L-Tryptophan transport in human red blood cells

Red blood cell metabolism under prolonged anaerobic storage

Oxygen dependent modulation of red blood cell metabolism is a long investigated issue. However, the recent introduction of novel mass spectrometry-based approaches lends itself to implement our understanding of the effects of red blood cell prolonged exposure to anaerobiosis. Indeed, most of the studies conducted so far have addressed the short term issue, while the limited body of literature covering a 42 days storage period only takes into account a handful of metabolic parameters (ATP, DPG, glucose, glyceraldehyde 3-phosphate, and lactate). We hereby performed a mass spectrometry-based untargeted metabolomics analysis in order to highlight metabolic species in erythrocyte concentrates stored anaerobically in SAGM additive solutions for up to 42 days, by testing cells on a weekly basis. We could confirm previous evidence about long term anaerobiosis promoting glycolytic metabolism in RBCs and prolonging the conservation of high energy phosphate reservoirs and purine homeostasis. In parallel, we evidenced that, in contrast to aerobic storage, anaerobiosis impairs erythrocyte capacity to cope with oxidative stress by blocking metabolic diversion towards the pentose phosphate pathway, which negatively affects glutathione homeostasis. Therefore, although oxidative stress was less sustained than in aerobically stored counterparts, oxidative stress markers still accumulate over anaerobic storage progression.

Amino acid transport in human red blood cells

Rafaelsen (1) has proposed a membrane transport model involving transport function deficiencies in affective disorders. The deficiencies of transport concern a group pump as well as a specific pump in the cell membrane. As peripheral blood cells are important for the study of basic features essential to biological transport functions and have been suggested as models for membrane mechanisms in the CNS, a series of studies was undertaken to characterize amino acid transport in the human red blood cell (h-RBC). Five different transport systems were identified by kinetic analysis and studies of inhibition and sodium dependence of transport: the L-, T-, Ly-, ASC- and Gly-system. The T-system which appeared specific to L-tryptophan transport has not been identified in other cells. The L-system for large neutral amino acids and the T-system correspond to the group and specific pump, respectively, of the model of Rafaelsen. The importance of the h-RBC as a tool of biological psychiatry is discussed.

Reabsorption of neutral amino acids mediated by amino acid transporter LAT2 and TAT1 in the basolateral membrane of proximal tubule

In order to understand the renal reabsorption mechanism of neutral amino acids via amino acid transporters, we have isolated human L-type amino acid transporter 2 (hLAT2) and human T-type amino acid transporter 1 (hTAT1) in human, then, we have examined and compared the gene structures, the functional characterizations and the localization in human kidney. Northern blot analysis showed that hLAT2 mRNA was expressed at high levels in the heart, brain, placenta, kidney, spleen, prostate, testis, ovary, lymph node and the fetal liver. The hTAT1 mRNA was detected at high levels in the heart, placenta, liver, skeletal muscle, kidney, pancreas, spleen, thymus and prostate. Immunohistochemical analysis on the human kidney revealed that the hLAT2 and hTAT1 proteins coexist in the basolateral membrane of the renal proximal tubules. The hLAT2 transports all neutral amino acids and hTAT1 transports aromatic amino acids. The basolateral location of the hLAT2 and hTAT1 proteins in the renal proximal tubule as well as the amino acid transport activity of hLAT2 and hTAT1 suggests that these transporters contribute to the renal reabsorption of neutral and aromatic amino acids in the basolateral domain of epithelial proximal tubule cells, respectively. Therefore, LAT2 and TAT1 play essential roles in the reabsorption of neutral amino acids from the epithelial cells to the blood stream in the kidney. Because LAT2 and TAT1 are essential to the efficient absorption of neutral amino acids from the kidney, their defects might be involved in the pathogenesis of disorders caused by a disruption in amino acid absorption such as blue diaper syndrome.

New mechanism for amino acid influx into human epidermal Langerhans cells: L-dopa/proton counter-transport system

We have characterized a stereospecific transport mechanism for L-dopa into human epidermal Langerhans cells (LCs). It is different from any other amino acid transport system. It is highly concentrative, largely pH-independent, and independent of exogenous Na+, glucose and oxygen, and fuelled by a renewable intracellular energy source inhibited by iodoacetate but not by arsenate. We propose that the mechanism is a unidirectional L-dopa/proton counter-transport system. We have recently demonstrated anaerobic glycolysis in human epidermis, which substantiates the need of proton pumps for resident LCs. The findings prompt a re-evaluation of the profound changes LCs undergo when exposed to oxygen in aerobic culture. L-dopa is not metabolized by LCs but can rapidly be dislocated to the intercellular space by certain extracellular amino acids, i.e. LCs can profit by L-dopa in a dualistic way, altogether a remarkable biological phenomenon.

Elevated free tyrosine in rhinoceros erythrocytes

Red blood cells of African black rhinoceroses (Diceros bicornis) are highly sensitive to oxidant-induced hemolysis and they possess a number of enzymatic and biochemical features that differ radically from other mammals. Here we show concentrations of free tyrosine in rhinoceros red blood cells which can approach levels as high as 1 mM, 50-fold higher than in human red blood cells. Elevated levels of tyrosine are also observed in red blood cells of other members of the order Perissodactyla such as the horse and zebra. Captive black rhinoceroses have significantly lower levels of red blood cell tyrosine than black rhinoceroses in the wild. Tyrosine transport studies indicate that black rhinoceros red blood cells have lost the ability to transport tyrosine as efficiently as human red blood cells.

Characterization of tryptophan high affinity transport system in pinealocytes of the rat. Day-night modulation

Tryptophan is required in the pineal gland for the formation of serotonin, precursor of melatonin biosynthesis. The level of this amino acid in the serum and in the pineal gland of the rat undergoes a circadian rhythm, and reduced plasma tryptophan concentration decreases secretion of melatonin in humans. Tryptophan is transported into the cells by the long chain neutral amine acid system T and by the aromatic amino acid system T. The high affinity component of [(3)H]tryptophan uptake was studied in pinealocytes of the rat. Inhibition was observed in the presence of phenylalanine or tyrosine, but not in the presence of neutral amino acids, alanine, glycine, serine, lysine or by 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid, a substrate specific for system L. The transport of tryptophan was temperature-dependent and trans-stimulated by phenylalanine and tyrosine, but was energy-, sodium-, chloride-, and pH-independent. In addition, the sulphydryl agent N-ethylmaleimide did not modify the high affinity transport of tryptophan in pinealocytes. The kinetic parameters were not significantly different at 12:00 as compared to 24:00 h. The treatment with the inhibitor of tryptophan hydroxylase, p-chlorophenylalanine, produced an increase in the maximal velocity of the uptake and a reduction in the affinity at 12:00, but not at 24:00 h, probably indicating that during the day, the formation of serotonin in the pineal gland is favoured by elevating the uptake of tryptophan, whereas at 24:00 h other mechanisms, such as induction of enzymes are taking place. High affinity tryptophan uptake in the rat pineal gland occurs through system T and is upregulated during the day when the availability of serotonin is reduced.

Centrifugal and chromatographic analyses of tryptophan and tyrosine uptake by red blood cells and GLUT1 proteoliposomes with permeability estimates and observations on dihydrocytochalasin B

We analyzed transport into liposomes and proteoliposomes, separated the free and internalized radioactively labeled substrates by size-exclusion chromatography (SEC) and observed a net influx owing to nonfacilitated diffusion across the lipid bilayers during the separation. The permeabilities (10(-9) cm/s) of glucose transporter (GLUT1) proteoliposomes were estimated to be 4.6, 1.0, 1.4 and 2.1 for D-glucose, L-glucose, L-Tyr and L-Trp, respectively; 15, 3.3, 5.1 and 2.1 times higher than the corresponding permeabilities of liposomes. These values indicated that GLUT1 did not transport Tyr or Trp, or transported Tyr, and only Tyr, slowly. This interpretation was supported by further analyses. Dihydrocytochalasin B inhibited the transport of Tyr and, partially, Trp into human red blood cells (centrifugal analyses). It did not inhibit Tyr and Trp influx into GLUT1 proteoliposomes, but partitioned strongly into the bilayers and seemed to make them fragile. The GLUT1 inhibitor cytochalasin B and the GLUT1 substrate 2-deoxy-D-glucose did not inhibit Tyr transport into the cells. Upon immobilized biomembrane affinity chromatography, Trp decreased the cytochalasin B retardation by GLUT1 only at levels far above the physiological Trp concentration. Ethanol (commonly added to aqueous solutions for enhancing a compound's solubility) halved the retardation at 4% (v/v) concentration. Drastic modification of the SEC method is required to allow permeability measurements with nonlabeled and highly permeable substrates.

Functional properties of multispecific amino acid transporters and their implications to transporter-mediated toxicity

The absorption, distribution and excretion of most of xenobiotics, drugs, environmental toxins and their metabolites are mediated by membrane transporters. Recent advances in the transporter molecular biology have made it possible to investigate the mechanisms of transport of those exogenous compounds and their transporter-mediated toxicity at the molecular level. Exogenous compounds including drugs and toxic substances occurring in the environment pass through the transporters with broad substrate selectivity, namely "multispecific" transporters, taking advantage of the multispecific nature to exert their toxic effects. The remarkable examples of such transporter-mediated toxicity are 1-methyl-4-phenyl-2,3-dihydropyridinium (MPP+)-neurotoxicity mediated by dopamine transporters, cephaloridine-nephrotoxicity mediated by organic anion transporters and methylmercury-toxicity mediated by system L amino acid transporters. The molecular identification of system L transporter LAT1 (L-type amino acid transporter 1) has lead to the understanding of the mechanisms of their multispecific substrate recognition and revealed their localization at the blood-brain barrier and placental barrier. LAT1 relies on the hydrophobic interaction between substrate amino acid side chains and the substrate binding site, so that many variations are possible for the substrate amino acid side chains, which is the basis of the broad substrate selectivity. System L transporters, thus, function as a path for the membrane permeation of drugs and toxic compounds occurring in the environment with amino acid-related structures. Beside methylmercury-cysteine conjugate, amino acid-related neurotoxins such as beta-N-methylamino-L-alanine, S-(1,2-dichlorovinyl)-L-cysteine and 3-hydroxykynurenine are proposed to pass through system L transporters to exert their toxicity. Because the presence of such transporters is crucial for the manifestation of the organ toxicity, the inhibition of the transporters would be expected to be beneficial to prevent the disorders caused by the transporter-mediated toxicity.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

The human T-type amino acid transporter-1: characterization, gene organization, and chromosomal location

System T is a Na+-independent transport system that selectively transports aromatic amino acids. Here, we determined the structure of the human T-type amino-acid transporter-1 (TAT1) cDNA and gene (SLC16A10). The human TAT1 cDNA encoded a 515-amino-acid protein with 12 putative membrane-spanning domains. Human SLC16A10 was localized on human chromosome 6, mapped to 6q21-q22. SLC16A10 contains six exons spanning 136 kb. In contrast to rat TAT1, which is mainly present in the intestine, human TAT1 was strongly expressed in human kidney as well as in human intestine. Expression of human TAT1 in Xenopus laevis oocytes demonstrated the Na+-independent transport of tryptophan, tyrosine, phenylalanine, and L-dopa, indicating that human TAT1 is a transporter subserving system T. Because human TAT1 is proposed to be crucial to the efficient absorption of aromatic amino acids from intestine and kidney, its defect could be involved in the disruption of aromatic amino-acid transport, such as in blue diaper syndrome.

Expression cloning of a Na+-independent aromatic amino acid transporter with structural similarity to H+/monocarboxylate transporters

A cDNA was isolated from rat small intestine by expression cloning which encodes a novel Na+-independent transporter for aromatic amino acids. When expressed in Xenopus oocytes, the encoded protein designated as TAT1 (T-type amino acid transporter 1) exhibited Na+-independent and low-affinity transport of aromatic amino acids such as tryptophan, tyrosine, and phenylalanine (Km values: approximately 5 mm), consistent with the properties of classical amino acid transport system T. TAT1 accepted some variations of aromatic side chains because it interacted with amino acid-related compounds such as l-DOPA and 3-O-methyl-DOPA. Because TAT1 accepted N-methyl- and N-acetyl-derivatives of aromatic amino acids but did not accept their methylesters, it is proposed that TAT1 recognizes amino acid substrates as anions. Consistent with this, TAT1 exhibited sequence similarity (approximately 30% identity at the amino acid level) to H+/monocarboxylate transporters. Distinct from H+/monocarboxylate transporters, however, TAT1 was not coupled with the H+ transport but it mediated an electroneutral facilitated diffusion. TAT1 mRNA was strongly expressed in intestine, placenta, and liver. In rat small intestine TAT1 immunoreactivity was detected in the basolateral membrane of the epithelial cells suggesting its role in the transepithelial transport of aromatic amino acids. The identification of the amino acid transporter with distinct structural and functional characteristics will not only facilitate the expansion of amino acid transporter families but also provide new insights into the mechanisms of substrate recognition of organic solute transporters.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

Neutral amino acid transport in bovine articular chondrocytes

1. The sodium-dependent amino acid transport systems responsible for proline, glycine and glutamine transport, together with the sodium-independent systems for leucine and tryptophan, have been investigated in isolated bovine chondrocytes by inhibition studies and ion replacement. Each system was characterized kinetically. 2. Transport via system A was identified using the system-specific analogue alpha-methylaminoisobutyric acid (MeAIB) as an inhibitor of proline, glycine and glutamine transport. 3. Uptake of proline, glycine and glutamine via system ASC was identified by inhibition with alanine or serine. 4. System Gly was identified by the inhibition of glycine transport with excess sarcosine (a substrate for system Gly) whilst systems A and ASC were inhibited. This system, having a very limited substrate specificity and tissue distribution, was also shown to be Na+ and Cl- dependent. Evidence for expression of the system Gly component GLYT-1 was obtained using the reverse transcriptase-polymerase chain reaction (RT-PCR). 5. System N, also of narrow substrate specificity and tissue distribution, was shown to be present in chondrocytes. Na+-dependent glutamine uptake was inhibited by high concentrations of histidine (a substrate of system N) in the presence of excess MeAIB and serine. 6. System L was identified using the system specific analogue 2-aminobicyclo(2,2, 1)heptane-2-carboxylic acid (BCH) and D-leucine as inhibitors of leucine and tryptophan transport. 7. The presence of system T was tested by using leucine, tryptophan and tyrosine inhibition. It was concluded that this system was absent in the chondrocyte. 8. Kinetic analysis showed the Na+-independent chondrocyte L system to have apparent affinities for leucine and tryptophan of 125 +/- 27 and 36 +/- 11 microM, respectively. 9. Transport of the essential amino acids leucine and tryptophan into bovine chondrocytes occurs only by the Na+-independent system L, but with a higher affinity than the conventional L system.

Increase in red blood cell triiodothyronine uptake in untreated unipolar major depressed patients compared to healthy volunteers

1. Kinetic parameters of red blood cell (RBC) L-triiodothyronine (T3) initial uptake (Vmax, maximal velocity and Km, Michaelis constant) were determined in 34 untreated inpatients suffering from unipolar depression and in 40 healthy volunteers. 2. Both Vmax and Km were significantly increased in depressed patients as compared to controls. The alterations in kinetic parameters were not associated with the severity of depression. 3. Out of the 19 depressed patients who were submitted to TRH test, 7 of them (36%) showed a blunted TRH-induced TSH response associated with a Vmax situated outside the control mean value +/- 1 S.D. 4. The authors found a significant positive correlation between Vmax of RBC L-T3 and L-tryptophan (TRP) uptakes which is in agreement with the assumption that L-T3 and L-TRP share a common carrier system at the erythrocyte level. 5. The results indicate that the uptake of L-T3 by RBC is increased in major depression. These transport perturbations might reflect alterations in the plasmatic metabolism of L-T3. Evaluation of RBC L-T3 uptake could be useful in a best biological characterization of the depressed patients with regard to their thyroid function.

Red blood cell L-tryptophan uptake in depression. II. Effect of an antidepressant treatment

The evolution of the kinetic parameters, maximal velocity (Vmax) and Michaelis constant (Km), of L-tryptophan (L-TRP) uptake into red blood cells (RBC) was studied in 30 depressed patients in a drug-free state (D0) and after 1 week (D7) and 4 weeks (D28) of a treatment involving a variety of antidepressant drugs, including SSRIs and tricyclics. At D0, 76% of patients exhibited abnormal values of Vmax, which were either higher (36%) or lower (40%) than the control range (control Vmax mean +/- 1 S.D.). High and low Km values were observed in parallel with high and low Vmax values. At D7, individual values of Vmax varied drastically compared to their corresponding value at D0, whatever the pretreatment value of the parameter. The magnitude of the Vmax variation during the first week of treatment was found to be significantly larger in the treatment responders than in the non-responders. At D28, Vmax values of all the responders to treatment were within the control range, whatever their pretreatment Vmax value. On the contrary, non-responders had Vmax values that were significantly lower than those of the controls. Changes in Km followed changes in Vmax during antidepressant treatment. In conclusion, normalization of L-TRP transport kinetics was concomitant with a clear alleviation of depressive symptoms, indicating that RBC L-TRP uptake is dependent on clinical state. Moreover, early reactivity of the Vmax as soon as the first week of treatment may be useful as a predictive index of clinical outcome at D28.

Red blood cell L-tryptophan uptake in depression: kinetic analysis in untreated depressed patients and healthy volunteers

Kinetic parameters (Vmax and K(m)) of L-tryptophan (TRP) uptake into red blood cells (RBC) were measured in 72 drug-free depressed inpatients and 35 healthy volunteers. Mean Vmax and K(m) values were not significantly different between patients and volunteers. The frequency distributions of Vmax values, however, largely differed in the two groups: Vmax values were homogeneous in the volunteers while they were widely scattered in the depressed patients. Only 15 out of the 72 depressed patients (21%) had Vmax values within 1 SD from the mean control value. Forty-four percent of the patients (n = 32) had Vmax values above the control mean + 1 SD in 11 patients and above the control mean + 2 SD in 21 patients. Thirty-five depressed patients (n = 25) had Vmax values below the control mean - 1 SD in 8 patients and below the control mean - 2 SD in 17 patients. High and low K(m) values were observed in combination with high and low Vmax values. The alterations in kinetic parameters were neither associated with severity of depression nor with a specific diagnostic subtype of depression. The data show abnormalities in RBC L-TRP uptake in most depressed patients that likely reflect a disturbance in peripheral availability of TRP on which central serotonin synthesis closely depends.

Decrease in red blood cell L-tryptophan uptake in schizophrenic patients: possible link with loss of impulse control

1. Kinetic parameters of erythrocyte L-tryptophan (TRP) uptake (Vmax, maximal velocity and Km, Michaelis constant) were determined in 19 neuroleptic-free schizophrenic patients and in 19 healthy volunteers. Both Vmax and Km values were significantly lower in schizophrenic patients than in controls. 2. Mean Vmax value was found to be lower in patients who had attempted suicide than in patients who had not. No difference was observed when patients were subdivided on the basis of the violence of suicide attempts. 3. A significant negative correlation was observed between Vmax and scores on the loss of impulse control item as assessed on the PANS scale. 4. Decrease in red blood cell L-TRP uptake reflects a disturbance in the peripheral metabolism of TRP that may result in a deficiency of the plasma L-TRP availability on which the central serotonin (5HT) synthesis closely depends. 5. In addition, the results suggest that the alteration in RBC L-TRP uptake is associated with loss of impulse control in schizophrenic patients.

Neutral amino acid transport characterization of isolated luminal and abluminal membranes of the blood-brain barrier

The neutral amino acid carrier composition of luminal and abluminal membranes of the blood-brain barrier has been studied using isolated membrane vesicles. Phenylalanine was carried almost exclusively by a high affinity (Km = 10 +/- 2 microM), Na(+)-independent amino acid transport system, presumably L1 system, that was found to be symmetrically distributed between luminal and abluminal membranes. Inhibition of phenylalanine uptake was used to determine the affinities (Ki values) toward leucine (17 +/- 3 microM), tryptophan (8 +/- 1), 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BCH) (11 +/- 2), alanine (628 +/- 117), and glutamine (228 +/- 51). Alanine was found to be transported by two Na(+)-dependent transport systems that were located exclusively on the abluminal membrane. Kinetic and inhibition experiments indicated that one of these activities was due to system A, which is probably the main route for Na(+)-dependent alanine transport (Km = 0.6 +/- 0.2 mM) under physiological conditions. The other Na(+)-dependent activity was attributed to a B(o,+)-like system based on its sensitivity toward BCH. This latter system showed greater affinity for large neutral amino acids. The affinities of these two transport systems for several other amino acids were also studied.

Amino acid transport by human erythrocyte membranes

The human erythrocyte plasma membrane is permeable to several free amino acids usually present in the bloodstream. Seven distinct routes of entry have been described which represent both secondary active transport and facilitated diffusion (passive transport). Additionally, certain amino acids can enter the cell by simple diffusion, at least to a limited extent. The function of most of these transport systems is unclear, although it has been suggested that the cell can take up certain amino acids and carry them to various parts of the body. In the case of glutamine, cysteine, and glycine, however, it is believed that the biosynthesis of the tripeptide glutathione is the primary reason for their uptake into the cell. Much of the amino acid transport probably has no function in mature red cells, but might be a remnant of the immature cell's needs. This review discusses the various amino acid transport systems known to be present in the red cell plasma membrane.

Reconstitution studies of amino acid transport system L in rat erythrocytes

In many cell types, including human erythrocytes, membrane transport of hydrophobic amino acids such as leucine and phenylalanine is mediated primarily by Na(+)-independent system L. In this paper we demonstrate that erythrocytes from the rat have a 400-fold higher system L transport capacity than human erythrocytes. We have exploited this high transport activity to achieve the first successful reconstitution of an erythrocyte amino acid transporter into phospholipid vesicles. Rat erythrocyte membranes were depleted of extrinsic membrane proteins, solubilized in 50 mM n-octyl glucoside and reconstituted into egg-yolk phospholipid vesicles by a gel filtration freeze-thaw protocol. Optimal reconstitution of transport activity occurred at lipid/protein ratios of 25-35:1. At a lipid/protein ratio of 25:1, one-half of the total uptake of L-[14C]leucine (0.2 mM, 25 degrees C) was inhibited by 2 mM phloretin and thus judged to be carrier-mediated. This component of L-leucine uptake was inhibited by non-radioactive L-phenylalanine and L-leucine, and only to a very much weaker extent by glycine and L-alanine. Two other inhibitors of system L in intact cells, MK196 and PCMBS (p-chloromercuriphenylsulphonate), were also effective inhibitors of phloretin-sensitive L-leucine transport in reconstituted proteoliposomes. Phloretin-insensitive uptake of L-leucine in proteoliposomes occurred by simple diffusion across the lipid bilayer.

Triiodothyronine is a high-affinity inhibitor of amino acid transport system L1 in cultured astrocytes

The relationship between the transport of thyroid hormones and that of amino acids was examined by measuring the uptake of amino acids that are characteristic substrates of systems L, A, and N, and the effect of 3,3',5-triiodo-L-thyronine (T3) on this uptake, in cultured astrocytes. Tryptophan and leucine uptakes were rapid, Na(+)-independent, and efficiently inhibited by T3 (half-inhibition at approximately 2 microM). Two Na(+)-independent L-like systems (L1 and L2), common to leucine and aromatic amino acids, were characterized kinetically. System L2 had a low affinity for leucine and tryptophan (Km = 0.3-0.9 mM). The high-affinity system L1 (Km approximately 10 microM for both amino acids) was competitively inhibited by T3 with a Ki of 2-3 microM (close to the T3 transport Km). Several T3 analogues inhibited system L1 and the T3 transport system similarly. Glutamine uptake and alpha-(methylamino)isobutyric acid uptake were, respectively, two and 200 times lower than tryptophan and leucine uptakes. T3 had little effect on the uptakes of glutamine and alpha -(methylamino)isobutyric acid. The results indicate that the T3 transport system and system L1 are related.

Erythrocyte uptake kinetics and cell to plasma gradients of leucine and phenylalanine in fed and fasted rats

The kinetic parameters of the L-phenylalanine and L-leucine uptake by isolated erythrocytes in fed and 24 hour starved rats have been determined. In addition, the in vivo compartmentation between blood cells and plasma of the above amino acids in arterial and venous blood vessels has also been studied under the above physiological situations. Both the L-phenylalanine and L-leucine uptake by erythrocytes was saturable and non concentrative. Starvation increased the Km value for the leucine uptake and did not significantly affect that of phenylalanine uptake. The in vivo blood cell/plasma (C/P) concentration ratio of both amino acids was higher than the unit. The starvation-induced changes in the relative distribution of these amino acids between the blood cell and the plasma compartments were significant for the phenylalanine in the aortic artery but not in venous blood. The transport system capabilities measured in vitro can not account for the maintenance of both the leucine and phenylalanine gradient between blood cells and plasma, and the starvation-induced changes in the blood amino acids compartmentation are not directly related entirely to the transport system capabilities.

Identification of a new transport system (y+L) in human erythrocytes that recognizes lysine and leucine with high affinity

1. The effect of neutral amino acids on the transport of L-lysine across the human erythrocyte membrane was studied. 2. All neutral amino acids tested (range 0.3-5 mM) inhibit the influx of L-[14C]lysine (1 microM). The inhibition pattern is biphasic, and tends to reach a maximum at approximately 50% of the original flux. The concentrations that give 25% inhibition are (mM): L-cysteine (2.7), L-alanine (1.3), L-serine (0.9), L-isoleucine (0.6), L-phenylalanine (0.35), L-methionine (< 0.3), L-leucine (< 0.3). L-lysine and L-arginine completely inhibit the rate at the highest concentration. 3. These results can be explained by assuming that L-lysine transport occurs through two independent transporters that differ in their affinity for neutral amino acids. A detailed kinetic analysis of the effect of L-leucine on L-lysine entry is consistent with this hypothesis. 4. Using a new experimental strategy, the substrate and inhibitor transport parameters for the two systems were determined. The half-saturation constants for lysine (+/- S.E.M.) are found to be: KmA, 0.014 +/- 0.002 mM and KmB, 0.112 +/- 0.017 mM. The maximum rates differ by a factor of 8.2 (VmaxB/VmaxA). The leucine inhibition constants are: KiA, 0.022 +/- 0.003 mM and KiB, 30.36 +/- 7.9 mM. If the sodium in the incubation medium is replaced by potassium, the apparent affinity for leucine (1/KiA) is reduced approximately 30-fold. 5. The maximum inhibition caused by leucine decreases as the lysine concentration is raised, showing that leucine acts upon the higher affinity system. 6. When added to the trans side, L-leucine, L-phenylalanine and L-isoleucine do not cause inhibition, but stimulate the flux by approximately 30%. This indicates that these analogues are also transported. 7. In conclusion, in the concentration range 1-100 microM, lysine crosses the red cell membrane through two distinct transport systems, one of which recognizes both neutral and cationic amino acids with high affinity.

Tyrosine transport in a human melanoma cell line as a basis for selective transport of cytotoxic analogues

Tyrosine is an essential amino acid for the initial step of melanin synthesis, yet little is known concerning its transport in melanocytes. As an important first step in the development of new anti-melanoma agents based upon chemical and pharmacological modifications of melanin synthesis, the present study characterized the transport mechanism of tyrosine in vitro using the human melanoma cell line SK-MEL 23. Several tyrosine transport systems may be involved in melanocytes: systems L and T, which transport neutral amino acids with branched or aromatic side chains, and systems A and ASC, which transport neutral amino acids with smaller side chains. In order to determine which system or combination of systems is involved in tyrosine transport in melanoma cells, studies of kinetics, Na(+)-dependence and competitive inhibition were undertaken. The Km and Vmax. for the Na(+)-independent transport system were found to be 0.164 +/- 0.016 mM and 21.6 +/- 1.1 nmol/min per mg of protein respectively. This transport was preferentially inhibited by the system L specific analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, the system T substrate tryptophan, and the sulphur homologue of tyrosine, 4-S-cysteinylphenol. Sequential addition of these inhibitors at increasing concentrations indicated that they inhibit the same transporter. Our results suggest that tyrosine transport in SK-MEL 23 melanoma cells is similar to system L transport previously characterized in other cell types. This one transport system appears to supply all the tyrosine required for both cell growth and melanin synthesis. The transport system may be subject to manipulation by melanogenic stimulating factors, making the transport of cytotoxic tyrosine analogues an important area for further study.

Transport of L-tyrosine by B16/F10 malignant melanocytes: characterization of the process

The main characteristics of L-tyrosine (L-Tyr) uptake by B16/F10 malignant melanocytes are reported. This amino acid can be taken up by two systems, both of them being saturable. The first one would be system L. This system can be studied in cells preloaded with amino acids that are a good substrate for system L, such as L-methionine or L-tryptophan. The kinetic parameters for L-Tyr uptake by this transport system are Vm = 6.5 pmol L-Tyr/10(3) cells.min and Km around 130 microM. The second system, probably the system ASC, shows lower capacity but higher affinity than the former. This system can be detected only in cells previously depleted of amino acids, showing approximate kinetic values of Vm 0.05 pmol L-Tyr/10(3) cells.min and Km around 5 microM. It is shown that the increase in cell density yields a decrease in the rate of L-Tyr uptake by system L, but this increase does not affect the high affinity system, alpha-MSH does not affect significantly the L-Tyr uptake by both systems. 2-Amino bicyclo-(2,2,1)-heptane-2-carboxylic acid produces a remarkable inhibition of the rate of L-Tyr uptake, but alpha-methylaminoisobutyric acid does not affect the rate of transport of this amino acid. The absence of sodium produces a slight but reliable decrease in the rate of L-Tyr uptake, supporting the involvement of two different transport systems. The ionophores monensin and nigericin enhance the transport by system L, but this effect is suppressed by the presence of ouabain. This finding indicates that the (Na+ -K+)-ATPase is essential for the stimulating action of ionophores.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of benzenoid amino acids by system T and four broad scope systems in preimplantation mouse conceptuses

We have studied transport of L-tryptophan, L-tyrosine and L-phenylalanine as factors contributing to homeostasis of these amino acids in preimplantation mouse conceptuses. Benzenoid amino acids were transported by the Na(+)-independent systems L and b0,+ in 1-cell conceptuses, and by these systems plus the Na(+)-dependent systems B0,+ and B in blastocysts. In addition, a component of Na(+)-independent tryptophan, tyrosine and phenylalanine transport in 1-cell and 2-cell conceptuses and in blastocysts resisted inhibition by L-leucine. The latter component of transport not only preferred benzenoid amino acids and in particular tryptophan as substrates, but it also was inhibited strongly and competitively by alpha-N-methyl-L-tryptophan. The leucine-resistant component of tryptophan transport also was inhibited strongly by N-ethylmaleimide and D-tryptophan, and it appeared to be inhibited weakly by 3-amino-endo-bicyclo[3.2.1]octane-3-carboxylic acid (BCO) but not by other amino acids tested as inhibitors. By these criteria, the leucine-resistant component of transport of benzenoid amino acids resembled system T in human red blood cells and rat hepatocytes. It is not entirely clear why preimplantation blastocysts have five good systems for transport of tryptophan. It is possible, however, that tryptophan homeostasis is particularly important during preimplantation development since it has been shown elsewhere that tryptophan availability in blood increases within one day after rat eggs are fertilized.

L-tyrosine and L-tryptophan membrane transport in erythrocytes and antidepressant drug choice

In the treatment of depression, when antidepressant drug choice is made according to alterations of erythrocyte membrane transport of L-tyrosine and L-tryptophan in the individual patient, the clinical results are superior to those obtained when drugs are prescribed according to the physician's judgment. This is demonstrated by comparing three experimental groups: I, 100 patients treated in relation to their L-tyrosine and L-tryptophan transport; II, 30 patients treated according to the clinician's experience; III, 38 subjects treated against the L-tyrosine and L-tryptophan transport indications. In these groups, the frequency of patients improved by more than 70% is 77%, 47%, and 16%, respectively.

Uptake of chloroquine by human erythrocytes

Analysis of studies of the pH dependence of the kinetics of chloroquine (CQ) uptake by human erythrocytes indicates that the unionised CQ species is the major membrane permeant at physiological pH even though the concentration of this species as a fraction of the total CQ concentration in solution is extremely small (0.01% at pH 7.4). CQ concentration-dependence studies and studies performed in the presence of various substrates and inhibitors of erythrocyte membrane transport failed to provide evidence of saturation or inhibition of CQ transport, which suggests that the likely mechanism of CQ transport across human erythrocyte membranes is by passive diffusion. Results of equilibrium binding studies of CQ to intact and lysed human erythrocytes indicated that the mechanism of CQ accumulation in intact human erythrocytes appears to be by a combination of ion trapping (a consequence of the basic nature of the drug and the pH gradient across the human erythrocyte membrane) and binding of CQ to cell components.

Cytotoxicity of S-(1,2,3,4,4-pentachlorobutadienyl)-L cysteine after apical and basolateral exposure of LLC-PK monolayers. Involvement of an amino acid transport system

Glutathione conjugation and subsequent formation of cysteine conjugates are key steps in the nephrotoxicity of halogenated alkenes. In this metabolic activation several organs are involved. However little is known about the transporters responsible for the uptake of cysteine conjugates. Recent evidence suggest that amino acid transporters play a role in this uptake. Monolayers of LLC-PK1 cells, a kidney cell line, were exposed to S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine (PCBD-CYS). Cytotoxicity was used as a parameter for PCBD-CYS uptake. Basolateral exposure (1 h: 400 microM and 16 h: 25 microM) to PCBD-CYS resulted in a much higher aminooxyacetic acid inhibitable cytotoxicity than apical exposure, suggesting a preferential basolateral uptake of PCBD-CYS. Exposure to PCBD-CYS in the absence of sodium did not result in a decrease of the cytotoxicity, suggesting a sodium independency of the PCBD-CYS uptake. Amino acids and amino acid analogues were used as diagnostic compounds in the further identification of the PCBD-CYS transporter. In cis-inhibition experiments monolayers were co-incubated with PCBD-CYS and these diagnostic compounds during one hour. System L substrates such as 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH) and cycloleucine did not inhibit cytotoxicity. D-Tryptophan, a model inhibitor of System T, caused a strong inhibition. System L has, in contrast to System T, a high sensitivity to trans-stimulation. Pre-loading the monolayers with the diagnostic compounds should cause an increase in cytotoxicity when System L is involved. Neither System L substrates such as BCH and cycloleucine nor D-tryptophan increased cytotoxicity. These results suggest a preferential basolateral uptake of PCBD-CYS in LLC-PK1 monolayers and involvement of an amino acid transporter with characteristics of System T.

Separate and shared lysosomal transport of branched and aromatic dipolar amino acids

Transport systems analogous to the T and L carriers for aromatic and bulky dipolar amino acids in plasma membranes have been characterized in the membranes of intact lysosomes isolated from human fetal skin fibroblasts. While system L appears ubiquitous in plasma membranes, system T has previously been discriminated only in the plasmalemma of human red blood cells and freshly isolated rat hepatocytes. Our findings with the lysosomal systems, provisionally designated t and l, reveal both shared and dissimilar properties with the plasma membrane systems. These properties include a lack of dependency on extralysosomal Na+, differential sensitivities to the classical system L analog, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), and the system T analog, D-tryptophan, as well as susceptibility to thiol modification at the membrane by reactivity with N-ethylmaleimide. A transport system in lysosomes from the FRTL-5 rat thyroid cell line has been described by Bernar et al. ((1986) J. Biol. Chem. 261, 17107-17112) resembles a composite of both carrier systems reported in this work.

How does displacement of albumin-bound tryptophan cause sustained increases in the free tryptophan concentration in plasma and 5-hydroxytryptamine synthesis in brain?

Models of tryptophan catabolism and binding to serum albumin are presented to explain the observed effect of displacement of tryptophan from albumin on the concentrations of free and bound tryptophan and on the rate of 5-hydroxytryptamine (5-HT) synthesis from tryptophan in the brain. A rapid rate of dissociation of tryptophan from albumin (compared to the transit time of tryptophan through the liver) and a large fractional extraction of the free pool of tryptophan during passage through the liver are shown to be necessary factors in determining the effects observed. Because of the low fractional extraction of free tryptophan in the brain, the synthesis of 5-HT will be dependent only upon the free pool of tryptophan. Dissociation of tryptophan from albumin only causes a sustained increase in 5-HT synthesis in the brain because of the effect that this dissociation has on hepatic tryptophan catabolism and thereby on the free pool of tryptophan.

Metabolic origins of 5-hydroxytryptamine in enteric neurons in a teleostean fish (Platycephalus bassensis), a toad (Bufo marinus) and the guinea-pig

1. 5-Hydroxytryptamine (5-HT) content and synthesis in mucosa-free intestine of guinea-pig, the teleost Platycephalus bassensis and the amphibian Bufo marinus was studied by HPLC with electrochemical detection or by TLC. 2. The 5-HT content of small intestine was: guinea-pig 0.58; Bufo: 1.23; Platycephalus: 26.88 nmol/g. 3. Intestine from each species synthesized 5-HT from exogenous 5-HTP. 4. Platycephalus preparations synthesized labelled 5-HT from 14C-tryptophan, but no labelled 5-HT was detected after similar incubation of guinea-pig or Bufo preparations. 5. Incubation of guinea-pig preparations with tryptophan did not increase tissue 5-HT or 5-HIAA content. 6. 5-HT in Platycephalus enteric neurons may be synthesized from tryptophan in situ; 5-HT in Bufo and guinea-pig neurons may be synthesized elsewhere, perhaps in enterochromaffin cells.

Alpha-aminoisobutyric acid transport in isolated rat submandibular salivary acinar cells

Na+-dependent alpha-aminoisobutyric acid (AIB) transport by isolated submandibular cell aggregates (pmol min-1 mg protein-1) was greater in the presence than in the absence of insulin, Vmax (5220 compared with 2900). Km (1.78 and 1.40 mM, respectively) was unaffected by insulin. Na+-dependent methyl-aminobutyric acid (MeAIB) transport was also greater in the presence of insulin (V max, 3120 compared with 2010 pmol min-1 mg protein-1; Km, 1.03 and 0.93 mM). In the presence of 10 mM MeAIB, Na+-dependent AIB transport was reduced to 76 pmol min-1 mg protein in both control and insulin-treated cells. The remaining Na+-dependent uptake of AIB was inhibited by 10 mM serine. Na+-independent AIB transport was unaffected by insulin, and in the presence of 5 mM 2-aminobicyclo-[2,2,1]-heptane-carboxylic acid (BCH) AIB uptake was reduced to 10% of that observed under Na+-replete conditions. In the absence of insulin, the rate of Na+-dependent AIB uptake rapidly decayed; however, following the addition of hormone the rate of transport was maintained. Thus in the rat submandibular gland AIB uptake is mediated by at least three transport systems (A, L and ASC), and maintenance of normal system A activity requires insulin.

Neutral amino acid transport systems of microvillous membrane of human placenta

Placental transport produces concentrations of amino acids in fetal blood greater than those of maternal blood. Competitive inhibition studies of zwitterionic amino acid transport in isolated vesicles from the microvillous (maternal facing) plasma membranes of syncytiotrophoblast defined three transport systems: 1) a sodium-dependent system that supports methylaminoisobutyric acid (MeAIB) transport and has the characteristics of an A system; 2) a sodium-independent system with a high affinity for leucine and other amino acids with branched or aromatic side chains; and 3) a sodium-independent system with a preference for alanine as a substrate. The two sodium-independent systems could be further discriminated by marked specificity for trans stimulation with alanine or with leucine. System ASC, known to be present in whole placenta, and the neutral brush-border or imino systems of other polarized epithelia were apparently absent. Kinetic characteristics of the A system make it the probable primary driving force for concentrative transfer of its substrate amino acids to the fetus. Characteristics of the high-affinity leucine system demonstrated that it is saturated by normal serum leucine concentrations. Regulation of either system has the potential to alter placental amino acid uptake and transfer to the fetus.

Decreased tyrosine transport in fibroblasts from schizophrenic patients

Amino acid transport was studied in vitro in cultured fibroblasts from schizophrenic patients and controls. An isolated decrease in the transport capacity (Vmax) for tyrosine was observed in cells from the patients. The Km for tyrosine transport was unaffected. The kinetic parameters for phenylalanine, tryptophan, leucine and glycine transport did not differ between patients and controls. Competitive inhibition among the amino acids transported by the L-system and its exchange properties were normal in cells from the patients. No differences in intracellular levels of amino acids between patients and controls were observed. The decreased tyrosine transport in the cells from schizophrenic patients appears not to be related to any known amino acid transport system and may reflect a more general defect in plasma membrane function in schizophrenia.

Heterogeneity of amino acid transport in horse erythrocytes: a detailed kinetic analysis of inherited transport variation

1. Thoroughbred horses were divisible into five distinct amino acid transport subgroups on the basis of their erythrocyte permeability to L-alanine, measured uptake rates ranging from 5 to 625 mumol l cells-1 h-1 (0.2 mM-extracellular L-alanine, 37 degrees C). 2. Erythrocytes from animals belonging to the lowest L-alanine permeability subgroup (5-15 mumol l cells-1 h-1) (transport-deficient type) exhibited slow nonsaturable transport of this amino acid. In contrast, cells from horses of the four transport-positive subgroups possessed additional high-affinity (apparent L-alanine Km (Michaelis constant) congruent to 0.3 mM) and/or low-affinity (apparent L-alanine Km congruent to 13 mM) Na+-independent transport routes selective for L-neutral amino acids of intermediate size. The two transporters, designated systems asc1 and asc2, respectively, also possessed a significant affinity for dibasic amino acids. 3. Amino acid transport activity in horse erythrocytes behaved as if controlled by three co-dominant alleles (s, h and l), where s is a silent allele, and h and l code for the functional presence of systems asc1 and asc2, respectively. 4. At physiological temperature, system asc1 operated preferentially in an exchange mode. In contrast, system asc2 did not participate in exchange reactions at 37 degrees C, but did exhibit significant trans-acceleration at 25 degrees C. 5. Reduction of the incubation temperature also resulted in dramatic decreases in apparent Km and Vmax for L-alanine uptake by system asc2, whereas the effects of temperature on system asc1 were much less marked. At 5 degrees C the two transporters exhibited equivalent kinetic constants for L-alanine influx. L-Alanine uptake by transport-deficient cells was relatively insensitive to temperature. Influx by this route may represent the ground-state permeability of the lipid bilayer. 6. The effects of low temperature on system asc2 suggest a preferential impairment of the mobility of the unloaded carrier relative to that of the loaded transporter. Similarly, the different kinetic properties of systems asc1 and asc2 at physiological temperature are attributed to a difference in the mobilities of the empty carriers, this difference being minimized at 5 degrees C.

Characterization of tryptophan transport in human placental brush-border membrane vesicles

The characteristics of tryptophan uptake in isolated human placental brush-border membrane vesicles were investigated. Tryptophan uptake in these vesicles was predominantly Na+-independent. Uptake of tryptophan as measured with short incubations occurred exclusively by a carrier-mediated process, but significant binding of this amino acid to the membrane vesicles was observed with longer incubations. The carrier-mediated system obeyed Michaelis-Menten kinetics, with an apparent affinity constant of 12.7 +/- 1.0 microM and a maximal velocity of 91 +/- 5 pmol/15 s per mg of protein. The kinetic constants were similar in the presence and absence of a Na+ gradient. Competition experiments showed that tryptophan uptake was effectively inhibited by many neutral amino acids except proline, hydroxyproline and 2-(methylamino)isobutyric acid. The inhibitory amino acids included aromatic amino acids as well as other system-1-specific amino acids (system 1 refers to the classical L system, according to the most recent nomenclature of amino acid transport systems). The transport system showed very low affinity for D-isomers, was not affected by phloretin or glucose but was inhibited by p-azidophenylalanine and N-ethylmaleimide. The uptake rates were only minimally affected by change in pH over the range 4.5-8.0. Tryptophan uptake markedly responded to trans-stimulation, and the amino acids capable of causing trans-stimulation included all amino acids with system-1-specificity. The patterns of inhibition of uptake of tryptophan and leucine by various amino acids were very similar. We conclude that system t, which is specific for aromatic amino acids, is absent from human placenta and that tryptophan transport in this tissue occurs via system 1, which has very broad specificity.

Quantification of the importance of individual steps in the control of aromatic amino acid metabolism

The quantitative importance of the individual steps of aromatic amino acid metabolism in rat liver was determined by calculation of the respective Control Coefficients (Strengths). The Control Coefficient of tryptophan 2,3-dioxygenase for tryptophan degradation was determined in a variety of physiological conditions and with a range of activities of tryptophan 2,3-dioxygenase. The Control Coefficient varied from 0.75 with basal enzyme activity to 0.25 after maximal induction of the enzyme by dexamethasone. The remainder of the control for tryptophan degradation was associated with the transport of the amino acid across the plasma membrane, with only very small contributions from kynureninase and kynurenine hydroxylase. The Control Coefficients of tyrosine aminotransferase for tyrosine degradation were approx. 0.70 and 0.20 with basal and dexamethasone-induced tyrosine aminotransferase activities respectively; the Control Coefficients of the transport of the amino acid into the cell were 0.22 and 0.58 respectively. Phenylalanine hydroxylase was found to have a Control Coefficient for the degradation of phenylalanine of approx. 0.50 under conditions of basal enzyme activity; after maximal activation by glucagon, the Control Coefficient decreased to 0.12. The transport of phenylalanine was responsible for the remaining control in the pathway. These results have important implications, directly for the regulation of aromatic amino acid metabolism in the liver, and indirectly for the regulation of neuroamine synthesis in the brain.

Transport of the aromatic amino acids into isolated rat liver cells. Properties of uptake by two distinct systems

The transport of the aromatic amino acids into isolated rat liver cells was studied. There was a rapid and substantial binding of the aromatic amino acids, L-alanine and L-leucine to the plasma membrane. This has important consequences for the determination of rates of transport and intracellular concentrations of the amino acids. Inhibition studies with a variety of substrates of various transport systems gave results consistent with aromatic amino acid transport being catalysed by two systems: a 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)-insensitive aromatic D- and L-amino acid-specific system, and the L-type system (BCH-sensitive). The BCH-insensitive component of transport was Na+-independent and facilitated non-concentrative transport of the aromatic amino acids; it was unaffected by culture of liver cells for 24 h, by 48 h starvation, dexamethasone phosphate or glucagon. Kinetic properties of the BCH-inhibitable component were similar to those previously reported for the L2-system in liver cells. The BCH-insensitive component was a comparatively low-Km low-Vmax. transport system that we suggest is similar to the T-transport system previously seen only in human red blood cells. The results are discussed with reference to the importance of the T- and L-systems in the control of aromatic L-amino acid degradation in the liver.

Comparative aspects of the apparent Michaelis constant for neutral amino acid transport in several animal tissues

The apparent Michaelis constant, Km, for transport of a number of neutral amino acids has been compared between intestine, heart, brain and erythrocytes among a variety of animals using values available in the literature. Neutral amino acids with side chains containing 3, 4, 7 and 9 carbon atoms had approximately equal mean Km values when tested for intestinal transport among a variety of species; alanine appeared to have a mean Km value that was larger than those found for the first group, and glycine had a significantly greater mean Km than all of the other compounds tested. Km values for phenylalanine and tryptophan measured in rat heart were found to be close to the means measured for these substrates in intestine. The mean Km values measured in mammalian brain for each of the neutral amino acid substrates were found not be significantly different from each other. When the means of Km values for the neutral amino acids tested were compared between intestine and brain, only the glycine means were shown to differ significantly between the organs. Based on data for several mammalian species, brain appears to have a greater average apparent affinity for glycine than does intestine. In the human erythrocytes and in a few other mammalian species, Km values for all neutral amino acids tested with exception of glycine were found to be similar in magnitude to each other and to the Km averages of neutral amino acids found in intestine for the series containing 3-9 carbon atoms. The Km value for glycine in the human erythrocyte was noted to be substantially lower in value than the averages for glycine in brain or intestine. Avian red blood cells appear to have high apparent affinity for neutral amino acid transport when compared with red cells of several mammalian species.

Tryptophan transport through transport system T in the human erythrocyte, the Ehrlich cell and the rat intestine

We studied the transport of tryptophan through transport system T in the human red cell, the Ehrlich ascites-tumour cell and in everted sacs of rat intestine. In red cells we confirmed earlier results on Na+-independence and aromatic amino acid specificity (Rosenberg, R., Young, J.D. and Ellory, J.C. (1980) Biochim. Biophys. Acta 598, 375-384). In addition we observed that N-methylation or N-acetylation did not reduce the affinity of the substrates for system T, hydroxylation could increase or decrease substrate affinity, and system T was insensitive to pH changes in the medium. These results characterized reactive differences between system T and other known amino acid transport systems. We also found that D-isomers were about 1/3 as effective as L-isomers to inhibit L-tryptophan uptake. D-Tryptophan competitively inhibited L-tryptophan uptake, but was not taken up by system T. L-Tryptophan produced trans-stimulation of the uptake (influx) and trans-inhibition of the release (efflux) of L-[3H]tryptophan; D-tryptophan produced trans-inhibition of the efflux but did not affect significantly the uptake. These results show that in red cells the transport properties of transport system T are asymmetric. Transport system T seems to be absent in the other two preparations studied, the Ehrlich ascites-tumour cell and the rat intestine.

Incorporation and translocation of aminophospholipids in human erythrocytes

Cell morphology changes are used to examine the interaction of exogenous phosphatidylserine and phosphatidylethanolamine with human erythrocytes. Short-chain saturated lipids transfer from liposomes to cells, inducing shape changes that are indicative of their incorporation into, and in some cases translocation across, the cell membrane bilayer. Dioleoylphosphatidylserine and low concentrations of dilauroyl- and dimyristoylphosphatidylserine induce stomatocytosis. At higher concentrations, dilauroylphosphatidylserine and dimyristoylphosphatidylserine induce a biphasic shape change: the cells crenate initially but rapidly revert to a discocytic and eventually stomatocytic shape. The extent of these shape changes is dose dependent and increases with increasing hydrophilicity of the phospholipid. Cells treated with dilauroylphosphatidylethanolamine and bovine brain lysophosphatidylserine exhibit a similar biphasic shape change but revert to discocytes rather than stomatocytes. These shape changes are not a result of vesicle--cell fusion nor can they be accounted for by cholesterol depletion. The reversion from crenated to stomatocytic forms is dependent on intracellular ATP and Mg2+ concentrations and the state of protein sulfhydryl groups. The present results are consistent with the existence of a Mg2+- and ATP-dependent protein in erythrocytes that selectively translocates aminophospholipids to the membrane inner monolayer engendering aminophospholipid asymmetry.

Characterization of quinolinic acid phosphoribosyltransferase in human blood and observations in Huntington's disease

Quinolinic acid (QUIN), an excitotoxic compound present in the mammalian CNS and periphery, has been hypothetically linked to human neurodegenerative disorders such as Huntington's disease and epilepsy. Quinolinic acid phosphoribosyltransferase (QPRT), the catabolic enzyme of QUIN, is found in the CNS and peripheral organs where it may be a major influence on the tissue levels of QUIN. We have measured QPRT activity in human blood as a means of assessing one aspect of QUIN metabolism in humans. The enzyme was present in blood cells, platelets having a sixfold greater activity than erythrocytes, but was essentially absent from the plasma. In a blood cell fraction, enzyme activity was potently inhibited by phthalic acid (IC50 = 6.1 microM). Kinetic analyses conducted over a range of QUIN concentrations yielded Km values of 1.89-3.75 microM and Vmax values of 33.4-72.5 fmol nicotinic acid mononucleotide/h/mg protein. Enzyme activity varied 2.2-fold between normal individuals, was reasonably constant over a series of sampling intervals, and showed some diminution when blood was stored for 1 month at -20 degrees C. No differences of enzyme activity in erythrocytes or platelets were apparent between three Huntington's disease patients and their unaffected spouses. These data indicate that measurements of QPRT activities in blood are a convenient means to monitor QUIN metabolism in human subjects and that a deficiency of the enzyme is not apparent in Huntington's disease.