The transport of alanine, serine, and cysteine in cultured human fibroblasts

The mechanism of action of N-acetylcysteine (NAC): The emerging role of H2S and sulfane sulfur species

Initially adopted as a mucolytic about 60 years ago, the cysteine prodrug N-acetylcysteine (NAC) is the standard of care to treat paracetamol intoxication, and is included on the World Health Organization's list of essential medicines. Additionally, NAC increasingly became the epitome of an "antioxidant". Arguably, it is the most widely used "antioxidant" in experimental cell and animal biology, as well as clinical studies. Most investigators use and test NAC with the idea that it prevents or attenuates oxidative stress. Conventionally, it is assumed that NAC acts as (i) a reductant of disulfide bonds, (ii) a scavenger of reactive oxygen species and/or (iii) a precursor for glutathione biosynthesis. While these mechanisms may apply under specific circumstances, they cannot be generalized to explain the effects of NAC in a majority of settings and situations. In most cases the mechanism of action has remained unclear and untested. In this review, we discuss the validity of conventional assumptions and the scope of a newly discovered mechanism of action, namely the conversion of NAC into hydrogen sulfide and sulfane sulfur species. The antioxidative and cytoprotective activities of per- and polysulfides may explain many of the effects that have previously been ascribed to NAC or NAC-derived glutathione.

Cysteine Uptake for Accumulation of Glutathione by the Cyanobacterium Synechocystis strain PCC 6803

By incubation with precursor amino acids, L-glutamate, L-cysteine, and glycine, or with L-cysteine, the unicellular cyanobacterium Synechocystis strain PCC 6803 increased intracellular glutathione levels, under illumination with white light. L-Cystine could not serve as the precursor. Two transport systems for L-cysteine were evident in the strain: a high (Ks; L-cysteine concentration giving one-half of the maximum uptake, 21~35 μM) and a low (Ks, 1.0~1.8 mM) affinity transport system. The latter, responsible for glutathione accumulation, was an energy-requiring process. L-Cysteine uptake by the two transport systems were Na(+)-independent, and inhibited by the presence of neutral amino acids and less inhibited by basic or acidic amino acids. These results suggested that the neutral amino acid symport(s) is involved in L-cysteine uptake for the GSH accumulation in this strain.

Rate of transport of l-arginine is independent of the expression of inducible nitric oxide synthase in HEK 293 cells

Expression of inducible nitric oxide synthase (iNOS) is generally accompanied by a parallel upregulation in l-arginine transport which is dependent, at least in part, on the synthesis of new carrier proteins. It is not clear however whether the induction of iNOS and its subsequent utilisation of l-arginine for NO synthesis contribute to the enhancement in l-arginine transport rates observed following induction of cells with pro-inflammatory mediators. To address this issue, we have transfected an iNOS construct in a pEGFP-N1 vector into HEK-293 cells and investigated the effects this has on l-arginine transport. The expression of iNOS through transfection resulted in the production of significant quantities of NO as detected by the standard Griess assay. Under these conditions, the transport of l-arginine was found to be unaltered, with rate of uptake being comparable in both transfected and non-transfected cells. Characterisation of the transporter(s) involved with uptake of l-arginine revealed features characteristic of the classical cationic amino acid transport system y(+). Further analysis of the expression profile of the cationic amino acid transporter (CAT) involved revealed the presence of transcripts for CAT-1 and CAT-2B. These data demonstrate that iNOS activity does not drive or enhance l-arginine transport despite the fact that HEK-293 cells transport l-arginine via the CATs, including CAT-2B which is thought to be critical for supply of substrate to iNOS.

New inhibitors for the neutral amino acid transporter ASCT2 reveal its Na+-dependent anion leak

The neutral amino acid transporter ASCT2 catalyses uncoupled anion flux across the cell membrane in the presence of transported substrates, such as alanine. Here, we report that ASCT2 conducts anions already in the absence of transported substrates through a leak anion-conducting pathway. The properties of this leak anion conductance were studied by electrophysiological recording from ASCT2-expressing HEK293 cells. We found that the leak anion conductance was inhibited by the binding of the newly characterized inhibitors benzylserine and benzylcysteine to ASCT2. These inhibitors competitively prevent binding of transported substrates to ASCT2, suggesting that they bind to the ASCT2 binding site for neutral amino acid substrates. The leak anion conductance exhibits permeation properties that are similar to the substrate-activated anion conductance of ASCT2, preferring hydrophobic anions such as thiocyanate. Inhibition of the leak anion conductance by benzylserine requires the presence of extracellular, but not intracellular Na(+). The apparent affinity of ASCT2 for extracellular Na(+) was determined as 0.3 mm. Interestingly, a Na(+)-dependent leak anion conductance with similar properties was previously reported for the related excitatory amino acid transporters (EAATs), suggesting that this leak anion conductance is highly conserved within the EAAT protein family.

Identification and characterization of uptake systems for cystine and cysteine in cultured astrocytes and neurons: evidence for methylmercury-targeted disruption of astrocyte transport

Maintenance of appropriate intracellular glutathione (GSH) levels is crucial for cellular defense against oxidative damage. A suggested mechanism of methylmercury (MeHg) neurotoxicity implicates the involvement of oxygen radical formation and a decrease in cellular levels of GSH. Astrocytes play an important role in providing GSH precursors to neurons, and as will be discussed in this review, altered GSH homeostasis likely leads to impairment of astrocytic handling of glutamate, and neuronal energy metabolism. The review summarizes recent observations on transport systems for cysteine and cystine, precursors of GSH, in primary cultures of astrocytes and neurons, and their sensitivity to MeHg treatment.

Electrogenic Na(+)-dependent L-alanine transport in the lizard duodenum. Involvement of systems A and ASC

L-Alanine transport across the isolated duodenal mucosa of the lizard Gallotia galloti has been studied in Ussing chambers under short-circuit conditions. Net L-alanine fluxes, transepithelial potential difference (PD), and short-circuit current (Isc) showed concentration-dependent relationships. Na(+)-dependent L-alanine transport was substantially inhibited by the analog alpha-methyl aminoisobutyric acid (MeAIB). Likewise, MeAIB fluxes were completely inhibited by L-alanine, indicating the presence of system A for neutral amino acid transport. System A transport activity was electrogenic and exhibited hyperbolic relationships for net MeAIB fluxes, PD, and Isc, which displayed similar apparent K(m) values. Na(+)-dependent L-alanine transport, but not MeAIB transport, was partially inhibited by L-serine and L-cysteine, indicating the participation of system ASC. This transport activity represents the major pathway for L-alanine absorption and seemed to operate in an electroneutral mode with a negligible contribution to the L-alanine-induced electrogenicity. It is concluded from the present study that the active Na(+)-dependent L-alanine transport across the isolated duodenal mucosa of Gallotia galloti results from the independent activity of systems A and ASC for neutral amino acid transport.

Astrocytes provide cysteine to neurons by releasing glutathione

Cysteine is the rate-limiting precursor of glutathione synthesis. Evidence suggests that astrocytes can provide cysteine and/or glutathione to neurons. However, it is still unclear how cysteine is released and what the mechanisms of cysteine maintenance by astrocytes entail. In this report, we analyzed cysteine, glutathione, and related compounds in astrocyte conditioned medium using HPLC methods. In addition to cysteine and glutathione, cysteine-glutathione disulfide was found in the conditioned medium. In cystine-free conditioned medium, however, only glutathione was detected. These results suggest that glutathione is released by astrocytes directly and that cysteine is generated from the extracellular thiol/disulfide exchange reaction of cystine and glutathione: glutathione + cystine<-->cysteine + cysteine-glutathione disulfide. Conditioned medium from neuron-enriched cultures was also assayed in the same way as astrocyte conditioned medium, and no cysteine or glutathione was detected. This shows that neurons cannot themselves provide thiols but instead rely on astrocytes. We analyzed cysteine and related compounds in rat CSF and in plasma of the carotid artery and internal jugular vein. Our results indicate that cystine is transported from blood to the CNS and that the thiol/disulfide exchange reaction occurs in the brain in vivo. Cysteine and glutathione are unstable and oxidized to their disulfide forms under aerobic conditions. Therefore, constant release of glutathione by astrocytes is essential to maintain stable levels of thiols in the CNS.

Comparison of sulfur amino acid utilization for GSH synthesis between HepG2 cells and cultured rat hepatocytes

HepG2 cells are widely used as a model of human hepatocytes for studies of drug metabolism and toxicity. However, GSH metabolism in HepG2 cells is poorly characterized. This report describes the utilization of sulfur amino acids for GSH synthesis in HepG2 cells. In contrast to primary cultures of rat hepatocytes, which rely mostly on methionine for GSH synthesis, HepG2 cells use cystine. Their inability to utilize methionine for GSH synthesis was not due to lack of methionine uptake or low cellular ATP levels, but rather to the lack of S-adenosyl-methionine synthetase activity. When HepG2 cells were cultured overnight in medium containing cystine as the only sulfur amino acid, addition of glutamate or acivicin had minimal to no effect on cell GSH; however, addition of threonine significantly depleted cell GSH. When cystine (0.18 mM) uptake was measured, glutamate (2.5 mM), which inhibited cystine uptake in cultured rat hepatocytes, had a minimal effect in HepG2 cells. Instead, threonine (20 mM) strongly inhibited the apparent uptake of cystine by HepG2 cells. Strong inhibition by threonine of apparent cystine uptake was actually due to inhibition of cysteine uptake, which resulted from GSH-cystine mixed disulfide exchange. Radio-HPLC confirmed this. After incubating cells with [35S]cystine (0.18 mM) for 10 min, the total counts inside the cell matched the counts in the uptake medium in the form of GSH-cysteine mixed disulfide. Finally, HepG2 cells took up cysteine by both Na(+)-dependent and -independent mechanisms. The former exhibited high affinity and low capacity, whereas the latter exhibited the opposite. At a physiologic concentration of cysteine (10 microM), 68% of cysteine uptake occurred via the Na(+)-dependent system and 32% via system L1.

Cystine uptake and glutathione level in fetal brain cells in primary culture and in suspension

The glutathione level and the factors affecting this level were investigated in fetal rat brain cells in a primary culture. Early in the culture, the glutathione level of the brain cells decreased, but after 5 h it began to increase. This increase was not observed in a cystine-free medium and was prevented by excess glutamate. Cystine was taken up in freshly isolated brain cell suspensions, and its rate increased during the culture. The cystine uptake was mediated by a Na(+)-independent, glutamate-sensitive route previously found in various types of cells and designated as system X-c. The uptake of cystine is a crucial factor in maintaining the glutathione level of the cells under culture, because it provides cysteine for the cells for glutathione synthesis. Cysteine was undetectable in the medium before the culture, but it appeared, though at a very low level, when the brain cells were cultured there. The source of this cysteine was the cystine in the medium. Presumably the decrease in the glutathione level of the cells in the early stage of the culture resulted from the fact that the medium did not contain cysteine. The enhancement of the cystine uptake during culture may constitute a protective mechanism against the oxidative stress to which the cultured cells are exposed. Regulation of the glutathione level in fetal brain cells in vivo by the transport of cystine and cysteine is discussed.

Characterization of amino acid transport in human endothelial cells

The transport of amino acids has been studied in human umbilical vein endothelial cells. Neutral amino acids enter human umbilical vein endothelial cells through three distinct agencies endowed with the characteristics of systems A, ASC, and L. Each system has been studied by evaluating the influx of preferential substrates. The influx of L-proline and 2-methylaminoisobutyric acid occurs through an Na(+)-dependent adaptively regulated trans-inhibited agency identifiable with system A. L-Threonine influx occurs mainly through a distinct Na(+)-dependent trans-stimulated pathway corresponding to system ASC. System L accounts for Na(+)-independent influx of L-leucine. These systems cooperate for the transport of L-glutamine, which is due mainly to system ASC, whereas the component due to the operation of system A increases upon amino acid starvation. No clear evidence was found for a glutamine-specific system ("system N"). Two systems, one Na+ dependent (system XAG-) and the other Na+ independent (system xc-), transport anionic amino acids. L-Arginine influx exhibits a poor dependence on extracellular Na+, whereas it is sensitive to conditions known to change membrane potential and to trans-stimulation by intracellular amino acids. These features are consistent with a process mediated by system y+ and may be of significance for the regulation of the intracellular concentration of L-arginine.

Numerical analysis reveals complexities of glutamate transport

The uptake of radiolabeled glutamate into cultured human (HeLa S3) and hamster (CHO-K1) cells was analyzed according to modified Michaelis-Menten models fit by the Marquardt least-squares method. Kinetic parameters not previously reported for these cells were obtained. Some rarely used features available with this fitting method proved to be extremely helpful. Most importantly, a goodness-of-fit measure revealed a significant alteration of glutamate uptake in HeLa cells that was induced by starvation. This apparent regulation, unexpected for glutamate transport, might have been missed if the fit had been judged by eye or by the magnitude of parameter standard deviations. Techniques for analyzing parameter distributions, improving experimental design and performing tests of significance are also described.

Triphasic vascular effects of thiol compounds and their oxidized forms on dog coronary arteries

The vascular effects of 2-mercaptoethanol, cysteamine, L-cysteine, glutathione (GSH), cystamine and oxidized GSH (GSSG) on the isometric tension of isolated dog coronary arterial strips were examined, and these effects were compared with the triphasic response induced by dithiothreitol (DTT); a rapid and weak contraction (phase A), an intervening slow relaxation (phase B) and a slowly-developing strong contraction (phase C) which we previously reported. The responses of the arteries induced by 2-mercaptoethanol, cysteamine and L-cysteine consisted of phases A, B and C. The order of contractile potency (ED50 of phase C) was DTT approximately L-cysteine > 2-mercaptoethanol approximately cysteamine, while the order of relaxant potency (ED50 of phase B) was DTT > cysteamine approximately 2-mercaptoethanol. GSSG and cystamine mainly produced relaxation, which corresponded to phase B. The phase C contraction was specific to the reduced forms of thiols, except for GSH, which produced only relaxation. The participation of endothelial cells was not essential for the contracting or relaxing effects of the thiol compounds. The phase C contraction was depressed by W-7, a calmodulin antagonist, while phase A was not. Therefore calmodulin-dependent protein kinases may participate in phase C, not in phase A.

The preferential interaction of L-threonine with transport system ASC in cultured human fibroblasts

The transport of L-threonine was studied in cultured human fibroblasts. A kinetic analysis of L-threonine transport in a range of extracellular concentrations from 0.01 to 20 mM indicated that this amino acid enters cells through both Na(+)-independent and Na(+)-dependent routes. These routes are: (1) a non-saturable, Na(+)-independent route formally indistinguishable from diffusion; (2) a saturable, Na(+)-independent route inhibitable by the analog BCH and identifiable with system L; (3) a low-affinity, Na(+)-dependent component (Km = 3 mM) which can be attributed to the activity of system A since it is adaptively enhanced by amino acid starvation and suppressed by the characterizing analog MeAIB and (4) a high-affinity, Na(+)-dependent route (Km = 0.05 mM). This latter route is identifiable with system ASC since it is insensitive to adaptive regulation, uninhibited by MeAIB, trans-stimulated by intracellular substrates of system ASC, markedly stereoselective, and relatively insensitive to changes in external pH. At an external concentration of 0.05 mM more than 90% of L-threonine transport is referrable to the activity of system ASC; in these conditions, the transport of the amino acid exhibits typical ASC-features even in the absence of inhibitors of other transport agencies, and, therefore, it can be employed as a reliable indicator of the activity of transport system ASC in cultured human fibroblasts.

Phorbol esters stimulate the transport of anionic amino acids in cultured human fibroblasts

The effect of phorbol esters on the transport of amino acids has been evaluated in cultured human fibroblasts. The activity of the Na(+)-dependent system XAG- for anionic amino acids is selectively and markedly stimulated by phorbol esters. The effect is maximal within 15 min; it is attributable to an increase in transport maximum (Vmax) and not prevented by inhibitors of protein synthesis. The half-maximal stimulation is observed at concentrations of phorbol 12,13-dibutyrate lower than 100 nM. Prolonged incubations in the presence of 1 microM phorbol 12,13-dibutyrate lower the binding of the ligand to its receptor with a loss of the stimulatory effect on transport. The results presented indicate that the stimulation of amino acid transport through system XAG- by phorbol esters requires the activation of protein kinase C.

Homocysteine uptake by human umbilical vein endothelial cells in culture

The characteristics of the uptake of L-homocysteine by cultures of human umbilical vein endothelial cells have been examined. Uptake occurred by Na(+)-dependent and Na(+)-independent systems, but was essentially independent of the pH of the uptake medium. The Na(+)-independent system corresponded to system L, being totally inhibited by the presence of beta-2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) a system L analogue. It was concluded on the basis of starvation experiments coupled with failure to detect any inhibition in the presence of 2-methylaminoisobutyric acid (MeAIB), a system A analogue, that the Na(+)-dependent uptake was wholly accounted for by system ASC. The kinetic properties of systems L and ASC were determined by omitting Na+ from the uptake medium and incorporating BCH in the medium, respectively. It has been concluded on the basis of the inhibitory effects of a number of amino acids that uptake of homocysteine occurs by those systems which transport cysteine.

Rapid and steady-state amino acid transport in perfused human fibroblasts and colon adenocarcinoma cells: effects of methotrexate

Initial and steady-state uptakes of serine and phenylalanine by human fibroblasts and human colon tumour cells were studied applying a double isotope dilution technique to perfused populations of cultivated cells retained on microcarrier beads. This new method permits the differentiation of the unidirectional transport parameters and can also distinguish between membrane-associated processes and independently intracellular events in isolated cells. High initial L-serine uptake values in colon adenocarcinoma cells became negative under steady-state conditions. To determine if the observed negative L-serine uptake was produced by the rapid efflux of intracellular L-[3H]serine, the cells were treated with methotrexate (MTX) (an inhibitor of cytosolic dihydrofolate reductase). The modified curve of L-[3H]serine uptake after MTX treatment suggests that, under these experimental conditions, net serine transport is non concentrative in colon tumour cells and could be modulated by the rate of intracellular serine metabolism; it also suggests that MTX does not directly affect serine transport in perfused human colon adenocarcinoma cells. Initial and steady-state uptakes of phenylalanine were high in both fibroblasts and tumour cells and were unaffected by MTX treatment.

The transport of L-glutamine into cultured human fibroblasts

The transport of L-glutamine has been studied in diploid human fibroblasts in culture. Mathematical discrimination by nonlinear regression, competition analysis, and conditions varying the relative contribution of the various mediations have been used to characterize the systems engaged in the inward transport of this amino acid. The adopted criteria showed that L-glutamine enters the fibroblast by the Na(+)-dependent systems ASC and A and by a Na(+)-independent route identified as system L. The relative contribution of these agencies to the total saturable uptake of glutamine varied with the concentration of the amino acid and with the nutritional state of the cell. At amino acid concentrations approaching those encountered in human plasma: (1) system ASC represented the primary mediation for entry of L-glutamine in human fibroblasts; (2) the contribution of system A was lower, though significant, in unstarved repressed cells and became predominant in starved derepressed cells; (3) the Na(+)-dependent system L accounted for less than one-fifth of glutamine uptake in either nutritional condition. The changes in the relative contribution of the various systems to the uptake of glutamine as a function of its concentration may have implications in pathophysiology under conditions associated with enhanced glutamine concentrations in the extracellular fluids.

Inhibition of sodium-dependent L-leucine uptake in rat brain synaptosomes

Synaptosomes isolated from adult rat cerebral cortices were used for studying the uptake of L-leucine by the Na(+)-dependent route. Three non-metabolizable amino acid analogues, which had been used previously to discriminate the Na(+)-dependent A-type uptake system of animal cells, were employed in this study. It was found that Na(+)-dependent uptake of leucine was insensitive to inhibition by 2-aminoisobutyric acid (AIB) and N-methylaminoisobutyric acid (MeAIB) whereas N-methylalanine (NMA) was markedly inhibitory. Inhibition by NMA was stereospecific--only the L-isomer had a pronounced effect. Na(+)-dependent uptake of leucine as well as its inhibition by L-NMA were rather insensitive to changes in pH from 6 to 9. Kinetic analysis of inhibition by L-NMA of Na(+)-dependent uptake revealed a non-competitive type of inhibition with a Ki value of approximately 0.5 mM.

Selective potentiation by L-cysteine of apparent binding activity of [3H]glutathione in synaptic membranes of rat brain

Significant apparent binding activity of [3H]glutathione was detected in synaptic membranous preparations of the rat brain. In vitro addition of sucrose (50-1000 mM) and Triton X-100 (0.02-0.1%) significantly diminished the apparent binding activity, whereas pretreatment of the membranes with Triton X-100 (0.01-0.4%) did not affect the activity. A slight but statistically significant reduction of the apparent binding activity was induced by the in vitro addition (1 mM) of two constituent amino acids, L-glutamic acid and glycine. In contrast, another constituent amino acid, L-cysteine, potently enhanced the binding activity at a concentration higher than 0.1 mM. No prominent alteration of the activity occurred following the inclusion of structurally-related amino acids, dithiothreitol, dithioerythritol and numerous other amino acids. Scatchard analysis revealed that the apparent binding consisted of two independent separate components with Kd values of 0.76 and 11.0 microM, and Bmax values of 4.00 and 27.0 pmol/mg protein respectively. In vitro addition of 1 mM L-cysteine resulted in a single component with a Kd of 8.5 microM and a Bmax of 105 pmol/mg protein. Pretreatment of the membranes with 1 mM L-cysteine potentiated the apparent binding, with a further addition of L-cysteine having no effect. The retina had the highest activity followed by the hypothalamus, striatum, spinal cord, midbrain, hippocampus, medulla-pons, cerebellum and cerebral cortex, which occurred independently of the incubation temperature. In peripheral organs examined, the pituitary possessed higher activity than the retina, with progressively lower activities in the adrenal, liver, spleen, skeletal muscle and heart. No significant activity was detected in the kidney. Addition of 1 mM L-cysteine significantly potentiated the activities at 30 degrees, but not at 2 degrees, in the hippocampus and cerebral cortex without affecting those in other central structures. In contrast, a profound inhibition of the activity was induced by the addition of L-cysteine in the pituitary, adrenal, intestinal mucosa, skeletal muscle and retina independently of the temperature. These results suggest that L-cysteine may selectively potentiate the apparent binding activity of [3H]glutathione in particular regions of the brain, while eliminating that in the peripheral excitable tissues.

Temperature-dependent and -independent apparent binding activities of [3H]glutathione in brain synaptic membranes

An apparent binding activity of [3H]glutathione was examined by using synaptic membrane preparations of the rat brain. The activity was found to be more than two times as high at 30 degrees C as that found at 2 degrees C. At 2 degrees C, the apparent binding sites consisted of a single component with a Kd of 0.77 microM and a Bmax of 5.60 pmol/mg protein. In contrast, two independent separate sites with Kds of 0.56 and 12.6 microM and Bmaxs of 2.50 and 28.5 pmol/mg protein were observed at 30 degrees C. In vitro addition of Triton X-100 significantly inhibited the apparent binding activities detected at both temperatures, whereas pretreatment of the membranes with the detergent did not significantly affect both binding activities. Among 3 constituent amino acids of glutathione, L-cysteine induced a selective and irreversible potentiation of the apparent activities, which occurred independently of the incubation temperature. Scatchard analysis revealed that L-cysteine drastically increased the number of the low affinity sites without significantly altering their affinity. Apparent binding activities determined at both incubation temperatures were unevenly distributed in the central and peripheral structures. Distribution profile of the temperature-dependent activities was found to be closely related to that of the basal binding activity of [3H]L-glutamic acid, a putative central excitatory neurotransmitter. These results suggest that brain synaptic membranes may indeed contain specific binding sites of [3H]glutathione which have an interaction with the glutamate binding sites. Possible presence of two distinctly different apparent binding sites of [3H]glutathione, such as temperature-independent high affinity sites and temperature-dependent low affinity sites, is also suggested.

Glycine transport by cultured human fibroblasts

The transport of glycine was studied in cultured human fibroblasts. The amino acid entered the cell by Na+-dependent and Na+-independent mechanisms. Na+-independent glycine (0.1 mM) transport was less than 10% of total uptake and occurred by a mechanism formally indistinguishable from diffusion. Two distinct routes contributed to Na+-dependent glycine transport. The first route was identified with system A because it was inhibited by MeAIB and underwent adaptive regulation. The second route was identified with system ASC as it was inhibited by L-alanine, but not by MeAIB. Kinetic analysis revealed that the two systems operated glycine transport with the same Km of 1.6 mM, a value unusually high for system ASC.

Evidence for an alanine, serine, and cysteine system of transport in isolated brain capillaries

Brain capillaries isolated from 2-month-old male and female Sprague-Dawley rats were used to study the transport of neutral amino acids. The uptake of alanine, leucine, and alpha-methylaminoisobutyric acid (MeAIB) was a linear function of time for the first minute of incubation. Based on these observations, an incubation time of 1 min was used to measure transport activities. The intracellular water volume of the isolated capillaries was 2.2 microliters/mg protein. This value was significantly lower (1.8 microliter/mg protein) when measured in the absence of sodium. L-Alanine, L-serine, and L-cysteine were taken up from the abluminal surface of brain capillaries by a sodium- and energy-dependent, carrier-mediated system. This uptake, for the most part, was not inhibited by MeAIB. System ASC (alanine-serine-cysteine) appeared to be of primary importance for the transport of these amino acids in isolated brain capillaries. The apparent Km and Vmax values for L-alanine uptake by ASC transport, based on the Hofstee plot presentation, were 1.3 mM and 0.975 nmol/microliter water content/min, respectively. The results also indicate that the transport of MeAIB and 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) was limited to the sodium-dependent system A (alanine) and the sodium-independent system L (leucine), respectively.

L-proline uptake in human fibroblasts: evidence for a high-affinity system in addition to system A

Proline uptake was studied in human skin fibroblasts by simultaneous running of kinetic and inhibition experiments on the same cell lines. Two systems for proline uptake were shown: a high-affinity system not inhibited by alpha-(methylamino)isobutyric acid and a low affinity system inhibited by this amino acid (i.e. system A). These results appear to be of interest, firstly because up till now, system A was considered preferable for proline uptake in human fibroblasts, and secondly because they illustrate the need for combined inhibition and kinetic studies of amino acid uptake, especially when the substrate concentration range used and the respective Km of the systems do not allow their detection by kinetic analysis alone. Furthermore, this high-affinity system may have major physiological implications.

Characterization of L-threonine and L-glutamine transport in murine P388 leukemia cells in vitro. Presence of an N-like amino acid transport system

The transport of L-threonine and L-glutamine into murine P388 leukemia cells has been characterized. Threonine appears to be a specific substrate for a Na+-dependent amino acid transport system similar to system ASC of the HTC hepatoma cell. Threonine transport is uninhibited by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid and alpha-(methylamino)isobutyric acid, shows a pattern of transport similar to that seen in HTC hepatoma cells over the pH range of 5.5-7.5, and is inhibited by L-serine and L-cysteine. Approximately two-thirds of glutamine transport into P388 cells also appears to enter P388 cells via this ASC-analogous system. However, based upon (a) inhibition studies with threonine (where the K1 of threonine inhibition of glutamine transport was 7-fold the Km of threonine transport), (b) inhibition analysis of glutamine transport with various amino acids and amino acid analogues, and (c) different patterns of transport between threonine and glutamine over the pH range of 5.5-7.5, approximately one-third of glutamine transport can be attributed to a second Na+-dependent amino acid transport system. This system appears to be similar to the system N of rat hepatocytes. Glutamine and threonine do not appear to enter P388 cells via systems A or L to any significant degree. P388 cells do not appear to exhibit 'adaptive regulation' of amino acid transport. Differences in 'adaptive regulation' could therefore not be utilized for comparing threonine and glutamine transport.

Effect of extracellular potassium on amino acid transport and membrane potential in fetal human fibroblasts

The distribution ratio of the lipophilic cation tetraphenylphosphonium (TPP+) has been used to estimate the electrical potential difference across the plasma membrane in cultured human fibroblasts. These cells exhibit a membrane potential markedly influenced by the diffusion potential of K+. High extracellular potassium concentrations depolarize human fibroblasts and depress the activity of transport systems A, ASC (both serving for zwitterionic amino acids), X-AG (for anionic amino acids), and y+ (for cationic amino acids). High doses (100 microM) of the K+-ionophore valinomycin hyperpolarize the cells. This condition enhances the activity of systems A, ASC and y+. Transport systems L (for neutral amino acids) and x-C (for anionic amino acids) are insensitive to changes in extracellular K+ or to valinomycin. System X-AG is inhibited by the addition of 100 microM valinomycin, but the effect of the ionophore appears to be potential-independent. These results indicate that: (a) the activity of systems L and x-C is potential-independent and (b) the activity of systems A, ASC, X-AG and y+ is sensitive to alterations of external [K+] associated to changes in membrane potential.

Effect of insulin on the activity of amino acid transport systems in cultured human fibroblasts

The regulation of amino acid transport by insulin has been studied in cultured human fibroblasts. Among the six amino acid transport systems operating in cultured human fibroblasts, two systems (A and X-C) are strongly stimulated by insulin and four (ASC, X-AG, y+ and L) are essentially not sensitive to the presence of the hormone in the incubation medium. The hormonal stimulation of system A and system X-C became significant after 3 h of incubation and increased up to 12 h. The stimulatory effect was related to insulin concentration, with a half-maximal stimulation at 10(-9) M hormone concentration. Insulin enhanced transport activity by increasing the maximal velocity (Vmax) of transport, without significant changes in Km values.