Reconstitution and characterization of a sodium-stimulated active aminoisobutyric acid transport system derived from partially purified plasma membranes from mouse fibroblasts transformed by simian virus 40: comparison of reconstituted vesicles with native membrane vesicles

A Na⁺-specific and Na⁺-stimulated active α-aminoisobutyric acid transport system was reconstituted from plasma membranes isolated from mouse fibroblast BALB/c 3T3 cells transformed by simian virus 40. The plasma membranes were treated with dimethylmaleic anhydride and then extracted with 2% cholate. The cholate-solubilized supernatant proteins were combined with exogenous phospholipids and eluted through a Sephadex G-50 column. This yielded reconstituted vesicles which in the presence of Na⁺ could actively transport α-aminoisobutyric acid as shown by the transient accumulation above the equilibrium level (overshoot). The overshoot was not obtained with other monovalent cations such as K⁺, Li⁺, and choline⁺. The electrochemical effect of the lipophilic anion, SCN^?, led to greater α-aminoisobutyric acid uptake as compared to that observed with Cl^? or SO₄^2?. The Na⁺-stimulated transport of a-aminoisobutyric acid was a saturable process with an apparent K_m of 2 mm. Studies of the inhibition of α-aminoisobutyric acid transport by other amino acids showed that methylaminoisobutyric acid [specifically transported by A system (alanine preferring)]had a pronounced inhibitory effect on a-aminoisobutyric acid uptake in contrast to the slight inhibitory effect produced by phenylalanine [primarily transported by L system (leucine preferring)]. The results show that the reconstituted vesicles, prepared from partially purified membrane proteins and exogenous phospholipids, regained the same important transport properties of native membrane vesicles, i.e., Na⁺-specific and Na⁺-stimulated concentrative α-aminoisobutyric acid uptake.     

Neutral amino acid transport in surface membrane vesicles isolated from mouse fibroblasts: Intrinsic and extrinsic models of regulation

Membrane transport carrier function, its regulation and coupling to metabolism, can be selectively investigated dissociated from metabolism and in the presence of a defined electrochemical ion gradient driving force, using the single internal compartment system provided by vesiculated surface membranes. Vesicles isolated from nontransformed and Simian virus 40-transformed mouse fibroblast cultures catalyzed carrier-mediated transport of several neutral amino acids into an osmotically-sensitive intravesicular space without detectable metabolic conversion of substrate. When a Na⁺ gradient, external Na⁺ > internal Na⁺, was artifically imposed across vesicle membranes, accumulation of several neutral amino acids achieved apparent intravesicular concentrations 6- to 9-fold above their external concentrations. Na⁺-stimulated alanine transport activity accompanied plasma membrane material during subcellular fractionation procedures. Competitive interactions among several neutral amino acids for Na⁺-stimulated transport into vesicles and inactivation studies indicated that at least 3 separate transport systems with specificity properties previously defined for neutral amino acid transport in Ehrlich ascites cells were functional in vesicles from mouse fibroblasts: the A system, the L system and a glycine transport system. The pH profiles and apparent K_m values for alanine and 2-aminoisobutyric acid transport into vesicles were those expected of components of the corresponding cellular uptake system. Several observations indicated that both a Na⁺ chemical concentration gradient and an electrical membrane potential contribute to the total driving force for active amino acid transport via the A system and the glycine system. Both the initial rate and quasi-steady-state of accumulation were stimulated as a function of increasing concentrations of Na⁺ applied as a gradient (external > internal) across the membrane. This stimulation was independent of endogenous Na⁺, K⁺-ATPase activity in vesicles and was diminished by monensin or by preincubation of vesicles with Na⁺. The apparent K_m for transport of alanine and 2-aminoisobutyric acid was decreased as a function of Na⁺ concentration. Similarly, in the presence of a standard initial Na⁺ gradient, quasi-steady-state alanine accumulation in vesicles increased as a function of increasing magnitudes of interior-negative membrane potential imposed across the membrane by means of K⁺ diffusion potentials (internal > external) in the presence of valinomycin; the magnitude of this electrical component was estimated by the apparent distributions of the freely permeant lipophilic cation triphenylme thylphosphonium ion. Alanine transport stimulation by charge asymmetry required Na⁺ and was blocked by the further addition of either nigericin or external K⁺. As a corollary, Na⁺-stimulated alanine transport was associated with an apparent depolarization, detectable as an increased labeled thiocyanate accumulation. Permeant anions stimulated Na⁺-coupled active transport of these amino acids but did not affect Na⁺-independent transport. Translocation of K⁺, H⁺, or anions did not appear to be directly involved in this transport mechanism. These characteristics support an electrogenic mechanism in which amino acid translocation is coupled t o an electrochemical Na⁺ gradient by formation of a positively charged complex, stoichiometry unspecified, of Na⁺, amino acid, and membrane component. Functional changes expressed in isolated membranes were observed t o accompany a change in cellular proliferative state or viral transformation. Vesicles from Simian virus 40-transformed cells exhibited an increased Vmax of Na⁺-stimulated 2-aminoisobutyric acid transport, as well as an increased capacity for steady-state accumulation of amino acids in response t o a standard Na⁺ gradient, relative t o vesicles from nontransformed cells. Density-inhibition of nontransformed cells was associated with a marked decrease in these parameters assayed in vesicles. Several possibilities for regulatory interactions involving gradient-coupled transport systems are discussed.     

The efficiency of energetic coupling between Na flow and amino acid transport in ehrlich cells — A revised assessment

The overall efficiency of the coupling between transport of α-aminoisobutyrate and the entry of Na⁺ in Ehrlich cells has previously been determined to be 8–10%. It was concluded that the efficiency is grossly inadequate to account for the energization of amino acid transport by the electrochemical potential gradient of Na⁺, as postulated by the “gradient hypothesis”. This conclusion had, however, not taken into account that a major part of the Na⁺ entry is not coupled to a α-aminoisobutyrate transport. The “intrinsic efficiency”, which relates the amino acid transport to the coupled Na⁺ entry only, has now been evaluated from available experimental data and found to be approximately adequate to account for the highest accumulation ratios for this amino acid reported. It is concluded that the gradient hypothesis cannot be rejected on energetic grounds.     

The degree and the efficiency of coupling between the influxes of Na and α-aminoisobutyrate in Ehrlich cells

The coupling between the influx of Na⁺ and the transport of α-aminoisobutyrate into Ehrlich cells has been investigated as to its degree and efficiency, with and without metabolic inhibition.     

Transport of β-alanine in renal brush border membrane vesicles

The findings that the equilibrium uptake of β-alanine decreased with increasing medium osmolarity and preincubation with β-alanine increased uptake of the amino acid indicate that the uptake of β-alanine by rabbit renal brush border membranes represents transport into membrane vesicles. A Na⁺ electrochemical gradient (extravesicular > intravesicular) stimulated the initial rate of β-alanine uptake about three times and effected a transient accumulation of the amino acid twice the equilibrium value. Stimulation of the uptake was specific for Na⁺. Gramicidin abolished the overshoot, presumably by dissipating the gradient by accelerating the electrogenic entrance of Na⁺ into the vesicle via a pathway not coupled to uptake of β-alanine. In K⁺-loaded vesicle, valinomycin enhanced the Na⁺ gradient-dependent uptake of β-alanine. These findings indicate that the Na⁺ gradient-dependent transport of β-alanine is an electrogenic process and suggest that the membrane potential is a determinant of β-analine transport. Uptake of β-aniline, at a given concentration, reflected the sum of contributions from Na⁺ gradient-dependent and -independent transport systems. The dependent system saturated at 100 μM. The independent system did not saturate. At physiological concentrations the rate of the Na⁺ gradient-dependent uptake was four times that in the absence of the gradient. The Na⁺ gradient-dependent rate of β-alanine uptake was strongly inhibited by taurine, suggesting that β-amino acids have a common transport system, α-Amino acids, i.e. l-arginine, l-glutamate, l-proline, and glycine, representing previously reported specific α-amino acid transport systems in the brush border membrane, did not inhibit the uptake of β-alanine. These findings indicate that the brush border membrane has a distinct transport system for β-amino acids.     

Na+-independent l-arginine transport in rabbit renal brush border membrane vesicles

Na⁺-independent l-arginine uptake was studied in rabbit renal brush border membrane vesicles. The finding that steady-state uptake of l-arginine decreased with increasing extravesicular osmolality and the demonstration of accelerative exchange diffusion after preincubation of vesicles with l-arginine, but not d-arginine, indicated that the uptake of l-arginine in brush border vesicles was reflective of carrier-mediated transport into an intravesicular space. Accelerative exchange diffusion of l-arginine was demonstrated in vesicles preincubated with l-lysine and l-ornithine, but not l-alanine or l-proline, suggesting the presence of a dibasic amino acid transporter in the renal brush border membrane. Partial saturation of initial rates of l-arginine transport was found with extravesicular [arginine] varied from 0.005 to 1.0 mM. l-Arginine uptake was inhibited by extravesicular dibasic amino acids unlike the Na⁺-independent uptake of l-alanine, l-glutamate, glycine or l-proline in the presence of extravesicular amino acids of similar structure. l-Arginine uptake was increased by the imposition of an H⁺ gradient (intravesicular pH<extravesicular pH) and H⁺ gradient stimulated uptake was further increased by FCCP. These findings demonstrate membrane-potential-sensitive, Na⁺-independent transport of l-arginine in brush border membrane vesicles which differs from Na⁺-independent uptake of neutral and acidic amino acids. Na⁺-independent dibasic amino acid transport in membrane vesicles is likely reflective of Na⁺-independent transport of dibasic amino acids across the renal brush border membrane.     

The Na+ gradient hypothesis in cytoplasts derived from Ehrlich ascites tumor cells

The concentration gradients of Na⁺ and the non-metabolizable amino acid, α-aminoisobutyric acid, and the membrane potential were measured in cytoplasts derived from Ehrlich ascites tumor cells in order to test the Na⁺ gradient hypothesis for the active transport of neutral amino acids in animal cells. According to this hypothesis, the Na⁺ electrochemical gradient and the amino acid activity gradient should be equal at the steady state. It has been difficult to measure the Na⁺ electrochemical gradient in intact Ehrlich cells because Na⁺ may be sequestered in the nuclei of these cells. This problem is avoided with cytoplasts derived from Ehrlich cells because they do not contain internal compartments where Na⁺ could be sequestered. Since these cytoplasts also maintain steady state concentrations of Na⁺, K⁺, and α-aminoisobutyric acid similar to those found in whole Ehrlich cells, they are uniquely suited for testing the Na⁺ gradient hypothesis. Assuming the activity coefficients of external and cytoplasmic Na⁺ are equal, the energy in the Na⁺ electrochemical gradient of cytoplasts was 90% of that in the α-aminoisobutyric acid concentration gradient at the steady state. If the Na⁺ gradient hypothesis is correct, the 10% difference between these two gradients cannot be explained in terms of the sequestration of Na⁺ in the nucleus because cytoplasts do not contain internal compartments.     

Sodium-coupled electrogenic transport of pyroglutamate (5-oxoproline) via SLC5A8, a monocarboxylate transporter

Pyroglutamate, also known as 5-oxoproline, is a structural analog of proline. This amino acid derivative is a byproduct of glutathione metabolism, and is reabsorbed efficiently in kidney by Na⁺-coupled transport mechanisms. Previous studies have focused on potential participation of amino acid transport systems in renal reabsorption of this compound. Here we show that it is not the amino acid transport systems but instead the Na⁺-coupled monocarboxylate transporter SLC5A8 that plays a predominant role in this reabsorptive process. Expression of cloned human and mouse SLC5A8 in mammalian cells induces Na⁺-dependent transport of pyroglutamate that is inhibitable by various SLC5A8 substrates. SLC5A8-mediated transport of pyroglutamate is saturable with a Michaelis constant of 0.36 ± 0.04 mM. Na⁺-activation of the transport process exhibits sigmoidal kinetics with a Hill coefficient of 1.8 ± 0.4, indicating involvement of more than one Na⁺ in the activation process. Expression of SLC5A8 in Xenopuslaevis oocytes induces Na⁺-dependent inward currents in the presence of pyroglutamate under voltage-clamp conditions. The concentration of pyroglutamate necessary for induction of half-maximal current is 0.19 ± 0.01 mM. The Na⁺-activation kinetics is sigmoidal with a Hill coefficient of 2.3 ± 0.2. Ibuprofen, a blocker of SLC5A8, suppressed pyroglutamate-induced currents in SLC5A8-expressing oocytes; the concentration of the blocker necessary for causing half-maximal inhibition is 14 ± 1 μM. The involvement of SLC5A8 can be demonstrated in rabbit renal brush border membrane vesicles by showing that the Na⁺-dependent uptake of pyroglutamate in these vesicles is inhibitable by known substrates of SLC5A8. The Na⁺ gradient-driven pyroglutamate uptake was stimulated by an inside-negative K⁺ diffusion potential induced by valinomycin, showing that the uptake process is electrogenic.     

Inhibition of Na+-coupled solute transport by calcium in brush border membrane vesicles

Intracellular Ca⁺⁺ is known to influence Na⁺ flux in luminal membranes. Abnormally elevated Ca⁺⁺ levels in some cells is believed to be the primary pathophysiologic defect in cystic fibrosis (CF). This in turn is thought to alter Na⁺ transport which accounts for certain clinical manifestations of this disease. Two Na⁺-dependent intestinal transport mechanisms have been reported to be suppressed or missing in CF. To examine whether alterations in cell Ca⁺⁺ may account for these findings, studies were performed to examine the influence of Ca⁺⁺ on Na⁺-solute co-transport across intestinal luminal membranes. Purified brush border membrane vesicles prepared from rat small bowel were preincubated in either Ca⁺⁺-free buffer or buffer containing 2.5 mM CaCl₂. Ca⁺⁺ loaded vesicles showed marked inhibition of Na⁺ co-transport of taurocholic acid, taurochenodeoxycholic acid, glucose and valine when compared to controls. The uptake of Na⁺ was also significantly reduced by intravesicular Ca⁺⁺. These data demonstrate that intravesicular Ca⁺⁺ inhibits Na⁺-coupled solute transport as well as Na⁺ influx across intestinal brush border membranes. These data suggest that intracellular Ca⁺⁺ may suppress Na⁺-dependent solute absorption in the intestine. Results presented here further support the theory that elevated intracellular Ca⁺⁺ may account for intestinal malabsorption and other altered transport phenomena reported in CF.     

Uptake of proline by renal brushborder vesicles: A mathematical analysis

A mathematical model for amino acid uptake by membrane vesicles is described which includes two components, a Na⁺ dependent and a Na⁺ independent system. Uptake in the model is a function of both initial external Na⁺ and amino acid concentrations. Sodium dependence of amino acid transport in the model is manifested by changing affinity constants for amino acid uptake under different Na⁺ concentrations. The differing affinities for influx and efflux caused by increasing internal Na⁺ concentrations with time during transport incubations result in an “overshoot” for amino acid accumulation. For inwardly directed Na⁺ gradients, the model predicts the dependence of the occurrence of the overshoot on initial external substrate concentration and the dependence of the height of the overshoot on initial external Na⁺ concentration. This model has been used to describe experimental data on proline uptake by rat renal brushborder membrane vesicles.     

The influence of cellular amino acids and the Na+: K+ pump on the membrane potential of the Ehrlich ascites tumor cell

The membrane potential of the Ehrlich ascites tumor cell was shown to be influenced by its amino acid content and the activity of the Na⁺: K⁺ pump. The membrane potential (monitored by the fluorescent dye, 3,3′-dipropylthiodicarbocyanine iodide) varied with the size of the endogenous amino acid pool and with the concentration of accumulated 2-aminoisobutyrate. When cellular amino acid content was high, the cells were hyperpolarized; as the pool declined in size, the cells were depolarized. The hyperpolarization seen with cellular amino acid required cellular Na⁺ but not cellular ATP. Na⁺ efflux was more rapid from cells containing 2-aminoisobutyrate than from cells low in internal amino acids. These observations indicate that the hyperpolarization recorded in cells with high cellular amino acid content resulted from the electrogenic co-efflux of Na⁺ and amino acids.Cellular ATP levels were found to decline rapidly in the presence of the dye and hence the influence of the pump was seen only if glucose was added to the cells. When the cells contained normal Na⁺ (approx. 30 mM), the Na⁺: K⁺ pump was shown to have little effect on the membrane potential (the addition of ouabain had little effect on the potential). When cellular Na⁺ was raised to 60 mM, the activity of the pump changed the membrane potential from the range ?25 to ?30 mV to ?44 to ?63 mV. This hyperpolarization required external K⁺ and was inhibited by ouabain.     

Na+-dependent transport of glycine in renal brush border membrane vesicles: Evidence for a single specific transport system

The uptake of glycine in rabbit renal brush border membrane vesicles was shown to consist of glycine transport into an intravesicular space. An Na⁺ electrochemical gradient (extravesicular>intravesicular) stimulated the initial rate of glycine uptake and effected a transient accumulation of intravesicular glycine above the steady-state value. This stimulation could not be induced by the imposition of a K⁺, Li⁺ or choline⁺ gradient and was enhanced as extravesicular Na⁺ was increased from 10 mM to 100 mM. Dissipation of the Na⁺ gradient by the ionophore gramicidin D resulted in diminished Na⁺-stimulated glycine uptake. Na⁺-stimulated uptake of glycine was electrogenic. Substrate-velocity analysis of Na⁺-dependent glycine uptake over the range of amino acid concentrations from 25 μM to 10 mM demonstrated a single saturable transport system with apparent K_m = 996 μM and V_max = 348 pmol glycine/mg protein per min. Inhibition observed when the Na⁺-dependent uptake of 25 μM glycine was inhibited by 5 mM extravesicular test amino acid segregated dibasic amino acids, which did not inhibit glycine uptake, from all other amino acid groups. The amino acids d-alanine, d-glutamic acid, and d-proline inhibited similarly to their l counterparts. Accelerative exchange of extravesicular [³H]glycine was demonstrated when brush border vesicles were preloaded with glycine, but not when they were preloaded with l-alanine, l-glutamic acid, or with l-proline. It is concluded that a single transport system exists at the level of the rabbit renal brush border membrane that functions to reabsorb glycine independently from other groups of amino acids.     

Regulation of the electrogenic (Na+ + K+)-pump of Ehrlich cells by intracellular cation levels

Ehrlich cells actively accumulate neutral amino acids even if both the Na⁺ and K⁺ gradients are inverted. The seeming contradiction of this observation to the gradient hypothesis is, however, explained by the presence of a powerful electrogenic Na⁺ pump, which stongly raises the electrochemical potential gradient of Na⁺ under these conditions. Since the evidence of this pump has so far been found only during abnormal concentrations of alkali ions (low K⁺, high Na⁺) in these cells, the question arises whether the pump is equally powerful with completely normal cells, when the pump is not ‘needed’ for amino acid transport. Using the initial rate of uptake of the test amino acid (2-aminoisobutyrate) as a sensitive monitor of the electrical potential at constant cation distribution between cell and medium, a procedure has been devised to split the overall electrical potential into the diffusional and the pump component. With this procedure it could be shown that the electrogenic pump per se is most powerful in K⁺-depleted and Na⁺-rich cells but declines to a lower ‘resting’ value according as the electrolyte content of the cell approaches normality. A strong positive correlation between cellular Na⁺ content and the electrogenic pumping activity suggests that the intracellular activity of this ion regulates the rate of the electrogenic pump. The low activity of the pump under normal conditions may explain why the existance of this pump has rarely come to attention previously.     

Na+/solute symport in membrane vesicles from Bacillus alcalophilus

The characteristics of α-aminoisobutyric acid translocation were examined in membrane vesicles from obligately alkalophilic Bacillus alcalophilus and its non-alkalophilic mutant derivative, KM23. Vesicles from both strains exhibited α-aminoisobutyric acid uptake upon energization with ascorbate and N,N,N′,N′-tetramethyl-p-phenylenediamine. The presence of Na⁺ caused a pronounced reduction in the Km for α-aminoisobutyric acid in wild-type but not KM23 vesicles; the maximum velocity (V) was unaffected in vesicles from both strains. Passive efflux and exchange of α-aminoisobutyric acid from wild-type vesicles were Na⁺-dependent and occurred at comparable rates (with efflux slightly faster than exchange). This latter observation suggests that the return of the unloaded carrier to the inner surface is not rate-limiting for efflux. The rates of α-aminoisobutyric acid efflux and exchange were also comparable in KM23 vesicles, but were Na⁺-independent. Furthermore, in vesicles from the two strains, both efflux and exchange were inhibited by generation of a transmembrane electrochemical gradient of protons, outside positive. This suggests that the ternary complex between solute, carrier, and coupling ion bears a positive charge in both strains even though the coupling ion is changed. Evidence from experiments with an alkalophilic strain that was deficient in l-methionine transport indicated that the porters, i.e., the solute-translocating elements, used by non-alkalophilic mutants are not genetically distinct from those used by the alkalophilic parent; that is, the change in coupling ion cannot be explained by the expression of a completely new set of Na⁺-independent, H⁺-coupled porters upon mutation of B. alcalophilus to non-alkalophily.     

Energization of amino acid transport in energy-depleted Ehrlich cells and plasma membrane vesicles

We redirect attention to contributions to the energization of the active transport of amino acids in the Ehrlich cell, beyond the known energization by down-gradient comigration of Na⁺, beyond possible direct energization by coupling to ATP breakdown, and beyond known energization by exchange with prior accumulations of amino acids. We re-emphasize the uphill operation of System L, and by prior depletion of cellular amino acids show that this system must receive energy beyond that made available by their coupled exodus. After this depletion the Na⁺-independent accumulation of the norbornane amino acid, 2-aminobicycloheptane-2-carboxylic acid becomes strongly subject to stimulation by incubation with glucose. Energy transfer between Systems A and L through the mutual substrate action of ordinary amino acids was minimized although not entirely avoided by the use of amino acid analogs specific to each system.When 2,4-dinitrophenol was included in the depleting treatment, and pyruvate, phenazine methosulfate, or glucose used for restoration, recovery of uptake of the norbornane amino acid was independent of external Na⁺ or K⁺ levels. Restoration of the uptake of 2-(methylamino)isobutyric acid was, however, decreased by omission of external K⁺. Contrary to an earlier finding, restoration of uptake of each of these amino acids was associated with distinct and usually correlated rises in cellular ATP levels. ATP addition failed to stimulate exodus of the norbornane amino acid from plasma membrane vesicles, although either NADH or phenazine methosulfate did stimulate exodus. ATP production and use is thus associated with transport energization, although evidence for a direct role failed to appear.     

l-Alanine uptake in brush border membrane vesicles from the gill of a marine bivalve

Summary Brush border membrane vesicles were prepared from mussel gills using differential and sucrose density gradient centrifugation. These vesicles contained both the maximal Na⁺-dependent alanine transport activity found in the gradient and the maximal activities of -glutamyl transpeptidase and alkaline phosphatase. Electron micrographs showed closed vesicles of approximately 0.1–0.5 m diameter. Transport experiments using these vesicles demonstrated a transient 18-fold overshoot in intravesicular alanine concentration in the presence of an inwardly directed Na⁺ gradient, but not under Na⁺ equilibrium conditions. A reduced overshoot (10-fold) was seen with an inwardly directed K⁺ gradient. Further studies revealed a broad cation selectivity, with preference for Na⁺, which was characteristic of alanine transport but not glucose transport in these membranes. The apparent amino acid specificity of the uptake pathway(s) was similar to that of intact gills and supported the idea of at least four separate pathways for amino acid transport in mussel gill brush border membranes. The apparent Michaelis constant for alanine uptake was approximately 7m, consistent with values forK _t determined with intact tissue.     

Glutamate,aspartate, and γ-aminobutyrate transport by membrane vesicles prepared from rat brain

To prepare membrane vesicles, nerve terminal preparations (synaptosomes) isolated from rat cerebral cortex were first subjected to hypotonic lysis. After collecting the membranes contained in this fraction by centrifugation, membrane vesicles were then reconstituted during incubation in a potassium salt solution at 37 °C. The transport of glutamate, aspartate, or γ-aminobutyric acid (GABA) was measured by transferring vesicles to 10 vol of 0.1 m NaCl solution containing the radioactive substrate. Transport was temperature dependent and exhibited saturation kinetics with an apparent K_m of 2.5 μm. The rates and extent of l-glutamate and l-aspartate uptake were equivalent and were greater than those for GABA. Valinomycin increased the rate of uptake of each of these substances suggesting a role for an electrogenic component in transport. Consonant with this notion, external K⁺ and Rb⁺ decreased uptake of all three compounds. External thiocyanate also increases the rate of glutamate, aspartate, and GABA transport. Uptake of these neuroactive amino acids was absolutely dependent on external Na⁺; no other monovalent cation tested substitutes for it. Gramicidin D and nigericin inhibit glutamate transport by abolishing both the Na⁺ and K⁺ gradients. Monensin inhibits uptake by selectively dissipating the Na⁺ gradient. For both glutamate and GABA transport, the Na⁺ and K⁺ gradients are synergistic and not additive.     

Isolation and function of the amino acid transporter PAT1 (slc36a1) from rabbit and discrimination between transport via PAT1 and system IMINO in renal brush-border membrane vesicles

Reabsorption of amino acids is an important function of the renal proximal tubule. pH-dependent amino acid transport has been measured previously using rabbit renal brush-border membrane vesicles (BBMV). The purpose of this investigation was to determine whether this pH-dependent uptake represents H⁺/amino acid cotransport via a PAT1-like transport system. The rabbit PAT1 cDNA was isolated (2296bp including both 5′ and 3′ untranslated regions and poly(A) tail) and the open reading frame codes for a protein of 475 amino acids (92% identity to human PAT1). Rabbit PAT1 mRNA was found in all tissues investigated including kidney. When expressed heterologously in a mammalian cell line, rabbit PAT1 mediates pH-dependent, Na⁺-independent uptake of proline, glycine, l-alanine and α-(methylamino)isobutyric acid. Proline uptake was maximal at pH?5.0 (K_m?2.2±0.7?mM). A transport system with identical characteristics (ion dependency, substrate specificity) was detected in rabbit renal BBMV where an overshoot was observed in the absence of Na⁺ but in the presence of an inwardly directed H⁺ gradient. In the presence of Na⁺ and under conditions in which PAT1 transport function was suppressed, a second proline uptake system was detected that exhibited functional characteristics similar to those of the IMINO system. The functional characteristics of rabbit PAT1 in either mammalian cells or renal BBMV suggest that PAT1 is the low-affinity transporter of proline, glycine and hydroxyproline believed to be defective in patients with iminoglycinuria.     

The characterisation of two partially purified systems for Na+-dependent amino acid transport

Two systems mediating the transport of amino acids were studied in vesicles derived from protein-depleted membranes of pigeon erythrocytes. One system (ASC system) catalysed the Na⁺-dependent exchange of small neutral amino acids, such as alanine, serine and cysteine. The other system, also Na⁺-dependent, mediated the active transport of glycine. The ASC and glycine systems were distinguished by the sensitivity of the latter to the anion present, by the former's requirement for an exchangeable amino acid and by the inability of alanine to inhibit the transport of glycine. Preliminary results indicated that the influx of glycine was electrically silent. The only major integral protein retained in the vesicles was the band 3 protein, but that could not be unequivocally identified as the transporter.     

Evidence for the transport of neutral as well as cationic amino acids by ATA3, a novel and liver-specific subtype of amino acid transport system A

We report here on the cloning and functional characterization of the third subtype of amino acid transport system A, designated ATA3 (amino acid transporter A3), from a human liver cell line. This transporter consists of 547 amino acids and is structurally related to the members of the glutamine transporter family. The human ATA3 (hATA3) exhibits 88% identity in amino acid sequence with rat ATA3. The gene coding for hATA3 contains 16 exons and is located on human chromosome 12q13. It is expressed almost exclusively in the liver. hATA3 mediates the transport of neutral amino acids including α-(methylamino)isobutyric acid (MeAIB), the model substrate for system A, in a Na⁺-coupled manner and the transport of cationic amino acids in a Na⁺-independent manner. The affinity of hATA3 for cationic amino acids is higher than for neutral amino acids. The transport function of hATA3 is thus similar to that of system y⁺L. The ability of hATA3 to transport cationic amino acids with high affinity is unique among the members of the glutamine transporter family. hATA1 and hATA2, the other two known members of the system A subfamily, show little affinity toward cationic amino acids. hATA3 also differs from hATA1 and hATA2 in exhibiting low affinity for MeAIB. Since liver does not express any of the previously known high-affinity cationic amino acid transporters, ATA3 is likely to provide the major route for the uptake of arginine in this tissue.