Electric fields in coupling membranes

Noncoupled Mitochondrial Respiration as Therapeutic Approach for the Treatment of Metabolic Diseases: Focus on Transgenic Animal Models

Mitochondrial dysfunction contributes to numerous chronic diseases, and mitochondria are targets for various toxins and xenobiotics. Therefore, the development of drugs or therapeutic strategies targeting mitochondria is an important task in modern medicine. It is well known that the primary, although not the sole, function of mitochondria is ATP generation, which is achieved by coupled respiration. However, a high membrane potential can lead to uncontrolled reactive oxygen species (ROS) production and associated dysfunction. For over 50 years, scientists have been studying various synthetic uncouplers, and for more than 30 years, uncoupling proteins that are responsible for uncoupled respiration in mitochondria. Additionally, the proteins of the mitochondrial alternative respiratory pathway exist in plant mitochondria, allowing noncoupled respiration, in which electron flow is not associated with membrane potential formation. Over the past two decades, advances in genetic engineering have facilitated the creation of various cellular and animal models that simulate the effects of uncoupled and noncoupled respiration in different tissues under various disease conditions. In this review, we summarize and discuss the findings obtained from these transgenic models. We focus on the advantages and limitations of transgenic organisms, the observed physiological and biochemical changes, and the therapeutic potential of uncoupled and noncoupled respiration.

Fifty Years of Research on Protonophores: Mitochondrial Uncoupling As a Basis for Therapeutic Action

Protonophores are compounds capable of electrogenic transport of protons across membranes. Protonophores have been intensively studied over the past 50 years owing to their ability to uncouple oxidation and phosphorylation in mitochondria and chloroplasts. The action mechanism of classical uncouplers, such as DNP and CCCP, in mitochondria is believed to be related to their protonophoric activity; i.e., their ability to transfer protons across the lipid part of the mitochondrial membrane. Given the recently revealed deviations in the correlation between the protonophoric activity of some uncouplers and their ability to stimulate mitochondrial respiration, this review addresses the involvement of some proteins of the inner mitochondrial membrane, such as the ATP/ADP antiporter, dicarboxylate carrier, and ATPase, in the uncoupling process. However, these deviations do not contradict the Mitchell theory but point to a more complex nature of the interaction of DNP, CCCP, and other uncouplers with mitochondrial membranes. Therefore, a detailed investigation of the action mechanism of uncouplers is required for a more successful pharmacological use, including their antibacterial, antiviral, anticancer, as well as cardio-, neuro-, and nephroprotective effects.

Novel oxaloacetate effect on mitochondrial Ca2+ movement

Mitochondrial Ca2+ movement was investigated in the presence of oxaloacetate, which is widely known as a 'Ca(2+)-releasing' agent [1978, Proc. Natl. Acad. Sci. USA 75, 1690-1694]. It is demonstrated that rat liver mitochondrial are capable of net Ca2+ accumulation from the oxaloacetate supplemented assay mixture. Both the membrane energization and the cation uniport at the expense of oxaloacetate are shown to be specifically blocked by either arsenite or ammonium chloride. With respiratory inhibitors present, ADP is shown to be a prerequisite for a high Ca2+ capacity, which can be alternatively enlarged with a concomitant loss of the arsenite effect by an addition of an NADP(+)-specific reductant (isocitrate). Arsenite-sensitive production of NADPH is observed, thus suggesting coupling between pyridine nucleotide transhydrogenation and the cation uniport in mitochondria. The role of such a coupling mechanism in the uniporter-mediated Ca2+ fluxes in mitochondria is discussed.

The proton-translocating nicotinamide adenine dinucleotide transhydrogenase

H(+)-transhydrogenase couples the reversible transfer of hydride ion equivalents between NAD(H) and NADP(H) to the translocation of protons across a membrane. There are separate sites on the enzyme for the binding of NAD(H) and of NADP(H). There are some indications of the position of the binding sites in the primary sequence of the enzymes from mitochondria and Escherichia coli. Transfer of hydride ion equivalents only proceeds when a reduced and an oxidized nucleotide are simultaneously bound to the enzyme. When delta p = 0 the rate of interconversion of the ternary complexes of enzyme and nucleotide substrates is probably limiting. An increase in delta p accelerates the rate of interconversion in the direction of NADH----NADP+ until another kinetic component, possibly product release, becomes limiting. The available data are consistent with either direct or indirect mechanisms of energy coupling.

Charge changes in sarcoplasmic reticulum and Ca2+-ATPase induced by calcium binding and release: a study using lipophilic ions

Changes in the charge of sarcoplasmic reticulum (SR) vesicles are studied using lipophilic ions, which are adsorbed by the membrane phase. Upon addition of MgATP, phenyldicarbaundecaborane (PCB-) and tetraphenylboron (TPB-) are taken up by the SR vesicles, while tetraphenylphosphonium (TPP+) is released into the water phase. The PCB- uptake occurs as well under conditions when SR membrane is shunted by high Cl- concentration. MgATP induces minor additional binding of PCB- in the presence of oxalate and it is followed by release of the lipophilic anion from the vesicles. EGTA partly reverses the ATP effect, and calcium ionophore A23187 plus EGTA reverses it completely. Vesicles that were preliminarily loaded by Ca2+ demonstrated higher passive and lower ATP-dependent PCB- binding. Activation of isolated Ca2+-ATPase in the presence of 0.1 mM EGTA results in PCB- release into the medium and additional TPP+ binding to the enzyme. We suggest that the redistribution of the lipophilic ions between the water phase and SR membrane reflects charge changes in Ca2+-binding sites inside both SR vesicles and Ca2+-ATPase molecules in the course of Ca2+ translocation.

Cloning and expression of the transhydrogenase gene of Escherichia coli

Based on the rationale that Escherichia coli cells harboring plasmids containing the pnt gene would contain elevated levels of enzyme, we have isolated three clones bearing the transhydrogenase gene from the Clarke and Carbon colony bank. The three plasmids were subjected to restriction endonuclease analysis. A 10.4-kilobase restriction fragment which overlapped all three plasmids was cloned into the PstI site of plasmid pUC13. Examination of several deletion derivatives of the resulting plasmid and subsequent treatment with exonuclease BAL 31 revealed that enhanced transhydrogenase expression was localized within a 3.05-kilobase segment. This segment was located at 35.4 min in the E. coli genome. Plasmid pDC21 conferred on its host 70-fold overproduction of transhydrogenase. The protein products of plasmids carrying the pnt gene were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membranes from cells containing the plasmids. Two polypeptides of molecular weights 50,000 and 47,000 were coded by the 3.05-kilobase fragment of pDC11. Both polypeptides were required for expression of transhydrogenase activity.

Triphenyltetrazolium and its derivatives are anisotropic inhibitors of energy transduction in oxidative phosphorylation in rat liver mitochondria

Triphenyltetrazolium and its derivatives inhibited energy transduction in mitochondria but not in submitochondrial particles, which are inside-out relative to the membranes of mitochondria. Triphenyltetrazolium incorporated into the inside of submitochondrial particles inhibited ATP synthesis in the particles. Triphenyltetrazolium also inhibited the reduction of NAD by succinate coupled with oxidation of succinate by O2 and hydrolysis of ATP. Energization of mitochondrial inner membranes with succinate and with ATP induced sites on the membranes for triphenyltetrazolium and its derivatives. The maximum amounts of energy-dependent binding sites for triphenyltetrazolium on membranes energized with succinate and ATP, respectively, were 14 and 4 nmol/mg protein. Triphenyltetrazolium also induced H+ ejection from the energized membranes. The maximum amounts of H+ ejection from membranes energized with succinate and ATP, respectively, were 4 and 2.4 nmol/mg protein. Triphenyltetrazolium also decreased the membrane potential up to about half the control value and caused shrinkage of mitochondria in an energy-dependent fashion. Comparison of the Hammett's sigma constants of triphenyltetrazolium derivatives with various substituents on the 3-benzene ring showed that lower concentrations of triphenyltetrazolium derivatives with a stronger positive charge were required for inhibition of energy transduction. The present findings show that triphenyltetrazolium and its derivatives act as anisotropic inhibitors of energy transduction by binding to negative charges created on the outer side (C-side) of energized mitochondria, and that the positive charge of these inhibitors is one of important factors for their inhibitory activity. These negative charges may be an essential part of the H+ pump.

Active transport of calcium in membrane vesicles from Mycobacterium phlei

Active transport of calcium ions has been demonstrated in inside-out membrane vesicles from Mycobacterium phlei mediated by respiratory linked substrates as well as by ATP hydrolysis. The uptake of calcium exhibited an apparent Km of 80 microM and V of 16.6 nmol calcium uptake x min-1 x mg protein-1. A fortyfold concentration gradient for calcium ions was calculated for both the ATP-induced and the respiration-induced transport of calcium. Removal of coupling-factor-latent ATPase resulted in the complete loss of ATP-driven Ca2+ transport whereas the respiration-driven uptake was reduced by 40-50%. The uptake of calcium was inhibited by the proton conducting ionophores carbonylcyanide m-chlorophenylhydrazone and Gramicidin-D. The accumulated calcium was freely exchangeable with external calcium and was rapidly released by the addition of inhibitors of energy transduction, proton-translocating uncouplers or the ionophore A23187. The uptake of the weak base, methylamine, upon the oxidation of respiratory-linked substrates or the hydrolysis of ATP showed the generation of a protein gradient (inside acidic) which was partially collapsed on the addition of calcium ions. These results suggest that a Ca2+/H+ antiport mechanism may be responsible for the transport of calcium.

Localized energization of the mitochondrial inner membrane by ATP

Studies were made to determine whether the energy-dependent binding of ethidium to the mitochondrial inner membrane reflects the membrane potential or the energization of localized regions of the membrane. The number of binding sites of ethidium in mitochondria energized with ATP was 72 nmol/mg protein and decreased with increase in the amount of the ATPase system (F1 . F0) inactivated by oligomycin. These findings clearly show that the energy-dependent binding of ethidium to the mitochondrial inner membrane energized with ATP does not reflect the membrane potential, in good accord with the previous conclusion (Higuti, T., Yokota, M., Arakaki, N., Hattori, A. and Tani, I. (1978) Biochim. Biophys. Acta 503, 211-222), but that ethidium binds to localized regions of the energized membrane that are directly affected by ATPase (F1), reflecting the localized energization of the membrane by ATP.

Sidedness of inhibition of energy transduction in oxidative phosphorylation in rat liver mitochondria by ethidium bromide

Ethidium bromide, a new type of inhibitor of energy transduction in oxidative phosphorylation, inhibited ATP synthesis in intact mitochondria but not in submitochondrial particles, the latter being inside-out relative to the membranes of intact mitochondria. Ethidium bromide incorporated inside the submitochondrial particles inhibited ATP synthesis in the particles. The decrease of the membrane potential by valinomycin (plus KCl) inhibited only slightly the energy-dependent binding of ethidium bromide to the mitochondria. The present results show clearly that ethidium bromide inhibited energy transduction in oxidative phosphorylation by acting on the outer side (C-side) of the inner mitochondrial membrane, perhaps by neutralizing negative charges created on the surface of the C-side, and that it had no inhibitory activity on the inner side (M-side) of the membrane. Th present results show also that the energy-dependent binding of ethidium is not due to electrophoretic transport down the membrane potential; ethidium may bind to negative charges on the surface of the C-side. The present study suggest that an anisotropic distribution of electric charge in the inner mitochondrial membrane is an intermediary high energy state of oxidatvie phosphorylation.

An analysis of the binding of fluorescence probes in mitochondrial systems

Measurements of the binding of the fluorescent probes 8-anilinonaphthalene-1-sulfonate (ANS) and ethidium ions to whole and disruped mitochondria and submitochondrial particles suggest that the inner mitochondrial membrane is freely permeable to the two probes. Equations relating the binding of permeant probes to the electro-chemical balance across the membrane of vesicular systems are derived and these equations used to analyze Scatchard plots of the binding of the two probes to energized and nonenergized mitochondria and EDTA particles.

Effects of substrate and inhibitor binding on thermal and proteolytic inactivation of rat liver transhydrogenase

The thermostability and proteolytic inactivation of rat liver submitochondrial particle transhydrogenase was studied in the presence of pyridine dinucleotide substrates and a variety of divalent metal and nucleotide inhibitors. Relative to the unliganded enzyme, the NADPH-enzyme complex was more thermostable and showed a twofold greater rate of tryptic inactivation, while the NADP+-enzyme complex was more thermolabile and only slightly more susceptible to tryptic inactivation. Neither NAD+ nor NADH significantly affected thermostability or proteolysis. Similar effects of these ligands were observed for the non-energy-linked and energy-linked transhydrogenase reactions, indicating that both activities are catalyzed by the same enzyme. In thermal experiments, acetyl-CoA, 2'-AMP, and NMNH stabilized, palmitoyl-CoAlabilized, and dephospho-CoA, CoA, NMN+, and 5'-AMP had little effect on enzyme stability. Tryptic inactivation was inhibited by 2'-AMP and NMN+ but was not influenced by the other nucleotide inhibitors. Divalent metal ion inhibitors (Mg2+, Ca2+, Mn2+, Ba2+, and Sr2+) stabilized transhydrogenase against thermal inactivation and promoted tryptic inactivation.

8-Anilinonaphthalene-1-sulphonate interaction with whole and disrupted mitochondria: a re-evaluation of the use of double-reciprocal plots in the derivation of binding parameters for fluorescent probes binding to mitochondrial membranes

It is shown that 8-anilinonaphthalene-1-sulphonate is a permeant anion of whole mitochondrial membranes. It is also shown experimentally and algebraically that plots of reciprocal fluorescence against reciprocal membrane concentration, at a fixed 8-anilino-naphthalene-1-sulphonate concentration, are straight lines even when more than one binding site is involved.

The proton-translocating nicotinamide-adenine dinucleotide (phosphate) transhydrogenase of rat liver mitochondria

1. The NAD(P) transhydrogenase activity of the soluble fraction of sonicated rat liver mitochondrial preparations was greater than the NAD-linked isocitrate dehydrogenase activity, and the NAD-linked and NADP-linked isocitrate dehydrogenase activities were not additive. The NAD-linked isocitrate dehydrogenase activity was destroyed by an endogenous autolytic system or by added nucleotide pyrophosphatase, and was restored by a catalytic amount of NADP. 2. We concluded that the isocitrate dehydrogenase of rat liver mitochondria was exclusively NADP-specific, and that the oxoglutarate/isocitrate couple could therefore be used unequivocally as redox reactant for NADP in experiments designed to operate only the NAD(P) transhydrogenase (or loop 0) segment of the respiratory chain in intact mitochondria. 3. During oxidation of isocitrate by acetoacetate in intact, anaerobic, mitochondria via the rhein-sensitive, but rotenone- and arsenite-insensitive, NAD(P) transhydrogenase, measurements of the rates of carbonyl cyanide p-trifluoromethoxyphenylhydrazone-sensitive and carbonyl cyanide p-trifluoromethoxyphenylhydrazone-insensitive pH change in the presence of various oxoglutarate/isocitrate concentration ratios gave an -->H(+)/2e(-) quotient of 1.94+/-0.12 for outward proton translocation by the NAD(P) transhydrogenase. 4. Measurements with a K(+)-sensitive electrode confirmed that the electrogenicity of the NAD(P) transhydrogenase reaction corresponded to the translocation of one positive charge per acid equivalent. 5. Sluggish reversal of the NAD(P) transhydrogenase reaction resulted in a significant inward proton translocation. 6. The possibility that isocitrate might normally be oxidized via loop 0 at a redox potential of -450mV, or even more negative, is discussed, and implies that a P/O quotient of 4 for isocitrate oxidation might be expected.