Uncoupling of oxidative phosphorylation

Vesicular preparation of a highly coupled ATPase-ATP synthase complex from pig heart mitochondria

1. A method is described to prepare an ATPase-ATP synthase complex from pig heart mitochondria exhibiting a very high ATP-32Pi exchange activity (1.6 mumol/min per mag protein in optimal conditions). 2. The preparation is virtually devoid of nucleoside diphosphokinase and adenylate kinase activities. 3. Freeze-fracture studies show that the ATPase-ATP synthase complex is integrated in lipid vesicles of 400-600 A in diameter. 4. It contains the endogenous natural proteic inhibitor which seems to behave as a coupling factor. 5. The rate of ATP hydrolysis catalyzed by the ATPase-ATP synthase complex is competitively inhibited by ADP, while the presence of ADP increases the initial rate of 32Pi incorporation into ATP. 6. The 32Pi incorporation into ATP can occur at a rate almost equal to that of nucleoside triphosphate (NTP) hydrolysis provided that the rate of NTP hydrolysis is kept low and that the ADP concentration is high enough. In these conditions, a very high coupling between NTP hydrolysis and ATP synthesis can be demonstrated.

Picrate as an inhibitor of photosystem II in photosynthetic electron transport

2,4,6-Trinitrophenol (picric acid) is an effective inhibitor of photosynthetic electron transport (pI50 value 6.82). By means of artificial donor and acceptor systems its site of inhibition was located at the reducing side of photosystem II. In addition, picric acid also uncouples photophosphorylation but a much higher concentration is required than for inhibition of electron transport (pI50 value 4.40). By use of radioactivity labelled [U-14C]picric acid (spec. act. 2.1 Ci/mol) a binding constant of 0.25 microM and a number of binding sites of 2.84 nmol/mg chlorophyll was measured. This corresponds to one molecule of picric acid per 394 molecules of chlorophyll or one molecule per electron transport chain. Picric acid is competitively displaced from the thylakoid membrane by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (and functionally related compounds) and phenolic inhibitors as well. In this property picric acid, despite its phenol character, is distinct from other phenolic inhibitors.

Effects of anionic xenobiotics on rat kidney. I--Tissue and mitochondrial respiration

The polar 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) metabolite, 2,2-bis(p-chlorophenyl)acetic acid (DDA), and the phenoxyacetic acid herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), were previously shown to be accumulated to high levels in liver and kidney via the organic acid transport system, raising the possibility of organ-specific toxicity secondary to transport. In these studies, accumulation of DDA was shown to depress oxygen consumption by renal cortical slices at high doses (0.1 and 1mM). Isolated renal and hepatic mitochondria were uncoupled by low doses of DDA (5-10 nmoles/mg mitochondrial protein. Maximal uncoupling was seen at 50-70 nmoles/mg. 2,4-D and 2,4,5-T also produced uncoupling, but at doses of 70 nmoles/mg or higher. All agents were more effective with alpha-ketoglutarate or pyruvate-malate), all three agents also depressed State 3 (ADP-stimulated) respiration. Again, DDA was more effective than 2,4-D or 2,4,5-T. These results suggest that accumulation of these or other anionic xenobiotics may lead to toxicity in those tissues possessing the organic anion transport system.

The hypothesis of localized chemiosmosis is unsatisfactory

The hypothesis of 'localized' chemiosmosis have been put to an experimental test in the system of oxidative phosphorylation by rat liver mitochondria. We find that the variation of the ratio of phosphate potential to delta muH with delta muH does not depend on how delta muH is varied. This is in conflict with hypotheses of localized chemiosmosis. Of all coupling hypotheses, only the parallel-coupling hypothesis can explain the observations, unless variation of the H+/ATP stoichiometry of the ATPase proton pump is accepted. In the latter case. 'macroscopic' chemiosmosis can explain the observations equally well as hypotheses of localized chemiosmosis. It is concluded that either variation of H+/ATP stoichiometries must be accepted, or that the parallel-coupling hypothesis should be reformulated so that it becomes open to experimental tests.

The mechanism of uncoupling by picrate in Escherichia coli K-12 membrane systems

The mechanism of action of the uncoupler picrate on intact cells and everted membrane vesicles of Escherichia coli K-12 was investigated. Like in mitochondria [Hanstein, W. G. and Hatefi, Y. (1974) Proc. Natl Acad. Sci. USA, 71, 288-292], it was observed that picrate uncoupled energy-linked functions only in everted, but not in intact membrane systems. In the vesicles picrate also decreased the magnitude of the transmembrane proton-motive force at concentrations similar to those at which it caused uncoupling. Experiments with 14C-labelled picrate showed that this compound bound both to deenergized intact cells and everted vesicles. However, upon energization of the membrane, picrate was extruded from the intact cell and taken up to a larger extent by the vesicles. These energy-dependent changes in picrate uptake correlated with the magnitude of the transmembrane electrical potential, delta psi. It is therefore proposed that picrate is a permeant uncoupler, that delta psi is the driving force for picrate movement across biological membranes, and that the uncoupling activity of picrate in everted membrane systems is due to its protonophoric action.

Studies of the kinetics of the isolated mitochondrial ATPase using dinitrophenol as a probe

1. Dinitrophenol and maleate anions increase VATP on the 'washed', isolated, mitochondrial ATPase. Hydrolyses of iso-GTP and 2'-deoxy ATP are also stimulated, while hydrolyses of other nucleoside triphosphates (ITP, GTP etc.) are not. 2. Preincubation with ATP, iso-GTP or 2'-deoxy ATP results in a metastable enzyme form with a raised V and a reduced Km. Dinitrophenol stimulates both ATP and ITP hydrolyses by this form. 3. The Arrhenius plot of ATP (but not ITP) hydrolysis by the isolated ATPase shows a break at about 18 degrees C, apparently because the rate limiting step of hydrolysis changes as the temperature rises. 4. Adenylyl beta, gamma-imidodiphosphate (AdoPP[NH]P) inhibits ITP hydrolysis in a pseudofirst order reaction. Its binding is competitive with ITP. If the enzyme is preincubated with ATP, the rate of AdoPP[NH]P binding increases. It is concluded that AdoPP[NH]P inhibits by binding to the hydrolytic site of the enzyme. 5. We conclude that ATP hydrolysis is limited by diphosphate release and ITP hydrolysis by bond splitting. Energy release during ATP hydrolysis is maximal at the ATP binding step, and during ITP hydrolysis at bond splitting.

A specific uncoupler-binding protein in Tetrahymena pyriformis and Paracoccus denitrificans

The uncoupler of mitochondrial oxidative phosphorylation, 2-nitro-4-azido-carbonylcyanide phenylhydrazone (N3CCP) which is capable of photoaffinity labeling has been used to examine the effect of uncouplers on the energy conserving membrane of Paracoccus denitrificans and Tetrahymena pyriformis. The N3CCP uncouples respiration in P. denitrificans and T. pyriformis cells with U1/2 values of 1.05 microM and 0.24 microM, respectively. Binding studies show the presence of 0.65 +/- 0.05 high affinity sites per cytochrome alpha with Kd of 0.5 +/- 0.1 microM in P. denitrificans membranes and 1.4 +/- 0.2 sites per cytochrome alpha 2 with a Kd of 0.4 +/- 0.1 microM in T. pyriformis membranes. Irradiation of [3H]-N3CCP bound to the membranes leads to a covalent linking of the radioactive uncoupler to a peptide of 10--15 kdaltons as analyzed by SDS-polyacrylamide gel electrophoresis. It is concluded that these two microbial systems contain a specific high affinity uncoupler binding site very similar to that of mammalian mitochondria (Katre, N.V. and Wilson, D.F. (1978) Arch. Biochem. Biophys. 191, 647--656).

Quercetin interaction with the chloroplast ATPase complex

1. Quercetin, a flavonoid which acts as an energy transfer inhibitor in photophosphorylation is shown to inhibit the P-ATP exchange activity of membrane-bound CF1 and the ATPase activity of isolated CF1. Quercetin, affects also the proton uptake in chloroplasts in a manner similar to that of dicyclohexylcarbodiimide. 2. The light-dependent proton uptake in EDTA-treated chloroplasts is stimulated by quercetin. In untreated chloroplasts quercetin has a dual effect: it enhances at pH above 7.5 while at lower pH values it decreases the extent of H+ uptake. Similar effects were obtained with dicyclohexylcarbodiimide. 3. Like quercetin, dicyclohexylcarbodiimide was also found to inhibit the ATPase activity of isolated CF1. 4. Quercetin inhibits uncoupled electron transport induced by either EDTA-treatment of chloroplasts or by addition of uncouplers. Quercetin restores H+ uptake in both types of uncoupled chloroplasts. 5. The mode of action of quercetin and dicyclohexylcarbodiimide in photophosphorylation is discussed, and interaction with both CF1 and F0 is suggested.

Structural requirements of alkyl acyldithiocarbazates for the uncoupling of oxidative phosphorylation in mitochondria

A structure-activity relationship study on the uncoupling of alkyl acyldithiocarbazates was carried out. Greater activity was observed with increasing alkyl chain length, the optimum being C9. A further increase in alkyl chain length caused a decrease in the activity. Thione-thiol tautomeric forms with a dissociable proton were dound to be of primary importance for the uncoupling and the role of the acyl group was auxiliary. The reactivity of the SH group of alkyl acyldithiocarbazates with an SH-reagent was very low. These compounds facilitated the valinomycin-induced swelling of non-respiring mitochondria and non-sonicated lecithin liposomes in isotonic potassium acetate solution.

Effect of inhibitors on acid production by baker's yeast

Glucose-induced acid extrusion, respiration and anaerobic fermentation in baker's yeast was studied with the aid of sixteen inhibitors. Uranyl(2+) nitrate affected the acid extrusion more anaerobically than aerobically; the complexing of Mg2+ and Ca2+ by EDTA at the membrane had no effect. Inhibitors of glycolysis (iodoacetamide, N-ethylmaleimide, fluoride) suppressed acid production markedly, and so did the phosphorylation-blocking arsenate. Fluoroacetate, inhibiting the citric-acid cycle, had no effect. Inhibition by uncouplers depended on their pKa values: 2,4,6-trinitrophenol (pKa 0.4) less than 2,4-dinitrophenol (4.1) less than azide (4.7) less than 3-chlorophenylhydrazonomalononitrile (6.0). Inhibition by trinitrophenol was only slightly increased by its acetylation. Cyanide and nonpermeant oligomycin showed practically no effect; inhibition by dicyclohexylcarbodiimide was delayed but potent. The concentration profiles of inhibition of acid production differed from those of respiration and fermentation. Thus, though the acid production is a metabolically dependent process, it does not reflect the intensity of metabolism, except partly in the first half of glycolysis.

Current-voltage relationships for the plasma membrane and its principal electrogenic pump in Neurospora crassa: I. Steady-state conditions

The nonlinear membrane current-voltage relationship (I-V curve) for intact hyphae of Neurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus "quasi-linear" cable theory. Under normal conditions of growth and respiration, the membrane I-V curve is best described as a parabolic segment convex in the direction of depolarizing current. At the average resting potential of - 174 mV, the membrane conductance is approximately 190 micronhos/cm2; conductance increase to approximately 240 micronhos/cm2 at -300 mV, and decreases to approximately 130 micronhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range. Inhibition of the primary electrogenic pump in Neurospora by ATP withdrawal (with 1 mM KCN) depolarizes the membrane to the range of -40 to -70 mV and reduces the slope of the I-V curve by a fixed scaling factor of approximately 0.8. For wild-type Neurospora, compared under control conditions and during steady-state inhibition by cyanide, the I-V difference curve--presumed to define the current-voltage curve for the electrogenic pump--is a saturation function with maximal current of approximately 20 muA/cm2, a half saturation potential near -300 mV, and a projected reversal potential of ca. -400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split. The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-state I-V curves, under the assumption the cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects. The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 muF/cm/2); a second of 50-75 msec; and a third of 20-30 sec, perhaps representing changes of intracellular composition.

The kinetic mechanism of action of an uncoupler of oxidative phosphorylation

The chemiosmotic hypothesis predicts that the mechanism by which weak acids uncouple oxidative phosphorylation in mitochondria is identical to the mechanism by which they transport hydrogen ions across artificial bilayer membranes. We report here the results of a kinetic study of uncoupler-mediated hydrogen ion transport across bilayer membranes. We made electrical relaxation measurements on black lipid membranes exposed to the substituted benzimidazole 5,6-dichloro-2-trifluoromethylbenzimidazole. The simplest model consistent with our experimental data allowed us to deduce values for adsorption coefficients and rate constants. Our major conclusions are that the back diffusion of the neutral species is the rate limiting step for the steady state transport of hydrogen ions, that both the neutral and charged forms of the uncoupler adsorb strongly to the interfaces, and that the reactions at the membrane-solution interfaces occur sufficiently rapidly for equilibrium to be maintained. Independent measurements of the adsorption coefficients of both the neutral and anionic forms of the weak acid and also of the permeability of the membrane to the neutral form agreed well with the values deduced from the kinetic study.

Mechanism of action of EM 49, membrane-active peptide antibiotic

EM 49 (recently renamed octapeptin) is a membrane-active peptide antibiotic that has been reported to affect the structure of bacterial membranes (K. S. Rosenthal, P. E. Swanson, and D. R. Storm, Biochemistry 15:5783-5792, 1976). In this study, it is shown that the effects of EM 49 on bacterial metabolism are similar to those of uncouplers of oxidative phosphorylation. EM 49 stimulated bacterial respiration within a narrow concentration range corresponding to minimum inhibitory concentrations and inhibited respiration at concentrations comparable to minimum biocidal concentrations. In addition, the peptide increased membrane proton permeability and lowered the adenosine 5'-triphosphate pool size. Parallel studies done with the related antibiotic polymyxin B demonstrated that the two peptides differed considerably in their effects on bacterial respiration. In contrast to EM 49, polymyxin B did not stimulate respiration at any concentration. It is proposed that the primary action of EM 49 is to disrupt the selective ion permeability of the cytoplasmic membrane, thereby relaxing the membrane potential.

Relationship between the structure of carbonylcyanide phenylhydrazones and inhibition of growth of microorganisms, stimulation of respiration of yeast cells and rat liver mitochondria

The effect of ten derivatives of carbonylcyanide phenylhydrazone on growth of bacteria, yeast and different species of filamentous fungi was investigated. In yeast and mitochondria isolated from rat liver the effect of these derivatives on the respiratory activity was also followed. The relative efficiency of the individual derivatives of carbonylcyanide phenylhydrazone was determined on the basis of the results obtained. It was shown that derivatives, in which the substituent on the benzene ring causes simultaneously an increase of acidity and lipophilicity of the derivative as compared with the non-substituted carbonylcyanide phenylhydrazone (4-trifluoromethoxy-, 3-chloro-, 4-chloro and 3,4-dichloro-derivatives) were most effective.