Effect of ATPase inhibitors on the proton pump of respiratory-deficient yeast

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

Influence of copper on proton efflux from Saccharomyces cerevisiae and the protective effect of calcium and magnesium

The inhibitory effect of Cu on glucose-dependent H+ efflux from Saccharomyces cerevisiae was manifest at low (micromolar) concentrations, with the time period between the addition of glucose and commencement of H+ efflux, H+ efflux rate and duration all being affected with increasing Cu concentration (5-100 microM). Ca, at a concentration of 0.5 mM, completely removed the inhibitory effect of Cu at concentrations up to 50 microM and considerably reduced it at higher concentrations (up to 150 microM). Mg exhibited a similar but weaker protective effect against the influence of Cu. The protective effect of Ca against 50 microM Cu was evident at low Ca concentrations (2.5-5 microM), whereas Mg was effective at > or = 50 microM. In order to prevent the inhibitory effect of Cu, it was necessary to add Ca or Mg to the cell suspension before Cu addition. It is concluded that the protective effect of Ca and Mg is mediated by competitive and stabilizing interactions at the cell surface as well as physiological functions of Ca and Mg.

Energy-dependent, carrier-mediated extrusion of carboxyfluorescein from Saccharomyces cerevisiae allows rapid assessment of cell viability by flow cytometry

Carboxyfluorescein diacetate is a nonfluorescent compound which can be used in combination with flow cytometry for vital staining of yeasts and bacteria. The basis of this method is the assumption that, once inside the cell, carboxyfluorescein diacetate is hydrolyzed by nonspecific esterases to produce the fluorescent carboxyfluorescein (cF). cF is retained by cells with intact membranes (viable cells) and lost by cells with damaged membranes. In this report, we show that Saccharomyces cerevisiae extrudes cF in an energy-dependent manner. This efflux was studied in detail, and several indications that a transport system is involved were found. Efflux of cF was stimulated by the addition of glucose and displayed Michaelis-Menten kinetics. A Km for cF transport of 0.25 mM could be determined. The transport of cF was inhibited by the plasma membrane H(+)-ATPase inhibitors N,N'-dicyclohexylcarbodiimide and diethylstilbestrol and by high concentrations of tetraphenylphosphonium ions. These treatments resulted in a dissipation of the proton motive force, whereas the intracellular ATP concentration remained high. Transport of cF is therefore most probably driven by the membrane potential and/or the pH gradient. The viability of S. cerevisiae was determined by a two-step procedure consisting of loading the cells with cF followed by incubation at 40 degrees C in the presence of glucose. Subsequently, the fluorescence intensity of the cells was analyzed by flow cytometry. The efflux experiments showed an excellent correlation between the viability of S. cerevisiae cells and the ability to translocate cF. This method should prove of general utility for the rapid assessment of yeast vitality and viability.

Interactions between glucose metabolism and oxidative phosphorylations on respiratory-competent Saccharomyces cerevisiae cells

The purpose of this work was to analyze the interactions between oxidative phosphorylations and glucose metabolism on yeast cells aerobically grown on lactate as carbon source and incubated in a resting cell medium. On such respiratory-competent yeast cells, four different metabolic steady states have particularly been studied: (a) glucose feeding under anaerobiosis, (b) ethanol supply under aerobiosis, (c) glucose supply under aerobiosis and (d) glucose plus ethanol under aerobiosis. For each condition, we measured: (a) the cellular ATP/ADP ratio and NADH content sustained under these conditions, (b) the glucose consumption rate (glucose conditions) and the respiratory rate (aerobic conditions). Under aerobic conditions, when ethanol is used as substrate, the ATP/ADP ratio and NADH level are very high as compared with glucose feeding. However, the rate of oxygen consumption is similar under both conditions. The main observation is a large increase in the respiratory rate when both glucose and ethanol are added. This increase corresponds to an ATP/ADP ratio and a NADH level lower than those observed with ethanol but higher than those with glucose. Therefore the response of the respiratory rate to the ATP/ADP ratio depends on the redox potential. We studied the way in which the ATP-consuming activity was increased under glucose+ethanol conditions. By NMR experiments, it appears that neither the futile cycle at the level of the phosphofructo-1-kinase/fructo-1,6-bisphosphatase couple nor the synthesis of carbohydrate stores could account for the increase in oxidative phosphorylation. However, it is shown that, in the presence of glucose+ethanol, ATP consumption is strongly stimulated. It is hypothesized that this consumption is essentially due to the combination of the well-known plasma membrane proton-ATPase activation by glucose and the high phosphate potential due to oxidative ethanol metabolism. While it is well documented that oxidative phosphorylations inhibit the glycolytic flux, i.e. the Pasteur effect, we clearly show in this work that the glycolytic pathway limits the ability of mitochondria to maintain a cellular phosphate potential.

Amiloride and 5-(N-ethyl-N-isopropyl) amiloride inhibit medium acidification and glucose metabolism by the fission yeast Schizosaccharomyces pombe

We have investigated the mechanism by which amiloride and 5-(N-ethyl-N-isopropyl)amiloride (EIPA) inhibit glucose-stimulated medium acidification in the fission yeast Schizosaccharomyces pombe. The addition of glucose to an unbuffered suspension of cells results in the extrusion of acid. This process was inhibited by diethylstilbestrol (DES), an inhibitor of the H(+)-ATPase (IC50 71 microM), and also by amiloride (IC50 824 microM) and EIPA (IC50 203 microM). The presence of 100 mM NaCl reduced the degree of inhibition observed for amiloride and EIPA, but had no effect on inhibition by DES. N-Methylglucosamine partially protected the cells against the effect of amiloride, but choline chloride did not, suggesting that sodium may be important in the action of amiloride. To establish the site of action of amiloride and EIPA, ATP hydrolysis assays were performed on isolated plasma membranes. H(+)-ATPase activity was inhibited by orthovanadate, but not by amiloride or EIPA. However, both amiloride and EIPA were found to inhibit the incorporation of radioactivity from labelled glucose in S. pombe, with IC50 values of 879 and 272 microM for amiloride and EIPA respectively. Again, 100 mM NaCl was found to reduce the effectiveness of inhibition. Amiloride had no effect on the uptake of 2-deoxyglucose under the same conditions, indicating that amiloride does not inhibit the glucose transporter. We propose that amiloride and EIPA disrupt glucose-induced acidification by inhibiting glucose metabolism.

Phenotypes of sphingolipid-dependent strains of Saccharomyces cerevisiae

To study sphingolipid function(s) in Saccharomyces cerevisiae, we have investigated the effects of environmental stress on mutant (SLC) strains (R. C. Dickson, G. B. Wells, A. Schmidt, and R. L. Lester, Mol. Cell. Biol. 10:2176-2181, 1990) that either contain or lack sphingolipids, depending on whether they are cultured with a sphingolipid long-chain base. Strains lacking sphingolipid were unable to grow at low pH, at 37 degrees C, or with high salt concentrations in the medium; these environmental stresses are known to inhibit the growth of some S. cerevisiae strains with a defective plasma membrane H(+)-ATPase. We found that sphingolipids were essential for proton extrusion at low pH and furthermore found that cells lacking sphingolipid no longer exhibited net proton extrusion at normal pH after a 1-min exposure to pH 3. Cells lacking sphingolipid appeared to rapidly become almost completely permeable to protons at low pH. The deleterious effects of low pH could be partially prevented by 1 M sorbitol in the suspension of cells lacking sphingolipid. Proton extrusion at normal pH (pH 6) was significantly inhibited at 39 degrees C only in cells lacking sphingolipid. Thus, the product of an SLC suppressor gene permits life without sphingolipids only in a limited range of environments. Outside this range, sphingolipids appear to be essential for maintaining proton permeability barriers and/or for proton extrusion.

Activation of the potassium uptake system during fermentation in Saccharomyces cerevisiae

Fermentable sugars activated the K+ uptake system, increasing the Vmaxs of Rb+, Na+, and Li+ influxes, but sugars did not affect the effluxes of these cations. This activation seems to be a direct effect of fermentation and not the consequence of the H+ pump ATPase activation or internal pH decrease produced by fermentation.

Dimorphism-associated changes in plasma membrane H(+)-ATPase activity of Candida albicans

In situ plasma membrane H(+)-ATPase activity was monitored during pH-regulated dimorphism of Candida albicans using permeabilized cells. ATPase activity was found to increase in both the bud and germ tube forming populations at 135 min which coincides with the time of evagination. Upon reaching the terminal phenotype the mycelial form exhibited higher H(+)-ATPase activity as compared to the yeast form. At the time of evagination H(+)-efflux exhibited an increase. K+ depletion resulted in attenuated ATPase activity and glucose induced H(+)-efflux. The results demonstrate that ATPase may play a regulatory role in dimorphism of C. albicans and K+ acts as a modulator.

Plasma membrane Mg(2+)-ATPase of Pachysolen tannophilus: characterization and role in alcohol tolerance

Following cell fractionation in sucrose density gradients, plasma membrane Mg(2+)-ATPase from Pachysolen tannophilus was studied. The ATPase displayed an apparent Km for ATP of 1.42 mM and was inhibited by high concentrations of Mg2+. The inhibitory effects of ethanol, 1-propanol, 1-butanol, and benzyl alcohol on Mg(2+)-ATPase were evaluated, and the concentration of each alcohol that inhibited ATPase activity by 50% (IC50) was determined. The IC50 decreased as the chain length of the alcohol increased. Moreover, the IC50 for ATPase activity was similar to the IC50 for growth rate, suggesting an association between impaired growth and ATPase inhibition. Almost complete inhibition of ATPase activity occurred at temperatures approaching 60 degrees C, and the optimal temperature was around 44 degrees C for ATPase from both control and ethanol-treated cells. Inclusion of 50 mM MgCl2 or CaCl2 in the medium did not rescue cells from the deleterious effects of ethanol.

Domains of yeast plasma membrane and ATPase-associated glycoprotein

In yeast homogenates the plasma membrane H(+)-ATPase and a major surface glycoprotein of about 115 kDa are present in two membrane fractions with peak densities in sucrose gradients of 1.17 and 1.22. Immunogold electron microscopy of frozen yeast sections indicates that the ATPase is exclusively (greater than 95%) present at the surface membrane. Therefore the two ATPase-containing fractions appear to correspond to different domains of the plasma membrane. The 115 kDa glycoprotein is tightly associated with the ATPase during solubilization and purification of the enzyme. However, in a mutant lacking the glycoprotein the activity of the plasma membrane H(+)-ATPase is similar to wild type, suggesting that this association is fortuitous. The ATPase and the glycoprotein are difficult to separate by electrophoresis and therefore binding of concanavalin A to the ATPase cannot be unambiguously demonstrated in wild-type yeast. By utilizing the mutant without glycoprotein it was shown that the ATPase band of 105 kDa binds concanavalin A.

Growth control strength and active site of yeast plasma membrane ATPase studied by site-directed mutagenesis

Several amino acids which are conserved in cation-pumping ATPases with phosphorylated intermediate have been mutagenized in the yeast plasma membrane H+-ATPase. The mutant genes have been selectively expressed in a yeast strain where the wild-type ATPase is only expressed in galactose medium. A series of mutants with decreasing levels of activity demonstrates that the ATPase is rate-limiting for growth and that decreased ATPase activity correlates with decreased intracellular pH. Enzymatic and transport studies of mutant ATPases indicate that (a) Lys474 is the target for the inhibitor fluorescein 5'-isothiocyanate and this residue can be replaced by either arginine or histidine with partial retention of activity; (b) the sensitivity to inhibition by vanadate is affected by the mutations Thr231----Gly, Cys376----Leu, Lys379----Gln and Asp634----Asn; (c) the mutation Ser234----Ala causes uncoupling between ATP hydrolysis and proton transport and reduces the ATP content of the cells; (d) the mutation Asp730----Asn, which affects a polar residue conserved in hydrophobic stretches of H+-ATPases, abolishes ATPase activity and proton transport but not the formation of a phosphorylated intermediate.

Physiology of mutants with reduced expression of plasma membrane H+-ATPase

Two mutations containing insertions and deletions in the promoter in the plasma membrane H+-ATPase gene (PMA1) of Saccharomyces cerevisiae have been introduced into the genome by homologous recombination, replacing the wild-type gene. The resulting strains have 15 and 23% of the wild-type ATPase content. Decreased levels of ATPase correlate with decreased rates of proton efflux and decreased uptake rates of amino acids, methylamine, hygromycin B and tetraphenylphosphonium. This supports a central role of the enzyme in yeast bioenergetics. However, the final accumulation gradient of tetraphenylphosphonium is not affected by the mutations and that of methylamine and 2-aminoisobutyric acid is only decreased in the most extreme mutant. Apparently, kinetic constraints seem to prevent the equilibration of yeast active transports with the electrochemical proton gradient. As expected from their transport defects, the ATPase-deficient mutants are more resistant to hygromycin B and more sensitive to acidification than wild-type yeast. Mutant cells are very elongated, suggesting a structural role of the ATPase in the yeast surface.

Ferrous iron uptake by Bifidobacterium breve

Bifidobacterium breve transports ferrous iron in preference to the ferric form in a saturable, concentration-dependent manner with an optimum pH of 6. Iron transport is highly temperature sensitive. Two transport systems with apparent Km's of 86 +/- 27 and 35 +/- 20 microM (p greater than 0.01) were distinguished, one operating at high iron concentrations, the other at low iron concentrations. Iron uptake could not be accounted for by surface binding. Uptake of iron was inhibited by iron chelators, a protein ionophore, and ATPase inhibitors, and it was stimulated by potassium ionophores. The presence of a ferri reductase in the insoluble cell fraction of B. breve and its "spent" growth medium was demonstrated. The hypothesis is presented that iron uptake by bifidobacteria is related to the nutritional immunity phenomenon.

Chitin synthase 1, an auxiliary enzyme for chitin synthesis in Saccharomyces cerevisiae

Previously, we showed that chitin synthase 2 (Chs2) is required for septum formation in Saccharomyces cerevisiae, whereas chitin synthase 1 (Chs1) does not appear to be an essential enzyme. However, in strains carrying a disrupted CHS1 gene, frequent lysis of buds is observed. Lysis occurs after nuclear separation and appears to result from damage to the cell wall, as indicated by osmotic stabilization and by a approximately 50-nm orifice at the center of the birth scar. Lysis occurs at a low pH and is prevented by buffering the medium above pH 5. A likely candidate for the lytic system is a previously described chitinase that is probably involved in cell separation. The chitinase has a very acidic pH optimum and a location in the periplasmic space that exposes it to external pH. Accordingly, allosamidin, a specific chitinase inhibitor, substantially reduced the number of lysed cells. Because the presence of Chs1 in the cell abolishes lysis, it is concluded that damage to the cell wall is caused by excessive chitinase activity at acidic pH, which can normally be repaired through chitin synthesis by Chs1. The latter emerges as an auxiliary or emergency enzyme. Other experiments suggest that both Chs1 and Chs2 collaborate in the repair synthesis of chitin, whereas Chs1 cannot substitute for Chs2 in septum formation.

TRK1 encodes a plasma membrane protein required for high-affinity potassium transport in Saccharomyces cerevisiae

We identified a 180-kilodalton plasma membrane protein in Saccharomyces cerevisiae required for high-affinity transport (uptake) of potassium. The gene that encodes this putative potassium transporter (TRK1) was cloned by its ability to relieve the potassium transport defect in trk1 cells. TRK1 encodes a protein 1,235 amino acids long that contains 12 potential membrane-spanning domains. Our results demonstrate the physical and functional independence of the yeast potassium and proton transport systems. TRK1 is nonessential in S. cerevisiae and maps to a locus unlinked to PMA1, the gene that encodes the plasma membrane ATPase. Haploid cells that contain a null allele of TRK1 (trk1 delta) rely on a low-affinity transporter for potassium uptake and, under certain conditions, exhibit energy-dependent loss of potassium, directly exposing the activity of a transporter responsible for the efflux of this ion.

Diethylstilbestrol. Interactions with membranes and proteins and the different effects upon Ca2+- and Mg2+-dependent activities of the F1-ATPase from Rhodospirillum rubrum

The hydrophobic compound diethylstilbestrol inhibits the generation of the proton gradient and the membrane potential in chromatophores from Rhodospirillum rubum and dissipates proton gradients over asolectin vesicle membranes. The Ca2+-ATPase activity of chromatophores, of purified F0F1-ATPase and of purified F1-ATPase is also decreased in the presence of diethylstilbestrol. Other repressed activities are the pyrophosphatase activity of soluble pyrophosphatase from yeast and the NADH oxidation by L-lactate:NAD oxidoreductase. We have previously reported that also ATP synthesis, PPi synthesis and PPi hydrolysis of R. rubrum chromatophores are inhibited by diethylstilbestrol [Strid et al. (1987) Biochim. Biophys. Acta 892, 236-244]. Addition of bovine serum albumin reverses or prevents diethylstilbestrol-induced inhibition of the activities tested. On the other hand, the Mg2+-ATPase activity of chromatophores, purified F0F1-ATPase and purified F1-ATPase are stimulated by low concentrations of diethylstilbestrol. On the basis of its hydrophobicity and the reversal of its inhibition by bovine serum albumin, diethylstilbestrol is proposed to act unspecifically on membranes and at hydrophobic domains of proteins. Such an attack upon the subunits of the F1-ATPase, altering the subunit interactions, is proposed to explain the different results obtained for the Ca2+-ATPase and the Mg2+-ATPase.

TRK1 encodes a plasma membrane protein required for high-affinity potassium transport in Saccharomyces cerevisiae

We identified a 180-kilodalton plasma membrane protein in Saccharomyces cerevisiae required for high-affinity transport (uptake) of potassium. The gene that encodes this putative potassium transporter (TRK1) was cloned by its ability to relieve the potassium transport defect in trk1 cells. TRK1 encodes a protein 1,235 amino acids long that contains 12 potential membrane-spanning domains. Our results demonstrate the physical and functional independence of the yeast potassium and proton transport systems. TRK1 is nonessential in S. cerevisiae and maps to a locus unlinked to PMA1, the gene that encodes the plasma membrane ATPase. Haploid cells that contain a null allele of TRK1 (trk1 delta) rely on a low-affinity transporter for potassium uptake and, under certain conditions, exhibit energy-dependent loss of potassium, directly exposing the activity of a transporter responsible for the efflux of this ion.

Activation of yeast plasma membrane ATPase by acid pH during growth

When yeast grows on media in which a final external pH lower than 4 is attained there is a 2-3-fold increase in plasma membrane ATPase activity. This acid-mediated activation produces a 2-fold increase in the ATPase affinity for ATP but does not modify its optimum pH. The acid-mediated activation is dependent on the stage of growth, only late logarithmic or stationary cells being capable of being activated by incubation in an acidic buffer. Only cells able to activate the ATPase maintain a constant internal pH when incubated in acidic buffers. It is concluded that acid-mediated activation of the plasma membrane ATPase is a mechanism of maintaining constant internal pH during growth of yeast on acid media.

Tight control of the amount of yeast plasma membrane ATPase during changes in growth conditions and gene dosage

The activity of the plasma membrane ATPase in growing yeast is increased by low pH and by glucose, conditions which result in a higher demand for proton pumping. The amount of enzyme is not significatively modified under these conditions. The amount of ATPase is only slightly increased by introducing extra copies of its gene in autonomous plasmids. In addition, the expression of the ATPase gene in a multi-copy plasmid causes a reduction of the copy number of the plasmid and slows growth. Therefore, overexpression of the ATPase is detrimental for the cell, justifying a regulatory mechanism based on increasing the catalytic activity and not the amount of enzyme.

Replacement of the promoter of the yeast plasma membrane ATPase gene by a galactose-dependent promoter and its physiological consequences

In order to probe the physiological role of the yeast plasma membrane ATPase we have replaced the constitutive promoter of its gene by a galactose-dependent promoter. The resulting cells stop growing on glucose medium when the preformed ATPase is diluted to 20% of normal. There is a correlation between ATPase activity and both proton efflux from the cells and amino acid transport. A large proportion of growth-arrested cells appear enlarged and with several buds containing nuclei.

Electrochemical potential and ion transport in vesicles of yeast plasma membrane

Vesicles from yeast plasma membrane were prepared according to Franzusoff and Cirillo [1983) J. Biol. Chem. 258, 3608), with slight modifications. When Mg-ATP was added, this preparation was able to generate a membrane potential, that was sensitive to inhibitors of the yeast H+-ATPase and uncouplers, and could be decreased by the addition of permeant anions, as measured by the fluorescence changes of the dye oxonol V. The addition of ATP could also generate a pH gradient, detectable by the fluorescence changes of the monitor aminochloromethoxyacridine. This gradient was sensitive to inhibitors of ATPase and uncouplers, and could be increased by the addition of permeant anions to the incubation mixture. When the vesicles were loaded with KCl, an increased rate of K+ efflux was produced upon the addition of ATP. Cytochrome oxidase from bovine heart could be reconstituted into the vesicles and was shown to generate a membrane potential difference, negative inside, evidenced by the fluorescence quenching of the cyanide dipropylthiacarbocyanine and the uptake of tetraphenylphosphonium. Besides, in these vesicles, K+ and Rb+, but not Na+ or NH+4 could decrease the quenching of fluorescence and the uptake of tetraphenylphosphonium produced when the electron-donor system was present. In the vesicles in which cytochrome oxidase was incorporated, upon the addition of cytochrome c and ascorbate, the uptake of 86Rb+ could be demonstrated also. This uptake was found to be saturable and inhibited by K+, and to a lesser degree by Na+. The results obtained indicate that these vesicles are reasonably sealed and capable of generating and maintaining a membrane potential. The membrane potential could be used to drive ions across the membrane of the vesicles, indicating the presence and functionality of the monovalent cation carrier. The vesicles, in general terms seem to be suitable for studying transport of ions and metabolites in yeast.

The Saccharomyces cerevisiae start mutant carrying the cdc25 mutation is defective in activation of plasma membrane ATPase by glucose

Activation of plasma membrane ATPase by the addition of glucose was examined in several cell division cycle mutants of Saccharomyces cerevisiae. The start mutant carrying the cdc25 mutation was shown to be defective in ATPase activation at the restrictive temperature. Genetic analysis showed that lack of growth and defective activation of ATPase at the restrictive temperature were caused by the same mutation. It was also found that CDC25 does not map at the same locus as the structural gene of plasma membrane ATPase (PMA1). We conclude that the product of CDC25 controls the activation of ATPase.

Intracellular ATP in a glucosephosphate isomerase mutant of Saccharomyces cerevisiae

The degree of ATP depletion caused by glucose in a glucosephosphate isomerase-deficient strain of Saccharomyces cerevisiae was determined. Even in the presence of a sugar normally fermentable by the mutant, the addition of glucose can decrease the intracellular ATP, depending on the competition of the sugars for transport and subsequent phosphorylation. For both parent and mutant cells, a correlation exists between the calculated velocity of ATP formation or ATP consumption during the utilization of different concentrations of sugars and the experimental intracellular ATP level. For initially resting yeast cells, a rate increase of 35 mumol per min per g ATP was calculated to increase the intracellular level of this nucleotide by 1 mumol per g cell mass.

Proton pump activity and Mg-ATPase activity in rat kidney cortex brushborder membranes: effect of 'proton ATPase' inhibitors

In order to further characterize the ATP driven proton pump present in the luminal membrane of the renal proximal tubule, brushborder membranes were isolated from rat kidney cortex and the effect of various proton ATPase inhibitors on intravesicular ATP hydrolysis in sealed brushborder membrane vesicles and on Mg-ATPase activity in permeabilized brushborder membranes was investigated. The protonophor induced intravesicular ATP hydrolysis (ATP driven proton pump) was inhibited by DCCD and filipin but not by diethylstilbestrol and duramycin. All four compounds decreased Mg-ATPase activity, the two former inhibited the ATPase activity with a lower potency than the proton pump. NEM--up to 10 mM--and orthovanadate did not affect intravesicular ATP hydrolysis nor Mg-ATPase activity. From the relative sensitivity of the proton pump and the Mg-ATPase activity to the inhibitors it is concluded that about 35% of the Mg-ATPase activity found in the brushborder membrane can be attributed to the ATP-driven proton pump. Furthermore, the results obtained with NEM and duramycin suggest that the brushborder membrane proton pump has different properties than the proton pump in clathrin-coated vesicles or endosomes. The results presented above raise the possibility that the brushborder membrane proton pump is predominantly involved in acid secretion by the proximal convoluted tubule whereas the proton pump in clathrin-coated vesicles may be predominantly involved in the endocytosis of larger peptides and proteins.

Uptake of phenol by Trichosporon cutaneum

The soil yeast Trichosporon cutaneum, which is distinguished by having a strictly oxidative metabolism, can be induced to utilize phenol as a sole carbon source. The present paper shows that such phenol-induced cells contain a specific, energy-dependent uptake system for phenol. Phenol uptake is not directly linked to its o-hydroxylation inside the cell, the first step of phenol metabolism. The Km for uptake is 235 +/- 30 microM, that for hydroxylation only 4.5 +/- 0.5 microM. Further, the phenol analog 2,6-dimethylphenol, which can not be hydroxylated, competes with phenol for the uptake system. The pH dependence of uptake indicates that phenolate is an essential form during the uptake process. The energy requirement for phenol uptake is indicated by effects of various inhibitors of energy generation, including proton-conducting uncouplers. Direct monitoring of proton movements in a pH-stat during phenol uptake indicates a phenol-proton symport. One proton is cotransported with every phenol molecule. Phenol competes with the uptake of sucrose and glycerol by cells grown on these substrates. Under such conditions the uptake of phenol seems to proceed through a different system, with lower affinity for phenol than in phenol-grown cells.

The proton pumps of the plasmalemma and the tonoplast of higher plants

Studies on intact cells, membrane vesicles, and reconstituted proteoliposomes have demonstrated in higher plants the existence of an ATP-driven electrogenic proton pump operating at the plasmalemma. There is also evidence of a second ATP-driven H+ pump localized at the tonoplast. The characteristics of both these ATP-driven pumps closely correspond to those of the plasmalemma and tonoplast proton pumps of Neurospora and yeasts.

Intracellular pH distribution and transmembrane pH profile of yeast cells

The pH-dependent fluorescence excitation of fluorescein located intracellularly and in the vicinity of cells of the yeast Saccharomyces cerevisiae and Endomyces magnusii was used to obtain local pH values at a linear resolution 0.2 micron. Cells suspended in water or in a diluted (5 mM) acidic buffer had a relatively alkaline interior (about 7.0-7.5) with pH decreasing gradually toward the periphery and further out through the cell wall to the value of the bulk solution. In slightly alkaline weak buffers the cells also showed an alkaline center and a slightly acidic ring-shaped area, but the peripheral region close to the membrane was again alkaline with pH increasing toward the bulk solution. The heterogeneity of intracellular pH was reduced or nearly abolished in starved or antimycin-treated cell. Suspension of cells in strong (200 mM) buffer resulted within 15-20 min in a nearly homogeneous pH pattern throughout the cell, attaining pH values of 5.5-7.5, depending on the pH of the buffer. Addition of glucose with concomitant pH decrease of the extracellular medium did not change appreciably the intracellular pattern for 20-30 min, except with diethylstilbestrol (inhibitor of proton-extruding ATPase) when the cell became more acidic. It appears that the delta pH measurements between the cell as a whole and the bulk solution (as are used for the calculation of the electrochemical potential of protons in proton-driven transports) are not substantiated, the probable pH difference across the plasma membrane being substantially smaller than previously supposed.