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At least 19 recordsLinked to original sources
PubMed2004

Selective expression of a novel high-affinity transport system for acidic and neutral amino acids in the tapetum cells of Arabidopsis flowers.

Within the flower, microsporogenesis represents a major sink for nitrogen, but knowledge on how the imported nitrogen is transferred from the anther cell layers to developing pollen is lacking. Here, we provide information on characterization of a transporter (AtLHT2) that might play an important role in partitioning of amino acids for microspore development. Biochemical analysis in yeast showed that AtLHT2 transports proline and aspartate with high affinity. However, other neutral and acidic amino acids act as strong competitors for proline and aspartate uptake indicating that AtLHT2 generally transports uncharged and negatively charged amino acids. Comparison of the apparent K(m) values of AtLHT2 with previously characterized amino acid transporters clearly demonstrated that AtLHT2 represents a novel high-affinity system for neutral and acidic amino acids. Northern blot analysis showed strong expression of the amino acid transporter in flower buds. Cellular expression could be resolved by using RNA in situ hybridization and in situ RT-PCR methods, which localized AtLHT2 specifically to the tapetum tissue of the anthers. Developing pollen grains are symplasmically isolated from the sporophytic tissue and rely on the nutrients and other compounds secreted from the tapetum cells. Thus, the functional characterization of AtLHT2, together with our expression and localization studies, strongly suggest that in Arabidopsis flowers, AtLHT2 has a critical function in import of neutral and acidic amino acids into the tapetum cells for synthesis of compounds important for microspore structure and in transfer of organic nitrogen to the locule for pollen development.

Amino Acid Transport Systems↗
PubMed1995

Osmoregulation of neutral amino acid transport.

Neutral amino acids are important organic osmolytes found in most osmotically stressed cells. During prolonged hypertonic stress, accumulation of neutral amino acids contributes to the cell volume recovery and may protect cellular proteins against salt denaturation. Hypertonicity activates system A transport, a major neutral amino acid transporter in mammalian cells, by a microtubule-dependent mechanism. Similar to the regulation of amino acid efflux induced by hypotonic-swelling, influx via system A may be under the control of intracellular ionic strength. An osmotically sensitive repressor may negatively regulate the gene expression of a regulatory protein which activates the preexisting system A transporter protein. Hypertonically activated protein kinases may be involved in this osmoregulation of amino acid transport.

Amino Acids↗
PubMed1982

Milk protein quantity and quality in the term infant. II. Effects on acidic and neutral amino acids.

The response of the plasma and urine concentrations of acidic and neutral amino acids to two bovine protein formulas (1.5 gm/100 ml) containing, respectively, 60% whey proteins and 40% caseins or 18% whey proteins an 82% caseins was measured in term infants. These two groups of infants were compared with a group of infants that were breast-fed; all infants were fed ad libitum. Concentrations of citrulline, threonine, phenylalanine, and tyrosine in the plasma and urine were higher in the formula-fed infants than in the breast-fed infants and were a reflection of the amount of protein provided. Concentrations of tyrosine and phenylalanine were higher in the plasma and urine of infants fed the casein-predominant formulas than they were in those fed whey-predominant formulas. The opposite was true for threonine, which was present in higher concentrations in plasma and urine of infants fed whey-predominant formulas than it was in infants fed casein-predominant formulas. Concentrations of taurine were lower in the plasma and urine of formula-fed infants than they were in breast-fed infants. These differences give further evidence that formulas now in common use may provide a protein intake in excess of protein requirements and that there is a dietary requirement for taurine in man which is not satisfied by such formulas. Although these differences have not been established as nutritionally deleterious, neither can they be assumed to be entirely acceptable.

Amino Acids↗
PubMed2002

Functional characterization of monocarboxylic acid, large neutral amino acid, bile acid and peptide transporters, and P-glycoprotein in MDCK and Caco-2 cells.

Bidirectional transport studies were conducted to determine whether substrates of five intestinal transporters showed carrier-mediated asymmetric transport across MDCK (Madin-Darby canine kidney) cell monolayers grown under standard conditions. Drug concentrations were quantitated using liquid scintillation counting, liquid chromatography/mass spectrometry/mass spectrometry, or liquid chromatography/mass spectrometry. In the presence of a pH gradient, benzoic acid exhibited net apical-to-basolateral transport, with apparent permeability ratios (apical-to-basolateral permeability/basolateral-to-apical permeability) ranging from 14 to 25. The addition of valproic acid reduced the permeability ratio by 70-90%. Cephalexin transport also exhibited net absorption in the presence of a pH gradient, with apparent permeability ratios ranging from 14 to 71, depending on growth conditions. Radiolabeled phenylalanine exhibited a low level of carrier-mediated absorption with an apparent permeability ratio of 1.8 that was reduced to 1.0 in the presence of unlabeled L-phenylalanine. Taurocholic acid did not exhibit carrier-mediated absorption. Cyclosporine and fexofenadine exhibited P-glycoprotein-mediated efflux from both MDCK and Caco-2 cells, which was more sensitive to inhibition in MDCK cells. These results suggest that although MDCK cell monolayers may be a useful model for evaluating transport by the absorptive monocarboxylic acid and peptide transporters and the efflux transporter, P-glycoprotein, they are not useful for predicting large neutral amino acid or bile acid transport in the intestine.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1984

Electrophysiological evidence for acidic, basic, and neutral amino acid olfactory receptor sites in the catfish.

Electrophysiological experiments indicate that olfactory receptors of the channel catfish, Ictalurus punctatus, contain different receptor sites for the acidic (A), basic (B), and neutral amino acids; further, at least two partially interacting neutral sites exist, one for the hydrophilic neutral amino acids containing short side chains (SCN), and the second for the hydrophobic amino acids containing long side chains (LCN). The extent of cross-adaptation was determined by comparing the electro-olfactogram (EOG) responses to 20 "test" amino acids during continuous bathing of the olfactory mucosa with water only (control) to those during each of the eight "adapting" amino acid regimes. Both the adapting and test amino acids were adjusted in concentrations to provide approximately equal response magnitudes in the unadapted state. Under all eight adapting regimes, the test EOG responses were reduced from those obtained in the unadapted state, but substantial quantitative differences resulted, depending upon the molecular structure of the adapting stimulus. Analyses of the patterns of EOG responses to the test stimuli identified and characterized the respective "transduction processes," a term used to describe membrane events initiated by a particular subset of amino acid stimuli that are intricately linked to the origin of the olfactory receptor potential. Only when the stimulus compounds interact with different transduction processes are the stimuli assumed to bind to different membrane "sites." Four relatively independent L-alpha-amino acid transduction processes (and thus at least four binding sites) identified in this report include: (a) the A process for aspartic and glutamic acids; (b) the B process for arginine and lysine; (c) the SCN process for glycine, alanine, serine, glutamine, and possibly cysteine; (d) the LCN process for methionine, ethionine, valine, norvaline, leucine, norleucine, glutamic acid-gamma-methyl ester, histidine, phenylalanine, and also possibly cysteine. The specificities of these olfactory transduction processes in the catfish are similar to those for the biochemically determined receptor sites for amino acids in other species of fishes and to amino acid transport specificities in tissues of a variety of organisms.

Adaptation, Physiological↗
PubMed2004

Molecular cloning of mouse amino acid transport system B0, a neutral amino acid transporter related to Hartnup disorder.

Resorption of amino acids in kidney and intestine is mediated by transporters, which prefer groups of amino acids with similar physico-chemical properties. It is generally assumed that most neutral amino acids are transported across the apical membrane of epithelial cells by system B(0). Here we have characterized a novel member of the Na(+)-dependent neurotransmitter transporter family (B(0)AT1) isolated from mouse kidney, which shows all properties of system B(0). Flux experiments showed that the transporter is Na(+)-dependent, electrogenic, and actively transports most neutral amino acids but not anionic or cationic amino acids. Superfusion of mB(0)AT1-expressing oocytes with neutral amino acids generated inward currents, which were proportional to the fluxes observed with labeled amino acids. In situ hybridization showed strong expression in intestinal microvilli and in the proximal tubule of the kidney. Expression of mouse B(0)AT1 was restricted to kidney, intestine, and skin. It is generally assumed that mutations of the system B(0) transporter underlie autosomal recessive Hartnup disorder. In support of this notion mB(0)AT1 is located on mouse chromosome 13 in a region syntenic to human chromosome 5p15, the locus of Hartnup disorder. Thus, the human homologue of this transporter is an excellent functional and positional candidate for Hartnup disorder.

Amino Acid Sequence↗
PubMed1991

Affinity of antineoplastic amino acid drugs for the large neutral amino acid transporter of the blood-brain barrier.

The relative affinity of six anticancer amino acid drugs for the neutral amino acid carrier of the blood-brain barrier was examined in rats using an in situ brain perfusion technique. Affinity was evaluated from the concentration-dependent inhibition of L-[14C]-leucine uptake into rat brain during perfusion at tracer leucine concentrations and in the absence of competing amino acids. Of the six drugs tested, five, including melphalan, azaserine, acivicin, 6-diazo-5-oxo-L-norleucine, and buthionine sulfoximine, exhibited only low affinity for the carrier, displaying transport inhibition constants (Ki, concentrations producing 50% inhibition) ranging from 0.09 to 4.7 mM. However, one agent - D,L-2-amino-7-bis[(2-chloroethyl)amino]- 1,2,3,4-tetrahydro-2-naphthoic acid (D,L-NAM) - demonstrated remarkably high affinity for the carrier, showing a Ki value of approximately 0.2 microM. The relative affinity (1/Ki) of D,L-NAM was greater than 100-fold that of the other drugs and greater than 10-fold that of any compound previously tested. As the blood-brain barrier penetrability of most endogenous neutral amino acids is related to their carrier affinity, the results suggest that D,L-NAM may be a promising agent which may show enhanced uptake and distribution to brain tumors.

2-Naphthylamine↗
PubMed1989

Effects of running the Boston marathon on plasma concentrations of large neutral amino acids.

Plasma large neutral amino acid concentrations were measured in thirty-seven subjects before and after completing the Boston Marathon. Concentrations of tyrosine, phenylalanine, and methionine increased, as did their "plasma ratios" (i.e., the ratio of each amino acid's concentration to the summed plasma concentrations of the other large neutral amino acids which compete with it for brain uptake). No changes were noted in the plasma concentrations of tryptophan, leucine, isoleucine, nor valine; however, the "plasma ratios" of acid patterns may influence neurotransmitter synthesis.

Amino Acids↗
PubMed2004

Na+ -dependent neutral amino acid transporters A, ASC, and N of the blood-brain barrier: mechanisms for neutral amino acid removal.

Four Na+ -dependent transporters of neutral amino acids (NAA) are known to exist in the abluminal membranes (brain side) of the blood-brain barrier (BBB). This article describes the kinetic characteristics of systems A, ASC, and N that, together with the recently described Na+ -dependent system for large NAA (Na+ -LNAA), provide a basis for understanding the functional organization of the BBB. The data demonstrate that system A is voltage dependent (3 positive charges accompany each molecule of substrate). Systems ASC and N are not voltage dependent. Each NAA is a putative substrate for at least one system, and several NAA are transported by as many as three. System A transports Pro, Ala, His, Asn, Ser, and Gln; system ASC transports Ser, Gly, Met, Val, Leu, Ile, Cys, and Thr; system N transports Gln, His, Ser, and Asn; Na+ -LNAA transports Leu, Ile, Val, Trp, Tyr, Phe, Met, Ala, His, Thr, and Gly. Together, these four systems have the capability to actively transfer every naturally occurring NAA from the extracellular fluid (ECF) to endothelial cells and thence to the circulation. The existence of facilitative transport for NAA (L1) on both membranes provides the brain access to essential NAA. The presence of Na+ -dependent carriers on the abluminal membrane provides a mechanism by which NAA concentrations in the ECF of brain are maintained at approximately 10% of those of the plasma.

Algorithms↗
PubMed2006

Amino acid starvation induces the SNAT2 neutral amino acid transporter by a mechanism that involves eukaryotic initiation factor 2alpha phosphorylation and cap-independent translation.

Nutritional stress caused by amino acid starvation involves a coordinated cellular response that includes the global decrease of protein synthesis and the increased production of cell defense proteins. Part of this response is the induction of transport system A for neutral amino acids that leads to the recovery of cell volume and amino acid levels once extracellular amino acid availability is restored. Hypertonic stress also increases system A activity as a mechanism to promote a rapid recovery of cell volume. Both a starvation-dependent and a hypertonic increase of system A transport activity are due to the induction of SNAT2, the ubiquitous member of SLC38 family. The molecular mechanisms underlying SNAT2 induction were investigated in tissue culture cells. We show that the increase in system A transport activity and SNAT2 mRNA levels upon amino acid starvation were blunted in cells with a mutant eIF2alpha that cannot be phosphorylated. In contrast, the induction of system A activity and SNAT2 mRNA levels by hypertonic stress were independent of eIF2alpha phosphorylation. The translational control of the SNAT2 mRNA during amino acid starvation was also investigated. It is shown that the 5'-untranslated region contains an internal ribosome entry site that is constitutively active in amino acid-fed and -deficient cells and in a cell-free system. We also show that amino acid starvation caused a 2.5-fold increase in mRNA and protein expression from a reporter construct containing both the SNAT2 intronic amino acid response element and the SNAT2-untranslated region. We conclude that the adaptive response of system A activity to amino acid starvation requires eukaryotic initiation factor 2alpha phosphorylation, increased gene transcription, and internal ribosome entry site-mediated translation. In contrast, the response to hypertonic stress does not involve eukaryotic initiation factor 2alpha phosphorylation, suggesting that SNAT2 expression can be modulated by specific signaling pathways in response to different stresses.

5' Untranslated Regions↗
PubMed1977

The effects of hypothyroidism and fasting on electrogenic amino acid transfer: possible evidence for multiple neutral amino acid carrier systems in rat jejunum.

The jejunal mechanisms for the electrogenic transfer of four neutral amino acids (alanine, leucine, methionine, valine) and for sarcosine were characterised by an electrical method in vitro. The values for apparent Km obtained electrically agree well with those assessed by conventional chemical techniques. Hypothyroidism and/or fasting rats for 3 days induced differential changes in the apparent Km and p.d.max for the various amino acids. These alterations were interpreted as indicating the presence of at least three mechanisms for neutral amino acid transfer and one for sarcosine. In euthyroid rats, only alanine showed changes in apparent Km (decrease) and p.d.max (decrease) after fasting for 3 days. With hypothyroidism the kinetic parameters of electrogenic transfer for alanine, valine and sarcosine were significantly altered while those for leucine and methionine were unaffected.

Alanine↗