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

Chloroplast membrane sidedness. Location of plastocyanin determined by chemical modifiers

Intact grana and stroma membranes (outer membrane absent) and detergent or sonication disrupted thylakoid membranes were treated with the hydrophilic covalent chemical modifiers [35S]diazonium benzene sulfonic acid ([35S]DABS) and [14C]glycine ethylester plus 1-cyclohexyl-3-(2-morpholinoethyl)-carbodiimide metho-p-toluenesulfonate (CDIS). Plastocyanin was purified using column chromatography followed by polyacrylamide gel electrophoresis and the incorporation of [35S]DABS and [14C]glycine ethylester into plastocyanin was determined by slicing the gels and counting the radioactivity in the plastocyanin band. Plastocyanin isolated from thylakoids disrupted prior to chemical modification binds two to four times as much of either modifier than the plastocyanin isolated from intact chloroplasts. This ratio is five to ten times lower than the ratio expected for a component buried behind the permeability barrier of a membrane. The data suggest that plastocyanin is partially exposed at the external surface of the thylakoid membrane rather than being completely buried in, or behind, the lipo-protein membrane.

Energy conservation in chemotrophic anaerobic bacteria

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The behavior of 9-aminoacridine as an indicator of transmembrane pH difference in liposomes of natural bacterial phospholipids

The behavior of 9-aminoacridine as an indicator of pH differences artificially set across a membrane has been reexamined in liposomes prepared from bacterial phospholipids extracted from chromatophores of Rhodopseudomonas capsulata grown photoheterotrophically. The dye behaves as an ideal indicator for pH differences lower than about three units; at higher pH's the expected linear dependence of Q/(100-Q) vs. pH is no longer strictly observed. Similarly a linear dependence upon the volume of the liposomes added has been verified. The amine ceases to respond to pH changes when the pH of the external medium exceeds the value of 10, corresponsing to the pKa of 9-aminoacridine. The apparent volume of the inner phase of liposomes, as calculated from fluorescence quenching, but not the slope of dependence of fluorescence on pH, appears to be affected by several factors, including the ionic composition, the osmolarity of the external medium, and the microscopic structure of the liposomes. Millimolar concentrations of earth-alkaline cations diminish the apparent internal volume of liposomes, in agreement with the complexing effect of these ions on phospholipid bilayers. The osmotic response of the apparent inner volume has also been verified; this parameter decreases linearly with the reciprocal of the external osmolarity, as expected from the van't Hoff relation; an osmolarity exceeding 0.3 M is, however, necessary in order to observe this effect.

Generation of a transmembrane electric potential during respiration by Azotobacter vinelandii membrand vesicles

Membrane vesicles isolated from Azotobacter vinelandii strain O by lysis of spheroplasts in potassium of sodium phosphate buffer develop a transmembrane electric potential during respiration. The magnitude of this potential was determined by three independent methods: (i) fluorescence of 3,3'-dipropylthiodicarbocyanine and 3,3'-dihexyloxacarbocyanine; (ii) uptake of 86Rb+ in the presence of valinomycin; and (iii) uptake of [3H]triphenylmethyl phosphonium. In method (i), the relative fluorescence of these cyanine dyes in the presence of intact cells or derived vesicles is quenched during oxication of electron donors. A linear relationship between this quenching and a potassium diffusion potential was employed to calibrate the probe response. In method (ii), the steady-state concentration ratio of rubidium across the vesicle membrane during oxidation of L-malate was converted to potential by the Nernst equation. In method (iii), the steady-state concentration ratio of this lipophilic cation was likewise converted to a potential. With the exception of 3,3'-dihexyloxacarbocyanine fluorescence, these methods gave good agreement for the potential developed during L-malate oxidation by membrane vesicles. A value of 75 to 80 mV (inside negative) was obtained for vesicles prepared in potassium phosphate, and 104 mV (inside negative) was obtained for vesicles prepared in sodium phosphate. Electrogenic expulsion of hydrogen ion was observed during L-malate oxidation, and the amount of proton exodus was greater in potassium rather than the sodium-containing vesicles. This indicates the presence of a sodium-proton antiport mechanism. In addition, D-glucose uptake was observed during development of a potassium diffusion potential that was artificially imposed across the vesicle membrane. These observations suggest the presence of a glucose-proton symport mechanism in accordance with the principles of Mitchell.

Thermoplasma acidophilum: intracellular pH and potassium concentration

Thermoplasma acidophilum is a free-living thermophilic mycoplasma. Although the organism lacks a cell wall, it can grow in medium as dilute as 66 mosM. The intracellular K+ concentration can be as low as 17 mM, but varies according to the osmolality of the culture medium. The internal pH can be measured by taking advantage of the fact that T. acidophilum undergoes lysis when the pH is adjusted to neutrality. Thus, by appropriate analysis of titration curves, it is possible to conclude that the internal pH is near 5.5. This result was confirmed by a second type of experiment in which the internal pH was analyzed by rupturing the cells in a French Pressure Cell.

A method for measuring the internal pH in illuminated chloroplasts based on the stimulation of proton uptake by amines

A simple method for measuring the internal pH of chloroplasts during steady-state illuminations based on the stimulation of proton uptake by monofunctional amines was developed. Predictions of a mathematical derivation concerning the dependence of the stimulation on the amine concentration, the internal volume, the pK of the amine and the external pH have been verified experimentally. To circumvent uncoupling and swelling due to large internal accumulation of amines extrapolation of the stimulation to low amine concentrations was suggested and shown to lead to valid values. Alternatively, swelling could be largely reduced in a medium containing potassium aspartate and valinomycin.

Photophosphorylation as a function of illumination time. I. Effects of permeant cations and permeant anions

(1) Very brief periods of illumination do not initiate photophosphorylation in isolated chloroplast lamellae. The time of illumination required before any phosphorylation can be detected is inversely proportional to the light intensity. At very high intensities, phosphorylation is initiated after illumination for about 4 ms. (2) There is no similar delay in the initiation of electron transport. The rate of electron transport is very high at first but declines at about the time the capacity for ATP synthesis develops. When the chloroplasts are uncoupled with gramicidin the high initial rate persists. (3) Various ions which permeate the thylakoid membrane (K+ or Rb+ in the presence of valinomycin, SCN-, I-, or C1O4-) markedly increase the time of illumination required to initiate phosphorylation. Potassium ions in the presence of valinomycin increase the delay to a maximum of about 50 ms whereas thiocyanate ions increase the delay to a maximum of about 25 ms. The effects of K+ with valinomycin and the effect of SCN- are not additive. Permeant ions and combinations of permeant ions have little or no effect on phosphorylation during continuous illumination. (4) The reason for the threshold in the light requirement and the reason for the effect of permeant ions thereon are both obscure. However, it could be argued that the energy for phosphorylation initially resides in an electric potential gradient which is abolished by migration of ions in the field, leaving a more slowly developing proton concentration gradient as the main driving force for phosphorylation during continuous illumination. If so, the threshold in the presence of permeant ions should depend on internal hydrogen ion buffering.

Photophosphorylation as a function of illumination time. II. Effects of permeant buffers

(1) The amounts of orthophosphate, bicarbonate and tris (hydroxymethyl)-aminomethane found inside the thylakoid are almost exactly the amounts predicted by assuming that the buffers equilibrate across the membrane. Since imidazole and pyridine delay the development of post-illumination ATP formation while increasing the maximum amount of ATP formed, it follows that such relatively permeant buffers must also enter the inner aqueous space of the thylakoid. (2) Photophosphorylation begins abruptly at full steady-state efficiency and full steady-state rate as soon as the illumination time exceeds about 5 ms when permeant ions are absent or as soon as the time exceeds about 50 ms if valinomycin and KC1 are present. In either case, permeant buffers have little or no effect on the time of illumination required to initiate phosphorylation. A concentration of bicarbonate which would delay acidification of the bulk of the inner aqueous phase for at least 350 ms has no effect at all on the time of initiation of phosphorylation. In somewhat swollen chloroplasts, the combined buffering by the tris(hydroxymethyl) aminomethane and orthophosphate inside would delay acidification of the inside by 1500 ms but, even in the presence of valinomycin and KC1, the total delay in the initiation of phosphorylation is then only 65 ms. Similar discrepancies occur with all of the other buffers mentioned. (3) Since these discrepancies between internal acidification and phosphorylation are found in the presence of saturating amounts of valinomycin and KC1, it seems that photophosphorylation can occur when there are no proton concentration gradients and no electrical potential differences across the membranes which separate the medium from the greater part of the internal aqueous phase. (4) We suggest that the protons produced by electron transport may be used directly for phosphorylation without even entering the bulk of the inner aqueous phase of the lamellar system. If so, phosphorylation could proceed long before the internal pH reflected the proton activity gradients within the membrane.

An estimation of the light-induced electrochemical potential difference of protons across the membrane of Halobacterium halobium

The light-dependent uptake of triphenylmethylphosphonium (TPMP+) and of 5,5-dimethyloxazolidine-2,4-dione (DMO) by starved purple cells of Halobacterium halobium was investigated. DMO uptake was used to calculate the pH difference (deltapH) across the membrane, and TPMP+ was used as an index of the electrical potential difference, deltapsi. Under most conditions, both in the light and in the dark, the cells are more alkaline than the medium. In the light at pH 6.6, deltapH amounts to 0.6-0.8 pH unit. Its value can be increased to 1.5-2.0 by either incubating the cells with TPMP+ (10(-3) M) or at low external pH (5.5). --deltapH can be lowered by uncoupler or by nigericin. The TPMP+ uptake by the cells indicates a large deltapsi across the membrane, negative inside. It was estimated that in the light, at pH 6.6, deltapsi might reach a value of about 100 mV and that consequently the electrical equivalent of the proton electrochemical potential difference, deltamuH+/F, amounts under these conditions to about 140 mV. The effects of different ionophores on the light-drive proton extrusion by the cells were in agreement with the effects of these compounds on --deltapH.

Age-related increase of cuticle permeability in the nematode Caenorhabditis briggsae

The external cuticular surface of nematodes, which resembles cellular membranes in certain ways, appears to deteriorate with age. For example, when the permeabilities to radioactive water of young and old nematodes were compared, and the data were corrected for the different surface: volume ratios, the older nematodes were significantly more permeable. In both living and dead nematodes, the same rates of water exchange were observed, indicating that the major route of exchange was probably by passive diffusion through the cuticle rather than by active processes such as swallowing or excreting water.

Stimulation of photosystem I-induced oxidation of chloroplast cytochrome b-559 by pre-illumination and by low pH

(1) The proportion of higher plant chloroplast cytochrome b-559 oxidizable during illumination by low intensity 732 nm light increases as the pH is decreased below 6.5. At pH 5.0-5.3 total oxidation is seen and subsequent red light can cause reduction of up to 2/3 of the oxidized cytochrome. The oxidation by far red light at pH 5 is inhibited by 2 muM 2,5-dibromo-3-methyl-6-isopropyl-rho-benzoquinone whereas the red light-induced reduction is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In this pH range ferricyanide-oxidized cytochrome b-559 exists in a form not reducible by ferrocyanide. (2) An increase in the amplitude of far-red induced oxidation also occurs at higher pH (up to pH 7.8) after pre-treatment of chloroplasts with substantially higher levels of light (approx. 10(6) ergs-cm-2-s-1). The degree of light activation is pH dependent, being more pronounced at lower pH. After light activation, cytochrome b-559 can be completely oxidized by far-red light in a manner reversible by red light up to pH values of 6, and the curve describing the amplitude of far-red oxidation as a function of pH is shifted by 0.5-1.0 pH unit toward higher pH. Far-red oxidation and red light reduction are again inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, respectively. (3) Light activation at pH 5.2-6.0 is also manifested in a small decrease in the amplitude of subsequent dark ferrocyanide reduction, and this decrease is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (10 muM). (4) The effect of intramembranal acidity on the effective redox potential of cytochrome b-559 and its function is discussed.

The proton electrochemical gradient in Escherichia coli cells

The internal pH of Escherichia coli cells was estimated from the distribution of either 5,5-[14C]dimethyl-2,4-oxazolidinedione or [14C]methylamine. EDTA/valinomycin treatment of cells was employed to estimate delta psi from 86Rb+ distribution concomitant with the delta pH for calculation of delta muH. Respiring intact cells maintained an internal pH more alkaline by 0.63-0.75 unit than that of the milieu at extracellular pH 7, both in growth medium and KCl solutions. The delta pH decreased when respiration was inhibited by anaerobiosis or in the presence of KCN. The delta muH, established by EDTA/valinomycin-treated cells, was constant (122-129 mV) over extracellular potassium concentration of 0.01 mM-1 mM. At the lower potassium concentration delta psi (110-120 mV) was the predominant component, and at the higher concentration delta pH increased to 0.7 units (42 mV). At 150 mM potassium delta muH was reduced to 70 mV mostly due to a delta pH component of 0.89 (53 mV). The interchangeability of the delta muH components is consistent with an electronic proton pump and with potassium serving as a counter ion in the presence of valinomycin. Indeed both parameters of delta muH decreased in the presence of carbonylcyanide p-trifluoromethoxyphenylhydrazone. The highest delta pH of 2 units was observed in the intact cells at pH 6; increasing the extracellular pH decreased the delta pH to 0 at pH 7.65 and to -0.51 at pH 9. A similar pattern of dependence of delta pH on extracellular pH was observed in EDTA/valinomycin-treated cells but the delta psi was almost constant over the whole range of extracellular pH values (6-8) implying electroneutral proton movement. Potassium is specifically required for respiration of EDTA-treated E. coli K12 cells since other monovalent or divalent cations could not replace potassium and valinomycin was not required.

Determination of H+/e- ratios in chloroplasts with flashing light

Using a rapid pH electrode, measurements were made of the flash-induced proton transport in isolated spinach chloroplasts. To calibrate the system, we assumed that in the presence of ferricyanide and in steady-state flashing light, each flash liberates from water one proton per reaction chain. We concluded that with both ferricyanide and methylviologen as acceptors two protons per electron are translocated by the electron transport chain connecting Photosystem II and I. With methyl viologen but not with ferricyanide as an acceptor, two additional protons per electron are taken up due to Photosystem I activity. One of these latter protons is translocated to the inside of the thylakoid while the other is taken up in H2O2 formation. Assuming that the proton released during water splitting remains inside the thylakoid, we compute H+/e- ratios of 3 and 4 for ferricyanide and methylviologen, respectively. In continuous light of low intensity, we obtained the same H+/e- ratios. However, with higher intensities where electron transport becomes rate limited by the internal pH, the H+/e- ratio approached 2 as a limit for both acceptors. A working model is presented which includes two sites of proton translocation, one between the photoacts, the other connected to Photosystem I, each of which translocates two protons per electron. Each site presents a approximately 30 ms diffusion barrier to proton passage which can be lowered by uncouplers to 6-10 ms.

Relations between the electrical potential, pH gradient, proton flux and phosphorylation in the photosynthetic membrane

The transmembrane electrical potential (deltaphi), the proton flux (H+), the rate of electron transport (e), the pH gradient (deltapH) and the rate of phosphorylation (ATP) were measured in chloroplasts of spinach. Photosynthesis was excited periodically with flashes of variable frequencies and intensities. A new method is described for determining the rate of electron transport and proton flux. Under conditions where the rate of electron transport and proton flux are not pH controlled the following correlations were found in the range 50 mV less than or equal to deltaphi less than or equal to 125 mV and 1.8 less than or equal to deltapH less than or equal to 2.7: (1) The pH gradient, deltapH, increases with H+ independently of Phout between 7-9. (2) The rate of phosphorylation, ATP, depends exponentially on deltapH (at constant deltaphi) and is independent of pHout between 7-9. (3) The rate of phosphorylation, ATP, depends also on deltaphi (at constant deltapH and at constant proton flux H+). (4) The proton flux via the ATPase pathway, Hp+, depends non-linearly on the ratio of the proton concentrations: Hp+ approximately (Hin+/Hout+)b, (b=2.3--2.6). The proton flux via the basal pathway, Hb+, depends linearly on the ratio of the proton concentrations: Hb+ approximately (Hin/Hout). (5) The ratio deltaH+/ATP (e/ATP, i.e. the ratio of the total proton flux, Hp+ + Hb+, and the rate of ATP formation, ATP, depends strongly on deltaphi and on deltapH. The ratio is deltaH+/ATP approximately 3 (e/ATP approximately 1.5) at deltapH 2.7 and deltaphi = 125 mV. (6) It is supposed that the reason for the dependence of deltaH+/ATP on deltaphi anddeltapH is the different functional dependence of the basal proton flux Hb+ and the phosphorylating proton flux Hp+ on deltapH and deltaphi. The calculation of deltaH+/ATP on the basis of this assumption is in fair agreement with the experimental values. Also the "threshold" effects can be explained in this way. (7) The ratio of deltaHp+/ATP, i.e. the ratio of the phosphorylating proton flux Hp+ and ATP, is deltaHp+/ATP APPROXIMATELY 2.4.

Influence of unsaturated fatty acids in chloroplasts. Shift of the pH optimum of electron flow and relations to deltapH, thylakoid internal pH and proton uptake

Linolenic acid (C18:3) is the main endogenous unsaturated fatty acid of thylakoid membrane lipids, and seems in its free form to exert significant effects on the structure and function of photosynthetic membranes. In this investigation the effect of linolenic acid was studied at various pH values on the electron flow rate in isolated spinach chloroplasts and related to deltapH, the proton pump and the pH of the inner thylakoid space (pHi). The deltapH and pHi were estimated from the extent of the fluorescence quenching of 9-aminoacridine. Linolenic acid caused a shift (approximately one unit) of the pH optimum for electron flow toward acidity in the following systems: (a) photosystems II + I (from H2O to NADP+ or to 2,6-dichlorophenolindophenol) coupled or non-coupled; (b) photosystem II (from H2O to 2,6-dichlorophenolindophenol in the presence of dibromothymoquinone). In photosystem I conditions (phenazine methosulphate), the deltapH of the control increased as a function of external pHo with a maximum around pH 8.8. When linolenic acid was added, the deltapH dropped, but its optimum was shifted toward more acidic pHo. The same phenomena were also observed in photosytems II + I (from H2O to ferricyanide) and in photosystem II conditions (from H2O to ferricyanide in the presence of dibromothymoquinone). However, the deltapH was smaller and the sensitivity of the proton gradient toward linolenic acid was eventually higher than for photosystem I electron flow activity. The proton pump which might be considered as a measure of the internal buffering capacity of thylakoids was optimum at pHo, 6.7 in the controls. An addition of linolenic acid diminished the proton pump and shifted its optimum toward higher pHo. As a consequence, pHi increased when pHo was raised. At the optimal pHo 8.6 to 9, pHi were 5 to 5.5. Additions of increasing concentrations of linolenic acid displaced the curves toward higher pHi. A decrease of pHo was therefore required to maintain the pHi in the range of 5-5.5 for maximum electron flow. In conclusion, the electron flow activity seems to be delicately controlled by the proton pump (buffer capacity), deltapH, pHi and pHo. Fatty acids damage the membrane integrity in such a way that the subtile equilibrium between the factors is disturbed.

Identification of the 120 mus phase in the decay of delayed fluorescence in spinach chloroplasts and subchloroplast particles as the intrinsic back reaction. The dependence of the level of this phase on the thylakoids internal pH

After a 500 mus laser flash a 120 mus phase in the decay of delayed fluorescence is visible under a variety of circumstances in spinach chloroplasts and subchloroplast particles enriched in Photosystem II prepared by means of digitonin. The level of this phase is high in the case of inhibition of oxygen evolution at the donor side of Photosystem II. Comparison with the results of Babcock and Sauer (1975) Biochim. Bio-phys. Acta 376, 329-344, indicates that their EPR signal IIf which they suppose to be due to Z+, the oxidized first secondary donor of Photosystem II, is well correlated with a large amplitude of our 120 mus phase. We explain our 120 mus phase by the intrinsic back reaction of the excited reaction center in the presence of Z+, as predicted by Van Gorkom and Donze (1973) Photochem. Photobiol. 17, 333-342. The redox state of Z+ is dependent on the internal pH of the thylakoids. The results on the effect of pH in the mus region are compared with those obtained in the ms region.

ATP synthesis driven by a protonmotive force in Streptococcus lactis

An electrochemical potential difference for hydrogen ions ( a protonmotive force) was artifically imposed across the membrane of the anaerobic bacterium Streptococcus lactis. When cells were exposed to the ionophore, valinomycin, the electrical gradient was established by a potassium diffusion potential. A chemical gradient of protons was established by manipulating the transmembrane pH gradient. When the protonmotive force attained a value of 215 mV or greater, net ATP synthesis was catalyzed by the membrane-bound Ca++, Mg++ -stimulated ATPase. This was true whether the protonmotive force was dominated by the membrane potential (negative inside) or the pH gradient (alkaline inside). Under these conditions, ATP synthesis could be blocked by the ATPase inhibitor, dicyclohexylcarbodiimide, or by ionophores which rendered the membrane specifically permeable to protons. These observations provide strong evidence in support of the chemiosmotic hypothesis, which states that the membrane-bound ATPase couples the inward movement of protons to the synthesis of ATP.

pH gradient across the lysosomal membrane generated by selective cation permeability and Donnan equilibrium

The pH within isolated Triton WR 1339-filled rat liver lysosomes was determined by measuring the distribution of [14C]methylamine between the intra- and extralysosomal space. The intralysosomal pH was found to be approximately one pH unit lower than that of the surrounding medium. Increasing the extralysosomal cation concentration lowered the pH gradient by a cation exchange indicating the presence of a Donnan equilibrium. The lysosomal membrane was found to be significantly more permeable to protons than to other cations. The relative mobility of cations through the lysosomal membrane is H+ greater than Cs+ greater than Rb+ greater than K greater than Na+ greater than Li+ greater than Mg2+, Ca2+. The presented data suggest that the acidity within isolated Triton WR 1339-filled lysosomes is maintained by: (1) a Donnan equilibrium resulting from the intralysosomal accumulation of nondiffusible anions and (2) a selective permeability of the lysosomal membrane to cations.

The internal-alkaline pH gradient, sensitive to uncoupler and ATPase inhibitor, in growing Clostridium pasteurianum

1. The intracellular pH was measured in growing Clostridium pasteurianum with and acid-base equilibrium distribution method. [14C]Dimethyloxazolidinedione, [14]methylamine and [14C]acetic acid were used as "deltapH-indicators". During growth the extracellular pH decreased from 7.1 to 5.1; simultaneously the intracellular pH changed from 7.5 to 5.9. Thus, the intracellular pH was more alkaline than the extracellular pH by 0.4 to 0.8 pH-units. 2. This pH gradient (interior alkaline) was abolished by the proton conductor carbonylcyanide m-chlorophenylhydrazone and the ATPase inhibitor N,N'-dicyclohexylcarbodiimide. The pH gradient could not be demonstrated in cells depleted of an energy substrate. These results suggest that the pH gradient is formed by an ATPase-driven extrusion of protons from the cells rather than by a Donnan potential. 3. Growth of the organism was inhibited by low concentrations of both carbonylcyanide m-chlorophenylhydrazone (5 muM) and dicyclohexylcarbodiimide (5 muM). This finding suggests that the pH gradient is essential for the growing cell as it may be required for substrate accumulation and other types of transport processes.

On the steady-state electrical potential difference across the thylakoid membranes of chloroplasts in illuminated plant cells

The potential defference across the thyladoid membranes under steady-state saturating light conditions, measured with microcapillary glass electrodes, was found to be small as compared to the potential initially generated at the onset of illunimation. This result is discussed to be in agreement with quantitative estimates on the approximate magnitudes of the potential generating electron flux through the photo-synthetic electron transport chain and of the potential dissipating ion fluxes across the thylakoid membrane under steady-state conditions. It is concluded that a pH gradient of approx. 3-3.4 units is built up in the light across the membrane. The negative diffusion potential associated with this gradient is suggested to cause the transient negative potential observed in the dark after illumination.

A partial reaction in photosystem II: reduction of silicomolybdate prior to the site of dichlorophenyldimethylurea inhibition

Silicomolybdate functions as an electron acceptor in a Photosystem II water oxidation (measured as O2 evolution) partial reaction that is 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) insensitive, that is, reduction os silicomolybdate occurs at or before the level of Q, the primary electron acceptor for Photosystem II. This report characterizes the partial reaction with the principal findings being as follows: 1. Electron transport to silicomolybdate significantly decreased room temperature Photosystem I side of the DCMU had no effect on the fluorescence level, consistent with silicomolybdate accepting electrons at or before Q. In the absence of DCMU, silicomolybdate is also reduced at a site on the Photosystem I side of the DCMU block, prior to or at plastoquinone, since the plastoquinone antagonist dibromothymoquinone (DBMIB) did not affect the electron transport rate. 3. Electron transport from water to silicomolybdate (+ DCMU) is not coupled to ATP formation, nor is there a measurable accumulation of protons within the membrane (measured by amine uptake). Silicomolybdate is not inhibitory to phosphorylation per se since neither cyclic nor post-illumination (XE) phosphorylation were inhibited. 4. Uncouplers stimulated electron transport from water to silicomolybdate in the pH range of 6 to 7, but inhibited at pH values near 8. These data are consistent with the view that when electron flow is through the abbreviated sequence of water to Photosystem II to silicomolybdate (+ DCMU), conditions are not established for the water protons to be deposited within the membrane. Experiments reported elsewhere (Fiaquinta, R.T., Dilley, R.A. and Horton, P.(19741 J. Bioenerg. 6, 167-177) and these data, are consistent with the hypothesis that electron transport between Q and plastoquinone energizes a membrane conformational change that is required to interact with the water oxication system so as to result in the deposition of water protons either within the membrane itself or within the inner oxmotic space.

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.

Control of proton translocation induced by ATPase activity in chloroplasts

1. Proton uptake was induced by ATP in the dark following light triggering of ATPase activity in chloroplasts. The accumulated protons were released when ATPase activity was inhibited by the energy transfer inhibitor DIO-9. 2. Approximately two protons were taken up for each ATP hydrolyzed at pH 8. A drop in H+/ATP ratio was caused by uncouplers such as NH4Cl and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. These uncouplers caused an increase in the rate of ATP hydrolysis without a corresponding increase in proton uptake. 3. The energy transfer inhibitor dicyclocarbodiimide inhibited both ATPase activity and the rate of proton uptake without changing the H+/ATP ratio. 4. The antibiotic valinomycin caused an increase in the rate of both proton uptake and ATP hydrolysis without altering the ratio of H+/ATP. The H+/ATP ratio varied with changes in the external pH. The results were discussed in view of the chemiosmotic theory of oxidative and photosynthetic phosphorylation.