Na+/L+ selectivity in transport systemA: Effects of substrate structure

Christensen, Halvor N.; Handlogten, Mary E.

doi:10.1007/bf01940932

Cited by 45 publications

(15 citation statements)

References 18 publications

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“…3A). The SNAT1 transport cycle does not require any other ionic species; however, Li ϩ can substitute for Na ϩ in driving transport and (contrary to common thinking) classic System A is tolerant of a Li ϩ for Na ϩ substitution (63). However, we propose that Li ϩ tolerance is not in itself a useful criterion with which to classify amino acid transporters, since our data for SNAT1 illustrated that: (i) Li ϩ tolerance is incomplete and (ii) the apparent effectiveness of Li ϩ as a replacement for Na ϩ depends both on membrane potential and on the parameters (e.g.…”

Section: Mechanisms Of Snat1-mediated Amino Acid Transport-mentioning

confidence: 89%

Functional Properties and Cellular Distribution of the System A Glutamine Transporter SNAT1 Support Specialized Roles in Central Neurons

Mackenzie

Schäfer

Erickson³

et al. 2003

Journal of Biological Chemistry

133

149

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Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K 0.5 Ϸ 0.3 mM), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na ؉ electrochemical gradient. The voltage-dependent binding of Na ؉ precedes that of the amino acid in a simultaneous transport mechanism. Li ؉ (but not H ؉ ) can substitute for Na ؉ but results in reduced V max . In the absence of amino acid, SNAT1 mediates Na ؉ -dependent presteady-state currents (Q max Ϸ 9 nC) and a nonsaturable cation leak with selectivity Na ؉ , Li ؉ > > H ؉ , K ؉ . Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na ؉ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via ␣-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

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Section: Mechanisms Of Snat1-mediated Amino Acid Transport-mentioning

confidence: 89%

Functional Properties and Cellular Distribution of the System A Glutamine Transporter SNAT1 Support Specialized Roles in Central Neurons

Mackenzie

Schäfer

Erickson³

et al. 2003

Journal of Biological Chemistry

133

149

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“…3A). However, the ability of Li+ to replace Na+ is partially dependent on the amino acid sidechain structure (Christensen and Handlogten, 1977).…”

Section: Effect Of Cell Density On the Adaptive Regulation Of System Amentioning

confidence: 99%

Adaptive regulation of neutral amino acid transport system A in rat H4 hepatoma cells

Kilberg

Han

Barber

et al. 1985

Journal Cellular Physiology

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Substrate regulation of System A transport activity in rat H4 hepatoma cells is described. The uptake of several amino acids was tested in the presence of system-specific inhibitors. System A activity was increased in a RNA- and protein synthesis-dependent manner by amino acid deprivation of the cells (adaptive regulation), whereas transport by Systems ASC, N, y+, and L was unaffected. Unlike human fibroblasts, the H4 cells did not require serum to exhibit the depression of System A. At cell densities between 88 X 10(3) and 180 X 10(3) cells/cm2, the degree of adaptive regulation was inversely related to cell density. Both transport of AIB and adaptive regulation of System A were nearly abolished if either K+ or Li+ was substituted for Na+ in the medium. The presence of cycloheximide or tunicamycin blocked further increases in starvation-induced activity within 1 hr of addition, suggesting the involvement of a plasma membrane glycoprotein. In contrast, if the medium was supplemented with actinomycin after the stimulation of System A had begun, the activity continued to increase for an additional 2 hr before being slowed by the inhibitor. The contributions of trans-inhibition and repression to the amino acid-induced decay of System A activity were estimated for several representative amino acids. In general, the System A activity in normal rat hepatocytes was much less sensitive to trans-inhibition than the corresponding activity in H4 hepatoma cells. The half-life values for the amino acid-dependent decay of System A ranged from 0.5 to 2.0 hr.

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“…Its differences from System A leave no doubt of the independence of the 2: System ASC is less sensitive to H+ and will not accept Li+ as a substitute for Na+. The receptor site of System ASC binds the alkali ion at a closely specified point in juxtaposition t o the hydroxyl group of ordinary (trans) 4-hydroxyproline (4), whereas System A binds Na+ or Li+ at a different, less precisely localized point rather nearer the P-carbon atom of the bound amino acid substrate (5). The ASC system is conspicuous in immature and nucleated red cells (6)(7)(8), leukemia cells (9), and lymphocytes (10); it participates in placental transport (1 l), and in general it appears to be ubiquitous.…”

Section: :Jssmentioning

confidence: 99%

Amino acid transport systems in animal cells: Interrelations and energization

Christensen

1977

J Supramol Struct

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After summarizing the discrimination of the several transport systems for neutral amino acids in the cell of the higher animal, I discuss here the ways in which 2 dissimilar transport systems interact, so that one tends t o run torward for net entry and the other backwards for net exodus. An evaluation of the proposals for energization shows that uphill transport continues when neither alkali-ion gradients nor ATP levels are favorable. Evidence is presented that under these conditions a major contribution is made by another mode of energization, which may depend on the fueling of an oxidoreductase in the plasma membrane. This fueling may involve the export by the mitochondrion of the reducing equivalents of NADH by one of the known shuttles, e.g., the malate-aspartate shuttle. After depletion of the energy reserves in the Ehrlich cell by treating it with dinitrophenol plus iodoacetate concentrative uptake of test amino acids is restored by pyruvate, but in poor correlation with the restoration of alkali-ion gradients and ATP levels. This restoration by pyruvate but not by glucose is highly sensitive to rotenone. A combination of phenazine methosulfate and ascorbate will also produce transport restoration, before either the alkali-ion gradients or ATP levels have begun to rise. The restoration of transport applies to a model amino acid entering by the Na+-independent system, as well as to one entering by the principal Na+-dependent system, restoration being blocked by ouabain, despite the weak effect of ouabain on the alkali-ion gradients in the Ehrlich cell. Quinacrine terminates very quickly the uptake of model amino acids, before the alkali-ion gradients have begun to fall and before the ATP level has been halved. Quinacrine is also effective in blocking restoration of uphill transport by either pyruvate or the phenazine reagent. Preliminary results show that vesicles prepared from the plasma membrane of the Ehrlich cell quickly reduce cytochrome c or ferricyanide in the presence of NADH, and that the distribution of a test amino acid between the vesicle and its environment is influenced by NADH, quinacrine, and an uncoupling agent in ways consistent with the above proposal, assuming that a majority of the vesicles are everted.Key words: amino acid transport in animal cells, energization of transport systems, discrimination of transport systems, reverse operation of transport systems, Ehrlich cell, NADH dehydrogenase, alkali-ion gradients, phenazine methosulfate, ouabain TRANSPORT SYSTEMS AND THEIR INTERACTIONThe transport systems for neutral amino acids in the Ehrlich cell are taken t o be approximately representative of those of the cells of the higher animal in general, with more and more evidence supporting that interpretation. Most conspicuous are a broad-

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Na+/L+ selectivity in transport systemA: Effects of substrate structure

Cited by 45 publications

References 18 publications

Functional Properties and Cellular Distribution of the System A Glutamine Transporter SNAT1 Support Specialized Roles in Central Neurons

Functional Properties and Cellular Distribution of the System A Glutamine Transporter SNAT1 Support Specialized Roles in Central Neurons

Adaptive regulation of neutral amino acid transport system A in rat H4 hepatoma cells

Amino acid transport systems in animal cells: Interrelations and energization

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