Relations in the Transport of β-Alanine and the α-Amino Acids in the Ehrlich Cell

Characterization of a sodium-dependent taurine transporter in rabbit choroid plexus

Taurine, a beta-amino acid, plays an important role as a neuromodulator and is necessary for the normal development of the brain. Since de novo synthesis of taurine in the brain is minimal and in vivo studies suggest that taurine does not cross the blood-brain barrier, we examined whether the choroid plexus, the blood-cerebrospinal fluid barrier, plays a role in taurine transport in the central nervous system. The uptake of [3H]taurine into ATP-depleted choroid plexus from rabbit was substantially greater in the presence of an inwardly directed Na+ gradient, whereas in the absence of a Na+ gradient taurine accumulation was negligible. A transient inside-negative potential gradient enhanced the Na(+)-driven uptake of taurine into the tissue slices, suggesting that the transport process is electrogenic. Na(+)-driven taurine uptake was saturable with an estimated Vmax of 111 +/- 20.2 nmol/g per 15 min and a Km of 99.8 +/- 29.9 microM. The estimated coupling ratio of Na+ and taurine was 1.80 +/- 0.122. Na(+)-dependent taurine uptake was significantly inhibited by beta-amino acids, but not by alpha-amino acids, indicating that the transporter is selective for beta-amino acids. Na(+)-dependent taurine uptake showed some selectivity for anions: the accumulation was comparable in the presence of Cl-, Br- and thiocyanate whereas I-, SO4(2-) and gluconate did not stimulate the uptake significantly. Collectively, our results demonstrate that taurine is transported in the choroid plexus via a Na(+)-dependent, saturable and apparently beta-amino acid selective mechanism. This process may be functionally relevant to taurine homeostasis in the brain.

Osmotic regulation of taurine transport via system beta and novel processes in mouse preimplantation conceptuses

Taurine was shown recently to increase the frequency at which 2-cell mouse conceptuses develop into blastocysts in vitro. For this reason and because taurine helps cells adapt to external stresses, we studied transport of this and related amino acids by preimplantation mouse conceptuses. The most conspicuous component of taurine transport in conceptuses at the 1-cell through blastocyst stages of development was both Na(+)- and Cl(-)-dependent. This Na(+)- and Cl(-)-dependent transport system interacted relatively strongly with beta- but not alpha-amino acids. By these criteria, transport system beta is responsible for Na(+)-dependent taurine transport in preimplantation mouse conceptuses. Moreover, detection of mRNA encoding the taurine transport protein (TAUT) in early conceptuses supports the theory that TAUT is a major component of system beta. Transport of taurine by system beta in 1-cell conceptuses was slower in hypotonic than in hypertonic media, whereas the reverse was true for system beta in blastocysts. In contrast, hypotonically stimulated Na(+)-independent taurine transport was, of course, more rapid in hypotonic than in hypertonic media in both 1-cell conceptuses and blastocysts. Transport via this hypotonically stimulated process also showed no sign of saturation by up to 10 mM taurine. Hypotonically stimulated taurine transport appeared transiently in 1-cell conceptuses under hypotonic conditions until they had recovered their initial volumes. Hence, we suggest that a decrease in taurine uptake via system beta and an increase in taurine exodus via the Na(+)-independent, nonsaturable transport process could contribute to the regulatory volume decrease in 1-cell conceptuses in hypotonic medium. Since taurine uptake by system beta in blastocysts is, however, higher in hypotonic than in hypertonic media, taurine uptake by system beta in blastocysts might intensify a tendency to increase cell volume in hypotonic medium. Such an increase in taurine uptake could further favor anabolic changes associated with cell swelling. In addition to contributing to regulation of cellular volume and perhaps metabolism, the hypotonically stimulated Na(+)-independent transport processes in early embryos have novel characteristics. Hypotonically stimulated Na(+)-independent taurine transport was inhibited by niflumate, N-ethylmaleimide and NaN3 but not by furosemide, iodoacetate, KCN, ouabain or alpha- or beta-amino acids. Furthermore, 4,4'-diisothiocyanostilbene-2,2'-disulfonate inhibited this transport in 1-cell conceptuses but not in blastocysts. Hence, different hypotonically stimulated Na(+)-independent taurine transport processes appear to be present in 1-cell conceptuses vs. blastocysts. The functions of these and other instances of developmental regulation of expression of transport processes in preimplantation conceptuses remain largely to be elucidated.(ABSTRACT TRUNCATED AT 400 WORDS)

Is taurine a hypothalamic neurotransmitter?: A model of the differential uptake and compartmentalization of taurine by neuronal and glial cell particles from the rat hypothalamus

Although taurine has been postulated to be a neurotransmitter or neuromodulator in the mammalian CNS, little is known concerning its role in brain function. Evidence suggesting that taurine may influence endocrine and homeostatic mechanisms via the hypothalamus resulted in our investigations into its function in this brain region. The main objectives of the research were to characterize the specific binding, uptake, and release of taurine in the hypothalamus. A specific aim was to examine the proposed neurotransmitter role for taurine in the hypothalamus. This was accomplished by comparing the characteristics and properties of the binding, uptake, and release of taurine with those for the classical neurotransmitters which satisfy the criteria for a neurotransmitter. On such a comparative basis, the characteristics of taurine uptake satisfy the neurotransmitter criterion of inactivation of taurine in the hypothalamus. However, the observed characteristics of taurine binding and release in the hypothalamus do not satisfy the respective neurotransmitter criteria of specific receptors and Ca2+-dependent evoked release. Therefore, solely on the basis of the experimental observations reported herein, we must conclude that taurine apparently does not function as a neurotransmitter in the hypothalamus. Two uptake systems were found in the P2 fraction, a high affinity uptake system and a low affinity uptake system. Uptake systems for taurine have previously been reported in glial and nerve cell homogenates, and therefore, because of the known contamination of crude synaptosomal preparations with glial particles, we sought to determine the cellular origin of the two taurine uptake systems in our crude preparation. Using a variety of diverse biochemical techniques such as hypo-osmotic shock, release experiments and Arrhenius plots, we determined that physical changes of the media or depolarizing stimuli which would influence neuronal and glial cell particles differently, also had differing effects on high and low affinity taurine uptake or its release from the respective uptake compartments. We conclude that the high affinity taurine uptake system/compartment is located on/in neuronal membranes/particles/particles and that the low affinity taurine uptake system/compartment is located on/in neuronal membranes/particles and that model for the differential cellular transport and compartmentalization of taurine into neuronal and glial cells has important implications concerning its possible role in the CNS.

Further studies on amino acid transport in murine P388 leukemia cells in vitro. Presence of system y+

The transport of glycine and L-lysine into murine P388 leukemia cells has been examined. Glycine transport appears to be shared by both systems A and ASC in P388 cells. Glycine transport is Na+-dependent and is effectively blocked by alpha-(methylamino)isobutyric acid, threonine and alanine but only a marginal reduction in transport is seen with 100-fold excess cold 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid. System gly is not expressed in P388 cells. Lysine is largely transported by a Na+-independent, pH-insensitive system with a Km of 0.079 mM. Lysine transport is relatively unaffected by the addition of 100-fold excess cold alpha-(methylamino)isobutyric acid, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid and the anionic amino acids, L-glutamate and L-aspartate. A partial inhibition of lysine transport was observed with L-threonine and L-leucine while L-arginine and L-histidine radically decreased lysine transport. Lysine appears to be transported by a system similar to the system y+ seen in cultured human fibroblasts, Ehrlich ascites cells, and hepatoma cell lines.

Volume-sensitive taurine transport in fish erythrocytes

Taurine plays an important role in cell volume regulation in both vertebrates and invertebrates. Erythrocytes from two euryhaline fish species, the eel (Anguilla japonica) and the starry flounder (Platichthys stellatus) were found to contain high intracellular concentrations of this amino acid (approximately equal to 30 mmol per liter of cell water). Kinetic studies established that the cells possessed a saturable high-affinity Na+-dependent beta-amino-acid transport system which also required Cl- for activity (apparent Km (taurine) 75 and 80 microM; Vmax 0.85 and 0.29 mumol/g Hb per hr for eel (20 degrees C) and flounder cells (10 degrees C), respectively. This beta-system operated with an apparent Na+/Cl-/taurine coupling ratio of 2:1:1. A reduction in extracellular osmolarity, leading to an increase in cell volume, reversibly decreased the activity of the transporter. In contrast, low medium osmolarity stimulated the activity of a Na+-independent nonsaturable transport route selective for taurine, gamma-amino-n-butyric acid and small neutral amino acids, producing a net efflux of taurine from the cells. Neither component of taurine transport was detected in human erythrocytes. It is suggested that these functionally distinct transport routes participate in the osmotic regulation of intracellular taurine levels and hence contribute to the homeostatic regulation of cell volume. Volume-induced increases in Na+-independent taurine transport activity were suppressed by noradrenaline and 8-bromoadenosine-3', 5'-cyclic monophosphate, but unaffected by the anticalmodulin drug, pimozide.

Regulation of taurine transport in rat skeletal muscle

Taurine concentration of soleus muscle (SL, slow-twitch) was initially about twofold higher than that of extensor digitorum longus muscle (EDL, fast-twitch). Taurine concentration in gastrocnemius muscle (GC) was intermediate between that of EDL and SL. Four days after sciatic nerve section, taurine concentration in the EDL but not in the SL was increased by 2.5-fold. The increase was not due to the muscle atrophy and was observed 28 days after denervation. Tenotomy did not increase the total taurine content of the EDL. The increase in taurine concentration of the denervated EDL was prevented by simultaneous ingestion of guanidinoethane sulfonate, a competitive inhibitor of taurine transport. The initial and the maximal rates of [3H]taurine uptake were significantly higher in SL than in EDL. Denervation dramatically accelerated the initial and the maximal rates of the transport in EDL, whereas it significantly reduced those in SL. In contrast, the electrical stimulation of sciatic nerve accelerated the uptake of taurine by EDL and SL of the control but not of the curare-treated rats. These results suggest that transport of taurine into rat skeletal muscles is regulated differently by neural information and by muscular activity, and that the regulation is dependent on the muscle phenotype.

Transport of imino acids and non-alpha-amino acids across the brush-border membrane of the rabbit ileum

The transport of beta-alanine and MeAIB and their effects as inhibitors of the transport of alanine, leucine and lysine across the brush-border membrane of the intact epithelium from the rabbit's distal ileum has been examined. Two separate transport systems have been characterized: 1) A sodium-dependent, beta-alanine-accepting system, which is a high-affinity transport system for alpha-amino-monocarboxylic acids (neutral a.a.) and for cationic a.a., accepts non-alpha-amino acids as well as non-alpha-imino acids, is moderately stereospecific, and for which the affinity of a neutral a.a. is greatly reduced by N-methylation. 2) A sodium-dependent transport system for imino acids, which is inaccessible to cationic amino acids and non-alpha-amino acids but accepts cyclic, non-alpha-imino acids, is moderately stereospecific, and for which neutral a.a. have much lower affinities than their N-methylated derivatives. On the basis of the observations of this and the preceding paper five transport systems for amino acids are ascribed to the rabbit ileum. Some discrepancies between the present results and those obtained with brush-border membrane microvesicles from the rabbit small intestine are discussed.

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.

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

We have confirmed previous demonstrations of sodium gradient-stimulated transport of L-alanine, phenylalanine, proline, and beta-alanine, and in addition demonstrated transport of N-methylamino-isobutyric acid (MeAIB) and lysine in isolated rabbit kidney brush border vesicles. In order to probe the multiplicity of transport pathways available to each of these 14C-amino acids, we measured the ability of test amino acids to inhibit tracer uptake. To obtain a rough estimate of nonspecific effects, e.g., dissipation of the transmembrane sodium electrochemical potential gradient, we measured the ability of D-glucose to inhibit tracer uptake. L-alanine and phenylalanine were completely mutually inhibitory. Roughly 75% of the 14C-L-alanine uptake could be inhibited by proline and beta-alanine, while lysine and MeAIB were no more effective than D-glucose. Roughly 50% of the 14C-phenylalanine uptake could be inhibited by proline and beta-alanine; lysine was as effective as proline and beta-alanine, and the effects of pairs of these amino acids at 50 mM each were not cumulative. MeAIB was no more effective than D-glucose. We conclude that three pathways mediate the uptake of neutral L, alpha-amino acids. One system is inaccessible to lysine, proline, and beta-alanine. The second system carries a major fraction of the L-alanine flux; it is sensitive to proline and beta-alanine, but not to lysine. The third system carries half the 14C-phenylalanine flux, and it is sensitive to proline, lysine, and beta-alanine. Since the neutral, L, alpha-amino acid fluxes are insensitive to MeAIB, we conclude that they are not mediated by the classical A system, and since all of the L-alanine flux is inhibited by phenylalanine, we conclude that it is not mediated by the classical ASC system. L-alanine and phenylalanine completely inhibit uptake of lysine. MeAIB is no more effective than D-glucose in inhibiting lysine uptake, while proline and beta-alanine appear to inhibit a component of the lysine flux. We conclude that the 14C-lysine fluxes are mediated by two systems, one, shared with phenylalanine, which is inhibited by proline, beta-alanine, and L-alanine, and one which is inhibited by L-alanine and phenylalanine but inaccessible to proline, beta-alanine, and MeAIB. Fluxes of 14C-proline and 14C-MeAIB are completely inhibited by L-alanine, phenylalanine, proline, and MeAIB, but they are insensitive to lysine. Proline and MeAIB, as well as alanine and phenylalanine, but not lysine, inhibit 14C-beta-alanine uptake. However, beta-alanine inhibits only 38% of the 14C-proline uptake and 57% of the MeAIB uptake. We conclude that two systems mediate uptake of proline and MeAIB, and that one of these systems also transports beta-alanine.

GABA transport in the rat thyroid

1. The uptake of gamma-aminobutyric acid (GABA) into rat thyroid slices was studied. 2. Uptake of 14C-GABA was concentration-dependent: one unsaturable (diffusion) and two saturable components obeying Michaelis-Menten kinetics contributed to transport. 3. The kinetic constants of saturable GABA transport systems were: Km1 = 1.5 microM, V1 = 4.0 nmol x (g wet weight)-1 x (20 min)-1 (high-affinity uptake): Km2 = 800 microM, V2 = 260 nmol x (g wet weight)-1 x (20 min)-1 (low-affinity uptake). 4. Uptake mediated by each of the carrier systems was concentrative, entirely Na+-dependent, and required activation energies characteristic for active transport. 5. High-affinity transport was structurally specific for GABA. The substrate specificity of low-affinity uptake resembled that of beta-amino acid transport systems.

Taurine transport by rat hepatocytes in primary culture

Hepatic taurine concentration is 30--100 times that of plasma, suggesting an efficient taurine uptake mechanism in the hepatocyte. The characteristics of hepatic taurine transport were studied in primary cultures of adult rat hepatocytes. Taurine uptake was concentrative and linear for over 4 h. At taurine concentrations 2.5--100 microM, uptake was saturable with constants Km = 44 microM, V = 0.28 nmol/mg protein per min and EA = 13.2 kcal/mol. Uptake was inhibited 41% by incubation under N2 and was competitively inhibited by beta-alanine (Ki = 94 microM) and hypotaurine (Ki = 14 microM). Uptake was linearly dependent upon Na+ concentration from 0 to 140 mM. A second nonsaturable uptake process was identifiable only at taurine concentrations greater than 1 mM. This process was presumed to represent passive diffusion. At taurine concentrations existing in plasma, taurine enters the hepatocyte primarily by a single, Na+-dependent, carrier-mediated, oxygen-requiring transport process.

Isoproterenol-stimulated taurine influx in the perfused rat heart

Taurine influx in the perfused rat heart was characterized and the effect of isoproterenol on this process determined. Hearts were perfused by the Langendorff technique with [3H]-taurine in a non-recirculating system. The rate of taurine influx was constant for at least 20 min and the process was saturable. A Km of 45 micron indicated that taurine influx is mediated by a high affinity transport system. Competition between taurine and beta-alanine, but not alpha-amino acids, for influx indicated that the transport sites are specific for beta-amino acids. Isoproterenol (4 X 10(-7) M) stimulated the rate of taurine influx, but propranolol (1 X 10(-8) M) blocked this stimulation. The enhancement of influx by isoproterenol was specific for beta-amino acids in that alpha-amino acid influx was not affected. Dibutyryl cyclic AMP (1 X 10(-3) M) and theophylline (1 X 10(-3) M) also stimulated taurine influx, whereas alterations in heart rate had no effect on the rate of taurine influx. The results are suggestive of a beta-adrenergically activated, cyclic AMP-mediated mechanism controlling isoproterenol-stimulated taurine influx.

The effects of isoproterenol on taurine concentration in the rat heart

Administration of the beta-agonist isoproterenol to rats caused a sharp increase in the influx rate of taurine into the heart, but had no effect on the rate of biosynthesis of taurine in the heart. Influx rates of other amino acids were also stimulated, the distribution ratios, (dpm/g heart)/(dpm/ml plasma), of beta-amino acids being 3--4 times higher than alpha-amino acids. The doses of isoproterenol employed induced cardiac hypertrophy, but the stimulation of taurine influx appeared to be an independent phenomenon, in that near maximum stimulation of influx was achieved prior to stimulation of ornithine decarboxylase activity. Reserpine treatment also produced stimulation of taurine influx with no effect on biosynthesis. These results indicate a close connection between increased sympathomimetic activity and increased influx of amino acids.

Renal tubular reabsorption of taurine, gamma-aminobutyric acid (GABA) and beta-alanine studied by continuous microperfusion

Renal tubular reabsorption of taurine, gamma-aminobutyric acid (GABA), and beta-alanine was studied in vivo et situ by continuous microperfusion of single proximal tubules of the rat. In each case, reabsorption was much slower than that for other amino acids that have been studied. With a concentration of 0.1 mmol/l in initial load was reabsorbed over perfusion distance of 3 mm. Taurine reabsorption saturated with only 2.17 mmol/l in initial perfusate. Assuming simple two-parameter kinetics, upper limits for Km of 0.54 mmol/l and forVmax of 0.59 pmol-cm-1--s-1 for tubular reabsorption of taurine were estimated. High (20 mmol/l) concentrations of taurine or beta-alanine in perfusate completely inhibited GABA reabsorption, but L-phenylalanine (20 mmol/l) had no significant effect. The results indicate that the three amino acids are reabsorbed slowly from the proximal tubule by what may be a common transport system. This system appears to have a high affinity but low capacity and to be different from other known renal tubular transport systems for amino acids.

Transport of L-4-azaleucine in Escherichia coli

The uptake of L-4-azaleucine was examined in Escherichia coli K-12 strains to determine the systems that serve for its accumulation. L-4=Azaleucine in radio-labeled form was synthesized and resolved by the action of hog kidney N-acylamino-acid amidohydrolase (EC 3.5.1.B) on the racemic alpha-N-acetyl derivative of DL-[dimethyl-14C]4-azaleucine. L-4-Azaleucine is taken up in E. coli by energy-dependent processes that are sensitive to changes in the pH and to inhibition by leucine and the aromatic amino acids. Although a single set of kinetic parameters was obtained by kinetic experiments, other evidence indicates that transport systems for both the aromatic and the branched-chain amino acids serve for azaleucine. Azaleucine uptake in strain EO317, with a mutation leading to derepression and constitutive expression of branched-chain amino acid (LIV) transport and binding proteins, was not repressed by growth with leucine as it was in parental strain EO300. Lesions in the aromatic amino acid transport system, aroP, also led to changes in the regulation of azaleucine uptake activity when cells were grown on phenylalanine. Experiments on the specificity of azaleucine uptake and exchange experiments with leucine and phenylalanine support the hypothesis that both LIV and aroP systems transport azaleucine. The ability of external azaleucine to exchange rapidly with intracellular leucine may be an important contributor to azaleucine toxicity. We conclude from these and other studies that at least four other process may affect azaleucine sensitivity: the level of branched-chain amino acid biosynthetic enzymes; the level of leucine, isoleucine, and valine transport systems; the level of the aromatic amino acid, aroP, uptake system; and, possibly, the ability of the cell to racemize D and L amino acids. The relative importance of these processes in azaleucine sensitivity under various conditions is not known precisely.

Neutral amino acid transport in Pseudomonas fluorescens

Membrane transport of beta-alanine, l-alanine, and l-proline was studied in a beta-alanine transaminaseless mutant (strain 67) of Pseudomonas fluorescens. In this mutant beta-alanine is metabolically inert, and it was therefore possible to demonstrate active transport of this substrate in the absence of intracellular catabolism. The permease which catalyzes the uptake of beta-alanine also transports l-proline and l-alanine. This common transport system was distinguished from permeases which transport only l-alanine and only l-proline by competition studies in strain 67 and by studies of transport specificity in a permeaseless mutant (strain 67/4MTR).

Lysine transport in human kidney: evidence for two systems

Experiments with slices of human kidney cortex from two control subjects demonstrated two distinct transport systems for lysine (alpha and beta) which differ greatly in affinity and capacity. Both systems were found in kidney from two patients with cystinuria. Studies with rat kidney confirmed these findings. These experiments defined only a single transport system for cystine in kidney from both control and cystinuric subjects.

Amino acid transport: evidence for genetic control of two types in human kidney

A mutation affecting renal transport of proline, hydroxyproline, and glycerine occurs in man. In the presumed homozygote there is still significant residual transport of these compounds; however, this remaining function is saturated at normal concentrations of substrate in the plasma and is not inhibited by L-proline in the expected way. The presumed heterozygote has partial loss of a transport system common to the three substrates, which becomes saturated at high concentrations of substrate and is inhibited by L-proline. Two different types of transport systems are proposed: a common system and systems with lower capacity and greater specificity. The two types of transport appear to be controlled by separate genes.