Determination of pH in chloroplasts. I. Distribution of ( 14 C) methylamine

Evidence for complexed plastocyanin as the immediate electron donor of P-700

The reduction of P-700 by its electron donors shows two fast phases with half-times of 20 and 200 mus in isolated spinach chloroplasts. We have studied this electron transfer and the oxidation kinetics of cytochrome f. Incubation of chloroplasts with KCN or HgCl2 decreased the amplitude of the 20 mus phase. This provides evidence for a function of plastocyanin as the immediate electron donor of P-700. At low concentrations of salt and sugar the fast phases of P-700+ reduction were largely inhibited. Increasing concentrations of MgCl2, KCl and sorbitol (up to 5, 150 and 200 mM, respectively) were found to increase the relative amplitudes of the fast phases to about one-third of the total P-700 signal. Addition of both 3 mM MgCl2 and 200 mM sorbitol increased the relative amplitude of the 20 mus phase to 70%. The interaction between P-700 and plastocyanin is concluded to be favoured by a low internal volume of the thylakoids and compensation of surface charges of the membrane. The half-time of 20 mus was not changed when the amplitude of this phase was altered either by salt and sorbitol, or by inhibition of plastocyanin. This is evidence for the existence of a complex between plastocyanin and P-700 with a lifetime long compared to the measuring time. The 200 mus phase exhibited changes in its half-time that indicated the participation of a more mobile pool of plastocyanin. Cytochrome f was oxidized with a biphasic time course with half-times of 70--130 mus and 440--860 mus at different salt and sorbitol concentrations. The half-time of the faster phase and a short lag of 30--50 mus in the beginning of the kinetics indicate an oxidation of cytochrome f via the 20 mus electron transfer to P-700. An inhibition of this oxidation by MgCl2 suggests that the electron transfer from cytochrome f to complexed plastocyanin is not controlled by negative charges in contrast to that from plastocyanin to P-700.

The kinetics and thermodynamics of the reduction of cytochrome c by substituted p-benzoquinols in solution

1. The mechanisms by which p-benzoquinol and its derivatives reduce cytochrome c in solution have been investigated. 2. The two major reductants are the species QH- (anionic quinol) and Q.- (anionic semiquinone). A minor route of electron transfer from the fully protonated QH2 species can also occur. 3. The relative contributions of these routes to the overall reduction rate are governed by pH, ionic strength and relative reactant concentrations. 4. For a series of substituted p-benzoquinols, the forward rate constant, k1, of the anionic quinol-mediatd reaction is related to the midpoint potential of the QH-/QH. couple involved in the rate-limiting step, as predicted by the theory of Marcus for outer-sphere electron transfer reactions in a bimolecular collision process. 5. A mechanism for the biological quinol oxidation reactions in mitochondria and chloroplasts is proposed based upon the findings with these reactions in solution.

A novel method for measurement of intravesicular pH using fluorescent probes

A method for the measurement of intravesicular pH of phospholipid vesicles and gastric microsomes is described. The present method makes use of the well characterized pH-dependent shift of the emission maximum of two fluorescent amines, quinine and acridine. As the probes distribute into the intravesicular space, according to the existing pH gradient, they respond to the acidic environment and emit fluorescence at a longer wavelength than the external probes which sense the more alkaline medium. By measuring both the decrease in the alkaline fluorescence peak and the enhancement of the acidic peak, direct determination of the internal pH can be obtained. This method has the advantages of giving a positive signal (enhancement of fluorescence instead of quenching), measuring intravesicular pH directly without the need of independent measurement of the volume of the H+ space, and also enabling the possible use of a much lower probe concentration than that required by the fluorescence-quenching method. The accuracy of the present method was quantitatively verified using phospholipid vesicles as a well defined model system. Application to biological systems was demonstrated using gastric microsomes which actively transport H+ into the microsomal space via an H+-K+ exchange ATPase system. The reasoning of the present method is also extended to the monitoring of internal alkaline pH gradient by using a fluorescent weak acid, o-hydroxycinnamic acid. Some general criteria for the future search for better Ph gradient probes are presented.

Measurement of chloroplast internal protons with 9-aminoacridine. Probe binding, dark proton gradient, and salt effects

A defined ratio, gamma, of the total proton uptake to the concentration change of free internal H+ is observed for illuminated envelope-free chloroplasts (Haraux, F. and de Kouchkovsky, Y. (1979) Biochim. Biophys. Acta, 546, 455-471). Proton uptake is measured by the external pH shift, free internal H+ by 9-aminoacridine fluorescence quenching. Extension of this work leads to the following conclusions, which, in the case of 9-aminoacridine behaviour, should apply to any kind of diffusible protonizable delta pH probe: 1. The gamma constancy is preserved when the internal volume (Vi) is modulated by chlorophyll and osmolarity changes: thus, 9-aminoacridine behaves as expected from the delta pH distribution of an amine of high pK; previous doubts on this point are attributed to the lack of control of the external proton uptake. 2. With variable 9-aminoacridine concentration, however, some variation of gamma confirms the existence of slight light-induced probe-membrane interactions. 3. According to the diffuse layer theory, salts decrease the negative potential at the 'plane of closest approach' of the thylakoids, thereby releasing the excess 9-aminoacridine in this diffuse layer, which increases its fluorescence. Although of equal valency, NH4+ is more potent than K+, suggesting competition between amines for specific anionic binding sites. 4. Two categories of membrane modifications are induced by salts: in addition to the above-mentioned electrical effect, mono- and divalent cations at high concentration increase the chloroplast proton binding capacity. La3+ is only able to release the excess dye in the diffuse layer and leaves gamma unchanged. Therefore the probe-membrane interactions should have limited importance for steady-state delta pH measurement. 5. A Donnan-type dark pH difference, which could seriously bias these delta pH estimates, is found experimentally to be less than 2 (no significant gamma change when Vi varies) and even theoretically less than 1 (on the basis of the concentration of the non-diffusible internal protonizable groups). Similarly, the predictable errors of Vi and its possible light-induced variations must have a small effect on delta pH under present experimental conditions.

Inhibition of lactate transport in Ehrlich ascites tumor cells and human erythrocytes by a synthetic anhydride of lactic acid

The synthesis and some of the physical and biological characteristics of a new inhibitor of lactate transport are described. The inhibitor is isobutylcarbonyl lactayl anhydride (iBCLA). It is formed by the condensation of lactic acid and isobutylchloroformate. It inhibits lactate transport 50% at 0.5 microgram/mg of protein in both Ehrlich ascites tumor cells and human erythrocytes. In contrast, 15 microgram of iBCLA/mg of protein is required for 50% inhibition of phosphate transport in erythrocytes, and phosphate transport in Ehrlich ascites tumor cells is unaffected at levels as high as 50 microgram of iBCLA/mg of protein. A time-dependent and concentration-dependent reversal of lactate transport inhibition took place on exposure of iBCLA-treated Ehrlich ascites cells to hydroxylamine or dithiothreitol. These data, along with the observed sensitivity of the lactate transporter to sulfhydryl reagents [Spencer, T. L., & Lehninger, A. L. (1976) Biochem. J. 154, 405-414], suggest that iBCLA acylates an essential sulfhydryl group on the transporter. When glycolyzing Ehrlich ascites tumor cells were treated with concentrations of iBCLA sufficient for complete inhibition of lactate transport, intracellular lactate levels increased, intracellular pH and extra-cellular lactate levels decreased, and overall lactate production was inhibited.

Electrochemical proton gradient in Micrococcus lysodeikticus cells and membrane vesicles

Using the distribution of weak acids to measure the pH gradient (delta pH; interior alkaline) and the distribution of the lipophilic cation [3H]tetraphenylphosphonium+ to monitor the membrane potential (delta psi; interior negative), we studied the electrochemical gradient or protons (delta mu- H+) across the membrane of Micrococcus lysodeikticus cells and plasma membrane vesicles. With reduced phenazine methosulfate as electron donor, intact cells exhibited a relatively constant delta mu- H+ (interior negative and alkaline) of -193 mV to -223 mV from pH 5.5 to pH 8.5. On the other hand, in membrane vesicles under the same conditions, delta mu- H+ decreased from a maximum value of -166 mV at pH 5.5 to -107 mV at pH 8.0 and above. This difference is related to a differential effect of external pH on the components of delta mu- H+. In intact cells, delta pH decreased from about -86 mV (i.e., 1.4 units) at pH 5.5 to zero at pH 7.8 and above, and the decreases in delta pH was accompanied by a reciprocal increase in delta psi from -110 mV at pH 5.5 to -211 mV at pH 8.0 and above. In membrane vesicles, the decrease in delta pH with increasing external pH was similar to that described for intact cells; however, delta psi increased from -82 mV at pH 5.5 to only -107 mV at pH 8.0 and above.

Measurement of net proton-hydroxyl permeability of large unilamellar liposomes with the fluorescent pH probe, 9-aminoacridine

The fluorescent probe 9-aminoacridine was used to measure the rate of decay of experimentally established pH gradients across liposome membranes. From the rate of decay, separate permeability coefficients for protons (PH) and hydroxyls (POH) were calculated and summed to yield the net proton-hydroxyl permeability (Pnet). The net permeability of protons and hydroxyls was found to be approximately 10(-4) cm/s, six orders of magnitude greater than that measured for sodium and pyrophosphate ions under similar conditions. This suggests that protons and/or hydroxyls cross lipid bilayers by a different mechanism than do other monovalent cations and anions. In addition, the measurements provide a standard for net proton-hydroxyl permeability in pure phospholipid bilayers for comparison with biological membranes.

Chromium oxalate: a new spin label broadening agent for use with thylakoids

Potassium tris(oxalato)chromate(III) trihydrate (chromium ixalate) has been shown to be a more useful broadening agent than potassium ferricyanide for the spin label 2,2,6,6-tetramethylpiperidine-N-oxyl-4-amine (Tempamine) in thylakoid suspensions. Our data show that chromium oxalate is less permeable than ferricyanide, does not inhibit thylakoid electron transport or photophosphorylation, and is not photoreduced by thylakoids.

Quantitative estimation of the photosynthetic proton binding inside the thylakoids by correlating internal acidification to external alkalinisation and to oxygen evolution in chloroplasts

The external alkalinisation delta pHe, or the rate of oxygen evolution vO2, of a suspension of envelope-free chlorplasts was correlated with their internal acidification, estimated from the transmembrane delta pHei. Knowing the external buffer value, the concentration of the total protons moved Hi was calculated from the delta pHe, measured with a glass electrode ([Hi] was also obtained from vO2), and the free proton concentration [Hi+] was determined from delta pHei, measured with 9-aminoacridine. This gives a ratio gamma i = theta [Hi]/theta [Hi+], which is independent of the thylakoids internal volume. Within a large pHi range, scanned by varying the light intensity, gamma i was kept reasonably constant; it was hardly sensitive to pHi. This apparent invariability implies a continuous change of the internal buffer value beta i with pHi, since beta i/gamma i = -2.3.....10pHi, a relationship which inlcudes neither the total concentration of protonizable groups [Ai] nor pKi. As gamma i approximately Ki[Ai]/(Ki + [Hi+i]2, to keep gamma i constant when pHi drops, pKi and [Ai] must increase. This may be achieved by a progressive unmasking of anionic functions, initially inaccessible in the membrane. The relative slowness of this process may explain why gamma i calculated from the initial kinetics was sometimes smaller in high than in low light, where it always equalled that measured from the steady-state amplitude at all intensities. A small deficit of [Hi+] deduced from what could have been expected from delta pHe may reflect a limited binding of protons in the membrane itself, about 1 H+ for 30--130 chlorophylls (gamma i could be between 70 and 240, more frequently around 100); these numbers varied depending on the samples, but were constant for a given preparation.

The buffering capacity of the internal phase of thylakoids and the magnitude of the pH changes inside under flashing light

The buffering capacity inside thylakoids is determined and the magnitude of flash-induced pH changes inside is calibrated in the pH range from 6.4 to 8.1. The work is based on flash-induced absorption changes of neutral red in a chloroplast suspension in which the outer phase is strongly buffered by bovine serum albumin. It is shown that neutral red is bound inside thylakoids. The binding can be described by a simple isotherm with an apparent Km = 4 microM and satruation at 1 neutral red per 17 chlorophylls. The apparent pK of neutral red is shifted from 6.6 in solution to 7.25 when bound inside. It is demonstrated that neutral red is a clean indicator of pH changes inside, i.e. when properly used it shows no response to other events. Although bound it reports pH changes which occur in the internal osmolar (aqueous) volume of thylakoids. This is obvious from the influence of chemically very different buffers on the magnitude of the absorption changes of neutral red. These act in a manner proportional to their calculated buffering capacity in aqueous solution. The intrinsic buffering capacity of the internal phase is determined with the aid of these buffers, at pH 7.2 it is between 0.8 and 1 mM (at 60 mosM). The absence of large variations in the buffering capacity in the range from pH 6.4 to 8.1 suggests that proteinaceous groups are involved in addition to the lipids which may dominate the buffering capacity at lower pH. The magnitude of the internal pH change is arrpox. 0.6 (at pH 7.3) under stimulation of both photosystems with a short xenon flash of light.

Transmembrane electrical potential and transmembrane pH gradient in the acidophile Thiobacillus ferro-oxidans

Thiobacillus ferro-oxidans is capable of using the oxidation of Fe2+ by O2 at pH 2.0 as the sole source of energy for growth and CO2 fixation. The bacterium maintains an intracellular pH of 6.5 over a range of external pH from 1.0 to 8.0, as measured by [14C]acetate and [3H]methylamine distribution. The membrane potential was estimated by the distribution of the lipid-soluble cation dibenzyldimethylammonium and the anion SCN-. At pH 2.0 (the pH of growth) during Fe2+ oxidation the transmembrane pH gradient is 4.5 units with an opposing membrane potential of -10mV, giving a proton electrochemical gradient of +256mV. This gradient is actively maintained.

The accumulation of neutral red in illuminated thylakoids

Thylakoids isolated from spinach (Spinacia oleracea L.) bind only a small fraction of neutral red in the dark whereas they accumulate large amounts of the protonated dye in their inner space under light. Light-induced neutral red uptake depends on the size of the proton gradient across the thylakoid membrane but does not follow the mechanism established for amines. Instead, the correlation between pH gradient and neutral red uptake can be predicted quantitatively assuming that protonated neutral red is accumulated mainly as dimer species. Under appropriate conditions, accumulation of protonated neutral red in the inner thylakoid space is proportional to an absorbance increase at 520 nm. This 520-nm change can be used for the continuous measurement of pH changes in thylakoids during steady-state illumination.

The physical aspects of energy transduction in biological systems

The primary energy sources for all the organisms living on the Earth are either sunlight or the energy liberated during chemical transformations (mainly, oxidation) of certain substances – food. Within the cell this energy is transformed, accumulated, and then utilized to ensure a multitude of processes (synthesis of new low- and high-molecular compounds, muscle contraction, luminescence, transfer of ions counter to their concentration gradients, etc.).

The role of universal ‘energy keeper’, of the, as it were, ‘energy small change’ in biology is played by the molecules of adenosine tri- phosphate (ATP) whose hydrolytic dissociation in water solutions with the formation of adenosine diphosphate (ADP) and inorganic phosphate (P1) is accompanied by a rather strong decrease of system energy.†

Accumulation of thallous ions (Tl+) as a measure of the electrical potential difference across the cytoplasmic membrane of bacteria

The accumulation of thallous ions (204Tl+) by intact bacteria was investigated. I conclude that Tl+ is a permeant cation, and that it therefore accumulates in response to the electrical potential difference (delta psi) across the cytoplasmic membrane (interior negative). A comparison with other methods shows that the distribution ratio of 204Tl+ serves as a reasonably satisfactory method for measuring the membrane potential of Streptococcus faecalis. Glycolyzing cells of this organism develop membrane potentials of up to 180 mV. Preliminary experiments with Escherichia coli, especially those with a mutant defective in the proton-translocating ATPase, indicate that the Tl+ distribution also serves as a measure of the membrane potential in this organism. The particular advantage of Tl+ over other indicators of the membrane potential is that the cells need not be pretreated in any way. By use of the Tl+ distribution, it was calculated that respiring cells of E. coli develop a membrane potential of 160 mV with D-lactate and 180 mV with glucose as a substrate, respectively.

Effect of nucleotides on potential and pH changes across the thylakoid membrane of spinach chloroplasts

With appropriate preparations of spinish chloroplasts we observe three distinct effects of the nucleotides: 1. An accelaration of the dark decay of the light induced 520 nm absorbance change after ATP addition. 2. An acidification of the internal space of the thylakoids after ATP addition in darkness. 3. A dark ATPase activity which is regulated by the deltapH across the membrane. We conclude that the effect of the nucleoside triphosphates on the 520 nm signal is linked to a change of the proton conductivity of the membrane, induced by the formation of a deltapH across the membrane in consequence of the dark ATPase activity. The mode of action of the nucleoside diphosphates in the presence of inorganic phosphate on the 520 nm signal is discussed. It is proposed that the effects observed are linked to the hydrolysis of the newly formed nucleoside triphosphates.

Use of salicylic acid to measure the apparent intracellular pH in the Ehrlich ascites-tumor cell and Escherichia coli

The distribution of salicylic acid between the intracellular and extracellular phases has been used to estimate the intracellular pH in the Ehrlich cell and Escherichia coli. The validity of the method was established by: (i) comparison of the results obtained with salicylic acid with those obtained with 5,5-dimethyloxazolidine-2,4-dione; (ii) by following changes of the apparent intracellular pH under circumstances in which such changes are predictable, e.g., the addition of weak acids or proton conductors to the incubation medium during incubation at acidic pH; (iii) by comparison of the apparent intracellular pH changes with the uptake of H+ by the cells estimated from the changes of the medium pH. Optimal results are obtained with this indicator when the extracellular pH is below 5.5, because in this case the indicator is to a sufficient extent in its penetrating form, so that its movement can reflect intracellular pH changes occurring in less than 30 s. When the intracellular pH falls below 5.2 measurable binding of salicylic acid to the intracellular material of the Ehrlich cell takes place, but above this pH no binding has been found. The Ehrlich cell and cells of Escherichia coli behaved similarly under various experimental circumstances tested, but striking difference were found in the inherent permeability of the membrane to H+ and in the changes in this parameter by lowering the temperature to 2 degrees C.

Proton electrochemical gradient and phosphate potential in submitochondrial particles

The aerobic uptake of inorganic ions, such as 86Rb+ or 125I-, by submitochondrial particles, is about one order of magnitude lower than the uptake of organic ions, such as acridines or 8-anilino-1-naphthalene sulphonate. The values of deltapH, the transmembrane pH differential, and deltapsi, the transmembrane membrane potential are between 60 and 100 mV when calculated on the inorganic ions and between 150 and 240 mV when calculated on the organic ions. The discrepancy between the deltapH and deltapsi values from organic and inorganic ions is large at high but not at low ion/protein ratios. 2. In the absence of weak bases and strong acids the values of deltamuH, the proton electrochemical potential difference, are close to 100 mV and the magnitude of deltapH and deltapsi are similar. Weak bases decrease deltapH and enhance deltapsi. Strong acids decrease deltapsi and enhance deltapH. Interchangeability of deltapH with deltapsi occurs at low concentrations of weak bases and strong acids. High concentrations of weak bases and strong acids cause depression of deltamuH. 3. Concentrations of weak bases capable of abolishing deltapH, do not affect ATP synthesis. Concentrations of strong acids capable of abolishing deltapsi affect only slightly ATP synthesis. Concentrations of weak bases and strong acids capable of causing a decline of deltapH + deltapsi inhibit ATP synthesis. 4. Depression of deltamuH is paralleled by inhibition of ATP synthesis and decline of deltaGp, the phosphate potential. Abolition of ATP synthesis occurs only when deltamuH is below 20 mV. The deltaGp/deltamuH ratio increases hyperbolically with the decrease of deltamuH.

Phosphate transport in Micrococcus lysodeikticus

Phosphate accumulates in Micrococcus lysodeikticus cells against a concentration gradient, by an energy-dependent process. The phosphate transport is derepressed during phosphate deprivation. The depression process is inhibited by chloramphenicol. The apparent Km of phosphate transport is 4.3 micronM. The activation energy of the transport is 21 kcal per mol in the temperature range of 0-29degrees C, and 4.9 kcal per mol between 29 and 40degrees C. The rate of the transport increases in presence of K+ and Mg2+. Arsenate is a competitive inhibitor of phosphate transport, having an apparent Ki of 6.0 micronM. Sulfhydryl reagents, respiratory inhibitors and uncouplers of oxidative phosphorylation inhibit phosphate transport.

Oxidation-reduction coupled phosphorylation in the dark with isolated spinach chloroplasts

1. Spinach chloroplasts, pre-incubated with ferricyanide, acquire the ability to make ATP in the dark provided they are supplied with a reductant and a lipophilic mediator that can penetrate the membrane. The mediator must be of the type that, upon oxidation, releases protons into the surrounding medium such as 2,3,5,6-tetramethyl-p-phenylenediamine (DAD). 2. Dark phosphorylation is not affected by the electron transport inhibitor, 3(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), but is inhibited by uncouplers of photophosphorylation (e.g. NH4Cl and carbonylcyanide-m-chlorophenylhydrazone (CCCP)) and high concentrations of the energy transfer inhibitor, Dio-9. 3. Because only catalytic amounts of the mediator DAD are required to saturate dark phosphorylation, it is concluded that DAD shuttles reducing equivalents across the membrane from the reductant, ascorbate, on the outside to ferricyanide, the oxidant, trapped on the inside. 4. The results are interpreted within the framework of the chemiosmotic hypothesis for the coupling of electron transport to phosphorylation.