Photoelectric response generated under non-heme iron reduction on the photosystem II acceptor side

Proteoliposomes containing oxygen-evolving particles of Photosystem II and associated with a planar phospholipid membrane generate a transmembrane electric potential difference (DeltaPsi) induced by a laser flash. With direct electrometrical technique, it was shown that the direction of the electrical field ("minus" inside the proteoliposome) corresponds to acceptor side of the Photosystem II complex facing inside and donor side facing outside of the liposomes. In addition to the fast phase (tau < 0.1 microsec) of the DeltaPsi generation due to electron transfer between YZ of the water-oxidizing complex and the primary plastoquinone QA, a phase with tau approximately 120 microsec and maximum amplitude approximately 30% of the amplitude of the fast phase was observed under the first flash in proteoliposomes containing potassium ferricyanide, which is known as an oxidant of the non-heme iron (Fenh) on the acceptor side of Photosystem II. This additional phase was absent under the second laser flash but was completely restored after 5 min dark adaptation. The phase of the photoelectric response with tau approximately 120 microsec is probably due to electron transfer from QA to Fenh(III) and likely includes a component related to H+ transfer.     

Electrogenic Protonation of the Secondary Quinone Acceptor Q<Subscript>B</Subscript> in Spinach Photosystem II Complexes Incorporated into Lipid Vesicles

The generation of transmembrane electric potential difference (delta psi) in quinone acceptor complex of proteoliposomes containing core complexes of photosystem II from spinach was studied using for the measurements a direct electrometric technique. Besides the fast increase in the membrane potential associated with the electron transfer between the redox-active tyrosine 161 residue (Y(Z)) in D1 polypeptide and the primary quinone acceptor Q(A), an additional electrogenic phase with tau approximately 0.85 msec at pH 7.3 and the maximal relative amplitude of approximately 11% of the Y(Z)ox Q(A)- phase was observed after the second light flash. The sensitivity of this phase to diuron (an inhibitor of electron transfer between Q(A) and the secondary quinone acceptor Q(B)), the dependence of its amplitude on the light flash parity, and also a decrease in its rate constant with increase in pH indicated that it was due to dismutation of Q(A)- and Q(B)- with the subsequent protonation of a doubly reduced plastoquinone molecule: Q(A)- Q(B)- + 2H+ --> Q(A)Q(B)H2.     

Transmembrane outward hydrogen current in intracellularly perfused neurones of the snail Helix pomatia

The ionic nature and pharmacological properties of the outward current activated by membrane depolarization were studied on isolated neurones of the snail Helix pomatia, placed in Na+- and Ca2+-free extracellular solutions and intracellularly perfused with K+-free solution ("nonspecific outward current"). It was shown that the amplitude and reversal potential of this current (estimated from instantaneous current-voltage characteristics) are determined mainly by the transmembrane gradient for H+ ions. Lowering of pHi induced an increase in the current amplitude and a shift of the reversal potential to more negative values; the shift magnitude was comparable with that predicted for the hydrogen electrode. Raising pHi, as well as lowering pHo, induced a decrease in the current amplitude and a displacement of the current activation curve to more positive potentials. Addition of EGTA (8 mmol/l) to the intracellular perfusate did not affect the current amplitude. Extracellular 4-aminopyridine (10 mmol/l), verapamil (0.25 mmol/l) or Cd2+ (0.5 mmol/l) blocked the current. It is concluded that the current studied is carried mainly by H+ ions. In the same neurones the nature of the fast decay of the calcium inward current was also studied (in the presence of extracellular Ca2+ ions). This decay considerably slowed when pHi was raised or pHo was lowered, and it became less pronounced upon extracellular application of 4-aminopyridine or upon intracellular introduction of phenobarbital (4 mmol/l) and tolbutamide (3 mmol/l). It is suggested that the fast decay of the calcium inward current is due to activation of a Ca-sensitive component of the hydrogen current which depends on accumulation of Ca2+ ions. The possible physiological role of the transmembrane hydrogen currents is discussed.     

ATP synthesis in Staphylococcus aureus cells during induction of membrane potentials and proton gradient

The dynamics of ATP synthesis in Staphylococcus aureus cells was studied during membrane potential induction and K+ gradient generation in the presence of valinomycin. The starting level of intracellular ATP was 0.05 mM. Valinomycin (30 micrograms/ml) caused an increment of the intracellular ATP level up to 0.25 mM. The protonophore uncoupler, m-chlorinecarbonylcyanidephenylhydrazonium, and the H+-ATPase inhibitor, N,N'-dicyclohexylcarbodiimide, effectively suppress ATP synthesis induced by valinomycin. No ATP synthesis occurs at K+ concentration of 200 mM. The transmembrane gradient formation results in the synthesis of a smaller amount of ATP (0.10 mM).     

Effects of hyperthermia on the membrane potential and Na+ transport of V79 fibroblasts

The effects of hyperthermia (41-43 degrees C) on the membrane potential (calculated from the transmembrane distribution of [3H]tetraphenylphosphonium) and Na+ transport of Chinese hamster V79 fibroblasts were studied. At 41 degrees C, hyperthermia induced a membrane hyperpolarization of log phase cells (5 to 26 mV) that was reversible upon returning to 37 degrees C. The hyperpolarization was inhibited 50% by 1 mM ouabain or 0.25 mM amiloride, an inhibitor of Na+:H+ exchange. Shifting temperature to 41 degrees C increased ouabain-sensitive Rb+ uptake indicating activation of the electrogenic Na+ pump. At 43 degrees C for 60 min, the membrane potential of log phase cells depolarized (20-35 mV). Parallel studies demonstrated enhanced Na+ uptake at 41 degrees C only in the presence of ouabain. At 43 degrees C, Na+ uptake was increased relative to controls with or without ouabain present. At both 41 and 43 degrees C, 0.25 mM amiloride inhibited heat-stimulated Na+ uptake. Na+ efflux was enhanced at 41 degrees C in a process inhibited by ouabain. Thus, one consequence of heat treatment at 41 degrees C is activation of Na+:H+ exchange with the resultant increase in cytosolic [Na+] activating the electrogenic Na+ pump. At temperatures greater than or equal to 43 degrees C, the Na+ pump is inhibited.     

The mechanism of lead-induced mitochondrial Ca2+ efflux

Addition of Pb²⁺ to rat kidney mitochondria is followed by induction of several reactions: inhibition of Ca²⁺ uptake, collapse of the transmembrane potential, oxidation of pyridine nucleotides, and a fast release of accumulated Ca²⁺. When the incubation media are supplemented with ruthenium red, the effect of Pb²⁺ on NAD(P)H oxidation, membrane , and Ca²⁺ release are not prevented if malate-glutamate are the oxidizing substrates; however, the latter two lead-induced reactions are prevented by ruthenium red if succinate is the electron donor. It is proposed that in mitochondria oxidizing NAD-dependent substrates, Pb²⁺ induces Ca²⁺ release by promoting NAD(P)H oxidation and a parallel drop in due to its binding to thiol groups, located in the cytosol side of the inner membrane. In addition, it is proposed that with succinate as substrate, the Ca²⁺-releasing effect of lead is due to the collapse of the transmembrane potential as a consequence of the uptake of Pb²⁺ through the calcium uniporter, since such effect is ruthenium red sensitive.     

Na/H exchange in cultured chick heart cells. pHi regulation

The purpose of this study was to establish the existence of Na/H exchange in cardiac muscle and to evaluate the contribution of Na/H exchange to pHi regulation. The kinetics of pHi changes in cultured chick heart cells were monitored microfluorometrically with 6-carboxyfluorescein and correlated with Nai content changes analyzed by atomic absorption spectrophotometry; transmembrane H+ movements were evaluated under pH stat conditions. After induction of an intracellular acid load by pretreatment with NH4Cl, a regulatory cytoplasmic alkalinization occurred with a t1/2 of 2.9 min. pHi regulation required external Na+ and was concomitant with transmembrane H+ extrusion as well as a rapid rise in Nai content in an Na/H ratio of 1:1. Microelectrode recordings of membrane potential demonstrated directly the electroneutral character of pHi regulation. Acid-induced net Na+ uptake could be either stimulated by further decreasing pHi or inhibited by decreasing pHo; Na+ uptake was unaffected by tetrodotoxin (10 micrograms/ml), quinidine (10(-3) M), DIDS (10(-4) M), Clo-free solution, or HCO3-free solution. Amiloride (10(-3) M) maximally inhibited both pHi regulation and Na+ uptake; the ID50 for amiloride inhibition of Na+ uptake was 3 microM. Nao-dependent H+ extrusion showed half-maximal activation at 15 mM Nao; Li+, but not K+ or choline+, could substitute for Na+ to support H+ extrusion. Cao-free solution also stimulated acid-induced Na+ uptake. We conclude that pHi regulation following an acid load in cardiac muscle cells is by an amiloride-sensitive, electroneutral Na/H exchange. Stimulation of Na/H exchange up to 54 pmol/cm2 X s indicates the rapidity of this exchange across cardiac cell membranes. Na/H exchange may also participate in steady state maintenance of pHi.     

Electrical potential changes, H+ translocation and phosphorylation induced by short flash excitation in Rhodopseudomonas sphaeroides chromatophores.

1. The basal decay of the carotenoid shift of chromatophores from photosynthetic bacteria following short flash excitation is approximately biphasic. The decay indicates the dissipation of the transmembrane electrical potential. 2. The H+ efflux following rapid H+ binding after a flash, measured from the colour change of added cresol red, shows very similar kinetics to the carotenoid shift decay suggesting that the dissipation of the electric potential decay is a consequence of the H+ efflux. 3. The electric potential decay is stimulated when the chromatophore suspension is supplemented with ADP and Pi (in either the presence or absence of antimycin A). 4. The stimulated electric potential decay by ADP and Pi has a similar pH dependence to that of phosphorylation in continuous light. 5. The stimulation of the electric potential decay by ADP and Pi is reversed, by aurovertin, an antibiotic which inhibits phosphorylation. 6. The stimulation of the electric potential decay by ADP+Pi is also reversed by the inhibitors oligomycin and venturicidin. These inhibitors, but not aurovertin, also inhibit the fast phase of the decay under non-phosphorylating conditions. 7. Valinomycin accelerates the overall rate of decay of the electric potential, inhibits the ADP and Pi stimulated electric potential decay, and inhibits the flash-induced phosphorylation. The decay rate of the H+ efflux however, is slower in the presence of this ionophore. 8. Nigericin-type ionophores accelerate the overall decay rate of the H+ efflux and inhibit the ADP and Pi stimulated electric potential decay. The basal rate of the electric potential decay is unaffected by treatment with these ionophores. 9. When a coupling factor associated with the chromatophore ATPase is removed from the membrane, both the stimulation of the electric potential decay by ADP and Pi and ADP phosphorylation are inhibtied. Both reactions are completely restored after reconstitution with the crude coupling factor extract. The basal electric potential decay rate is not affected by the removal of coupling factor.     

A model of photosystem II for the analysis of a fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a plant efficiency analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (delta psi(t)) as well as pH in the lumen (pHL(t)) and stroma (pHs(t)). PS II model parameters and numerical coefficients in delta psi(t), and pHs(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when delta psi = 0 and pHL(t), pHS(t) altered to small extent relative to control values in vivo, with maximum delta psi(t) approximately 90 MB and delta psi(t) approximately 40 MB, for the stationary state at deltapH aproximately equal to 1.8. As the light intensity was increased from 300 to 1200 micromol x m(-2) x s(-1), the heat dissipation rate constants increased threefold for nonradiative recombination of P680+Phe- and by approximately 30% for P680+Q(A)-. The parameters delta psi, pH(S) and pH(L) were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of qE quenching is discussed, which is related with light induced pH(L) lumen acidification, when Q(A)- and P680+ recombination probability increases to regulate the QA reduction.     

The oxygen dependence of the mitochondrial respiration rate in ascites tumor cells.

The effect of the oxygen concentration on the rate of oxygen consumption by 786 and TA3 ascites tumor cell lines has been determined under steady-flow conditions with a membraneless fast-responding O2 electrode and using ascorbate and N,N,N',N'-tetramethyl-p-phenylenediamine as electron donors. The reaction was initiated by rapid injection of O2 into anaerobically incubated test system. The time-dependence of the intact cell respiration showed three distinct phases; an early very fast but short duration phase, a subsequent slow phase that prevailed for most of the reaction period and a third phase which preceded the reestablishment of anaerobiosis. Kinetic analysis of the reaction indicated a linkage between the catalytic efficiency and the transmembrane electrochemical potential. The rates of O2 uptake, obtained in the presence of both protonophores and ionophores, were monotonic and pseudo-first order over 90% of the course of O2 consumption. Extrapolation of the observed rates to zero time, at which zero delta mu H+ and thus constant flow prevails, was used to calculate the oxygen concentration for the half-maximal respiratory rate, which was found to be in the range 1.55-2.10 microM O2. No noticeable variation in the value of this kinetic parameter was found between the two cell lines used. Possible reasons for discrepancies in published reports on the oxygen dependence of the cytochrome c oxidase activity in various mitochondrial and reconstituted systems are discussed.     

Distribution and Activity of the Plasma Membrane H+-ATPase in Mimosa pudica L. in Relation to Ionic Fluxes and Leaf Movements

Plasma membrane H+-ATPase was immunolocalized in several cell types of the sensitive plant Mimosa pudica L., and transmembrane potentials were measured on cortical cells. In comparison with the nonspecialized cortical cells of the petiole or stem, the proton pump was highly expressed in motor cells. These immunological data are in close agreement with electrophysiological data, because the active component of the transmembrane potential was low in the nonspecialized cortical cells and high in motor cells. Therefore, motor cells contain the plasma membrane H+-ATPase required to mediate the ionic fluxes that are involved in circadian leaf movements and that are necessary to recover the turgor potential that is considerably affected by the large K+ and Cl- efflux associated with seismonastic movement. With the exception of sieve tubes, the phloem also had a high density of H+-ATPase. This suggests that the recovery of the transmembrane ionic gradients (K+ and Cl-), which is affected by various stimuli, is more energized by the companion and parenchyma cells than by the sieve elements. In addition, at the phloem/cortex interface collocytes displayed the required properties for lateral transduction of the action potential toward the pulvinal motor cells.     

Superstoichiometric Ca2+ uptake supported by hydrolysis of endogenous ATP in rat liver mitochondria.

The nature of the energy store causing rapid superstoichiometric leads to H+/2e minus ejection and leads to Ca2+/2e minus uptake ratios in rat liver mitochondria pulsed with Ca2+ has been investigated. The extent and the rate of the initial fast superstoichiometric phase of H plus ejection were greatly reduced by oligomycin and other ATPase inhibitors; the subsequent shoichiometric phase was unaffected. No such inhibition was seen with atractyloside. Similarly, the initial fast phase of Ca2+ uptake was reduced in extent by oligomycin, whereas the slower stoichiometric phase was unaffected. Moreover, the ATP content of mitochondria previously incubated with succinate decreased by about 80% within 5 s after pulsing with Ca2+. The energy store for superstoichiometric Ca2+ uptake and H plus injection is thus identified as endogenous ATP.     

Reduced capacity of Ca<Superscript>2+</Superscript> retention in liver as compared to kidney mitochondria. ADP requirement

Ca²⁺ loading in mitochondria promotes the opening of a non-selective transmembrane pathway. Permeability transition is also associated with the interaction of cyclophilin D at the internal surface of the non-specific transmembrane pore. This interaction is circumvented by cyclosporin A and ADP. Our results show that, in the absence of ADP, liver mitochondria were unable to retain Ca²⁺, they underwent a fast and large amplitude swelling, as well as a rapid collapse of the transmembrane potential. In contrast, in the absence of ADP, kidney mitochondria retained Ca²⁺, swelling did not occur, and the collapse of the membrane potential was delayed. Ca²⁺ efflux was reversed by the addition of ADP and cyclosporin A. Our findings indicate that the differences between liver and kidney mitochondria are due to the low association of cyclophilin D to the ADP/ATP carrier found in kidney mitochondria as compared to liver mitochondria.     

Characterization of Ca2+ transients induced by intracellular photorelease of InsP3 in mouse ovarian oocytes

Ca2+ transients (measured with Fluo-3) were induced in single mouse ovarian oocytes by photolytic liberation of InsP3. The time course of cytosolic Ca2+ changes induced in this way is composed of distinct phases: upstroke, fast decline, slow declining plateau and fast decline to rest level. All the phases reflect mainly intracellular redistributions of the ion and not influx, since they are not strongly dependent on external Ca2+ or on changes in transmembrane potential. Often sustained Ca2+ oscillations followed the first InsP3-induced Ca2+ transient. These persisted for several minutes in the absence of external Ca2+. The initial rate of Ca2+ rise and the delay between the InsP3 stimulus and Ca2+ upstroke are correlated with the amount of liberated InsP3. A second InsP3 stimulation, applied during the plateau, causes only small Ca2+ elevations, lacking the upstroke phase. A second, full sized, transient could be elicited only after a complete return to the basal level. Vanadate, applied intracellularly, appeared to inhibit the re-uptake phase into the stores, stabilizing the plateau level. The present observations suggest that in mouse oocytes the InsP3-sensitive stores provide only a small and graded Ca2+ release which may then act as a trigger for a more substantial Ca(2+)-induced Ca2+ release (CICR) process.     

Tonoplast Na+/H+ Antiport Activity and Its Energization by the Vacuolar H+-ATPase in the Halophytic Plant Mesembryanthemum crystallinum L

Tonoplast vesicles were isolated from leaf mesophyll tissue of the inducible Crassulacean acid metabolism plant Mesembryanthemum crystallinum to investigate the mechanism of vacuolar Na+ accumulation in this halophilic species. In 8-week-old plants exposed to 200 mM NaCl for 2 weeks, tonoplast H+-ATPase activity was approximately doubled compared with control plants of the same age, as determined by rates of both ATP hydrolysis and ATP-dependent H+ transport. Evidence was also obtained for the presence of an electroneutral Na+/H+ antiporter at the tonoplast that is constitutively expressed, since extravesicular Na+ was able to dissipate a pre-existing transmembrane pH gradient. Initial rates of H+ efflux showed saturation kinetics with respect to extravesicular Na+ concentration and were 2.1-fold higher from vesicles of salt-treated plants compared with the controls. Na+-dependent H+ efflux also showed a high selectivity for Na+ over K+, was insensitive to the transmembrane electrical potential difference, and was more than 50% inhibited by 200 [mu]M N-amidino-3,5-diamino-6-chloropyrazinecarboxamide hydrochloride. The close correlation between increased Na+/H+ antiport and H+-ATPase activities in response to salt treatment suggests that accumulation of the very high concentrations of vacuolar Na+ found in M. crystallinum is energized by the H+ electrochemical gradient across the tonoplast.     

Formation of the transmembrane electrochemical potential on Staphylococcus cells and membrane vesicles

The generation of transmembrane difference of electrochemical potentials was registered on the intact cells and ultrasonication-obtained membrane vesicles of Staphylococcus aureus with the application of transmembrane electrophoresis of permeant anions, potassium transport in the presence of valinomycin and 8-anilinonaphthalene-1-sulphonate fluorescence. The membrane potential is formed when the chain of electron transfer or H+-ATPase functions or when the pH gradient varies (the nonenzymic pathway). M-chlorinecarbonylcyanidephenylhydrazonium, a protonophore uncoupler potassium cyanide, an inhibitor of the respiratory chain, N',N-dicyclohexylcarbodiimide, an inhibitor of ATPase, cause the membrane potential dissipation. The orientation of the transmembrane electric field is as follows: "minus" inside cells and "plus" inside membrane vesicles.     

Effect of oxytocin on the electrical potential and ion permeability of the apical membrane of frog gall bladder epithelial cells

The influence of oxytocin on the intracellular Na+ and K+ concentrations, the level of transmembrane potential differences, and on the relative ionic permeability (PNa/PK) of the apical zones of the superficial epithelium membrane was studied in experiments on the isolated frog gallbladder (GB). Oxytocine introduced into the outer incubation solution in a dose of 20 mulliunits/ml caused a reduction of transmembrane potential difference, and an increase of PNa/pk coefficient and an insignificant shift of the Na+ and K+ concentrations in the intracellular medium. Thirty minutes after the oxytocine action of the organ the membrane potential (MP) of the cells decreased from 52.7 mV to 38.7 mV (the cell is negatively charged inside), and PNa/PK increased from 0,083 (control) to 0,175 (test) with a simultaneous increase in the intracellular Na+ concentration by 18.3 milliequiv./kg of (H2O)i. Such a shift in the intracellular Na+ and K+ concentrations may cause a decrease of the MP by only--0.7 mV, but actually the membrane potential decreased by--14.0 mV. Thus, the reduction of the transmembrane potential difference results from increase of PNa/PK under the influence of oxytocine. No electrogenic ionic transport through the apical membrane of frog gallbladder epithelial cells was revealed.     

H~+诱导心磷脂的六角形Ⅱ相和H~+的跨膜转运

本文报告H~ 能诱导心磷脂由双层排列转变为六角形Ⅱ相.含心磷脂的多层脂囊泡的~(31)P中核磁共振谱显示高场峰低场肩的双层排列特点,当pH降到2时,~(31)P核磁共振谱表现为低场峰高场肩的六角形Ⅱ相特点,表明H~ 对心磷脂多形性转变的诱导作用.用oxonol-V作为探剂.H~ 可使结合在人工脂膜上的oxonl-V的吸收峰红移和光吸收增加,表明心磷脂的六角形Ⅱ相在人工脂膜上具有H~ 的载体特性,易化H~ 的跨膜转运.     

Oscillations in H+ and Ca2+ Ion Fluxes around the Elongation Region of Corn Roots and Effects of External pH

Net fluxes of H+ and Ca2+ around the elongation region of low-salt corn (Zea mays L.) roots were measured using the microelectrode ion flux estimation (MIFE) technique. At pH 5.2 two oscillatory components were found. Fast, 7-min oscillations in H+ flux were superimposed on slow oscillations of about 1.5 h. Fast oscillations in Ca2+ flux showed a strong dependence on the H+ oscillations and were normally leading in phase by about 1 to 1.5 min. Both oscillatory components were strongly affected by external pH values. Preincubation for 20 h in buffered pH 4.0 solution suppressed the slow oscillatory component and caused huge H+ influxes in the elongation region. The fast oscillations were 8 times larger in amplitude and were slightly lengthened. Preincubation at pH 6.0 did not suppress the rhythmic character of the ion fluxes but it shifted the average H+ flux to greater efflux. The fast and slow oscillatory components of H+ flux seem to relate to biophysical and biochemical mechanisms of intracellular pH homeostasis, respectively. The origin of the Ca2+ flux oscillations is discussed in terms of the "weak acid Donnan Manning" model of cell wall ion exchanges.     

Intrinsic characteristics of the proton pump in the luminal membrane of a tight urinary epithelium. The relation between transport rate and delta mu H

A number of tight urinary epithelia, as exemplified by the turtle bladder, acidify the luminal solution by active transport of H+ across the luminal cell membrane. The rate of active H+ transport (JH) decreases as the electrochemical potential difference for H+ [delta mu H = mu H(lumen) - mu H(serosa)] across the epithelium is increased. The luminal cell membrane has a low permeability for H+ equivalents and a high electrical resistance compared with the basolateral cell membrane. Changes in JH thus reflect changes in active H+ transport across the luminal membrane. To examine the control of JH by delta mu H in the turtle bladder, transepithelial electrical potential differences (delta psi) were imposed at constant acid-base conditions or the luminal pH was varied at delta psi = 0 and constant serosal PCO2 and pH. When the luminal compartment was acidified from pH 7 to 4 or was made electrically positive, JH decreased as a linear function of delta mu H as previously described. When the luminal compartment was made alkaline from pH 7 to 9 or was made electrically negative, JH reached a maximal value, which was the same whether the delta mu H was imposed as a delta pH or a delta psi. The nonlinear JH vs. delta mu H relation does not result from changes in the number of pumps in the luminal membrane or from changes in the intracellular pH, but is a characteristic of the H+ pumps themselves. We propose a general scheme, which, because of its structural features, can account for the nonlinearity of the JH vs. delta mu H relations and, more specifically, for the kinetic equivalence of the effects of the chemical and electrical components of delta mu H. According to this model, the pump complex consists of two components: a catalytic unit at the cytoplasmic side of the luminal membrane, which mediates the ATP-driven H+ translocation, and a transmembrane channel, which mediates the transfer of H+ from the catalytic unit to the luminal solution. These two components may be linked through a buffer compartment for H+ (an antechamber).