The membrane structure studied with cationic dyes. 2. Aggregation, metachromatic effects and pK a shifts

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.

Dissecting the pattern of proton release from partial process involved in ubihydroquinone oxidation in the Q-cycle

A key feature of the modified Q-cycle of the cytochrome bc₁ and related complexes is a bifurcation of QH₂ oxidation involving electron transfer to two different acceptor chains, each coupled to proton release. We have studied the kinetics of proton release in chromatophore vesicles from Rhodobacter sphaeroides, using the pH-sensitive dye neutral red to follow pH changes inside on activation of the photosynthetic chain, focusing on the bifurcated reaction, in which 4H⁺are released on complete turnover of the Q-cycle (2H⁺/ubiquinol (QH₂) oxidized). We identified different partial processes of the Q_o-site reaction, isolated through use of specific inhibitors, and correlated proton release with electron transfer processes by spectrophotometric measurement of cytochromes or electrochromic response. In the presence of myxothiazol or azoxystrobin, the proton release observed reflected oxidation of the Rieske iron‑sulfur protein. In the absence of Q_o-site inhibitors, the pH change measured represented the convolution of this proton release with release of protons on turnover of the Q_o-site, involving formation of the ES-complex and oxidation of the semiquinone intermediate. Turnover also regenerated the reduced iron-sulfur protein, available for further oxidation on a second turnover. Proton release was well-matched with the rate limiting step on oxidation of QH₂ on both turnovers. However, a minor lag in proton release found at pH 7 but not at pH 8 might suggest that a process linked to rapid proton release on oxidation of the intermediate semiquinone involves a group with a pK in that range.

Effects of pathogenic mutations in membrane subunits of mitochondrial Complex I on redox activity and proton translocation studied by modeling in Escherichia coli

Effects of Complex I mutations were studied by modeling in NuoH, NuoJ or NuoK subunits of Escherichia coli NDH-1 by simultaneous optical monitoring of deamino-NADH oxidation and proton translocation and fitting to the data a model equation of transmembrane proton transport. A homolog of the ND1-E24 LHON/MELAS mutation caused 95% inhibition of d-NADH oxidation and proton translocation. The NuoJ-Y59F replacement decreased proton translocation. The NuoK-E72Q mutation lowered the enzyme activity, but proton pumping could be rescued by the double mutation NuoK-E72Q/I39D. Moving the NuoK-E72/E36 pair one helix turn towards the periplasm did not affect redox activity but decreased proton pumping.

Light-induced currents from oriented purple membrane: II. Proton and cation contributions to the photocurrent

The sign of B2, the micro-second component of the photocurrent from oriented purple membrane, is that of positive charge moving away from the purple membrane in the direction of proton release. B2 could be due to internal dipole or proton movement, proton release, or metal cation release. We found that the waveform of B2 is virtually insensitive to changes in the salt concentration as long as it is >40 mM KCl, >5 mM CaCl(2), or >0.5 mM LaCl(3). However, below these limits, B2's apparent rate of decay increases as the salt concentration decreases without any change in the initial amplitude. This salt dependence suggests that B2 is due to a positive charge, either a metal cation or a proton, moving from the membrane into the solution. That the positive charge is not a metal cation is suggested by the waveform of B2 remaining unchanged upon replacing the cations both in solution and in the binding sites of the purple membrane. Direct evidence that the positive charge movement is due to protons was obtained by examining the correlation of B2 with the proton dependent processes of bacteriorhodopsin in buffers and dyes. Based on these observations, we suggest that most, if not all, of the intrinsic B2 component of the photocurrent at moderate salt concentration is due to proton release.The photocurrents from purple membranes whose surface potential has been reduced by delipidation or chemical modification of carboxyl groups with methyl esters were found to be only modestly changed. This suggests that the salt effect is not through its modulation of the surface potential. Rather, we propose that in low salt B2 represents the sum of a proton release from the surface of the purple membrane and a second current component, due to cations moving back towards the membrane, which is only important in low salt. The cation counter current is induced by proton release which creates a transient uncompensated negative charge on the membrane.

Neutral red as a probe for confocal laser scanning microscopy studies of plant roots

BACKGROUND AND AIMS

Neutral red (NR), a lipophilic phenazine dye, has been widely used in various biological systems as a vital stain for bright-field microscopy. In its unprotonated form it penetrates the plasma membrane and tonoplast of viable plant cells, then due to protonation it becomes trapped in acidic compartments. The possible applications of NR for confocal laser scanning microscopy (CLSM) studies were examined in various aspects of plant root biology.

METHODS

NR was used as a fluorochrome for living roots of Phaseolus vulgaris, Allium cepa, A. porrum and Arabidopsis thaliana (wild-type and transgenic GFP-carrying lines). The tissues were visualized using CLSM. The effect of NR on the integrity of the cytoskeleton and the growth rate of arabidopsis primary roots was analysed to judge potential toxic effects of the dye.

KEY RESULTS

The main advantages of the use of NR are related to the fact that NR rapidly penetrates root tissues, has affinity to suberin and lignin, and accumulates in the vacuoles. It is shown that NR is a suitable probe for visualization of proto- and metaxylem elements, Casparian bands in the endodermis, and vacuoles in cells of living roots. The actin cytoskeleton and the microtubule system of the cells, as well as the dynamics of root growth, remain unchanged after short-term application of NR, indicating a relatively low toxicity of this chemical. It was also found that NR is a useful probe for the observation of the internal structures of root nodules and of fungal hyphae in vesicular-arbuscular mycorrhizas.

CONCLUSIONS

Ease, low cost and absence of tissue processing make NR a useful probe for structural, developmental and vacuole-biogenetic studies of plant roots with CLSM.

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.

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 single and continuous spectrophotometric assay for various lipolytic enzymes, using natural, non-labelled lipid substrates

A rapid, continuous spectrophotometric assay for measuring the amount and activity of several lipolytic enzymes is described. It is based on the metachromatic properties of the cationic dye safranine, and makes use of the fact that an adequate combination of a lipolytic enzyme with one of its substrates leads to a change in the net negative charge at the lipid/water interface, which is monitored by the absorbance change of safranine. Utilizing this method, most lipolytic enzymes can be detected in very low amounts (milliunit or less) in about 1 min without employing radiolabelled lipids or synthetic lipid analogues. Over a wide range of enzyme concentrations, there is a good linearity between the initial hydrolysis rate (determination by the safranine method) and the amount of enzyme. The versatility of the assay is illustrated by examples showing how phospholipase A2, triacylglycerol hydrolase, phospholipase D or phospholipase C (either general or phosphatidylinositol-specific) activities can be detected, either separately or sequentially. Due to its high sensitivity, simplicity, and rapidity, this assay should find its main application in monitoring column effluents during the purification steps of lipolytic enzymes.

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.

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

The fluorescence quenching of 9-aminoacridine (9-AA) after imposition of a transmembrane pH gradient (inside acidic) in liposomes has been investigated for a number of different lipid systems. The initial fluorescence decrease after a rapid pH jump, induced in the extravesicular medium by a stopped-flow mixing technique, was ascribed to a response of 9-AA to the imposed pH gradient and not to changes in the vesicular system itself. Time constants for this fluorescence quenching are in the range of several hundred milliseconds at 25 degrees C. Fluorescence recovery which should be correlated to the dissipation of the pH gradient occurs in the 100 s time range and is 10-30-times faster than the delta pH decay monitored with the entrapped hydrophilic pH-indicator dye pyranine. The quenching was severely hindered below the lipid phase transition of dipalmitoylphosphatidylglycerol. No delta pH-induced quenching was obtained in lipid vesicles containing only zwitterionic, net uncharged phosphatidylcholine headgroups. For the occurrence of quenching, the presence of negatively charged headgroups, i.e. phosphatidylglycerol or phosphatidylserine, was necessary. The extent of quenching, at a specific pH difference applied, had a cooperative dependency (Hill coefficient approximately 2) on the number of negative headgroups in the membrane and on the concentration of unquenched (unbound) 9-AA molecules. The concentration of quenched 9-AA molecules was furthermore proportional to the number of dimer-excimer complexes of 9-AA which are formed during the quenching process.

The use of neutral red as an intracellular pH indicator in rat brain cortex in vivo

Intracellular pH in the intact, normally perfused rat brain cortex was determined by rapid scanning reflectance spectrophotometry of Neutral Red. Neutral Red, a pH indicator dye, was administered intraperitoneally to rats. Reflectance spectra recorded from the exposed dural surface of 11 anesthetized rats were used to calculate an intracellular pH of 7.04 +/- 0.01. Detailed studies on the interactions of the dye with brain tissue were carried out in vitro to define the in vivo calibration curves. In addition, the physiological effect of dye administration on systemic blood pressure was determined, as well as uptake curves for Neutral Red into plasma and brain. It is concluded that Neutral Red can be used as an in vivo brain intracellular pH indicator and compares favorably with other methods of brain intracellular pH measurement with respect to accuracy, sensitivity, noninvasiveness, and stability and has the potential to exceed any existing method in time resolution.

Reconstituted amiloride-inhibited sodium transporter from rabbit kidney medulla is responsible for Na+-H+ exchange

Microsomes formed from rabbit kidney medulla and reconstituted proteoliposomes formed from these microsomes were capable of amiloride-inhibited Na+ transport that was insensitive to valinomycin either with or without K+. This indicated that the Na+ transport process was electroneutral. This Na+ transport process was insensitive to extravesicular Cl- or HCO-3 and not stimulated by high intravesicular gradients of K+, Ca2+ or Mg2+, which indicated that the process did not require NaCl or NaHCO3 co-transport or Na+/K+, Na+/Ca2+ or Na+/Mg2+ counter-transport. Na+ uptake into microsomes or proteoliposomes was inhibited by extravesicular K+, Ca2+, Mg2+ or La3+, which indicated that these ions interacted with the Na+-binding site on the transport protein. Na+ uptake into microsomes was stimulated by intravesicular protons and inhibited by extravesicular protons. This suggested that microsomes were capable of Na+-H+ exchange and this was confirmed when Na+ was shown to stimulate H+ efflux from microsomes. The amiloride-inhibited Na+ transporter from medulla microsomes which has been reconstituted into proteoliposomes is most likely a Na+-H+ exchanger. This Na+ transporter was totally insensitive to the uncoupler 1799, either in the presence or absence of valinomycin plus K+ and less sensitive to NH3 than to amiloride. This indicated that amiloride inhibited Na+ transport not merely by acting as a weak-base uncoupler but by directly interacting with the protein responsible for Na+-H+ exchange.

Mechanistic differences in the energy-linked fluorescence decreases of 9-aminoacridine dyes associated with bovine heart submitochondrial membranes

(1) The pH dependence of the fluorescence intensities of 9-aminoacridines associated with energized submitochondrial membranes suggests that a mechanism(s) other than protonation of the dye molecules, as is the case with quinacrine, is responsible for the energy-linked fluorescence decreases of 9-aminoacridine and 9-amino-3-chloro-7-methoxyacridine (9-ACMA). (2) That the fluorescence polarization of quinacrine associated with submitochondrial membranes more than doubles upon energization of the membranes is attributed to: (i) the bulky side chain at the 9-position of the acridine moiety which hinders the molecular rotation of quinacrine and (ii) electrostatic forces resulting from the protonation of quinacrine . H+ which induce tight binding between the dye molecules and the membranes. (3) The protonation of quinacrine associated with energized membranes, from the monoprotonated to the diprotonated species, takes place in the membrane phase, as evidence by the observation of a 'break' in both the Arrhenius plot of the respiratory rate and the plot of fluorescence polarization as a function of temperature. (4) That the measured fluorescence polarization of both 9-aminoacridine and 9-ACMA associated with both energized and nonenergized membranes is nearly zero suggests that the emitting species of these dye molecules are those in the 'free' form and that the membrane-bound molecules have formed nonfluorescent complexes; consequently no polarization can be measured.

The involvement of the electrical double layer in the quenching of 9-aminoacridine fluorescence by negatively charged surfaces

The addition of 9-aminoacridine monohydrochloride to carboxymethyl-cellulose particles or azolectin liposomes suspended in a low cation medium results in a quenching of its fluorescence. This quenching can be released on the addition of cations. The effectiveness of cations is related only to their valency in the series of salts tested, being monovalent less than divalent less than trivalent, and is independent of the associated anions. These results indicate an electrical rather than a chemical effect, and the relative effectiveness of the various cations can be predicted by the application of classical electrical double layer theory. Fluorescence quenching can also be released on protonation of the fixed negatively charged ionisable groups, and the quenching release curve follows the ionisation curve of these groups. We postulate that when 9-aminoacridine molecules are in the electrical diffuse layer adjacent to the charged surface their fluorescence is quenched, probably due to aggregate formation. As cations are added the 9-aminoacridine concentration at the surface falls as it is displaced into the bulk solution, where it shows a high fluorescence yield with a fluorescence lifetime of 16.3 ns. The fluorescence quenching is associated with an absorbance decrease, which is pronounced with carboxymethyl-cellulose particles and can probably be attributed to self-shielding. The negative charges carried by lipoprotein membranes are primarily due to carboxyl and phosphate groups. Therefore these results with carboxymethyl-cellulose (carboxyl) and azolectin (phosphate) support our earlier suggestion that 9-aminoacridine may be used to probe the electrical double layer associated with negatively charged biological membranes.

The light dependent uptake of N-methylphenazinium cations by the thylakoids of isolated chloroplasts

The absorption of N-methylphenazinium methylsulfate (MP+ methylsulfate) in suspensions of envelope-free chloroplasts is reversibly lowered in the light. When the electron transport system of the chloroplasts is inhibited by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), the photobleaching reflects an uptake of MP+ into the thylakoids. Its magnitude is a function of the composition and of the pH of the suspension medium and, most importantly, is controlled by the availability of permeant anions which apparently accompany MP+ into the thylakoid as counterions. Consequently, the rate of the bleaching is strongly dependent on the permeability of the thylakoid to the available anion. At pH 7.5, the thylakoids of DCMU poisoned pokeweed chloroplasts are able to hold at least 6 MP+/chlorophyll. It is proposed that, in the presence of MP+, the light reaction of Photosystem I in DCMU-inhibited chloroplasts causes a conformational change of the membranes which exposes nucleophilic sites inside the thylakoids. These sites appear to have a high affinity for MP+, but may bind protons or other cations under certain experimental conditions. The uptake of MP+ has a hypochromic effect on its absorption band in the near ultraviolet due to the resulting heterogeneous distribution of the dye cation between medium and chloroplast grana.

Energy-linked protonation of quinacrine in beef heart submitochondrial membranes

1. The absorption spectrum of quinacrine in aqueous solution, in the visible region, changes with the pH of the medium in the pH range from 6.0 to 9.0 with an isosbestic point at 353 nm. This indicates that the monoprotonated (quinacrine - H+) and the diprotonated (quinacrine - 2H+) forms of quinacrine at equilibrium in this pH range have a 1 to 1 stoichiometry. 2. The monoprotonated and the dipronated forms to quinacrine exhibit similar fluorescence emission spectra, but distinctive fluorescence excitation spectra. 3. The relative fluorescence quantum yields of quinacrine in aqueous media of various pH values are estimated. The relative fluorescence quantum yield of quinacrine at pH 9.0 is more than 3 fold of that at pH 6.0. 4. The fluorescence excitation and emission spectra, as well as the relative fluorescence quantum yield of quinacrine associated with non-energized submitochondrial membranes, are similar to those of quinacrine alone. 5. Analyses of the absorption spectra, the fluorescence excitation spectra and the relative fluorescence quantum yield indicate that the energy-linked fluorescence decrease of quinacrine associated with the energized submitochondrial membranes results from the protonation of quinacrine - H+ to form quinacrine - 2H+. 6. Quantitative data are provided indicating that the maximal efficiency of protonation of quinacrine - H+ to form quinacrine - 2H+ depends on the concentration of H+ in the membranes generated through energy coupling, and the concentration of quinacrine - H+ initially present in the reaction medium. Under optimal conditions virtually complete conversion of quinacrine - H+ into quinacrine - 2H+ is observed. 7. The fluorescence intensity of quinacrine, either alone or associated with non-energized submitochondrial membranes, decreases with increasing temperature. When quinacrine is associated with the energized membranes, however, its fluorescence intensity increases slightly with increasing temperature. This unusual fluorescence behavior towards temperature, together with the fact that under optimal conditions virtually all the quinacrine molecules associated with the energized membranes are in the diprotonated form, further substantiate our earlier conclusion that the diprotonated quinacrine molecules are tightly bound to the energized membranes in a fashion which does not permit ready equilibration with the external medium.

On the stimulation of the light-induced proton uptake by uncoupling aminoacridine derivatives in spinach chloroplasts

1. Light-induced proton uptake by spinach chloroplasts is enhanced several-fold by 9-(4-diethylamino-1-methylbutylamino)-6-chloro-2-methoxyacridine (atebrin). This stimulation does not depend on the chlorophyll concentration. The amount of extra protons taken up in the presence of atebrin is determined by the pKa values of atebrin and the pH of the incubation medium. 2. Both the stimulation of the proton uptake and the maximal binding capacity for atebrin is sensitive to uncouplers. However, the ratio of bound to free atebrin does not depend on the presence of uncoupler up to the saturating atebrin concentration. 3. From simultanious kinetic measurements of atebrin fluorescence and proton movement it seems that after binding of the completely protonated atebrin the dye and the protons can move separately. This can also be inferred from the spectral behaviour of atebrin in illuminated chloroplasts. 4. The stimulation of the proton uptake by atebrin does not depend on the presence of salts in the incubation medium. However, the 'saturating' atebrin concentration increases strongly with increasing salt concentration in the medium. 5. It is concluded that the interaction of atebrin and other acridines with energized chloroplasts most likely occurs at the level of the membrane proper. 6. It is proposed that uncoupling by atebrin is a consequence of the creation of a high proton activity at the periphery of the thylakoid membrane, which opposes a proton gradient across the membrane. The uncoupling by atebrin is not of the protonophoric type according to this mechanism.

Ca2+ transport by mitochondria from L1210 mouse ascites tumor cells

Mitochondria isolated from the ascites form of L1210 mouse leukemia cells readily accumulate Ca(2+) from the suspending medium and eject H(+) during oxidation of succinate in the presence of phosphate and Mg(2+), with normal stoichiometry between Ca(2+) uptake and electron transport. Ca(2+) loads up to 1600 ng-atoms per mg of protein are attained. As is the case in mitochondria from normal tissues, Ca(2+) uptake takes precedence over oxidative phosphorylation. However, Ca(2+) transport by the L-1210 mitochondria is unusual in other respects, which may possibly have general significance in tumor cells. The apparent affinity of the L1210 mitochondria for Ca(2+) in stimulation of oxygen uptake is about 3-fold greater than in normal liver mitochondria; moreover, the maximal rate of Ca(2+) transport is also considerably higher. Furthermore, when Ca(2+) pulses are added to L1210 mitochondria in the absence of phosphate or other permeant anions, much larger amounts of Ca(2+) are bound and H(+) ejected per atom of oxygen consumed than in the presence of phosphate; up to 7 Ca(2+) ions are bound per pair of electrons passing each energy-conserving site of the electron-transport chain. Such "superstoichiometry" of Ca(2+) uptake can be accounted for by two distinct types of respiration-dependent interaction of Ca(2+) with the L1210 mitochondria. One is the stimulation of oxygen consumption, which is achieved by relatively low concentrations of Ca(2+) (K(m) congruent with 8 muM) and is accompanied by binding of Ca(2+) up to 40 ng-atoms per mg of protein. The second process, also dependent on electron transport, is the binding of further Ca(2+) from the medium in exchange with previously stored membrane-bound protons, in which the affinity for Ca(2+) is much lower (K(m) congruent with 120 muM).