Membrane potentials and resistances of giant mitochondria. Metabolic dependence and the effects of valinomycin

Why do cells need oxygen? Insights from mitochondrial composition and function

Mitochondrial membrane-embedded redox proteins are classically perceived as deterministic "electron transport chain" (ETC) arrays cum proton pumps; and oxygen is seen as an "immobile terminal electron acceptor." This is untenable because: (1) there are little free protons to be pumped out of the matrix; (2) proton pumping would be highly endergonic; (3) ETC-chemiosmosis-rotary ATP synthesis proposal is "irreducibly complex"/"non-evolvable" and does not fit with mitochondrial architecture or structural/distribution data of the concerned proteins/components; (4) a plethora of experimental observations do not conform to the postulates/requisites; for example, there is little evidence for viable proton-pumps/pH-gradient in mitochondria, trans-membrane potential (TMP) is non-fluctuating/non-trappable, oxygen is seen to give copious "diffusible reactive (oxygen) species" (DRS/DROS) in milieu, etc. Quite contrarily, the newly proposed murburn model's tenets agree with known principles of energetics/kinetics, and builds on established structural data and reported observations. In this purview, oxygen is needed to make DRS, the principal component of mitochondrial function. Complex V and porins respectively serve as proton-inlet and turgor-based water-exodus portals, thereby achieving organellar homeostasis. Complexes I to IV possess ADP-binding sites and their redox-centers react/interact with O₂ /DRS. At/around these complexes, DRS cross-react or activate/oxidize ADP/Pi via fast thermogenic one-electron reaction(s), condensing to form two-electron stabilized products (H₂ O₂ /H₂ O/ATP). The varied architecture and distribution of components in mitochondria validate DRS as (i) the coupling agent of oxidative reactions and phosphorylations, and (ii) the primary reason for manifestation of TMP in steady-state. Explorations along the new precepts stand to provide greater insights on mitochondrial function and pathophysiology.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Electrophysiology clarifies the megariddles of the mitochondrial permeability transition pore

After a brief review of the early history of mitochondrial electrophysiology, the contribution of this approach to the study of the mitochondrial permeability transition (MPT) is recapitulated. It has for example provided evidence for a dimeric nature of the MPT pore, allowed the distinction between two levels of control of its activity, and underscored the relevance of redox events for the phenomenon. Single-channel recording provides a means to finally solve the riddle of the biochemical entity underlying it by comparing the characteristics of the pore with those of channels formed by candidate molecules or complexes. The possibility that this entity may be the protein import machinery of the inner mitochondrial membrane is emphasized.

Incorporation of transmembrane hydroxide transport into the chemiosmotic theory

A cornerstone of textbook bioenergetics is that oxidative ATP synthesis in mitochondria requires, in normal conditions of internal and external pH, a potential difference (delta psi) of well over 100 mV between the aqueous compartments that the energy-transducing membrane separates. Measurements of delta psi inferred from diffusion of membrane-permeant ions confirm this, but those using microelectrodes consistently find no such delta psi--a result ostensibly irreconcilable with the chemiosmotic theory. Transmembrane hydroxide transport necessarily accompanies mitochondrial ATP synthesis, due to the action of several carrier proteins; this nullifies some of the proton transport by the respiratory chain. Here, it is proposed that these carriers' structure causes the path of this "lost" proton flow to include a component perpendicular to the membrane but within the aqueous phases, so maintaining a steady-state proton-motive force between the water at each membrane surface and in the adjacent bulk medium. The conflicting measurements of delta psi are shown to be consistent with the response of this system to its chemical environment.

A characterization of cuprizone-induced giant mouse liver mitochondria

Cuprizone affects the liver of treated mice in a random manner, causing no appreciable change in some cases and inducing the formation of megamitochondria with altered properties in others. Lack of a full appreciation of this variability may be at the origin of some discrepancies in published work dealing with the properties of cuprizone mouse liver mitochondria (CMLM). CMLM from fully affected livers were remarkably labile and difficult to isolate in a coupled state by homogenization and centrifugation techniques. The integral respiratory chain proteins of CMLM were functionally normal, with the exception of succinic dehydrogenase which showed considerable inhibition. Coupled morphological and functional analysis provided evidence that these properties were independent of CMLM size, a matter which had remained doubtful thus far and bears on the validity of literature reports.

Synthesis, location, and lateral mobility of fluorescently labeled ubiquinone 10 in mitochondrial and artificial membranes

To explore the influence of the long isoprene chain of ubiquinone 10 (UQ) on the mobility of the molecule in a phospholipid bilayer, we have synthesized a fluorescent derivative of the head-group moiety of UQ and measured its lateral diffusion in inner membranes of giant mitochondria and in large unilamellar vesicles. The diffusion coefficients, determined by the technique of fluorescence redistribution after photobleaching, were 3.1 X 10(-9) cm2 s-1 in mitochondria and 1.1 X 10(-8) cm2 s-1 in vesicles. Similar diffusion rates were observed for fluorescently labeled phosphatidylethanolamine (PE) with the same moiety attached to its head group (4-nitro-2,1,3-benzooxadiazole: NBD). Fluorescence emission studies carried out in organic solvents of different dielectric constants, and in vesicles and mitochondrial membranes, indicate that NBDUQ is located in a more hydrophobic environment than NBDPE or the starting material IANBD (4-[N-[(iodoacetoxy)ethyl]-N-methylamino]-7-nitro-2,1,3- benzoxadiazole). Fluorescence quenching studies carried out with CuSO4, a water-soluble quenching agent, also indicate that NBDUQ is located deeper in the membrane than NBDPE. These results suggest that ubiquinone and PE are oriented differently in a membrane, even though their diffusion rates are similar. Conclusions regarding whether or not diffusion of UQ is a rate-limiting step in electron transfer must await a more detailed knowledge of the structural organization and properties of the electron transfer components.

Interaction of intravenously injected liposomes with mouse liver mitochondria. A fluorescence and electron microscopy study

Megamitochondria, resulting from cuprizone feeding of Swiss ICR mice, were fluorescent in hepatocytes after the intravenous injection to mice of a liposome-encapsulated acridine orange-DNA complex (AO-DNA). Flow cytofluorimetric analysis of isolated megamitochondria showed that the proportion of liposome-encapsulated AO-DNA which localized in megamitochondria increased from 0.02% of the dose injected per liver cell at 3 min after injection to an average of 0.34% at 1 h after injection. Megamitochondria showed negligible fluorescence by fluorescence activated cell sorting (FACS) analysis when free AO-DNA was intravenously injected. Transmission electron micrographs of mouse liver tissue after intravenous injection of liposomes encapsulating iron dextran showed an association of the liposomes with megamitochondria which appeared identical to liposome association with normal mitochondria. These results support and extend our earlier observation that a fraction of the liposomes injected intravenously into mice associate with mitochondria in the liver, and possibly deliver their aqueous contents there.

Protonmotive force and photophosphorylation in single swollen thylakoid vesicles

Swollen vesicles generally 40 micron in diameter were prepared from spinach chloroplasts. These vesicles appear to originate from thylakoids. The present study reports results obtained with individual vesicles using micromanipulative procedures. The electric potential across the membrane was measured with microelectrodes and the pH of the internal space was calculated from the fluorescence of the pH indicator pyranine. The individual vesicles photophosphorylate as measured with luciferin-luciferase. Impalement with microelectrodes did not affect the ability of individual vesicles to photophosphorylate. However, there was no significant membrane potential either with continuous illumination or light flashes. In contrast, we found a delta pH of 3.7 under photophosphorylative conditions and the incubation with the appropriate buffers blocked photophosphorylation presumably by preventing formation of a pH gradient. We propose that, in these vesicles, the membrane potential plays no role in photophosphorylation, whereas a pH gradient is obligatory.

Protonmotive force in giant mitochondria

The accompanying communication [Eur. J. Biochem. 141 (1984) 1-4] indicates that the microinjection of the pH fluorescent indicator pyranine (8-hydroxy-1,2,6- pyrenesulfonate ) into giant mitochondria or mitoplasts does not affect their ability to carry out oxidative phosphorylation. The dye can therefore be used as a quantitative indicator of internal mitochondrial pH. We found that activation of metabolism in rotenone-inhibited giant mitochondria by the addition of succinate produces an internal pH change corresponding to a pH shift of 0.3 to the alkaline range, approximately the same value found previously for conventional rat liver mitochondria.

Kinetics of Lucifer yellow CH efflux in giant mitochondria

The fluorescent dye Lucifer yellow CH was microinjected electrophoretically into giant mitochondria isolated from mice maintained on a diet containing cuprizone. The dye was retained by the mitochondria, indicating that it was contained in a space bounded by a selectively permeable membrane. The labelling was reversible by reversing the polarity of the current. A study of the disappearance of the fluorescence indicates that the permeability of the mitochondrial membrane to the dye (probably the lithium and/or the potassium salts) ranges from 10(-7) to 10(-8) cm/s.

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.

Electrophoretic injection of a fluorescent dye into giant mitochondria and mitoplasts

Studies of the electrical properties of giant mitochondria and mitoplasts with microelectrodes have indicated that there are no significant metabolically dependent membrane potentials. The internal location of the microelectrode has been confirmed by electrophoretically microinjecting the water-soluble dye Lucifer yellow CH into giant mitochondria or mitoplasts.

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.

Membrane potential, chloride exchange, and chloride conductance in Ehrlich mouse ascites tumour cells

1. The steady-state tracer exchange flux of chloride was measured at 10-150 mM external chloride concentration, substituting either lactate or sucrose for chloride. The chloride flux saturates in both cases with a K 1/2 about 50 and 15 mM, respectively. 2. The inhibitory effect of other monovalent anions on the chloride transport was investigated by measuring the 36Cl- efflux into media where either bromide, nitrate, or thiocyanate had been substituted for part of the chloride. The sequence of increasing affinity for the chloride transport system was found to be: Br- less than Cl- less than SCN- = NO3-. 3. The chloride steady-state exchange flux in the presence of nitrate can be described by Michaelis-Menten kinetics with nitrate as a competitive inhibitor of the chloride flux. 4. The apparent activation energy (EA) was determined to be 67 +/- 6.2 kJ/mole, and was constant between 7 and 38 degrees C. 5. The membrane potential (Vm) was measured as a function of the concentration of external K+, substituting K+ for Na+. The transference number of K+ (tK) was estimated from the slope of Vm vs. log10 (K+)e, and tCl and tNa were calculated, neglecting current carried by ions other than Cl-, K+, and Na+. The diffusional net flux of K+ was calculated from the steady-state exchange flux of 42K+, assuming the flux ratio equation to be valid. From this value the K+ conductance and the Na+ and Cl- conductances were calculated. The experiments showed that GCl, GNa, and GK are all about 14 muS/cm2. 6. The net (conductive) chloride permeability derived from the chloride conductance was 4 x 10(-8) cm/sec compared with the apparent permeability of 6 x 10(-7) cm/sec as calculated from the chloride tracer exchange flux. These data suggest that about 95% of the chloride transport is mediated by an electrically silent exchange diffusion. 7. Comparable effects of phloretin (0.25 mM) on the net (conductive) permeability and the apparent permeability to chloride (about 80% inhibition) may indicate that the chloride exchange and conductance pathways are not completely separate and distinct modes of transport, but may involve common elements. The reduced chloride permeability in the presence of phloretin is estimated to be two orders of magnitude larger than the ground permeability of the cell membrane.

Electron probe analysis of vascular smooth muscle. Composition of mitochondria, nuclei, and cytoplasm

Electron probe analysis of dry cryosections was used to determine the composition of the cytoplasm and organelles of rabbit portal-anterior mesenteric vein (PAMV) smooth muscle. All analytical values given are in mmol/kg wt +/- SEM. Cytoplasmic concentrations in normal, resting muscles were: K, 611 +/- 1.7; Na, 167 +/- 2.7; Cl, 278 +/- 1.0; Mg, 36 +/- 1.1; Ca, 1.9 +/- 0.5; and P, 247 +/- 1.1. Hence, the sum of intracellular Na + K exceeded cytoplasmic Cl by 500 mmol/kg dry wt, while the calculated total, nondiffusible solute was approximately 50 mmol/kg. Cytoplasmic K and Cl were increased in smooth muscles incubated in solutions containing an excess (80 mM) of KCl. Nuclear and cytoplasmic Na and Ca concentrations were not significantly different. The mitochondrial Ca content in normal fibers was low, 0.8 +/- 0.5, and there was no evidence of mitochondrial Ca sequestration in muscles frozen after a K contracture lasint 30 min. Transmitochondrial gradients of K, Na, and Cl were small (0.9--1.2). In damaged fibers, massive mitochondrial Ca accumulation of up to 2 mol/kg dry wt in granule form and associated with P could be demonstrated. Our findings suggest (a) that the nonDonnan distribution of Cl in smooth muscle is not caused by sequestration in organelles, and that considerations of osmotic equilibrium and electroneutrality suggest the existence of unidentified nondiffusible anions in smooth muscle, (b) that nuclei do not contain concentrations of Na or Ca in excess of cytoplasmic levels, (c) that mitochondria in PAMV smooth muscle do not play a major role in regulating cytoplasmic Ca during physiological levels of contraction but can be massively Ca loaded in damaged cells, and (d) that the in situ transmitochondrial gradients of K, Na, and Cl do not show these ions to be distributed according to a large electromotive Donnan force.

Use of dyes to estimate the electrical potential of the mitochondrial membrane

A number of cationic or anionic fluorescent dyes were investigated as possible monitors of the membrane potential of rat liver mitochondria, and giant mitochondria isolated from the liver of mice maintained on a diet containing cuprizone. The fluorescence of four dyes (8-anilino-1-naphthalenesulfonic acid, merocyanine 540, 3,3'-dipropyl-thiocarbocyanine, and bis[1,3-dibutylbarbituric acid-(5)]-pentamethine oxonol) was found to respond appropriately to changes in an apparent K+ diffusion potential. Generally, valinomycin-induced K+ diffusion potentials as calculated using the Nernst equation were used to calibrate the dependence of the fluorescence on the membrane potential. The appropriateness of this approach was verified for two dyes using microelectrodes in giant mitochondria. The apparent membrane potential change induced by the addition of succinate was variable but was very low and generally less than 60 mV in magnitude. The results are consistent with the notion that a large membrane potential is not established upon the initiation of metabolism and that the membrane potential does not play a significant role in the observed ADP phosphorylation.

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.