Tyrosine transport by membrane vesicles isolated from rat brain

Tyrosine uptake by membrane vesicles derived from rat brain has been investigated. The uptake is dependent on an Na⁺ gradient ([$\text{Na}{}^{\mathrm{+}}\text{]}\text{outside}\text{> [}\text{Na}{}^{\mathrm{+}}\text{]}\text{inside}$). The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. The process is stimulated by a membrane potential (negative inside) as demonstrated by the effect of the ionophores valinomycin and carbonyl cyanide $\text{m-}\text{chlorophenylhydrazone}$ and anions with different permeabilities. Kinetic data show that tyrosine is accumulated by two systems with different affinities. Tyrosine uptake is inhibited by the presence of phenylalanine and tryptophan.     

Active transport of manganese in isolated membrane vesicles of Bacillus subtilis.

Membrane vesicles isolated from cells of bacillus subtilis W23 accumulate manganese in the presence of an energy source. The artificial electron donor system ascorbate and phenazine methosulfate or reduced nicotinamide adenine dinucleotide and phenazine methosulfate can supply the energy for the uptake. D-Lactate in the presence or absence of phenazine methosulfate would not support manganese accumulation. Anaerobiosis, cyanide, m-chlorophenyl carbonylcyanide hydrozone, valinomycin, gramicidin, and p-hydroxy-mercuribenzoate inhibit the uptake. The inhibition by p-hydroxymercuribenzoate is prevented by excess dithiothreitol. Potassium fluoride or sodium arsenate has no effect on the uptake. The manganese transport system in the B. subtilis vesicles exhibits Michaelis-Menten kinetics with a Km of 13 muM and a Vmax of 1.7 nmol/min per mg (dry weight) of membranes. The uptake of manganese is specific and is not inhibited by 0.1 mM CaCL2 or Mgcl2.     

Phosphate transport into brush-border membrane vesicles isolated from rat small intestine.

Uptake of Pi into brush-border membrane vesicles isolated from rat small intestine was investigated by a rapid filtration technique. The following results were obtained. 1. At pH 7.4 in the presence of a NaCl gradient across the membrane (sodium concentration in the medium higher than sodium concentration in the vesicles), phosphate was taken up by a saturable transport system, which was competitively inhibited by arsenate. Phosphate entered the same osmotically reactive space as D-glucose, which indicates that transport into the vesicles rather than binding to the membranes was determined. 2. The amount of phosphate taken up initially was increased about fourfold by lowering the pH from 7.4 to 6.0.3. When Na+ was replaced by K+, Rb+ or Cs+, the initial rate of uptake decreased at pH 7.4 but was not altered at pH 6.0.4. Experiments with different anions (SCN-,Cl-, SO42-) and with ionophores (valinomycin, monactin) showed that at pH 7.4 phosphate transport in the presence of a Na+ gradient is almost independent of the electrical potential across the vesicle membrane, whereas at pH 6.0 phosphate transport involves the transfer of negative charge. It is concluded that intestinal brush-border membranes contain a Na+/phosphate co-transport system, which catalyses under physiological conditions an electroneutral entry of Pi and Na+ into the intestinal epithelial cell. In contrast with the kidney, probably univalent phosphate and one Na+ ion instead of bivalent phosphate and two Na+ ions are transported together.     

Sodium-dependent methyl 1-thio-beta-D-galactopyranoside transport in membrane vesicles isolated from Salmonella typhimurium.

Membrane vesicles isolated from Salmonella typhimurium G-30 grown in the presence of melibiose catalyze methyl 1-thio-beta-D-galactopyranoside (TMG) transport in the presence of sodium or lithium, as shown initially with intact cells by Stock and Roseman (Stock, J., and Roseman, S. (1971), Biochem. Biophys. Res. Commun. 44, 132). TMG-dependent sodium uptake is also observed, but only when a potassium diffusion potential (interior negative) is induced across the vesicle membrane. Cation-dependent TMG accumulation varies with the electrochemical gradient of protons generated as a result of D-lactate oxidation, and the vesicles catalyze D-lactate-dependent sodium efflux in a manner which is consistent with the operation of a proton-sodium exchange mechanism. Although the stoichiometry between sodium and TMG appears to be 1:1 when transport is induced by a potassium diffusion potential, evidence is presented which indicates that the relationship may exceed unity under certain conditions. The results are explained in terms of a model in which TMG-sodium (lithium) symport is driven by an electrochemical gradient of protons which functions to maintain a low intravesicular sodium or lithium concentration through proton--sodium (lithium) antiport.     

Active phosphate ion transport in plasma membrane vesicles isolated from mouse fibroblasts.

Inorganic phosphate accumulated 8-fold in plasma membrane vesicles derived from simian virus 40-transformed 3T3 mouse fibroblasts when a NaCl gradient (external greater than internal) was artificially imposed across the membrane. Preincubation with Na+ or addition of monensin markedly reduced phosphate accumulation. Na+-stimulated phosphate transport was not affected by addition of either dicarboxylic acids, antimycin A, or ouabain and persisted after addition of proton ionophores. The coupling of phosphate transport to Na+ gradients was pH-dependent, with maximal stimulation by Na+ below pH 7. These findings suggest that monovalent phosphate anion moves across the plasma membrane in co-transport with sodium ion.     

The use of isolated membrane vesicles to study epithelial transport processes

Summary Epithelia are multicompartment and multicomponent systems performing transcellular and paracellular transport in a very complex manner. One way to get a deeper understanding of the function of such a complex system is to dissect it into the single components and then, after having defined the components under well-controlled conditions, to try to describe the behavior of the whole system on the basis of the properties of the single components.This article deals with the analysis of isolated plasma membranes derived from the luminal and contraluminal face of epithelial cells, predominantly renal proximal tubular and small intestinal cells. It is aimed to give an overview of methods used to isolate and separate plasma membranes, to study their transport properties as membrane vesicles, and also to address the question of how information gained with the isolated membranes corresponds to observations made in the intact cell using other, notably electrophysiological, measurements. The review also critically evaluates the limitations of the approach and thereby tries to set the work on isolated membranes in the proper perspective within the field of transport physiology.     

Proton transport by gastric membrane vesicles.

A highly purified membrane fraction was derived from hog gastric mucosa by a combination of differential and density gradient centrifugation and free flow electrophoresis. This final fraction was 35-fold enriched with respect to cation activated ouabain-insensitive ATPase. Antibody against this fraction was shown to be bound to the luminal surface of the gastric glands. The addition of ATP to this fraction or the density gradient fraction resulted in H+ uptake into an osmotically sensitive space. The apparent Km for ATP was 1.7-10(-4) M in the absence of a K+ gradient similar to that found for ATPase activity. The reaction is specific for ATP and requires cation in the sequence K+ greater than Rb+ greater than Cs+ greater than Na+ greater than Li+ and inhibited by ATPase inhibitors such as N,N'-dicylclohexyl-carbodiimide. Maximal H+ uptake occurs with an outward K+ gradient but the minimal apparent KA is found in the absence of a K+ gradient. The pH optimum for H+ uptake is between 5.8 and 6.2 which corresponds to the pH range for phosphroylation of the enzyme, but is considerably less than the pH maximum of the K+ dependent dephosphorylation. In the presence of an inward K+ gradient, protonophores such as tetrachlorsalicylanilide only partially abolish the H+ gradient but valinomycin dissipates 75% of the gradient, and nigericin abolishes the gradient. The vesicles therefore have a low K+ conductance but a measurable H+ conductance, hence a K+ gradient can produce an H+ gradient in the presence of valinomycin. The uptake and spontaneous leak of H+ are temperature sensitive with a similar transition temperature. Ultraviolet irradiation inactivates ATPase and proton transport at the same rate, approximately at twice the rate of p-nitrophenylphosphatase inactivation. It is concluded that H+ uptake by these vesicles is probably due to a dimeric (H+ + K+)-ATPase and is probably non-electrogenic.     

Choline transport by synaptosomal membrane vesicles isolated from insect nervous tissue

Membrane vesicles derived from insect nervous tissue are capable of accumulating choline via a high affinity, carrier-mediated process with ion gradients as the sole driving force. The transport is strictly dependent on the presence of Na+ and Cl- in the medium and is stimulated by a membrane potential.     

Active transport of alanine by thermostable membrane vesicles isolated from a thermophilic bacterium.

1. Thermostable membrane vesicles which were capable of active transport of alanine dependent on either respiration or an artificial membrane potential were isolated from the thermophilic aerobic bacterium PS3. 2. Uptake of alanine was dependent on the oxidation of ascorbate-phenazine methosulfate or on generated or exogenous NADH, but succinate and malate failed to drive the uptake. The optimum temperature for respiration-driven uptake of alanine was 45 to 60 degrees. 3. Potassium ion-loaded vesicles were prepared by incubating vesicles at 55 degrees in 0.5 M potassium phosphate. The addition of valinomycin elicited rapid and transient uptake of alanine under the test conditions. Uptake of alanine in response to valinomycin was progressively enhanced by the addition of dicylohexylcarbodiimide, but was completely abolished in the presence of a proton conductor or synthetic permeable cation. The effect of dicyclohexylcarbodiimide was dependent on its concentration and was maximal at a concentration of 0.4 mM. 4. The proton permeability of membrane vesicles was reduced by the addition of dicyclohexylcarbodiimide. A small but significant difference was found in the initial rates of proton uptake in the presence of dicyclohexylcarbodiimide with and without alanine. The results suggest that protons alanine are transported simultaneously in a stoichiometric ratio of 1 : 1. 5. The uptake of alanine was also driven by a pH gradient induced by an instantaneous pH drop in a suspension of alkali-loaded vesicles. Thus, alanine accumulation was driven not only by an electrical potential but also by a pH gradient. 6. Addition of ATP resulted in the inhibition of alanine uptake dependent on artificial membrane potential. ATP hydrolysis by membrane ATPase created a membrane potential which was inside-positive, and this might decrease the effective membrane potential (generated by K+ efflux mediated by valinomycin) available to drive alanine uptake.     

Glycodeoxycholate transport in brush border membrane vesicles isolated from rat jejunum and ileum.

The transport of the bile salt, glycodeoxycholate, was studied in vesicles derived from rat jejunal and ileal brush border membranes using a rapid filtration technique. The uptake was osmotically sensitive, linearly related to membrane protein and resembled D-glucose transport. In ileal, but not jejunal, vesicles glycodeoxycholate uptake showed a transient vesicle/medium ratio greater than 1 in the presence of an initial sodium gradient. The differences between glycodeoxycholate uptake in the presence and absence of a Na+ gradient yielded a saturable transport component. Kinetic analysis revealed a Km value similar to that described previously in everted whole intestinal segments and epithelial cells isolated from the ileum. These findings support the existence of a transport system in the brush border membrane that: (1) reflects kinetics and characteristics of bile salt transport in intact intestinal preparations, and (2) catalyzes the co-transport of Na+ and bile salt across the ileal membrane in a manner analogous to D-glucose transport.     

Rotavirus interaction with isolated membrane vesicles.

To gain information about the mechanism of epithelial cell infection by rotavirus, we studied the interaction of bovine rotavirus, RF strain, with isolated membrane vesicles from apical membrane of pig enterocytes. Vesicles were charged with high (quenching) concentrations of either carboxyfluorescein or calcein, and the rate of fluorophore release (dequenching) was monitored as a function of time after mixing with purified virus particles. Purified single-shelled particles and untrypsinized double-shelled ones had no effect. Trypsinized double-shelled virions induced carboxyfluorescein release according to sigmoid curves whose lag period and amplitude were a function of virus concentration and depended on both temperature and pH. The presence of 100 mM salts (Tris Cl, NaCl, or KCl) was required, since there was no reaction in isoosmotic salt-free sorbitol media. Other membrane vesicle preparations such as apical membranes of piglet enterocyte and rat placenta syncytiotrophoblasts, basolateral membranes of pig enterocytes, and the undifferentiated plasma membrane of cultured MA104 cells all gave qualitatively similar responses. Inhibition by a specific monoclonal antibody suggests that the active species causing carboxyfluorescein release is VP5*. Ca2+ (1 mM), but not Mg2+, inhibited the reaction. In situ solubilization of the outer capsid of trypsinized double-shelled particles changed release kinetics from sigmoidal to hyperbolic and was not inhibited by Ca2+. Our results indicate that membrane destabilization caused by trypsinized outer capsid proteins of rotavirus leads to fluorophore release. From the data presented here, a hypothetical model of the interaction of the various states of the viral particles with the membrane lipid phase is proposed. Membrane permeabilization induced by rotavirus may be related to the mechanism of entry of the virus into the host cell.     

Glucose transport activity in isolated plasma membrane vesicles from Saccharomyces cerevisiae

Purified plasma membranes prepared from yeast cells by mechanical agitation with glass beads exhibit no detectable sugar transport activity. However, the addition of phospholipid (asolectin) liposomes to the purified plasma membranes followed by freezing, thawing, and brief sonication produces membrane vesicles which exhibit D-glucose-specific transport activity. The characteristics of zero trans, equilibrium exchange, and influx counterflow exhibited by the membrane vesicles are similar to those of intact cells.     

Cl- transport in apical plasma membrane vesicles isolated from bovine tracheal epithelium

Gradually altered synthetic entities were employed as molecular probes, and arachidonic acid, ADP, human alpha-thrombin and the Ca2+ ionophore A23187 as aggregation-inducing agents, in a comprehensive study on the response profile of human blood platelets with an emphasis on the effects of exogenous and increased intracellular Ca2+. Corroborating further previous conclusions, some representative carbamoylpiperidine derivatives, at concentrations effecting substantial inhibition of ADP-induced aggregation, failed to retain that effect when 5.0 mM Ca2+ was introduced into the otherwise identical test medium; reference compounds chlorpromazine and propranolol registered corresponding inhibitory patterns. At increased concentrations the compounds' inhibitory potency was regenerated even in the presence of 5 mM Ca2+. In fact, in sufficiently high concentrations, the compounds were even capable of inhibiting aggregation elicited by 15 microM of the ionophore A23187; so did chlorpromazine and propranolol. Another set of congeners revealed the striking sensitivity of ionophore A23187-induced human blood platelet aggregation to the surface active potencies of inhibitor molecules. The loss in inhibitory potency was directly related to the lesser hydrophobic character of the molecule.