The mechanism of mitochondrial swelling. VIII. Permeability of mitochondria to alkali metal acetates

The capacity of beef heart mitochondria to undergo osmotically induced volume changes in decimolar M⁺-acetate or other weak acid anion media is characterized by the following features: (1) mitochondria resist swelling when suspended in potassium or rubidium acetate media in the presence of respiratory inhibitors; (2) mitochondria swell extensively when suspended in ammonium or sodium acetate media in the presence of respiratory inhibitors; and (3) actively respiring mitochondria swell extensively whether suspended in ammonium, sodium, potassium, or rubidium acetate media. These findings have been interpreted to mean that (1) the nonenergized mitochondrial inner membrane is permeable to acetate anions, (2) the nonenergized mitochondrial inner membrane is permeable to ammonium and sodium ions in the presence of acetate or other weak acid anions, (3) the nonenergized mitochondrial inner membrane is relatively impermeable to potassium and rubidium ions in the presence of acetate or other weak acid anions, and (4) energized mitochondria are considerably more permeable to potassium and rubidium (acetate) ions than are non-energized mitochondria. The experiments described in this communication which provide the evidence for these interpretations involve methods which are independent of volume changes. The results confirm the first three of the above interpretations but are inconsistent with the fourth. A general theory for passive ion movements in mitochondria is presented and the results are discussed in terms of the development of an energy dependent ion gradient as the key to energized swelling in potassium or rubidium acetate.     

The morphology and configurational states of isolated heavy beef heart mitochondria by the freeze fracture technique

Configurational changes of glutaraldehyde fixed heavy beef heart mitochondria are confirmed using the freeze fracture technique. Large amplitude swelling occurred after unfixed mitochondria were suspended in 30% glycerol. Fine structure of the outer and inner mitochondrial membranes is described using unfixed heavy beef heart mitochondria by the freeze fracture technique. The matrix side of the inner membrane appears to be covered with 90 Å particles while the opposite side (cytochromec side) is also particulate covered by a high density of lower profile particles with a smooth underlying mosaic layer beneath. The outer surface of the outer membrane is smooth with particles embedded within the membrane. Possible structure of the membrane is discussed.     

Membrane instability in respiring mitochondria: role of phosphate.

Metabolically-induced (spontaneous) high amplitude swelling of mitochondria has been shown to be due to a serial disruption of the mitochondrial membranes [D. Sambasivarao & V. Sitaramam (1985), Biochim Biophys Acta, 806, 195-209]. Phosphate- and arsenate-induced swelling was investigated in mitochondria to evaluate the role of phosphate transport in the instability created in the mitochondrial membranes. Phosphate-induced swelling in respiring mitochondria was similar to spontaneous swelling. Both represent essentially colloidal swelling due to the variable porosity induced in the inner membrane to polyols by respiration. Swelling of non-respiring mitochondria at high ammonium phosphate concentrations was, on the other hand, primarily due to high permeability to phosphate. This membrane instability created by phosphate transport in the surrounding lipid involves neither the endogenous nor the exogenous Ca2+.     

Mode of Methomyl and Bipolaris maydis (race T) Toxin in Uncoupling Texas Male-Sterile Cytoplasm Corn Mitochondria

Bipolaris maydis race T toxin (BmT), and its functional analog, methomyl, uncoupled Texas male-sterile (T) cytoplasm mitochondria by decreasing the resistance of the inner membrane to protons. However, unlike protonophoric or ionophoric agents, BmT toxin and methomyl induced irreversible swelling. Packed volume measurements showed that mitochondrial volume was irreversibly increased by methomyl and BmT toxin indicating that mitochondria no longer functioned as differentially permeable osmometers. The decreased resistance of inner mitochondrial membranes to protons and the loss of osmotic volume regulation suggests that methomyl and BmT toxin induced the formation of hydrophilic pores in T mitochondrial inner membranes.     

Respiration induces variable porosity to polyols in the mitochondrial inner membrane

The osmotic basis of low and high amplitude swelling in mitochondria was investigated in detail using sucrose and mannitol as external osmolytes. Osmotic behaviour of mitochondria in various respiratory states was consistent with significant changes in the porosity of the inner membrane corresponding to the rate of respiration. The stoichiometry of oxidative phosphorylation was confirmed to be dependent on the physical state (i.e., osmotic stretch) of the inner membrane regardless of the external polyol used. High amplitude swelling in polyol media was shown to arise from a sequential disruption of the outer and inner mitochondrial membranes, due to a dynamic instability induced by a combination of respiration, unscreened (fixed) surface charge density and the consequent variable porosity of the inner membrane. These novel experimental findings based on the physical theory of osmosis emphasize the need to define the fine structural changes of the inner membrane associated with oxidative phosphorylation to arrive at a comprehensive mechanism.     

Inhibition of the transport of citrate during ADP-ribosylation of inner membrane proteins of the mitochondria

The ability of rat liver submitochondrial particles to catalyze NAD+ hydrolysis with a transfer of ADP-ribose residues to protein membranes has been demonstrated ADP-ribosylation is directly dependent on NAD+ concentration upon saturation with 1 mM NAD+ and is inhibited by physiological compounds (e.g., ATP, 10 mM; nicotinamide, 10 mM); besides, it is an artificial acceptor of ADP-ribose, arginine methyl ester. It was found that ADP-ribose is accepted by inner mitochondrial membrane protein, whose molecular masses amount to 25-30 kDa. The fact that 5'-AMP is a product of ADP-ribose degradation by snake venom phosphodiesterase suggests that the inner membrane vesiculate proteins are modified by mono(ADP-ribose). Covalent modification of membrane proteins by ADP-ribose leads to citrate transport inhibition in inner membrane vesicles the [14C]citrate uptake is significantly decreased thereby. The ability of ADP-ribosylation inhibitors to restore the citrate transport rate is suggestive of a direct regulatory effect of NAD+-dependent ADP-ribosylation on the activity of citrate-translocating system of inner mitochondrial membranes.     

Understanding ion transport by quantification of mitochondrial swelling

Suspensions of mitochondria are turbid and scatter light. An increase in the matrix volume (swelling) due to the influx of permeable solutes results in a decrease in the amount of light scattered. This property can be used to study solute fluxes across the mitochondrial inner membrane. A rapid method for isolating mitochondria is presented along with three swelling experiments using energized and non-energized mitochondria to illustrate ion transport across energy transducing membranes.     

Uncoupling Action of Antibiotic Sporaviridins with Rat-liver Mitochondria

The effects of the glycoside antibiotic sporaviridins (SVDs) on oxidative phosphorylation of rat-liver mitochondria were examined. SVDs released state 4 respiration, dissipated transmembrane electrical potential, and accelerated ATPase activity. These facts demonstrated that SVDs are potent uncouplers of oxidative phosphorylation. During the uncoupling caused by SVDs, large amplitude swelling and oxidation of intramitochondrial NAD(P)H occurred, suggesting that SVDs greatly enhanced nonspecific permeability of the inner mitochondrial membrane. It is suggested that the uncoupling action of SVDs might be caused by dissipation of proton electrochemical potential due to an increase in the permeability of inner mitochondrial membrane.     

Uncoupling action of antibiotic sporaviridins with rat-liver mitochondria.

The effects of the glycoside antibiotic sporaviridins (SVDs) on oxidative phosphorylation of rat-liver mitochondria were examined. SVDs released state 4 respiration, dissipated transmembrane electrical potential, and accelerated ATPase activity. These facts demonstrated that SVDs are potent uncouplers of oxidative phosphorylation. During the uncoupling caused by SVDs, large amplitude swelling and oxidation of intramitochondrial NAD(P)H occurred, suggesting that SVDs greatly enhanced nonspecific permeability of the inner mitochondrial membrane. It is suggested that the uncoupling action of SVDs might be caused by dissipation of proton electrochemical potential due to an increase in the permeability of inner mitochondrial membrane.     

Stilbene derivatives inhibit the activity of the inner mitochondrial membrane chloride channels

Ion channels selective for chloride ions are present in all biological membranes, where they regulate the cell volume or membrane potential. Various chloride channels from mitochondrial membranes have been described in recent years. The aim of our study was to characterize the effect of stilbene derivatives on single-chloride channel activity in the inner mitochondrial membrane. The measurements were performed after the reconstitution into a planar lipid bilayer of the inner mitochondrial membranes from rat skeletal muscle (SMM), rat brain (BM) and heart (HM) mitochondria. After incorporation in a symmetric 450/450 mM KCl solution (cis/trans), the chloride channels were recorded with a mean conductance of 155 ± 5 pS (rat skeletal muscle) and 120 ± 16 pS (rat brain). The conductances of the chloride channels from the rat heart mitochondria in 250/50 mM KCl (cis/trans) gradient solutions were within the 70–130 pS range. The chloride channels were inhibited by these two stilbene derivatives: 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS). The skeletal muscle mitochondrial chloride channel was blocked after the addition of 1 mM DIDS or SITS, whereas the brain mitochondrial channel was blocked by 300 μM DIDS or SITS. The chloride channel from the rat heart mitochondria was inhibited by 50–100 μM DIDS. The inhibitory effect of DIDS was irreversible. Our results confirm the presence of chloride channels sensitive to stilbene derivatives in the inner mitochondrial membrane from rat skeletal muscle, brain and heart cells.     

Cholesterol as activator of ADP-ATP exchange in reconstituted liposomes and in mitochondria

The influence of cholesterol on ADP-ATP exchange activity was measured in the reconstituted system, submitochondrial (sonic) particles and mitoplasts (isolated inner mitochondrial membranes). In the reconstituted system, cholesterol markedly enhanced the nucleotide-uptake rate, when added to membranes of various compositions i.e., pure phosphatidylcholine, phosphatidylcholine/phosphatidylethanolamine mixtures and crude egg yolk phospholipids. The stimulation was linearly dependent on the amount of incorporated cholesterol up to 7–13% added sterol, depending on the type of phospholipids. Cholesterol influenced neither the amount of actively reconstituted carrier proteins nor the affinity of the carrier towards nucleotides nor the breakpoint of temperature dependence in the Arrhenius plot. The stimulation could be correlated with an increase in the molecular activity of the carrier protein. The influence of cholesterol was also measured in the natural environment of the carrier protien, i.e., the inner mitochondrial membrane. Both with submitochondrial particles from beef heart and especially with mitoplasts from rat liver, incorporation of cholesterol by fusion with sterol-containing liposomes led to a stimulation of ADP-ATP exchange activity, comparable to the effect in the reconstituted system. These results are discussed in relation to the absence of cholesterol in the inner mitochondrial membrane and in the view of the generally accepted ordering effect of cholesterol on phospholipid bilayers.     

LOCALIZATION OF THE ENZYMES OF KETOGENESIS IN RAT LIVER MITOCHONDRIA

The localization of the enzymes of ketogenesis in isolated rat liver mitochondria has been investigated. Mitochondrial subfractions were isolated after disruption of this subcellular organelle by (a) hypotonic lysis in water, which permitted the ultracentrifugal separation of the soluble and membranous compartments of the mitochondrion, or by (b) a procedure involving swelling, contraction, and ultrasonic treatment, which permitted the isolation from discontinuous sucrose gradients of subfractions rich in intermembrane space protein, outer membrane, and inner membrane-matrix particles. Two membrane subfractions were invariably present as distinct bands at the lower interface of the discontinuous gradient. The upper of these two bands was found to be a highly purified preparation of outer mitochondrial membrane. Subfractions rich in matrix and in inner membrane were isolated from inner membrane-matrix particles after hypotonic treatment. The content of the various mitochondrial compartments in all subfractions was assessed from their enzymic and electron microscopic characteristics. The ketogenic activity of each subfraction was determined by measuring its capacity to form ketone bodies from acetyl CoA. The activity of this process was markedly enhanced by dithiothreitol. These measurements of ketone body formation, together with assays of individual enzymes of the ketogenic pathway, show that thiolase, HMGCoA synthase, and HMGCoA cleavage enzyme are localized in the matrix of the inner membrane-matrix particles. The rates of ketone body formation indicate that the HMGCoA synthase is the rate-limiting enzyme of the pathway in subfractions of high matrix content. Studies with sodium chloride indicate that a large portion of the HMGCoA synthase, which remains present in membrane subfractions derived from water-treated mitochondria, is bound by ionic interaction to component(s) of the membrane.     

Protein import into mitochondria: ATP-dependent protein translocation activity in a submitochondrial fraction enriched in membrane contact sites and specific proteins

To identify the membrane regions through which yeast mitochondria import proteins from the cytoplasm, we have tagged these regions with two different partly translocated precursor proteins. One of these was bound to the mitochondrial surface of ATP-depleted mitochondria and could subsequently be chased into mitochondria upon addition of ATP. The other intermediate was irreversibly stuck across both mitochondrial membranes at protein import sites. Upon subfraction of the mitochondria, both intermediates cofractionated with membrane vesicles whose buoyant density was between that of inner and outer membranes. When these vesicles were prepared from mitochondria containing the chaseable intermediate, they internalized it upon addition of ATP. A non-hydrolyzable ATP analogue was inactive. This vesicle fraction contained closed, right-side-out inner membrane vesicles attached to leaky outer membrane vesicles. The vesicles contained the mitochondrial binding sites for cytoplasmic ribosomes and contained several mitochondrial proteins that were enriched relative to markers of inner or outer membranes. By immunoelectron microscopy, two of these proteins were concentrated at sites where mitochondrial inner and outer membranes are closely apposed. We conclude that these vesicles contain contact sites between the two mitochondrial membranes, that these sites are the entry point for proteins into mitochondria, and that the isolated vesicles are still translocation competent.     

Regulation of mitochondrial matrix volume

Mitochondrial volume homeostasis is a housekeeping cellular function essential for maintaining the structural integrity of the organelle. Changes in mitochondrial volume have been associated with a wide range of important biological functions and pathologies. Mitochondrial matrix volume is controlled by osmotic balance between cytosol and mitochondria. Any dysbalance in the fluxes of the main intracellular ion, potassium, will thus affect the osmotic balance between cytosol and the matrix and promote the water movement between these two compartments. It has been hypothesized that activity of potassium efflux pathways exceeds the potassium influx in functioning mitochondria and that potassium concentration in matrix could be actually lower than in cytoplasm. This hypothesis provides a clear-cut explanation for the mitochondrial swelling observed after mitochondrial depolarization, mitochondrial calcium overload, or opening of permeability transition pore. It should also be noted that the rate of water flux into or out of the mitochondrion is determined not only by the osmotic gradient that acts as the driving force for water transport but also by the water permeability of the inner membrane. Recent data suggest that the mitochondrial inner membrane has also specific water channels, aquaporins, which facilitate water movement between cytoplasm and matrix. This review discusses different phases of mitochondrial swelling and summarizes the potential effects of mitochondrial swelling on cell function. potassium homeostasis; depolarization; mitochondrial swelling     

Virus-induced permeability transition in mitochondria

Isolated rat liver mitochondria undergo permeability transition after supplementation with a suspension of tobacco mosaic virus. Four mitochondrial parameters proved the opening of the permeability transition pore in the inner mitochondrial membrane: increased oxygen consumption, collapse of the membrane potential, release of calcium ions from mitochondria, and high amplitude mitochondrial swelling. All virus-induced changes in mitochondria were prevented by cyclosporin A. These effects were not observed if the virus was treated with EGTA or disrupted by heating. Protein component of the virus particle in the form of 20S aggregate A-protein, or helical polymer, as well as supernatant of the heat-disrupted virus sample, had no effect on mitochondrial functioning. Electron microscopy revealed the direct interaction of the virus particles with isolated mitochondria. The possible role of the mitochondrial permeability transition pore in virus-induced apoptosis is discussed.     

Disruption of the structural organization of brain mitochondrial membranes during deprivation of the paradoxal sleep phase and correction for it using lithonite]

Interrelation between the structure-functional state of membranes of the brain mitochondria and intensity of metabolic processes in their lipid matrix has been studied during long-term deprivation of the paradoxic sleep and against a background of preventive administration of atypical tranquillizer, lithonite, a derivative of nicotinic acid. It is shown that sleep deprivation for four days is accompanied by expressed activation of lipids' oxidation by free radicals, inhibition of anti-oxidant system and inactivation of the marker enzymes of mitochondrial membranes. The character of binding and distribution of fluorescent probes 1-anilinonaphthalene-8-sulphonate (ANS) and N-phenyl-1-naphthalamine (PNA) in different layers of mitochondrial membranes is broken, which evidences for deep rearrangement of the lipid matrix. Sleep deprivation causes an increase in the relative volume and swelling of mitochondria but a decrease in the inner membrane area and the number of crusts. A course of lithonite administration exerts a selective membrane-protecting effect stabilizing structure-functional state of the brain mitochondria by non-specific protection of their lipid component and stimulation of the antioxidant system. An expediency to use lithonite for correction of other deadaptive processes induced by the membrane destruction is substantiated.     

Ca2+ Induces a Cyclosporin A-Insensitive Permeability Transition Pore in Isolated Potato Tuber Mitochondria Mediated by Reactive Oxygen Species

Oxidative damage of mammalian mitochondria induced by Ca²⁺ and prooxidants is mediated by the attack of mitochondria-generated reactive oxygen species on membrane protein thiols promoting oxidation and cross-linkage that leads to the opening of the mitochondrial permeability transition pore (Castilho et al., 1995). In this study, we present evidence that deenergized potato tuber (Solanum tuberosum) mitochondria, which do not possess a Ca²⁺ uniport, undergo inner membrane permeabilization when treated with Ca²⁺ (>0.2 mM), as indicated by mitochondrial swelling. Similar to rat liver mitochondria, this permeabilization is enhanced by diamide, a thiol oxidant that creates a condition of oxidative stress by oxidizing pyridine nucleotides. This is inhibited by the antioxidants catalase and dithiothreitol. Potato mitochondrial membrane permeabilization is not inhibited by ADP, cyclosporin A, and ruthenium red, and is partially inhibited by Mg²⁺ and acidic pH, well known inhibitors of the mammalian mitochondrial permeability transition. The lack of inhibition of potato mitochondrial permeabilization by cyclosporin A is in contrast to the inhibition of the peptidylprolyl cis–trans isomerase activity, that is related to the cyclosporin A-binding protein cyclophilin. Interestingly, the monofunctional thiol reagent mersalyl induces an extensive cyclosporin A-insensitive potato mitochondrial swelling, even in the presence of lower Ca²⁺ concentrations (>0.01 mM). In conclusion, we have identified a cyclosporin A-insensitive permeability transition pore in isolated potato mitochondria that is induced by reactive oxygen species.     

Electrophysiological characterization of contact sites in brain mitochondria.

From morphological and biochemical studies it has been recognized that the regions where the outer and inner membranes of mitochondria come in close contact (contact sites) can be the route mechanism through which mitochondria interact directly with the cytoplasm. We have studied these regions electrophysiologically with the patch clamp technique, with the aim of understanding if this direct interaction is mediated by high conductance ion channels similar to the channel already detected in the inner membrane of mitochondria (Sorgato M. C., Keller, B. U., and Stühmer, W. (1987) Nature 330, 498-500). Contact sites isolated from rat brain mitochondria were thus incorporated into liposomes subsequently enlarged sufficiently to be patch clamped. This study shows that these particular fractions contain ion channels with conductances ranging from approximately 5 picosiemens to 1 nanosiemens (in symmetrical 150 mM KCl). Most of these channels are not voltage-dependent and can be open at physiological potentials sustained by respiring mitochondria. The lack of voltage sensitivity seems not to be the outcome of methodological artifacts, as voltage-gated channels are detected in giant liposomes containing either the outer mitochondrial membrane or a partially purified fraction of the inner mitochondrial membrane. These data therefore indicate that channels present in mitochondrial contact sites have properties which render them amenable to perform several of the functions hypothesized for these regions, particularly that of translocating macromolecules from the cytoplasm to the matrix of mitochondria.     

The mechanism of mitochondrial swelling. V. Permeability of mitochondria to alkali metal salts of strong acid anions

Mitochondria do not swell appreciably when suspended in media containing the chlorides or bromides of alkali metal or ammonium ions. On the other hand, extensive swelling takes place when mitochondria are suspended in ammonium or sodium acetate. These findings have been widely interpreted to mean that the mitochondrial membrane is impermeable to chloride and bromide ions. However, the resistance of the mitochondria to volume changes is not necessarily a valid criteria of impermeability to a given ion pair. Such a conclusion presumes the as yet untested assumptions that (1) permeability to the ion pair is pair is always the rate-limiting step in swelling, and (2) permeability to the ion pair is equivalent to the driving force for water influx. We have conducted experiments addressed to the question of mitochondrial permeability by methods (tracer exchange diffusion) which are independent of volume changes. Our findings indicate that the mitochondrial membrane is very readily penetrated by alkali metal chloride and bromide salts. Further, we have concluded that the resistance to swelling in such media is associated with a lack of driving force.     

Effect of yttrium on calcium-dependent processes in vertebrate myocardium

Inoptopic effect of yttrium acetate (Y³⁺) on myocardium of the marsh frog Rana ridibunda and its effect on ion transport across the inner mitochondrial membrane (IMM) of rat heart was studied. Y³⁺ was found to decrease the rate of heart contractions and to stimulate ion transport in the rat heart mitochondria in media with 10 mM glutamate and 2 mM malate. Presence of Y³⁺ induced inhibition of energy-dependent Ca²⁺ transport into mitochondria, which was expressed as a marked decrease of their swelling in the media containing 125 mM NH₄NO₃ and Ca²⁺ or 25 mM potassium acetate, 100 mM sucrose and Ca²⁺. It is suggested that the Y³⁺-induced decrease in rat muscle contractions is determined not only by direct suppressing effect of Y³⁺ on potential-modulated Ca²⁺-channels of pacemaker and contractile cardiomyocytes (CM), but also by its indirect effect on Ca²⁺-carrier in IMM. The data confirming that Y³⁺ activates energy-dependent K⁺ transport catalyzed by mitochondrial uniporter and blocks Ca²⁺-channels in the mitochondrial membrane are important for more complete understanding of mechanisms of the Y³⁺ action on vertebrates and human CM.