Identification of a basolateral Cl-/HCO3- exchanger specific to gastric parietal cells

The basolateral Cl(-)/HCO(3)(-) exchanger in parietal cells plays an essential role in gastric acid secretion mediated via the apical gastric H(+)-K(+)-ATPase. Here, we report the identification of a new Cl(-)/HCO(3)(-) exchanger, which shows exclusive expression in mouse stomach and kidney, with expression in the stomach limited to the basolateral membrane of gastric parietal cells. Tissue distribution studies by RT-PCR and Northern hybridizations demonstrated the exclusive expression of this transporter, also known as SLC26A7, to stomach and kidney, with the stomach expression significantly more abundant. No expression was detected in the intestine. Cellular distribution studies by RT-PCR and Northern hybridizations demonstrated predominant localization of SLC26A7 in gastric parietal cells. Immunofluorescence labeling localized this exchanger exclusively to the basolateral membrane of gastric parietal cells, and functional studies in oocytes indicated that SLC26A7 is a DIDS-sensitive Cl(-)/HCO(3)(-) exchanger that is active in both acidic and alkaline pH(i). On the basis of its unique expression pattern and function, we propose that SLC26A7 is a basolateral Cl(-)/HCO(3)(-) exchanger in gastric parietal cells and plays a major role in gastric acid secretion.     

Site-directed mutagenesis identifies residues in uncoupling protein (UCP1) involved in three different functions

Using site-specific mutagenesis, we have constructed several mutants of uncoupling protein (UCP1) from brown adipose tissue to investigate the function of acidic side chains at positions 27, 167, 209, and 210 in H(+) and Cl(-) transport as well as in nucleotide binding. The H(+) transport activity was measured with mitochondria and with reconstituted vesicles. These mutant UCPs (D27N, D27E, E167Q, D209N, D210N, and D209N + D210N) are expressed at near wt levels in yeast. Their H(+) transport activity in mitochondria correlates well with the reconstituted protein except for D27N (intrahelical), which shows strong inhibition of H(+) transport in the reconstituted system and only 50% decrease of uncoupled respiration in mitochondria. In the double adjacent acidic residues (between helix 4 and helix 5), mutation of D210 and of D209 decreases H(+) transport 80% and only 20%, respectively. These mutants retain full Cl(-) transport activity. The results indicate that D210 participates in H(+) uptake at the cytosolic side and D27 in H(+) translocation through the membrane. Differently, E167Q has lost Cl(-) transport activity but retains the ability to transport H(+). The separate inactivation of H(+) and Cl(-) transport argues against the fatty acid anion transport mechanism of H(+) transport by UCP. The mutation of the double adjacent acidic residues (D209, D210) decreases pH dependency for only nucleoside triphosphate (NTP) but not diphosphate (NDP) binding. The results identify D209 and D210 in accordance with the previous model as those residues which control the location of H214 in the binding pocket, and thus contribute to the pH control of NTP but not of NDP binding.     

Characteristics of weak base-induced vacuoles formed around individual acidic organelles

We have previously found that the weak base 4-aminopyridine induces Brownian motion of acidic organelles around which vacuoles are formed, causing organelle traffic disorder in neurons. Our present study investigated the characteristics of vacuoles induced by weak bases (NH(4)Cl, aminopyridines, and chloroquine) using mouse cells. Individual vacuoles included acidic organelles identified by fluorescent protein expression. Mitochondria and actin filaments were extruded outside the vacuoles, composing the vacuole rim. Staining with amine-reactive fluorescence showed no protein/amino acid content in vacuoles. Thus, serous vacuolar contents are probably partitioned by viscous cytosol, other organelles, and cytoskeletons, but not membrane. The weak base (chloroquine) was immunochemically detected in intravacuolar organelles, but not in vacuoles. Early vacuolization was reversible, but long-term vacuolization caused cell death. The vacuolization and cell death were blocked by the vacuolar H(+)-ATPase inhibitor and Cl--free medium. Staining with LysoTracker or LysoSensor indicated that intravacuolar organelles were strongly acidic and vacuoles were slightly acidic. This suggests that vacuolization is caused by accumulation of weak base and H(+) in acidic organelles, driven by vacuolar H(+)-ATPase associated with Cl(-) entering, and probably by subsequent extrusion of H(+) and water from organelles to the surrounding cytoplasm.     

Thiocyanate transport in resting and IL-4-stimulated human bronchial epithelial cells: role of pendrin and anion channels

SCN(-) (thiocyanate) is an important physiological anion involved in innate defense of mucosal surfaces. SCN(-) is oxidized by H(2)O(2), a reaction catalyzed by lactoperoxidase, to produce OSCN(-) (hypothiocyanite), a molecule with antimicrobial activity. Given the importance of the availability of SCN(-) in the airway surface fluid, we studied transepithelial SCN(-) transport in the human bronchial epithelium. We found evidence for at least three mechanisms for basolateral to apical SCN(-) flux. cAMP and Ca(2+) regulatory pathways controlled SCN(-) transport through cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, respectively, the latter mechanism being significantly increased by treatment with IL-4. Stimulation with IL-4 also induced the strong up-regulation of an electroneutral SCN(-)/Cl(-) exchange. Global gene expression analysis with microarrays and functional studies indicated pendrin (SLC26A4) as the protein responsible for this SCN(-) transport. Measurements of H(2)O(2) production at the apical surface of bronchial cells indicated that the extent of SCN(-) transport is important to modulate the conversion of this oxidant molecule by the lactoperoxidase system. Our studies indicate that the human bronchial epithelium expresses various SCN(-) transport mechanisms under resting and stimulated conditions. Defects in SCN(-) transport in the airways may be responsible for susceptibility to infections and/or decreased ability to scavenge oxidants.     

Proton pump-driven cutaneous chloride uptake in anuran amphibia

Krogh introduced the concept of active ion uptake across surface epithelia of freshwater animals, and proved independent transports of Na(+) and Cl(-) in anuran skin and fish gill. He suggested that the fluxes of Na(+) and Cl(-) involve exchanges with ions of similar charge. In the so-called Krogh model, Cl(-)/HCO(3)(-) and Na(+)/H(+) antiporters are located in the apical membrane of the osmoregulatory epithelium. More recent studies have shown that H(+) excretion in anuran skin is due to a V-ATPase in mitochondria-rich (MR) cells. The pump has been localized by immunostaining and H(+) fluxes estimated by pH-stat titration and mathematical modelling of pH-profiles in the unstirred layer on the external side of the epithelium. H(+) secretion is voltage-dependent, sensitive to carbonic-anhydrase inhibitors, and rheogenic with a charge/ion-flux ratio of unity. Cl(-) uptake from freshwater is saturating, voltage independent, and sensitive to DIDS and carbonic-anhydrase inhibitors. Depending on anuran species and probably on acid/base balance of the animal, apical exit of protons is coupled to an exchange of Cl(-) with base (HCO(3)(-)) either in the apical membrane (gamma-type of MR cell) or in the basolateral membrane (alpha-type MR cell). The gamma-cell model accounts for the rheogenic active uptake of Cl(-) observed in several anuran species. There is indirect evidence also for non-rheogenic active uptake accomplished by a beta-type MR cell with apical base secretion and basolateral proton pumping. Several studies have indicated that the transport modes of MR cells are regulated via ion- and acid/base balance of the animal, but the signalling mechanisms have not been investigated. Estimates of energy consumption by the H(+)-ATPase and the Na(+)/K(+)-ATPase indicate that the gamma-cell accomplishes uptake of NaCl in normal and diluted freshwater. Under common freshwater conditions with serosa-positive or zero V(t), the K(+) conductance of the basolateral membrane would have to maintain the inward driving force for Na(+) uptake across the apical membrane. With the K(+) equilibrium potential across the basolateral membrane estimated to -105 mV, this would apply to external Na(+) concentrations down to 40-120 micromol/l. NaCl uptake from concentrations down to 10 micromol/l, as observed by Krogh, presupposes that the H(+) pump hyperpolarizes the apical membrane, which would then have to be associated with serosa-negative V(t). In diluted freshwater, exchange of cellular HCO(3)(-) with external Cl(-) seems to be possible only if the proton pump has the additional function of keeping the external concentration of HCO(3)(-) low. Quantitative considerations also lead to the conclusion that with the above extreme demand, at physiological intracellular pH of 7.2, the influx of Cl(-) via the apical antiporter and the passive exit of Cl(-) via basolateral channels would be possible within a common range of intracellular Cl(-) concentrations.     

Quantitative analyses of the uncoupling activity of substituted phenols with mitochondria from flight muscles of house flies

Uncoupling activity with flight-muscle mitochondria from house flies was measured for a series of weakly acidic uncouplers (substituted phenols) and compared with the protonophoric potency across lecithin liposomal membranes. The activity was linearly related to the protonophoric potency when such factors as the stability of anionic species in the membrane phase and the difference in the pH conditions of the extramembranous aqueous phase were taken into account. Relationships of the flight-muscle activity with activities measured previously with rat-liver mitochondria and spinach chloroplasts were linear. Our findings were further evidence for the shuttle-type mechanism of the uncoupling action of weakly acidic uncouplers.     

Determinants of anion-proton coupling in mammalian endosomal CLC proteins

Many proteins of the CLC gene family are Cl(-) channels, whereas others, like the bacterial ecClC-1 or mammalian ClC-4 and -5, mediate Cl(-)/H(+) exchange. Mutating a "gating glutamate" (Glu-224 in ClC-4 and Glu-211 in ClC-5) converted these exchangers into anion conductances, as did the neutralization of another, intracellular "proton glutamate" in ecClC-1. We show here that neutralizing the proton glutamate of ClC-4 (Glu-281) and ClC-5 (Glu-268), but not replacing it with aspartate, histidine, or tyrosine, rather abolished Cl(-) and H(+) transport. Surface expression was unchanged by these mutations. Uncoupled Cl(-) transport could be restored in the ClC-4(E281A) and ClC-5(E268A) proton glutamate mutations by additionally neutralizing the gating glutamates, suggesting that wild type proteins transport anions only when protons are supplied through a cytoplasmic H(+) donor. Each monomeric unit of the dimeric protein was found to be able to carry out Cl(-)/H(+) exchange independently from the transport activity of the neighboring subunit. NO(3)(-) or SCN(-) transport was partially uncoupled from H(+) countertransport but still depended on the proton glutamate. Inserting proton glutamates into CLC channels altered their gating but failed to convert them into Cl(-)/H(+) exchangers. Noise analysis indicated that ClC-5 switches between silent and transporting states with an apparent unitary conductance of 0.5 picosiemens. Our results are consistent with the idea that Cl(-)/H(+) exchange of the endosomal ClC-4 and -5 proteins relies on proton delivery from an intracellular titratable residue at position 268 (numbering of ClC-5) and that the strong rectification of currents arises from the voltage-dependent proton transfer from Glu-268 to Glu-211.     

The antibacterial action of lactoperidoxase. The nature of the bacterial inhibitor

Lactoperoxidase (EC 1.11.1.7), an enzyme present in various mammalian glands and in their secretions, catalyses the oxidation of thiocyanate by hydrogen peroxide to form a compound that inhibits the growth, oxygen uptake and acid production of certain bacteria. This compound was found to be too unstable to isolate in pure form, but its properties in dilute aqueous solution were studied with a view to establishing its identity. At thiocyanate concentrations of approximately 1mm, formation of the inhibitor, which took place by a nonstoicheiometric reaction, was maximal when an approximately equimolar amount of hydrogen peroxide was added. Excess of hydrogen peroxide oxidized the inhibitor to sulphate and cyanate. The inhibitor displayed a polarographic reduction wave of which the half-wave potential was pH-dependent. Studies of the variation of the polarographic half-wave potential and of the u.v. extinction with pH indicated that the inhibitor existed in an acid-base equilibrium (pK(a) 5.1+/-0.1). The inhibitor decomposed by a mechanism involving H(+) ions and thiocyanate, the kinetics varying according to whether the inhibitor was in its acidic or basic form. From these studies it was concluded that the inhibitor was either cyanosulphurous acid (HO(2)SCN) or cyanosulphuric acid (HO(3)SCN).     

Membrane potential of Plasmodium-infected erythrocytes

The membrane potential (Em) of normal and Plasmodium chabaudi-infected rat erythrocytes was determined from the transmembrane distributions of the lipophilic anion, thiocyanate (SCN), and cation, triphenylmethylphosphonium (TPMP). The SCN- and TPMP-measured Em of normal erythrocytes are -6.5 +/- 3 mV and -10 +/- 4 mV, respectively. The TPMP-measured Em of infected cells depended on parasite developmental stage; "late" stages (schizonts and gametocytes) were characterized by a Em = -35 mV "early stages (ring and copurifying noninfected) by a low Em (-16 mV). The SCN-determined Em of infected cells was -7 mV regardless of parasite stage. Studies with different metabolic inhibitors including antimycin A, a proton ionophore (carbonylcyanide m-chlorophenylhydrazone [CCCP] ), and a H+ -ATPase inhibitor (N,N'-dicyclohexylcarbodiimide, [DCCD] ) indicate that SCN monitors the Em across the erythrocyte membrane of infected and normal cells whereas TPMP accumulation reflects the Em across the plasma membranes of both erythrocyte and parasite. These inhibitor studies also implicated proton fluxes in Em-generation of parasitized cells. Experiments with weak acids and bases to measure intracellular pH further support this proposal. Methylamine distribution and direct pH measurement after saponin lysis of erythrocyte membranes demonstrated an acidic pH for the erythrocyte matrix of infected cells. The transmembrane distributions of weak acids (acetate and 5,5-dimethyloxazolidine-2,4-dione) indicated a DCCD-sensitive alkaline compartment. The combined results suggest that the intraerythrocyte parasite Em and delta pH are in part the consequence of an electrogenic proton pump localized to the parasite plasma membrane.     

The anti-cancer agent nemorosone is a new potent protonophoric mitochondrial uncoupler

Nemorosone, a natural-occurring polycyclic polyprenylated acylphloroglucinol, has received increasing attention due to its strong in vitro anti-cancer action. Here, we have demonstrated the toxic effect of nemorosone (1-25 μM) on HepG2 cells by means of the MTT assay, as well as early mitochondrial membrane potential dissipation and ATP depletion in this cancer cell line. In mitochondria isolated from rat liver, nemorosone (50-500 nM) displayed a protonophoric uncoupling activity, showing potency comparable to the classic protonophore, carbonyl cyanide m-chlorophenyl hydrazone (CCCP). Nemorosone enhanced the succinate-supported state 4 respiration rate, dissipated mitochondrial membrane potential, released Ca(2+) from Ca(2+)-loaded mitochondria, decreased Ca(2+) uptake and depleted ATP. The protonophoric property of nemorosone was attested by the induction of mitochondrial swelling in hyposmotic K(+)-acetate medium in the presence of valinomycin. In addition, uncoupling concentrations of nemorosone in the presence of Ca(2+) plus ruthenium red induced the mitochondrial permeability transition process. Therefore, nemorosone is a new potent protonophoric mitochondrial uncoupler and this property is potentially involved in its toxicity on cancer cells.     

Cytochemical demonstration of NPPase activity for detecting proton-translocating ATPase of Golgi complex in rat pancreatic acinar cells

An attempt at cytochemical demonstration of acidification proton-translocating ATPase (H(+)-ATPase) of Golgi complex in rat pancreatic acinar cells has been made by using p-nitrophenylphosphatase (NPPase) cytochemistry which is used for detecting of Na(+)-K(+)-ATPase (Mayahara et al. 1980) and gastric H(+)-K(+)-ATPase (Fujimoto et al. 1986). K(+)-independent NPPase activity was observed on the membrane of the trans cisternae of Golgi complex, but not inside of cisternae. The localization of NPPase activity is different from that of acid phosphatase activity where reaction products were seen on the inside of the trans Golgi cisternae. Since this activity was insensitive to vanadate, ouabain and independent of potassium ions, it was distinct from plasma membranous ATPases such as Na(+)-K(+)-ATPase and Ca2(+)-ATPase. The K(+)-independent NPPase activity was diminished by the inhibitors of H(+)-ATPase such as N-ethylmaleimide (NEM) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The NPPase reaction products were also seen on the membranes of other acidic organelles, i.e., lysosomes, endosomes, autophagosomes and coated vesicles. These results suggest that NPPase activity on the membrane of the Golgi complex and other acidic organelles corresponds with H(+)-ATPase which plays a role in acidification.     

The role of chloride in taurine transport across the human placental brush-border membrane.

Taurine, a sulfated beta-amino acid, is conditionally essential during development. A maternal supply of taurine is necessary for normal fetal growth and neurologic development, suggesting the importance of efficient placental transfer. Uptake by the brush-border membrane (BBM) in several other tissues has been shown to be via a selective Na(+)-dependent carrier mechanism which also has a specific anion requirement. Using BBM vesicles purified from the human placenta, we have confirmed the presence of Na(+)-dependent, carrier-mediated taurine transport with an apparent Km of 4.00 +/- 0.22 microM and a Vmax of 11.72-0.36 pmol mg-1 protein 20 s-1. Anion dependence was examined under voltage-clamped conditions, in order to minimize the contribution of membrane potential to transport. Uptake was significantly reduced when anions such as thiocyanate, gluconate, or nitrate were substituted for Cl-. In addition, a Cl(-)-gradient alone (under Na(+)-equilibrated conditions) could energize uphill transport as evidenced by accelerated uptake (3.13 +/- 0.8 pmol mg-1 protein 20 s-1) and an overshoot compared to Na+, Cl- equilibrated conditions (0.60 +/- 0.06 pmol mg-1 protein 20 s-1). A Cl(-)-gradient (Na(+)-equilibrated) also stimulated uptake of [3H]taurine against its concentration gradient. Analysis of uptake in the presence of varying concentrations of external Cl- suggested that 1 Cl- ion is involved in Na+/taurine cotransport. We conclude that Na(+)-dependent taurine uptake in the placental BBM has a selective anion requirement for optimum transport. This process is electrogenic and involves a stoichiometry of 2:1:1 for Na+/Cl-/taurine symport.     

Electroneutral sodium absorption and electrogenic anion secretion across murine small intestine are regulated in parallel

Electrolyte transport processes of small intestinal epithelia maintain a balance between hydration of the luminal contents and systemic fluid homeostasis. Under basal conditions, electroneutral Na(+) absorption mediated by Na(+)/H(+) exchanger 3 (NHE3) predominates; under stimulated conditions, increased anion secretion mediated by CFTR occurs concurrently with inhibition of Na(+) absorption. Homeostatic adjustments to diseases that chronically affect the activity of one transporter (e.g., cystic fibrosis) may include adaptations in the opposing transport process to prevent enterosystemic fluid imbalance. To test this hypothesis, we measured electrogenic anion secretion (indexed by the short-circuit current) across NHE3-null [NHE3(-)] murine small intestine and electroneutral Na(+) absorption (by radioisotopic flux analysis) across small intestine of mice with gene-targeted disruptions of the anion secretory pathway, i.e., CFTR-null [CFTR(-)] or Na(+)-K(+)-2Cl(-) cotransporter-null [NKCC1(-)]. Protein expression of NHE3 and CFTR in the intestinal epithelia was measured by immunoblotting. In NHE3(-), compared with wild-type small intestine, maximal and bumetanide-sensitive anion secretion following cAMP stimulation was significantly reduced, and there was a corresponding decrease in CFTR protein expression. In CFTR(-) and NKCC1(-) intestine, Na(+) absorption was significantly reduced compared with wild-type. NHE3 protein expression was decreased in the CFTR(-) intestine but was unchanged in the NKCC1(-) intestine, indicating that factors independent of expression also downregulate NHE3 activity. Together, these data support the concept that absorptive and secretory processes determining NaCl and water movement across the intestinal epithelium are regulated in parallel to maintain balance between the systemic fluid volume and hydration of the luminal contents.     

Rapid release of Mg(2+) from liver mitochondria by nonesterified long-chain fatty acids in alkaline media

Long-chain fatty acids induce a rapid release of Mg(2+) from both energized and nonenergized rat liver mitochondria suspended at pH 8 in isotonic saline but not sucrose media. The effect is observed only with fatty acids that possess protonophoric activity. The most active saturated fatty acids are myristic and palmitic, while the most active unsaturated acids are oleic, linolenic, and arachidonic. The rate of Mg(2+) release drastically decreases with decreasing medium pH to 7.2-7.6. However, at those pH values this rate is doubled by energization of mitochondria with respiratory substrates. Mg(2+) release is accompanied by cyclosporin A-insensitive large-amplitude swelling of mitochondria. This swelling is similar to that produced by the divalent metal ionophore A23187 and is interpreted as being due to activation of the inner membrane anion channel, the K(+) uniporter, and the K(+)/H(+) exchanger. In energized mitochondria, both swelling and Mg(2+) release are blocked by the exogenous K(+)/H(+) exchanger nigericin. It is proposed that fatty acids under conditions of alkaline mitochondrial matrix activate latent Mg(2+)-sensitive ion-conducting pathways in the inner mitochondrial membrane, which mediate swelling and Mg(2+) release. It is hypothesized that fatty acids activate an intrinsic Mg(2+)/H(+) exchanger that is related to, or identical with, the K(+)/H(+) exchanger.     

Cerebrocrast promotes the cotransport of H+ and Cl- in rat liver mitochondria

Considering that cerebrocrast stimulates oligomycin-inhibited state 3 respiration simultaneously with mitochondrial transmembrane potential (Deltapsi) dissipation, the mechanism underlying the uncoupler activity of cerebrocrast was assessed by its ability to permeabilize the mitochondrial inner membrane to H(+) or to K(+) or to cotransport anions with H(+). The partition coefficient of cerebrocrast in mitochondrial membrane and its ability to act as a membrane-active compound disturbing membrane lipid organization were also investigated. Cerebrocrast induced no permeabilization of mitochondrial inner membrane to H(+) or K(+), but it was able to transport H(+) in association with Cl(-). Cerebrocrast showed a strong incorporation into the mitochondrial membrane, with a partition coefficient (Kp(m/w)) of 2.7(+/-0.1)x10(5). Cerebrocrast also reduced, in a concentration dependent manner, the phase transition temperature, the cooperative unit size, and the enthalpy associated with the phase transition temperature of DMPC membrane bilayers. It was concluded that the uncoupler activity of cerebrocrast is due to its ability to promote the cotransport of H(+) with Cl(-) through the rat liver mitochondrial inner membrane, and that this cerebrocrast mechanism of action may be potentiated by alterations of membrane lipid organization and membrane lateral heterogeneity.     

Factors affecting the permeability of isolated fat-cells from the rat to [42K]potassium and [36Cl]chloride ions

1. The effluxes of (42)K(+) and (36)Cl(-) from isolated fat-cells from the rat were studied under a variety of conditions known to affect the metabolism of the cells. 2. (42)K(+) efflux from isolated fat cells was increased in a Na(+)-free-high-K(+) medium and decreased in a K(+)-free medium. The existence of K(+) exchange diffusion across the fat-cell membrane is suggested. 3. (36)Cl(-) efflux from isolated fat-cells was decreased when the Cl(-) component of the wash medium was replaced by acetate. The basal (36)Cl(-) efflux is suggested to be partly by Cl(-) exchange diffusion and partly in company with a univalent cation. 4. A variety of lipolytic stimuli, adrenaline, adrenocorticotrophic hormone, N-6,O-2'-dibutyryladenosine cyclic 3':5'-monophosphate and theophylline, increased (42)K(+) efflux from isolated fat-cells. The adrenaline stimulation was biphasic; an initial, rapid and transient increase in (42)K(+) loss from the fat-cells was followed by a slower, more prolonged, increase in (42)K(+) efflux. The initial phase was inhibited by phentolamine but not by propranolol. 5. Insulin increased (42)K(+) efflux only after preincubation with the cells.     

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.     

Involvement of H(+)-ATPase and carbonic anhydrase in inorganic carbon uptake for endosymbiont photosynthesis

Symbiotic cnidarians absorb inorganic carbon from seawater to supply intracellular dinoflagellates with CO(2) for their photosynthesis. To determine the mechanism of inorganic carbon transport by animal cells, we used plasma membrane vesicles prepared from ectodermal cells isolated from tentacles of the sea anemone, Anemonia viridis. H(14)CO(-)(3) uptake in the presence of an outward NaCl gradient or inward H(+) gradient, showed no evidence for a Cl(-)- or H(+)- driven HCO(-)(3) transport. H(14)CO(-)(3) and (36)Cl(-) uptakes were stimulated by a positive inside-membrane diffusion potential, suggesting the presence of HCO(-)(3) and Cl(-) conductances. A carbonic anhydrase (CA) activity was measured on plasma membrane (4%) and in the cytoplasm of the ectodermal cells (96%) and was sensitive to acetazolamide (IC(50) = 20 nM) and ethoxyzolamide (IC(50) = 2.5 nM). A strong DIDS-sensitive H(+)-ATPase activity was observed (IC(50) = 14 microM). This activity was also highly sensitive to vanadate and allyl isothiocyanate, two inhibitors of P-type H(+)-ATPases. Present data suggest that HCO(-)(3) absorption by ectodermal cells is carried out by H(+) secretion by H(+)-ATPase, resulting in the formation of carbonic acid in the surrounding seawater, which is quickly dehydrated into CO(2) by a membrane-bound CA. CO(2) then diffuses passively into the cell where it is hydrated in HCO(-)(3) by a cytosolic CA.     

Differential sensitivity of the cellular compartments of Saccharomyces cerevisiae to protonophoric uncoupler under fermentative and respiratory energy supply.

The effect of a protonophoric uncoupler (CCCP) on the different cellular compartments was investigated in yeast grown aerobically on lactate. These cells were incubated in a resting cell medium under three conditions; in aerobiosis with lactate or glucose or in anaerobiosis with glucose as energetic substrate. For each condition, in vivo 31P NMR was used to measure pH gradients across vacuolar and plasma membrane and phosphorylated compound levels. Respiratory rate (aerobic conditions) and TPP+ uptake were measured independently. Concerning the polyphosphate metabolism, spontaneous NMR-detected polyphosphate breakdown occurred, in anaerobiosis and in the absence of CCCP. In contrast, in aerobiosis, polyphosphate hydrolysis was induced by addition of either CCCP or a vacuolar membrane ATPase-specific inhibitor, bafilomycin A1. Moreover, polyphosphates were totally absent in a null vacuolar ATPase activity mutant. The vacuolar polyphosphate content depended on two factors: vacuolar pH value, strictly linked to the vacuolar H(+)-ATPase activity, and inorganic phosphate concentration. CCCP was more efficient in dissipating the proton electrochemical gradient across vacuolar and mitochondrial membranes than across the plasma membrane. This discrepancy can be essentially explained by a difference of stimulability of each proton pump involved. As long as the energetic state (measured by NDP + NTP content) remains high, the plasma membrane proton ATPase is able to compensate the proton leak. Moreover, this ATPase contributes only partially to the generation of delta pH. The maintenance of the delta pH across the plasma membrane, that of the energetic state, and the cellular TPP+ uptake depend on the nature of the ATP-producing process.(ABSTRACT TRUNCATED AT 250 WORDS)