Respiration-dependent uncoupler-stimulated ATPase activity in castor bean endosperm mitochondria and submitochondrial particles

1. The uncoupler-stimulated ATPase activity of castor bean endosperm mitochondria and submitochondrial particles has been studied. The rate of ATP hydrolysis catalyzed by intact mitochondria was slow and little enhanced by addition of uncouplers at the concentration required for uncoupling the oxidative phosphorylation. ATPase activity was stimulated at higher concentrations of uncouplers.

2. 1-Anilinonaphthalene 8-sulfonate fluorescence was decreased when the mitochondria were oxidizing succinate. Carbonylcyanide-p-trifluoromethoxyphenylhydrazone and antimycin reversed the succinate-induced fluorescence diminution. ATP did not induce the fluorescence response.

3. The addition of succinate, NADH or ascorbate/N,N,N′,N′-tetramethyl-p-phenylenediamine as electron donor induced high ATPase activity in the presence of low concentrations of uncouplers. Stimulating effect of uncouplers was completely abolished by further addition of antimycin.

4. Submitochondrial particles were prepared by sonication. The particles catalyzed a rapid hydrolysis of ATP and carbonylcyanide-p-trifluoromethoxyphenylhydrazone at 10^-8 M did not stimulate the ATPase activity. Addition of succinate induced uncoupler-stimulated ATPase activity. The effect of succinate was completely abolished by further addition of antimycin.

5. The treatment of submitochondrial particles by trypsin or high pH also induced uncoupler-stimulated ATPase activity.

6. The above results were interpreted to indicate that ATPase inhibitor regulated the back-flow reaction of mitochondrial oxidative phosphorylation.     

Mutual regulation between mitochondrial ATPase and respiratory chain activities

Compounds made from the reaction of fluorescamine with simple primary amines and with mycosamine-containing macrolide antibiotics (e.g., amphotericin B) are used to investigate possible interactions between ATPase and respiration enzymes in rat liver mitochondria. The following observations have been made. (1) The acyclic form of the benzyl amine-fluorescamine compound stimulates the ATPase-linked inorganic phosphate formation, and this stimulation is not affected by rotenone, antimycin A, and potassium cyanide. In contrast, the respiratory inhibitors are able to prevent the stimulation of ATPase activity that is caused by conventional uncouplers e.g., 2,4-dinitrophenol. (2) The acyclic form of the amphotericin B-fluorescamine compound has no effect on ATPase-linked inorganic phosphate formation rate. However, in the presence of the antibiotic-fluorescamine compounds, the respiratory inhibitors are no longer able to prevent the uncoupler-stimulated ATPase activity. (3) The amine-fluorescamine modifiers have no effect on rotenone-sensitive NADH-cytochrome c reductase, on succinate-cytochrome c reductase, and on cytochrome oxidase in submitochondrial particles. (4) The amine-fluorescamine modifiers decrease the rate of the ATP-driven NAD⁺ reduction by succinate in submitochondrial particles. (5) The amine-fluorescamine modifiers inhibit the stimulation of respiration that is caused by conventional uncouplers, although the modifiers have no effect on the kinetics of the proton influx induced by uncouplers. The data are consistent with the hypothesis that the ATPase-linked and respiration-linked proton pumps may interact directly with each other, and this step establishes the mutual regulation between ATPase and respiratory activities.     

Energy-linked electron transfer reactions in Rhodopseudomonas viridis

The particulate fraction of Rhodopseudomonas viridis when supplied with succinate catalyses the reduction of NAD⁺ by light; this reaction is inhibited by uncouplers of oxidative phosphorylation but not by oligomycin. Formation of NADH takes place in the dark when ATP or PP_i is supplied. Both light and dark reactions are inhibited by valinomycin and nigericin, when added together, but not by either separately. NADH formation in R. viridis appears to take place by an energy-dependent reversal of electron flow and energy may be conserved in the form of a membrane potential. The addition of ATP caused the oxidation of both C553 and C558 in chromatophores; carbonylcyanide p-trifluoromethoxyphenylhydrazone and oligomycin abolished this oxidation.

The NAD⁺ and NADH concentrations at equilibrium in the light-dependent reaction were determined and the oxidation-reduction potential of this couple calculated. From this value it was calculated that under these experimental conditions the energy requirement to form NADH from the succinate/fumarate couple at E_h = o V was 9.4 kcal.

Particles of R. viridis contained an active transhydrogenase, driven by either light or ATP, that was sensitive to uncouplers of oxidative phosphorylation; the light-driven reaction was insensitive to oligomycin and was inhibited by antimycin A and 2-heptyl-4-hydroxyquinone-N-oxide.

R. viridis did not grow aerobically but particles contained NADH oxidase activity that was cyanide sensitive. There was no spectroscopic evidence for cytochromes of the b-type in reduced-minus-oxidised spectra of particles or in pyridine haemochrome spectra of whole cells.     

ATP synthesis and hydrolysis in submitochondrial particles subjected to an acid—base transition: Effects of the ATPase inhibitor protein

ATP hydrolysis or succinate oxidation by inhibitor-rich submitochondrial particles leads to a 3-fold increase in ATPase activity, with concomitant loss of about 30% of bound inhibitor protein. An acid—base transition causes similar, but smaller, effects (a 30% ATPase increase, and a loss of 8% of the inhibitor). Omitting the electrical component of the gradient completely abolished these effects. The inhibitor protein inhibits ADP phosphorylation induced by an acid—base transition but not by NADH oxidation. This is suggested to reflect the slow movement of the inhibitor protein and the brief period of acid—base jump phosphorylation.     

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.     

Oxidative phosphorylation in Micrococcus denitrificans. I. Preparation and properties of phosphorylating membrane fragments

A procedure was described to prepare stable membrane fragments from aerobically grown cells of Micrococcus denitrificans. This preparation contained flavins, cytochromes b, c, a and o, and catalyzed the synthesis of ATP coupled to the oxidation of NADH and succinate. The P:O ratios were about 1.0 for NADH and 0.4 for succinate oxidation. The electron-transfer pathways responsible for these oxidations were similar to, though not identical with, those of mammalian mitochondria in their construction and sensitivity to inhibitors. Oxidative phosphorylation by the membrane fragments was uncoupled by the usual uncouplers and energy-transfer inhibitors, though 2,4-dinitrophenol was much less effective and higher concentrations of oligomycin and tributyltin chloride were required for complete inhibition as compared with the mitochondrial system. Oleate also caused uncoupling, which was relieved by serum albumin. Treatment with high concentrations of LiCl yielded an essentially uncoupled preparation, but this treatment as well as many other procedures failed to yield soluble coupling factors. Unlike the mitochondrial ATPase activity, ATP hydrolysis by the membrane fragments was inhibited to about 50% by uncouplers and energy-transfer inhibitors. It seems that the bacterial preparation possessed two types of ATPase, one of which was sensitive to these reagents as well as to LiCl treatment and probably to high concentrations of ADP. The advantage of this preparation for the study of the mechanism of oxidative phosphorylation is discussed.     

Energy-dependent dissociation of ATP from high affinity catalytic sites of beef heart mitochondrial adenosine triphosphatase

Incubation of [gamma-32P]ATP with a molar excess of the membrane-bound form of mitochondrial ATPase (F1) results in binding of the bulk of the radioactive nucleotide in high affinity catalytic sites (Ka = 10(12) M-1). Subsequent initiation of respiration by addition of succinate or NADH is accompanied by a profound decrease in the affinity for ATP. About one-third of the bound radioactive ATP appears to dissociate, that is, the [gamma-32P]ATP becomes accessible to hexokinase. The NADH-stimulated dissociation of [gamma-32P]ATP is energy-dependent since the stimulation is inhibited by uncouplers of oxidative phosphorylation and is prevented by respiratory chain inhibitors. The rate of the energy-dependent dissociation of ATP that occurs in the presence of NADH, ADP, and Pi is commensurate with the measured initial rate of ATP synthesis in NADH-supported oxidative phosphorylation catalyzed by the same submitochondrial particles. Thus, the rate of dissociation of ATP from the high affinity catalytic site of submitochondrial particles meets the criterion of kinetic competency under the conditions of oxidative phosphorylation. These experiments provide evidence in support of the argument that energy conserved during the oxidation of substrates by the respiratory chain can be utilized to reduce the very tight binding of product ATP in high affinity catalytic sites and to promote dissociation of the nucleotide.     

Effect of the protonmotive force on ATP-linked processes and mobilization of the bound natural ATPase inhibitor in beef heart submitochondrial particles

In an attempt to determine whether the natural ATPase inhibitor (IF₁) plays a role in oxidative phosphorylation, the time course of ATP synthesis and ATP hydrolysis in inside-out submitochondrial particles from beef heart mitochondria either possessing IF₁ (Mg-ATP particles) or devoid of IF₁ (AS particles) was investigated and compared to movements of IF₁, as assessed by an isotopic assay. The responses of the above reactions to preincubation of the particles in aerobiosis with NADH or succinate were as follows: (1) The few seconds lag that preceded the steady-rate phase of ATP synthesis was shortened and even abolished both in Mg-ATP particles and AS particles. The rate of ATP synthesis in the steady state was independent of the length of the lag. (2) ATPase was slowly activated, maximal activation being obtained after a 50-min preincubation; there was no direct link between the development of the protonmotive force (maximal within 1 sec) and ATPase activation. (3) Bound IF₁ was slowly released; the release of bound IF₁ as a function of the preincubation period was parallel to the enhancement of ATPase activity; the maximal amount of IF₁ released was a small fraction of the total IF₁ bound to the particles (less than 20%). (4) The double reciprocal plots of the rates of ATP and ITP hydrolysis vs. substrate concentrations that were curvilinear in the absence of preincubation with a respiratory substrate became linear after aerobic preincubation with the substrate. The data conclusively show that only ATPase activity in submitochondrial particles is correlated with the release of IF₁, and that the total extent of IF₁ release induced by respiration is limited. On the other hand, the kinetics of ATPase in control and activated particles are consistent with the existence of two conformations of the membrane-bound F₁-ATPase, directed to ATP synthesis or ATP hydrolysis and distinguishable by their affinity for IF₁.     

Oxidative phosphorylation and ATPase activities of human tumor mitochondria

Studies were carried out with intact mitochondria isolated from human astrocytoma, oat cell carcinoma and melanoma which were propagated in athymic mice. These human tumor mitochondria were capable of coupled oxidative phosphorylation. They also showed significant uncoupler-stimulated ATPase if defatted bovine serum albumin was included in the assay media. However, the uncoupler response curves were different and the magnitude of the ATPase activity was lower than could be obtained with mitochondria of a normal tissue, such as liver. Some of these characteristics were also exhibited by mitochondria from several animal hepatomas and Ehrlich ascites tumor. In the three tumors studied, mitochondria from oat cell carcinoma were more labile, whereas higher respiratory control ratios and greater stimulation of ATPase by uncouplers were obtained with melanoma mitochondria.The mitochondrial ATPase was not the major cellular ATPase in any of the three tumors. This was indicated by a low inhibition of the ATPase activity of tumor cell homogenates by oligomycin. A very large fraction of the cellular ATPase activities was recovered in the microsomal fractions.     

Effect of uncouplers and inhibitors of oxidative phosphorylation on the reduced and oxidized forms of mitochondiral ATPase.

A series of uncouplers and inhibitors of oxidative phosphorylation have been studied with regard to their effect on the hydrolytic activity of the reduced and oxidized forms of isolated or membrane-bound mitochondrial ATPase. Uncouplers (2,4-dinitrophenol, dicoumarol), which are also activators of the hydrolytic activity of ATPase, were more potent activators on the oxidized form of the enzyme. Inhibitors of oxidative phosphorylation (oligomycin, azide and amytal) had a more potent inhibitory effect on the hydrolytic activity of ATPase in its reduced form. Purified F1-ATPase, oligomycin insensitive in the oxidized form of the enzyme, became sensitive to oligomycin in the reduced form. An interpretation of the results suggests the presence of a mechanism that unifies the action of these different compounds on the synthesis and hydrolysis of ATP catalyzed by mitochondrial ATPase.     

Inhibition of oxidative phosphorylation by Ca2+ or Sr2+: a competition with Mg2+ for the formation of adenine nucleotide complexes

Intramitochondrial Sr2+, similar to Ca2+, inhibits oxidative phosphorylation in intact rat-liver mitochondria. Both Ca2+ and Sr2+ also inhibit the hydrolytic activity of the ATPase in submitochondrial particles. Half-maximal inhibition of ATPase activity was attained at a concentration of 2.5 mM Ca2+ or 5.0 mM Sr2+ when the concentration of Mg2+ in the medium was 1.0 mM. The inhibition of ATPase activity by both cations was strongly decreased by increasing the Mg2+ concentration in the reaction medium. In addition, kinetical data and the determination of the concentration of MgATP, the substrate of the ATPase, in the presence of different concentrations of Ca2+ or Sr2+ strongly indicate that these cations inhibit ATP hydrolysis by competing with Mg2+ for the formation of MgATP. On the basis of a good agreement between these results with submitochondrial particles and the results of titrations of oxidative phosphorylation with carboxyatractyloside or oligomycin in mitochondria loaded with Sr2+ it can be concluded that intramitochondrial Ca2+ or Sr2+ inhibits oxidative phosphorylation in intact mitochondria by decreasing the availability of adenine nucleotides to both the ADP/ATP carrier and the ATP synthase.     

Assays of the metabolic viability of single giant mitochondria. Experiments with intact and impaled mitochondria

Single giant mitochondria isolated from mice fed cuprizone were assayed for their metabolic viability. Two tests were devised. One test optically detected the accumulation of calcium phosphate within the mitochondria under massive loading conditions (including the presence of succinate and ATP). The accumulation corresponds to a test of energy coupling from either electron transport or the hydrolysis of ATP since it is blocked by either antimycin A or oligomycin. The other assay tested for the production of ATP from ADP and Pi, using myofibrils. Myofibrils prepared from glycerinated rabbit psoas muscle contract only in the presence of ATP and not in the presence of ADP. Myofibrillar contraction is unaffected by the presence of antimycin A or oligomycin. However, myofibrils in the presence of mitochondria that are phosphorylating ADP to ATP do contract. This contraction is blocked by antimycin A and/or oligomycin. Hence, the ATP which causes myofibrillar contraction is produced by oxidative phosphorylation. At low mitochondrial concentration, only the myofibrils in close proximity with mitochondria contract in the presence of ADP. Therefore the assay can be used to test the viability of individual mitochondria. Individual giant mitochondria were found to be viable, using both of these assays. Comparable results were obtained in mitochondria impaled with microelectrodes. The potentials and resistances were unaffected by concomitant calcium phosphate accumulation or oxidative phosphorylation.     

The possible role of palmitoyl-CoA in the regulation of the adenine nucleotides transport in mitochondria under different metabolic states. I. Comparison of liver mitochondria from starved and fed rats.

It has been shown that KM values for ADP when rat liver mitochondria oxidized succinate were strictly dependent on the values of the respiratory control ratios. The Ki values for palmitoyl-CoA inhibition of the ADP-stimulated succinate oxidation and the inhibition of the uncoupler-stimulated ATPase activity were equal to 0.5 muM. Mitochondria from livers of starved rats showed 30% inhibition of the state 3 respiratory rate (compared to the uncoupled respiratory rate) which was abolished by addition of carnitine. It was supposed that this inhibition was due to the influence of acyl-CoAs bound to the inner mitochondrial membrane on the adeninenucleotide translocase. Mitochondria from livers of fed rats showed a strong inhibition of succinate oxidation both in state 4 and state 3, although the rate of uncoupled respiration was normal. It was assumed that in this case the changes in mitochondrial behaviour was caused by the decrease in the concentration of ADP and ATP in the matrix space of mitochondria.     

Unique action of a modified weakly acidic uncoupler without an acidic group, methylated SF 6847, as an inhibitor of oxidative phosphorylation with no uncoupling activity: possible identity of uncoupler binding protein

The potent weakly acidic uncoupler SF 6847 was modified by methylation of its phenolic OH group, and the effect of the resulting derivative, with no acid-dissociable group, on oxidative phosphorylation in rat liver mitochondria was examined. The methylated SF 6847 did not induce uncoupling at up to 40 microM, while SF 6847 uncoupled oxidative phosphorylation completely at about 20 nM, indicating that the acid-dissociable group is essential for uncoupling. The O-methylated SF 6847 at 20 microM did, however, inhibit state 3 respiration of mitochondria, although it did not inhibit electron-flow through the respiratory chain, ATPase activated by weakly acidic uncouplers or Pi-ATP exchange. At the same concentration, it also inhibited ATP synthesis in submitochondrial particles. These features are different from those of known inhibitors of oxidative phosphorylation. Thus, O-methylated SF 6847 is a unique inhibitor of oxidative phosphorylation. The possible identity of the uncoupler binding protein is discussed on the basis of these results.     

Inhibition of oxidative phosphorylation by Ca or Sr: A competition with Mg for the formation of adenine nucleotide complexes

Intramitochondrial Sr²⁺, similar to Ca²⁺, inhibits oxidative phosphorylation in intact rat-liver mitochondria. Both Ca²⁺ and Sr²⁺ also inhibit the hydrolytic activity of the ATPase in submitochondrial particles. Half-maximal inhibition of ATPase activity was attained at a concentration of 2.5 mM Ca²⁺ or 5.0 mM Sr²⁺ when the concentration of Mg²⁺ in the medium was 1.0 mM. The inhibition of ATPase activity by both cations was strongly decreased by increasing the Mg²⁺ concentration in the reaction medium. In addition, kinetical data and the determination of the concentration of MgATP, the substrate of the ATPase, in the presence of different concentrations of Ca²⁺ or Sr²⁺ strongly indicate that these cations inhibit ATP hydrolysis by competing with Mg²⁺ for the formation of MgATP. On the basis of a good agreement between these results with submitochondrial particles and the results of titrations of oxidative phosphorylation with carboxyatractyloside or oligomycin in mitochondria loaded with Sr²⁺ it can be concluded that intramitochondrial Ca²⁺ or Sr²⁺ inhibits oxidative phosphorylation in intact mitochondria by decreasing the availability of adenine nucleotides to both the ADP/ATP carrier and the ATP synthase.     

Effect of heliomycin on the respiration and oxidative phosphorylation of the liver mitochondria of the rat

The effect of heliomycin and known uncouplers of oxidative phosphorylation on respiration and oxidative phosphorylation was studied comparatively. Heliomycin, as well as 2,4-dinitrophenol, valinomycin and gramicidin S inhibited the mitochondrial synthesis of ATP. This process was inhibited completely by heliomycin at a concentration of 1.5 x 10(-5) M. The synthesis of inorganic pyrophosphate, the other macroergic compound, was also inhibited by heliomycin, ATPase and pyrophosphatase of uncoupled mitochondria being not inhibited by the antibiotic. Like 2,4-dinitrophenol, heliomycin stimulated the synthesis of ATPase and respiration in intact mitochondria. Probably, heliomycin inhibited the synthesis of ATP and pyrophosphate by uncoupling the processes of respiration and oxidative phosphorylation. It was shown earlier that heliomycin, a specific inhibitor of bacterial RNA synthesis, also affected energy metabolism of bacterial cells by inhibiting the synthesis of ATP and active transport.     

Effects of chronic ethanol treatment of mitochondrial functions damage to coupling site I

Chronic ethanol feeding to rats produces changes in hepatic mitochondria which persist in the absence of ethanol metabolism. The integrity of isolated mitochondria is well preserved, as evidenced by unchanged activities of latent, Mg²⁺- and dinitrophenol-stimulated ATPase activity, and unaltered permeability to NADH. With succinate or ascorbate as substrates, oxygen uptake by mitochondria from ethanol-fed rats was decreased compared to pair-fed controls. The decrease was comparable under state 4 or state 3 conditions, or in the presence of an uncoupler. However, with the NAD⁺-dependent substrates, ADP-stimulated oxygen consumption (state 3) was decreased to a greater extent than state 4 or uncoupler-stimulated oxygen consumption in mitochondria from ethanol-fed rats. This suggests that the decrease in energy-dependent oxygen consumption at site I may be superimposed upon damage to the respiratory chain. Using NAD⁺-dependent substrates (glutamate, α-ketoglutarate or β-hydroxybutyrate) the respiratory control ratio and the $\text{P}\text{O}$ ratio of oxidative phosphorylation were significantly decreased in mitochondria isolated from the livers of rats fed ethanol. By contrast, when succinate or ascorbate served as the electron donor these functions were unchanged. The rate of phosphorylation is decreased 70% with the NAD⁺-dependent substrates because of a decreased flux of electrons, as well as a lower efficiency of oxidative phosphorylation. With succinate and ascorbate as substrates, the rate of phosphorylation is decreased 20–30%, owing to a decreased flux of electrons. These data suggest the possibility that, in addition to effects on the respiratory chain, energy-coupling site I may be damaged by ethanol feeding. Energy-dependent Ca²⁺ uptake, supported by either substrate oxidation or ATP hydrolysis, was inhibited by chronic ethanol feeding.Concentrations of acetaldehyde (1–3 mm) which inhibited phosphorylation associated with the oxidation of NAD⁺-dependent substrates had no effect on that of succinate or ascorbate. Many of the effects of chronic ethanol feeding on mitochondrial functions are similar to those produced by acetaldehyde in vitro.     

Action of different nucleotides on the last step of aldosterone synthesis

The effect of various nucleotides on the last step of aldosterone biosynthesis, the so-called "18 oxidation" (transformation of 18-hydroxycorticosterone to aldosterone), was studied by incubation of tritiated 18-hydroxycorticosterone with untreated duck adrenal mitochondria in vitro. The study was carried out in the absence or in the presence of antimycin A which blocks the respiratory chain. Results show that, when oxidative phosphorylation chain functions normally, GTP and CTP had no effect, UTP stimulated this reaction but ADP and ATP inhibited the transformation of 18-hydroxycorticosterone into aldosterone to the same extent. For this reason ATP is included in all controls for experiments studying the effect of ATP when "18 oxidation" is inhibited by antimycin A. When oxidative phosphorylation chain is inhibited by antimycin A, ATP is able to reverse the inhibition of "18 oxidation" induced by antimycin A, in the presence of succinate. Under these conditions UTP is not able to reverse the inhibition induced by antimycin A; GTP and CTP had no effect. Effects of ATP and UTP on the last step of aldosterone biosynthesis are related to different mechanisms. ATP clearly acts as an energy source for "18 oxidation" in the presence of succinate. The role of UTP must still be determined.     

Inhibition of mitochondrial bioenergetics: the effects on structure of mitochondria in the cell and on apoptosis

The effects of specific inhibitors of respiratory chain, F(o)F(1)ATP synthase and uncouplers of oxidative phosphorylation on survival of carcinoma HeLa cells and on the structure of mitochondria in the cells were studied. The inhibitors of respiration (piericidin, antimycin, myxothiazol), the F(1)-component of ATP synthase (aurovertin) and uncouplers (DNP, FCCP) did not affect viability of HeLa cells, apoptosis induced by TNF or staurosporin and the anti-apoptotic action of Bcl-2. Apoptosis was induced by combined action of respiratory inhibitors and uncouplers indicating possible pro-apoptotic action of reactive oxygen species (ROS) generated by mitochondria. Short-term incubation of HeLa cells with the mitochondrial inhibitors and 2-deoxyglucose followed by 24-48 h recovery resulted in massive apoptosis. Apoptosis correlated to transient (3-4 h) and limited (60-70%) depletion of ATP. More prolonged or more complete transient ATP depletion induced pronounced necrosis. The inhibitors of respiration and uncouplers caused fragmentation of tubular mitochondria and formation of small round bodies followed by swelling. These transitions were not accompanied with release of cytochrome c into the cytosol and were fully reversible. The combined effect of respiratory inhibitors and uncouplers developed more rapidly indicating possible involvement of ROS generated by mitochondria. More prolonged (48-72 h) incubation with this combination of inhibitors caused clustering and degradation of mitochondria.     

Effect of Ca ions on the transmembrane electric potential, synthesis and hydrolysis of ATP in brain mitochondria

The transmembrane potential (delta psi) of rabbit brain mitochondria was measured with the fluorescent dye dis--C2--5. During oxidative phosphorylation a fall in delta psi in the order of 20% was observed. In the presence of inhibitors of ATP synthesis, there was a good correlation between the fall in delta psi and the ADP-stimulated increase in respiration rate. The influence of endogenous calcium on the energetic metabolism of mitochondria was studied by measuring the changes of delta psi. An amount of 12 nmol Ca2+/mg protein cause half-inhibition of the ATP synthesis rate; 50 nmol/mg completely inhibits oxidative phosphorylation. The effect of the Ca2+ load on the ATPase activity of intact mitochondria was studied. It was found that endogenous calcium inhibits in a similar degree synthesis and hydrolysis of ATP. It was shown that both Ca ATP and Mg ATP can serve as a substrate for the mitochondrial ATPase.