, A. B., Karliner, J. S., and Cecchini, G. (2005) FEBS Lett. 579, 4861-4866). Here we present a quantitative analysis of the interaction of NADH-OH and other nucleotides with oxidized and reduced complex I in tightly coupled submitochondrial particles. Both the rate of the NADH-OH binding and its affinity to complex I are strongly decreased in the presence of succinate. The effect of succinate is completely reversed by rotenone, antimycin A, and uncoupler. The relative affinity of ADP-ribose, a competitive inhibitor of NADH oxidation, is also shown to be significantly affected by enzyme reduction (KD of 30 and 500 microM for oxidized and the succinate-reduced enzyme, respectively). Binding of NADH-OH is shown to abolish the succinate-supported superoxide generation by complex I. Gradual inhibition of the rotenone-sensitive uncoupled NADH oxidase and the reverse electron transfer activities by NADH-OH yield the same final titration point (approximately 0.1 nmol/mg of protein). The titration of NADH oxidase appears as a straight line, whereas the titration of the reverse reaction appears as a convex curve. Possible models to explain the different titration patterns for the forward and reverse reactions are briefly discussed.     

DCCD-sensitive, Na+-dependent H+-influx process coupled to membrane potential formation in membrane vesicles of Halobacterium halobium

The effects of N,N'-dicyclohexylcarbodiimide (DCCD) on light-induced H+-transport and transmembrane electric potential (delta phi) formation were studied in the membrane vesicles of Halobacterium halobium R1M1. In accordance with our previous finding of the existence of two DCCD-binding components in vesicle membrane using 14C-DCCD (Konishi & Murakami FEBS Lett. 169, 283-286 (1984)), DCCD inhibited the H+-influx process biphasically; that is, the H+-influx process which is electrically silent was initially inhibited at concentrations below 30 nmol of DCCD/mg vesicle protein, while another H+-influx process which is coupled to delta phi formation was secondarily inhibited above this concentration of DCCD. The latter H+-influx process was highly dependent on the Na+ concentration. The extents of Na+-dependent recovery of delta phi formation and H+-influx were quantitatively correlated. From these results, it was concluded that the second DCCD-sensitive H+-influx process which is coupled to delta phi formation is due to the hypothetical Na+/H+-antiporter postulated by Lanyi and MacDonald (Biochemistry 15, 4608-4614 (1976)). It was also found that Li+ can be substituted for Na+ in this system, as is the case with Na+/H+-antiporters found in other organisms.     

Modulation of the mitochondrial cyclosporin A-sensitive permeability transition pore by the proton electrochemical gradient. Evidence that the pore can be opened by membrane depolarization.

This paper reports an investigation on the relationship between the proton electrochemical gradient (delta mu H+) and the cyclosporin A-sensitive permeability transition pore (PTP) in rat liver mitochondria. Using the SH group cross-linker phenylarsine oxide as the inducer, we show that both matrix pH and the membrane potential can modulate the process of PTP induction independently of Ca2+. We find that membrane depolarization induces the PTP per se when pHi is above 7.0, while at acidic matrix pH values PTP induction is effectively prevented. Since Ca2+ uptake leads to major modifications of the delta mu H+ (i.e. matrix alkalinization and membrane depolarization), we have explored the possibility that the Ca(2+)-induced changes of the delta mu H+ may contribute to PTP induction by Ca2+. Our data in mitochondria treated with Ca2+ plus N-ethylmaleimide and Ca2+ plus phosphate show that membrane depolarization is a powerful inducer of the PTP. Taken together, our observations indicate that the PTP can be controlled directly by the delta mu H+ both in the absence and presence of Ca2+, and suggest that a collapse of the membrane potential may be the cause rather than the consequence of PTP induction under many experimental conditions. Thus, many inducers may converge on dissipation of the membrane potential component of the delta mu H+ by a variety of mechanisms.