Acriflavine: anisotropic inhibitor of energy transduction in oxidative phosphorylation of rat liver mitochondria

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.

Anti-Cancer Activity of Acriflavine as Metabolic Inhibitor of OXPHOS in Pancreas Cancer Xenografts

Purpose

All currently available therapies for the treatment of pancreatic ductal adenocarcinoma (PDAC) show limited success. PDACs fast progression depends on the tumor characteristics and can be influenced by the microenvironment. The antibacterial drug acriflavine (ACF) has been shown to have anti-cancer activities in cell lines.

Materials and Methods

To understand the working mechanism of ACF on cancer progression and tumor-stromal interplay, we evaluated pancreatic cancer in cell culture (Panc-1) (morphology, cell invasion and RNA expression) and the macrophage cell line THP1 (representing innate immune stromal cells). In the translational arm, the activity of ACF on xenograft models of human PDAC tumors representing different clinical subclasses was investigated (tumor growth, morphology and immunofluorescence, RNA expression and pathway analysis).

Results

In vitro, ACF reduces epithelial-to-mesenchymal transition (EMT) and invasion of Panc-1 cells and shifts macrophage polarization to a M1-like anti-tumoral phenotype. At non-toxic concentrations, ACF downregulates cell metabolism. In xenografts, effect on tumor growth and metabolism could be confirmed; however, the innate immune stromal cells did not respond. Importantly, only in the moderately differentiated PDAC model, ACF could significantly suppress tumor growth and not in the fast-growing EMT-high model. Pathway analysis shows that ACF highly significantly downregulates metabolic pathways, especially OXPHOS and MYC/cell proliferation pathways in xenografts.

Conclusion

ACF, with known pleiotropic effects on cancer cells, is in this study shown to be an attractive therapeutic based on its novel observed metabolic activity. Repurposing this compound for cancer treatment should be in the setting with other targeting agents, which offers reduced chance of resistance development in PDAC. Further evaluation should best be done in biological complex models such as human xenografts or syngeneic cancer models.

H(+)-ATP synthase from rat liver mitochondria. A simple, rapid purification method of the functional complex and its characterization

A novel, simple, and rapid preparative method for purification of rat liver H(+)-ATP synthase by anion-exchange HPLC was developed. The H(+)-ATP synthase purified had higher ATPase activity in the absence of added phospholipids than any preparation reported previously, and this activity was completely inhibited by oligomycin. When reconstituted into proteoliposomes, the H(+)-ATP synthase showed an ATP-dependent 8-anilinonaphthalene-1-sulfonate response and ATP-Pi exchange activity, both of which were also completely inhibited by oligomycin and an uncoupler, indicating the intactness of the H(+)-ATP synthase. An immunochemical study and a labeling experiment with N,N'-[14C]dicyclohexylcarbodiimide ([14C]DCCD) demonstrated the presence of chargerin II ( a product of mitochondrial A6L DNA) and DCCD-binding protein (subunit c) in the complex. The subunits of the complex were separated into 11 main fractions by reverse-phase HPLC, and 3 of them and the delta subunit in F1 were partially sequenced. A search for sequence homologies indicated that these components were subunit b, coupling factor 6, subunit delta, and subunit epsilon. This is the first report of the existence of subunit b, factor 6, and chargerin II in H(+)-ATP synthase purified from rat liver mitochondria.

Silymarin-induced mitochondrial Ca2+ release

The effect of silymarin on different functions of mitochondria isolated from rat kidneys was studied. Addition of silymarin to mitochondria oxidizing succinate, induced stimulation of the respiratory State 4; while in mitochondria oxidizing NAD-dependent substrates, the drug produced inhibition of the oxygen consumption. It is also shown that silymarin induces mitochondrial swelling, a drop in the transmembrane potential, as well as Ca2+ release. It is proposed that due to its hydrophobic character, silymarin produces an alteration in the lipidic milieu of the inner membrane which is conductive to an inhibition of the electron transport in the NAD-CoQ span of the respiratory chain, as well as to the loss of the energy dependent accumulated Ca2+.

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.

Use of the fluorescent probe, 1-anilino-8-naphthalene sulfonate, to monitor the interaction of pesticide chemicals with mitochondrial membranes

1. The interaction of mitochondrial and submitochondrial membranes with DDT, methoxychlor and kelthane as monitored by fluorescence changes of ANS was studied. 2. The three organochlorine pesticide chemicals reduced the succinate- and ATP-dependent quenching of ANS fluorescence of mitochondria in a concentration-dependent manner. The results suggest that they blocked the supply of energy to the inner membrane. 3. The reversal of the substrate-induced quenching of ANS by uncouplers was abolished by DDT and kelthane probably as a consequence of the inhibition of protonophoric activity of the uncoupler. 4. Both DDT and kelthane were relatively ineffective inhibitors of substrate-induced quenching of ANS fluorescence of submitochondrial particles. The results suggest an asymmetry of the inner membrane with respect to DDT-inhibition of substrate-induced energization. 5. The possible basis of this asymmetry as well as of inhibition of energy supply to the membrane is discussed.

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.

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.