Delineation of sodium-stimulated amino acid transport pathways in rabbit kidney brush border vesicles

A Non-Targeted Capillary Electrophoresis-Mass Spectrometry Strategy to Study Metabolic Differences in an In vitro Model of High-Glucose Induced Changes in Human Proximal Tubular HK-2 Cells

Diabetic nephropathy is characterized by the chronic loss of kidney function due to high glucose renal levels. HK-2 proximal tubular cells are good candidates to study this disease. The aim of this work was to study an in vitro model of high glucose-induced metabolic alterations in HK-2 cells to contribute to the pathogenesis of this diabetic complication. An untargeted metabolomics strategy based on CE-MS was developed to find metabolites affected under high glucose conditions. Intracellular and extracellular fluids from HK-2 cells treated with 25 mM glucose (high glucose group), with 5.5 mM glucose (normal glucose group), and with 5.5 mM glucose and 19.5 mM mannitol (osmotic control group) were analyzed. The main changes induced by high glucose were found in the extracellular medium where increased levels of four amino acids were detected. Three of them (alanine, proline, and glutamic acid) were exported from HK-2 cells to the extracellular medium. Other affected metabolites include Amadori products and cysteine, which are more likely cause and consequence, respectively, of the oxidative stress induced by high glucose in HK-2 cells. The developed CE-MS platform provides valuable insight into high glucose-induced metabolic alterations in proximal tubular cells and allows identifying discriminative molecules of diabetic nephropathy.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Mechanism and putative structure of B(0)-like neutral amino acid transporters

The Na(+)-dependent transport of neutral amino acids in epithelial cells and neurons is mediated by B(0)-type neutral amino acid transporters. Two B(0)-type amino acid transporters have been identified in the neurotransmitter transporter family SLC6, namely B(0)AT1 (SLC6A19) and B(0)AT2 (SLC6A15). In contrast to other members of this family, B(0)-like transporters are chloride-independent. B(0)AT1 and B(0)AT2 preferentially bind the substrate prior to the Na(+)-ion. The Na(+)-concentration affects the K ( m ) of the substrate and vice versa. A kinetic scheme is proposed that is consistent with the experimental data. An overlapping binding site of substrate and cosubstrate has been demonstrated in the bacterial orthologue LeuT( Aa ) from Aquifex aeolicus, which elegantly explains the mutual effect of substrate and cosubstrate on each other's K ( m )-value. LeuT( Aa ) is sequence-related to transporters of the SLC6 family, allowing homology modeling of B(0)-like transporters along its structure.

Deciphering the mechanisms of intestinal imino (and amino) acid transport: The redemption of SLC36A1

The absorption of zwitterionic imino and amino acids, and related drugs, is an essential function of the small intestinal epithelium. This review focuses on the physiological roles of transporters recently identified at the molecular level, in particular SLC36A1, by identifying how they relate to the classical epithelial imino and amino acid transporters characterised in mammalian small intestine in the 1960s-1990s. SLC36A1 transports a number of D- and L-imino and amino acids, beta- and gamma-amino acids and orally-active neuromodulatory and antibacterial agents. SLC36A1 (or PAT1) functions as a proton-coupled imino and amino acid symporter in cooperation with the Na+/H+ exchanger NHE3 (SLC9A3) to produce the imino acid carrier identified in rat small intestine in the 1960s but subsequently ignored because of confusion with the IMINO transporter. However, it is the sodium/imino and amino acid cotransporter SLC6A20 which corresponds to the betaine carrier (identified in hamster, 1960s) and IMINO transporter (identified in rabbit and guinea pig, 1980s). This review summarises evidence for expression of SLC36A1 and SLC6A20 in human small intestine, highlights the differences in functional characteristics of the imino acid carrier and IMINO transporter, and explains the confusion surrounding these two distinct transport systems.

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

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

Steady-state kinetic characterization of the mouse B(0)AT1 sodium-dependent neutral amino acid transporter

The members of the neurotransmitter transporter family SLC6A exhibit a high degree of structural homology; however differences arise in many aspects of their transport mechanisms. In this study we report that mouse B(0)AT1 (mouse Slc6a19) mediates the electrogenic transport of a broad range of neutral amino acids but not of the chemically similar substrates transported by other SLC6A family members. Cotransport of L: -Leu and Na(+) generates a saturable, reversible, inward current with Michaelis-Menten kinetics (Hill coefficient approximately 1) yielding a K(0.5) for L: -Leu of 1.16 mM and for Na(+) of 16 mM at a holding potential of -50 mV. Changing the membrane voltage influences both substrate binding and substrate translocation. Li(+) can substitute partially for Na(+) in the generation of L: -Leu-evoked inward currents, whereas both Cl(-) and H(+) concentrations influence its magnitude. The simultaneous measurement of charge translocation and L: -Leu uptake in the same cell indicates that B(0)AT1 transports one Na(+) per neutral amino acid. This appears to be accomplished by an ordered, simultaneous mechanism, with the amino acid binding prior to the Na(+), followed by the simultaneous translocation of both co-substrates across the plasma membrane. From this kinetic analysis, we conclude that the relatively constant [Na(+)] along the renal proximal tubule both drives the uptake of neutral amino acids via B(0)AT1 thermodynamically and ensures that, upon binding, these are translocated efficiently into the cell.

Effects of pH changes on systems ASC and B in rabbit ileum

Influx of D-aspartate (D-Asp), L-glutamate (L-Glu), and serine (Ser) across the brush-border membrane of the intact mucosa from rabbit ileum has been examined. L-Glu influx is chloride independent and completely sodium dependent. D-Asp and L-Glu share a transport system with a maximum transport rate of 1 micromol. cm-2. h-1 and an apparent affinity constant (K1/2) of approximately 0.3 mM. The function of this transport system is pH insensitive between pH 5.65 and 8.2, and bipolar amino acids do not affect the way in which the transport system handles D-Asp and L-Glu. The characteristics of this transport system match those of system X-AG. L-Glu and Ser share a transporter for which the inhibitor constant (Ki) of L-Glu against Ser decreases from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum rate of transport remains unaffected at approximately 10 micromol. cm-2. h-1. The Ki values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive contribution to Ser influx (0.8 micromol. cm-2. h-1 at 1 mM Ser) are the same at both pH values. The L-Glu-sensitive transport of Ser together with the contribution of system bo,+ account for approximately 50% of Ser influx at pH 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by system B is reduced by >95% at pH 5.65. At pH 7. 2 Ki of Ser against transport of leucine (Leu) by system B is 18 mM and Ki of Leu against transport of Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitive transport of Ser are performed by an equivalent of system ASC. Supplementary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small intestine.

A new prenylated flavone from Artocarpus champeden inhibits the K(+)-dependent amino acid transport in Bombyx mori midgut

The effect of some flavonoids on the K(+)-dependent and K(+)-independent leucine uptake into brush border membrane vesicles from Bombyx mori larval midgut was investigated. Among the compounds tested, cyclochampedol, recently purified from Artocarpus champeden, was able to inhibit in micromolar range the leucine transport. The inhibition occurred both in the absence and in the presence of potassium and was not affected by leucine concentration. The apparent Ki was 0.25 mM. Cyclochampedol represents the first non-competitive inhibitor of an amino acid transport system in Lepidoptera. The relevance of this result is discussed.

Effect of a lysine-enriched diet on L-lysine transport by the brush-border membrane of the chicken jejunum

The influx of L-lysine into apical vesicles from the chicken jejunum occurs through two systems, one with low Michaelis constant (K(m)) and features of system b0,+ and the other with relatively high K(m) for L-lysine and with properties of system y+. In the present study the effect of a lysine-enriched diet (Lys, containing 68 g L-lysine/kg dietary protein, control animals 48 g/kg) on L-lysine uptake through both transport systems was investigated. Results show that 1) lysine enrichment had no effect on either body weight or the efficiency of food utilization. 2) In Lys-fed animals, the mediated L-lysine influx was best fitted to the two-system model with y+ and b0,+ activity. 3) In the presence of an Na+ gradient, total L-lysine uptake is significantly higher in Lys-fed animals than in control birds (about 40% increase). 4) Lys diet increases K(m)b0,+ 6-fold (KSCN gradient) and 12-fold (NaSCN gradient) and maximum velocity (Vmax) by 6- and 20-fold, respectively. The effects of Lys enrichment on the y(+)-like system are only observed on the Vmax and in the presence of a Na+ gradient (30% increase). 5) Na+ is involved in the activation of the transport process in the Lys-fed chickens, but there is no correlation between external Na+ concentration and L-lysine influx. In conclusion, both b(0,+)- and y(+)-like transport systems are upregulated by dietary lysine but with different kinetic profiles; the high-capacity y(+)-like carrier shows a Vmax increase without changes in K(m), whereas the low-capacity b(0,+)-like system shows an increase in Vmax as well as in the K(m).

A candidate mouse model for Hartnup disorder deficient in neutral amino acid transport

The mutant mouse strain HPH2 (hyperphenylalaninemia) was isolated after N-ethyl-N-nitrosourea (ENU) mutagenesis on the basis of delayed plasma clearance of an injected load of phenylalanine. Animals homozygous for the recessive hph2 mutation excrete elevated concentrations of many of the neutral amino acids in the urine, while plasma concentrations of these amino acids are normal. In contrast, mutant homozygotes excrete normal levels of glucose and phosphorus. These data suggest an amino acid transport defect in the mutant, confirmed in a small reduction in normalized values of 14C-labeled glutamine uptake by kidney cortex brush border membrane vesicles (BBMV). The hyperaminoaciduria pattern is very similar to that of Hartnup Disorder cases also show niacin deficiency symptoms, of Hartnup Disorder cases also show niacin deficiency symptoms, which are thought to be multifactorially determined. Similarly, the HPH2 mouse exhibits a niacin-reversible syndrome that is modified by diet and by genetic background. Thus, HPH2 provides a candidate mouse model for the study of Hartnup Disorder, an amino acid transport deficiency and a multifactorial disease in the human.

Allosteric regulation of [3H]vinblastine binding to P-glycoprotein of MCF-7 ADR cells by dexniguldipine

Plasma membranes were prepared from the P-glycoprotein expressing human breast cancer cell line MCF-7 ADR. [3H]Vinblastine bound to these membranes saturably with a Bmax of 24 pmol/mg of protein and KD of 23 nM. In contrast, membranes from the parent cells MCF-7 WT, which do not express P-glycoprotein, did not bind [3H]vinblastine with high affinity. Cytotoxics known to be transported by P-glycoprotein inhibited the binding of [3H]vinblastine, as did multidrug reversing agents including the 1,4-dihydropyridine, dexniguldipine-HCl (Ki, 15 nM). In dissociation kinetic experiments, dexniguldipine-HCl accelerated the dissociation of [3H]vinblastine from P-glycoprotein, indicating a negative heterotropic allosteric mechanism of action through a drug binding site distinct from that of vinblastine. Other 1,4-dihydropyridines tested also accelerated [3H]vinblastine dissociation from P-glycoprotein, however, multidrug reversing drugs of different chemical classes, including quinidine, verapamil and cyclosporin A did not. These results suggest that P-glycoprotein of MCF-7 ADR cell membranes possesses at least two drug acceptor sites which are allosterically coupled: receptor site-1 which binds vinca alkaloids, and receptor site-2 which binds 1,4-dihydropyridines such as dexniguldipine-HCl, which had the highest affinity of the tested derivatives.

H(+)-coupled alpha-methylaminoisobutyric acid transport in human intestinal Caco-2 cells

Transepithelial apical-to-basal transport and cellular uptake of the non-metabolisable amino acid alpha-methylaminoisobutyric acid (MeAIB) across confluent monolayers of the human intestinal epithelial cell line Caco-2 are enhanced by a transepithelial pH gradient (apical pH 6.0, basolateral pH 7.4). In Na(+)-free conditions (apical pH 7.4, basolateral pH 7.4), net absorption (120 +/- 58 pmol/cm2 per h, n = 13) and uptake across the apical membrane (cell/medium ratio 0.56 +/- 0.06, n = 13) are low. However, in Na(+)-free conditions with apical pH 6.0, net absorption (685 +/- 95 pmol/cm2 per h, n = 15) and intracellular accumulation (cell/medium ratio 3.63 +/- 0.29, n = 14) were marked. Continuous monitoring of intracellular pH (pHi) in BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein)-loaded Caco-2 cell monolayers indicated that apical addition of MeAIB (20 mM) was associated with H(+)-flow across the apical membrane in both Na+ and Na(+)-free conditions. This transport process is rheogenic in Na(+)-free media, stimulating an inward short-circuit current in voltage-clamped Caco-2 cell monolayers. On the basis of competition for MeAIB accumulation and pHi experiments, L-proline, glycine, L-alanine and beta-alanine are also substrates for H(+)-linked transport at the apical membrane of Caco-2 cells but L-valine, L-leucine and L-phenylalanine are not. These data are consistent with the expression, in the apical brush-border membrane of Caco-2 cells, of a H(+)-coupled, Na(+)-independent MeAIB carrier.

Poly(A)+ RNA from the mucosa of rat jejunum induces novel Na(+)-dependent and Na(+)-independent leucine transport activities in in oocytes of Xenopus laevis

Complementary DNA clones have been isolated recently from rat (D2) and rabbit kidney (rBAT) which induce increased Na(+)-independent Leu and Lys transport activities (System b0, +) when expressed in oocytes of Xenopus laevis. These cDNAs encode type II membrane glycoproteins which show significant homology to the heavy chain of the human and mouse 4F2 surface antigen (4F2hc). Injection of human 4F2hc cRNA into oocytes also results in induction of Leu/Lys transport activity, but with differing cation requirements for the two amino acids (Na(+)-dependent for Leu, Na(+)-independent for Lys: system y+L). System y+L is a newly discovered zwitterionic/cationic amino acid transporter first described in human erythrocytes. Here we have examined the characteristics of Leu transport in Xenopus oocytes microinjected with mRNA from the mucosa of rat jejunum. L-Leu uptake during 10 min (0.2 mM, 20 degrees C) reached 20 pmol/oocyte compared with endogenous uptake by water-injected oocytes of typically 3-4 pmol/oocyte. The expressed transport activity was 80% Na(+)-dependent. The Na(+)-dependent component of the expressed flux was saturable (Km app 0.20 mM) and inhibited by Lys, but not by Ala or Phe. The minor Na(+)-independent component of expressed Leu transport activity was also saturable (Km app 0.10 mM). Amino acid inhibition studies resolved this flux into two main components, one of which was inhibited by Lys, Ala and Phe and another which was only inhibited by Lys. There was a small residual component of Na(+)-independent Leu transport which was insensitive to inhibition by Lys. Experiments utilizing polymerase chain reaction (PCR) demonstrated the presence of both D2 and 4F2hc message in rat jejunum. Hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to D2 had no effect on the expression of Na(+)-linked Leu transport activity, but reduced the smaller Na(+)-independent component of Leu transport by 40%, suggesting only a minor role of D2 in the expression of rat intestinal Leu transport activity. Although the properties of Na(+)-dependent Leu transport were, with the exception of a lack of inhibition by Ala and Phe, consistent with erythrocyte y+L, hybrid-depletion of jejunal mRNA with an antisense oligonucleotide complementary to 4F2hc had no detectable effect on the expressed transport activity. We conclude, therefore, that mRNA from rat jejunum encodes novel Na(+)-dependent and Na(+)-independent transport activities unrelated to the D2/4F2hc glycoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Complex subcellular distribution of sodium-dependent amino acid transport systems in kidney cortex and LLC-PK1/Cl4 cells

To characterize the amino acid transport system in basolateral membranes and to test for possible intracellular loci of amino acid transport activity, we surveyed the distribution of L-alanine transport activity in rabbit proximal tubular cells and LLC-PK1/Cl4 cells. A three-dimensional separation procedure based on differential sedimentation, density gradient centrifugation, and counter-current distribution resolved 21 physically and biochemically distinct membrane populations from rabbit cortex. Inhibition of L-alanine transport by phenylalanine and N-(methylamino)isobutyric acid was used to delineate parallel amino acid transport pathways. Population n was identified as brush border membranes by virtue of its 16-fold maltase enrichment; 94% of its Na(+)-dependent alanine transport activity was mediated by systems previously shown to be characteristic of brush border membranes. Two populations, c' and c", which accounted for 25% of the total Na,K-ATPase activity, were identified as basalateral membranes on the basis of Na,K-ATPase cumulative enrichment factors of 15 and 21; 82% of the total alanine transport in these populations was mediated by a Na(+)-independent system similar to the classical system L. Na,K-ATPase, Na(+)-independent and Na(+)-dependent alanine transport activities were associated with intracellular membrane populations as well as with the plasma membranes. The major intracellular locus of Na,K-ATPase activity, population i accounted for roughly 31% of the Na,K-ATPase, maximally enriched ninefold; it contained 29% of the total system L transport activity. Population l, which was identified as endoplasmic reticulum because it was the major locus of membrane-bound NADPH cytochrome c reductase activity, contained 44% of the total system A transport. Three distinct Golgi-derived populations, m', m", and o, accounted for 39% of the total system A transport. A survey of the amino acid transport systems in LLC-PK1/Cl4 cells showed that the majority of system A-mediated amino acid transport was present in membranes of intracellular and possibly apical origin. The presence of large intracellular pools of amino acid transport activities might reflect newly synthesized transport proteins, ongoing membrane recycling or, perhaps, intracellular reserves available for rapid recruitment to the plasma membrane.

Ultrastructural localization of a neutral and basic amino acid transporter in rat kidney and intestine

A sodium-independent neutral and basic amino acid transporter (NBAT) from rat kidney was recently cloned and its amino acid sequence deduced. We used light and electron microscopic immunoperoxidase labeling to determine the cellular localization of NBAT in rat kidney and small intestine. The localization was carried out using site-directed antisera raised against synthetic peptides within NBAT. The most prominent localization of NBAT was in microvilli of epithelial cells lining renal proximal tubules. Microvilli of small intestinal epithelia were less frequently immunoreactive. Unexpectedly, the most intense labeling in the small intestine was seen within enteroendocrine cells and submucosal neurons. The neuronal labeling was highly localized within dense core vesicles in axon terminals apposed to the basal lamina near fenestrated blood vessels. These results support the proposal that NBAT plays a role in reabsorption of amino acids in renal tubules. In addition, they suggest that NBAT (or NBAT-like proteins) may have multiple functions in the small intestine, including luminal uptake of amino acids and vesicular uptake of related substrates into enteroendocrine cells and enteric neurons.

Molecular cloning and expression of a high affinity L-proline transporter expressed in putative glutamatergic pathways of rat brain

We have used the polymerase chain reaction (PCR) with degenerate oligonucleotides derived from two conserved regions of the norepinephrine and gamma-aminobutyric acid transporters to identify novel Na(+)-dependent transporters in rat brain. One PCR product hybridized to a 4.0 kb RNA concentrated in subpopulations of putative glutamatergic neurons including mitral cells of the olfactory bulb, pyramidal cells of layer V of the cerebral cortex, pyramidal cells of the piriform cortex, and pyramidal cells of field CA3 of the hippocampus. Transient expression of the cognate cDNA conferred Na(+)-dependent L-proline uptake in HeLa cells that was saturable (Km = 9.7 microM) and exhibited a pharmacological profile similar to that for high affinity L-proline transport in rat brain slices. The cloned transporter cDNA predicts a 637 aa protein with 12 putative transmembrane domains and exhibits 44%-45% amino acid sequence identity with other members of the emerging family of neurotransmitter transporters. These findings support a synaptic role for L-proline in specific excitatory pathways in the CNS.

Cloning of a rat kidney cDNA that stimulates dibasic and neutral amino acid transport and has sequence similarity to glucosidases

The transport of amino acids across cell membranes is believed to be mediated by integral membrane proteins with distinct substrate specificities. Using expression cloning in Xenopus oocytes and assaying for the uptake of 14C-labeled cystine, we isolated a 2.3-kilobase cDNA (D2) from a rat kidney library. D2 is expressed specifically in kidney and intestine and induces the transport of both neutral and cationic amino acids. The deduced amino acid sequence predicts a 78-kDa protein with a single transmembrane domain, a structure not typical of the known membrane transport proteins, which generally have multiple membrane-spanning regions. The putative extracellular region is highly similar to the 4F2 heavy-chain cell surface antigen and to a family of alpha-glucosidases, which raises the possibility that D2 encodes a transport activator or regulatory subunit.

Expression cloning of a cDNA from rabbit kidney cortex that induces a single transport system for cystine and dibasic and neutral amino acids

We have isolated a cDNA clone by screening a rabbit kidney cortex cDNA library for expression of sodium-independent transport of L-arginine and L-alanine in Xenopus laevis oocytes. Expressed uptake relates to a single component of sodium-independent transport for dibasic and neutral amino acids. This transport activity resembles the functionally defined system b0,+ and carries cystine and dibasic amino acids with high affinity. The rBAT (b0,+ amino acid transporter-related) mRNA is found mainly in kidney and intestinal mucosa. It encodes a predicted 77.8-kDa protein with only one putative transmembrane domain and seven potential N-glycosylation sites. This protein could either be a constitutive element or a specific activator of system b0,+.

Expression of Na(+)-independent amino acid transport in Xenopus laevis oocytes by injection of rabbit kidney cortex mRNA

Poly(A)+ mRNA was isolated from rabbit kidney cortex and injected into Xenopus laevis oocytes. Injection of mRNA resulted in a time- and dose-dependent increase in Na(+)-independent uptake of L-[3H]alanine and L-[3H]arginine. L-Alanine uptake was stimulated about 3-fold and L-arginine uptake was stimulated about 8-fold after injection of mRNA (25-50 ng, after 3-6 days) as compared with water-injected oocytes. T.I.C. of oocyte extracts suggested that the increased uptake actually represented an increase in the oocyte content of labelled L-alanine and L-arginine. The expressed L-alanine uptake, obtained by subtracting the uptake in water-injected oocytes from that in mRNA-injected oocytes, showed saturability and was inhibited completely by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) and L-arginine. The expressed L-arginine uptake in mRNA-injected oocytes also showed saturability, being completely inhibited by L-dibasic amino acids) and partially inhibited by BCH. Expression of both L-alanine and L-arginine uptake showed clear cis-inhibition by cationic (e.g. L-arginine) and neutral (e.g. L-leucine) amino acids. In all, this points to the expression of a Na(+)-independent transport system with broad specificity (i.e. b degree, (+)-like). In addition, part of the expressed uptake of L-arginine could be due to a system y(+)-like transporter. After size fractionation through a sucrose density gradient, the mRNA species encoding these increased transport activities (Na(+)-independent transport of L-alanine and of L-arginine) were found in fractions of an average mRNA chain-length of 1.8-2.4 kb. On the basis of these results, we conclude that Na(+)-independent transport system(s) for L-alanine and L-arginine from rabbit renal cortical tissues, most likely proximal tubules, are expressed in Xenopus laevis oocytes. These observations may represent the first steps towards expression and cloning of these transport pathways.

Sodium-alanine cotransport in renal proximal tubule cells investigated by whole-cell current recording

Sodium-alanine cotransport was investigated in single isolated proximal tubule cells from rabbit kidney with the whole-cell current recording technique. Addition of L-alanine at the extracellular side induced an inward-directed sodium current and a cell depolarization. The sodium-alanine cotransport current was stereospecific and sodium dependent. Competition experiments suggested a common cotransport system for L-alanine and L-phenylalanine. Sodium-alanine cotransport current followed simple Michaelis-Menten kinetics, with an apparent Km of 6.6 mM alanine and 11.6 mM sodium and a maximal cotransport current of 0.98 pA/pF at -60 mV clamp potential. Hill plots of cotransport current suggested a potential-independent coupling ratio of one sodium and one alanine. The apparent Km for sodium and the maximal cotransport current were potential dependent, whereas the apparent Km for L-alanine was not affected by transmembrane potential. The increase in Km for alanine with decreasing inward-directed sodium gradients suggested a simultaneous transport mechanism. These results are consistent with a cotransport model with potential-dependent binding or unbinding of sodium (high-field access channel) and a potential-dependent translocation step.

Molecular sizes of amino acid transporters in the luminal membrane from the kidney cortex, estimated by the radiation-inactivation method

Renal brush-border membrane vesicles from rat kidney cortex were irradiated in frozen state with a gamma-radiation source. Initial rates of influx into these vesicles were estimated for substrates such as L-glutamic acid, L-alanine, L-proline and L-leucine to establish the molecular sizes of their carriers. Transport was measured in initial-rate conditions to avoid artifacts arising from a decrease in the driving force caused by a modification of membrane permeability. Initial rates of Na(+)-independent uptakes for those four substrates appeared unaffected in the dose range used (0-6 Mrad), indicating that the passive permeability of the membrane towards these substrates was unaffected. However, at higher doses of irradiation the Na+ influx and the intravesicular volume evaluated by the uptake of glucose at equilibrium were altered by radiation. Thus Na(+)-dependent influx values were corrected for volume changes, and the corrected values were used to compute radiation-inactivation sizes of the transport systems. Their respective values for L-glutamic acid, L-proline, L-leucine and L-alanine carriers were 250, 224, 293 and 274 kDa. The presence of the free-radicals scavenger benzoic acid in the frozen samples during irradiation did not affect the uptake of glucose, phosphate and alkaline phosphatase activity. These results indicate that freezing samples in a cryoprotective medium was enough to prevent secondary inactivation of transporters by free radicals. Uptakes of beta-alanine and L-lysine were much less affected by radiation. The radiation-inactivation size of the Na(+)-dependent beta-alanine carrier was 127 kDa and that of the L-lysine carrier was 90 kDa.

Characterization of amino acid transport systems in human placental basal membrane vesicles

The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na(+)-independent and Na(+)-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. L-Glutamate and alpha-(methylamino)isobutyrate were transported only Na(+)-independently, while the transport of L-alanine was dependent solely on an Na+ gradient from the outside to the inside of the vesicles. L-Methionine, L-leucine, glycine and L-proline transport were supported by both Na(+)-independent and Na(+)-dependent systems. L-Lysine transport was decreased in the presence of cations, an inwardly directed Na+ gradient was much more effective than a K+ gradient at slowing L-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na(+)-independent and five Na(+)-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na(+)-independent system interacts with all substrates tested. Another Na(+)-independent system carries glycine, L-methionine, L-leucine and L-lysine; it is sensitive to L-glutamate, but not to L-proline or alpha-(methylamino)isobutyrate. The third system is selective for L-lysine, which is inhibited by L-methionine, glycine and L-leucine, but inaccessible to L-glutamate, L-proline and alpha-(methylamino)isobutyrate. One Na(+)-dependent system carries L-alanine, glycine, L-methionine and L-leucine, and it is sensitive to L-proline. The second system mediates transport of L-alanine, glycine, L-methionine and L-proline, but is not sensitive to L-leucine. The third system carries L-alanine, glycine and L-proline, and is inaccessible to L-methionine and L-leucine. The fourth system is responsible for L-methionine and L-leucine; it is sensitive to L-alanine and glycine, but not to L-proline. The fifth system is selective for L-proline.

Characterization of amino-acid transport systems in guinea-pig intestinal brush-border membrane

The amino-acid transport systems have been characterized in brush-border membrane vesicles prepared from guinea-pig small intestine. Uptake of all amino acids tested was measured at the initial velocity for 5 s. L-Proline, alpha-(methylamino)isobutyrate, glycine, L-alanine and L-methionine were transported dependent solely on an Na+ gradient from the outside to the inside of the vesicles, and L-cysteine, L-phenylalanine and L-leucine were transported dependent largely on the Na+ gradient with a small fraction of Na+-independent transport. The transport of L-aspartic acid and L-lysine was independent of the Na+ gradient and L-lysine transport was somewhat inhibited by the presence of cations, including Na+, K+ and Li+. A cross-inhibition study of the uptake of these amino acids in the brush border of guinea-pig intestine revealed the presence of at least three Na+-dependent and three Na+-independent carrier-mediated systems. One Na+-dependent system interacted mainly with imino acid. Another Na+-dependent system interacted with neutral amino acids, while a third system was selective for glycine. One Na+-independent system is for acidic amino acids, another is responsible for neutral amino acids and a third for cationic amino acids. These transport systems of amino acids in guinea-pig small intestine are compared with those in rabbit and mouse intestine.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Uptake of lysine and proline via separate alpha-neutral amino acid transport pathways in Mytilus gill brush border membranes

Brush border membrane vesicles (BBMV) were prepared from the gills of the marine mussel, Mytilus edulis. These membranes contained two distinct pathways for cotransport of Na+ and alpha-neutral amino acids. The major pathway in mussel gill BBMV was the alanine-lysine (AK) pathway, which had a high affinity for alanine and for the cationic amino acid, lysine. The AK pathway was inhibited by nonpolar alpha-neutral amino acids and cationic amino acids, but was not affected by beta-neutral amino acids or imino acids. The kinetics of lysine transport were consistent with a single saturable process, with a Jmax of 550 pmol/mg-min and a Kt of 5 microM. The AK pathway did not have a strict requirement for Na+, and concentrative transport of lysine was seen in the presence of inwardly directed gradients of Li+ and K+, as well as Na+. Harmaline inhibited the transport of lysine in solutions containing either Na+ or K+. The alanine-proline (AP) pathway transported both alanine and proline in mussel gill BBMV. The AP pathway was strongly inhibited by nonpolar alpha-neutral amino acids, proline, and alpha-(methylamino)isobutyric acid (Me-AIB). The kinetics of proline transport were described by a single saturable process, with a Jmax of 180 pmol/mg-min and Kt of 4 microM. In contrast to the AK pathway, the AP pathway appeared to have a strict requirement for Na+. Na+-activation experiments with lysine and proline revealed sigmoid kinetics, indicating that multiple Na+ ions are involved in the transport of these substrates. The transport of both lysine and proline was affected by membrane potential in a manner consistent with electrogenic transport.

Alanine transport systems in isolated basal plasma membrane of human placenta

Concentrative transfer of amino acids from mother to fetus is affected by transport across both microvillous (maternal-facing) and basal (fetal-facing) plasma membranes of the human placental syncytiotrophoblast. Isolated basal plasma membrane vesicles were used to elucidate transport systems for neutral amino acids across this membrane. The concentration dependence and inhibition of zero-trans-alanine uptake were studied and four pathways for alanine uptake were defined as follows: 1) a sodium-dependent system shared by methylaminoisobutyric acid, which has the characteristics of an A system; 2) a sodium-dependent system resistant to inhibition by methylaminoisobutyric acid, which has the characteristics of an ASC system; 3) a sodium-independent system which may resemble an L system; 4) nonsaturable uptake. The microvillous membrane of the syncytiotrophoblast possesses systems similar to 1 and 3, but system 2 is unique to the basal plasma membrane. Active and passive transport of amino acids across both microvillous and basal plasma membranes may contribute to trophoblast amino acid uptake and nutrition and to the transfer of amino acids to the fetus.

Basolateral amino acid and glucose transport by the intestine of the teleost, Anguilla anguilla

1. D-glucose transport into BLMV was osmotically reactive, sodium independent, and inhibited by phloretin but not by phloridzin. 2. The survey of 6 L-amino acids identified three groups with respect to transfer across the basolateral cell border. Transport of proline and glutamate occurred by Na-dependent carriers and by apparent simple diffusion. Alanine, lysine and phenylalanine were transported by Na-independent carriers and apparent simple diffusion. Glycine transport was stimulated above apparent simple diffusion only by a simultaneous inwardly-directed Na gradient and outwardly-directed K gradient. 3. Only proline and glutamate demonstrated the ability to depolarize the membrane potential, consistent with Na-dependent rheogenic transport.

Characterization of amino acid transport systems in human placental brush-border membrane vesicles

Brush-border microvillous plasma membrane vesicles were prepared from human full-term placental syncytiotrophoblasts and purified 33-fold from the homogenate with reference to a membrane marker enzyme, alkaline phosphatase (EC 3.1.3.1). Transport of alpha-(methylamino)isobutyrate by the membrane vesicles was stimulated in the presence of an Na+ gradient from the outside to the inside of the vesicles. The initial rate of uptake in a 10-s period was enhanced with increasing concentration of Na+ in the external medium. The level of alpha-(methylamino)isobutyrate transported into the vesicles reached a maximum 1 min after the start of incubation at 37 degrees C, and then decreased with time due to efflux. Extrapolation to infinite medium osmolarity showed no uptake, indicating transport of alpha-(methylamino)isobutyrate into membrane vesicles. The initial rate of uptake was dependent on temperature and pH: the highest rate occurred at 37 degrees C and the optimal pH was 8.0. When the alpha-(methylamino)isobutyrate concentration was varied, the initial rate of uptake dependent on an Na+ gradient (out greater than in) obeyed Michaelis-Menten kinetics with Km and Vmax values of 1.07 mM and 3.23 nmol/10 s per mg of protein, respectively. Cross-inhibition patterns indicated that at least three Na+-dependent and two Na+-independent carrier-mediated pathways existed in the human placental brush border. One Na+-dependent pathway interacted with all substrates tested. Another Na+-dependent route interacted with L-proline, alpha-(methylamino)isobutyrate, and L-methionine, while a third pathway was selective for L-methionine. One Na+-independent pathway was selective for L-cysteine, while the other pathway interacted with all substrates tested.

A rapid method for the reconstitution of Na+-dependent neutral amino acid transport from bovine renal brush-border membranes

1. A simple and rapid method for the reconstitution of Na+-dependent neutral amino acid transport activity from bovine renal brush border membranes is described. 2. The neutral detergent decanoyl-N-methylglucamide ('MEGA-10') was employed to solubilize the membrane protein. This obviated the necessity for a prolonged dialysis step. 3. The properties of amino acid transport in these vesicles were similar to those observed in native membranes. 4. This should be a useful procedure in the eventual identification and isolation of amino acid transport proteins.

Evidence for a single common Na+-dependent transport system for alanine, glutamine, leucine and phenylalanine in brush-border membrane vesicles from bovine kidney

The characteristics of the Na+-dependent transport of alanine, glutamine, leucine and phenylalanine were studied in bovine renal brush-border membrane vesicles. Inhibition of the transport of any one of these amino acids by any other was mutually competitive. The Ki value for the inhibition of alanine transport by leucine was similar to the Km for leucine transport; similar interrelationships existed for the other amino acids. Each amino acid was shown to exchange with each of the other amino acids across the membrane. From these and other results it is concluded that the Na+-dependent transport of these four amino acids is catalysed by a single common transport system.

Inhibition of L-alanine uptake into bovine jejunal brush border membrane vesicles by L-amino acids

Jejunal epithelial cells from slaughtered Holstein cows were fractionated to obtain purified brush border membranes from which membrane vesicles were prepared for use in amino acid uptake studies. Uptake of alanine was determined by incubation of vesicles with a solution containing radiolabelled alanine, isolation of vesicles and accumulated alanine by filtration, and detection of accumulated alanine by liquid scintillation counting. Uptake studies were conducted under conditions shown to provide linear rates of accumulation. Sodium-dependent active transport was determined as the difference between uptake measured in the presence and absence of sodium in the extravesicular buffer. Inhibition of alanine uptake increased with increasing extra-vesicular inhibitor concentration until a plateau value was reached. Inhibition of sodium-dependent alanine uptake by 100 mM glycine was 72%; 25 mM isoleucine, valine, or methionine completely inhibited initial alanine uptake. These results indicate the existence of at least two sodium-dependent transport systems, one capable and one incapable of accepting glycine for transport. At concentrations designed to represent expected concentrations of free amino acids in intestinal digesta, several equimolar mixtures (.2 to 5 mM) of 20 amino acids inhibited alanine uptake, suggesting that significant interaction among amino acids for uptake may be occurring under in vivo conditions.

Effects of succinylacetone on amino acid uptake in the rat kidney

Infants with hereditary tyrosinemia also have a renal Fanconi syndrome and excrete succinylacetone (SA). We have studied the effects of SA on rat renal tubular amino acid transport in vivo and in vitro using isolated renal tubules. Injection of SA produces increased clearance of several amino acids in the intact animal. In vitro SA causes a reversible inhibition of alpha-aminoisobutyric acid uptake, resulting from depressed low- and high-affinity transport systems. Addition of glutamate, succinate, or glucose, alone or in combination, did not restore transport. These observations suggest the usefulness of SA in the production of a physiologic animal model for the study of the human Fanconi syndrome.

Neutral amino acid influx in developing rabbit blastocysts

The influx of the neutral amino acids glycine, aminoisobutyric acid (AIB), and leucine into rabbit blastocysts was measured. In day 6 postcoitus (pc) embryos, glycine influx was Na+ independent, whereas AIB and leucine influx involved both Na+-dependent and independent components. From days 5 to 7 pc, the leucine and AIB influx remained constant, although the Na+-dependent fraction decreased and the Na+-independent fraction increased with age. None of the Na+-independent influx was inhibited by methylaminoisobutyric acid (MeAIB), an amino acid analogue specific for the system A of neutral amino acid uptake. In addition, MeAIB influx was Na+ independent, implying that system A is not involved in leucine or AIB uptake. All Na+-dependent influx is thus considered to occur via system ASC. System L contributed only to the influx of leucine at days 6 and 7 pc, as measured by inhibition of Na+-independent influx by 2-amino-bicyclo-(2,2,1)-heptane-2-carboxylic acid.

Cationic amino acid transport by two renal epithelial cell lines: LLC-PK1 and MDCK cells

LLC-PK1 and MDCK cells take up cationic amino acids (lysine and arginine) by a specific sodium independent transport system. Uptake is inhibited by ornithine in LLC-PK1 and MDCK cells either in the presence or absence of sodium and by glutamine or homoserine in MDCK cells in the presence of sodium. Trans-stimulation of uptake occurs in the presence of intracellular cationic amino acids. Experiments with valinomycin or with different extracellular potassium concentrations suggest that uptake is dependent on the membrane potential of these cells. These transport features are similar to those previously ascribed to a transport system denominated y+ in other cells. Further experiments suggested that this carrier system is localised to the basolateral membrane in each cell type.