Tyrosine transport by membrane vesicles isolated from rat brain

Further studies on the relationship between tyrosine supply and catecholamine production in cultured adrenal chromaffin cells

To elucidate a possible role of tyrosine supply as a factor modulating catecholamine biosynthesis in the adrenergic cell, the transport of [14C]tyrosine into cultured bovine adrenal chromaffin cells was first examined, and the relationship between [14C]tyrosine transport and [14C]catecholamine formation was then investigated. Under the conditions which were routinely employed to determine the rate of catecholamine biosynthesis, tyrosine was taken up into the cells in a manner independent of extracellular Na+ and Ca2+, and this uptake was also insensitive to ouabain and various metabolic inhibitors. The stimulation of these cells with high K+ and other secretagogues caused no significant alteration in the uptake. While, tyrosine transport was markedly inhibited by tyrosine analogues and other L-aromatic amino acids, and this inhibition was accompanied by the reduction of [14C]catecholamine formation. In contrast, tyrosine transport was markedly enhanced by flavone, and this enhancement was also accompanied by the augmentation of catecholamine production under the same experimental conditions. These results seem to indicate that the transport of tyrosine into the cells may be closely related to catecholamine formation within the cells, thus providing an evidence for a possible role of tyrosine supply as one of the factors affecting catecholamine production in the adrenal chromaffin cell.

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.

Na+ dependence of tyrosine transport across the synaptosomal membrane reflects changes in the morphology of synaptosomes

The Na+ dependence of tyrosine uptake into rat brain synaptosomes and synaptosomal plasma membrane vesicles (SPMV) was examined in the present study. At low tyrosine concentrations, the isoosmotic substitution of Na+ by sucrose in the incubation medium led to an increase of tyrosine uptake in synaptosomes and to a decrease in SPMV. The removal of extracellular Ca2+ and Mg2+ and addition of isoosmotic sucrose completely prevented the augmented tyrosine uptake in Na+-free incubated synaptosomes. Morphological differences were found at the electron-microscopic level when synaptosomes were incubated in Na+-free and Na+-containing media. The internal volume measured for synaptosomes incubated in a Na+-free medium was almost half of that obtained in a Na+-containing medium, in good agreement with the observations made with the electron microscope. Also, the omission of Ca2+ and Mg2+ resulted in a specific swelling of only the synaptosomes incubated in Na+-free medium. When synaptosomes and SPMV were preloaded with several neutral amino acids, the tyrosine uptake rate was greatly increased, indicating fully operational exchange mechanisms for these amino acids. We propose that the enhancement of high-affinity synaptosomal tyrosine uptake observed in Na+-free medium is a consequence of a specific shrinkage of the synaptosomes and a parallel increase of the exchange rate with endogenous neutral amino acids.

A protein-free diet changes synaptosomal membrane fluidity and tyrosine and glutamate transport

Synaptosomes were isolated from cerebrums of rats fed standard (20% protein) or protein-free diets for 30 days. Arrhenius plots of their (Na+/K+)ATPase activities revealed a transition temperature of 25.5 degrees C for control rats and 23.4 degrees C for rats on protein-free diet, indicating that the latter increases synaptosomal membrane fluidity. The only change observed in the composition of the synaptosomal membranes was a 26% decrease of sialic acid. In synaptosomes from rats on protein-free diet the uptake of tyrosine was slightly reduced while that of glutamate was not affected. However, the exit of glutamate was reduced.

Efflux and exchange of glycine by plasma membrane vesicles isolated from glioblastoma cells

The efflux and exchange of glycine were studied in plasma membrane vesicles isolated from cultured glioblastoma cells. The mechanism of glycine translocation has been probed by comparing the ion dependence of net efflux to that of exchange. Dilution-induced efflux requires the simultaneous presence of internal sodium and chloride, while influx is dependent on the presence of these two ions on the outside (Zafra, F. and Giménez, C. (1986) Brain Res. 397, 108-116). Glycine efflux from the membrane vesicles is stimulated by external glycine, this exchange being dependent on external sodium, but not on external chloride. The parallelism observed in influx and efflux processes suggests that glycine is translocated in both directions across the membrane, probably by interacting with the carrier. To account for all the observed effects of external ions, glycine concentrations and membrane potential on glycine influx and efflux, a kinetic model of the Na+/Cl-/glycine cotransport system is discussed.

Effects of systematically administered lithium on tryptophan transport and exchange in plasma-membrane vesicles isolated from rat brain

The effect of lithium on the sodium-dependent high-affinity system for tryptophan uptake was examined in plasma membrane vesicles derived from rat brain. We demonstrated that Na+ could be replaced by lithium in the external medium and the presence of lithium produced an increase in the Vmax of the tryptophan transport whereas it had no significant effect on the Km for the substrate. Plasma membrane vesicles derived from synaptosomes obtained from long-term lithium-treated rats are able to accumulate tryptophan to a greater extent than normal rats and maintain a more negative membrane potential than controls. Our data support the idea that the stimulation by lithium of the high-affinity uptake system for tryptophan by maintaining adequate membrane potentials across the membrane, could lead to the stabilization of serotonin production, as has been demonstrated in long term-lithium treatment.

Calcium-dependent release of tyrosine in brain elicited by stimulation of neuropeptide receptors

We sought to establish whether the endogenous opiate-receptor agonist Met-enkephalin (m-ENK) selectively modulates the release of endogenous tyrosine (Tyr) from brain slices prepared from the corpus striatum (CS). Amino acids (AAs) released from slices of CS and, for comparison, cerebral cortex (Cx) were measured by HPLC. Incubation of slices with m-ENK (1-10 microM) increased the basal release of Tyr (up to 293% of control) from CS, but not Cx, whereas other nonneurotransmitter AAs, phenylalanine (Phe) and valine (Val), were unchanged. The release of the putative neurotransmitter AAs glutamate (Glu), taurine (Tau), and glycine (Gly) were similarly increased by 50-150% with m-ENK in slices of CS, but not Cx. The enhanced release of AAs by m-ENK was prevented by removal of extracellular Ca2+ or by preincubation with the opiate receptor antagonist naloxone. Neuronal depolarization by potassium (5-55 mM) in the presence of Ca2+ did not affect the release of Tyr, whereas release of neurotransmitter AAs such as gamma-aminobutyric acid (GABA) were markedly increased. The increase in basal Tyr release by m-ENK was not the result of a decreased uptake of Tyr. Relative to slices, the basal release of Tyr, Phe, and Val from a synaptosomal (P2) preparation of CS was small (8-51%) compared to that of GABA, Gly, Glu, and Tau (49-123%). Nonetheless, m-ENK (10 microM) markedly increased the release of Tyr (to 833%), but not Glu, Gly, and Tau from the P2 fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Stoichiometry of sodium- and chloride-coupled glycine transport in synaptic plasma membrane vesicles derived from rat brain

The stoichiometric properties of the glycine transporter were studied in synaptic plasma membrane vesicles from rat brain. The present results, together with previous data from our laboratory, allow us to suggest a stoichiometry of 2 Na+ and 1 Cl- per glycine zwitterion for the translocation cycle catalyzed by the glycine carrier. We propose a kinetic model with an ordered mechanism for the binding/debinding of solutes.

Tyrosine transport into isolated rat brain synaptosomes. Ionic dependence and kinetic studies

The ionic dependence and kinetics of the uptake of L-tyrosine into isolated rat brain synaptosomes has been investigated. L-Tyrosine has been found to enter the synaptosomes through three different transport systems showing distinct ionic requirements and kinetic characteristics. The one with the lowest affinity for tyrosine (Km 0.6 mM) showed a strong Na+ dependence. This system seems to provide the nerve cell with a safety mechanism that ensures the supply of tyrosine even in the presence of high levels of competing amino acids. The second one (Km 50 microM) does not appear to exhibit any strong ionic requirements and features most of the characteristics of the L-system for large neutral amino acids. Finally, the third shows the most interesting ionic dependence. Its activity increases at very low Na+ external concentrations, but this increase is prevented by the removal of divalent cations, Ca2+ and Mg2+. This ionic behaviour, along with the affinity constant of this system (Km 6 microM) (within the range of tyrosine extraneural concentrations), suggests that it is an initial regulatory step in the synthesis of catecholamines.

Efflux and exchange of glycine by synaptic plasma membrane vesicles derived from rat brain

The influx and exchange of glycine were studied in synaptic plasma membrane vesicles isolated from rat brain. The vesicles were loaded with [U-14C]glycine by active transport driven by an Na+ as well as a Cl- gradient (out greater than in). Dilution-induced efflux requires the simultaneous presence of internal Na+ and Cl-. As the efflux of glycine has been demonstrated to be strictly dependent on the presence of both ions (Mayor, F., Jr., Marvizón, J.G., Aragón, M.C., Giménez, C. and Valdivieso, F. (1981) Biochem. J. 198, 535-541), it can be concluded that the efflux of glycine is, in many aspects, symmetrical with its influx. Glycine efflux from the membrane vesicles is stimulated by external glycine, this exchange being partially dependent on external sodium and not on external chloride. The parallelism observed in influx and efflux processes shows that the release of glycine (by efflux and homoexchange) occurs via the carrier system. These results suggest that glycine is translocated in both directions across the membrane, probably by interacting with the carrier. Both sodium and chloride have to be present on the same side as glycine.

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.

Developmental studies on the uptake of tyrosine by synaptosomes and plasma membrane vesicles derived from rat brain. Effect of thyroid hormones

The uptake of L-tyrosine at various stages of development was examined in synaptosomes and in plasma membrane vesicles derived from rat brain. The total uptake has two components, Na+-dependent and Na+-independent, respectively. The Na+-dependent component of the transport system appears around the 5th postnatal day and increases with age. The affinity of the transport system for tyrosine does not vary substantially during development. The Vmax increases more than six-fold between day 15 and adulthood. Plasma membrane vesicles derived from T3-treated rats accumulate more tyrosine than those obtained from the control animals. The results support the view that thyroid hormones during development promote the establishment of the systems implicated in neurotransmission in the developing nervous system.

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

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

Ontogenetic studies on tryptophan transport into plasma membrane vesicles derived from rat brain synaptosomes: effect of thyroid hormones

The uptake of tryptophan at various stages of development was examined in plasma membrane vesicles derived from rat brain. The total uptake has two components Na+-dependent and Na+-independent respectively. The Na+-dependent component of the transport system appears around the 5th postnatal day and increases with the age. The Km value of the system does not vary during development. The Vmax increases five-fold between 14 and 35 day of postnatal life. Plasma membrane vesicles derived from T3-treated rats are able to accumulate nearly three-fold more tryptophan than nontreated rats. The results support the idea that thyroid hormones at the earlier stages of life, promote the establishment of neurotransmission in the developing nervous system.

Effectors of amino acid transport processes in animal cell membranes

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

beta-Alanine transport into plasma membrane vesicles derived from rat brain synaptosomes

Transport of beta-alanine has been demonstrated in membrane vesicles isolated from rat brain, using artificially imposed ion gradients as the sole energy source. The uptake of beta-alanine is strictly dependent on the presence of Na+ and Cl- in the medium, and the process can be driven either by an Na+ gradient (out greater than in) or by a Cl- gradient (out greater than in) when the other essential ion is present. The process is stimulated by a membrane potential (negative inside) as demonstrated by the effect of ionophore valinomycin and anions with different permeabilities. beta-Alanine uptake is inhibited by the presence of GABA.

Studies on the osmotic disruption and resealing of synaptosomes

The release of lactate dehydrogenase and K+ when synaptosomes are exposed to resuspension in media of various osmolarity has been investigated in order to measure their disruption. Even when resuspended in distilled water a significant percentage (10-20%) of lactate dehydrogenase and K+ remains unreleased. The particles containing these substances sediment to the same density as synaptosomes. Synaptosomes retaining their internal organelles after hypoosmotic treatment can be seen in electron micrographs. Resealing of disrupted synaptosomes was measured by the inclusion of [14C]sucrose. The resealing is spontaneous, essentially complete (80-90%) within 20 min and not noticeably affected by temperature, pH, or the addition of fusogen. The synaptosome preparation after hypoosmotic disruption will therefore contain some undisrupted synaptosomes with some or all of their complement of cytoplasmic constituents, as well as resealed synaptosomes. The retention of the ability of the hypoosmotically treated preparation to convert [14C]choline to [14C]acetylcholine is demonstrated as an example of the disproportionate effect these undisrupted particles have on its properties.

Presynaptic tyrosine availability in the phenylketonuric brain: a hypothetical evaluation

From measured effects of amino acids on synaptosomal tyrosine uptake and from published data on human CNS levels of amino acids hypothetical calculations were made to compare CNS tyrosine availability for catecholamine synthesis in the hyperphenylalaninemic and non-hyperphenylalaninemic condition. These calculations indicate an approximately two-fold reduction in the availability of tyrosine in phenylketonuria that is due solely to a reduction in CNS tyrosine alone.

Tryptophan transport into plasma membrane vesicles derived from rat brain synaptosomes

Tryptophan uptake by membrane vesicles derived from rat brain was investigated. The uptake is dependent on the Na+ gradient [Na+] outside greater than [Na+] inside, and is maximal when both Na+ and Cl- are present. The uptake represents transport into an osmotically active space and not a binding artifact, as indicated by the effect of increasing the medium osmolarity. The uptake of tryptophan is stimulated by a membrane potential (interior negative) as demonstrated by the effects of the ionophores valinomycin and carbonyl cyanide m-chlorophenylhydrazone and anions with different permeabilities. Kinetic data show that tryptophan is accumulated by two systems with different affinities. Ouabain, an inhibitor of Na+,K+-activated ATPase, does not affect tryptophan transport. The uptake of tryptophan is inhibited by high concentrations of phenylalanine, tyrosine, leucine, and 3,4-dihydroxyphenylalanine.

Inhibition by L-phenylalanine of tryptophan transport by synaptosomal plasma membrane vesicles: implications in the pathogenesis of phenylketonuria

Phenylalanine is accumulated in the genetically linked deficiency phenylketonuria. The effect of L-phenylalanine on the transport of tryptophan was studied using membrane vesicles from rat-brain synaptosomes. Phenylalanine at similar concentrations to those found in phenylketonuric patients competitively inhibits tryptophan uptake, with a Ki of the same order as the Km for tryptophan. This inhibition could be responsible for the depletion of serotonin found in phenylketonuria.

Inhibition by L-phenylalanine of tyrosine transport by synaptosomal plasma membrane vesicles: implications in the pathogenesis of phenylketonuria

The effect of L-phenylalanine on the transport of tyrosine was studied using membrane vesicles from rat brain synaptosomes. Phenylalanine, which is accumulated in phenylketonuria, competitively inhibits tyrosine uptake at concentrations similar to those found in phenylketonuric patients, with a Ki of the same order of the Km for tyrosine. This inhibition could be responsible for the depletion of catecholamines observed in phenylketonuria.