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PubMed1999

Interactions between the thiol-group reagent N-ethylmaleimide and neutral and basic amino acid transporter-related amino acid transport.

The neutral and basic amino acid transport protein (NBAT) expressed in renal and jejunal brush-border membranes is involved in amino acid and cystine absorption. NBAT mutations result in Type 1 cystinuria. A C-terminal myc-tagged NBAT (NBATmyc) retains the amino acid transport and protein-protein interaction properties of NBAT when expressed in Xenopus oocytes. Neutral amino acid (Ala, Phe)-cationic amino acid (Arg) heteroexchanges related to NBATmyc expression in oocytes are inactivated by treatment with the thiol-group reagent N-ethylmaleimide (NEM), although significant Arg-Arg and Ala-Ala homoexchanges persist. Inactivation of heteroexchange activity by NEM is accompanied by loss of >85% of alanine and cystine uptake, with smaller (<50%) inhibition of arginine and phenylalanine uptake. NEM-sensitive cystine uptake and arginine-alanine heteroexchange (system b(0,+) activity) are not expressed by an NBAT truncation mutant (NBATmyc-Sph1) lacking the 13 C-terminal amino acid residues, but the mutant expresses NEM-resistant transport activity (system y(+)L-like) equivalent to that of full-length NBATmyc. The deleted region of NBATmyc-Sph1 contains two cysteine residues (671/683) which may be the targets of NEM action. The synthetic amino acid 2-trifluoromethylhistidine (TFMH) stimulated alanine efflux at pH 7.5 and arginine at pH 5.5, but not vice versa, establishing the existence of distinct pathways for cationic and neutral amino acid homoexchange (TFMH is zwitterionic at pH 7.5 and cationic at pH 5.5). We suggest that NBAT expresses a combination of system b(0,+) and y(+)L-like activities, possibly by interacting with different light-chain subunits endogenous to oocytes (as does the homologous 4F2hc protein). The C-terminus of NBAT may also have an additional, direct role in the mechanism of System b(0,+) transport (the major transport activity that is defective in Type 1 cystinuria).

Amino Acid Sequence↗
PubMed2003

Amino acid transporters: roles in amino acid sensing and signalling in animal cells.

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Amino Acid Transport Systems↗
PubMed2004

Effect of treatment with the Molecular Adsorbents Recirculating System on arterial amino acid levels and cerebral amino acid metabolism in patients with hepatic encephalopathy.

BACKGROUND: Liver failure is associated with low concentrations of branched-chain amino acids and high concentrations of most other amino acids. In this study the effect of treatment with the Molecular Adsorbents Recirculating System (MARS) on arterial amino acid levels and cerebral amino acid metabolism was examined in patients with severe hepatic encephalopathy. METHODS: The study included seven patients with hepatic encephalopathy from fulminant hepatic failure (FHF) and five patients with hepatic encephalopathy from acute-on-chronic liver failure (AoCLF). Cerebral blood flow and cerebral arteriovenous differences in amino acids were measured before and after 6 h of treatment with MARS. RESULTS: During MARS treatment, the total arterial amino acid concentration decreased by 20% from 8.92 +/- 7.79 mmol/L to 7.16 +/- 5.64 mmol/L (P < 0.05). The concentration decreased in all amino acids with the exception of the branched-chain amino acids. Fischer's ratio of branched-chain to aromatic amino acids increased from 0.73 +/- 0.47 to 0.91 +/- 0.54 (P < 0.05). A net cerebral efflux of amino acids in patients with FHF (8.94 +/- 8.34 micromol/100 g/min) as well as AoCLF (7.35 +/- 24.97 micromol/100 g/min) was not affected by the MARS treatment. MARS had no effect on the cerebral metabolic rate of any single amino acid in either group. CONCLUSIONS: MARS treatment tends to normalize the arterial amino acid concentrations in patients with hepatic encephalopathy. Even though the overall reduction in plasma amino acids and improvement in amino acid dysbalance may well be beneficial, it was not accompanied by any immediate improvement in cerebral amino acid metabolism in patients with FHF or AoCLF.

Adult↗
PubMed1992

A comparative study of the neutrophil stimulatory activity in vitro and pro-inflammatory properties in vivo of 72 amino acid and 77 amino acid IL-8.

IL-8, a potent neutrophil-activating protein, can be produced by many cell types including monocytes, lymphocytes, fibroblasts, neutrophils, and endothelial cells. Depending on the cell source, the N-terminal amino acid sequence of IL-8 displays heterogeneity that has been shown to confer differences in its neutrophil stimulatory activity in vitro. Despite these observations the relative potency of different IL-8 molecules in vivo is unknown. To address this question we have investigated the biologic activity of the two predominant forms of IL-8, the 72 and the 77 amino acid proteins, in vitro and in vivo. In vitro, human rIL-8(72) and human rIL-8(77) dose dependently induced adherence of rabbit peritoneal neutrophils and human neutrophils to laminin-coated plates and elevated cytoplasmic levels of Ca2+ ([Ca2+]i) in fura-2 loaded neutrophils. In these in vitro assays human rIL-8(72) was more potent than human rIL-8(77) while inducing comparable responses to human rC5a. With respect to enhancing [Ca2+]i, neutrophils desensitized to human rIL-8(72) failed to respond to human rIL-8(77). However, neutrophils fully desensitized to human rIL-8(77) could exhibit a partial response to human rIL-8(72). Further, human rIL-8(72) was approximately 10-fold more effective than human rIL-8(77) in displacing human [125I]rIL-8(72) from rabbit peritoneal neutrophils in a receptor-binding assay. In vivo, intradermally administered human rIL-8(72) and human rIL-8(77) induced 111In-neutrophil accumulation and edema formation in rabbit skin. In contrast to the in vitro studies, the two forms of IL-8 gave identical responses in vivo although they were less potent than human rC5a. Our results demonstrate that, in vitro, human rIL-8(72) is more potent than human rIL-8(77) in stimulating neutrophils. It may be that IL-8)72) has a greater affinity and/or efficacy for the neutrophil IL-8 cell-surface receptors. One possibility for the observation that both forms of IL-8 are equipotent in inducing inflammatory responses in vivo is that the extended form is proteolytically cleaved to the more biologically active IL-8(72).

Animals↗
PubMed2002

[Determination of amino acids and glucose in amino acid injection by anion exchange chromatography with integrated pulsed amperometric detection].

A direct, sensitive, simple and practical method for the determination of amino acids and glucose in amino acid injection by anion exchange chromatography with integrated pulsed amperometric detection was developed. There is no need for sample pretreatment. The retention behavior of amino acids and glucose on the anion exchange column was investigated using sodium hydroxide and sodium acetate as eluent. When the concentration of sodium hydroxide was changed, the change in the retention times was generally greater for amino acids than for glucose. The difference can be used to improve the separation between amino acids and glucose. When the concentration of sodium acetate was changed, the change in the retention times was generally greater for amino acids containing two carboxyl groups than for amino acids containing only one carboxyl group. The difference can be used to improve the separation between di-carboxylic amino acids and mono-carboxylic amino acids. In order to separate amino acids and glucose, a ternary gradient elution method with water, sodium hydroxide and sodium acetate as mobile phases was employed. The optimized gradient elution condition for the analysis of 17 amino acids and glucose in amino acid injection was obtained. The time of the gradient elution program was 74 min. Under the optimized conditions and the column temperature of 30 degrees C, the detection limits for 17 amino acids and glucose were 0.3 pmol- 10.3 pmol. The calibration graphs of peak area for all the analytes were linear in the ranges about two orders of magnitude. The relative standard deviations (RSDs) (n = 5) were 0.7% - 3.8%. The recoveries of added standard were 88.3% - 104.6%.

Amino Acids↗
PubMed1982

Electrophysiological analysis of rat renal sugar and amino acid transport. IV. Basic amino acids.

Electrophysiological techniques were used to study the transport of the basic amino acids L-arginine, L-lysine and L-ornithine in rat kidney proximal tubule in vivo. Tubular cells were punctured with microelectrodes and the response of the cell membrane potential to sudden applications of the amino acids was measured. In the presence of physiological Na+ concentrations luminal perfusion with millimolar concentrations of basic amino acids depolarized the tubular cells in a concentration dependent fashion by up to 15 mV, while in the absence of Na+ no significant potential changes were observed. These observations indicate that the basic amino acids are taken up into the cell across the brushborder in coupling with Na+ ions in a similar way as neutral and acidic amino acids, and that simple conductive pathways for uncoupled flow of the basic amino acids do either not exist or are quantitatively negligible in the brushborder. From kinetic measurements and competition experiments it was concluded that all basic amino acids are transported by the same transport system, which however does not accept acidic or neutral amino acids (with the possible exception of L-cystine). Perfusion of the peritubular capillaries with millimolar concentrations of basic amino acids depolarized the cells only by approximately 1 mV, both in the presence and absence of Na+. This observation may indicate that a passive uncoupled transport pathway for basic amino acids is present in the peritubular cell membrane to allow exit from cell to interstitial space, if the intracellular concentration rises high enough to overcome the cell membrane potential.

Amino Acids↗
PubMed1991

[Preparation and analysis of hydroxycarboxylic acid, sugar acid and amino acid complexes of gallium(III)].

Neutral, stable in water and not hydrolysing gallium(III) complexes were prepared and analysed by instrumental analytical methods. Nine hydroxy-carboxylic acids, four sugar acids and four amino acids were used as ligands. The chemical and physical characteristics of complexes (composition, stability, water solubility, pH in aqueous solution) significantly depend on the quality of gallium salt and ligand, the ratio of gallium and ligand, the medium used during the preparation (pH and temperature).

Amino Acids↗
PubMed1974

Substrate turnover during prolonged exercise in man. Splanchnic and leg metabolism of glucose, free fatty acids, and amino acids.

Arterial concentrations and substrate exchange across the leg and splanchnic vascular beds were determined for glucose, lactate, pyruvate, glycerol, individual acidic and neutral amino acids, and free fatty acids (FFA) in six subjects at rest and during 4 h of exercise at approximately 30% of maximal oxygen uptake. FFA turnover and regional exchange were evaluated using (14)C-labeled oleic acid. The arterial glucose concentration was constant for the first 40 min of exercise, but fell progressively thereafter to levels 30% below basal. The arterial insulin level decreased continuously, while the arterial glucagon concentration had risen fivefold after 4 h of exercise. Uptake of glucose and FFA by the legs was markedly augmented during exercise, the increase in FFA uptake being a consequence of augmented arterial levels rather than increased fractional extraction. As exercise was continued beyond 40 min, the relative contribution of FFA to total oxygen metabolism rose progressively to 62%. In contrast, the contribution from glucose fell from 40% to 30% between 90 and 240 min. Leg output of alanine increased as exercise progressed. Splanchnic glucose production, which rose 100% above basal levels and remained so throughout exercise, exceeded glucose uptake by the legs for the first 40 min but thereafter failed to keep pace with peripheral glucose utilization. Total estimated splanchnic glucose output was 75 g in 4 h, sufficient to deplete approximately 75% of liver glycogen stores. Splanchnic uptake of gluconeogenic precursors (lactate, pyruvate, glycerol, alanine) had increased 2- to 10-fold after 4 h of exercise, and was sufficient to account for 45% of glucose release at 4 h as compared to 20-25% at rest and at 40 min of exercise. In the case of alanine and lactate, the increase in precursor uptake was a consequence of a rise in splanchnic fractional extraction. It is concluded that during prolonged exercise at a low work intensity (a) blood glucose levels fall because hepatic glucose output fails to keep up with augmented glucose utilization by the exercising legs; (b) a large portion of hepatic glycogen stores is mobilized and an increasing fraction of the splanchnic glucose output is derived from gluconeogenesis; (c) blood-borne substrates in the form of glucose and FFA account for a major part of leg muscle metabolism, the relative contribution from FFA increasing progressively; and (d) augmented secretion of glucagon may play an important role in the metabolic adaptation to prolonged exercise by its stimulatory influence on hepatic glycogenolysis and gluconeogenesis.

Abdomen↗
PubMed1994

Comparative responses of genetically lean and fat chickens to lysine, arginine and non-essential amino acid supply. II. Plasma amino acid responses.

1. Three experiments performed to study the effects of amino acid imbalances on the growth of genetically lean (LL) and fat (FL) male chickens from 28 to 42 d of age were described by Leclercq et al. (1994). The plasma amino acid concentrations of birds on selected treatments from that paper are reported here. In experiment 1, three dietary concentrations of digestible lysine were compared (4.75, 6.75 and 7.75 g/kg). In experiment 2, two dietary concentrations of digestible arginine were compared (6.53 and 10.00 g/kg). In experiment 3, three diets were compared: a high-protein diet (189 g CP/kg), a low-protein diet containing added essential amino acids (144 g CP/kg), and this low-protein diet supplemented with 40 g/kg of non-essential amino acids (NEAA; glutamic and aspartic acids). 2. The present results are compared with two earlier reports on the same genotypes. The LL consistently had lower plasma concentrations of methionine, cystine, phenylalanine, isoleucine and valine, and higher concentrations of histidine, than the FL chickens. In 4 of 5 experiments, LL leucine concentrations were lower, and glutamic acid, tyrosine, glutamine and alanine were higher, than in the FL. The other amino acids measured; arginine, lysine, aspartic acid, glycine and serine, exhibited variable responses among the experiments. 3. When the limiting essential amino acids, lysine and arginine, were added to a deficient diet, the plasma concentration of the supplemented amino acid increased while the others remained constant or decreased. 4. When glutamic and aspartic acids were added to the low protein diet, plasma amino acid responses were similar to those of adding a limiting amino acid to a deficient diet, except that alanine exhibited a dramatic increase. 5. Although there were genotype by diet interactions for several amino acids, the interactions were caused by differences in the degree of the responses, not in their direction. 6. These results suggest that the FL and LL genotypes do not utilise various amino acids with the same efficiency and, as a consequence, the ideal profile of dietary amino acids should not be the same for both lines. The results support the hypothesis that selection for fatness and leanness changed the amino acid requirements independently of the effects of food intake.

Amino Acids↗
PubMed1986

Response of rats to diets of equal chemical score: effect of lysine or threonine as the limiting amino acid and of an amino acid excess.

The effect of lysine or threonine deficiency with or without excesses of all other amino acids was evaluated in a 21-d feeding study with male rats. Four amino acid mixtures were designed to be first limiting for the rat in lysine or threonine and contained either 0 or 50% excess of nonlimiting amino acids. These mixtures were incorporated into purified diets to provide seven levels [20-140% of the National Research Council (NRC) requirement] of the limiting amino acid. Food intake, body weight gain and carcass composition were measured for each rat to determine the effects of the identity of the limiting amino acid and of amino acid excess on the response to dietary chemical score. Significant effects and/or interactions of the identity of the limiting amino acid (i.e., Lys or Thr) and the presence of excess amino acids were seen for each of the measured responses. At equivalent dietary percentages of the NRC requirement, threonine deficiency supported greater body weight gain than did lysine deficiency. At equivalent deficiencies (Lys vs. Thr) threonine-deficient rats were more susceptible to adverse effects of excess amino acids. When the limiting amino acids were incorporated into the diet through incremental addition of the deficient amino acid mixture, rats responded to levels of lysine or threonine in excess of the NRC requirement. These results suggest that the current NRC requirements for these amino acids are too low and that aspects of the dietary amino acid composition other than the percentage deficit of the limiting amino acid can be important determinants of animal response.

Animals↗
PubMed1997

Molecular characteristics of mammalian and insect amino acid transporters: implications for amino acid homeostasis.

In mammalian cells, the uptake of amino acids is mediated by specialized, energy-dependent and passive transporters with overlapping substrate specificities. Most energy-dependent transporters are coupled either to the cotransport of Na+ or Cl- or to the countertransport of K+. Passive transporters are either facilitated transporters or channels. As a prelude to the molecular characterization of the different classes of transporters, we have isolated transporter cDNAs by expression-cloning with Xenopus laevis oocytes and we have characterized the cloned transporters functionally by uptake studies into oocytes using radiolabelled substrates and by electrophysiology to determine substrate-evoked currents. Mammalian transporters investigated include the dibasic and neutral amino acid transport protein D2/NBAT (system b0+) and the Na(+)- and K(+)-dependent neuronal and epithelial high-affinity glutamate transporter EAAC1 (system XAG-). A detailed characterization of these proteins has provided new information on transport characteristics and mechanisms for coupling to different inorganic ions. This work has furthermore advanced our understanding of the roles these transporters play in amino acid homeostasis and in various pathologies. For example, in the central nervous system, glutamate transporters are critically important in maintaining the extracellular glutamate concentration below neurotoxic levels, and defects of the human D2 gene have been shown to account for the formation of kidney stones in patients with cystinuria. Using similar approaches, we are investigating the molecular characteristics of K(+)-coupled amino acid transporters in the larval lepidopteran insect midgut. In the larval midgut, K+ is actively secreted into the lumen through the concerted action of an apical H+ V-ATPase and an apical K+/2H+ antiporter, thereby providing the driving force for absorption of amino acids. In vivo, the uptake occurs at extremely high pH (pH 10) and is driven by a large potential difference (approximately -200 mV). Studies with brush-border membrane vesicles have shown that there are several transport systems in the larval intestine with distinct amino acid and cation specificities. In addition to K+, Na+ can also be coupled to amino acid uptake at lower pH, but the Na+/K+ ratio of the hemolymph is so low that K+ is probably the major coupling ion in vivo. The neutral amino acid transport system of larval midgut has been studied most extensively. Apart from its cation selectivity, it appears to be related to the amino acid transport system B previously characterized in vertebrate epithelial cells. Both systems have a broad substrate range which excludes 2-(methylamino)-isobutyric acid, an amino acid analog accepted by the mammalian Na(+)-coupled system A. In order to gain insights into the K(+)-coupling mechanism and into amino acid and K+ homeostasis in insects, current studies are designed to delineate the molecular characteristics of these insect transporters. Recent data showed that injection of mRNA prepared from the midgut of Manduca sexta into Xenopus laevis oocytes induced a 1.5- to 2.5-fold stimulation of the Na(+)-dependent uptake of both leucine and phenylalanine (0.2 mmoll-1, pH 8). The molecular cloning of these transporters is now in progress. Knowledge of their unique molecular properties could be exploited in the future to control disease vectors and insect pests.

Amino Acid Transport Systems↗
PubMed1999

Reactivating tammar wallaby blastocysts oxidize fatty acids and amino acids.

The tammar wallaby, Macropus eugenii, has a ruminant-like digestive system which may make a significant concentration of amino acids and fatty acids available to the blastocyst via uterine fluids. Fluorescent and radioisotope analyses were performed to determine the rate of glutamine and palmitate use by blastocysts recovered on day 0, 3, 4, 5 and 10 after reactivation induced by removal of pouch young (RPY). Between day 0 and 4 glutamine uptake increased from 15.6 +/- 6.6 to 36.1 +/- 2.7 pmol per embryo h-1 (P < 0.01) and ammonium production increased from 8.2 +/- 4.3 to 26.6 +/- 3.0 pmol per embryo h-1 (P < 0.01). Glutamine oxidation did not increase until day 10 after RPY (P < 0.01), but the percentage of glutamine oxidized increased from 4.5 +/- 3.1% during diapause to 31.2 +/- 12.6% (P < 0.01) by day 5 after RPY and increased further to 51.0 +/- 15.8% (P < 0.01) by day 10 after RPY. Palmitate oxidation also increased from 0.3 +/- 0.1 by day 0 blastocysts to 3.8 +/- 1.7 pmol per embryo h-1 (P < 0.01) by day 4 blastocysts. This increase provides a greater potential for ATP production, possibly to supply increased demand due to the coincident resumption of mitoses. The ATP:ADP ratio within blastocysts had reduced by the time of the first measurement at day 3 (0.5 +/- 0.2 pmol per embryo h-1; P < 0.01) compared with day 0 blastocysts (1.4 +/- 0.3 pmol per embryo h-1). It is likely that metabolism of amino acids and fatty acids contributes to the energy supply during reactivation of tammar wallaby blastocysts after embryonic diapause.

Adenosine Diphosphate↗
PubMed2002

Rapid simultaneous determination of organic acids, free amino acids, and lactose in cheese by capillary electrophoresis.

A capillary electrophoresis (CE) method for the simultaneous separation of 11 metabolically important organic acids (oxalic, formic, citric, succinic, orotic, uric, acetic, pyruvic, propionic, lactic, and butyric), 10 amino acids (Asp, Glu, Tyr, Gly, Ala, Ser, Leu, Phe, Lys, and Trp), and lactose has been optimized, validated, and tested in dairy products. Repeatability and linearity were calculated for each compound, with detection limit values as low as 0.2 x 10(-2) mM for citric acid and Gly. The method was applied to analyze yogurt and different varieties of commercial cheeses. This method yielded specific CE patterns for different varieties of cheese. Also, it has been shown to be sensitive enough to measure small changes in composition of some of those compounds in fresh cheese stored under accelerated ripening conditions for 2 d at 32 degrees C (e.g., from 1728.3 +/- 45.0 to 1166.7 +/- 4.5 mg/100 g of DM in the case of lactose, or from 23.5 +/- 0.6 to 76.8 +/- 16.7 mg/100 g of DM in the case of acetic acid).

Acetic Acid↗
PubMed1990

Kinetics of influx of peptides and amino acids into hamster jejunum in vitro: physiological and theoretical implications.

1. This paper reports a comparison of the kinetics of influx into hamster jejunum of a series of dipeptides of neutral, basic and acidic amino acids, and a tripeptide of neutral amino acids, with those of corresponding free amino acids. 2. Kt, the substrate concentration at which the transport rate is half the maximal transport rate, and Vmax, the maximal transport rate, were more similar from one peptide to another than among amino acids, with the result that, over a wide range of concentrations, rates of influx of individual peptides varied much less than those of amino acids. 3. It is suggested that this may account for the rates of absorption of amino acids being closely related to the amino acid composition of the protein fed, instead of being widely dissimilar as with corresponding mixtures of free amino acids. 4. With neutral amino acids, both Kt and Vmax. fell with increasing length of the side-chain, as observed on many previous occasions. This did not occur with the corresponding homologous dipeptides, which shows that the hypothesis that the apparent affinity for transport is related to the lipophilic properties of the side-chain cannot be applied to peptides.

Amino Acids↗
PubMed1996

Amino-acid-dependent modulation of amino acid transport in Xenopus laevis oocytes.

We have measured rates of uptake of arginine, glutamine, glutamate, serine, phenylalanine and glycine in Xenopus laevis oocytes cultured for periods of up to 24h in saline in the presence or absence of a mixture of 20 amino acids at concentrations approximating those in Xenopus plasma. Amino acid supplementation increased the total intracellular amino acid concentration from 8.2 to 18.4 nmol per oocyte. Specific Na(+)-dependent amino acid transporters (systems B0,+, Xag-) exhibit 'adaptive regulation' (up-regulation during amino acid deprivation and down-regulation during amino acid supplementation). Na(+)-independent transporters of glutamate, glutamine and glycine (including system asc) display an opposite modulation in activity, which may help to combat amino-acid-induced oxidative stress by increasing the supply of glutathione precursors. Single amino acids at physiological plasma concentrations (0.47 mmol l-1 L-alanine, 0.08 mmol l-1 L-glutamate) mimicked at least some effects of the amino acid mixture. The mechanisms of transport modulation do not appear to include trans-amino acid or membrane potential effects and, in the case of Na(+)-independent transport, are independent of protein or mRNA synthesis. Furthermore, activation of protein kinase C by phorbol 12-myristate 13-acetate did not significantly affect endogenous glutamine and glutamate transport. The Xenopus oocyte appears to possess endogenous signalling mechanisms for selectively modulating the activity of amino acid transport proteins expressed in its surface membranes, a factor for consideration when using oocytes as an expression system for structure-function studies of cloned amino acid transporters.

Amino Acid Transport Systems↗