Lysine Efflux Research Articles

Delayed skeletal development and IGF-1 deficiency in a mouse model of lysinuric protein intolerance.

SLC7A7 deficiency, or lysinuric protein intolerance (LPI), causes loss of function of the y+LAT1 transporter critical for efflux of arginine, lysine and ornithine in certain cells. LPI is characterized by urea cycle dysfunction, renal disease, immune dysregulation, growth failure, delayed bone age, and osteoporosis. We previously reported that Slc7a7 knockout mice (C57BL/6 x 129/SvEv F2) recapitulate LPI phenotypes, including growth failure. Our main objective was to characterize the skeletal phenotype in these mice. Compared to wild type littermates, juvenile Slc7a7 knockout mice demonstrated 70% lower body weights, 87% lower plasma IGF-1 concentrations, and delayed skeletal development. Because poor survival prevents evaluation of mature knockout mice, we generated a conditional Slc7a7 deletion in mature osteoblasts or mesenchymal cells of the osteo-chondroprogenitor lineage, butno differences in bone architecture were observed. Overall, global Slc7a7 deficiency caused growth failure with low plasma IGF-1 concentrations and delayed skeletal development, but Slc7a7 deficiency in the osteoblastic lineage was not a major contributor to these phenotypes. Future studies utilizing additional tissue-specific Slc7a7 knockout models may help dissect cell autonomous and non-cell autonomous mechanisms underlying phenotypes in LPI.

The Transport of Lysine across Monolayers of Human Cultured Intestinal Cells (Caco-2) Depends on Na+-Dependent and Na+-Independent Mechanisms on Different Plasma Membrane Domains

To characterize the mechanisms involved in the intestinal absorption of the essential amino acid L-lysine from the diet, the transepithelial transport of L-lysine was studied in monolayers of cultured human intestinal cells (Caco-2) grown and differentiated on microporous membrane supports. L-lysine was transported mainly in the apical (AP) to basolateral (BL) direction and the BL to AP transport was ∼ one order of magnitude lower at all concentrations tested. Nonlinear regression analysis of the transport in the AP to BL and the BL to AP direction identified, in both cases, single saturable components with similar Km but different Vmax and a nonsaturable diffusional component. The AP to BL L-lysine transport was highly energy- and sodium-dependent and was unaffected by an unfavorable concentration gradient. Selective replacement of sodium ions in the AP or the BL compartment and determination of both AP to BL transport and the intracellular soluble lysine pool showed that uptake occurs via a sodium-independent mechanism, not significantly influenced by membrane potential, whereas efflux is a sodium-dependent process. Competition experiments showed that L-lysine uptake is highly stereospecific and is shared by cationic and large neutral amino acids. This study demonstrates the presence of a sodium-dependent mechanism of lysine efflux across the BL membrane of intestinal cells, which may be essential for lysine transport into the blood circulation. Overall, these results support the use of the Caco-2 cell model for studies of intestinal nutrient transport.

Cloning and functional expression of a cDNA from rat jejunal epithelium encoding a protein (4F2hc) with system y+L amino acid transport activity.

Two different protein families, designated CAT (cationic amino acid transporter) and BAT (broad-specificity amino acid transporter) mediate the plasma membrane transport of cationic amino acids in animal cells. CAT transporters have 12-14 transmembrane domains and are selective for cationic amino acids. BAT proteins, in contrast, have one to four transmembrane domains and induce the transport of both cationic and zwitterionic amino acids when expressed in Xenopus oocytes. Mutations in the human BAT gene cause type I cystinuria, a disease affecting the ability of intestinal and renal brush border membranes to transport cationic amino acids and cystine. We have used functional expression cloning in oocytes to isolate a BAT-related cDNA from rat jejunal epithelium. The cDNA encodes the rat 4F2 heavy chain (4F2hc) cell-surface antigen, a 527-residue (60 kDa) protein that is 26% identical in amino acid sequence with rat renal BAT (also known as NBAT/D2). Expression of rat jejunal 4F2hc in oocytes induced the lysine-inhibitable Na+-dependent influx of leucine and the leucine-inhibitable Na+-independent influx of lysine. Lysine efflux was stimulated by extracellular (Na+ plus leucine). These characteristics identify the expressed amino acid transport activity as system y+L, a transporter that has been implicated in basal membrane transport of cationic amino acids in intestine, kidney and placenta.

Characterization of Vacuolar Arginine Uptake and Amino Acid Efflux inNeurospora crassaUsing Cupric Ion to Permeabilize the Plasma Membrane

Treatment ofNeurospora crassamycelia with cupric ion has been shown to permeabilize the plasma and mitochondrial membranes. Permeabilized mycelia were shown to take up arginine into the vacuoles. Uptake was ATP-independent and appeared to be driven by an existing K+-gradient. The kinetic characteristics of the observed uptake were similar to those observed using vacuolar membrane vesicles: theKmfor arginine uptake was found to be 4.2–4.5 mM. Permeabilized mycelia were used to study the regulation of arginine uptake into vacuoles. The results suggest that uptake is relatively indifferent to the contents of the vacuoles and is not affected by growth of mycelia in amino acid-supplemented medium. Efflux of arginine, lysine, and ornithine from vacuoles was also measured using mycelia permeabilized with cupric ion. Arginine release was shown to be specifically enhanced by cytosolic ornithine and/or increases in the vacuolar pool of arginine or ornithine. Lysine efflux was shown be indifferent to the presence of other amino acids. These observations emphasize the importance of vacuolar compartmentation in controlling arginine and ornithine metabolism and suggest that vacuolar compartmentation may play an important role in nitrogen homeostasis of filamentous fungi.

The Efflux of Lysine from the Basolateral Membrane of Human Cultured Intestinal Cells (Caco-2) Occurs by Different Mechanisms Depending on the Extracellular Availability of Amino Acids ,

The efflux of the nutritionally essential amino acid, L-lysine from the basolateral (BL) membrane was characterized in human cultured intestinal cells (Caco-2) grown and differentiated on permeable filter supports. Cells were loaded by incubating with 3H-lysine from the apical (AP) side in the absence of sodium (substituted with choline) in the BL medium; under these conditions, cells accumulated lysine in the intracellular soluble pool to 10- to 20-fold the extracellular concentration. L-Lysine efflux in the BL medium was then followed, and initial rates of efflux were calculated under different experimental conditions. L-Lysine efflux exhibited a strong energy dependence. The presence of an inwardly directed gradient of sodium or lithium stimulated lysine efflux; ouabain reduced efflux in both sodium- and lithium-containing medium. When zwitterionic or cationic amino acids were added to the BL medium, L-lysine efflux was strongly stimulated. The most efficient trans-stimulating amino acids were L-leucine > L-methionine = L-ornithine = L-arginine. In the presence of trans-stimulating amino acids in the BL medium, L-lysine efflux exhibited energy independence and was not affected by the presence of a sodium gradient. In addition, the sensitivity of efflux to N-ethylmaleimide was different in the absence or in the presence of amino acids in the BL medium. These results suggest that different mechanisms may operate in the BL efflux of L-lysine from human intestinal epithelial cells, depending on the extracellular availability of other amino acids, to guarantee optimal bioavailability of this essential amino acid both in the postprandial absorptive period and between meals.

Na + -independent Lysine Transport in Human Intestinal Caco-2 Cells

The nature of transepithelial and cellular transport of the dibasic amino acid lysine in human intestinal epithelial Caco-2 cells has been characterized. Intracellular accumulation of lysine across both the apical and basolateral membranes consists of a Na+-independent, membrane potential-sensitive uptake. Na+-independent lysine uptake at the basolateral membrane exceeds that at the apical membrane. Lysine uptake consists of both saturable and nonsaturable components. Na+-independent lysine uptake at both membranes is inhibited by lysine, arginine, alanine, histidine, methionine, leucine, cystine, cysteine and homoserine. In contrast, proline and taurine are without inhibitory effects at both membranes. Fractional Na+-independent lysine efflux from preloaded epithelial layers is greater at the basolateral membrane and shows trans-stimulation across both epithelial borders by lysine, arginine, alanine, histidine, methionine, and leucine but not proline and taurine. Na+-independent lysine influx (10 micron) in the presence of 10 mm homoserine shows further concentration dependent inhibition by lysine. Taken together, these data are consistent with lysine transport being mediated by systems bo,+, y+ and a component of very low affinity (nonsaturable) at both membranes. The relative contribution to lysine uptake at each membrane surface (at 10 micron lysine), normalized to total apical uptake (100%), is apical bo,+ (47%), y+ (27%) and the nonsaturable component (26%), and basal bo,+ (446%), y+ (276%) and the nonsaturable component (20%). Northern analysis shows hybridization of Caco-2 poly(A)+RNA with a human rBAT cDNA probe.

Transport of nitric oxide synthase inhibitors through cationic amino acid carriers in human erythrocytes

The interaction of arginine analogues, which are known to inhibit nitric oxide synthase, with two cationic amino acid transporters of human erythrocytes (systems y + and y +L) was studied. Arginine and relevant analogues [ N G-monomethyl-L-arginine (L-NMMA); N G-monomethyl-D-arginine (D-NMMA) and N G-nitro-L-arginine (L-NOARG)] were found to inhibit labeled lysine influx into intact erythrocytes. As expected, the pattern of inhibition reflected the contribution of the two distinct transport systems. All analogues showed a higher affinity for system y +L than for system y +. The half-saturation (inhibition) constants estimated for systems y + and y +L (±SEM) were (μM): l-arginine, 55.7 ± 5.4 and 2.4 ± 0.1; l-NMMA, 151 ± 13 and 7.5 ± 0.5; d-NMMA, 2660 ± 404 and 269 ± 25; l-NOARG, 9414 ± 169 and 594 ± 35. The transport properties of the analogues were investigated using an assay based on the trans-stimulation of lysine efflux. The addition of saturating concentrations of unlabeled analogues to the external medium stimulated efflux of labeled lysine through systems y +L and y +, showing that the analogues can enter the cell through these pathways.

Influence of organic cations on basic amino-acid uptake by human placental villi.

Human placental chorionic villi were incubated for 30 min with [3H]lysine or [3H]arginine and the distribution ratios (intracellular:extracellular concentrations) were determined. The ratios remained unchanged when Na+ in Earle's buffered salt solution was replaced with Li+. When Na+ was replaced with choline there was a significant increase is distribution ratios (lysine 1.34 +/- 0.33 v. 3.99 +/- 0.15, arginine 1.95 +/- 0.37 v. 5.05 +/- 1.16). Leucine, a neutral amino acid with a Na(+)-independent transport system, was unaffected by choline substitution. The distribution ratio for alanine, which is Na(+)-dependent, was reduced (2.50 +/- 0.41 v. 1.45 +/- 0.20). Two other quarternary amines, acetyl-beta-methylcholine and tetraethylammonium chloride (TEA) caused similar increases in the distribution ratios of the basic amino acids. Hordenine, a tertiary amine, was less effective and there was little or no effect with ephedrine, a secondary amine. The choline effect was first observable at concentrations of 105 mM. With TEA, there was a progressive increase in distribution ratios beginning at 29 mM. Lysine efflux was measured after incubation of villi with lysine in Earle's buffer or choline buffer. Lysine was rapidly released to the fresh medium with 25% more retained in choline-exposed villi. The amines may cause alterations in the kinetics of basic amino-acid transporters or may modify other aspects of placental physiology permitting an increase retention of the basic amino acids.

Lysine transport in lactating rat mammary tissue: evidence for an interaction between cationic and neutral amino acids

The transport of lysine by the lactating rat mammary gland has been examined to determine whether there is an interaction between cationic and neutral amino acids. Lysine uptake was time dependent and unaffected by replacing Na+ with choline. In the presence of Na+, lysine influx was inhibited by cationic amino acids (arginine, homoarginine, ornithine and lysine) and by a range of neutral amino acids (methionine, glutamine, leucine, phenylalanine, alanine, asparagine, alpha-aminoisobutyric acid (AIB), 2-aminobicyclo [2,2,1] heptane-2-carboxylic acid (BCH), proline and tryptophan). Leucine and glutamine also inhibited lysine influx in the absence of Na+ but phenylalanine and proline did not. Lysine efflux from mammary tissue was trans-accelerated by various cationic amino acids (lysine, arginine, homoarginine and ornithine). In addition, leucine and glutamine were capable of trans-stimulating lysine efflux in the presence and absence of Na+. It appears that cationic and neutral amino acids stimulated lysine efflux at a single locus.

Amino acid transport system y+L of human erythrocytes: specificity and cation dependence of the translocation step.

The transport specificity of system y+L of human erythrocytes was investigated and the carrier was found to accept a wide range of amino acids as substrates. Relative rates of entry for various amino acids were estimated from their trans-effects on the unidirectional efflux of L-[14C]-lysine. Some neutral amino acids, L-lysine and L-glutamic acid induced marked trans-acceleration of labeled lysine efflux; saturating concentrations of external L-leucine and L-lysine increased the rate by 5.3 +/- 0.63 and 6.2 +/- 0.54, respectively. The rate of translocation of the carrier-substrate complex is less dependent on the structure of the amino acid than binding. Translocation is slower for the bulkier analogues (L-tryptophan, L-phenylalanine); smaller amino acids, although weakly bound, are rapidly transported (L-alanine, L-serine). Half-saturation constants (+/- SEM) calculated from this effect (L-lysine, 10.32 +/- 0.49 microM and L-leucine, 11.50 +/- 0.50 microM) agreed with those previously measured in cis-inhibition experiments. The degree of trans-acceleration caused by neutral amino acids did not differ significantly in Na+, Li+ or K+ medium, whereas the affinity for neutral amino acids was dramatically decreased if Na+ or Li+ were replaced by K+. The observation that specificity is principally expressed in substrate binding indicates that the carrier reorientation step is largely independent of the forces of interaction between the carrier and the transport site.

Permeability of lipid bilayers to amino acids and phosphate

Permeability coefficients for amino acid classes, including neutral, polar, hydrophobic, and charged species, were measured and compared with values for other ionic solutes such as phosphate. The rates of efflux of glycine, lysine, phenylalanine, serine and tryptophan were determined after they were passively entrapped in large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine (EPC) or dimyristoylphosphatidylcholine (DMPC). The following permeability coefficients were obtained for: glycine, 5.7·10 −12 cm s −1 (EPC), 2.0·10 −11 cm s −1 (DMPC); serine, 5.5·10 −12 cm s −1 (EPC), 1.6·10 −11 cm s −1 (DMPC); lysine, 5.1·10 −12 cm s −1 (EPC), 1.9·10 −11 cm s −1 (DMPC); tryptophan, 4.1·10 −10 cm s −1 (EPC); and phenylalanine, 2.5·10 −12 cm s −1 (EPC). Decreasing lipid chain length increased permeability slightly, while variations in pH had only minor effects on the permeability coefficients of the amino acids tested. Phosphate permeability was in the range of 10 −12–10 −13 cm s −1 depending on the pH of the medium. The values for the polar and charged amino acids were surprisingly similar to those previously measured for monovalent cations such as sodium and potassium, which are in the range of 10 −12–10 −13 cm s −1, depending on conditions and the lipid species used. This observation suggests that the permeation rates for the neutral, polar and charged amino acids are controlled by bilayer fluctuations and transient defects, rather than partition coefficients and Born energy barriers. The results are relevant to the permeation of certain peptides into lipid bilayers during protein translocation and membrane biogenesis.

Ion dependence of cystine and lysine uptake by rat renal brush-border membrane vesicles

The shared transport system for uptake of l-cystine and l-lysine was examined in isolated rat renal brush-border membrane vesicles for the ionic requirements for activation of the system. No requirement for sodium was seen for either cystine or lysine influx. However, the efflux of lysine from the vesicle was stimulated by Na +. Therefore, the transport system appears to be asymmetric in its requirement for sodium. Two different divalent cations were used in the membrane isolations which resulted in different responses of cystine uptake to the electrogenic movement of K + out of the vesicle. Membranes prepared by Mg-aggregation showed no stimulation of cystine influx by the imposition of a transient interior negative potential while vesicles prepared by Ca-aggregation did respond to electrogenic stimulation by an outwardly directed K-diffusion potential in the presence of valinomycin. Lysine influx was stimulated by electrogenic potassium efflux in both Mg-prepared and Ca-prepared membranes. No difference in sodium requirement for cystine influx was seen between the vesicles isolated by different cation-aggregation methods.

Lysine excretion by Corynebacterium glutamicum

Corynebacterium glutamicum effectively excretes lysine when the internal lysine concentration is elevated. Lysine efflux was investigated using selected mutants which are not able to regulate lysine biosynthesis by feedback inhibition. Secretion of lysine is not the consequence of unspecific permeability of the plasma membrane but is mediated by a secretion carrier which is specific for lysine. Lysine export is characterized by high activation energy and follows Michaelis-Menten type kinetics with an internal Km of 20 mM and a Vmax of 12 nmol.min-1.mg dry cells-1. Excretion can proceed against a preexisting chemical gradient and against the electrical potential, which rules out a previously suggested pore model. Lysine excretion can also be observed in the wild-type strain especially under conditions of peptide uptake. Its possible physiological function may be related to regulation of internal amino acid concentrations under special growth conditions.

Amino acid transport across the hen colon: interactions between leucine and lysine

Several neutral amino acids such as leucine added on the mucosal side stimulate the mucosal-to-serosal flux (Jm----s) of cationic amino acids such as lysine. We show here in the colon of the NaCl-loaded hen that this is a result of interactions at the basolateral membrane. The hen colon was used because it tolerates prolonged incubation and has a relatively short tip of villus-to-muscularis mucosa distance. Two models have been proposed for the interpretation of the effect of mucosal-side addition of leucine on Jm----s of lysine: 1) one of an allosteric effect of cytoplasmic leucine on lysine efflux across the basolateral membrane; and 2) one of asymmetric heteroexchange of leucine from outside the basolateral membrane for cytoplasmic lysine. These two models were tested by measurements of influx across the brush-border membrane, of Jm----s and serosal-to-mucosal flux, and of steady state epithelial uptake of leucine and lysine. The data favor the model of allosteric interaction and are inconsistent with the model of asymmetric heteroexchange.

Lysine and alanine transport in the perfused guinea-pig placenta

The characteristics of l-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional l-[ 3H]lysine uptake (%) (perfusate lysine 50 μM) was high on maternal ( M = 87 ± 1) and fetal ( F = 73 ± 2) sides. l-[ 3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05–8.00 mM) gave K m values of 1.75 ± 0.70 mM and 0.90 ± 0.25 mM at maternal and fetal sides, respectively; corresponding V max values were 1.95 ± 0.38 and 0.87 ± 0.10 μmol·min −1·g −1. At both sides, lysine influx (50 μM) could be inhibited (about 60–80%) by 4 mM l-lysine and l-ornithine and less effectively (about 10–40%) by l-citrulline, l-arginine, d-lysine and l-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM l-alanine ( K m = 6.25 ± 3.27 mM; V max = 2.62 ± 0.94 μmol·min −1 · g −1), but unchanged by equimolar l-phenylalanine or l-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either l-alanine or l-phenylalanine; (iii) l-lysine and l-alaninie influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of l-[ 3H]lysine uptake by 4 mM l-homoserine was partially (60%) Na +-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na +-dependent, while lysine influx was partially Na +-dependent only at low concentrations (0.05–0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced l-[ 3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y +) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.

Transport of leucine, lysine, glycine and aspartate in neuroblastoma C1300 and glioma C6 cells

Non-saturable penetration and the V and Km constants of saturable influx of leucine, lysine and glycine were always greater in cultured neuroblastoma (C1300) than in glioma (C6) cells. Aspartate uptake was detected only in glioma cells. Unstimulated efflux of the amino acids was initially fast in both cell types but soon slowed down. The efflux of glycine and aspartate exhibited no heteroexchange, the efflux of lysine was stimulated by extracellular leucine and that of leucine slightly by lysine and glycine but only in glioma cells.

Is leucine an allosteric modulator of the lysine transporter in the intestinal basolateral membrane?

The transport of the dibasic amino acid L-lysine was investigated using basolateral membrane vesicles prepared from rat jejunal mucosal scrapings. The majority of the carrier-mediated transport was unaffected by the presence of sodium in the incubation medium, but voltage clamping of the vesicles did increase lysine uptake, indicating an associated movement of charge. Kinetic analysis of lysine influx and efflux showed the system to be symmetrical, but although the Vmax was comparable to other amino acid transport systems in this membrane, the dissociation constant for the overall reaction (KT) was an order of magnitude larger. This low affinity for lysine would explain the relatively slow rate of transport of this amino acid across the basolateral membrane. Competition experiments indicated that this system has a relatively narrow specificity carrying only lysine, arginine, ornithine, and histidine. In contrast the presence of L-leucine caused a marked stimulation of lysine efflux and influx across the vesicles. This effect was observed with leucine concentrations as low as 0.1 microM. It is concluded that although the lysine transport system in the basolateral membrane is slow in its basal state it can be rapidly turned on by the presence of L-leucine. The remarkably low concentrations required to do this suggest a possible allosteric interaction between the transporter and this neutral amino acid.

8. LYSINURIC PROTEIN INTOLERANCE (LPI): THE DEFECT IN CATIONIC AMINO ACID TRANSPORT IS EXPRESSED IN PLASMA MEMBRANE OF CULTURED FIBROBLASTS

Efflux of cationic amino acids lysine, arginine and ornithine at the basolateral membrane (BLM) of intestinal and renal tubular epithelia is defective in patients with LPI. Assuming that BLM of epithelial cells and plasma membrane of parenchymal cells are functionally analogous, we studied Na+ - independent transport of cationic amino acids and homoarginine (a nonmefabolized analogue) in cultured LPI fibreblasts and measured L-leucine transport as internal control. Net uptake of cationic amino acids was increased in LPI cells; homoarginine leucine uptake ratio was 0.30 ± 0.07 and 0.26 ± 0.01 (mean ± SEM) (p < 0.05) in LPI and control cells, respectively, at 10 min. The intracellular pool of cationic amino acids was increased (lys/ala 0.47 ± 0.05 vs. 0.31 ± 0.02; p< 0.01). The % trans-stimulation of homoarginine efflux was 1.0 ± 0.5 % in LPI homozygotes, 10 ± 0.5 % in heterozygotes and 22 ± 0.5 % in control cells. Our findings imply that the transport mutation in LPI is expressed asymmetrically at the membrane and concordantly at the basolateral membrane of epithelial and plasma membrane of parenchymaL cells.

Characteristics of lysine transport by isolated rat renal cortical tubule fragments

The uptake of l-lysine was examined in isolated renal cortical tubules. Lysine was actively taken up by the renal tubule cells isolated from 7-week-old rats. No metabolism of the transported lysine was found. There was no evidence for sodium-dependence of lysine uptake. Concentration dependence studies revealed that the lysine was taken up by one saturable transport system with a K m of 1.66 mmol/l and V max of 7 mmol/l intracellular fluid per 10 min. Lysine also entered by a non-saturable pathway. Arginine and ornithine inhibited the initial uptake of lysine. Cystine increased the efflux of lysine from preloaded renal cells via hetero-exchange, indicating that a common system exists for these two amino acids.

Modulation of lysine transport in cultured rat astrocytes and astrocytoma cells.

In the previous paper, it was shown that the transport of lysine into astrocytes and astrocytoma cells obeys the classical enzyme kinetics. Although unmodulated lysine transport into both normal rat astrocytes and rat astrocytoma cells is somewhat slower than needed for observed growth in the culture, it is capable of a large degree of enhancement. Insulin increases the Vmax for lysine influx in astrocytes tenfold and in astrocytoma cells fivefold. Glutathione produces a Vmax enhancement of 80% for astrocytes and 70% for astrocytoma cells. gamma-Glutamyl hydrazide is a weak inhibitor of lysine transport. Diethyl maleate appears to break down the regulation of lysine transport and allows a large increase in lysine influx in both cell types studied. Basic amino acid analogues canaline and S-aminoethylcysteine are not potent inhibitors of lysine transport. Lysine efflux kinetics are slower for C6 cells than for astrocytes; this difference is abolished by diethyl maleate and by dithiothreitol.