Kinetics of Ca2+ and Sr2+ uptake by yeast. Effects of pH, cations and phosphate

Catabolite inactivation of wild-type and mutant maltose transport proteins in Saccharomyces cerevisiae

The maltose transporter of Saccharomyces cerevisiae is subject to rapid, irreversible inactivation in the presence of glucose. Loss of transport function was paralleled by a decrease in amount of transporter protein and most likely involves endocytosis and degradation of the protein in the vacuole. This (catabolite) inactivation of Mal61p was triggered not only by glucose but also by 2-deoxy-D-glucose, which cannot be metabolized beyond 2-deoxy-D-glucose phosphate. The signal that targets membrane proteins specifically for catabolite inactivation is unknown. To investigate whether or not specific modification of Mal61p triggers the inactivation, putative protein kinase A and C phosphorylation sites were removed, and the transport activities and levels of the mutant proteins upon addition of glucose were followed in time. Three Mal61p mutants, i.e. S295A, T363A, and S487A, exhibited significantly reduced rates of inactivation in the presence of glucose. Likewise, in wild-type Mal61p the rate of inactivation and degradation of the protein paralleled each other in the case of T363A. On the contrary, for the S295A and S487A mutants the rates of protein degradation were slowed down more profoundly than was the loss of transport activity. These observations indicate that (i) some form of modification (e.g. phosphorylation) of the protein precedes breakdown, (ii) the modification inactivates Mal61p, and (iii) the inactivation of Mal61p is not necessarily followed by proteolytic degradation.

Influence of monovalent cations on yeast cytoplasmic and vacuolar pH

The effects of monovalent cations on the internal pH of yeast were studied. Our former procedure was modified, inducing maximal alkalinization of the cells with 100 mM-NH4OH instead of Tris base. The pH values were lower than reported before (Peña et al., J. Baceteriol. 1995 177, 1017-1022). With glucose as substrate, the internal cytoplasmic pH reached higher values when incubating at an external pH of 6.0, as compared to pH 4.0. Monovalent cations added approximately 5 min after glucose produced a further increase in the internal pH, which was higher at a previous incubation pH of 4.0 than that observed at pH 6.0. The selectivity of the changes followed a similar order to that of the transport system for monovalent cations. When incubating cells with glucose for more than 30 min, the initial changes of the internal pH appeared to be regulated by the cell. However, under the fluorescence microscope, it was observed that pyranine, which was confined to the cytoplasm during the first 15 min, was progressively concentrated in the vacuole. By studying the fluorescence changes of cells electroporated and then incubated with glucose or glucose plus potassium, we could follow the internal pH of this organelle, obtaining values within the range reported by other authors. Also, in cells preincubated with glucose for 60 min, and electroporated afterwards, the fluorescence of pyranine, which only entered the cytoplasm, allowed us to measure the pH of this compartment, showing that it was more alkaline than the vacuole. Moreover, the cytoplasmic pH increased upon addition of glucose or potassium. The vacuolar pH, on the other hand, increased upon addition of potassium after glucose, but decreased upon addition of glucose. In addition, incubation of the cells with glucose with or without pyranine produced vesiculation of the vacuole.

Metal cation uptake by yeast: a review

This review addresses metal uptake specifically by yeast. Metal uptake may be passive, active or both, depending on the viability of the biomass, and is influenced by a number of environmental and experimental factors. Uptake is typically accompanied by a degree of ion exchange and, under certain conditions, may be enhanced by the addition of an energy source. Intracellularly accumulated metal is most readily associated with the cell wall and vacuole but may also be bound by other cellular organelles and biomolecules. The intrinsic biochemical, structural and genetic properties of the yeast cell along with environmental conditions are crucial for its survival when exposed to toxic metals. Conditions of pH, temperature and the presence of additional ions, amongst others, have varying effects on the metal uptake process. We conclude that yeasts have contributed significantly to our understanding of the metal uptake process and suggest directions for future work.

The roles of magnesium in biotechnology

This review highlights the important roles played by magnesium in the growth and metabolic functions of microbial and animal cells, and therefore assigns a key role for magnesium ions in biotechnology. The fundamental biochemical and physiological actions of magnesium as a regulatory cation are outlined. Such actions are deemed to be relevant in an applied sense, because Mg2+ availability in cell culture and fermentation media can dramatically influence growth and metabolism of cells. Manipulation of extracellular and intracellular magnesium ions can thus be envisaged as a relatively simplistic, but nevertheless versatile, means of physiological cell engineering. In addition, biological antagonism between calcium and magnesium at the molecular level may have profound consequences for the optimization of biotechnological processes that exploit cells. In fermentation, for example, it is argued that the efficiency of microbial conversion of substrate to product may be improved by altering Mg:Ca concentration ratios in industrial feedstocks in a way that makes more magnesium available to the cells. With particular respect to yeast-based biotechnologies, magnesium availability is seen as being crucially important in governing central pathways of carbohydrate catabolism, especially ethanolic fermentation. It is proposed that such influences of magnesium ions are expressed at the combined levels of key enzyme activation and cell membrane stabilization. The former ensures optimum flow of substrate to ethanol and the latter acts to protect yeasts from physical and chemical stress.

The COT2 gene is required for glucose-dependent divalent cation transport in Saccharomyces cerevisiae

Eleven cobalt-tolerant mutants were found to belong to a single complementation group, cot2. In addition to cobalt, the cot2 mutants were found to tolerate increased levels of the divalent cations Zn2+, Mn2+, and Ni2+ as well. All of the cot2 mutants exhibited a wiener-shaped cellular morphology that was exacerbated by the carbon and nitrogen source but was unaffected by metals. The rate of glucose-dependent transport of cobalt into cells was reduced in strains that carry mutations in the COT2 gene. COT2 is not essential for growth. Strains that carry a COT2 allele conferring complete loss of function are viable and exhibit phenotypes similar to those of spontaneous cot2 mutations. The sequence of the COT2 gene shows that it is identical to GRR1, which encodes a protein required for glucose repression. The glucose dependence of the transport defect implies that cot2 mutations affect the link between glucose metabolism and divalent cation active transport.

The COT2 gene is required for glucose-dependent divalent cation transport in Saccharomyces cerevisiae

Eleven cobalt-tolerant mutants were found to belong to a single complementation group, cot2. In addition to cobalt, the cot2 mutants were found to tolerate increased levels of the divalent cations Zn2+, Mn2+, and Ni2+ as well. All of the cot2 mutants exhibited a wiener-shaped cellular morphology that was exacerbated by the carbon and nitrogen source but was unaffected by metals. The rate of glucose-dependent transport of cobalt into cells was reduced in strains that carry mutations in the COT2 gene. COT2 is not essential for growth. Strains that carry a COT2 allele conferring complete loss of function are viable and exhibit phenotypes similar to those of spontaneous cot2 mutations. The sequence of the COT2 gene shows that it is identical to GRR1, which encodes a protein required for glucose repression. The glucose dependence of the transport defect implies that cot2 mutations affect the link between glucose metabolism and divalent cation active transport.

Mechanisms of strontium uptake by laboratory and brewing strains of Saccharomyces cerevisiae

Laboratory and brewing strains of Saccharomyces cerevisiae were compared for metabolism-independent and -dependent Sr2+ uptake. Cell surface adsorption of Sr2+ to live cells was greater in the brewing than in the laboratory strain examined. However, uptake levels were greater in denatured (dried and ground) S. cerevisiae, and the relative affinities of Sr2+ for the two strains were reversed. Results for the brewing S. cerevisiae strain were similar whether the organism was obtained fresh from brewery waste or after culturing under the same conditions as for the laboratory strain. Reciprocal Langmuir plots of uptake data for live biomass were not linear, whereas those for denatured biomass were. The more complex Sr2+ binding mechanism inferred for live S. cerevisiae was underlined by cation displacement experiments. Sr2+ adsorption to live cells resulted in release of Mg2+, Ca2+, and H+, suggesting a combination of ionic and covalent bonding of Sr2+. In contrast, Mg2+ was the predominant exchangeable cation on denatured biomass, indicating primarily electrostatic attraction of Sr2+. Incubation of live S. cerevisiae in the presence of glucose resulted in a stimulation of Sr2+ uptake. Cell fractionation revealed that this increased Sr2+ uptake was mostly due to sequestration of Sr2+ in the vacuole, although a small increase in cytoplasmic Sr2+ was also evident. No stimulation or inhibition of active H+ efflux resulted from metabolism-dependent Sr2+ accumulation. However, a decline in cytoplasmic, and particularly vacuolar, Mg2+, in comparison with that of cells incubated with Sr2+ in the absence of glucose, was apparent. This was most marked for the laboratory S. cerevisiae strain, which contained higher Mg2+ levels than the brewing strain.

COT1, a gene involved in cobalt accumulation in Saccharomyces cerevisiae

The COT1 gene of Saccharomyces cerevisiae has been isolated as a dosage-dependent suppressor of cobalt toxicity. Overexpression of the COT1 gene confers increased tolerance to cobalt and rhodium ions but not other divalent cations. Strains containing null alleles of COT1 are viable yet more sensitive to cobalt than are wild-type strains. Transcription of COT1 responds minimally to the extracellular cobalt concentration. Addition of cobalt ions to growth media results in a twofold increase in COT1 mRNA abundance. The gene encodes a 48-kDa protein which is found in mitochondrial membrane fractions of cells. The protein contains six possible membrane-spanning domains and several potential metal-binding amino acid residues. The COT1 protein shares 60% identity with the ZRC1 gene product, which confers resistance to zinc and cadmium ions. Cobalt transport studies indicate that the COT1 product is involved in the uptake of cobalt ions yet is not solely responsible for it. The increased tolerance of strains containing multiple copies of the COT1 gene is probably due to increased compartmentalization or sequestration of the ion within mitochondria.

COT1, a gene involved in cobalt accumulation in Saccharomyces cerevisiae

The COT1 gene of Saccharomyces cerevisiae has been isolated as a dosage-dependent suppressor of cobalt toxicity. Overexpression of the COT1 gene confers increased tolerance to cobalt and rhodium ions but not other divalent cations. Strains containing null alleles of COT1 are viable yet more sensitive to cobalt than are wild-type strains. Transcription of COT1 responds minimally to the extracellular cobalt concentration. Addition of cobalt ions to growth media results in a twofold increase in COT1 mRNA abundance. The gene encodes a 48-kDa protein which is found in mitochondrial membrane fractions of cells. The protein contains six possible membrane-spanning domains and several potential metal-binding amino acid residues. The COT1 protein shares 60% identity with the ZRC1 gene product, which confers resistance to zinc and cadmium ions. Cobalt transport studies indicate that the COT1 product is involved in the uptake of cobalt ions yet is not solely responsible for it. The increased tolerance of strains containing multiple copies of the COT1 gene is probably due to increased compartmentalization or sequestration of the ion within mitochondria.

The concentration dependence of the depolarization of yeast by monovalent cations

Monovalent cations decrease the initial rate of uptake of the membrane potential probe 2-(dimethylaminostyryl)-1-ethyl-pyridinium (DMP) into metabolizing cells, showing that the cells are depolarized. A steep decrease in this rate was found even at low cation concentrations, reaching 62%, 42%, 58%, 40% and 40% at high concentrations of K+, Rb+, Cs+, Na+ and Li+, respectively. The corresponding concentrations at which half-maximum decrease was found were 0.22, 0.36, 1.2, 17 and 17 mM. These values are of the same order of magnitude as the half-saturation concentrations for monovalent cation uptake by the yeast.

Effect of ruthenium red upon Ca2+ and Mn2+ uptake in Saccharomyces cerevisiae. Comparison with the effect of La3+

The initial rate of both Ca2+ and Mn2+ uptake is inhibited by ruthenium red to about the same extent as by equivalent concentrations of La3+. The inhibition of Ca2+ uptake, however, is relieved during further incubation with ruthenium red. On preincubating the cells with ruthenium red even a stimulation of divalent cation uptake can be found. Relieve of the inhibition of divalent cation uptake is accompanied by K+ efflux. Both ruthenium red and La3+ displace Ca2+ very effectively from binding sites at the cell surface. The inhibition of initial Ca2+ uptake is accompanied by a reduction in the binding of Ca2+.

Effect of ethidium bromide and DEAE-dextran on divalent cation accumulation in yeast. Evidence for an ion-selective extrusion pump for divalent cations

The larger accumulation of Mn2+ than of Sr2+ in Saccharomyces cerevisiae is ascribed to the operation of a specific extrusion pump, presumably a Ca2+ pump, which has a higher affinity for Sr2+ than for Mn2+. The differences in accumulation levels of Mn2+ and Sr2+ attained after prolonged incubation are completely abolished in cells of which the plasmamembrane has been permeabilized with the polybase DEAE-dextran under isotonic conditions. In the permeabilized cells Sr2+ and Mn2+ accumulation levels are attained as for Mn2+ in intact cells. It is suggested that the accumulation of divalent cations into the permeabilized cells mainly represents their accumulation into the vacuoles. Also the cationic dye ethidium abolishes the differences in Mn2+ and Sr2+ accumulation. The dye increases the accumulation of Sr2+ but decreases that of Mn2+ somewhat. It cannot be distinguished yet whether its action is due to an impairment of the efflux pump or to an increase in the permeability of the plasmamembrane facilitating the divalent cations to be accumulated into the vacuoles. Ethidium does not affect the initial rates of divalent cation uptake into the vacuoles, but it effectively reduces the ultimate accumulation of the divalent cations in the DEAE-dextran permeabilized cells, possibly by competing with the divalent cations for intravacuolar binding sites. Similar results are obtained for the accumulation of Ca2+. It is concluded that the efflux pump enables the yeast cell to regulate accumulation levels of the various divalent cations to different extents.

Apparent saturation kinetics of divalent cation uptake in yeast caused by a reduction in the surface potential

The concentration dependence of the uptake rate of divalent cations in yeast can be described by a simple diffusion process after accounting for the effect of the surface potential upon the divalent cation concentration near the membrane. It is also necessary to correct for the effect of the cell pH upon the rate of translocation. The apparent saturation kinetics is ascribed to the fact that the quotient of the concentration of the divalent cations near the cell membrane and the bulk aqueous phase concentration is reduced on increasing the divalent cation concentration in the medium. The diffusion process regulated by the surface potential even mimics the saturation kinetics of a two-carrier transport system. The selectivity found between Ca2+ and Sr2+ uptake can probably be traced to differences in their affinity for the negative groups on the cell membrane determining the surface potential rather than to differences in their affinity for a transport system. The enhancement of divalent cation uptake by loading the cells with phosphate is probably due to the concomitant increase in the net negative charge of the cell membrane.

Effects of phenothiazines on inhibition of plasma membrane ATPase and hyperpolarization of cell membranes in the yeast Saccharomyces cerevisiae

The transmembranal potential, in Saccharomyces cerevisiae, has been calculated from the distribution ratio of the lipophilic cation tetraphenylphosphonium (TPP+) between the intracellular and extracellular water. Trifluoperazine at concentrations of 10 to 50 microM, caused a substantial increase in the membrane potential (negative inside). This increase was observed only in the presence of a metabolic substrate and was eliminated by the addition of the protonophores 2,4-dinitrophenol and sodium azide, removal of glucose, replacement of glucose by the nonmetabolizable analog 3-O-methyl glucose, or by the addition of 100 mM KCl. An increase in 45CaCl2 accumulation from solutions of low concentrations (1 microM) was observed under all conditions where membrane potential was increased. Proton ejection activity was monitored by measurements of the rates of the decrease in the pH of unbuffered cell suspensions in the presence of glucose. Trifluoperazine inhibited the changes in medium pH; this inhibition was not the result of an increase in the permeability of cell membranes to protons since in the absence of glucose, trifluoperazine did not cause a change in the rate of pH change generated by proton influx. The activity of plasma membrane ATPase was measured in crude membrane preparations in the presence of sodium azide to inhibit mitochondrial ATPase. Trifluoperazine strongly inhibited the activity of the plasma membrane ATPase. The effect of phenothiazines on transport and on membrane potential reported in this study and in the previous one (Eilam, Y. (1983) Biochim. Biophys. Acta 733, 242-248) were observed only in the presence of a metabolic substrate. The possibility that energy is required for the uptake of phenothiazines into the cells was eliminated by results showing energy-independent uptake of [3H]chlorpromazine. The results strongly suggest that phenothiazines activate energy-dependent K+-extrusion pumps, which lead to increased membrane potential. Increased influx of calcium seems to be energized by membrane potential, and therefore stimulated under all conditions where membrane potential is increased. The analog which does not bind to calmodulin, trifluoperazine sulfoxide, had no effect on the cells, but the involvement of calmodulin in the processes altered by trifluoperazine cannot as yet, be determined.

Interaction of membrane surface charges with the reconstituted ADP/ATP-carrier from mitochondria

Various modulating influences of negative and positive membrane charges on binding and transport properties of the reconstituted ADP/ATP carrier from mitochondria were investigated. The results are interpreted in terms of functional and structural asymmetries of the adenine nucleotide carrier embedded in the liposomal membrane. The surface potential of liposomes was measured directly either by potential-dependent adsorption of the fluorescent dye 2-p-toluidinylnaphthalene 6-sulfonate (TNS) or by the pK shift of the lipophilic pH indicator pentadecylumbelliferone. These results were correlated with the following observations. (1) Negative surface potentials increase the apparent dissociation constant, Kd, for binding of the negatively charged inhibitor carboxyatractylate to the reconstituted carrier protein. (2) Surface potentials modulate the apparent transport affinity, Km, of the reconstituted adenine nucleotide carrier for ADP and ATP. The interaction of surface charges with the transport function was investigated with carrier proteins oriented both right-side-out and inside-out. Thus the influence of the surface potential on the function of the ADP/ATP carrier could be determined for the internal and external active sites of the translocator on the outer side of the membrane. Large discrepancies were observed not only between the potentials measured directly (fluorescent dyes) and those measured indirectly (binding and transport affinities), but also between the different surface potentials determined from the influence on the alternatively oriented carrier proteins. The effect of surface charges was rather weak on the cytosolic side of the translocator, whereas there was a strong influence of surface charges on the active site at the matrix side. The most obvious explanation, i.e., screening of negative membrane charges by positively charged amino acid residues at the protein surface, could be ruled out. Besides the modulation of binding affinities for substrates and inhibitors, an additional side-specific effect of surface charges on the transport velocity was observed. Again, the influence on the internal active site of the ADP/ATP carrier was found to be much higher than that on the cytosolic site. The observed effects can be explained by a definite structural asymmetry of the carrier embedded in the liposomal membrane. That site which is physiologically exposed to the cytosol is located at a considerable distance from the plane of the membrane, whereas the opposite site seems to be in close proximity to the membrane surface. Moreover, a spatial equivalence of carboxyatractylate binding site and nucleotide binding site at the external side of the carrier protein was concluded.

Ionophores and intact cells. II. Oleficin acts on mitochondria and induces disintegration of the mitochondrial genome in yeast Saccharomyces cerevisiae

The non-macrolid polyene antibiotic oleficin, which has been shown to function as an ionophore of Mg2+ in isolated rat liver mitochondria, preferentially inhibited growth of the yeast Saccharomyces cerevisiae on non-fermentable substrates. It uncoupled and inhibited respiration of intact cells and converted both growing and resting cells into respiration-deficient mutants. The mutants arose as a result of fragmentation of the mitochondrial genome. Another antibiotic known to be an ionophore of divalent cations, A23187, also selectively inhibited growth of the yeast on non-fermentable substrates, but did not produce the respiration-deficient mutants, neither antibiotic inhibited the energy-dependent uptake of divalent cations by yeast cells nor opened the plasma membrane for these cations. The results indicate that in Saccharomyces cerevisiae both oleficin and A23187 preferentially affected the mitochondrial membrane without acting as ionophores in the plasma membrane.

Nystatin effects on cellular calcium in Saccharomyces cerevisiae

The primary effects of nystatin, a polyene antibiotic, on the yeast Saccharomyces cerevisiae were investigated. Though K+ leakage was observed shortly after the addition of nystatin, Ca2+ leakage was delayed 2-3 h after its application and it occurred only at an acidic pH and in the absence of K+, Na+ or Mg2+ from the medium. However, within 4 min after application nystatin induced a passive influx of Ca2+ into the cells even at a concentration of 1 microM in the medium. These results led to the conclusion that the primary membranal lesion induced by nystatin is not restricted to monovalent cations but is also manifested by increased permeability to Ca2+. The delayed leakage of Ca2+ is explained by the assumption that the bulk of cellular calcium is sequestered so that the concentration of free Ca2+ in the cytoplasm is very low. The sequestered calcium may be liberated 2-3 h after the addition of nystatin as a consequence of secondary damage to the cells such as intracellular acidification and loss of cations.

The effect of monovalent cations on calcium efflux in yeasts

The properties of the calcium efflux system in the yeast Saccharomyces cerevisiae were investigated. After growing the cells overnight in medium containing 45Ca, the cells were transferred to medium containing glucose. Herpes buffer (pH 5.2) and monovalent cations. The presence of potassium or sodium in the medium induced efflux of calcium from the cells. The magnitude of the efflux was dependent on the concentration of these cations in the medium. The time course of calcium efflux was analyzed, and two types of exchangeable calcium pools, which turned over at different rates, were detected: 'Fast turnover' and 'slow turnover'. Increase in the concentration of monovalent cations in the medium caused an increase in the fraction of cellular calcium which turned over at a fast rate, and activation of calcium efflux from the 'slow turnover' calcium pool. The specific changes in the parameters of calcium efflux induced by monovalent cations were different from those reported previously to be induced by divalent cations. Both processes, i.e. activation of calcium efflux by monovalent and by divalent cations, were found to be additive, indicating that they operate via different mechanisms. Experiments using the respiratory inhibitor Antimycin A, showed that stimulation of calcium efflux by monovalent cations is energy dependent. Lanthanum ions which are known to inhibit calcium influx into yeast cells, inhibited the activation of calcium efflux by both divalent and monovalent cations. Determination of the cationic composition of the cells indicated that the stimulation of calcium efflux was accompanied by influx of potassium or sodium into the cells.

Uptake and accumulation of Mn2+ and Sr2+ in Saccharomyces cerevisiae

Initial uptake of Mn2+ and Sr2+ in the yeast Saccharomyces cerevisiae was studied in order to investigate the selectivity of the divalent cation uptake system and the possible involvement of the plasma-membrane ATPase in this uptake. The initial uptake rates of the two ions were not significantly different. This ruled out a direct role of the plasma-membrane ATPase, since this ATPase is specific for Mn2+ compared to Sr2+. After 1 h uptake, Mn2+ had accumulated 10-times more than Sr2+. Influx of Mn2+ and Sr2+ remained unchanged during that time, however. The differences in accumulation level found for Mn2+ and Sr2+ could be ascribed to a greater efflux of Sr2+ as compared with Mn2+. Probably this greater efflux of Sr2+ was only apparent, since differential extraction of the yeast cells revealed that Mn2+ is more compartmentalised than Sr2+, giving rise to a lower relative cytoplasmic Mn2+ concentration.

Some characteristics of Ca2+ uptake by yeast cells

Experiments were performed to obtain information on: (i) the specific properties of Ca2+ binding and transport in yeast; (ii) the relationship between both parameters; (iii) similarities to or differences from other biological systems as measured by the effects of inhibitors; and (iv) the effects of mono and divalent cations, in order to get some insight on the specificity and some characteristics of the mechanism of the transport system for divalent cations in yeast. The results obtained gave some kinetic parameters for a high affinity system involved in the transport of Ca2+ in yeast. These were obtained mainly by considering actual concentrations of Ca2+ in the medium after substracting the amounts bound to the cell. A km of 1.9 microM and a Vmax of 1.2 nmol (100 mg.3 min)-1 were calculated. The effects of some inhibitors and other cations on Ca2+ uptake allow one to postulate some independence between binding and transport for this divalent cation. Of the inhibitors tested, only lanthanum seems to be a potent inhibitor of Ca2+ uptake in yeast. The effects of Mg2+ on the uptake of Ca2+ agree with the existence of a single transport system for both divalent cations. The actions of Na+ and K+ on the transport of Ca2+ offer interesting possibilities to study further some of the mechanistic properties of this transport system for divalent cations.

Uptake of the lipophilic cation dibenzyldimethylammonium into Saccharomyces cerevisiae. Interaction with the thiamine transport system

The distribution ratio of the lipophilic cation dibenzyldimethylammonium between the cells of Saccharomyces cerevisiae and the medium appears to reflect changes in the membrane potential in a way that is qualitatively correct: the addition of a proton conductor or of an agent which blocks metabolism causes an apparent depolarization of the cell membrane; monovalent cations cause also a lowering of the equilibrium distribution, whereas the addition of divalent cations results in an increase of the partition ratio. However, uptake of dibenzyldimethylammonium and probably also of other liophilic cations proceeds via the thiamine transport system of the yeast. Dibenzyldimethylammonium transport is inducible, like thiamine transport. A kinetic analysis of the mutual interaction between thiamine and dibenzyldimethylammonium uptake shows that these compounds share a common transport system; moreover, dibenzyldimethylammonium uptake is inhibited complete by thiamine disulfide, a competitive inhibitor of thiamine transport and dibenzyldimethylammonium uptake in a thiamine-transport mutant is reduced considerably. It is concluded that one should be cautious when using lipophilic cations to measure the membrane potential of cells of S. cerevisiae.

Kinetics of sulfate uptake by yeast

Uptake of sulfate by yeast requires the presence of a metabolic substrate and is dependent on the time during which the cells have been metabolizing in the absence of sulfate. At low concentrations of sulfate, uptake can be described by simple saturation kinetics. Uptake of sulfate is accompanied by a net proton influx of 3 H+ and an efflux of 1 K+ for each sulfate ion taken up. Divalent cations stimulate sulfate uptake at low concentrations of sulfate; the maximal rate of uptake is not significantly affected but Km is lowered. Stimulation by divalent cations shows an optimum at a cation concentration of about 4 mM. Monovalent cations are less effective, trivalent cations are more effective in stimulating sulfate uptake. The results are qualitatively in accordance with the notion, that the effect of cations is due to an effect via the surface potential.