Transport of neutral amino acids by human erythrocytes

AKT activation because of PTEN loss upregulates xCT via GSK3β/NRF2, leading to inhibition of ferroptosis in PTEN-mutant tumor cells

Here, we show that the tumor suppressor phosphatase and tensin homolog deleted from chromosome 10 (PTEN) sensitizes cells to ferroptosis, an iron-dependent form of cell death, by restraining the expression and activity of the cystine/glutamate antiporter system Xc- (xCT). Loss of PTEN activates AKT kinase to inhibit GSK3β, increasing NF-E2 p45-related factor 2 (NRF2) along with transcription of one of its known target genes encoding xCT. Elevated xCT in Pten-null mouse embryonic fibroblasts increases the flux of cystine transport and synthesis of glutathione, which enhances the steady-state levels of these metabolites. A pan-cancer analysis finds that loss of PTEN shows evidence of increased xCT, and PTEN-mutant cells are resistant to ferroptosis as a consequence of elevated xCT. These findings suggest that selection of PTEN mutation during tumor development may be due to its ability to confer resistance to ferroptosis in the setting of metabolic and oxidative stress that occurs during tumor initiation and progression.

Cationic amino acid transporters and their modulation by nitric oxide in cardiac muscle cells

Cationic amino acid transporters (CATs) play a central role in the supply of the substrate L-arginine to intracellular nitric oxide synthases (NOS), the enzymes responsible for the synthesis of nitric oxide (NO). In heart, NO produced by cardiac myocytes has diverse and even opposite effects on myocardial contractility depending on the subcellular location of its production. Approximately a decade ago, using a combination of biophysical and biochemical approaches, we discovered and characterized high- and low-affinity CATs that function simultaneously in the cardiac myocyte plasma membrane. Later on, we reported a negative feedback regulation of NO on the activity of cardiac CATs. In this way, NO was found to modulate its own biosynthesis by regulating the amount of L-arginine that becomes available as NOS substrate. We have recently solved the molecular determinants for this NO regulation on the low-affinity high-capacity CAT-2A. This review highlights some biophysical and biochemical features of L-arginine transporters and their potential relation to cardiac muscle physiology and pathology.

L-Arginine currents in rat cardiac ventricular myocytes

L-Arginine (L-Arg) is a basic amino acid that plays a central role in the biosynthesis of nitric oxide, creatine, agmantine, polyamines, proline and glutamate. Most tissues, including myocardium, must import L-Arg from the circulation to ensure adequate intracellular levels of this amino acid. This study reports novel L-Arg-activated inward currents in whole-cell voltage-clamped rat ventricular cardiomyocytes. Ion-substitution experiments identified extracellular L-Arg as the charge-carrying cationic species responsible for these currents, which, thus, represent L-Arg import into cardiac myocytes. This result was independently confirmed by an increase in myocyte nitric oxide production upon extracellular application of L-Arg. The inward movement of Arg molecules was found to be passive and independent of Na(2+), K(2+), Ca(2+) and Mg(2+). The process displayed saturation and membrane potential (V(m))-dependent kinetics, with a K(0.5) for l-Arg that increased from 5 mm at hyperpolarizing V(m) to 20 mm at +40 mV. L-Lysine and L-ornithine but not D-Arg produced currents with characteristics similar to that activated by L-Arg indicating that the transport process is stereospecific for cationic L-amino acids. L-Arg current was fully blocked after brief incubation with 0.2 mm N-ethylmaleimide. These features suggest that the activity of the low-affinity, high-capacity CAT-2A member of the y(2+) family of transporters is responsible for L-Arg currents in acutely isolated cardiomyocytes. Regardless of the mechanism, we hypothesize that a low-affinity arginine transport process in heart, by ensuring substrate availability for sustained NO production, might play a cardio-protective role during catabolic states known to increase Arg plasma levels severalfold.

Role of the plasma and erythrocytes in veno-arterial portal changes during post prandial state in the rat

The determination of plasma and whole blood free amino acid concentrations in arterial and portal venous blood during post prandial state in the rat was used to estimate the role of the erythrocytes in amino acid exchanges. The erythrocyte contents were calculated from plasma, whole blood concentrations and the hematocrit. The veno-arterial concentration differences in plasma were significant for all amino acids except a-aminobutyrate and ornithine whereas in the erythrocytes only 8 amino acids exhibit significant differences (ASP, ALA, VAL, MET, ILE, LEU, TYR, PHE). For 6 amino acids, a significant correlation between the plasma and the erythrocyte concentration has been found (VAL, ILE, LEU, TYR, PHE, HIS). These data suggest that in vivo during the time of contact between blood and organ tissues, some amino acids but not all are significantly taken up by the erythrocytes. Thus, it may be concluded that erythrocyte amino acid blood transport in arterio-venous portal exchanges, concerns particularly tyrosine and essential amino acids. The erythrocyte amino acid transport represents quantitatively about 20 per cent of the total blood transport.

ASCT-1 is a neutral amino acid exchanger with chloride channel activity

The ubiquitous transport activity known as system ASC is characterized by a preference for small neutral amino acids including alanine, serine, and cysteine. ASCT-1 and ASCT-2, recently cloned transporters exhibiting system ASC-like selectivity, are members of a major amino acid transporter family that includes a number of glutamate transporters. Here we show that ASCT1 functions as an electroneutral exchanger that mediates negligible net amino acid flux. The electrical currents previously shown to be associated with ASCT1-mediated transport result from activation of a thermodynamically uncoupled chloride conductance with permeation properties similar to those described for the glutamate transporter subfamily. Like glutamate transporters, ASCT1 activity requires extracellular Na+. However, unlike glutamate transporters, which mediate net flux and complete a transport cycle by countertransport of K+, ASCT-1 mediates only homo- and heteroexchange of amino acids and is insensitive to K+. The properties of ASCT-1 suggest that it may function to equilibrate different pools of neutral amino acids and provide a mechanism to link amino acid concentration gradients.

Glycine transport by the red cells of channel catfish

In humans, glycine enters the red cell via four distinct plasma membrane carrier systems. The purpose of the present experiments was to measure the mode of transport of glycine by channel catfish (Ictalurus punctatus) red cells. About 54% of the glycine was transported by system L, while 16.1% of the glycine was transported by system Gly. A further 15.6% of transport was via system ASC and system asc together. An unidentified Na+-independent system was responsible for the transport of 7.2% of the glycine. No solute appeared to be carried into the cell by band 3. The remainder of the glycine entered the cell by diffusion. The Na+-independent system exhibited a Ktvalue of 57 ± 12 (mean ± standard deviation) μM and Vmaxof 142 ± 27 nmol∙g hemoglobin−1∙min−1(this compares with system L, which exhibited a Ktvalue of 65 ± 21 μM and Vmaxof 516 ± 117 nmol∙g hemoglobin−1∙min−1). These results demonstrate that channel catfish red cells are capable of transporting glycine by three of the four transporters involved in human red cells, although the relative contributions differ markedly, and by an additional unidentified transport system not requiring Na+. The differences in glycine transport between human and catfish red cell membranes can be attributed to evolutionary influences.

Screening of potential transport systems for methyl mercury uptake in rat erythrocytes at 5 degrees by use of inhibitors and substrates

The current study was designed to screen the potential transport systems for methyl mercury (MeHg) uptake by isolated erythrocytes from rats at 5 degrees. Several inhibitors and substrates were used to test which potential transport system might be involved in MeHg uptake. Probenecid was used to test the organic anion transport system, valinomycin was used to test the effect of the membrane potential, D-glucose and cytochalasin B were used to test the facilitated diffusive D-glucose transport system and colchicine and vinblastine were used to test the microtubule system. The effects of Ca++, Mg++ and Na+ on MeHg uptake have been examined. Ouabain, ATP and glucose were used to test the active transport system, cysteine for the cysteine-facilitated transport system, glycine for system Gly, DL-methionine for system L, and MeHgCl and 4',4-diisothiocyano-2',2-stilbenedisulfonic acid (DIDS) for the Cl- ion transport system. The results showed that MeHg uptake might be involved in the following transport systems at 5 degrees: 1) organic anion transport system; 2) facilitated diffusive D-glucose transport system; 3) cysteine-facilitated transport system; 4) Cl- ion transport system. Moreover, the transport systems for MeHg uptake were sensitive to the membrane potential. Although the mechanisms of interaction of transport systems have not been fully clarified, evidence has been presented which support the existence of several simultaneous transport systems for MeHg uptake.

Effect of inhibitors and substrates on methyl mercury uptake by rat erythrocytes

Methyl mercury (MeHg) uptake by isolated erythrocytes from rats was studied at 20 degrees C. Inhibitors and substrates were used to test which transport system was involved in MeHg uptake. Ouabain and ATP were used to test the active transport system. Glycine was used to test system Gly. DL-Methionine was used to test system L. Cysteine was used to test the cysteine-facilitated transport system. The effects of Ca2+, Mg2+ and Na+ on MeHg uptake have been examined. MeHgCl and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were used to test C1- ion transport system. D-Glucose and cytochalasin B were used to test the facilitated diffusive D-Glucose transport system. Colchicine and vinblastine were used to test the microtubule system. Probenecid was used to test the organic acid transport system. Valinomycin was used to test the effect of the membrane potential on MeHg uptake. The results showed that MeHg uptake at 20 degrees C might be involved in the following transport systems: 1) an active transport system; 2) a cysteine-facilitated transport system; 3) a C1- ion transport system; 4) a facilitated diffusive D-glucose transport system; 5) an organic acid transport system. The transport systems for MeHg uptake were sensitive to the membrane potential.

In vitro adsorption of amino acids onto isolated rat erythrocyte membranes

Amino acids adsorbed onto blood cell membranes represent about 8% of the total amino acids in blood. The aim of this study was to determine the in vitro adsorption kinetics of different amino acids (L-alanine, glycine, L-glutamate, L-glutamine, L-phenylalanine and L-leucine) onto rat erythrocyte membranes and to assess the effect of 24-hr starvation on these adsorption kinetics. Isolated red cell membranes were incubated at 37 degrees C for 10 sec in the presence of 14C-amino acids--with different specific radioactivity--the radioactivity retained in the membrane fraction measured and kinetic parameters of amino acid adsorption determined. With the exception of glutamate, where the adsorption was negligible, all amino acids studied were adsorbed onto isolated red cell membranes, adhering to simple Michaelis-Menten kinetics. Km' values of glycine, phenylalanine and leucine adsorption in control rats (14.7 +/- 3.8 mM, 8.41 +/- 0.95 mM and 4.65 +/- 0.46 mM respectively, SEM, n = 6-8) decreased in response to 24-hr starvation, giving the following values: 0.792 +/- 0.122 mM, 5.32 +/- 0.82 mM and 3.53 +/- 0.31 mM respectively (SEM, n = 6-8), Vmax' value of glycine adsorption of control rats decreased (from 61.0 +/- 15.5 mmol/mol P/sec to 4.25 +/- 0.70 mmol/mol P/sec, SEM, n = 7) and that of leucine increased (from 13.5 +/- 1.0 mmol/mol P/sec to 18.9 +/- 2.0 mmol/mol P/sec, SEM, n = 7) as an effect of 24-hr starvation. This study shows that alanine, glycine, glutamine, phenylalanine and leucine, but not glutamate, adsorbed onto erythrocyte membranes according to Michaelis-Menten-like kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino acid transport by human erythrocyte membranes

The human erythrocyte plasma membrane is permeable to several free amino acids usually present in the bloodstream. Seven distinct routes of entry have been described which represent both secondary active transport and facilitated diffusion (passive transport). Additionally, certain amino acids can enter the cell by simple diffusion, at least to a limited extent. The function of most of these transport systems is unclear, although it has been suggested that the cell can take up certain amino acids and carry them to various parts of the body. In the case of glutamine, cysteine, and glycine, however, it is believed that the biosynthesis of the tripeptide glutathione is the primary reason for their uptake into the cell. Much of the amino acid transport probably has no function in mature red cells, but might be a remnant of the immature cell's needs. This review discusses the various amino acid transport systems known to be present in the red cell plasma membrane.

N-ethylmaleimide discriminates between two lysine transport systems in human erythrocytes

1. The sulfhydryl reagent N-ethylmaleimide (NEM) was shown to inactivate the low affinity lysine transporter in human erythrocytes (system y+) without affecting the high affinity transporter (system y+L). 2. Pre-treatment of the cells with NEM reduced the rate of entry of L-[14C]lysine (1 microM) by approximately 50% (maximum effect). 3. NEM (0.2 mM) inhibited the NEM-sensitive component of the flux with mono-exponential kinetics. The inactivation rate constant (k, +/- S.E.M.) was 0.53 +/- 0.027 min-1 (25 degrees C). The substrate did not protect against inactivation. 4. Lysine self-inhibition experiments revealed two transport systems in untreated cells (half-saturation constants Km; +/- S.E.M.), 12.0 +/- 1.7 microM and 109 +/- 15.6 microM) and only one high affinity system in NEM-treated cells (Km 9.5 +/- 0.67 microM), indicating that NEM inactivates system y+. 5. The NEM-insensitive L-[14C]lysine influx (system y+L) was inhibited with high affinity by unlabelled neutral amino acids. The inhibition constant for L-leucine in sodium medium (Ki +/- S.E.M.) was 10.7 +/- 0.72 microM (37 degrees C). The system was also strongly inhibited by L-methionine, L-glutamine and with less affinity by L-phenylalanine and L-serine. N-methyl-L-leucine, L-proline and 2-amino-2-norbornane-carboxylic acid, a bicyclic analogue of leucine, did not exert a significant effect. 6. Lysine transport through system y+L occurred at the same rate in Na+, K+ or Li+ medium and the binding of lysine to the transporter was unaffected by Na+ replacement. 7. The interaction of system y+L with neutral amino acids was dependent on the cation present in the medium. The inhibition constant for leucine and glutamine increased approximately 90- and 60-fold respectively when Na+ was replaced by K+. Li+ was shown to be a very good substitute for Na+.

Erythrocyte uptake kinetics and cell to plasma gradients of leucine and phenylalanine in fed and fasted rats

The kinetic parameters of the L-phenylalanine and L-leucine uptake by isolated erythrocytes in fed and 24 hour starved rats have been determined. In addition, the in vivo compartmentation between blood cells and plasma of the above amino acids in arterial and venous blood vessels has also been studied under the above physiological situations. Both the L-phenylalanine and L-leucine uptake by erythrocytes was saturable and non concentrative. Starvation increased the Km value for the leucine uptake and did not significantly affect that of phenylalanine uptake. The in vivo blood cell/plasma (C/P) concentration ratio of both amino acids was higher than the unit. The starvation-induced changes in the relative distribution of these amino acids between the blood cell and the plasma compartments were significant for the phenylalanine in the aortic artery but not in venous blood. The transport system capabilities measured in vitro can not account for the maintenance of both the leucine and phenylalanine gradient between blood cells and plasma, and the starvation-induced changes in the blood amino acids compartmentation are not directly related entirely to the transport system capabilities.

Differences in erythrocyte sodium transport between human plasma and artificial medium: the role and character of sodium efflux and influx stimulating plasma factors

The main objective of this study was to further characterize the plasma factor(s) which stimulate sodium efflux from erythrocytes, which we reported previously. Dialysis of plasma against an artificial medium using membranes with varying molecular mass cut-off points revealed relative molecular mass(es) of the factor(s) of 100-1000 Da. The factor(s) could be absorbed on Dowex at pH 1.5 and Amberlite at pH 11.0, indicating 'Zwitterionic' character. They are hydrophilic and resistant to acid hydrolysis. These characteristics and direct measurements of contents made amino acids likely candidates for the efflux stimulating properties of the factor(s). Indeed, plasma amino acids added to artificial medium could abolish the sodium efflux difference between plasma and the artificial medium. The efflux stimulating effect of amino acids appeared not to be the result of sodium influx stimulation. A coincident finding was that plasma also contains dialyzable sodium influx stimulating factor(s) which are not amino acids.

Regulation of rat erythrocyte l-glutamine, 1-glutamate and l-lysine uptake by short term starvation

1. The kinetic parameters (Km, Vmax and Kd) of L-glutamine, L-glutamate and L-lysine uptake by isolated red blood cells in fed and 24 hr starved rats have been determined. 2. L-Lysine and L-glutamine uptake was best fitted by a two transport component: a saturable component and a diffusion one. 3. Starvation brought about important decreases in the Km and Vmax for both L-lysine and L-glutamine uptake. 4. The Kd for L-glutamine showed a significant increase whereas that corresponding to L-lysine did not change by starvation. 5. L-Glutamate uptake adjusted to diffusion kinetics, with a Kd which did not change due to starvation. 6. It is concluded that the amino acid uptake showed specific regulation by starvation. 7. The mechanism involved is not dependent on protein synthesis--given the unnucleated nature of mammal red cells. 8. The magnitude of the changes observed in the uptake kinetic parameters may account for the extent of the blood amino acid pool changes as those produced in vivo over physiological limits.

Changes in alanine and glutamine transport during rat red blood cell maturation

Alanine and glutamine transport have been studied during red blood cell maturation in the rat. Kinetic parameters of Na(+)-dependent L-alanine transport were: Km 0.43 and 1.88 mM and Vmax 158 and 45 nmoles/ml ICW/min for reticulocytes and erythrocytes, respectively. During red cell maturation in the rat there is a loss of capacity and affinity of the system ASC for L-alanine transport. The values for Na(+)-dependent L-glutamine transport in reticulocytes were Km 0.51 mM and Vmax 157 nmoles/ml ICW/min. On the other hand, a total loss of L-glutamine transport mediated by both N and ASC systems is demonstrated in mature red cells. This seems to indicate that during rat red cell maturation the system N disappears. Furthermore, the system ASC specificity in mature cells changes, and glutamine enters the red cell by non-mediated diffusion processes.

The preferential interaction of L-threonine with transport system ASC in cultured human fibroblasts

The transport of L-threonine was studied in cultured human fibroblasts. A kinetic analysis of L-threonine transport in a range of extracellular concentrations from 0.01 to 20 mM indicated that this amino acid enters cells through both Na(+)-independent and Na(+)-dependent routes. These routes are: (1) a non-saturable, Na(+)-independent route formally indistinguishable from diffusion; (2) a saturable, Na(+)-independent route inhibitable by the analog BCH and identifiable with system L; (3) a low-affinity, Na(+)-dependent component (Km = 3 mM) which can be attributed to the activity of system A since it is adaptively enhanced by amino acid starvation and suppressed by the characterizing analog MeAIB and (4) a high-affinity, Na(+)-dependent route (Km = 0.05 mM). This latter route is identifiable with system ASC since it is insensitive to adaptive regulation, uninhibited by MeAIB, trans-stimulated by intracellular substrates of system ASC, markedly stereoselective, and relatively insensitive to changes in external pH. At an external concentration of 0.05 mM more than 90% of L-threonine transport is referrable to the activity of system ASC; in these conditions, the transport of the amino acid exhibits typical ASC-features even in the absence of inhibitors of other transport agencies, and, therefore, it can be employed as a reliable indicator of the activity of transport system ASC in cultured human fibroblasts.

Transport of L-tyrosine by B16/F10 malignant melanocytes: characterization of the process

The main characteristics of L-tyrosine (L-Tyr) uptake by B16/F10 malignant melanocytes are reported. This amino acid can be taken up by two systems, both of them being saturable. The first one would be system L. This system can be studied in cells preloaded with amino acids that are a good substrate for system L, such as L-methionine or L-tryptophan. The kinetic parameters for L-Tyr uptake by this transport system are Vm = 6.5 pmol L-Tyr/10(3) cells.min and Km around 130 microM. The second system, probably the system ASC, shows lower capacity but higher affinity than the former. This system can be detected only in cells previously depleted of amino acids, showing approximate kinetic values of Vm 0.05 pmol L-Tyr/10(3) cells.min and Km around 5 microM. It is shown that the increase in cell density yields a decrease in the rate of L-Tyr uptake by system L, but this increase does not affect the high affinity system, alpha-MSH does not affect significantly the L-Tyr uptake by both systems. 2-Amino bicyclo-(2,2,1)-heptane-2-carboxylic acid produces a remarkable inhibition of the rate of L-Tyr uptake, but alpha-methylaminoisobutyric acid does not affect the rate of transport of this amino acid. The absence of sodium produces a slight but reliable decrease in the rate of L-Tyr uptake, supporting the involvement of two different transport systems. The ionophores monensin and nigericin enhance the transport by system L, but this effect is suppressed by the presence of ouabain. This finding indicates that the (Na+ -K+)-ATPase is essential for the stimulating action of ionophores.(ABSTRACT TRUNCATED AT 250 WORDS)

Cationic and anionic amino acid transport studies in rat red blood cells

The transport of L-proline, L-lysine and L-glutamate in rat red blood cells has been studied. L-proline and L-lysine uptake were Na(+)-independent. When the concentration dependence was studied both showed a non-saturable uptake assimilable to a difussion-like process, with high Kd values (0.718 and 0.191 min-1 for L-proline and L-lysine respectively). Rat red blood cells showed high impermeability to L-glutamate. No sodium dependence was observed and the Kd value was low (0.067 min-1). Our results show firstly, that rat red blood cells do not have amino acid transport systems for anionic and cationic amino acids and secondly that erythrocytes show no sodium-dependent L-proline transport, and that these cells are very permeable to this amino acid.

Uptake and metabolism of dipeptides by human red blood cells

A function of the abundant cytoplasmic peptidases in red blood cells could be hydrolysis of oligopeptides circulating in plasma. To investigate whether human red blood cells actively transport dipeptides for this purpose, these cells were incubated with 14C-labelled glycylproline, glycylsarcosine, glycine, proline and alanine. There was uptake of each dipeptide, as indicated by their recovery as dipeptides in the cell cytoplasm. However, after a brief time (1-2 min) uptake of dipeptides abruptly ceased, while that of amino acids continued. As a result, after 30 min red blood cell uptake of amino acids was 5-13-fold greater than that of any dipeptide. Investigation of intracellular contents after 1 min of incubation revealed different metabolism for different dipeptides. The composition of intracellular radioactivity was 19-71% as intact dipeptides, 0-20% as free amino acids and 8-77% as neither dipeptides nor constituent amino acids. Investigation of the mechanism of dipeptide uptake by red blood cells showed: (1) a lack of hydrolysis by the plasma membrane, (2) no non-specific binding to the plasma membrane, and (3) a lack of saturation over a wide range of concentrations (0.05-50 mM). The data suggest that the mechanism of uptake of trace amounts of dipeptides by human red blood cells is either by simple diffusion or by a carrier system which has a very weak affinity for dipeptides. Upon entry, depending on the molecular structure, dipeptides are either hydrolysed or transformed into new compounds. The red blood cell uptake, however, does not appear to play any appreciable role in clearance of dipeptides from the plasma in the human.

Changes in glycine and leucine transport during red cell maturation in the rat

Both glycine and leucine transport in rat red blood cells have been studied. The glycine uptake showed two different components, one sodium-dependent and another diffusion-like process. In contrast, leucine uptake was sodium independent. Both, Na(+)-dependent glycine and the overall leucine uptake in red blood cells showed a saturable pattern. Kinetic parameters in reticulocytes were: i) glycine: apparent Km 0.16 mM; Vmax 100.2 nmol/ml ICW/min; ii) leucine: apparent Km 2.11 mM; Vmax 3.88 mumol/ml ICW/min. The erythrocytes kinetic parameters were: i) glycine: apparent Km 0.17 mM; Vmax 9.47 nmol/ml ICW/min; leucine; apparent Km 4.77 mM; Vmax 7.42 mumol/ml ICW/min. The Kd values (sodium independent glycine uptake) were similar in both kind of cells, but the importance of this component in total glycine uptake in erythrocytes was much higher than in reticulocytes. Our results confirm that rat red blood cells have both saturable leucine and Na(+)-dependent glycine uptake, but some important changes occur during cell maturation.

Heterogeneity of amino acid transport in horse erythrocytes: a detailed kinetic analysis of inherited transport variation

1. Thoroughbred horses were divisible into five distinct amino acid transport subgroups on the basis of their erythrocyte permeability to L-alanine, measured uptake rates ranging from 5 to 625 mumol l cells-1 h-1 (0.2 mM-extracellular L-alanine, 37 degrees C). 2. Erythrocytes from animals belonging to the lowest L-alanine permeability subgroup (5-15 mumol l cells-1 h-1) (transport-deficient type) exhibited slow nonsaturable transport of this amino acid. In contrast, cells from horses of the four transport-positive subgroups possessed additional high-affinity (apparent L-alanine Km (Michaelis constant) congruent to 0.3 mM) and/or low-affinity (apparent L-alanine Km congruent to 13 mM) Na+-independent transport routes selective for L-neutral amino acids of intermediate size. The two transporters, designated systems asc1 and asc2, respectively, also possessed a significant affinity for dibasic amino acids. 3. Amino acid transport activity in horse erythrocytes behaved as if controlled by three co-dominant alleles (s, h and l), where s is a silent allele, and h and l code for the functional presence of systems asc1 and asc2, respectively. 4. At physiological temperature, system asc1 operated preferentially in an exchange mode. In contrast, system asc2 did not participate in exchange reactions at 37 degrees C, but did exhibit significant trans-acceleration at 25 degrees C. 5. Reduction of the incubation temperature also resulted in dramatic decreases in apparent Km and Vmax for L-alanine uptake by system asc2, whereas the effects of temperature on system asc1 were much less marked. At 5 degrees C the two transporters exhibited equivalent kinetic constants for L-alanine influx. L-Alanine uptake by transport-deficient cells was relatively insensitive to temperature. Influx by this route may represent the ground-state permeability of the lipid bilayer. 6. The effects of low temperature on system asc2 suggest a preferential impairment of the mobility of the unloaded carrier relative to that of the loaded transporter. Similarly, the different kinetic properties of systems asc1 and asc2 at physiological temperature are attributed to a difference in the mobilities of the empty carriers, this difference being minimized at 5 degrees C.

New permeability pathways induced by the malarial parasite in the membrane of its host erythrocyte: potential routes for targeting of drugs into infected cells

Malarial parasites propagate asexually inside the erythrocytes of their vertebrate host. Six hours after invasion, the permeability of the host cell membrane to anions and small nonelectrolytes starts to increase and reaches its peak as the parasite matures. This increased permeability differs from the native transport systems of the normal erythrocyte in its solute selectivity pattern, its enthalpy of activation and its susceptibility to inhibitors, suggesting the appearance of new transport pathways. A biophysical analysis of the permeability data indicates that the selectivity barrier discriminates between permeants according to their hydrogen bonding capacity and has solubilization properties compared to those of iso-butanol. The new permeability pathways could result from structural defects caused in the host cell membrane by the insertion of parasite-derived polypeptides. It is suggested that the unique transport properties of the new pathways be used to target drugs into infected cells, to affect the parasite either directly or through the modulation of the intraerythrocytic environment. The feasibility of drug targeting is demonstrated in in vitro cultures of the human malarial parasite Plasmodium falciparum.

Differential effects of transmembrane potential on two Na+-dependent transport systems for neutral amino acids

The effects of changes of membrane potential on amino acid transport through systems A, ASC and L was investigated in the Ehrlich cell and the human erythrocyte. Changes of membrane potential were produced by incubating cells whose K+ permeability had been increased, either by valinomycin or by activation of Ca2+-dependent K+ channels, in medium containing different K+ concentrations. The changes in membrane potential were followed by measuring the distribution ratio reached by lipophilic indicators. Transport through Na+-dependent system A was sensitive to the membrane potential, the rate of amino acid uptake increasing 2.2-3.1-times for each 60 mV-hyperpolarization. The Na+-dependent system ASC was insensitive to membrane potential. The Na+-independent system L was not directly affected by membrane potential, but the steady-state accumulation of system L substrates was increased by hyperpolarization.

Comparative aspects of the apparent Michaelis constant for neutral amino acid transport in several animal tissues

The apparent Michaelis constant, Km, for transport of a number of neutral amino acids has been compared between intestine, heart, brain and erythrocytes among a variety of animals using values available in the literature. Neutral amino acids with side chains containing 3, 4, 7 and 9 carbon atoms had approximately equal mean Km values when tested for intestinal transport among a variety of species; alanine appeared to have a mean Km value that was larger than those found for the first group, and glycine had a significantly greater mean Km than all of the other compounds tested. Km values for phenylalanine and tryptophan measured in rat heart were found to be close to the means measured for these substrates in intestine. The mean Km values measured in mammalian brain for each of the neutral amino acid substrates were found not be significantly different from each other. When the means of Km values for the neutral amino acids tested were compared between intestine and brain, only the glycine means were shown to differ significantly between the organs. Based on data for several mammalian species, brain appears to have a greater average apparent affinity for glycine than does intestine. In the human erythrocytes and in a few other mammalian species, Km values for all neutral amino acids tested with exception of glycine were found to be similar in magnitude to each other and to the Km averages of neutral amino acids found in intestine for the series containing 3-9 carbon atoms. The Km value for glycine in the human erythrocyte was noted to be substantially lower in value than the averages for glycine in brain or intestine. Avian red blood cells appear to have high apparent affinity for neutral amino acid transport when compared with red cells of several mammalian species.

Tryptophan transport through transport system T in the human erythrocyte, the Ehrlich cell and the rat intestine

We studied the transport of tryptophan through transport system T in the human red cell, the Ehrlich ascites-tumour cell and in everted sacs of rat intestine. In red cells we confirmed earlier results on Na+-independence and aromatic amino acid specificity (Rosenberg, R., Young, J.D. and Ellory, J.C. (1980) Biochim. Biophys. Acta 598, 375-384). In addition we observed that N-methylation or N-acetylation did not reduce the affinity of the substrates for system T, hydroxylation could increase or decrease substrate affinity, and system T was insensitive to pH changes in the medium. These results characterized reactive differences between system T and other known amino acid transport systems. We also found that D-isomers were about 1/3 as effective as L-isomers to inhibit L-tryptophan uptake. D-Tryptophan competitively inhibited L-tryptophan uptake, but was not taken up by system T. L-Tryptophan produced trans-stimulation of the uptake (influx) and trans-inhibition of the release (efflux) of L-[3H]tryptophan; D-tryptophan produced trans-inhibition of the efflux but did not affect significantly the uptake. These results show that in red cells the transport properties of transport system T are asymmetric. Transport system T seems to be absent in the other two preparations studied, the Ehrlich ascites-tumour cell and the rat intestine.

Sodium-induced conformation changes in membrane transport proteins

In the presence of KCl, tryptic digestion of vesicles derived from pigeon erythrocyte membranes inactivates sodium-dependent uptake of alanine by the vesicles, whereas digestion in the presence of NaCl does not. Extensive degradation of vesicle proteins occurs under both conditions. Similarly, the extent of inhibition by N-ethylmaleimide of the sodium-dependent influxes of both glycine and alanine into human erythrocytes is greater when the cells are exposed to the thiol reagent in the presence of KCl than when NaCl is used. These observations are interpreted as providing evidence for sodium-induced conformation changes in these transport proteins.

Characterization of a novel Na+-independent amino acid transporter in horse erythrocytes

Horse erythrocytes are polymorphic with respect to L-alanine permeability. The present investigation compared the specificity, kinetics and cation-dependence of erythrocyte amino acid transport in two groups of thoroughbred horses, those with erythrocyte L-alanine permeabilities in the range 5-15 mumol/h per litre of cells (0.2 mM extracellular L-alanine, 37 degrees C) (transport-negative type) and those with L-alanine permeabilities in the range 450-700 mumol/h per litre of cells (transport-positive type). Transport-positive cells are shown to possess a novel high-affinity, stereospecific, Na+-independent transporter selective for neutral amino acids of intermediate size. This carrier system (provisional designation asc) operates preferentially in an exchange mode and is functionally absent from erythrocytes of transport-negative-type horses.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

Red cell amino acid transport. Evidence for the presence of system Gly in guinea pig reticulocytes

Guinea pig reticulocytes are shown to possess an Na+-dependent glycine transporter which also requires Cl- for activity. Glycine transport by this route is saturable (apparent Km 98 microM; Vmax 24 mumol/g Hb per h) and inhibited by sarcosine. The properties of this carrier closely resemble those of System Gly previously demonstrated in pigeon and human erythrocytes. In contrast, no System Gly activity was detected in mature guinea pig erythrocytes.

Sodium-alanine cotransport in oocytes of Xenopus laevis: correlation of alanine and sodium fluxes with potential and current changes

The sodium-dependent L-alanine transport across the plasma membrane of oocytes of Xenopus laevis was studied by means of [14C]-L-alanine, 22Na+ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 2:1 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 2:1. The sodium-L-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess of D-alanine over L-alanine caused about 60% inhibition.

Red-cell amino acid transport. Evidence for the presence of system ASC in mature human red blood cells

The properties of Na+-dependent L-alanine transport in human erythrocytes were investigated using K+ as the Na+ substitute. Initial rates of Na+-dependent L-alanine uptake (0.2 mM extracellular amino acid) for erythrocytes from 22 donors ranged from 40 to 180 mumol/litre of cells per h at 37 degrees C. Amino acid uptake over the concentration range 0.1-8 mM was consistent with a single saturable component of Na+-dependent L-alanine transport. Apparent Km and Vmax. values at 37 and 5 degrees C measured in erythrocytes from the same donor were 0.27 and 0.085 mM respectively, and 270 and 8.5 mumol/litre of cells per h respectively. The transporter responsible for this uptake was identified as system ASC on the basis of cross-inhibition studies with a series of 42 amino acids and amino acid analogues. Apparent Ki values for glycine, L-alpha-amino-n-butyrate, L-serine and L-leucine as inhibitors of Na+-dependent L-alanine uptake at 37 degrees C were 4.2, 0.12, 0.16 and 0.70 mM respectively. Reticulocytes from a patient with inherited pyruvate kinase deficiency were found to have a 10-fold elevated activity of Na+-dependent L-alanine uptake compared with erythrocytes from normal donors. Separation of erythrocytes according to cell density (cell age) established that even the oldest mature erythrocytes retained significant Na+-dependent L-alanine transport activity. Amino acid transport was, however, a more sensitive indicator of cell age than acetylcholinesterase activity. Erythrocytes were found to accumulate L-alanine against its concentration gradient (distribution ratio approx. 1.5 after 4 h incubation), an effect that was abolished in Na+-free media. Na+-dependent L-alanine uptake was shown to be associated with L-alanine-dependent Na+ influx, the measured coupling ratio being 1:1.