Tyrosine Transport Research Articles

Clozapine but not lithium reverses aberrant tyrosine uptake in patients with bipolar disorder

RationaleAvailability of the dopamine and noradrenaline precursor tyrosine is critical for normal functioning, and deficit in tyrosine transport across cell membrane and the blood-brain barrier has been reported in bipolar disorder and schizophrenia. Clozapine and lithium are two psychoactive agents used to treat psychosis, mood disorders and suicidal behavior, but their mechanism of action remains largely unknown.ObjectiveTo characterize immediate and delayed differences in tyrosine uptake between healthy controls (HC) and bipolar patients (BP) and see if these differences could be normalized by either clozapine, lithium or both. A second objective was to see if clozapine and lithium have additive, antagonistic or synergistic effects in this.MethodFibroblasts from five HC and five BP were incubated for 5 min or 6 h with clozapine, lithium, or combination of both. Radioactive labelled tyrosine was used to quantify tyrosine membrane transport.ResultsThere was significantly reduced tyrosine uptake at baseline in BP compared to HC, a deficit that grew with increasing incubation time. Clozapine selectively increased tyrosine uptake in BP and abolished the deficit seen under baseline conditions, while lithium had no such effect. Combination treatment with clozapine and lithium was less effective than when clozapine was used alone.ConclusionsThere was significant deficit in tyrosine transport in BP compared to HC that was reversed by clozapine but not lithium. Clozapine was more effective when used alone than when added together with lithium. Potential clinical implications of this will be discussed.

Efficient Transport and Biotransformation of Dipeptide-like Tyrosine/ Phenylalanine-Conjugated Phenolic Amide Esters in THP-1 Cells and PBMCs: A Potential Means for Transporting Compounds Inside Monocytes/Macrophages

Background: Recent studies suggest that dipeptide-like tyrosine/phenylalanine-conjugated phenolic amide compounds may contain several biological activities, including anti-inflammatory activity. However, there is currently no information about their transport and biotransformation in monocytes/macrophages involved in inflammation process. Objective: The objective of this study was to investigate cell transport and biotransformation of the phenolic amides and esters in monocyte/macrophage-like cells. Methods: Cell transport and biotransformation of the phenolic amides and esters (N-coumaroylphenylalanine, N-caffeoylphenylalanine, N-feruloylphenylalanine, N-coumaroyltyrosine, Ncaffeoyltyrosine, N-feruloyltyrosine, and their O-methyl esters) were investigated in THP-1 cells and PBMCs using HPLC, cellular, and kinetics methods. Results: In THP-1 cells, the phenolic amides were not transported significantly, but their O-methyl esters were transported significantly (P < 0.02). Also, the transport of the esters was found to be sodium-independent and pH-dependent. Among the tested esters, N-feruloylphenylalanine-Omethyl ester showed the highest uptake (Km of 25 μM), and the uptake was inhibited by PepT1/2 substrate and blocker (GlySar and enalapril) in THP-1 cells. Particularly, enalapril competitively inhibited the uptake with Ki of 560 μM. The data also showed that N-feruloylphenylalanine-Omethyl ester and N-feruloyltyrosine-O-methyl ester could be biotransformed into parent phenolic amides in THP-1 cells. Similarly, these ester compounds were also found to be transported and biotransformed in PBMCs. Conclusion: The data suggest that dipeptide-like tyrosine/phenylalanine-conjugated phenolic amide esters may be transported and biotransformed in THP-1 cells and PBMCs.

Metabolism of Amino Acids in the Brain and Their Roles in Regulating Food Intake.

Amino acids (AAs) and their metabolites play an important role in neurological health and function. They are not only the building blocks of protein but are also neurotransmitters. In the brain, glutamate and aspartate are the major excitatory neurotransmitters, whereas γ-aminobutyrate (GABA, a metabolite of glutamate) and glycine are the major inhibitory neurotransmitters. Nitric oxide (NO, a metabolite of arginine), H2S (a metabolite of cysteine), serotonin (a metabolite of tryptophan) and histamine (a metabolite of histidine), as well as dopamine and norepinephrine (metabolites of tyrosine) are neurotransmitters to modulate synaptic plasticity, neuronal activity, learning, motor control, motivational behavior, emotion, and executive function. Concentrations of glutamine (a precursor of glutamate and aspartate), branched-chain AAs (precursors of glutamate, glutamine and aspartate), L-serine (a precursor of glycineand D-serine), methionine and phenylalanine in plasma are capable of affecting neurotransmission through the syntheses of glutamate, aspartate, and glycine, as well as the competitive transport of tryptophan and tyrosine across from the blood-brain barrier. Adequate consumption of AAs is crucial to maintain their concentrations and the production of neurotransmitters in the central nervous system. Thus, the content and balance of AAs in diets have a profound impact on food intake by animals. Knowledge of AA transport and metabolism in the brain is beneficial for improving the health and well-being of humans and animals.

Characterisation of the Toxoplasma gondii tyrosine transporter and its phosphorylation by the calcium-dependent protein kinase 3.

Summary Toxoplasma gondii parasites rapidly exit their host cell when exposed to calcium ionophores. Calcium‐dependent protein kinase 3 (TgCDPK3) was previously identified as a key mediator in this process, as TgCDPK3 knockout (∆cdpk3) parasites fail to egress in a timely manner. Phosphoproteomic analysis comparing WT with ∆cdpk3 parasites revealed changes in the TgCDPK3‐dependent phosphoproteome that included proteins important for regulating motility, but also metabolic enzymes, indicating that TgCDPK3 controls processes beyond egress. Here we have investigated a predicted direct target of TgCDPK3, ApiAT5‐3, a putative transporter of the major facilitator superfamily, and show that it is rapidly phosphorylated at serine 56 after induction of calcium signalling. Conditional knockout of apiAT5‐3 results in transcriptional upregulation of most ribosomal subunits, but no alternative transporters, and subsequent parasite death. Mutating the S56 to a non‐phosphorylatable alanine leads to a fitness cost, suggesting that phosphorylation of this residue is beneficial, albeit not essential, for tyrosine import. Using a combination of metabolomics and heterologous expression, we confirmed a primary role in tyrosine import for ApiAT5‐3. However, no significant differences in tyrosine import could be detected in phosphorylation site mutants showing that if tyrosine transport is affected by S56 phosphorylation, its regulatory role is subtle.

Proinflammatory Cytokines and Oxidative Stress Decrease the Transport of Dopamine Precursor Tyrosine in Human Fibroblasts

Background: Proinflammatory cytokines and oxidative stress responses have been extensively implicated in the pathophysiology of neuropsychiatric disorders over the past 2 decades. Moreover, disturbed transport of the dopamine precursor (i.e., the amino acid tyrosine) has been demonstrated, in different studies, across fibroblast cell membranes obtained from neuropsychiatric patients. However, the role and influences of proinflammatory cytokines and oxidative stress, and the reasons for disturbed tyrosine transport in neuropsychiatric disorders, are still not evaluated. Aims: The present study aimed to assess the role of proinflammatory cytokines and oxidative stress, indicated in many neuropsychiatric disorders, in tyrosine transportation, by using human skin-derived fibroblasts. Methods: Fibroblasts obtained from a healthy control were used in this study. Fibroblasts were treated with proinflammatory cytokines (IL-1β, IFN-γ, IL-6, TNF-α), their combinations, and oxidative stress, optimized for concentrations and incubation time, to analyze the uptake of ¹⁴C-tyrosine compared to untreated controls. Results and Conclusion: This study demonstrates that proinflammatory cytokines and oxidative stress decrease the transport of tyrosine (47% and 33%, respectively), which can alter dopamine synthesis. The functionality of the tyrosine transporter could be a new potential biomarker to target for discovering new drugs to counteract the effects of proinflammatory cytokines and oxidative stress in the pathophysiology of neuropsychiatric disorders.

A reassessment of the blood–brain barrier transport of large neutral amino acids during acute systemic inflammation in humans

We reassessed data from a previous study on the transcerebral net exchange of large neutral amino acids (LNAAs) using a novel mathematical model of blood-brain barrier (BBB) transport. The study included twelve healthy volunteers who received a 4-h intravenous lipopolysaccharide (LPS) infusion (total dose: 0·3ng/kg), a human experimental model of the systemic inflammatory response during the early stages of sepsis. Cerebral blood flow and arterial-to-jugular venous LNAA concentrations were measured prior to and after LPS, and the BBB transport and brain extracellular concentrations of LNAAs were calculated. The arterial concentration and unidirectional cerebral influx of phenylalanine increased after LPS. The BBB transport of tyrosine was unaffected, while its concentration in the brain extracellular fluid increased. These findings suggest that LPS infusion leads to an increased cerebral uptake of phenylalanine, which is then metabolized to tyrosine. This may reflect a neuroprotective mechanism that 'detoxifies' excess intracerebral phenylalanine in the clinical setting of sepsis.

Genetic and Functional Study of L-Type Amino Acid Transporter 1 in Schizophrenia

Schizophrenia involves neural catecholaminergic dysregulation. Tyrosine is the precursor of catecholamines, and its major transporter, according to studies on fibroblasts, in the brain is the <smlcap>L</smlcap>-type amino acid transporter 1 (LAT1). The present study assessed haplotype tag single-nucleotide polymorphisms (SNPs) of the SLC7A5/LAT1 gene in 315 patients with psychosis within the schizophrenia spectrum and 233 healthy controls to investigate genetic vulnerability to the disorder as well as genetic relationships to homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG), the major catecholamine metabolites in the cerebrospinal fluid (CSF). Moreover, the involvement of the different isoforms of the system L in tyrosine uptake and LAT1 tyrosine kinetics were studied in fibroblast cell lines of 10 patients with schizophrenia and 10 healthy controls. The results provide suggestive evidence of individual vulnerability to schizophrenia related to the LAT1 SNP rs9936204 genotype. A number of SNPs were nominally associated with CSF HVA and MHPG concentrations but did not survive correction for multiple testing. The LAT1 isoform was confirmed as the major tyrosine transporter in patients with schizophrenia. However, the kinetic parameters (maximal transport capacity, affinity of the binding sites, and diffusion constant of tyrosine transport through the LAT1 isoform) did not differ between patients with schizophrenia and controls. The present genetic findings call for independent replication in larger samples, while the functional study seems to exclude a role of LAT1 in the aberrant transport of tyrosine in fibroblasts of patients with schizophrenia.

System L amino acid transporter LAT1 accumulates O-(2-fluoroethyl)-L-tyrosine (FET).

O-(2-fluoroethyl)-L-tyrosine (FET) labeled with fluorine-18 is an important and specific tracer for diagnostics of glioblastoma via positron emission tomography (PET). However, the mechanism of its quite specific accumulation in tumor tissue has not been understood so far. In this work we demonstrate that [(3)H]L-tyrosine is primarily transported by the system L transporter LAT1 in human LN229 glioblastoma cells. FET reduced tyrosine transport, suggesting that it shares the same uptake pathway. More importantly, accumulation of FET was significantly reduced after siRNA-mediated downregulation of LAT1. Xenopus laevis oocytes expressing human LAT1 together with the glycoprotein 4F2hc (necessary to pull LAT-1 to the plasma membrane) exhibited a similar accumulation of FET as observed in glioblastoma cells. In contrast, no accumulation was observed in control oocytes, not overexpressing an exogenous transporter. Because LAT1 works exclusively as an exchanger of amino acids, substrates at one side of the membrane stimulate exchange against substrates at the other side. Extracellular FET stimulated the efflux of intracellular [(3)H]L-leucine, demonstrating that FETis indeed an influx substrate for LAT1. However, FET injected into oocytes was not able to stimulate uptake of extracellular [(3)H]L-leucine, indicating that FET is not a good efflux substrate. Our data, therefore, suggest that FET is trapped within cells due to the asymmetry of its intra- and extracellular recognition by LAT1. If also found for other transporters in tumor cells, asymmetric substrate recognition may be further exploited for tumor-specific accumulation of PET-tracers and/or other tumor-related drugs.

A simplified method to quantify dysregulated tyrosine transport in schizophrenia

BackgroundSchizophrenia is associated with altered tyrosine transport across plasma membranes. This is typically demonstrated by measuring the uptake of radiolabeled tyrosine in cultured human fibroblasts. Our primary goal was to determine whether tyrosine uptake could be characterized using unlabeled tyrosine. A secondary goal was to assess the effect of antipsychotic drugs added during the incubation. MethodEpithelium-derived fibroblast cultures were generated from patients with schizophrenia (n=6) and age-matched controls (n=6). Cells between cycles 8–12 were exposed to an amino acid free medium for 60min and then for 1min to media containing tyrosine (0.008–1.0mM). Amino acid levels were measured and Michaelis–Menten parameters determined. Uptake of tyrosine (0.5mM) was also measured in control cells after antipsychotic drugs were introduced during the depletion or uptake phases. ResultsTyrosine uptake was sodium-independent. The maximal transport velocity (Vmax) was significantly lower in patients with schizophrenia than in controls (p<0.01). The transporter affinity (Km) did not differ between the groups. Tyrosine uptake was differentially affected (p<0.001) by inclusion of 10−4M haloperidol, chlorpromazine or clozapine during different periods of incubation. ConclusionDysregulated tyrosine kinetics in schizophrenia can be readily studied without the use of radiolabeled tracers. The data also indicate that tyrosine uptake may be subject to complex pharmacological effects.

Toxicological effect of dexamethasone treatment on the transport of both amino-acids, tyrosine and tryptophan, across the blood brain barrier (BBB)

Toxicological effect of dexamethasone treatment on the transport of both amino-acids, tyrosine and tryptophan, across the blood brain barrier (BBB)

Altered tryptophan and alanine transport in fibroblasts from boys with attention-deficit/hyperactivity disorder (ADHD): an in vitro study

BackgroundThe catecholaminergic and serotonergic neurotransmitter systems are implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). The amino acid tyrosine is the precursor for synthesis of the catecholamines dopamine and norepinephrine, while tryptophan is the precursor of serotonin. A disturbed transport of tyrosine, as well as other amino acids, has been found in a number of other psychiatric disorders, such as schizophrenia, bipolar disorder and autism, when using the fibroblast cell model. Hence, the aim of this study was to explore whether children with ADHD may have disturbed amino acid transport.MethodsFibroblast cells were cultured from skin biopsies obtained from 14 boys diagnosed with ADHD and from 13 matching boys without a diagnosis of a developmental disorder. Transport of the amino acids tyrosine, tryptophan and alanine across the cell membrane was measured by the cluster tray method. The kinetic parameters, maximal transport capacity (Vmax) and affinity constant (Km) were determined. Any difference between the two groups was analyzed by Student's unpaired t-test or the Mann Whitney U test.ResultsThe ADHD group had significantly decreased Vmax (p = 0.039) and Km (increased affinity) (p = 0.010) of tryptophan transport in comparison to controls. They also had a significantly higher Vmaxof alanine transport (p = 0.031), but the Km of alanine transport did not differ significantly. There were no significant differences in any of the kinetic parameters regarding tyrosine transport in fibroblasts for the ADHD group.ConclusionsTryptophan uses the same transport systems in both fibroblasts and at the blood brain barrier (BBB). Hence, a decreased transport capacity of tryptophan implies that less tryptophan is being transported across the BBB in the ADHD group. This could lead to deficient serotonin access in the brain that might cause disturbances in both the serotonergic and the catecholaminergic neurotransmitter systems, since these systems are highly interconnected. The physiological importance of an elevated transport capacity of alanine to the brain is not known to date.

Maternal inheritance of aberrant tyrosine transport in patients with schizophrenia

The 15th Biennial Winter Workshop in Psychoses, 15–18 November 2009, Barcelona, Spain: GENETICS AND TRANSLATIONAL PSYCHIATRY

Aberrant tyrosine transport across the fibroblast membrane in patients with schizophrenia –indications of maternal inheritance

Background In previous studies of the present patients with schizophrenia, aberrant tyrosine transport across the fibroblast membrane was found. A low K m, a kinetic factor indicating high affinity between tyrosine and the binding site at the cell membrane, was found to be associated with poor cognitive functions in patients. The present study aimed at investigating possible relationships between patients with schizophrenia and their first-degree relatives in aberrant tyrosine transport indicating that it may be a biological marker for the genetic susceptibility. Methods Thirty-three parents, 13 fathers and 20 mothers, from 23 families with a schizophrenic patient agreed to enter the study. They underwent skin biopsies for fibroblast cultivation, neuropsychological and psychiatric investigations and were classified as family history positive or negative. Tyrosine transport kinetics (K m and V max) were calculated from in vitro trials of gradients of extracellular tyrosine concentrations in fibroblast cultures. Results An association between patients with schizophrenia and their mothers were found for a low K m indicating maternal inheritance. Mothers displaying a low K m performed worse on the neuropsychological tests compared to mothers with normal K m. Corresponding relationships between a low K m and neurocognitive dysfunction had previously been found for the patients. Conclusions An aberrant tyrosine transport across plasma membrane may constitute a biological marker for an endophenotype within the schizophrenia spectrum with low cognitive functioning. A plausible mode for genetic transmission is maternal inheritance.

Aberrant amino acid transport in fibroblasts from patients with bipolar disorder

Aberrant tyrosine transport is a repeated finding in fibroblasts from schizophrenic patients. The transport aberration could lead to disturbances in the dopaminergic and noradrenergic neurotransmitter systems. Tyrosine and tryptophan are the precursors of the neurotransmitters dopamine and serotonin. Disturbed dopaminergic, noradrenergic and serotoninergic systems are implicated as causes of bipolar disorder. Hence, the aim of this study was to explore whether patients with bipolar disorder have an aberrant transport of tyrosine and/or tryptophan. Fibroblast cell lines from patients with bipolar type-1 disorder ( n = 10) and healthy controls ( n = 10) were included in this study. All patients fulfilled the DSM-IV diagnostic criteria. The transport of amino acids across the cell membranes was measured by the cluster tray method. The kinetic parameters, maximal transport velocity ( V max ) and affinity constant ( K m ) were determined. A significantly lower V max for tyrosine ( p = 0.027) was found in patients with bipolar type-1 disorder in comparison to healthy controls. No significant differences in K m for tyrosine and in the kinetic parameters of tryptophan between patients with bipolar type-1 disorder and healthy controls were observed. The decreased tyrosine transport (low V max ) found in this study may indicate less access of dopamine in the brain, resulting in disturbed dopaminergic and/or noradrenergic neurotransmission, that secondarily could lead to disturbances in other central neurotransmitter systems, such as the serotoninergic system. However, as sample size was small in this study and an age difference between patients and controls existed, the present findings should be considered as pilot data. Further studies with larger sample number are needed to elucidate the transport aberration and the significance of these findings.

Separation of amino acids mixtures containing tyrosine in electromembrane system

The principal distinction of tyrosine transport in the electromembrane system is revealed. It lies in the absence of electromigration of this amino acid through the cation-exchange membrane MC-40 in a wide range of current density. This is true for any pH value, including the region where tyrosine exists mainly in the form of cations. Only insignificant diffusive transfer of this aromatic amino acid (pI=5.63) is observed. At the same time the dependence of the flux through the anion-exchange membrane on the current density has the traditional form for amino acids with the maximum corresponding to the limiting current. On the basis of the revealed regularity of tyrosine transport, we study the possibility of its separation from the basic amino acid (arginine or histidine) that migrates actively through the cation-exchange membrane.

P03-138 Aberrant tyrosine transport - a biological marker for an inherited susceptibility to schizophrenia

A biological marker may serve as a link between the clinical manifestation of schizophrenia and the underlying susceptibility gene (s) and it may also serve the purpose to delineate subgroups or “endophenotypes”. Aberrant tyrosine transport across the cell membrane in patients with schizophrenia is such a biological marker. Previous studies have showed a reduced tyrosine transport across the fibroblast cell membrane in patients with schizophrenia. the experiments were performed in fibrobasts that had grown for several generations in vitro. Thus, the aberrant tyrosine transportation is likely to reflect a genetic trait that is transmitted through several cell generations. an association between aberrant tyrosine transport across fibroblast membrane and neurocognitive dysfunction was found for patients with schizophrenia in a previous study indicating a connection between deficient tyrosine transport and cognitive dysfunction. the aim of the present study was to investigate the occurrence of aberrant tyrosine transport across the fibroblast membrane in first-degree relatives to patients with schizophrenia to find indications of inheritance. Significant correlations between patients and their mothers were found regarding aberrant tyrosine transport and, furthermore, the mothers with aberrant tyrosine transport showd signs of reduced cognitive functioning. Maternal inheritance is a possible mode for genetic transmissionssion of susceptibility to schizophrenia for a subgroup of patients with low cognitive functioning.

Dual Role for the Tyrosine Decarboxylation Pathway inEnterococcus faeciumE17: Response to an Acid Challenge and Generation of a Proton Motive Force

In this work we investigated the role of the tyrosine decarboxylation pathway in the response of Enterococcus faecium E17 cells to an acid challenge. It was found that 91% of the cells were able to remain viable in the presence of tyrosine when they were incubated for 3 h in a complex medium at pH 2.5. This effect was shown to be related to the tyrosine decarboxylation pathway. Therefore, the role of tyrosine decarboxylation in pH homeostasis was studied. The membrane potential and pH gradient, the parameters that compose the proton motive force (PMF), were measured at different pHs (pH 4.5 to 7). We obtained evidence showing that the tyrosine decarboxylation pathway generates a PMF composed of a pH gradient formed due to proton consumption in the decarboxylation reaction and by a membrane potential which results from electrogenic transport of tyrosine in exchange for the corresponding biogenic amine tyramine. The properties of the tyrosine transporter were also studied in this work by using whole cells and right-side-out vesicles. The results showed that the transporter catalyzes homologous tyrosine/tyrosine antiport, as well as electrogenic heterologous tyrosine-tyramine exchange. The tyrosine transporter had properties of a typical precursor-product exchanger operating in a proton motive decarboxylation pathway. Therefore, the tyrosine decarboxylation pathway contributes to an acid response mechanism in E. faecium E17. This decarboxylation pathway gives the strain a competitive advantage in nutrient-depleted conditions, as well as in harsh acidic environments, and a better chance of survival, which contributes to higher cell counts in food fermentation products.

Functional characterization of tyrosine transport in fibroblast cells from healthy controls

Human fibroblast cells are an advantageous model to study the transport of amino acids across cell membranes, since one can control the environmental factors. A major problem in all earlier studies is the lack of precise and detailed knowledge regarding the expression and functionality of tyrosine transporters in human fibroblasts. This motivated us to perform a systematic functional characterization of the tyrosine transport in fibroblast cells with respect to the isoforms of system-L (LAT1, LAT2, LAT3, LAT4), which is the major transporter of tyrosine. Ten (n=10) fibroblast cell lines from healthy volunteers were included in the study. Uptake of L-[U-14C] tyrosine in fibroblasts was measured using the cluster tray method in the presence and absence of excess concentrations of various combinations of inhibitors. This study demonstrated that LAT1 is involved in 90% of total uptake of tyrosine and also around 51% of alanine. Not more than 10% can be accounted for by LAT2, LAT3 and LAT4 isoforms. LAT2 seems to be functionally weak in uptake of tyrosine while LAT3 and LAT4 contributed around 7%. 10% could be contributed by system-A (ATA2 isoform). Alanine consequently inhibited the tyrosine transport by up to 60%. Tyrosine transport through the LAT1 isoform has a higher affinity compared to system-L. In conclusion, the LAT1 isoform is the major transporter of tyrosine in human fibroblast cells. Competition between tyrosine and alanine for transport is shown to exist, probably between LAT1 and LAT2 isoforms. This study established fibroblast cells as a suitable experimental model for studying amino acid transport defects in humans.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA(−)) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 μM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA(−) lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA(−) had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 μM) was administered in perfusate, LNAA(−) still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.

Characterization of a novel tyrosine permease of lager brewing yeast shared bySaccharomyces cerevisiaestrain RM11-1a

In Saccharomyces cerevisiae yeast, the uptake of aromatic amino acids is mediated by the relatively specific permeases Tat1p, Tat2p, Bap2p, and Bap3p, as well as by two other permeases with broader specificities: Gap1p and Agp1p. Here, a novel permease gene TAT3 (Tyrosine Amino acid Transporter) identified in the S. cerevisiae-type subset genome of the lager brewing yeast strain Weihenstephan Nr.34 (34/70) is reported. The TAT3 sequence was also found in the genome of S. cerevisiae strain RM11-1a, but not in S. cerevisiae strain S288C. Tat3p showed a significant similarity to Penicillium chrysogenum ArlP permease, which has transport activity for aromatic amino acids and leucine. When overexpressed in ssy1Delta gap1Delta mutant cells, Tat3p exhibited a tyrosine transport activity with an apparent K(m) of 160 microM. TAT3 transcription in lager brewing yeast was subjected to nitrogen catabolite repression in a manner similar to that of GAP1. Furthermore, the subcellular localization of Tat3p-green fluorescent protein (GFP) fusion protein was dependent on the quality of the nitrogen source, indicating a post-translational control of Tat3p function.