Kinetic and steady-state investigations of solute accumulation in bacterial membranes by continuously monitoring the radioactivity in the effluent of flow-dialysis experiments

Selection of Protease-Positive and Protease-Negative Variants of Streptococcus cremoris

Protease-negative variants were shown to outcompete the wild-type strains of Streptococcus cremoris E(8), HP, and Wg(2) at pH values higher than 6.0 in milk. For S. cremoris E(8) this process was studied in more detail. At lower pH values the wild type had a selective advantage. This pH-dependent selection was not found in all media tested. The poor growth of the protease-negative variant at low pH was not due to lower internal pH values. By growing S. cremoris E(8) and Wg(2) in acidified milk (pH 5.9) the proteolytic activity of the cultures could be stabilized. In continuous cultures under amino acid limitation the wild type S. cremoris E(8) and HP strains had a selective advantage over the protease-negative variants at low dilution rates (D < 0.2) at all pH values of the medium. This was apparently due to a lower affinity-constant (K(s)) of the protease-positive variants for amino acids. Finally, a high fraction of protease-positive variants could be maintained in continuous cultures by using a growth medium with low concentrations of casein as a nitrogen source. At high dilution rates nearly all cells were protease positive.

Evaluation of the flow-dialysis technique for analysis of protein-ligand interactions: an experimental and a monte carlo study

Flow dialysis has found widespread use in determining the dissociation constant (KD) of a protein-ligand interaction or the amount of available binding sites (E0). This method has the potency to measure both these parameters in a single experiment and in this article a method to measure simultaneously the KD and E0 is presented, together with an extensive error analysis of the method. The flow-dialysis technique is experimentally simple to perform. However, a number of practical aspects of this method can have a large impact on the outcome of KD and E0. We have investigated all sources of significant systematic and random errors, using the interaction between mannitol and its transporter from Escherichia coli as a model. Monte Carlo simulations were found to be an excellent tool to assess the impact of these errors on the binding parameters and to define the experimental conditions that allow their most accurate estimation.

On the evaluation of data from flow-dialysis experiments

Flow dialysis can be used to measure (i) ligand binding to macromolecules and (ii) the size of transmembrane ion gradients. Generally an approximate method is used to calculate the binding or gradient parameters from the raw data. Here we present a simple but exact method and evaluate the errors that may arise when the approximate method is used to calculate the magnitude of ion gradients. In addition, equations are presented that allow for a correction for sampling from or additions to the upper compartment of a flow-dialysis vessel during the measurements. Setty and Hendler [(1982) J. Biochem. Biophys. Methods 7, 35-46] have reported artifacts in the measurement of ion-gradients caused by the addition of electron donors to the upper compartment of a flow-dialysis cell. Here we extend their observations and suggest additional methods to prevent such artifacts.

Transport of branched-chain amino acids in membrane vesicles of Streptococcus cremoris

The kinetics, specificity, and mechanism of branched-chain amino acid transport in Streptococcus cremoris were studied in a membrane system of S. cremoris in which beef heart mitochondrial cytochrome c oxidase was incorporated as a proton motive force (delta p)-generating system. Influx of L-leucine, L-isoleucine, and L-valine can occur via a common transport system which is highly selective for the L-isomers of branched chain amino acids and analogs. The pH dependency of the kinetic constants of delta p-driven L-leucine transport and exchange (counterflow) was determined. The maximal rate of delta p-driven transport of L-leucine (Vmax) increased with increasing internal pH, whereas the affinity constant increased with increasing external pH. The affinity constant for exchange (counterflow) varied in a similar fashion with pH, whereas Vmax was pH independent. Further analysis of the pH dependency of various modes of facilitated diffusion, i.e., efflux, exchange, influx, and counterflow, suggests that H+ and L-leucine binding and release to and from the carrier proceed by an ordered mechanism. A kinetic scheme of the translocation cycle of H+-L-leucine cotransport is suggested.

The phosphoenolpyruvate-dependent fructose-specific phosphotransferase system in Rhodopseudomonas sphaeroides. Evidence for a shift in the midpoint potential of the dithiol redox center during turnover of the carrier

Redox titrations of the fructose-specific carrier protein, EFruII, in Rhodopseudomonas sphaeroides show that only the reduced form of the enzyme is active. The oxidized form of the enzyme can still be phosphorylated but is unable to transfer the phosphoryl group to fructose. The redox properties of the enzyme change upon phosphorylation. The reduction rate of EFruII is slower than that of EFruII-P, whereas the opposite is true for the oxidation rate. Consequently the midpoint potential of the redox centre is more negative on EFruII than EFruII-P, most likely due to an upwards pK shift of the thiols upon phosphorylation. The measurements indicate that the phosphotransferase system is regulated by the redox potential in a way that is dependent on the substrate concentrations. We propose that the change in the midpoint potential during turnover could be a mechanism for an electron transport function of the carrier. The binding of Zn2+ protects the carrier dithiol against oxidation but the presence of Zn2+ does not stimulate the reduction of the oxidized carrier.

Measurements of the proton motive force generated by cytochrome c oxidase from Bacillus subtilis in proteoliposomes and membrane vesicles

Cytochrome c oxidase from Bacillus subtilis was reconstituted in liposomes and its energy-transducing properties were studied. The reconstitution procedure used included Ca2+-induced fusion of pre-formed membranes. The orientation of the enzyme in liposomes is influenced by the phospholipid composition of the membrane. Negatively charged phospholipids are essential for high oxidase activity and respiratory control. Analyses of the proteoliposomes by gel filtration, density gradient centrifugation and electron microscopy indicated a heterogeneity of the proteoliposomes with respect to size and respiratory control. Cytochrome c oxidase activity in the proteoliposomes resulted in the generation of a proton motive force, internally negative and alkaline. In the presence of the electron donor, ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine/cytochrome c or ascorbate/phenazine methosulphate, the reconstituted enzyme generated an electrical potential of 84 mV which was increased by the addition of nigericin to 95 mV and a pH gradient of 32 mV which was increased by the addition of valinomycin to 39 mV. Similar results were obtained with beef-heart cytochrome c oxidase reconstituted in liposomes. The maximal proton motive force which could be generated, assuming no endogenous ion leakage, varied over 110-140 mV. From this the efficiency of energy transduction by cytochrome c oxidase was calculated to be 18-23%, indicating that the oxidase is an efficient proton-motive-force-generating system.

The phosphoenolpyruvate-dependent fructose-specific phosphotransferase system in Rhodopseudomonas sphaeroides. EIIFru possesses a Zn2+-binding site and a dithiol/disulfide redox centre

Two interrelated sites have been detected on the fructose carrier in Rhodopseudomonas sphaeroides: an activity-linked dithiol and a Zn2+-binding site. Binding of Zn2+ brings EIIFru into a new conformation that to some extent mimics the conformation of phosphorylated EIIFru, an essential intermediate in the turnover of the enzyme. Binding of zinc to EIIFru or phosphorylating the enzyme protects it against trypsin inactivation relative to the dephosphorylated zinc-free enzyme. A dithiol is essential for activity. Interchanges between the redox states of the enzyme can be brought about by dithiothreitol and ferricyanide, but not, or very slowly, by molecular oxygen. The dithiol is protected, in the EIIFru-Zn2+ complex, against alkylation by MalNEt, reversible oxidation by Fe(CN)6(3-) and Cu2+, irreversible oxidation by Cu2+. The pK value of the activity linked thiol is 7.8. Protection experiments show that the dithiol is not located in any of the substrate-binding sites. The redox state of the enzyme does not influence the rate of inactivation of EIIFru by trypsin.

Energetics of Leucyl-Leucine Hydrolysis in Streptococcus cremoris Wg 2

The hydrolysis of the dipeptide leucyl-leucine by whole cells of Streptococcus cremoris Wg 2 was dependent on the presence of the energy source lactose. Incubation of cells with uncouplers or ATPase inhibitors prevented the increase of peptidase activity upon the addition of lactose. Incubation with the ionophore nigericin resulted in decreased peptide hydrolysis activity, while incubation with valinomycin led to increased hydrolysis activity. In the presence of nigericin the ΔpH component of the proton motive force was decreased, while the electrical potential was increased. With valinomycin, the electrical potential was collapsed and the ΔpH was increased. When the external pH was decreased from 8 to 5, the rate of peptide hydrolyzing activity by whole cells increased with increasing ΔpH component. In contrast, the peptide hydrolyzing activity in the cell extract decreased with decreasing external pH. These results indicate that the ΔpH component of the proton motive force determines the leucyl-leucine hydrolyzing activity in S. cremoris Wg 2 .

The role of the proton motive force and electron flow in solute transport in Escherichia coli

Transport of lactose and methyl beta-D-thiogalactopyranoside, a melibiose analogue, was studied in intact cells of Escherichia coli. A proton motive force could drive the translocation of these solutes via these two transport systems, but the initial rates and steady-state levels of solute accumulation increased upon initiation of electron transfer. When the absolute value of the proton motive force was decreased by ionophores the steady-state levels of lactose accumulation did not decrease as expected if thermodynamic equilibrium with the proton motive force had existed. Accumulation of lactose was also observed in the absence of any measurable proton motive force as long as electron transfer took place. Since both proton/lactose and sodium/methyl beta-D-thiogalactopyranoside symport showed the same characteristics, an explanation based on local proton diffusion pathways is unlikely.

Incorporation of beef heart cytochrome c oxidase as a proton-motive force-generating mechanism in bacterial membrane vesicles

Membrane vesicles derived from the strictly fermentative lactic acid bacterium Streptococcus cremoris have been fused with proteoliposomes containing the beef heart mitochondrial cytochrome c oxidase by means of a freeze/thaw-sonication technique. Evidence that fusion has taken place was obtained by freeze-etch electron microscopy, showing a less-dense intramembranous particle distribution in the fused membranes than in the bacterial membranes, and by sucrose gradient centrifugation, indicating a buoyant density of the majority of the membranes after fusion that was between the buoyant densities of the starting membrane preparations. In the fused membranes, 55-60% of the cytochrome c oxidase molecules are oriented with the cytochrome c binding site at the outer surface of the membrane. With the electron-donor system ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine/cytochrome c, a high proton-motive force (greater than 130 mV), inside negative and alkaline, can be generated in the fused membrane, and this proton-motive force can drive secondary transport of several amino acids. The procedure described can be used for incorporating a proton-motive force-generating system in isolated membrane vesicles from bacterial or eukaryotic origin that lack a suitable primary proton pump.

A polyvinylchloride-membrane based anion selective electrode for continuous registration of delta pH (interior alkaline) with salicylate as the indicator probe

An anion sensitive electrode has been constructed with the use of the lipid soluble cation benzyl-dimethyl-hexadecylammonium analogous to the procedure described for tetraphenylphosphonium-sensitive electrodes [Shinbo, T., Kamo, N., Kurihara, K. and Kobatake, Y. (1978) Arch. Biochem. Biophys. 187, 414-422]. The anion electrode responds to salicylate concentrations above 400 microM with a Nernstian sensitivity. Less lipid soluble anions like chloride and phosphate do not interfere. Below 400 microM salicylate the response of the electrode decreases gradually so that the sensitivity of the electrode is less than 10 mV per decade change at concentrations of the anion of 50 microM. A computer program has been developed to fit the electrode response curve with a polynomal function of the fourth power. Additional software-allows calculation of changes in the concentration of the salicylate anion, also under conditions where the sensitivity of the electrode for the anion is not constant. In this way the electrode can be used to measure changes in salicylate concentration that occur in a suspension of bacteria when, upon energization, a pH gradient is generated. 31P nuclear magnetic resonance measurements demonstrated that the pH gradient measured with the salicylate-sensitive electrode in the phototrophic bacterium Rhodopseudomonas sphaeroides is quantitatively correct. The response time of the electrode decreases from 1 min at 20 microM salicylate to 10 s at concentrations greater than or equal to 200 microM.

Phosphoenolpyruvate-dependent fructose phosphotransferase system in Rhodopseudomonas sphaeroides. The coupling between transport and phosphorylation in inside-out vesicles

The bacterial phosphotransferase systems are believed to catalyze the concomitant transport and phosphorylation of hexoses and hexitols. The transport is from the outside to the inside of the cell. An absolute coupling between transport and phosphorylation has however been questioned in the literature. We have tested the coupling by analysing the kinetics of fructose phosphorylation by inside-out vesicles of Rhodopseudomonas sphaeroides. We conclude that fructose indeed has to enter the vesicle before it can be phosphorylated and therefore cannot be phosphorylated from the cytoplasmic side of the membrane. The Km of the phosphorylation reaction is 8 microM. The diffusion of fructose into the vesicle is a reaction that is also catalysed by the components of the phosphotransferase system. The undirectional flux from the cytoplasmic side of the membrane to the periplasmic side is a slow process with a Km of 4 mM and is rate-limiting over a large external fructose concentration range. In summary there is no phosphorylation without transport, but there is transport without phosphorylation.

Light-induced proton gradients and internal volumes in chromatophores of Rhodopseudomonas sphaeroides

To test the predictions of the chemiosmotic hypothesis, it is essential to have sensitive and accurate measures of the aqueous volume and pH within membrane compartments. One unique feature of the present investigation is the application of electron spin resonance probes to determine internal aqueous volume and pH changes in bacterial chromatophores under virtually identical conditions. Volumes of the chromatophores ranged from 6 to 16 microliter/mg bacteriochlorophyll among different preparations, and were sensitive to the osmolarity of the suspending buffer. pH gradients reached two units in illuminated chromatophores as determined with ESR methods, and increased when KCl and valinomycin were added to the assay. Measurements with the fluorescent dye 9-amino-acridine yielded similar pH gradients, provided that an operational vesicle volume, which corrected for the binding of the dye to the membrane, was used in the calculation. The sensitivity of the ESR method allowed the measurement of pH gradients resulting from only a few light flashes. A plot of pH gradients versus number of flashes was linear up to about 30 flashes, and intercepted the origin. This result is consistent with proton release into the bulk aqueous phase after only a single light flash. This ability to measure small pH gradients offers new opportunities for the study of energy-transducing mechanisms.

The properties of citrate transport in membrane vesicles from Bacillus subtilis

The uptake system for citrate is induced in Bacillus subtilis W23 by growth in the presence of citrate and only membrane vesicles isolated from these cells show energy-dependent citrate uptake. Citrate transport in membrane vesicles is strictly dependent on the presence of divalent cations such as Mg2+, Mn2+, Zn2+, Ba2+, Be2+, Ca2+, Cu2+, Co2+ or Ni2+. The initial rate of citrate transport increases with the divalent cation concentration up to a maximum. The maximum initial rate of citrate uptake is reached with 2 mM Mg2+. The cations form stable chelates with citrate. The metal citrate complex is the transported solute. This is demonstrated for citrate uptake in the presence of Ca2+. Membrane vesicles from citrate-grown cells accumulate Ca2+ and citrate only if both solutes are present. Citrate and Ca2+ are accumulated in equimolar quantities. The uptake of Ca2+ but not of citrate is inhibited by Mg2+. Uptake of the metal-citrate complex is inhibited by the uncoupler carbonylcyanide p-trifluoromethoxyphenyl-hydrazone and in the presence of K+ ions by valinomycin and nigericin. The inhibitory effects correlate with the effects observed on the components of the proton-motive force, indicating that the proton-motive force is a driving force for metal-citrate transport. The number of protons (n) symported with the metal-citrate complex has been determined under different experimental conditions from the steady state levels of citrate accumulation, the electrical potential and pH gradient. This number varies from 1 at pH 4.7 to 2 at pH 8.0.

Characterisation of membrane vesicles from Paracoccus denitrificans and measurements of the effect of partial uncoupling on their thermodynamics of oxidative phosphorylation

1. Vesicles from Paracoccus denitrificans were prepared by applying an osmotic shock to spheroplasts derived from cells that had been grown anaerobically with succinate as carbon source and nitrate as electron acceptor. In the presence of either phenazinemethosulphate or N,N,N' N',-tetramethyl-p-phenylenediamine, the oxidation of isoascorbate supported the uptake of both S14CN- and 86Rb+ (in the presence of valinomycin), whereas NADH and succinate oxidation resulted only in S14CN- uptake. These observations show that the preparations contain both right-side-out and inside-out vesicles, and are related to the earlier proposal that the stimulation of an NADH-2,6-dichloroindophenol reductase activity by bee venom is an indicator of the proportion of right-side-out vesicles present. The implications impinge on previous conclusions [Burnell, J. N., John. P. and Whatley, F. R. (1975) Biochem. J. 150, 527-536 and FEBS Lett. 58, 215-218] about the mechanisms of sulphate and phosphate transport in P. denitrificans. 2. The relationship between the protonmotive force (delta p; transmembrane proton electrochemical gradient expressed in mV) and the phosphorylation potential (delta Gp) generated by vesicles from P. denitrificans has been studied as a function of the concentration of an uncoupler of oxidative phosphorylation. With either NADH or succinate as substrate, the uncoupler had a more pronounced effect on delta p than on delta Gp, so that the ratio delta Gp/F x delta p increased within a limited range of values of delta p close to the maximum. delta Gp/F x delta p was, however, approximately constant over the remaining range of delta p that was titrated. A fraction of 'highly coupled' vesicles, separated from the initial preparation by centrifugation through a Ficoll pad, showed similar titration behaviour. This demonstrated that heterogeneity within a vesicle preparation was not responsible for significant distortion of the true relationship between delta p and delta Gp. Values of delta p and delta Gp/F x delta p (H+/ATP) from 143-108 mV and 3.9-4.4, respectively, were determined when NADH was substrate, whereas with succinate, delta p ranged from 123-88 mV and delta Gp/F x delta p (H+/ATP) from 4.5-5.6. The variation in the value of delta Gp/F x delta p, which can be equated with a minimum value for the H+/ATP of the ATP synthase enzyme, is similar to, but less pronounced than, some of the data previously reported for mitochondria. Thus the observations with these bacterial vesicles, which represent an experimentally simpler system than mitochondria, might be taken as further evidence that measurements of the bulk phase delta p might not truly reflect the driving force for ATP synthesis sensed by the ATP synthase enzyme. However, other explanations that would make the data consistent with a chemiosmotic mechanism cannot be eliminated...

Short-term regulation of the nitrogenase activity in Rhodopseudomonas sphaeroides

The nitrogenase activity in whole cells of Rhodopseudomonas sphaeroides could be inhibited by lowering the electrical potential across the cytoplasmic membrane. The membrane potential was partly dissipated either by lowering the light intensity or by the addition of a lipophilic cation, tetraphenylphosphonium. Under these circumstances, it was shown that the intracellular ATP/ADP ratio was not affected and that the inhibition of the whole cell nitrogenase activity was not due to an inactivation of the nitrogenase enzyme. From these results it is concluded that electron transport to nitrogenase in Rps. sphaeroides is dependent on a high membrane potential. The nitrogenase enzyme in whole cells could be inactivated by lowering the membrane potential across the cytoplasmic membrane by incubating the cells in the dark or in the light in the presence of uncouplers. Nitrogenase could be reactivated in the light in the absence of uncouplers. Some possible mechanisms of action of NH+4 inhibition of whole cell nitrogenase activity could be excluded. Inhibition by NH4Cl of whole cell nitrogenase activity in Rps. sphaeroides could neither be explained by a rapid inactivation of the nitrogenase enzyme, nor by an effect on the intracellular ATP/ADP ratio or the membrane potential. NH+4 inhibits whole cell nitrogenase activity not directly but probably after being assimilated by glutamine synthetase. The role of glutamine, glutamate and 2-oxoglutarate on the regulation of electron transport to nitrogenase will be discussed.

Energy coupling of facilitated transport of inorganic ions in Rhodopseudomonas sphaeroides

Within the scope of a study on the effects of changes in medium composition on the proton motive force in Rhodopseudomonas sphaeroides, the energy coupling of sodium, phosphate, and potassium (rubidium) transport was investigated. Sodium was transported via an electroneutral exchange system against protons. The system functioned optimally at pH 8 and was inactive below pH 7. The driving force for the phosphate transport varied with the external pH. At pH 8, Pi transport was dependent exclusively on delta psi (transmembrane electrical potential), whereas at pH 6 only the delta pH (transmembrane pH gradient) component of the proton motive force was a driving force. Potassium (rubidium) transport was facilitated by a transport system which catalyzed the electrogenic transfer of potassium (rubidium) ions. However, in several aspects the properties of this transport system were different from those of a simple electrogenic potassium ionophore such as valinomycin: (i) accumulated potassium leaked very slowly out of cells in the dark; and (ii) the transport system displayed a threshold in the delta psi, below which potassium (rubidium) transport did not occur.

The mechanism of uncoupling by picrate in Escherichia coli K-12 membrane systems

The mechanism of action of the uncoupler picrate on intact cells and everted membrane vesicles of Escherichia coli K-12 was investigated. Like in mitochondria [Hanstein, W. G. and Hatefi, Y. (1974) Proc. Natl Acad. Sci. USA, 71, 288-292], it was observed that picrate uncoupled energy-linked functions only in everted, but not in intact membrane systems. In the vesicles picrate also decreased the magnitude of the transmembrane proton-motive force at concentrations similar to those at which it caused uncoupling. Experiments with 14C-labelled picrate showed that this compound bound both to deenergized intact cells and everted vesicles. However, upon energization of the membrane, picrate was extruded from the intact cell and taken up to a larger extent by the vesicles. These energy-dependent changes in picrate uptake correlated with the magnitude of the transmembrane electrical potential, delta psi. It is therefore proposed that picrate is a permeant uncoupler, that delta psi is the driving force for picrate movement across biological membranes, and that the uncoupling activity of picrate in everted membrane systems is due to its protonophoric action.

31P nuclear magnetic resonance studies of energy transduction in Rhodopseudomonas sphaeroides

31P nuclear magnetic resonance spectra of th phototrophic bacterium Rhodopseudomonas sphaeroides reveal the presence of inorganic phosphate, sugar phosphates and two non-identified P,P1-diesterified pyrophosphate compounds. Due to the presence of paramagnetic cations the resonances of these compounds can only be detected after repeated washing of the bacterial cells with a buffer, containing EDTA plus excess Mg2+. Washing with Mg2+-free EDTA buffer deteriorates the structural integrity of the membranes of Rps. sphaeroides. This is indicated by the appearance of an extra resonance peak in the spectra of these cells in a region where the phospholipids absorb and by a fivefold increase in proton permeability of the cytoplasmic membrane of Rps. sphaeroides under these conditions. Upon illumination of the cell suspension in the NMR tube the generation of a transmembrane pH gradient can be inferred from the shift in the resonances of extracellular and intracellular inorganic phosphate. Intracellular inorganic phosphate shows one homogeneous resonance peak upon illumination. This demonstrates that the mixing system, which has been developed for this application, functions efficiently. The magnitude of the light-dependent pH difference is 0.8 at the external pH 6. The width at half height of the internal inorganic phosphate peak is essentially independent of internal pH from pH 5--8, remains unchanged upon addition of uncoupler and is inversely proportional to the number of EDTA washings applied. These observations indicate that the inorganic phosphate NMR peak width is predominantly determined by the presence of a residual amount of paramagnetic cations, rather than by a broad distribution of internal pH values over the cells. Ionophores have an effect on the light-dependent pH-gradient in accordance with the chemiosmotic theory: valinomycin increases, and carbonylcyanide p-trifluoromethoxyphenylhydrazone decreases, the magnitude of this gradient.

The electrochemical proton gradient generated by the fumarate-reductase system in Escherichia coli and its bioenergetic implications

Proton translocation, coupled to electron transfer in the fumarate reductase system, generates and electrochemical potential gradient for protons (delta approximately mu H+). The magnitude of this free energy gradient has been determined in the Escherichia coli strains ML 208-225 and AN 283. The measurements were performed in (inverted) membrane particles, right-side out membrane vesicles and EDTA-treated intact cells in external media of various ionic compositions and pH. The maximal values of delta approximately mu H+ in these three systems were +103, -101 and -105 mV, respectively. This implicates that in E. coli, upon transition from oxygen to fumarate as electron acceptor, the magnitude of the delta approximately mu H+ decreases considerably. This change of delta approximately mu H+ has substantial consequences for the cellular metabolism.