Exploiting amino acid structure to learn about membrane transport

Transport of the essential nutrient isoleucine in human erythrocytes infected with the malaria parasite Plasmodium falciparum

The intraerythrocytic malaria parasite derives much of its requirement for amino acids from the digestion of the hemoglobin of its host cell. However, one amino acid, isoleucine, is absent from adult human hemoglobin and must therefore be obtained from the extracellular medium. In this study we have characterized the mechanisms involved in the uptake of isoleucine by the intraerythrocytic parasite. Under physiologic conditions the rate of transport of isoleucine into human erythrocytes infected with mature trophozoite-stage Plasmodium falciparum parasites is increased to approximately 5-fold that in uninfected cells, with the increased flux being via the new permeability pathways (NPPs) induced by the parasite in the host cell membrane. Transport via the NPPs ensures that protein synthesis is not rate limited by the flux of isoleucine across the erythrocyte membrane. On entering the infected erythrocyte, isoleucine is taken up into the parasite via a saturable, ATP-, Na+-, and H+-independent system which has the capacity to mediate the influx of isoleucine in exchange for leucine (liberated from hemoglobin). The accumulation of radiolabeled isoleucine within the parasite is mediated by a second (high-affinity, ATP-dependent) mechanism, perhaps involving metabolism and/or the concentration of isoleucine within an intracellular organelle.

Manipulation of CD98 expression affects both trophoblast cell fusion and amino acid transport activity during syncytialization of human placental BeWo cells

The physiological importance of CD98 surface antigen in regulating placental trophoblast cell fusion and amino acid transport activity has been studied in parallel in a cell model of syncytialization (the cytotrophoblast cell line BeWo following increased intracellular cAMP by forskolin treatment) using antisense oligonucleotides. CD98 protein abundance (determined by Western blot) was decreased by 50 % following antisense oligonucleotide transfection. Transfection with antisense oligonucleotide altered the responses of BeWo to forskolin. Cell fusion (determined by a quantitative flow cytometry assay) was inhibited by 57 %, and both human chorionic gonadotropin secretion and L-leucine influx through system L were suppressed. These findings show that CD98 is involved in the process of cell fusion necessary for syncytiotrophoblast formation and that during this physiologically important event, amino acid transport activity is also regulated through expression of this membrane protein.

Characterisation of L-tryptophan transporters in human placenta: a comparison of brush border and basal membrane vesicles

The mechanisms responsible for L-tryptophan transport at both the maternal- and fetal-facing surfaces of the term placenta have been determined in isolated membrane vesicles as part of a study on placental indoleamine 2,3-dioxygenase, the L-tryptophan-catabolising enzyme recently shown to regulate feto-maternal immunology. Brush border vesicle uptake of L-tryptophan is substantially into an osmotically active space. It is sodium independent and N-ethylmaleimide sensitive. Uptake of L-tryptophan, which is markedly stereospecific, has a Km of 26.3 microM and Vmax of 1.72 pmol (mg protein)(-1) s(-1) and is completely abolished by the L-system-specific substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH). These findings are in keeping with L-tryptophan transport being exclusively via system L (induced by the heterodimeric heavy chain of CD98 and system L-amino acid transporter-1 (LAT-1)). 1-Methyl-tryptophan (which is a known competitive inhibitor of indoleamine 2,3-dioxygenase) is a competitive inhibitor of L-tryptophan flux through this transport system (Ki = 113 microM). Basal membrane transport of L-tryptophan is more complex. Uptake is slower than at the brush border and although, as in the brush border, uptake is sodium independent, it is less sensitive to N-ethylmaleimide. There is clear evidence that two systems contribute to basal membrane transport since BCH is (in sodium-free media) only a partial inhibitor whereas L-histidine and L-cysteine are fully effective. The simplest explanation of these and other findings is that the basal membrane possesses two systems, one of which is similar to that induced by the heavy chain of CD98 and system L-amino acid transporter-2 (LAT-2). The other appears to be system y+L since in the presence of BCH inhibition by L-leucine but not by L-lysine is sodium dependent. These findings suggest the existence of non-identical carrier-mediated transport systems for L-tryptophan in brush border and basal membranes. This asymmetry may explain net transplacental transfer of this amino acid.

Heterodimeric amino acid transporters: expression of heavy but not light chains of CD98 correlates with induction of amino acid transport systems in human placental trophoblast

1. Activity of amino acid transport and relative abundance of mRNAs encoding related transporters have been studied in parallel either before or following in vitro culture of explants of human placental chorionic villi. 2. Amino acid transport activities through systems L (1.9-fold), y+L (2.6-fold) and y+ (3.2-fold) were markedly enhanced following culture for 48 h. 3. Relative mRNA abundance (determined by reverse transcription-polymerase chain reaction) for the heavy chain of CD98 surface antigen and for the cationic amino acid transporter-1 were similarly stimulated (2.8-fold and 2.6-fold, respectively). In contrast, none of the mRNA levels for light chains of CD98 (system L-amino acid transporter-1, system L-amino acid transporter-2, system y+L-amino acid transporter-1 and system y+L-amino acid transporter-2) studied nor for the cationic amino acid transporter-2B were altered.

Comparison of the transport characteristics of D- and L-methionine in a human intestinal epithelial model (Caco-2) and in a perfused rat intestinal model

Absorption mechanisms of L- and D-methionine (MET) in an in vitro cultured human intestinal epithelial cell model (Caco-2) and an in situ perfused rat intestinal model were investigated to determine if the kinetic characteristics of absorption are comparable in these two popular absorption models. The results indicate that the transport of L- and D-MET were concentration-dependent in both model systems, and displayed comparable Km values. The Km value for L-MET is 1.34 mM in the Caco-2 model and 3.6 mM in the perfused rat intestinal model, while the Km value for D-MET is 1.79 mM in the Caco-2 model and 2.87 mM in the perfused rat intestinal model. Although the Jmax values were not comparable because of significant methodology differences, the Jmax values for L-MET were always higher than that for D-MET. In addition, transport of L- and D-MET across the Caco-2 cell monolayers were also inhibited by 10 mM Phe and Lys while MeAIB, Pro and Glu were generally ineffective. Similar results were also observed with these inhibitors in the perfused rat intestinal model with the exception that a combination of Pro and Glu stimulated the uptake of L-MET. In conclusion, the transport characteristics of L- and D-MET are comparable in both model systems.

Characteristics and regulation of proline transport in cultured glioblastoma cells

L-Proline transport in C6 glioblastoma cells takes place mainly via a saturable Na(+)-dependent mechanism. The uptake process can be discriminated into two components, system A and system ASC. A minor proportion of L-proline transport is carried out by the ASC system, which appears to be constitutively expressed by the cell, but most is by system A which shows adaptive responses to amino acid deprivation and sensitivity to N-methyl-alpha-aminoisobutyric acid. The transport system is inhibited by proline derivatives, such as methyl and benzyl esters, and also hydroxyproline, and is stereospecific. Incubation of glioblastoma cells with phorbol 12-myristate 13-acetate led to concentration- and time-dependent decreases in L-proline transport. This effect could be mimicked by exogenous phospholipase C. Proline transport is significantly stimulated in the presence of Ca(2+)-mobilization agents and strongly inhibited in the absence of Ca2+. The present data suggest a complex regulation of L-proline transport by different kinases in glioblastoma cells.

Reconstitution studies of amino acid transport system L in rat erythrocytes

In many cell types, including human erythrocytes, membrane transport of hydrophobic amino acids such as leucine and phenylalanine is mediated primarily by Na(+)-independent system L. In this paper we demonstrate that erythrocytes from the rat have a 400-fold higher system L transport capacity than human erythrocytes. We have exploited this high transport activity to achieve the first successful reconstitution of an erythrocyte amino acid transporter into phospholipid vesicles. Rat erythrocyte membranes were depleted of extrinsic membrane proteins, solubilized in 50 mM n-octyl glucoside and reconstituted into egg-yolk phospholipid vesicles by a gel filtration freeze-thaw protocol. Optimal reconstitution of transport activity occurred at lipid/protein ratios of 25-35:1. At a lipid/protein ratio of 25:1, one-half of the total uptake of L-[14C]leucine (0.2 mM, 25 degrees C) was inhibited by 2 mM phloretin and thus judged to be carrier-mediated. This component of L-leucine uptake was inhibited by non-radioactive L-phenylalanine and L-leucine, and only to a very much weaker extent by glycine and L-alanine. Two other inhibitors of system L in intact cells, MK196 and PCMBS (p-chloromercuriphenylsulphonate), were also effective inhibitors of phloretin-sensitive L-leucine transport in reconstituted proteoliposomes. Phloretin-insensitive uptake of L-leucine in proteoliposomes occurred by simple diffusion across the lipid bilayer.

Modulation of transforming growth factor-beta 1 antiproliferative effects on endothelial cells by cysteine, cystine, and N-acetylcysteine

Early passaged bovine pulmonary artery endothelial cells exposed to 0.1-2.0 ng/ml transforming growth factor-beta 1 (TGF-beta 1) showed concentration-dependent growth inhibition, as assessed by [3H]thymidine labeling and cell counts, over a 96-h interval. Most of the inhibition of [3H]thymidine labeling measured at 96 h persisted when the medium was replaced with TGF-beta 1-free medium after 24 h, but the inhibition of labeling was prevented by the presence of anti-TGF-beta 1 antibody in the replacement medium. Additions of 2 mM cysteine, 1 mM cystine, or 2 mM N-acetylcysteine at the time of the initial addition of TGF-beta 1 blocked the inhibitory effect of TGF-beta 1 on [3H]-thymidine labeling when this was assessed after 72-96 h, but not at earlier times. Prevention of the inhibitory effect on cellular proliferation produced by cysteine, cystine and N-acetylcysteine was associated with elevation of cellular glutathione that was present at 48-96 h. There was no evidence for direct inactivation of TGF-beta 1 by the thiol-amino acids. Conditioned medium from TGF-beta 1-treated endothelial cells inhibited proliferation of mink lung carcinoma (CCL64) cells, supporting a previously reported concept of autocrine production of TGF-beta 1 by the endothelial cells. The inhibitory action of the conditioned medium was partially prevented when 1 mM cysteine was added during conditioning. Thus, TGF-beta 1 treatment of endothelial cells appears to set off autocrine production by these cells of TGF-beta 1 that perpetuates the inhibition of cellular proliferation. Replenishment of cellular glutathione with thiol-amino acids counteracts the growth-inhibitory effect of TGF-beta 1 through a currently undefined mechanism.

Accumulation of pyrroline 5-carboxylic acid in conditioned medium of cultured fibroblast: stimulatory effects of serum, insulin, and IGF-1

Pyrroline 5-carboxylate, an intermediate of amino acid metabolism, is released into medium by cultured normal human fibroblasts. With cells made quiescent by serum starvation, the addition of 10% fetal bovine serum augmented the release of pyrroline 5-carboxylate into medium by 2.5-fold. Although platelet-derived growth factor was without effect, both insulin and insulinlike growth factor-1 nearly reproduced the serum effect. The dose-dependence of insulin and insulinlike growth factor 1 effects suggested their mediation by their own respective receptors. Although the mechanism for the stimulatory effect remains unknown, these effects of insulin and insulinlike growth factor 1 on pyrroline 5-carboxylate suggest hormonal regulation of pyrroline 5-carboxylate release.

Use of membrane vesicles to estimate the numbers of system y+ and system L amino acid transporters in human erythrocytes

We have used equilibrium values for L-leucine and L-lysine uptake by right-side-out vesicles to estimate the membrane abundance (sites/cell) of Na(+)-dependent amino acid transport systems L and y+ in human erythrocytes. All of the intravesicular space was accessible to L-leucine, as judged by comparisons with uridine uptake via the equilibrative nucleoside transporter (10(4) sites/cell). In contrast, only 28% of the total intravesicular space was accessible to L-lysine uptake via system y+. Since human erythrocyte membranes generate an average of approximately 1000 vesicles/cell, these data provide evidence that system L is a relatively high-abundance membrane transport protein in human erythrocytes, while system y+ is present in smaller amounts (approximately 300 copies/cell). Calculated turnover numbers for L-lysine transport by system y+ at 37 degrees C are 24 s-1 for zero-trans influx and 150 s-1 for equilibrium-exchange influx.

Mechanism of L-alpha-methyldopa transport through a monolayer of polarized human intestinal epithelial cells (Caco-2)

The Caco-2 model system (Hidalgo et al., Gastroenterology, 96:736-749, 1989), which is a monolayer of polarized intestinal epithelial cells grown onto a porous polycarbonate membrane, was used to study the mechanism of transcellular transport of an antihypertensive agent, L-alpha-methyldopa (L-alpha-MD). The results showed that the transport of L-alpha-MD was pH, glucose, concentration, and temperature dependent, and it could be inhibited by metabolic inhibitors (e.g., 2,4-dinitrophenol) and by amino acids (e.g., L-phenylalanine) which have an affinity for the large neutral amino acid (LNAA) carrier. In addition, the apparent kinetic constants describing the transcellular transport of L-alpha-MD were altered depending on the time interval between feeding the cells and the transport experiments (postfeeding time, PFT). The apparent maximum carrier flux (Jmax) of L-alpha-MD was significantly increased (from 155 to 547 pmol/mg protein/min) when PFT was prolonged from 8.5 to 56 hr. These results indicated that the transcellular transport of L-alpha-MD through the polarized Caco-2 cell monolayer was carrier mediated via the LNAA carrier. The similarities in the characteristics of L-alpha-MD transport exhibited by the Caco-2 model system and other intestinal models in vitro further substantiate the usefulness of this cell culture model for studying the intestinal transport of nutrients and drugs.

Human placental L-tyrosine transport: a comparison of brush-border and basal membrane vesicles

1. The mechanisms responsible for L-tyrosine transport at both the maternal-facing and fetal-facing surfaces of the human full-term placenta have been studied using isolated brush-border and basal membrane vesicles under conditions where a direct comparison of the transport properties of the two membranes can be made. 2. Brush-border vesicle uptake of L-tyrosine was substantially into an osmotically active space. Transport was Na(+)-independent, N-ethylmaleimide-sensitive (half-maximal inhibition, Ki = 1.1 mM), and insensitive to pH over the range 5.5-8.5. The initial rate of brush-border L-tyrosine uptake as a function of concentration showed saturation and obeyed Michaelis-Menten kinetics with Michaelis constant (Km) and maximum velocity (Vmax) values of 54.2 microM and 1.28 pmol (mg protein)-1 s-1, respectively. Influx of L-tyrosine was stereospecific and was virtually completely abolished by L-phenylalanine, L-tryptophan, L-leucine or by 2-aminobicycloheptane-2-carboxylic acid. These properties suggest that system L is responsible for brush-border L-tyrosine transport. 3. Basal membrane transport of L-tyrosine was more complex and uptake was slower than that found in the brush border. Although, as in the brush-border membranes, uptake was completely Na(+)-independent, N-ethylmaleimide was a less effective inhibitor, there was stimulation of transport at more alkaline pH and uptake did not show marked stereospecificity. An apparent Km of 168.9 microM and a Vmax of 0.31 pmol (mg protein)-1 s-1 were calculated for basal L-tyrosine transport. There was clear inhibition by L- and D-tyrosine, L-phenylalanine and L-tryptophan. 2-Aminobicycloheptane-2-carboxylic acid was not as effective. 4. These findings suggest the existence of non-identical carrier-mediated transport systems for L-tyrosine in brush-border and basal membranes. Brush-border transport resembles that by system L; L-tyrosine transport at the basal membrane may be via system t.

Characteristics and adaptive regulation of glycine transport in cultured glial cells

The transport of glycine in C6 glioma cells takes place mainly in a heterogeneous Na+-dependent manner which can be resolved into different components. A Na+- and Cl(-)-dependent component with high affinity for glycine is pH-sensitive and inhibited by sarcosine, all these characteristics corresponding to System Gly. The low-affinity component of the transport of glycine can be discriminated as two components, namely System A and System ASC. The main proportion of glycine transport through the low-affinity system is carried out by the ASC System, which appears to be constitutively expressed by the cells. The adaptive response of the low-affinity Na+-dependent transport of glycine to amino acid deprivation was identified with System A on the basis of its ion-dependency, pH-sensitivity and by inhibition analysis. The possible physiological role of the high- and low-affinity components of the transport system for glycine in glial cells is discussed.

Sodium-coupled glycine uptake by Ehrlich ascites tumor cells results in an increase in cell volume and plasma membrane channel activities

The addition of 10 mM glycine to a physiological saline bathing Ehrlich ascites tumor cells is followed by a slow increase in cell volume that plateaus between 15 and 30 min at a level approximately equal to 17% greater than the control volume; this increase is not observed when glycine is added to cells suspended in a Na+-free saline. The results of studies using the patch-clamp technique in the cell-attached mode indicate that, 0.5-3 min after the addition of glycine to the bathing solution, there is a marked increase in the activity of single channels, which is almost all instances were previously present and operant in the plasma membrane. Successfully excised patches of membrane that contained a channel stimulated by glycine fell into two categories. Some became inactive within 15 sec in spite of the fact that the G omega seal remained intact. Others persisted for the lifetime of the seal. All of the persistent channels had an 11-fold selectivity for Cl- over K+ and a conductance of 23 pS when bathed by symmetrical 150 mM KCl solutions. Although the ionic specificities of the other channels have not been identified, there is reason to suspect that they might be K+ channels whose activities are dependent on factors lost when the patch is excised. Swelling induced by exposing these cells to a 50% hypotonic perfusate stimulated the activities of Cl- channels whose properties closely resemble those stimulated by the addition of glycine to the perfusate, strongly suggesting that the glycine-induced stimulation of Cl- channel activity is part of a volume-regulatory response to cell swelling. If the increase in channel activity induced by the addition of glycine to the perfusate is indeed a response to cell swelling, then this perfusate is indeed a response to cell swelling, then this volume-regulatory response must be extremely sensitive inasmuch as it appears to be "triggered" by an average increase in cell volume that does not exceed 5%.

Cystathionase activity and glutathione metabolism in redifferentiating rat hepatocyte primary cultures

Capacity to incorporate methionine sulfur into glutathione as well as cystathionase activity were lost in cultured hepatocytes in a biphasic manner with 75% of the total capacity disappearing with a half-life of about 10.6 hr, the remainder with a half-life of greater than 20 hr. Nicotinamide, 25 mM, produced a single phase loss with a t 1/2 of approximately 21 hr for both transsulfuration and cystathionase activity. Loss of both methionine sulfur incorporation and cystathionase activity occurred in transferrin/sodium selenite-supplemented Williams Medium E (TS-HWME) with a t 1/2 of about 96 hr through 72 hr in culture. Addition of the cystathionase inhibitor, propargylglycine, blocked glutathione synthesis in TS-HWME cells through 48 hr in culture, while propargylglycine blocked glutathione synthesis only at 4 hr in HWME cultured cells. Further, the accumulation of gamma-glutamyl transpeptidase was delayed by 48 hr in TS-HWME versus unsupplemented medium. Variation in the transport of sulfur amino acids was also found to occur with culture age. The Km values for cysteine and methionine transport were found to be approximately 150 and 100 microM, respectively, and were unaffected by culture age or the presence of TS-HWME. However, the Vmax for transport of methionine declined from 0.29 to 0.012 nmol/min/mg protein over 48 hr in culture. In TS medium, the Vmax at 48 hr for methionine transport had only decreased to 0.20 nmol/min/mg protein and increased for cysteine transport to 0.17 nmol/min/mg protein. These data suggest that during the redifferentiation of hepatocytes in culture, transsulfuration is regulated by control of the flow of substrate through cystathionase and that cystathionase is regulated by alteration of enzyme activity or content. Variations in the rate of transport of precursor sulfur amino acids are also an important component of the regulation of the net glutathione status of the redifferentiating hepatocyte.

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.

Induction of cystine transport activity in mouse peritoneal macrophages

Uptake of cystine was investigated in mouse peritoneal macrophages. The rates of the uptake of cystine in resident macrophages or macrophages elicited by some irritants were very low, but a drastic increase was observed when the cells were cultured in vitro. This increase was time-dependent and required protein synthesis. In macrophages elicited by thioglycollate broth, the rate of the uptake of cystine increased by about 40-fold after 16 h in culture. Contrary to the uptake of cystine, the rates of uptake of some neutral amino acids did not change markedly during culture. We characterized the induced activity of the cystine uptake in macrophages elicited by thioglycollate broth. Cystine was taken up in an Na+-independent and pH-sensitive manner, and the uptake was potently inhibited by extracellular glutamate and the analogous anionic amino acids, but not by aspartate. The activity of the glutamate uptake was also induced during the culture in a way similar to that of cystine uptake, and the uptake of glutamate was potently inhibited by cystine. From these results we concluded that the uptake of cystine and glutamate in macrophages was mostly mediated by a single transport system similar to the ones previously reported in human fibroblasts and some other cells. As a consequence of the induction of the activity of the cystine uptake, glutathione levels in macrophages doubled during culture, and a thiol compound, presumably cysteine, was released into the culture medium and accumulated there. When the macrophages were cultured hypoxically, the induction of the cystine uptake activity was markedly depressed, suggesting an involvement of oxygen in the induction.

Characterization of tryptophan transport in human placental brush-border membrane vesicles

The characteristics of tryptophan uptake in isolated human placental brush-border membrane vesicles were investigated. Tryptophan uptake in these vesicles was predominantly Na+-independent. Uptake of tryptophan as measured with short incubations occurred exclusively by a carrier-mediated process, but significant binding of this amino acid to the membrane vesicles was observed with longer incubations. The carrier-mediated system obeyed Michaelis-Menten kinetics, with an apparent affinity constant of 12.7 +/- 1.0 microM and a maximal velocity of 91 +/- 5 pmol/15 s per mg of protein. The kinetic constants were similar in the presence and absence of a Na+ gradient. Competition experiments showed that tryptophan uptake was effectively inhibited by many neutral amino acids except proline, hydroxyproline and 2-(methylamino)isobutyric acid. The inhibitory amino acids included aromatic amino acids as well as other system-1-specific amino acids (system 1 refers to the classical L system, according to the most recent nomenclature of amino acid transport systems). The transport system showed very low affinity for D-isomers, was not affected by phloretin or glucose but was inhibited by p-azidophenylalanine and N-ethylmaleimide. The uptake rates were only minimally affected by change in pH over the range 4.5-8.0. Tryptophan uptake markedly responded to trans-stimulation, and the amino acids capable of causing trans-stimulation included all amino acids with system-1-specificity. The patterns of inhibition of uptake of tryptophan and leucine by various amino acids were very similar. We conclude that system t, which is specific for aromatic amino acids, is absent from human placenta and that tryptophan transport in this tissue occurs via system 1, which has very broad specificity.

Transport of the aromatic amino acids into isolated rat liver cells. Properties of uptake by two distinct systems

The transport of the aromatic amino acids into isolated rat liver cells was studied. There was a rapid and substantial binding of the aromatic amino acids, L-alanine and L-leucine to the plasma membrane. This has important consequences for the determination of rates of transport and intracellular concentrations of the amino acids. Inhibition studies with a variety of substrates of various transport systems gave results consistent with aromatic amino acid transport being catalysed by two systems: a 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)-insensitive aromatic D- and L-amino acid-specific system, and the L-type system (BCH-sensitive). The BCH-insensitive component of transport was Na+-independent and facilitated non-concentrative transport of the aromatic amino acids; it was unaffected by culture of liver cells for 24 h, by 48 h starvation, dexamethasone phosphate or glucagon. Kinetic properties of the BCH-inhibitable component were similar to those previously reported for the L2-system in liver cells. The BCH-insensitive component was a comparatively low-Km low-Vmax. transport system that we suggest is similar to the T-transport system previously seen only in human red blood cells. The results are discussed with reference to the importance of the T- and L-systems in the control of aromatic L-amino acid degradation in the liver.

Transport of a low molecular weight extracellular esterase into membrane vesicles of Candida lipolytica

The low mol. wt extracellular esterase of Candida lipolytica is actively transported into membrane vesicles. In the absence of metabolic energy, a proton gradient can drive the transport process. The transport system does not accumulate the enzyme at peak levels due to the presence of a leak pathway.

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.

Characterization of a Chinese hamster-human hybrid cell line with increased system L amino acid transport activity

We have studied leucine transport in several Chinese hamster-human hybrid cell lines obtained by fusion of a temperature-sensitive line of Chinese hamster ovary cells, ts025C1, and normal human leukocytes. A hybrid cell line exhibiting a twofold increase in L-leucine uptake over that in the parental cell line was found. This hybrid cell line, 158CnpT-1, was temperature resistant, whereas the parental Chinese hamster ovary mutant, ts025C1, contained a temperature-sensitive leucyl-tRNA synthetase mutation. An examination of the different amino acid transport systems in this hybrid cell line revealed a specific increase of system L activity with no significant changes in systems A and ASC. The Vmax for L-leucine uptake exhibited by the hybrid 158CnpT-1 was twice that in the CHO parental mutant, ts025C1. Cytogenetic analysis showed that the hybrid 158CnpT-1 contains four complete human chromosomes (numbers 4, 5, 10, and 21) and three interspecific chromosomal translocations in a total complement of 34 chromosomes. Biochemical and cytogenetic analysis of segregant clones obtained from hybrid 158CnpT-1 showed that the primary temperature resistance and high system L transport phenotypes can be segregated from this hybrid independently. The loss of the primary temperature resistance was associated with the loss of the human chromosome 5, as previously reported by other laboratories, whereas the loss of the high leucine transport phenotype, which is associated with a lesser degree of temperature resistance, was correlated with the loss of human chromosome 20.

Characterization of a Chinese hamster-human hybrid cell line with increased system L amino acid transport activity

We have studied leucine transport in several Chinese hamster-human hybrid cell lines obtained by fusion of a temperature-sensitive line of Chinese hamster ovary cells, ts025C1, and normal human leukocytes. A hybrid cell line exhibiting a twofold increase in L-leucine uptake over that in the parental cell line was found. This hybrid cell line, 158CnpT-1, was temperature resistant, whereas the parental Chinese hamster ovary mutant, ts025C1, contained a temperature-sensitive leucyl-tRNA synthetase mutation. An examination of the different amino acid transport systems in this hybrid cell line revealed a specific increase of system L activity with no significant changes in systems A and ASC. The Vmax for L-leucine uptake exhibited by the hybrid 158CnpT-1 was twice that in the CHO parental mutant, ts025C1. Cytogenetic analysis showed that the hybrid 158CnpT-1 contains four complete human chromosomes (numbers 4, 5, 10, and 21) and three interspecific chromosomal translocations in a total complement of 34 chromosomes. Biochemical and cytogenetic analysis of segregant clones obtained from hybrid 158CnpT-1 showed that the primary temperature resistance and high system L transport phenotypes can be segregated from this hybrid independently. The loss of the primary temperature resistance was associated with the loss of the human chromosome 5, as previously reported by other laboratories, whereas the loss of the high leucine transport phenotype, which is associated with a lesser degree of temperature resistance, was correlated with the loss of human chromosome 20.

Discrimination of parallel neutral amino acid transport systems in the basolateral membrane of cat salivary epithelium

Transport of short-chain and long-chain neutral amino acids across the basolateral membrane of the epithelium in the perfused cat salivary gland has been studied using a rapid (less than 30 s) single circulation paired-tracer dilution technique. Amino acid uptake was measured by comparing the venous dilution profiles for a tritiated amino acid and D-[14C]mannitol (extracellular reference) following a bolus intra-arterial injection of a mixture containing both molecules. Unidirectional influx (v) was estimated from the maximal tracer uptake (Umax), the perfusate flow (F) and the perfusate amino acid concentration (Ca): v = [-F . ln (1-Umax)] . Ca. L-alanine influx was saturable and apparently mediated by a single entry system (Km = 0.83 +/- 0.11 mM and Vmax = 655 +/- 32 nmol/min . g). These kinetic constants were considerably lower than our previously reported values for L-phenylalanine: Km = 6.4 mM and Vmax = 1719 nmol/min . g. In cross-inhibition experiments performed over a wide range of concentrations (0.05-24 mM), influx of L-alanine and L-phenylalanine could be further discriminated, since both L-phenylalanine (Ki = 22 mM) and L-alanine (Ki = 19 mM) behaved as poor competitors. Removal of Na+ from the perfusate resulted in a selective inhibition of L-alanine and L-serine influx, whereas influx of the long-chain neutral amino acids L-leucine, L-phenylalanine and L-tryptophan remained unaffected. Although prolonged perfusion of glands with dinitrophenol (0.8 mM for 20-30 min) caused a variable but net inhibition of unidirectional uptake, it markedly enhanced the tracer efflux of L-leucine, L-phenylalanine, L-tyrosine and the basic amino acid L-lysine. It appears that at least two separate neutral amino acid transport systems are operative at the blood-tissue interface of the salivary epithelium: (i) a Na+-dependent alanine-serine-cysteine preferring type of carrier exhibiting a high affinity for amino acids with short, polar or linear side chains and (ii) a Na+-independent leucine preferring type of carrier selective for large neutral amino acids.

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.

Multiple transport pathways for neutral amino acids in rabbit jejunal brush border vesicles

Amino acids enter rabbit jejunal brush border membrane vesicles via three major transport systems: (1) simple passive diffusion; (2) Na-independent carriers; and (3) Na-dependent carriers. The passive permeability sequence of amino acids is very similar to that observed in other studies involving natural and artificial membranes. Based on uptake kinetics and cross-inhibition profiles, at least two Na-independent and three Na-dependent carrier-mediated pathways exist. One Na-independent pathway, similar to the classical L system, favors neutral amino acids, while the other pathway favors dibasic amino acids such as lysine. One Na-dependent pathway primarily serves neutral L-amino acids including 2-amino-2-norbornanecarboxylic acid hemihydrate (BCH), but not beta-alanine or alpha-methylaminoisobutyric acid (MeAIB). Another Na-dependent route favors phenylalanine and methionine, while the third pathway is selective for imino acids and MeAIB. Li is unable to substitute for Na in these systems. Cross-inhibition profiles indicated that none of the Na-dependent systems conform to classical A or ACS paradigms. Other notable features of jejunal brush border vesicles include (1) no beta-alanine carrier, and (2) no major proline/glycine interactions.

Human platelet-derived growth factor stimulates amino acid transport and protein synthesis by human diploid fibroblasts in plasma-free media

Purified human platelet-derived growth factor (PDGF) induces an increase in amino acid uptake via system A in quiescent human diploid fibroblasts. Cells must be exposed to PDGF for 45 min to obtain maximum transport stimulation. Transport stimulation requires protein synthesis. Transient exposure to PDGF, alone, in the absence of plasma components can stimulate transport. Acid-insoluble [3H]leucine incorporation is also stimulated by PDGF treatment, and this event also does not require the presence of plasma components. Finally, antiserum to PDGF that blocks PDGF-stimulated DNA synthesis in these cells also blocks PDGF-stimulated amino acid uptake and protein synthesis. Increased amino acid uptake and protein synthesis that occur soon after addition of fresh serum to quiescent cells can be attributed to the action of PDGF acting alone and should be useful as markers for the investigation of early cellular events caused by PDGF.

Single-site uptake of neutral amino acids into guinea-pig intestinal rings

1. A kinetic analysis of amino acid influx into guinea-pig small intestinal rings has been performed in an attempt to ascertain whether one or more transport sites for these substrates exists in the luminal membrane of the enterocyte. 2. No indirect correction for uptake into the extracellular space was applied, but it was assumed in the analysis that the total uptake included a diffusion term. This procedure was vindicated by the results obtained. 3. All analyses were performed by non-linear regression techniques. In many experiments, both substrate and inhibitor concentrations were varied within the same experiment, thus giving rise to three-dimensional diagrams describing transport processes. 4. All results pointed to the existence of a single transport agency shared by all amino acids tested. The kinetic constants, Km and Vmax, were independent of the concentration range used for their estimation; such behaviour would not be expected if several sites with different kinetic constants were available for transport. The value of KD, the constant describing the diffusive component of uptake, was the same when estimated from the uptake of an amino acid alone or when determined as the asymptote of the curve describing the inhibition of this uptake by an analogue. Finally, the Ki for an amino acid when used as an inhibitor was identical to its Km when used as substrate. This property was maintained even when the most disparate pair of amino acids, threonine and isoleucine, was examined.

Transport specificity for neutral and basic amino acids at maternal and fetal interfaces of the guinea-pig placenta

1. The unidirectional influx of amino acids into the guinea-pig syncytiotrophoblast was measured using a single circulation paired-tracer dilution technique which allows separate characterization of both fetal and maternal interfaces. An in situ preparation perfused through the fetal circulation was used to examine the fetal side, while an isolated preparation perfused through both the fetal and maternal circulations was used to study both interfaces simultaneously.2. On the fetal side the maximal uptake (U(max)) determined at tracer concentrations was high for the short-chain neutral amino acid alanine (76%) and the long-chain neutrals, leucine (75%), phenylalanine (90%) and tyrosine (82%) and for the basic amino acid lysine (65%). In contrast, U(max) was negligible for alpha-methylaminoisobutyric acid and taurine, a beta-amino acid.3. The uptake of alanine and phenylalanine on the fetal side was inhibited by both short-chain (alanine, serine, cysteine) and long-chain (phenylalanine, methionine, leucine) neutral amino acids. d-alanine had no effect on l-alanine uptake whereas d-phenylalanine significantly inhibited that of l-phenylalanine. Diaminobutyric acid, lysine and arginine were effective inhibitors of alanine uptake but had no effect on phenylalanine uptake.4. On the maternal side uptake of alanine, phenylalanine and lysine was measured. Over a wide range of concentrations self-inhibition of alanine influx was similar to the cross-inhibition observed with phenylalanine. In contrast, the influx of phenylalanine, which was strongly self-inhibited, was only partially cross-inhibited by alanine.5. Influx of alanine and phenylalanine was measured at various perfusate concentrations and was found to be saturable on both maternal and fetal sides. The data were fitted to a single hyperbola and, on the maternal side, the K(m) for alanine (10.3+/-2.7 mm, mean+/-s.e., n = 3) was three-fold higher than the value measured for phenylalanine (3.1+/-0.8 mm). On the fetal side the K(m) values for alanine (8.4+/-1.4 mm, n = 4) and phenylalanine (11.9+/-1.9 mm, n = 3) were similar.6. The uptake of alanine, phenylalanine and lysine appeared to be highly sodium-dependent accounting for 40-70% of the total influx. However, the inhibited fractions were found to be different on the two sides of the placenta.7. The results of uptake, cross-inhibition and Na(+)-dependency experiments suggest the presence of an alanine-serine-cysteine (ASC) type system and a leucine (L) type system with markedly overlapping specificities at both the fetal and maternal interfaces. Separate kinetic characterization of a two carrier system was not possible under the conditions of these experiments. However, kinetic parameters for the over-all transport of alanine and phenylalanine were measured.

The accumulation of amino acids by mouse ascites-tumour cells. Dependence on but lack of equilibrium with the sodium-ion electrochemical gradient

1. The fluorescent dye 3,3'-dipropyloxadicarbocyanine was used to show that the tumour cells absorbed 2-aminoisobutyrate, glycine, L-leucine and L-isoleucine and certain other amino acids electrogenically. The Km values with respect to amino acid concentration ([A]o), obtained from the fluorescence assays, varied through the above series from 0.8 to 26 mM, with Vmax. fairly constant. 2. Similar Km values described the uptake of the 14C-labelled amino acids in five instances where this was measured. 3. Each amino acid lowered the membrane potential (E) by 10-20 mV when its cellular concentration ([A]i) had reached a steady value and [A]o was 10mM. In these experiments energy metabolism was maintained by glycolysis, 2,4-dinitrophenol was present and cellular respiration was inhibited. The corresponding net flow of amino acid through the Na+ symport was deduced by making use of the fact that the depolarization an amino acid initially caused was roughly proportional to the net influx of amino acid itself. 4. The steady-state depolarization was attributed to the presence of a leak pathway for the amino acid with a rate coefficient PA. As assayed in the absence of Na+, PA was about 5-fold larger for isoleucine than for glycine. 5. Direct estimates of Vmax./PA were similar to those inferred from the extent of depolarization in the steady state and [A]i. 6. A mathematical model was used to predict [A]i/[A]o in term of the measured values of [Na]o, [Na]i, E, Km and Vmax./PA. The predicted and observed values agreed fairly well when [A]o was 1 mM or 10 mM. 7. [A]i/[A]o varied from about 2.5 for 10 mM-isoleucine to 30 for 1 mM-2-aminoisobutyrate when delta microNa, expressed as a ratio, was ostensibly in the range 19-43. 8. The concentration of 2-aminoisobutyrate from a 0.1 mM solution in the presence or absence of ouabain was consistent with the model, whereas the concentration of isoleucine from a 0.1 mM solution exceeded the predicted values 2-5-fold. 9. The tumour cells concentrated 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid by a non-electrogenic mechanism, with which isoleucine may also interact.

Specificity of neutral amino acid uptake at the basolateral side of the epithelium in the cat salivary gland in situ

1. Amino acid uptake was measured in resting cat submandibular glands with either a natural blood supply or perfused at constant flow with a Krebs-albumin solution. Following a bolus arterial injection of a 3H-labelled amino acid and D-[14C]mannitol (extracellular reference tracer), the venous effluent was immediately sampled sequentially. The maximal uptake, Umax, from the blood or perfusate was determined from the paired-tracer dilution curves using the expression: uptake % = (1 -- (3H/14C) X 100). 2. In glands with a natural blood supply, Umax values up to 46% were measured for short-chain (serine and alanine) and long-chain (valine, methionine, leucine, isoleucine, 1-amino-cyclopentane cyclopentane carboxylic acid, phenylalanine, tryptophan, tyrosine, histidine and glutamine) neutral amino acids. In contrast, Umax was negligible for amino acids of the imino-glycine group (proline and glycine) and the nonmetabolized amino acids, 2-aminoisobutyric acid (AIB) and methylaminoisobutyric acid (MeAIB). 3. In glands with a natural blood supply addition of an unlabelled amino acid to the tracer injectate reduced Umax for the test acid by up to 80%. The pattern of these interactions suggested the presence of two transport systems for neutral amino acids, one preferring short-chain and the other long-chain amino acids. 4. In glands perfused at constant flow rates with an amino acid-free Krebs-albumin solution high Umax values were measured: L-serine (66%), L-alanine (54%), L-leucine (43%), L-phenylalanine (42%) and L-tyrosine (51%). Only a low uptake was observed for L-proline (8%) and glycine (14%). There was no uptake of methylaminoisobutyric acid which confirms the result obtained in glands with an intact circulation. 5. Saturation of L-phenylalanine influx was observed in perfused glands as the perfusate concentration of unlabelled L-phenylalanine was increased from 0.5 to 20 mmol . 1-1. A Michaelis--Menten analysis based on a single entry system indicated an apparent Km of 6.4 +/- 0.8 mmol . 1-1 and a Vmax of 1719 +/- 94 nmol . min-1g.-1 6. Since the fenestrated capillaries in the salivary gland are readily permeable to the test amino acid and D-mannitol, it is most probable that the amino acid carriers are located in the basolateral side of the epithelium. 7. The use of a paired-tracer dilution technique to measure uptake in a single circulatory passage has enabled a detailed characterization of neutral amino acid transport in the salivary gland and has overcome the limitation of previous studies based on solute transfer from blood to saliva.