Methylation of Phenyl Rings in Ester-Stabilized Phosphorus Ylides Vastly Enhances Their Protonophoric Activity

We have recently discovered that ester-stabilized phosphorus ylides, resulting from deprotonation of a phosphonium salt such as [Ph3PCH2COOR], can transfer protons across artificial and biological membranes. To create more effective cationic protonophores, we synthesized similar phosphonium salts with one ((heptyloxycarbonylmethyl)(p-tolyl)bromide) or two ((butyloxycarbonylmethyl)(3,5-xylyl)osphonium bromide) methyl substituents in the phenyl groups. The methylation enormously augmented both protonophoric activity of the ylides on planar bilayer lipid membrane (BLM) and uncoupling of mammalian mitochondria, which correlated with strongly accelerated flip-flop of their cationic precursors across the BLM.

Synthesis of Triphenylphosphonium-Linked Derivative of 3,5-Ditert-butyl-4-hydroxybenzylidene-malononitrile (SF6847) via Knoevenagel Reaction Yields an Effective Mitochondria-Targeted Protonophoric Uncoupler

Mitochondrial uncouplers are actively sought as potential therapeutics. Here, we report the first successful synthesis of mitochondria-targeted derivatives of the highly potent uncoupler 3,5-ditert-butyl-4-hydroxybenzylidene-malononitrile (SF6847), bearing a cationic alkyl(triphenyl)phosphonium (TPP) group. As a key step of the synthesis, we used condensation of a ketophenol with malononitrile via the Knoevenagel reaction. SF-C5-TPP with a pentamethylene linker between SF6847 and TPP, stimulating respiration and collapsing membrane potential of rat liver mitochondria at submicromolar concentrations, proved to be the most effective uncoupler of the series. SF-C5-TPP showed pronounced protonophoric activity on a model planar bilayer lipid membrane. Importantly, SF-C5-TPP exhibited rather low toxicity in fibroblast cell culture, causing mitochondrial depolarization in cells at concentrations that only slightly affected cell viability. SF-C5-TPP was more effective in decreasing the mitochondrial membrane potential in the cell culture than SF6847, in contrast to the case of isolated mitochondria. Like other zwitterionic uncouplers, SF-C5-TPP inhibited the growth of Bacillus subtilis in the micromolar concentration range.

The Imidazolium Ionic Liquids Toxicity is Due to Their Effect on the Plasma Membrane

Ionic liquids (ILs) are organic salts with a low melting point. This is due to the fact that their alkyl side chains, which are covalently connected to the ion, hinder the crystallization of ILs. The low melting point of ILs has led to their widespread use as relatively harmless solvents. However, ILs do have toxic properties, the mechanism of which is largely unknown, so identifying the cellular targets of ILs is of practical importance. In our work, we showed that imidazolium ILs are not able to penetrate model membranes without damaging them. We also found that inactivation of multidrug resistance (MDR) pumps in yeast cells does not increase their sensitivity to imidazolium ILs. The latter indicates that the target of toxicity of imidazolium ILs is not in the cytoplasm. Thus, it can be assumed that the disruption of the barrier properties of the plasma membrane is the main reason for the toxicity of low concentrations of imidazolium ILs. We also showed that supplementation with imidazolium ILs restores the growth of cells with kinetically blocked glycolysis. Apparently, a slight disruption of the plasma membrane caused by ILs can, in some cases, be beneficial for the cell.

Nanoparticles of Push-Pull Triphenylamine-Based Molecules for Light-Controlled Stimulation of Neuronal Activity

Organic semiconductor materials with a unique set of properties are very attractive for interfacing biological objects and can be used for noninvasive therapy or detection of biological signals. Here, we describe the synthesis and investigation of a novel series of organic push-pull conjugated molecules with the star-shaped architecture, consisting of triphenylamine as a branching electron donor core linked through the thiophene π-spacer to electron-withdrawing alkyl-dicyanovinyl groups. The molecules could form stable aqueous dispersions of nanoparticles (NPs) without the addition of any surfactants or amphiphilic polymer matrixes with the average size distribution varying from 40 to 120 nm and absorption spectra very similar to those of human eye retina pigments such as rods and green cones. Variation of the terminal alkyl chain length of the molecules forming NPs from 1 to 12 carbon atoms was found to be an efficient tool to modulate their lipophilic and biological properties. Possibilities of using the NPs as light nanoactuators in biological systems or as artificial pigments for therapy of degenerative retinal diseases were studied both on the model planar bilayer lipid membranes and on the rat cortical neurons. In the planar bilayer system, the photodynamic activity of these NPs led to photoinactivation of ion channels formed by pentadecapeptide gramicidin A. Treatment of rat cortical neurons with the NPs caused depolarization of cell membranes upon light irradiation, which could also be due to the photodynamic activity of the NPs. The results of the work gave more insight into the mechanisms of light-controlled stimulation of neuronal activity and for the first time showed that fine-tuning of the lipophilic affinity of NPs based on organic conjugated molecules is of high importance for creating a bioelectronic interface for biomedical applications.

Efficiency of mitochondrial uncoupling by modified butyltriphenylphosphonium cations and fatty acids correlates with lipophilicity of cations: Protonophoric vs leakage mechanisms

In order to determine the share of protonophoric activity in the uncoupling action of lipophilic cations a number of analogues of butyltriphenylphosphonium with substitutions in phenyl rings (C4TPP-X) were studied on isolated rat liver mitochondria and model lipid membranes. An increase in the rate of respiration and a decrease in the membrane potential of isolated mitochondria were observed for all the studied cations, the efficiency of these processes was significantly enhanced in the presence of fatty acids and correlated with the octanol-water partition coefficient of the cations. The ability of C4TPP-X cations to induce proton transport across the lipid membrane of liposomes loaded with a pH-sensitive fluorescent dye increased also with their lipophilicity and depended on the presence of palmitic acid in the liposome membrane. Of all the cations, only butyl[tri(3,5-dimethylphenyl)]phosphonium (C4TPP-diMe) was able to induce proton transport by the mechanism of formation of a cation-fatty acid ion pair on planar bilayer lipid membranes and liposomes. The rate of oxygen consumption by mitochondria in the presence of C4TPP-diMe increased to the maximum values corresponding to conventional uncouplers; for all other cations the maximum uncoupling rates were significantly lower. We assume that the studied cations of the C4TPP-X series, with the exception of C4TPP-diMe at low concentrations, cause nonspecific leak of ions through lipid model and biological membranes which is significantly enhanced in the presence of fatty acids.

Retinal-Based Anion Pump from the Cyanobacterium Tolypothrix campylonemoides

In this work, TcaR rhodopsin from the cyanobacterium Tolypothrix campylonemoides was characterized. Analysis of the amino acid sequence of TcaR revealed that this protein possesses a TSD motif that differs by only one amino acid from the TSA motif of the known halorhodopsin chloride pump. The TcaR protein was expressed in E. coli, purified, and incorporated into proteoliposomes and nanodiscs. Functional activity was measured by electric current generation through the planar bilayer lipid membranes (BLMs) with proteoliposomes adsorbed on one side of the membrane surface, as well as by fluorescence using the voltage-dependent dye oxonol VI. We have shown that TcaR rhodopsin functions as a powerful anion pump. Our results show that the novel microbial anion transporter, TcaR, deserves deeper investigation and may be of interest both for fundamental studies of membrane proteins and as a tool for optogenetics.

Selectivity of cation transport across lipid membranes by the antibiotic salinomycin

The ionophoric antibiotic salinomycin is in the phase of preclinical tests against several types of malignant tumors including breast cancer. Notwithstanding, the data on its ion selectivity, although being critical for its therapeutic activity, are rather scarce. In the present work, we studied the ability of salinomycin to exert cation/H+-exchange across artificial bilayer lipid membranes (BLM) by measuring electrical potential on planar BLM in the presence of a protonophore and fluorescence responses of the pH-sensitive dye pyranine entrapped in liposomes. The following order of ion selectivity was obtained by these two methods: K+ > Na+ > Rb+ > Cs+ > Li+. Measurements of the monovalent cation-induced quenching of fluorescence of thallium ions in methanol showed that salinomycin effectively binds potassium and calcium but poorly binds sodium and lithium ions. At high concentrations, salinomycin transports Ca2+ through membranes of liposomes and mitochondria, as measured by using the calcium-sensitive dye Fluo-5 N. The data obtained can be used in the mechanistic studies of the anti-tumor activity of salinomycin and its selective cytotoxicity towards cancer stem cells.

Mirror proteorhodopsins

Proteorhodopsins (PRs), bacterial light-driven outward proton pumps comprise the first discovered and largest family of rhodopsins, they play a significant role in life on the Earth. A big remaining mystery was that up-to-date there was no described bacterial rhodopsins pumping protons at acidic pH despite the fact that bacteria live in different pH environment. Here we describe conceptually new bacterial rhodopsins which are operating as outward proton pumps at acidic pH. A comprehensive function-structure study of a representative of a new clade of proton pumping rhodopsins which we name "mirror proteorhodopsins", from Sphingomonas paucimobilis (SpaR) shows cavity/gate architecture of the proton translocation pathway rather resembling channelrhodopsins than the known rhodopsin proton pumps. Another unique property of mirror proteorhodopsins is that proton pumping is inhibited by a millimolar concentration of zinc. We also show that mirror proteorhodopsins are extensively represented in opportunistic multidrug resistant human pathogens, plant growth-promoting and zinc solubilizing bacteria. They may be of optogenetic interest.

Ester-stabilized phosphorus ylides as protonophores on bilayer lipid membranes, mitochondria and chloroplasts

Triphenylphosphonium ylides are commonly used as key intermediates in the Wittig reaction. Based on the known acidities of stabilized ylide precursors, we proposed that a methylene group adjacent to phosphorus in these compounds can ensure proton shuttling across lipid membranes. Here, we synthesized (decyloxycarbonylmethyl)triphenylphosphonium bromide (CMTPP-C₁₀) by reaction of triphenylphosphine with decyl bromoacetate. This phosphonium salt precursor of the ester-stabilized phosphorus ylide along with its octyl (CMTPP-C₈) and dodecyl (CMTPP-C₁₂) analogues was found to be a carrier of protons in mitochondrial, chloroplast and artificial lipid membranes, suggesting that it can reversibly release hydrogen ions and diffuse through the membranes in both zwitterionic (ylide) and cationic forms. The CMTPP-C₁₀-mediated electrical current across planar bilayer lipid membranes exhibited pronounced proton selectivity. Similar to conventional protonophores, known to uncouple electron transport and ATP synthesis, CMTPP-C_n (n = 8, 10, 12) stimulated mitochondrial respiration, while decreasing membrane potential, at micromolar concentrations, thereby showing the classical uncoupling activity in mitochondria. CMTPP-C₁₂ also caused dissipation of transmembrane pH gradient on chloroplast membranes. Importantly, CMTPP-C₁₀ exhibited substantially lower toxicity in cell culture, than C₁₂TPP. Thus, we report the finding of a new class of ylide-type protonophores, which is of substantial interest due to promising therapeutic properties of uncouplers.

Mitochondrial ATP Synthase and Mild Uncoupling by Butyl Ester of Rhodamine 19, C4R1

The homeostasis of the transmembrane potential of hydrogen ions in mitochondria is a prerequisite for the normal mitochondrial functioning. However, in different pathological conditions it is advisable to slightly reduce the membrane potential, while maintaining it at levels sufficient to produce ATP that will ensure the normal functioning of the cell. A number of chemical agents have been found to provide mild uncoupling; however, natural proteins residing in mitochondrial membrane can carry this mission, such as proteins from the UCP family, an adenine nucleotide translocator and a dicarboxylate carrier. In this study, we demonstrated that the butyl ester of rhodamine 19, C4R1, binds to the components of the mitochondrial ATP synthase complex due to electrostatic interaction and has a good uncoupling effect. The more hydrophobic derivative C12R1 binds poorly to mitochondria with less uncoupling activity. Mass spectrometry confirmed that C4R1 binds to the β-subunit of mitochondrial ATP synthase and based on molecular docking, a C4R1 binding model was constructed suggesting the binding site on the interface between the α- and β-subunits, close to the anionic amino acid residues of the β-subunit. The association of the uncoupling effect with binding suggests that the ATP synthase complex can provide induced uncoupling.

Synchronization of opening and closing of two gramicidin A channels pulled together by a linker: possible relevance to channel clustering

The linear 15-mer peptide gramicidin A (gA) produced by Bacillus brevis is known to form the simplest natural ion channel in lipid membranes representing a head-to-head transmembrane dimer. Its incorporation into a planar lipid bilayer manifests itself in regular electrical current transitions. If two gA subunits are tightly connected by a water-soluble, flexible linker of a certain length, the current transitions become heterogeneous: in a part of them, the amplitude is almost twofold higher than that of a single channel, thereby demonstrating the synchronous opening of two single channels. The lifetime, i.e. the open-state duration, of this dual channel is by several orders of magnitude longer than that of the single channel. Here, we used the ideas of the theory of excitons to hypothesize about the mechanism of synchronous opening and closing of two adjacent channels. Two independent (uncoupled) single channels can be described by two independent conformational coordinates q1 and q2, while two closely located channels can exhibit collective behavior, if the coupling between them produces mixing of the individual states (q1,0) and (0,q2). We suppose that a similar phenomenon can occur not only with synthetic derivatives of gA, but also with such natural channel-forming peptide antibiotics and toxins as alamethicin and syringomycin. In particular, channel clustering observed with these peptides may be also associated with formation of collective conductance states, resulting from mixing of their monomeric states, which allows us to explain the fact that clusters of these channels transmit ions and nonelectrolytes of the same size as the original single channels.

Photodynamic activity rather than drilling causes membrane damage by a light-powered molecular nanomotor

The chase toward endowing chemical compounds with machine-like functions mimicking those of biological molecular machineries has yielded a variety of artificial molecular motors (AMMs). Pharmaceutical applications of photoexcited monomolecular unidirectionally-rotating AMMs have been envisioned in view of their ability to permeabilize biological membranes. Nonetheless, the mechanical properties of lipid membranes render the proposed drilling activity of AMMs doubtful. Here, we show that singlet oxygen released by a photoexcited "molecular drill" oxidized unsaturated lipids composing giant unilamellar vesicles. In contrast, giant liposomes built of saturated lipids were inert to AMM photoactuation. The AMM did not mechanically destroy gramicidin A ion channels in planar bilayer lipid membranes but instead photoinactivated them. Sodium azide, a singlet oxygen quencher, reduced both AMM-mediated light-induced dye release from unsaturated large unilamellar vesicles and protected gramicidin A from photoinactivation. Upon additional consideration of the underlying bilayer mechanics, we conclude that AMMs' envisioned therapeutic and pharmaceutical applications rely on their photodynamic activity rather than their nanomechanical drilling abilities.

Antibiotic Pyrrolomycin as an Efficient Mitochondrial Uncoupler

Pyrrolomycins C (Pyr_C) and D (Pyr_D) are antibiotics produced by Actinosporangium and Streptomyces. The mechanism of their antimicrobial activity consists in depolarization of bacterial membrane, leading to the suppression of bacterial bioenergetics through the uncoupling of oxidative phosphorylation, which is based on the protonophore action of these antibiotics [Valderrama et al., Antimicrob. Agents Chemother. (2019) 63, e01450]. Here, we studied the effect of pyrrolomycins on the isolated rat liver mitochondria. Pyr_C was found to be more active than Pyr_D and uncoupled mitochondria in the submicromolar concentration range, which was observed as the mitochondrial membrane depolarization and stimulation of mitochondrial respiration. In the case of mitoplasts (isolated mitochondria with impaired outer membrane integrity), the difference in the action of Pyr_C and Pyr_D was significantly less pronounced. By contrast, in inverted submitochondrial particles (SMPs), Pyr_D was more active as an uncoupler, which caused collapse of the membrane potential even at the nanomolar concentrations. The same ratio of the protonophoric activity of Pyr_D and Pyr_C was obtained by us on liposomes loaded with the pH indicator pyranine. The protonophore activity of Pyr_D in the planar bilayer lipid membranes (BLMs) was maximal at ~pH 9, i.e., at pH values close to pK_a of this compound. Pyr_D functions as a typical anionic protonophore; its activity in the BLM could be reduced by the addition of the dipole modifier phloretin. The difference between the protonophore activity of Pyr_C and Pyr_D in the mitochondria and BLMs can be attributed to a higher ability of Pyr_C to penetrate the outer mitochondrial membrane.

Membrane Permeability of Modified Butyltriphenylphosphonium Cations

The alkyltriphenylphosphonium (TPP) group is the most widely used vector targeted to mitochondria. Previously, the length of the alkyl linker was varied as well as structural modifications in the TPP phenyl rings to obtain the optimal therapeutic effect of a pharmacophore conjugated with a lipophilic cation. In the present work, we synthesized butyltriphenylphosphonium cations halogenated and methylated in phenyl rings (C₄TPP-X) and measured electrical current through a planar lipid bilayer in the presence of C₄TPP-X. The permeability of C₄TPP-X varied in the range of 6 orders of magnitude and correlates well with the previously measured translocation rate constant for dodecyltriphenylphosphonium analogues. The partition coefficient of the butyltriphenylphosphonium analogues obtained by calculating the difference in the free energy of cation solvation in water and octane using quantum chemical methods correlates well with the permeability values. Using an ion-selective electrode, a lower degree of accumulation of analogues with halogenated phenyl groups was found on isolated mitochondria of rat liver, which is in agreement with their permeability decrease. Our results indicate the translocation of the butyltriphenylphosphonium cations across the hydrophobic membrane core as rate-limiting stage in the permeability process rather than their binding/release to/from the membrane.

Electrophysiological Characterization of Microbial Rhodopsin Transport Properties: Electrometric and ΔpH Measurements Using Planar Lipid Bilayer, Collodion Film, and Fluorescent Probe Approaches

Electrophysiological approaches to the study of the activity of retinal-containing protein bacteriorhodopsin (bR) or other proteins of this family are based usually on measurements of electrical current through a planar bilayer lipid membrane (BLM) with proteoliposomes attached to the BLM surface at one side of the membrane. Here, we describe the measurements of the pumping activity of bR and channelrhodopsin 2 (ChR2) with special attention to the study of voltage dependence of the light-induced currents. Strong voltage dependence of ChR2 suggests light-triggered ion channel activity of ChR2. We also describe electrophysiological measurements with the help of collodion film instead of BLM for the measurements of fast stages of a rhodopsin photocycle as well as the estimation of the activity of proteoliposomes without a macro membrane using fluorescent probes such as oxonol VI or 9-aminoacridine.

Peptide-induced membrane elastic deformations decelerate gramicidin dimer-monomer equilibration

Gramicidin A (gA) is a hydrophobic pentadecapeptide readily incorporating into a planar bilayer lipid membrane (BLM), thereby inducing a large macroscopic current across the BLM. This current results from ion-channel formation due to head-to-head transbilayer dimerization of gA monomers with rapidly established monomer-dimer equilibrium. Any disturbance of the equilibrium, e.g., by sensitized photoinactivation of a portion of gA monomers, causes relaxation toward a new equilibrium state. According to previous studies, the characteristic relaxation time of the gA-mediated electric current decreases as the current increases upon elevating the gA concentration in the membrane. Here, we report data on the current relaxation kinetics for gA analogs with N-terminal valine replaced by glycine or tyrosine. Surprisingly, the relaxation time increased rather than decreased upon elevation of the total membrane conductance induced by these gA analogs, thus contradicting the classical kinetic scheme. We developed a general theoretical model that accounts for lateral interaction of monomers and dimers mediated by membrane elastic deformations. The modified kinetic scheme of the gramicidin dimerization predicts the reverse dependence of the relaxation time on membrane conductance for gA analogs, with a decreased dimerization constant that is in a good agreement with our experimental data. The equilibration process may be also modulated by incorporation of other peptides ("impurities") into the lipid membrane.

Pioglitazone Is a Mild Carrier-Dependent Uncoupler of Oxidative Phosphorylation and a Modulator of Mitochondrial Permeability Transition

Pioglitazone (PIO) is an insulin-sensitizing antidiabetic drug, which normalizes glucose and lipid metabolism but may provoke heart and liver failure and chronic kidney diseases. Both therapeutic and adverse effects of PIO can be accomplished through mitochondrial targets. Here, we explored the capability of PIO to modulate the mitochondrial membrane potential (ΔΨ_m) and the permeability transition pore (mPTP) opening in different models in vitro. ΔΨ_m was measured using tetraphenylphosphonium and the fluorescent dye rhodamine 123. The coupling of oxidative phosphorylation was estimated polarographically. The transport of ions and solutes across membranes was registered by potentiometric and spectral techniques. We found that PIO decreased ΔΨ_m in isolated mitochondria and intact thymocytes and the efficiency of ADP phosphorylation, particularly after the addition of Ca²⁺. The presence of the cytosolic fraction mitigated mitochondrial depolarization but made it sustained. Carboxyatractyloside diminished the PIO-dependent depolarization. PIO activated proton transport in deenergized mitochondria but not in artificial phospholipid vesicles. PIO had no effect on K⁺ and Ca²⁺ inward transport but drastically decreased the mitochondrial Ca²⁺-retention capacity and protective effects of adenine nucleotides against mPTP opening. Thus, PIO is a mild, partly ATP/ADP-translocase-dependent, uncoupler and a modulator of ATP production and mPTP sensitivity to Ca²⁺ and adenine nucleotides. These properties contribute to both therapeutic and adverse effects of PIO.

Trialkyl(vinyl)phosphonium Chlorophenol Derivatives as Potent Mitochondrial Uncouplers and Antibacterial Agents

Trialkyl phosphonium derivatives of vinyl-substituted p-chlorophenol were synthesized here by a recently developed method of preparing quaternary phosphonium salts from phosphine oxides using Grignard reagents. All the derivatives with a number (n) of carbon atoms in phosphonium alkyl substituents varying from 4 to 7 showed pronounced uncoupling activity in isolated rat liver mitochondria at micromolar concentrations, with a tripentyl derivative being the most effective both in accelerating respiration and causing membrane potential collapse, as well as in provoking mitochondrial swelling in a potassium-acetate medium. Remarkably, the trialkyl phosphonium derivatives with n from 4 to 7 also proved to be rather potent antibacterial agents. Methylation of the chlorophenol hydroxyl group suppressed the effects of P₅₅₅ and P₄₄₄ on the respiration and membrane potential of mitochondria but not those of P₆₆₆, thereby suggesting a mechanistic difference in the mitochondrial uncoupling by these derivatives, which was predominantly protonophoric (carrier-like) in the case of P₅₅₅ and P₄₄₄ but detergent-like with P₆₆₆. The latter was confirmed by the carboxyfluorescein leakage assay on model liposomal membranes.

Rhodopsin Channel Activity Can Be Evaluated by Measuring the Photocurrent Voltage Dependence in Planar Bilayer Lipid Membranes

The studies of the functional properties of retinal-containing proteins often include experiments in model membrane systems, e.g., measurements of electric current through planar bilayer lipid membranes (BLMs) with proteoliposomes adsorbed on one of the membrane surfaces. However, the possibilities of this method have not been fully explored yet. We demonstrated that the voltage dependence of stationary photocurrents for two light-sensitive proteins, bacteriorhodopsin (bR) and channelrhodopsin 2 (ChR2), in the presence of protonophore had very different characteristics. In the case of the bR (proton pump), the photocurrent through the BLM did not change direction when the polarity of the applied voltage was switched. In the case of the photosensitive channel protein ChR2, the photocurrent increased with the increase in voltage and the current polarity changed with the change in the voltage polarity. The protonophore 4,5,6,7-tetrachloro-2-trifluoromethyl benzimidazole (TTFB) was more efficient in the maximizing stationary photocurrents. In the presence of carbonyl cyanide-m-chlorophenylhydrazone (CCCP), the amplitude of the measured photocurrents for bR significantly decreased, while in the case of ChR2, the photocurrents virtually disappeared. The difference between the effects of TTFB and CCCP was apparently due to the fact that, in contrast to TTFB, CCCP transfers protons across the liposome membranes with a higher rate than through the decane-containing BLM used as a surface for the proteoliposome adsorption.

Linking 7-Nitrobenzo-2-oxa-1,3-diazole (NBD) to Triphenylphosphonium Yields Mitochondria-Targeted Protonophore and Antibacterial Agent

Appending lipophilic cations to small molecules has been widely used to produce mitochondria-targeted compounds with specific activities. In this work, we obtained a series of derivatives of the well-known fluorescent dye 7-nitrobenzo-2-oxa-1,3-diazole (NBD). According to the previous data [Denisov et al. (2014) Bioelectrochemistry, 98, 30-38], alkyl derivatives of NBD can uncouple isolated mitochondria at concentration of tens of micromoles despite a high pK_a value (~11) of the dissociating group. Here, a number of triphenylphosphonium (TPP) derivatives linked to NBD via hydrocarbon spacers of varying length (C5, C8, C10, and C12) were synthesized (mitoNBD analogues), which accumulated in the mitochondria in an energy-dependent manner. NBD-C10-TPP (C10-mitoNBD) acted as a protonophore in artificial lipid membranes (liposomes) and uncoupled isolated mitochondria at micromolar concentrations, while the derivative with a shorter linker (NBD-C5-TPP, or C5-mitoNBD) exhibited no such activities. In accordance with this data, C10-mitoNBD was significantly more efficient than C5-mitoNBD in suppressing the growth of Bacillus subtilis. C10-mitoNBD and C12-mitoNBD demonstrated the highest antibacterial activity among the investigated analogues. C10-mitoNBD also exhibited the neuroprotective effect in the rat model of traumatic brain injury.

Anomalous potentials on bilayer lipid membranes in the presence of usnic acid: Markin-Sokolov versus Nernst-Donnan equilibrium

To gain insight into the mechanisms of ionophoric activity of usnic acid (UA), we examined the UA-induced generation of potentials on a planar bilayer lipid membrane (BLM) in the presence of concentration gradients of hydrogen and magnesium or calcium ions under open-circuit conditions. Remarkably, the BLM potential generated by UA at the proton concentration gradient of 1 pH unit was approximately twice the Nernst equilibrium level. With a concentration gradient of magnesium or calcium ions, the BLM potential generated by UA had the opposite sign. The observed anomalies in the membrane potentials were consistent with a theory developed by Markin and Sokolov (Bioelectrochem. Bioenerg. 1990) for the case of ionophore-mediated coupled fluxes of several ions across a membrane.

Novel pH-Sensitive Microbial Rhodopsin from Sphingomonas paucimobilis

This work provides the first characteristics of the rhodopsin SpaR from Sphingomonas paucimobilis, aerobic bacteria associated with opportunistic infections. The sequence analysis of SpaR has shown that this protein has unusual DTS motif which has never reported in rhodopsins from Proteobacteria. We report that SpaR operates as an outward proton pump at low pH; however, proton pumping is almost absent at neutral and alkaline pH. The photocycle of this rhodopsin in detergent micelles slows down with an increase in pH because of longer Schiff base reprotonation. Our results show that the novel microbial ion transporter SpaR of interest both as an object for basic research of membrane proteins and as a promising optogenetic tool.

Lipophilic ion aromaticity is not important for permeability across lipid membranes

To clarify the contribution of charge delocalization in a lipophilic ion to the efficacy of its permeation through a lipid membrane, we compared the behavior of alkyl derivatives of triphenylphosphonium, tricyclohexylphosphonium and trihexylphosphonium both in natural and artificial membranes. Exploring accumulation of the lipophilic cations in response to inside-negative membrane potential generation in mitochondria by using an ion-selective electrode revealed similar mitochondrial uptake of butyltricyclohexylphosphonium (C₄TCHP) and butyltriphenylphosphonium (C₄TPP). Fluorescence correlation spectroscopy also demonstrated similar membrane potential-dependent accumulation of fluorescein derivatives of tricyclohexyldecylphosphonium and decyltriphenylphosphonium in mitochondria. The rate constant of lipophilic cation translocation across the bilayer lipid membrane (BLM), measured by the current relaxation method, moderately increased in the following sequence: trihexyltetradecylphosphonium ([P_6,6,6,14]) < triphenyltetradecylphosphonium (C₁₄TPP) < tricyclohexyldodecylphosphonium (C₁₂TCHP). In line with these results, measurements of the BLM stationary conductance indicated that membrane permeability for C₄TCHP is 2.5 times higher than that for C₄TPP. Values of the difference in the free energy of ion solvation in water and octane calculated using the density functional theory and the polarizable continuum solvent model were similar for methyltriphenylphosphonium, tricyclohexylmethylphosphonium and trihexylmethylphosphonium. Our results prove that both cyclic and aromatic moieties are not necessary for lipophilic ions to effectively permeate through lipid membranes.

Molecular Mechanisms Responsible for Pharmacological Effects of Genipin on Mitochondrial Proteins

Genipin, a natural compound from Gardenia jasminoides, is a well-known compound in Chinese medicine that is used for the treatment of cancer, inflammation, and diabetes. The use of genipin in classical medicine is hindered because of its unknown molecular mechanisms of action apart from its strong cross-linking ability. Genipin is increasingly applied as a specific inhibitor of proton transport mediated by mitochondrial uncoupling protein 2 (UCP2). However, its specificity for UCP2 is questionable, and the underlying mechanism behind its action is unknown. Here, we investigated the effect of genipin in different systems, including neuroblastoma cells, isolated mitochondria, isolated mitochondrial proteins, and planar lipid bilayer membranes reconstituted with recombinant proteins. We revealed that genipin activated dicarboxylate carrier and decreased the activity of UCP1, UCP3, and complex III of the respiratory chain alongside with UCP2 inhibition. Based on competitive inhibition experiments, the use of amino acid blockers, and site-directed mutagenesis of UCP1, we propose a mechanism of genipin’s action on UCPs. At low concentrations, genipin binds to arginine residues located in the UCP funnel, which leads to a decrease in UCP’s proton transporting function in the presence of long chain fatty acids. At concentrations above 200 μM, the inhibitory action of genipin on UCPs is overlaid by increased nonspecific membrane conductance due to the formation of protein-genipin aggregates. Understanding the concentration-dependent mechanism of genipin action in cells will allow its targeted application as a drug in the above-mentioned diseases.

Zwitterionic Protonophore Derived from 2-(2-Hydroxyaryl)alkenylphosphonium as an Uncoupler of Oxidative Phosphorylation

2-(2-Hydroxyaryl)alkenylphosphonium salts (here coined as PPR) representing derivatives of quaternary phosphonium with two phenyl (P) and one alkyl (R) substituents linked through alkenyl bridge to substituted phenol were applied here to planar bilayer lipid membranes (BLM), isolated mitochondria, and cell culture. PPR with six carbon atoms in R (PP6) induced proton-selective currents across BLM and caused mitochondrial uncoupling. In particular, PP6 at submicromolar concentrations accelerated respiration, decreased membrane potential, and reduced ATP synthesis in isolated rat liver mitochondria (RLM). Methylation of a hydroxyl group substantially suppressed the protonophoric activity of PP6 on BLM and its uncoupling potency in RLM. Of note, the methylated derivative PP6-OMe was synthesized here via a new synthetic route including cyclization of PP6 with subsequent ring opening. PPR were considered as protonophoric uncouplers of a zwitterionic type, capable of penetrating membranes both as a zwitterion composed of a deprotonated phenol and a cationic quaternary phosphonium, and as a protonated cation. The protonophoric and uncoupling properties of PPR found here were speculated to account for their strong antibacterial activity described previously.

Effect of methyl and halogen substituents on the transmembrane movement of lipophilic ions

The introduction of a halogen or a methyl substituent changes the speed of the flip-flop of the penetrating cations and anions in the opposite way.

Carborane derivatives of 1,2,3-triazole depolarize mitochondria by transferring protons through the lipid part of membranes

Boron containing polyhedra (carboranes) are three-dimensional delocalized aromatic systems. These structures have been shown to transport protons through lipid membranes and mitochondria. Conjugation of carboranes to various organic moieties is aimed at obtaining biologically active compounds with novel properties. Taking advantage of 1,2,3-triazoles as the scaffolds valuable in medicinal chemistry, we synthesized 1-(o-carboranylmethyl)-4-pentyl-1,2,3-triazole (c-triazole) and 1-(o-carboranylmethyl)-4-pentyl-1,2,3-triazolium iodide (c-triazolium). Both compounds interacted with model lipid membranes and exhibited a proton carrying activity in planar bilayers and liposomes in a concentration- and pH-dependent manner. Importantly, mechanisms of the protonophoric activity differed; namely, protonation-deprotonation reactions of the triazole and the o-carborane moieties were involved in the transport cycles of c-triazole and c-triazolium, respectively. At micromolar concentrations, c-triazole and c-triazolium stimulated respiration of isolated rat liver mitochondria and depolarized their membrane potential, with c-triazole being more potent. In living K562 (human chronic myelogenous leukemia) cells, both c-triazolium and c-triazole altered the mitochondrial membrane potential as determined by a decreased intracellular accumulation of the potential-dependent dye tetramethylrhodamine ethyl ester. Finally, cell viability testing demonstrated a cytotoxic potency of c-triazolium and, to a lesser extent, of c-triazole against K562 cells, whereas non-malignant fibroblasts were much less sensitive. In all tests, the reference boron-free benzyl-4-pentyl-1,2,3-triazole showed little-to-no effects. These results demonstrated that carboranyltriazoles carry protons across biological membranes, a property potentially important in anticancer drug design.

Effect of Alkyl Chain Length on Translocation of Rhodamine B n-Alkyl Esters across Lipid Membranes

Voltage-dependent translocation of a series of cationic rhodamine B derivatives differing in n-alkyl chain length (ethyl, butyl, octyl, dodecyl, octadecyl) from one lipid monolayer to another was studied by measuring electrical current relaxation after a voltage jump on a planar bilayer phosphatidylcholine (PC) membrane. The rate of the translocation decreased in the following series of lipids: diphytanyl-PC > dioleyl-PC > diphytanoyl-PC > dierucoyl-PC. For all the lipids studied, the rate increased with lengthening of the hydrocarbon chain of the rhodamine derivatives, with the increase being most pronounced for the compounds having a short alkyl chain. The results could be well explained by involvement of molecule reorientations in the process of transmembrane flip-flop of the hydrophobic membrane-bound compounds. However, an impact of membrane dipole potential on the translocation rate could not be excluded, because the dipole potential could contribute to the energy barrier for translocation of the compounds located at different depths in the water-membrane interface. Based on the data obtained, a difference in the dipole potential of ester diphytanoyl-PC membranes with respect to ether diphytanyl-PC was estimated to be 108 mV, highlighting the contribution of a layer of oriented carbonyl groups of the lipids to the membrane dipole potential.

Role of mitochondrial outer membrane in the uncoupling activity of N-terminally glutamate-substituted gramicidin A

Of a series of gramicidin A (gA) derivatives, we have earlier found the peptide [Glu1]gA exhibiting very low toxicity toward mammalian cells, although dissipating mitochondrial membrane potential with almost the same efficiency as gA. Substitution of glutamate for valine at position 1 of the gA amino acid sequence, which is supposed to interfere with the formation of ion-conducting gA channels via head-to-head dimerization, reduces both channel-forming potency of the peptide in planar lipid bilayer membranes and its photonophoric activity in unilamellar liposomes. Here, we compared [Glu1]gA and gA abilities to cause depolarization of the inner mitochondrial membrane in mitochondria and mitoplasts, the latter lacking the outer mitochondrial membrane. Importantly, much less gA was needed to decrease the membrane potential in mitoplasts than in mitochondria, whereas the depolarizing potency of [Glu1]gA was nearly the same in these systems. Moreover, in multilamellar liposomes, [Glu1]gA exhibited more pronounced protonophoric activity than gA, in contrast to the data for unilamellar liposomes. These results allowed us to conclude that [Glu1]gA has a much higher permeability between adjacent lipid membranes than gA. Therefore, the fraction of peptide molecules, reaching the inner mitochondrial membrane upon the addition to cells, is much higher for [Glu1]gA compared to gА. Under these conditions, the decreased cytotoxicity of [Glu1]gA could be associated with its low efficiency as a channel-former dissipating potassium and sodium ion gradients across plasma membrane. The present study highlighted the role of the ability to permeate among various biological membranes for intracellular efficiency of ionophores.

Effect of Cyanide on Mitochondrial Membrane Depolarization Induced by Uncouplers

In this work, it was found that the ability of common uncouplers - carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and 2,4-dinitrophenol (DNP) - to reduce membrane potential of isolated rat liver mitochondria was diminished in the presence of millimolar concentrations of the known cytochrome c oxidase inhibitor - cyanide. In the experiments, mitochondria were energized by addition of ATP in the presence of rotenone, inhibiting oxidation of endogenous substrates via respiratory complex I. Cyanide also reduced the uncoupling effect of FCCP and DNP on mitochondria energized by succinate in the presence of ferricyanide. Importantly, cyanide did not alter the protonophoric activity of FCCP and DNP in artificial bilayer lipid membranes. The causes of the effect of cyanide on the efficiency of protonophoric uncouplers in mitochondria are considered in the framework of the suggestion that conformational changes of membrane proteins could affect the state of lipids in their vicinity. In particular, changes in local microviscosity and vacuum permittivity could change the efficiency of protonophore-mediated translocation.

A conjugate of decyltriphenylphosphonium with plastoquinone can carry cyclic adenosine monophosphate, but not cyclic guanosine monophosphate, across artificial and natural membranes

The present study demonstrated for the first time the interaction between adenosine 3',5'-cyclic monophosphate (cAMP), one of the most important signaling compounds in living organisms, and the mitochondria-targeted antioxidant plastoquinonyl-decyltriphenylphosphonium (SkQ1). The data obtained on model liquid membranes and human platelets revealed the ability of SkQ1 to selectively transport cAMP, but not guanosine 3',5'-cyclic monophosphate (cGMP), across both artificial and natural membranes. In particular, SkQ1 elicited translocation of cAMP from the source to the receiving phase of a Pressman-type cell, while showing low activity with cGMP. Importantly, only conjugate with plastoquinone, but not dodecyl-triphenylphosphonium, was effective in carrying cAMP. In human platelets, SkQ1 also appeared to serve as a carrier of cAMP, but not cGMP, from outside to inside the cell, as measured by phosphorylation of the vasodilator stimulated phosphoprotein. The SkQ1-induced transfer of cAMP across the plasma membrane found here can be tentatively suggested to interfere with cAMP signaling pathways in living cells.

Blocking of Single α-Hemolysin Pore by Rhodamine Derivatives

Measurements of ion conductance through α-hemolysin pore in a bilayer lipid membrane revealed blocking of the ion channel by a series of rhodamine 19 and rhodamine B esters. The longest dwell closed time of the blocking was observed with rhodamine 19 butyl ester (C4R1), whereas the octyl ester (C8R1) was of poor effect. Voltage asymmetry in the binding kinetics indicated that rhodamine derivatives bound to the stem part of the aqueous pore lumen. The binding frequency was proportional to a quadratic function of rhodamine concentrations, thereby showing that the dominant binding species were rhodamine dimers. Two levels of the pore conductance and two dwell closed times of the pore were found. The dwell closed times lengthened as the voltage increased, suggesting impermeability of the channel for the ligands. Molecular docking analysis revealed two distinct binding sites within the lumen of the stem of the α-hemolysin pore for the C4R1 dimer, but only one binding site for the C8R1 dimer. The blocking of the α-hemolysin nanopore by rhodamines could be utilized in DNA sequencing as additional optical sensing owing to bright fluorescence of rhodamines if used for DNA labeling.

Fast flip–flop of halogenated cobalt bis(dicarbollide) anion in a lipid bilayer membrane

Halogenation dramatically affects the flip–flop of cobalt bis(dicarbollide) across the lipid membrane causing acceleration (Cl, Br, I) or deceleration (F).

Uncoupling and Toxic Action of Alkyltriphenylphosphonium Cations on Mitochondria and the Bacterium Bacillus subtilis as a Function of Alkyl Chain Length

A series of permeating cations based on alkyl derivatives of triphenylphosphonium (C(n)-TPP(+)) containing linear hydrocarbon chains (butyl, octyl, decyl, and dodecyl) was investigated in systems of isolated mitochondria, bacteria, and liposomes. In contrast to some derivatives (esters) of rhodamine-19, wherein butyl rhodamine possessed the maximum activity, in the case of C(n)-TPP a stimulatory effect on mitochondrial respiration steadily increased with growing length of the alkyl radical. Tetraphenylphosphonium and butyl-TPP(+) at a dose of several hundred micromoles exhibited an uncoupling effect, which might be related to interaction between C(n)-TPP(+) and endogenous fatty acids and induction of their own cyclic transfer, resulting in transport of protons across the mitochondrial membrane. Such a mechanism was investigated by measuring efflux of carboxyfluorescein from liposomes influenced by C(n)-TPP(+). Experiments with bacteria demonstrated that dodecyl-TPP(+), decyl-TPP(+), and octyl-TPP(+) similarly to quinone-containing analog (SkQ1) inhibited growth of the Gram-positive bacterium Bacillus subtilis, wherein the inhibitory effect was upregulated with growing lipophilicity. These cations did not display toxic effect on growth of the Gram-negative bacterium Escherichia coli. It is assumed that the difference in toxic action on various bacterial species might be related to different permeability of bacterial coats for the examined triphenylphosphonium cations.

A long-linker conjugate of fluorescein and triphenylphosphonium as mitochondria-targeted uncoupler and fluorescent neuro- and nephroprotector

BACKGROUND

Limited uncoupling of oxidative phosphorylation is known to be beneficial in various laboratory models of diseases. Linking a triphenyl-phosphonium cation to fluorescein through a decyl (C10) spacer yields a fluorescent uncoupler, coined mitoFluo, that selectively accumulates in energized mitochondria (Denisov et al., Chem.Commun. 2014).

METHODS

Proton-transport activity of mitoFluo was tested in liposomes reconstituted with bacteriorhodopsin. To examine the uncoupling action on mitochondria, we monitored mitochondrial membrane potential in parallel with oxygen consumption. Neuro- and nephroprotecting activity was detected by a limb-placing test and a kidney ischemia/reperfusion protocol, respectively.

RESULTS

We compared mitoFluo properties with those of its newly synthesized analog having a short (butyl) spacer (C4-mitoFluo). MitoFluo, but not C4-mitoFluo, caused collapse of mitochondrial membrane potential resulting in stimulation of mitochondrial respiration. The dramatic difference in the uncoupling activity of mitoFluo and C4-mitoFluo was in line with the difference in their protonophoric activity on a lipid membrane. The accumulation of mitoFluo in mitochondria was more pronounced than that of C4-mitoFluo. MitoFluo decreased the rate of ROS production in mitochondria. MitoFluo was effective in preventing consequences of brain trauma in rats: it suppressed trauma-induced brain swelling and reduced a neurological deficit. Besides, mitoFluo attenuated acute kidney injury after ischemia/reperfusion in rats.

CONCLUSIONS

A long alkyl linker was proved mandatory for mitoFluo to be a mitochondria- targeted uncoupler. MitoFluo showed high protective efficacy in certain models of oxidative stress-related diseases.

GENERAL SIGNIFICANCE

MitoFluo is a candidate for developing therapeutic and fluorescence imaging agents to treat brain and kidney pathologies.

Single channel activity of OmpF-like porin from Yersinia pseudotuberculosis

To gain a mechanistic insight in the functioning of the OmpF-like porin from Yersinia pseudotuberculosis (YOmpF), we compared the effect of pH variation on the ion channel activity of the protein in planar lipid bilayers and its binding to lipid membranes. The behavior of YOmpF channels upon acidification was similar to that previously described for Escherichia coli OmpF. In particular, a decrease in pH of the bathing solution resulted in a substantial reduction of YOmpF single channel conductance, accompanied by the emergence of subconductance states. Similar subconductance substates were elicited by the addition of lysophosphatidylcholine. This observation, made with porin channels for the first time, pointed to the relevance of lipid-protein interactions, in particular, the lipid curvature stress, to the appearance of subconductance states at acidic pH. Binding of YOmpF to membranes displayed rather modest dependence on pH, whereas the channel-forming potency of the protein tremendously decreased upon acidification.

Fullerenol C60(OH)24 increases ion permeability of lipid membranes in a pH-dependent manner

Fullerenols are water-soluble analogs of fullerene exhibiting both antioxidant and prooxidant activities in vitro and in vivo. Here we report, for the first time, that fullerenol C60(OH)24 can induce ion permeability of a planar lipid bilayer membrane via the formation of ion pores or conductive defects with a preference for cations over anions. The fullerenol-mediated electrical current displayed non-linear concentration dependence and was reversibly enhanced by alkalinization. Calcium and magnesium ions decreased the fullerenol-induced potassium ion permeability. Voltage dependence of the current was sensitive to membrane composition, with the conductance being well pronounced in fully saturated diphytanoylphosphatidylcholine. Fullerenol did not induce carboxyfluorescein leakage from liposomes, suggesting a small size of fullerenol-induced pores. In contrast to ion permeability, the binding of C60(OH)24 to liposomes increased at acidic pH, as measured by fluorescence quenching of pyrene-labeled lipid. In line with this, the photodynamic action of fullerenol on the peptide gramicidin A also increased at low pH. It is hypothesized that aggregates of fullerenol may stabilize transient conductive lipid defects or pores formed under a variety of stress conditions.

Weak C–H acids as protonophores can carry hydrogen ions through lipid membranes and mitochondria: a case of o-carborane

ortho-Carborane (1,2-C₂B₁₀H₁₂) was found to be a carrier of protons in lipid membranes and mitochondria, suggesting that this dicarborane can reversibly release hydrogen ions and diffuse through the membrane.

Intramitochondrial accumulation of cationic Atto520-biotin proceeds via voltage-dependent slow permeation through lipid membrane

Conjugation to penetrating cations is a general approach for intramitochondrial delivery of physiologically active compounds, supported by a high membrane potential of mitochondria having negative sign on the matrix side. By using fluorescence correlation spectroscopy, we found here that Atto520-biotin, a conjugate of a fluorescent cationic rhodamine-based dye with the membrane-impermeable vitamin biotin, accumulated in energized mitochondria in contrast to biotin-rhodamine 110. The energy-dependent uptake of Atto520-biotin by mitochondria, being slower than that of the conventional mitochondrial dye tetramethyl-rhodamine ethyl ester, was enhanced by the hydrophobic anion tetraphenylborate (TPB). Atto520-biotin also exhibited accumulation in liposomes driven by membrane potential resulting from potassium ion gradient in the presence valinomycin. The induction of electrical current across planar bilayer lipid membrane by Atto520-biotin proved the ability of the compound to permeate through lipid membrane in a cationic form. Atto520-biotin stained mitochondria in a culture of L929 cells, and the staining was enhanced in the presence of TPB. Therefore, the fluorescent Atto520 moiety can serve as a vehicle for intramitochondrial delivery of hydrophilic drugs. Of importance for biotin-streptavidin technology, binding of Atto520-biotin to streptavidin was found to cause quenching of its fluorescence similar to the case of fluorescein-4-biotin.

Methodology for use of mitochondria-targeted cations in the field of oxidative stress-related research

For many pathological conditions, reactive oxygen species (ROS) generated in mitochondria are considered to have a role as a trigger. When mitochondrial ROS (mROS) are formed in the inner mitochondrial membrane, they initiate free radical-mediated chain reactions of lipid peroxidation and are thus especially damaging. The consequences of membrane damage are decreased electrical resistance of the membrane, oxidative damage to cardiolipin (a mitochondria specific lipid essential for functioning of respiratory chain proteins and H(+)-ATP synthase), and damage to mitochondrial DNA localized in close vicinity to the inner membrane, with consequent mitochondrial dysfunction and induction of apoptotic cascade and cell death. To target the starting point of such undesirable events, antioxidants conjugated with mitochondria-targeted, membrane-penetrating cations can be used to scavenge ROS inside mitochondria. The most demonstrative indications favoring this conclusion originate from recent discoveries of the in vivo effects of such cations belonging to the MitoQ and SkQ groups. Here we describe some essential methodological aspects of the application of mitochondria-targeted cations promising in treating oxidative stress-related pathologies.

A short-chain alkyl derivative of Rhodamine 19 acts as a mild uncoupler of mitochondria and a neuroprotector

Limited uncoupling of oxidative phosphorylation is known to be beneficial in various laboratory models of diseases. The search for cationic uncouplers is promising as their protonophorous effect is self-limiting because these uncouplers lower membrane potential which is the driving force for their accumulation in mitochondria. In this work, the penetrating cation Rhodamine 19 butyl ester (C4R1) was found to decrease membrane potential and to stimulate respiration of mitochondria, appearing to be a stronger uncoupler than its more hydrophobic analog Rhodamine 19 dodecyl ester (C12R1). Surprisingly, C12R1 increased H(+) conductance of artificial bilayer lipid membranes or induced mitochondria swelling in potassium acetate with valinomycin at concentrations lower than C4R1. This paradox might be explained by involvement of mitochondrial proteins in the uncoupling action of C4R1. In experiments with HeLa cells, C4R1 rapidly and selectively accumulated in mitochondria and stimulated oligomycin-sensitive respiration as a mild uncoupler. C4R1 was effective in preventing oxidative stress induced by brain ischemia and reperfusion in rats: it suppressed stroke-induced brain swelling and prevented the decline in neurological status more effectively than C12R1. Thus, C4R1 seems to be a promising example of a mild uncoupler efficient in treatment of brain pathologies related to oxidative stress.

Mild uncoupling of respiration and phosphorylation as a mechanism providing nephro- and neuroprotective effects of penetrating cations of the SkQ family

It is generally accepted that mitochondrial production of reactive oxygen species is nonlinearly related to the value of the mitochondrial membrane potential with significant increment at values exceeding 150 mV. Due to this, high values of the membrane potential are highly dangerous, specifically under pathological conditions associated with oxidative stress. Mild uncoupling of oxidative phosphorylation is an approach to preventing hyperpolarization of the mitochondrial membrane. We confirmed data obtained earlier in our group that dodecylrhodamine 19 (C(12)R1) (a penetrating cation from SkQ family not possessing a plastoquinone group) has uncoupling properties, this fact making it highly potent for use in prevention of pathologies associated with oxidative stress induced by mitochondrial hyperpolarization. Further experiments showed that C(12)R1 provided nephroprotection under ischemia/reperfusion of the kidney as well as under rhabdomyolysis through diminishing of renal dysfunction manifested by elevated level of blood creatinine and urea. Similar nephroprotective properties were observed for low doses (275 nmol/kg) of the conventional uncoupler 2,4-dinitrophenol. Another penetrating cation that did not demonstrate protonophorous activity (SkQR4) had no effect on renal dysfunction. In experiments with induced ischemic stroke, C(12)R1 did not have any effect on the area of ischemic damage, but it significantly lowered neurological deficit. We conclude that beneficial effects of penetrating cation derivatives of rhodamine 19 in renal pathologies and brain ischemia may be at least partially explained by uncoupling of oxidation and phosphorylation.

Boronated derivatives of chlorin e(6) and fluoride-containing porphyrins as penetrating anions: a study using bilayer lipid membranes

Boronated derivatives of porphyrins are studied extensively as promising compounds for boron-neutron capture therapy and photodynamic therapy. Understanding of the mechanism of their permeation across cell membranes is a key step in screening for the most efficient compounds. In the present work, we studied the ability of boronated derivatives of chlorin e(6) and porphyrins, which are mono-, di-, and tetra-anions, to permeate through planar bilayer lipid membranes (BLM). The translocation rate constants through the hydrophobic part of the lipid bilayer were estimated for monocarborane and its conjugate with chlorin e(6) by the method of electrical current relaxation. They were similar, 6.6 and 6.8 sec(-1), respectively. Conjugates of porphyrins carrying two and four carborane groups were shown to permeate efficiently through a BLM although they carry two charges and four charges, respectively. The rate of permeation of the tetraanion estimated by the BLM current had superlinear dependence on the BLM voltage. Because the resting potential of most mammalian cells is negative inside, it can be concluded that the presence of negatively-charged boronated groups in compounds should hinder the accumulation of the porphyrins in cells.

Unsaturated lipids protect the integral membrane peptide gramicidin A from singlet oxygen

In contrast to expectations that unsaturated fatty acids contribute to oxidative stress by providing a source of lipid peroxides, we demonstrated the protective effect of double bonds in lipids on oxidative damage to membrane proteins. Photodynamic inactivation of gramicidin channels was decreased in unsaturated lipid compared to saturated lipid bilayers. By estimating photosensitizer (boronated chlorine e6 amide) binding to the membrane with the current relaxation technique, the decrease in gramicidin photoinactivation was attributed to singlet oxygen scavenging by double bonds in lipids rather than to the reduction in photosensitizer binding. Gramicidin protection by unsaturated lipids was also observed upon induction of oxidative stress with tert-butyl hydroperoxide.