The 'delta pH'-probe 9-aminoacridine: response time, binding behaviour and dimerization at the membrane

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

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.

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

Photosystem I (PSI) complexes can support a light-driven electrochemical gradient for protons, which is the driving force for energy-conserving reactions across biological membranes. In this work, a computational model that enables a quantitative description of the light-induced proton gradients across the membrane of PSI proteoliposomes is presented. Using a set of electrodiffusion equations, a compartmental model of a vesicle suspended in aqueous medium was studied. The light-mediated proton movement was modeled as a single proton pumping step with backpressure of the electric potential. The model fits determinations of pH obtained from PSI proteoliposomes illuminated in the presence of mediators of cyclic electron transport. The model also allows analysis of the proton gradients in relation to the transmembrane ion fluxes and electric potential. Sensitivity analysis enabled a determination of the parameters that have greater influence on steady-state levels and onset/decay rates of transmembrane pH and electric potential. This model could be used as a tool for optimizing PSI proteoliposomes for photo-electrochemical applications.

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pHin established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pHout=7.8, we found that pHin approximately 5.4-5.7 in the state of photosynthetic control, and pHin approximately 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference DeltapH approximately 1.8-2.1. These values of DeltapH are consistent with a point of view that under steady-state conditions the proton gradient DeltapH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H+/ATP is n> or =4-4.7.

Interaction of antitumoral 9-aminoacridine drug with DNA and dextran sulfate studied by fluorescence and surface-enhanced Raman spectroscopy

Fluorescence spectroscopy and surface-enhanced Raman spectroscopy are applied to study the interaction of the drug 9-aminoacridine (9AA) with DNA and dextran sulfate. The effect of the electrostatic interaction between the positively charged 9AA and negatively charged groups in relation to the excimer or exciplex emission is investigated. The exciplex emission of 9AA is connected to the intercalation of this drug between nucleic base residues. The importance of negative groups in this interaction is evaluated by using dextran and dextran sulfate as model polymers. The existence of negative charges seems to induce an increase of the drug concentration in the vicinity of the polymers. The role of electrostatic attraction in the 9AA dimerization is confirmed by the excimer emission of 9AA in the presence of dextran sulfate. In the case of DNA, the phosphate groups may induce the drug approach to the DNA chain, but the exciplex fluorescence emission could be due to a charge transfer between the drug and adenine-rich sequences of DNA.

Energetics of protein transport across biological membranes. a study of the thylakoid DeltapH-dependent/cpTat pathway

Among the pathways for protein translocation across biological membranes, the DeltapH-dependent/Tat system is unusual in its sole reliance upon the transmembrane pH gradient to drive protein transport. The free energy cost of protein translocation via the chloro-plast DeltapH-dependent/Tat pathway was measured by conducting in vitro transport assays with isolated thylakoids while concurrently monitoring energetic parameters. These experiments revealed a substrate-specific energetic barrier to cpTat-mediated transport as well as direct utilization of protons from the gradient, consistent with a H+/protein antiporter mechanism. The magnitude of proton flux was assayed by four independent approaches and averaged 7.9 x 10(4) protons released from the gradient per transported protein. This corresponds to a DeltaG transport of 6.9 x 10(5) kJ.mol protein translocated(-1), representing the utilization of an energetic equivalent of 10(4) molecules of ATP. At this cost, we estimate that the DeltapH-dependent/cpTat pathway utilizes approximately 3% of the total energy output of the chloroplast.

Distinguishing between luminal and localized proton buffering pools in thylakoid membranes

The dual gradient energy coupling hypothesis posits that chloroplast thylakoid membranes are energized for ATP formation by either a delocalized or a localized proton gradient geometry. Localized energy coupling is characterized by sequestered domains with a buffering capacity of approximately 150 nmol H(+) mg(-1) chlorophyll (Chl). A total of 30 to 40 nmol mg(-1) Chl of the total sequestered domain buffering capacity is contributed by lysines with anomolously low pK(a)s, which can be covalently derivatized with acetic anhydride. We report that in thylakoid membranes treated with acetic anhydride, luminal acidification by a photosystem I (duraquinol [DQH(2)] to methyl viologen [MV]) proton pumping partial reaction was nearly completely inhibited, as measured by three separate assays, yet surprisingly, H(+) accumulation still occurred to the significant level of more than 100 nmol H(+) mg Chl(-1), presumably into the sequestered domains. The treatment did not increase the observed rate constant of dark H(+) efflux, nor was electron transport significantly inhibited. These data provide support for the existence of a sequestered proton translocating pathway linking the redox reaction H(+) ion sources with the CF(0) H(+) channel. The sequestered, low-pK(a) Lys groups appear to have a role in the H(+) diffusion process and chemically modifying them blocks the putative H(+) relay system.

Fluorescence and surface-enhanced Raman study of 9-aminoacridine in relation to its aggregation and excimer emission in aqueous solution and on silver surface

Fluorescence spectroscopy and surface-enhanced Raman spectroscopy (SERS) have been applied to study the aggregation and excimer emission of 9-aminoacridine (9AA) and 9-aminoacridine hydrochloride (9AA-HCl) in aqueous solution and on silver colloids. The effect of the drug concentration, pH, and chloride concentration on these processes has been investigated. The excimer emission of 9AA is connected to the dimerization of this drug in solution: the formation of 9AA dimers is greatly favored when the drug is under the amino form at neutral and acidic pH, while at alkaline pH the imino 9AA form tends to form large-size aggregates which cannot be excited to render excimer emission. 9AA is adsorbed on the silver surface under two different forms: strongly and weakly attached 9AA, each one corresponding to the different drug tautomers: imino and amino. The interaction of 9AA with silver induces a charge transfer from the adsorbate to the metal leading to a remarkable fluorescence quenching, a basicity decrease of the adsorbed drug and a considerable weakening of the dimer-excimer emission. Furthermore, an attribution of the main Raman features appearing in the SERS spectra has been proposed, providing marker bands for the imino and amino 9AA tautomers, and a mechanism for the molecular dimerization is also suggested.

A quantitative model for using acridine orange as a transmembrane pH gradient probe

Monitoring the acidification of the internal space of membrane vesicles by proton pumps can be achieved easily with optical probes. Transmembrane pH gradients cause a blue-shift in the absorbance spectrum and the quenching of the fluorescence of the cationic dye acridine orange. It has been postulated that these changes are caused by accumulation and aggregation of the dye inside the vesicles. We tested this hypothesis using liposomes with transmembrane concentration gradients of ammonium sulfate as model system. Fluorescence intensity of acridine orange solutions incubated with liposomes was affected by magnitude of the gradient, volume trapped by vesicles, and temperature. These experimental data were compared to a theoretical model describing the accumulation of acridine orange monomers in the vesicles according to the inside-to-outside ratio of proton concentrations, and the intravesicular formation of sandwich-like piles of acridine orange cations. This theoretical model predicted quantitatively the relationship between the transmembrane pH gradients and spectral changes of acridine orange. Therefore, adequate characterization of aggregation of dye in the lumen of biological vesicles provides the theoretical basis for using acridine orange as an optical probe to quantify transmembrane pH gradients.

Monovalent cations differentially affect membrane surface properties and membrane curvature, as revealed by fluorescent probes and dynamic light scattering

The effects of monovalent cations on the interfacial electrostatic potential (psi d), hydrodynamic shear boundary distance (ds), and membrane curvature were studied in large unilamellar phospholipid and galacto/sulfolipid liposomes containing different fractions of negatively charged lipids. The differential effects of alkali metal ions on psi d could be accurately determined at physiological surface charge densities with a surface-anchored fluorescent probe. Li+ and Na+ more effectively decrease psi d and exhibit higher association constants (Kas) than K+ and Cs+. These two groups of cations display qualitatively different perturbations of the interfacial structure. Combining Kas values with the electrokinetic (zeta) potentials yielded the respective ds values. At low ionic strength ds more substantially increases with Li+ or Na+ than with K+ or Cs+. Increasing surface charge density causes increased membrane curvature in the presence of K+ or Cs+, but this is largely prevented by Li+ or Na+. Membrane binding of the amphiphilic cation acridine orange decreases surface charge and membrane curvature more extensively than H3O+, Li+, and Na+. The differential interface-perturbing behavior of monovalent cations is discussed with regard to their different hydration tendencies that will modulate the extent and stability of the hydrogen-bond network along the charged membrane surface.

Characterization of 9-aminoacridine interaction with chromatophore membranes and modelling of the probe response to artificially induced transmembrane delta pH values

We analyze the adsorption of the fluorescent monoamine 9-aminoacridine to the membrane phase of photosynthetic chromatophores, in the physiological interval of pH values ranging from 5.5 to 8.5 and at ionic strengths of 0.005 and 0.150 M. The interaction of the probe with the membrane phase is described with S-shaped isotherms of the Hill type and is modulated by electrostatic effects as modelled with the Gouy-Chapman-Boltzman theory. This description is consistent with different values of the surface change density of the chromatophore membranes decreasing from about 1.3 x 10(-3) to about 0.5 x 10(-3) e-/A2, on changing the pH from 8.5/7.5 to 6.5/5.5, respectively. Furthermore we show that, when the free concentrations of the probe in the inner and outer vesicle compartments are computed from the adsorbing isotherms at the proper pH values, the model considering the equilibrium distribution of the neutral monoamine following the onset of a delta pH is sufficient to describe the dependence of the artificially induced transmembrane delta pH values on the observed quenching of the probe fluorescence.

The delta pH-driven, ATP-independent protein translocation mechanism in the chloroplast thylakoid membrane. Kinetics and energetics

Previous studies have shown that proteins are transported across the chloroplast thylakoid membrane by two very different mechanisms, one of which requires stromal factors and ATP, whereas the other mechanism is ATP independent but completely reliant on the thylakoidal delta pH. We have examined the role of the delta pH in the latter mechanism by simultaneously monitoring the magnitude of delta pH (by 9-aminoacridine fluorescence quenching) and the rate of import of the 23-kDa photosystem II protein into isolated pea thylakoids. We show that protein import can take place, at low but significant rates, at very low values of delta pH (in the region of 1.2-1.4), and that plots of the rate of protein import against proton concentration gradient are probably hyperbolic in nature. There is no evidence for a threshold level of delta pH which is required to drive translocation of the 23-kDa protein. Addition of uncouplers midway during import incubations results in a rapid and complete inhibition of translocation, showing that the continuous presence of the delta pH is required for translocation to take place. During import into intact chloroplasts, the intermediate-size 23-kDa protein substrate for the thylakoidal protein transport machinery is found only in the stromal fraction at all values of delta pH, suggesting that the initial interaction with the machinery is relatively weak, reversible and delta pH-independent. We therefore propose that the delta pH is required for both the initiation and completion of translocation; these roles are in marked contrast to the roles of protonmotive force in mitochondrial and sec-dependent bacterial protein transport.

Cercospora beticola toxins. VII. Fluorometric study of their interactions with biological membranes

The interactions of two beticolins, Cercospora beticola toxins, and of their magnesium complexes with liposomes or plasma membrane were studied. The fluorometric pH titration curves of beticolins in liposomes and in plasma membranes reveal the presence of the dissociated form of beticolins. The concentration of the magnesium complex in these membranes increases at high pH. The partition coefficient of beticolin-1 on liposomes is 3-fold higher than that of beticolin-2 and the fluorescence of both compounds on liposomes is similar. The addition of magnesium to liposomes causes a 40-fold and 20-fold increase in the partition coefficient of beticolin-1 and -2, respectively, as a result of the interactions between membrane, magnesium and beticolins. Beticolins react to a delta pH across the liposome membrane but the formation of the magnesium complex completely abolishes this effect.

The proton pumping activity of H(+)-ATPases: an improved fluorescence assay

A new method for the estimation of steady-state delta pH, and the rate of acidification, by H(+)-ATPases (and other proton transporters) in inverted membrane vesicles is described. The method is based on a combination of two widely used fluorescent delta pH probes, 9-aminoacridine and 9-amino-6-chloro-2-methoxyacridine. It is demonstrated that 9-amino-6-chloro-2-methoxyacridine fluorescence quenching, which is very sensitive to small pH gradients, is not sensitive to the magnitude of large pH gradient, while 9-aminoacridine, which does not sense small gradients, is very sensitive to large pH gradients. A proper mixture of the two probes provides a method which is equally sensitive to pH gradients from very small values up to 3.5 pH units. The probe response was evaluated by titrations of the fluorescence signal with nigericin and adjusted by changing the concentration ratio and the emission wavelength. In liposomes, submitochondrial particles and bacterial vesicles an almost linear dependence of quenching on delta pH over the entire range can be obtained with this method. It is demonstrated that the new method can be used to obtain more reliable estimates of the rate of acidification as well as the magnitude of delta pH, whereas each of these and similar probes, by themselves are not as reliable. A determination of the ratio delta Gp/delta muH over a wide range of values reveal that this ratio is not constant but decreases with delta Gp. This finding should be taken into consideration when attempting to estimate the H+/ATP ratio form the measurement of delta Gp/delta muH.

Probing biomembrane interfacial potential and pH profiles with a new type of float-like fluorophores positioned at varying distance from the membrane surface

Fluorophores of a new type were synthesized to probe the electrostatic potential or pH profiles in the external interface of biomembranes. The probes consist of the pH-sensitive fluorophore 7-hydroxycoumarin, coupled to a tetradecyl (myristyl) tail by a spacer group of varying length. A positively charged group is included between the tetradecyl and spacer groups to encourage a float-like alignment in the membrane head-group region. Three probes of this type were compared with 4-heptadecyl-7-hydroxycoumarin the fluorophore of which is embedded in the lipid head-group domain. Thus, a ruler-type positioning of the fluorophores was obtained at about 0.2, 0.6, 1.0, and 1.3 nm from the surface. The membrane-bound probes were tested in well-defined liposomes prepared by extrusion with different surface charge densities and size. The predicted positioning of the float-like probes is supported by their binding behavior in liposomes and by steady-state and nanosecond time-resolved fluorescence anisotropy, as well as by their accessibility to different quenchers. The interfacial electrostatic potential (psi d) and pH (pHd) values were derived from the observed apparent pKa shifts of the probes. The obtained psi d and pHd profiles as function of the surface potential (psi 0) and distance from the membrane surface are in good harmony with predictions from nonlinear Gouy-Chapman theory. The electrokinetic potentials (zeta) of the liposome series, measured by Doppler-electrophoretic frequency shift of laser light scattering, are in good proportion to the probe data. When bound to yeast cells, these probes monitor interfacial changes in parallel with glucose-induced medium acidification.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

Co-reconstitution of plasma membrane H(+)-ATPase from yeast and bacteriorhodopsin into liposomes. ATP hydrolysis as a function of external and internal pH

The H(+)-ATPase from the plasma membrane of Saccharomyces cerevisiae was isolated and purified. The enzyme was reconstituted with bacteriorhodopsin into asolectin liposomes by detergent dialysis at a molar ratio of 1 H(+)-ATPase to 50 bacteriorhodopsins. The overall orientation of the proteins is such that proton pumping to the vesicle interior occurs upon illumination and after addition of ATP. All liposomes which contain H(+)-ATPase also contain bacteriorhodopsin. The rate of ATP hydrolysis was measured as function of pH in the dark and during illumination of the proteoliposomes. The pH dependency can be described by the protonation of a monovalent group from the outside with an apparent pK of 7.3 and the deprotonation of a monovalent group at the inside with an apparent pK of 3.7. Inside and outside refer to the orientation of the H(+)-ATPase in the liposomes which is opposite to that occurring in vivo. It is concluded that the first step in the reaction cycle is the binding of a proton from the cytosol which is followed by ATP binding, ATP hydrolysis on the enzyme and the release of ADP and phosphate, and finally the proton is released from the enzyme into the external medium.

Determination of transmembrane pH gradients and membrane potentials in liposomes

Techniques for determining large transbilayer pH gradients (delta pH) and membrane potentials (delta psi) induced in response to delta pH in large unilamellar vesicle liposomal systems by measuring the transbilayer redistribution of radiolabeled compounds have been examined. For liposomes with acidic interiors, it is shown that protocols using radiolabeled methylamine in conjunction with gel filtration procedures to remove untrapped methylamine provide accurate measures of delta pH in most situations. Exceptions include gel state lipid systems, where transbilayer equilibration processes are slow, and situations where the interior buffering capacity is limited. These problems can be circumvented by incubation at elevated temperatures and by using probes with higher specific activities, respectively. Determination of delta pH in vesicles with a basic interior using weak acid probes such as radiolabeled acetate in conjunction with gel filtration was found to be less reliable, and an alternative equilibrium centrifugation protocol is described. In the case of determinations of the membrane potentials induced in response to these pH gradients, probes such as tetraphenylphosphonium and thiocyanate provide relatively accurate measures of the delta psi induced. It is shown that the maximum transmembrane pH gradient that can be stably maintained by an egg phosphatidylcholine-cholesterol 100-nm-diam large unilamellar vesicle is approximately 3.7 units, corresponding to an induced delta psi of 220 mV or transbilayer electrical field of 5 x 10(5) V/cm.

The possible role of redox-associated protons in growth of plant cells

The protons excreted by plant cells may arise by two different mechanisms: (1) by the action of the plasma membrane H(+)-ATPase and (2) by plasma membrane redox reactions. The exact proportion from each source is not known, but the plasma membrane H(+)-ATPase is, by far, the major contributor to proton efflux. There is still some questions of whether the redox-associated protons produced by NADH oxidation on the inner side of the plasma membrane traverse the membrane in a 1:1 relationship with electrons generated in the redox reactions. Membrane depolarization observed in the presence of ferricyanide reduction by plasma membranes of whole cells or tissues or the lag period between ferricyanide reduction and medium acidification argue that only scalar protons may be involved. The other major argument against tight coupling between protons and electrons involves the concept of strong charge compensation. When ferricyanide is reduced to ferrocyanide on the outside of cells or tissues, an extra negative charge arises, which is compensated for by the release of H+ or K+, so that the total ratio of increased H+ plus K+ equals the electrons transferred by transmembrane electron transport. These are strong arguments against a tight coupling between electrons and protons excreted by the plasma membrane. On the other hand, there is no question that inhibitor studies provide evidence for two mechanisms of proton generation by plasma membranes. When the H(+)-ATPase activity is totally inhibited, the addition of ferricyanide induces a burst of extra proton excretion, or vice versa, when plasma membrane redox reactions are inhibited, the H(+)-ATPase can function normally. Since plasma membrane redox reactions and associated H+ excretion are related to growth it is possible that in plants the ATPase-generated protons have a different function from redox-associated protons. The H(+)-ATPase-generated protons have been considered for many years to be necessary for cell wall expansion, allowing elongation to take place. A special function of the redox-generated protons may be in initiating proliferative cell growth, based on the presence of a hormone-stimulated NADH oxidase in membranes of soybean hypocotyls and stimulation of root growth by low concentrations of oxidants. Here we propose that this NADH oxidase and the redox protons released by its action control growth. The mechanism for this may be the evolution of protons into a special membrane domain, from which a signal to initiate cell proliferation may originate, independent of the action of the H(+)-ATPase-generated protons.(ABSTRACT TRUNCATED AT 400 WORDS)

Acridine orange as a probe for measuring pH gradients across membranes: mechanism and limitations

Acridine orange is an optical probe commonly used to monitor pH gradients across membranes. In the present study, the changes observed in the visible absorption spectrum of acridine orange during intravesicular acidification of oat root plasma membrane vesicles are shown to be identical with those obtained by increasing the free dye concentration, adding anions, or lowering the temperature, but different from those obtained on addition of biological membranes. It is therefore suggested that the absorbance changes observed during the formation of the pH gradient are simply due to accumulation of free dye inside the vesicles and subsequent dimerization, and not the result of dye-membrane interactions. The proportion of monomeric acridine orange that could undergo dimerization decreased with decreasing temperature. Furthermore, in a membrane-free system different anions induced the formation of dimer-excimer complexes to different degrees. During the formation of the pH gradient permeant anions present in the reaction medium follow the movement of protons into the vesicles, and the intravesicular accumulation of anions thereby amplifies acridine orange quenching, the degree of amplification being dependent on the anion species. Therefore, the use of acridine orange, and probably all metachromatic dyes, as probes for monitoring pH gradients is limited, since these probes neither reflect quantitatively the amount of H+ pumped nor the effect of anions and temperature on transmembrane H+ transport.

A delta pH clamp method for analysis of steady-state kinetics of photophosphorylation

An instrumental device is described which allows steady-state kinetic measurements of photophosphorylation at a desired proton gradient which can be maintained throughout the course of the experiment ('delta pH clamp'). This is achieved by electronic regulation of light intensity using the calibrated 9-aminoacridine fluorescence signal as sensor of the gradient. The instrument is suitable for determination of kinetic parameters of the proton-translocating ATPase in isolated envelope-free chloroplasts under defined conditions. At clamped delta pH, phosphorylation as a function of substrate concentration shows Michaelis-Menten kinetics. The true Michaelis constants and the dissociation constants for phosphate and ADP are reported. The Michaelis constants are not affected by the magnitude of the proton gradient in the investigated range. The significance of these results is discussed.

Evaluation of the buffer capacity and permeability constant for protons in chromatophores from Rhodobacter capsulatus

1. The kinetics of decay in the dark of the transmembrane pH difference (delta pH) induced by light in nonphosphorylating chromatophores of Rhodobacter capsulatus were studied using the fluorescent probe 9-aminoacridine, in the presence of 50 mM KCl and 2 microM valinomycin. The transient fluorescence changes induced by acid to base transitions of chromatophore suspensions were used as an empirical calibration [Casadio, R. & Melandri, B. A. (1985) Arch. Biophys. Biochem. 238, 219-228]. The kinetic competence of the probe response was tested by accelerating the delta pH decay with the ionophore nigericin. 2. The time course in the dark of the increase in the internal pH in pre-illuminated chromatophores was analyzed on the basis of a model which assumes a certain number of internal buffers in equilibrium with the free protons and a diffusion-controlled H+ efflux [Whitmarsh, J. (1987) Photosynt. Res. 12, 43-62]. This model was extended to include the effects of the transmembrane electric potential difference on the H+ efflux. 3. The diffusion constant for proton efflux was measured at different values of the internal pH by evaluating the frequency of trains of single-turnover flashes capable of maintaining different delta pH in a steady state. The steady-state equation derived from the model does not include any parameter relative to the internal buffers and allows unequivocal determination of the diffusion constant on the basis of the known H+/e- ratio (equal to two) for the active proton translocation by the bacterial photosynthetic chain. A value for the first-order diffusion constant corresponding to a permeability coefficient, PH = 0.2 micron.s-1, was obtained at an external pH of 8.0; this value was constant for an internal pH ranging over 7.0-4.7. 4. Using the value of the diffusion constant determined experimentally, a satisfactory fitting of the kinetics of delta pH decay in the dark could be obtained when the presence of two internal buffers (with pK values of 3.6 and 6.7, respectively) was assumed. For these calculations, the time course of the transmembrane electric potential difference was evaluated from the electrochromic signal of carotenoids, calibrated with K(+)-induced diffusion potentials. The two internal buffers, suitable for modelling the behaviour of the system, were at concentrations of 250 mM (pK = 3.6) and 24 mM (pK = 6.7) respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-proton exchange in human ileal brush-border membrane vesicles

This study examines the characteristics of Na+ and H+ transport as well as Na+-H+ exchange in human ileal brush-border membrane vesicles from organ donor intestine. 22Na+ uptake into vesicles and the fluorescence quenching of Acridine orange were employed to measure Na+ and H+ transport, respectively. Concentrative uptake of 22Na+ (4-fold overshoot above equilibrium) was observed under conditions of an outward proton gradient (pHi 5.5; pHo 7.5). Voltage-clamping (Ki+ = Ko+ + valinomycin) reduced the uptake of 22Na+ by 40-50% indicating the presence of Na+ conductance. Dissipation of the Acridine orange fluorescence quench in ileal vesicles with a preformed pH gradient (pHi 5.5; pHo 7.5) was accelerated by either external Na+ or voltage-clamping in the absence of Na+. The effects of Na+ and voltage-clamping were additive under the above conditions. In the absence of a pH gradient, Acridine orange quenching was induced by intravesicular Na+ as well as an interior negative K+ diffusion potential. In voltage-clamped BBMV, pH-driven Na+ uptake was inhibited by amiloride (Ki = 140 microM). The initial rate of pH-driven Na+ uptake was saturable and conformed to Michaelis-Menten kinetics with apparent Km and Vmax values of 27 +/- 1 mM and 47 +/- 1 nmol.(mg protein)-1.(3 s)-1, respectively. Li+ and NH4+, but not Cs+, K+, Rb+ or choline+ inhibited pH gradient-driven 22Na+ uptake. The results demonstrate in human ileal brush-border membrane vesicles the presence of an Na+/H+ exchanger and conductive transport pathways for Na+ and H+.

delta pH-induced fluorescence quenching of 9-aminoacridine in lipid vesicles is due to excimer formation at the membrane

The fluorescence of 9-aminoacridine (9-AA) is quenched in vesicular suspensions containing negatively charged lipid headgroups (e.g., phosphatidylserine) upon imposition of a transmembrane (inside acidic) pH-gradient. It is shown that this fluorescence loss is accompanied by the formation of 9-AA dimers that undergo a transition in the dimer excited state to a dimer-excimer state. This result has been obtained on the basis of the specific dimer fluorescence excitation and hypochromic absorbance spectra that are redshifted by maximally 275 cm-1 (4.4 nm) with respect to the corresponding monomer spectra, as well as by the detection of the characteristic broad excimer emission band, centered at 560 nm. The existence of the spectrally distinct dimer-excimer is further corroborated by fluorescence life-time measurements that indicate an increased lifetime of up to 24 ns for this complex as compared with the normal monomer fluorescence lifetime of 16 ns. The formation of this dimer-excimer complex from the monomers can be reversed completely and the original monomeric spectral properties restored after the abolishment of the electrochemical proton gradient. In addition to the delta pH-induced dimer redshift in absorbance and fluorescence excitation, a further small redshift in monomer absorbance, fluorescence excitation, and emission spectra is observed due solely to the presence of the negatively charged phospholipid headgroups.