Transmembrane electrical potential formation in spinach chloroplasts

Cationic penetrating antioxidants switch off Mn cluster of photosystem II in situ

Mitochondria-targeted antioxidants (also known as 'Skulachev Ions' electrophoretically accumulated by mitochondria) exert anti-ageing and ROS-protecting effects well documented in animal and human cells. However, their effects on chloroplast in photosynthetic cells and corresponding mechanisms are scarcely known. For the first time, we describe a dramatic quenching effect of (10-(6-plastoquinonyl)decyl triphenylphosphonium (SkQ1) on chlorophyll fluorescence, apparently mediated by redox interaction of SkQ1 with Mn cluster in Photosystem II (PSII) of chlorophyte microalga Chlorella vulgaris and disabling the oxygen-evolving complex (OEC). Microalgal cells displayed a vigorous uptake of SkQ1 which internal concentration built up to a very high level. Using optical and EPR spectroscopy, as well as electron donors and in silico molecular simulation techniques, we found that SkQ1 molecule can interact with Mn atoms of the OEC in PSII. This stops water splitting giving rise to potent quencher(s), e.g. oxidized reaction centre of PSII. Other components of the photosynthetic apparatus proved to be mostly intact. This effect of the Skulachev ions might help to develop in vivo models of photosynthetic cells with impaired OEC function but essentially intact otherwise. The observed phenomenon suggests that SkQ1 can be applied to study stress-induced damages to OEC in photosynthetic organisms.

Ion transport across a phospholipid membrane mediated by the peptide trichogin GA IV

Trichogin GA IV is a special member of a class of peptaibols that are linear peptide antibiotics of fungal origin, characterised by the presence of a variable number of alpha-aminoisobutyric acid residues, an acyl group at the N-terminus and a 1,2-amino alcohol at the C-terminus. Most of the peptaibols display ion-channel-forming or at least membrane-modifying properties. The 11-residue-long trichogin GA IV is not only one of shortest peptaibols, but it is also unique for its n-octanoyl group instead of the more common found acetyl group at the N-terminus. For the first time we have found that this lipopeptaibol is able to enhance conduction of monovalent cations through membranes of large unilamellar vesicles (LUVs). The influence of the [Leu-OMe]trichogin GA IV analogue (TRI) on ion permeation was studied under a variety of conditions (lipid composition, lipid-to-peptide ratio and a transmembrane potential). Parallel experiments were performed with the 16-residue long, channel-forming peptaibol, zervamicin (ZER). For the two peptides, the permeability between K(+) and Na(+) was found to be different. In addition, the ion diffusion rate dependencies on the peptide concentration are observed to be different. This might indicate that a different number of aggregated molecules are involved in the rate-limiting step, i.e. 3-4 (TRI) and 4-7 (ZER). In the presence of TRI, dissipation of the transmembrane potential, Delta psi, was observed with a rate to be dependent on the magnitude of both initial Delta psi and peptide concentration. Both peptides were activated by a cis-positive but not by cis-negative Delta psi. Under identical conditions the ion-conducting efficiency of zervamicin was 100-200 times higher than that of trichogin. Our results show that, unlike for zervamicin, the membrane-modifying activity of trichogin is not associated with a channel mechanism.

Fluorescent probes for non-invasive bioenergetic studies of whole cyanobacterial cells

Fluorescent DeltapH and DeltaPsi indicators have been screened for the non-invasive monitoring of bioenergetic processes in whole cells of the cyanobacterium Synechocystis sp. PCC 6803. Acridine yellow and Acridine orange proved to be the best DeltapH indicators for the investigation of thylakoid and cytoplasmic membrane energization: While Acridine yellow indicated only cytosolic energization, Acridine orange showed signals from both the thylakoid lumen and the cytosol that could be separated kinetically. Both indicators were applied successfully to monitor cellular energetics, such as the interplay of linear and cyclic photosynthetic electron transport, osmotic adaptation and solute transport across the cytoplasmic membrane. In contrast, useful membrane potential indicators were more difficult to find, with Di-4-ANEPPS and Brilliant cresyl blue being the only promising candidates for further studies. Finally, Acridine yellow and Acridine orange could also be applied successfully for the thermophilic cyanobacterium Synechococcus elongatus. Different from Synechocystis sp. PCC 6803, where both respiration and ATP hydrolysis could be utilized for cytoplasmic membrane energization, proton extrusion at the cytoplasmic membrane in Synechococcus elongatus was preferentially driven by ATP hydrolysis.

Voltage-dependent interaction of the peptaibol antibiotic zervamicin II with phospholipid vesicles

The effect of a transmembrane potential on ion channel formation by zervamicin II (ZER-II) was studied in a vesicular model system. The dissipation of diffusion potential caused by addition of ZER-II to small phosphatidylcholine vesicles was monitored using fluorescent (Safranine T) and optical (Oxonol YI) probes. Cis-positive potentials facilitated channel formation, while at cis-negative potentials, ion fluxes were inhibited. A potential-independent behavior of ZER-II was observed at high peptide concentrations, most likely due to its membrane modifying property.

Patch-clamp study of vascular plant chloroplasts: ion channels and photocurrents

This article presents direct measurements of large-conductance cation channel (G = 730 pS in 100/50 mM KCl; PK+/PCl- = 2.8) and light-induced current (photocurrent) in chloroplasts from C4 plants, Amaranthus hybridus and Zea mays, using the conventional patch-clamp technique. It was shown that preillumination of chloroplast gave rise to a fast decaying transient (tau approximately equal to 6 ms) in the light-induced current for the second and following light pulses. This transient increase of photocurrent was interpreted as a consequence of photoreducing of the redox pool between photosystems.

Electrogenic and electroneutral transport modes of renal Na/K ATPase reconstituted into proteoliposomes

This paper describes measurements of electrical potentials generated by renal Na/K-ATPase reconstituted into proteoliposomes, utilizing the anionic dye, oxonol VI. Calibration of absorption changes with imposed diffusion potentials allows estimation of absolute values of electrogenic potentials. ATP-dependent Nacyt/Kexc exchange in K-loaded vesicles generates large potentials, up to 250 mV. By comparing initial rates or steady-state potentials with ATP-dependent 22Na fluxes in different conditions, it is possible to infer whether coupling ratios are constant or variable. For concentrations of Nacyt (2-50 mM) and ATP (1-1000 microM) and pH's (6.5-8.5), the classical 3Nacyt/2Kexc coupling ratio is maintained. However, at low Nacyt concentrations (less than 0.8 mM), the coupling ratio is apparently less than 3Nacyt/2Kexc. ATP-dependent Nacyt/congenerexc exchange in vesicles loaded with Rb, Cs, Li and Na is electrogenic. In this mode congeners, including Naexc, act as Kexc surrogates in an electrogenic 3Nacyt/2congenerexc exchange. (ATP + Pi)-dependent Kcyt/Kexc exchange in K-loaded vesicles is electroneutral. ATP-dependent "uncoupled" Na flux into Na- and K-free vesicles is electroneutral at pH 6.5-7.0 but becomes progressively electrogenic as the pH is raised to 8.5. The 22Na flux shows no anion specificity. We propose that "uncoupled" Na flux is an electroneutral 3Nacyt/3Hexc exchange at pH 6.5-7.0 but at higher pH's the coupling ratio changes progressively, reaching 3Na/no ions at pH 8.5. Slow passive pump-mediated net K uptake into Na- and K-free vesicles is electroneutral, and may also involve Kcyt/Hexc exchange. We propose the general hypothesis that coupling ratios are fixed when cation transport sites are saturated, but at low concentrations of transported cations, e.g., Nacyt in Na/K exchange and Hexc in "uncoupled" Na flux, coupling ratios may change.

Oxonol VI as an optical indicator for membrane potentials in lipid vesicles

Experiments with large unilamellar dioleoylphosphatidylcholine vesicles were carried out in order to study the effect of membrane potential on the fluorescence of Oxonol VI. A partition equilibrium of dye between membrane and water was found to exist with a partition coefficient gamma identical to c lipid/c water of about 19,000 (at zero voltage). In the presence of an inside-positive membrane potential, the negatively charged dye accumulates in the intravesicular aqueous space according to a Nernst equilibrium. This leads to an increased adsorption of dye to the inner lipid monolayer and to a concomitant increase of fluorescence. The fluorescence change can be calibrated as a function of transmembrane voltage by generating a potassium diffusion potential in the presence of valinomycin. The intrinsic fluorescence of the membrane-bound dye is not affected by voltage; the whole influence of voltage on the fluorescence results from voltage-dependent partitioning of the dye between water and membrane. The voltage dependence of the apparent partition coefficient can be quantitatively described by a three-capacitor model in which the dye is assumed to bind to adsorption planes located on the hydrocarbon side of the membrane/solution interface. Oxonol VI was found to be suitable for detecting changes of membrane potential associated with the activity of the (Na+ + K+)-ATPase in reconstituted vesicles. When ATP is added to the external medium, pump molecules with the ATP-binding side facing outward become activated; this results in a translocation of net positive charge towards the vesicle interior. Under this condition, fluorescence changes corresponding to (inside-positive) potentials of up to 150-200 mV are observed. After the build-up of the membrane potential, a quasi-stationary state is reached in which the pump current is compensated by a back-flow of charge through passive conductance pathways.

The interaction of potential-sensitive molecular probes with dimyristoylphosphatidylcholine vesicles investigated by 31P-NMR and electron microscopy

The effect of a number of commonly employed potential-sensitive molecular probes on the 31P-NMR properties of dimyristoylphosphatidylcholine vesicles at two field strengths has been investigated in order to obtain information on the location and effect of these probes on the membrane bilayer. In comparison to the control dye-free vesicle spectrum, the probes diS-C3-(5) and diS-C4-(5), when added to a vesicle suspension, cause a substantial broadening of the 31P resonance with no detectable chemical shift within an uncertainty of +/- 0.05 ppm at 24 MHz. The spin-lattice and spin-spin relaxation times are also reduced when the cyanines are present by well over 20% relative to those of the control vesicle preparation. The addition of anionic probes, including several oxonol derivatives and merocyanine 540, causes no chemical shift, line broadening, or changes in the relaxation times. Possible explanations for the failure of the anionic probes to alter the vesicle 31P-NMR properties include charge repulsion between these dyes and the phosphate group that prevents the probes from penetrating the bilayer to a depth sufficient to alter the local motion of the phosphate moiety. The 31P resonance broadening and reduction in the relaxation times caused by the two cyanines is at least in part due to an increase in vesicle size as judged by electron microscopy measurements, although an inhibition of the local phosphate motion as well cannot be completely eliminated. The cyanine-mediated increase in vesicle size appears to be due to an irreversible vesicle-fusion process possibly initiated by the screening of surface charge by these probes. The implications of these observations in relation to functional energy-transducing preparations is discussed.

Reverse electron flow in chloroplasts

Energy dependent reverse electron flow reactions in isolated thylakoids provide a unique tool to study, in the dark, the coupling between the ATP synthase, proton transport and the electron transfer system. Appropriate experimental conditions have been established to follow experimentally the following reactions: 1. ATP driven proton uptake into the inner-thylakoid space, which requires preactivation of the ATP synthase. 2. ATP driven reverse electron transport, which involves proton transport as an intermediate, and results in the reduction of QA by an externally added electron donor. 3. ATP driven luminescence, which requires the presence of an oxidized partner on the water side of photosystem II, and involves electron transport from QB to QA. 4. ΔpH driven reverse electron flow, which does not require the participation of the ATP synthase, and uses reduced intermediates between the two photosystems as electron donors for the reduction of QA. 5. ΔpH driven luminescence which again uses reduced intermdiates between the two photosystems as electron donors for QA reduction, and requires the presence of an oxidized partner on the water side of photosystem II. Several of these reactions have been shown to occur in intact chloroplasts and may provide an important regulatory mechanism in vivo.

Oxonol dyes as monitors of membrane potential: the effect of viruses and toxins on the plasma membrane potential of animal cells in monolayer culture and in suspension

Optical indicators of the cationic, cyanine and anionic oxonol classes were used to evaluate the plasma membrane potential of animal cells in suspension and in monolayer culture. The optical signals were calibrated by using diffusion potentials either of K+ (in the presence of valinomycin) or of H+ (in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone; FCCP); both classes of dye gave similar values of plasma membrane potential, in the range -40 to -90 mV for different cell types. Addition of haemolytic Sendai virus or Staphylococcus aureus alpha-toxin depolarizes cells and causes them to leak monovalent cations; these effects are antagonized by extracellular Ca2+. Cells infected with vesicular stomatitis or Semliki Forest virus become depolarized during an infectious cycle; infection with other viruses was without affect on plasma membrane potential.

The use of carotenoids and oxonol VI as probes for membrane potential in proteoliposomes

Carotenoids present in lipids extracted from the cyanobacterium Synechococcus 6716 indicate trans-membrane potential in proteoliposomes reconstituted from these lipids and the ATPase complex isolated from the same organism. A carotenoid absorbance band shift to a longer wavelength is obtained with valinomycin-induced potassium ion diffusion potentials, irrespective of the polarity of the potassium gradient. In contrast to this, the (externally added) probe oxonol VI only shows an absorbance band shift when the external potassium ion concentration is higher than the internal one. In liposomes without ATPase complex, no carotenoid absorbance band shifts were observed.

A minimal hypothesis for membrane-linked free-energy transduction. The role of independent, small coupling units

Experimental data are reviewed that are not in keeping with the scheme of 'delocalized' protonic coupling in membrane-linked free-energy transduction. It turns out that there are three main types of anomalies: (i) rates of electron transfer and of ATP synthesis do not solely depend on their own driving force and on delta mu H, (ii) the ('static head') ratio of delta Gp to delta mu H varies with delta mu H and (iii) inhibition of either some of the electron-transfer chains or some of the H+-ATPases, does not cause an overcapacity in the other, non-inhibited proton pumps. None of the earlier free-energy coupling schemes, alternative to delocalized protonic coupling, can account for these three anomalies. We propose to add a fifth postulate, namely that of the coupling unit, to the four existing postulates of 'delocalized protonic coupling' and show that, with this postulate, protonic coupling can again account for most experimental observations. We also discuss: (i) how experimental data that might seem to be at odds with the 'coupling unit' hypothesis can be accounted for and (ii) the problem of the spatial arrangement of the electrical field in the different free-energy coupling schemes.

Proton movements and electric potential generation in reconstituted ATPase proteoliposomes from the thermophilic cyanobacterium Synechococcus 6716

ATP hydrolysis-induced proton translocation and electric potential generation have been studied in ATPase proteoliposomes by means of various optical probes. The proteoliposomes consisted of reconstituted ATPase complex and native lipid mixture isolated from the thermophilic cyanobacterium Synechococcus 6716 [Van Walraven et al. (1983) Eur. J. Biochem. 137, 101-106]. The native cartenoids and added oxonol VI served as probes for the electric membrane potential generated by the net charge separation (negative outside, positive inside). Their responses, with similar half-times as 9-tetradecylamino-6-chloro-2-methoxyacridine, are sensitive to valinomycin and stimulated by nigericin, as expected. The proton concentrations of extraliposomal and intraliposomal aqueous spaces were monitored by neutral red and cresol red; for internal measurements these pH indicators were trapped inside the vesicles during detergent dialysis. Internal acidification and external alkalinization induced by ATP hydrolysis are inhibited by nigericin and enhanced by valinomycin; at the commonly used higher valinomycin concentrations the neutral red response becomes transient, while the much slower cresol red response is diminished right from its onset. At smaller preset pH gradients both ATP hydrolysis activity and neutral red response are diminished. At increasing MgCl2 concentrations the neutral red responses are slowed down and the cresol red responses are slightly enhanced; this is observed for both internal and external dye responses. Neutral red permeation through the membrane is insignificant under our experimental conditions but is enhanced at temperatures below the lipid-phase transition. In the case of externally added neutral red the non-permeant buffer Hepes is only effective at high MgCl2 concentration, whereas some external cresol red response is visible only at high MgCl2 concentration in the presence of Hepes. The kinetics of the pH indicator and electric potential probe responses clearly distinguish fast interfacial and intra-membrane proton displacements from slow bulk proton equilibration. The data are summarized in a model that supports the importance of localized proton displacements for the primary energy-transducing events.

ATP-dependent spectral response of oxonol VI in an ATP-Pi exchange complex

Energy transduction in an ATPase complex (complex V) has been studied in two reactions catalyzed by this system, i.e., ATP-dependent spectral shift of oxonol VI, and ATP-Pi exchange activity. Aurovertin alone inhibits 50% of the oxonol shift at 2 microM, and no further inhibition occurs at up to 12 microM. In combination with even weakly effective uncouplers, 4 microM aurovertin fully abolishes the oxonol response. No such effects are observed in the presence of oligomycin and uncouplers. No pH gradient is detectable by quenching of 9-amino-6-chloro-2-methoxyacridine; and nigericin is without effect on the oxonol response. Valinomycin is inhibitory even in the absence of added potassium, due to ammonium ions introduced during the purification steps. Thiocyanate inhibits the dye response by only 10-27%, depending on the preparation. The extent of the oxonol response depends on the ATP/ADP ratio rather than the phosphorylation potential. The dye response in the ATPase complex is 4-7-times less sensitive to bile salts than in submitochondrial particles. The inhibition by cardiolipin can be reversed by the addition of phospholipids. The possibility is discussed that the oxonol response in the ATPase complex reflects, at least in part, a more local, ATP-dependent and energy-related process.

Uncoupler-stimulated Na+ pump and its possible role in the halotolerant bacterium, Ba

In cells of Ba1 suspended in K salt as the osmoticum, the respiratory rate declined by 80% between the pH values of 6.5 and 8.5. Catalytic amounts of Na+ ions prevented this drop. The possibility that Na+ exerted its effect by an influence on proton fluxes across the membrane (Na+/H+ exchange) was explored. Addition of catalytic amounts of Na+ ions to cells respiring at pH 8.5 elicited an influx of protons and, as a result, the delta pH across the membrane became diminished. delta psi (membrane potential) was not affected by Na+. At pH 6.5, Na+ caused no proton influx. FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) collapsed delta psi, but the Na+-dependent proton influx observed at pH 8.5 became enhanced, leading to an inversion of delta pH (more acid inside). When a Na salt was used as the osmoticum, delta pH of reversed polarity was generated by respiration also in the absence of FCCP. Respiring, inverted membrane vesicles responded to a Na+ pulse essentially as the intact cells. Based on the above and some additional findings it is suggested that these Na+-dependent effects are suited to prevent a raise in the intracellular pH over the level which hinders the respiratory activity. It may also play a role in the regulation of intracellular Na salt content.

Characterization of reconstituted ATPase complex proteoliposomes prepared from the thermophilic cyanobacterium Synechococcus 6716

The preparation and some properties are described of proteoliposomes consisting of the ATPase complex and lipids from the thermophilic cyanobacterium Synechococcus 6716. In the proteoliposomes (about 200 nm in diameter) only a low amount of protein can be incorporated (protein/lipid ratio of 0.01 w/w) and they show very few protein particles on freeze-fracture replicas. The octyl glucoside and cholate dialysis method of reconstitution yielded stable proteoliposomes with a relatively low proton permeability. ATP hydrolysis and 32Pi/ATP exchange activities were about 400 and 120 nmol X min-1 X mg protein-1, respectively; the former was strongly stimulated by an uncoupler. ATP hydrolysis induces membrane energization as monitored by membrane-potential- and surface-potential-indicating probes and by different pH indicators trapped inside the vesicles. The probes used were a membrane-bound fluorescent aminoacridine, which monitors surface charge-density changes, the native carotenoids and added oxonol VI for monitoring electrical potential in the membrane and the pH indicators neutral red and cresol red. The different rise kinetics of these probes indicate that proton accumulation upon ATP hydrolysis involves at least two steps: a membrane-localized potential charge and proton transfer followed by a much slower acidification of the bulk intravesicular space. Internal neutral red and cresol red seem to discriminate between proton translocation to the internal interface and bulk space, respectively.

A time lag in the onset of ATP-Pi exchange catalyzed by purified ATP-synthase (CF0-CF1) proteoliposomes and by chloroplasts

The time course of ATP-Pi exchange which is catalyzed by the isolated chloroplast ATP synthase in phospholipid vesicles was studied. The following observations were made. (i) The onset of 32Pi incorporation into ATP lags behind ATP hydrolysis. The lag lasts for about 2 min at 37 degrees C and is followed by a steady-state rate which is constant for more than 30 min. Under the same experimental conditions, ATP hydrolysis shows an initial burst followed by a constant, slower rate. (ii) The initial lag is independent of Mg-ATP concentration in the range 0.2-5 mM and of the presence of ADP. In contrast, the steady-state rate of ATP-Pi exchange has an apparent Km of 0.3 mM for Mg-ATP and is stimulated by ADP. (iii) Increasing the temperature from 30 to 45 degrees C decreases the lag from 6 min to zero. The steady-state rate of ATP-Pi exchange is affected to a much smaller extent by the temperature in this range. (iv) The lag is insensitive to valinomycin or tetraphenylboron, while the steady-state rate is partially inhibited. Nigericin and protonophores affect both the lag and steady-state rate. (v) ATP-induced membrane potential formation, as followed by oxonol VI, does not correlate with the lag in its kinetics and temperature dependence. ATP-induced pH gradient formation could not be detected in the proteoliposome system. (vi) Light-triggered ATP-Pi exchange in chloroplasts shows essentially the same time course as the proteoliposome system, but the lag lasts for only about 20 s at room temperature and is unaffected by a preexisting proton gradient. These results suggest that the initial lag in ATP-Pi exchange does not reflect the time required for the buildup of a protomotive force (delta - mu H+) nor the time required to produce ADP. It is suggested, therefore, that the lag reflects an internal autocatalytic conformational change in the ATP-synthase complex which is initiated by ATP hydrolysis and which converts the enzyme from an "exclusive ATPase state" to a "reversible ATP-synthase state". This slow transition is not directly coupled to a trans-membrane pH or potential gradient.