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At least 649 records · Page 36Linked to original sources
PubMed1992

Dopamine transport: pharmacological distinction between the synaptic membrane and the vesicular transporter in rat striatum.

The pharmacological properties of the monoamine transporters in the synaptic vesicles and of the dopamine transporters in the synaptic plasma membrane were compared. Tetrabenazine, an inhibitor of the vesicular transporter did not block ligand binding to the plasma membrane transporter. Various potent cocaine analogues and other compounds active at the plasma membrane transporter did not block ligand binding to the vesicular transporter. These data indicate pharmacological differences between the vesicular and synaptic membrane transporters.

Animals↗
PubMed1980

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 1. Inhibition of glucose transport.

Maltosyl isothiocyanate (MITC) has been synthesized from maltose with an overall yield of 88%. It has been found to be a potent irreversible inhibitor of zero trans influx of glucose with human erythrocytes. Kinetic analysis of glucose transport after treatment of erythrocytes with MITC revealed that VT was diminished while KT was unchanged. Transportable sugars and competitive inhibitors of monosaccharide transport protected against MITC inhibition, while carbohydrates which do not interact with the transporter gave no protection. Covalent inhibitors of anion transport were without effect on glucose transport. MITC fulfilled the kinetic requirements for an affinity label of the glucose transporter of human erythrocytes [Groman, E. V., Schultz, R. M., & Engel, L. L. (1977) Methods Enzymol. 46, 54].

Affinity Labels↗
PubMed1980

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 2. Identification of the transporter.

Maltosyl isothiocyanate (MITC), a potent irreversible inhibitor of glucose transport in human erythrocytes [Mullins, R. E., & Langdon, R. G. (1980) Biochemistry (preceding paper in this issue)], has been found to react almost exclusively with band 3 of the human erythrocyte membrane. The incorporation of [14C]MITC into band 3 was found to be antagonized by transportable sugars or competitive inhibitors of transport. On the basis of [14C]MITC incorporation into band 3 and MITC inhibition of transport, it is estimated that there are 3 x 10(5) glucose transporters present in the erythrocyte membrane. It was found that [14C]MITC-labeled band 3 could be converted into 14C-labeled band 4.5 during the Triton X-100 extraction procedure described by Kasahara & Hinkle [Kasahara, M., & Hinkel, P. C. (1977) J. Biol. Chem. 252, 7384]. On the basis of the evidence presented here and in the preceding paper, it is suggested that in the native erythrocyte membrane a component of band 3 is the glucose transport protein and that during purification with nonionic detergents the transport protein may be enzymatically degraded with some retention of activity.

Affinity Labels↗
PubMed2003

Tea polyphenols inhibit the transport of dietary phenolic acids mediated by the monocarboxylic acid transporter (MCT) in intestinal Caco-2 cell monolayers.

It was previously reported that a fluorescent marker dye, fluorescein, is transported via the monocarboxylic acid transporter (MCT). Fluorescein transport was competitively inhibited by MCT substrates such as ferulic and salicylic acids. Tea polyphenols, in particular, epigallocatechin gallate (EGCg) and epicatechin gallate (ECg), inhibited the transport of fluorescein. Tea polyphenols also inhibited the transport of salicylic and ferulic acids, suggesting tea polyphenols might be substrates of MCT. However, the transepithelial flux of tea polyphenols was much lower than that of the MCT substrates and was inversely correlated with the paracellular permeability of Caco-2 cell monolayers. These findings suggest that tea polyphenols are not substrates but inhibitors of MCT. Furthermore, the transepithelial transport of these polyphenols is mainly via paracellular diffusion. However, directional transport of ECg and EGCg from the basolateral to the apical side was observed, indicating that the behavior of tea polyphenols in the intestinal epithelium is complex.

Biological Transport↗
PubMed1996

Synthesis and transporter binding properties of 3 beta-(4'-alkyl-, 4'-alkenyl-, and 4'-alkynylphenyl)nortropane-2 beta-carboxylic acid methyl esters: serotonin transporter selective analogs.

New methods for the synthesis of 3 beta-(4'-alkyl-, 4'-alkenyl-, and 4'-alkynylphenyl)nortropane-2 beta-carboxylic acid methyl esters 2-4, respectively, were developed. These methods involved coupling of the appropriate organometallic reagents to 3 beta-(4'-iodophenyl)tropane-2 beta-carboxylic acid methyl ester (6a, RTI-55) or to an N-protected derivative of 6a followed by N-demethylation or removal of the protecting group. Some analogs were prepared by catalytic reduction of the alkene and alkene analogs 3 and 4 or by isomerization of the alkenes 3. The analogs 2-4 were evaluated for inhibition of radioligand binding to the serotonin (5-HT), dopamine (DA), and norepinephrine (NE) transporters. 3 beta-(4'-Isopropenyl- and 4'-cis-propenylphenyl)nortropane-2 beta-carboxylic acid methyl esters (3b,d), which possessed IC50 values of 0.6 and 1.15 nM, respectively, were the most potent analogs at the 5-HT transporter, and with NE/5-HT IC50 ratios of 240 and 128 nM, respectively, they were selective for the 5-HT relative to the NE transporter. Since interaction with the serotonin transporter may modulate the pharmacological effects resulting from binding to the dopamine transporter, 3 beta-(4'-isopropenylphenyl)tropane-2 beta-carboxylic acid methyl ester (11b) which has good affinity for both the 5-HT and DA transporters but low affinity at the NE transporter may be useful for studying this interaction.

Animals↗
PubMed1999

Transport of N(G)-nitro-L-arginine across intestinal brush border membranes by Na+ -dependent and Na+-independent amino acid transporters.

PURPOSE: To clarify the transport mechanism of NG-nitro-L-arginine (L-NNA), a potent NO-synthase inhibitor, across intestinal brush border membranes (BBM). METHODS: Dog intestinal BBM vesicles were used. RESULTS: The time course of L-NNA uptake showed a Na+ -dependent overshoot phenomenon. Concentration-dependence curves of L-NNA initial uptake were saturable in the presence and absence of Na+, indicating participation of Na+ -dependent and Na+ -independent carrier-mediated transport systems. The calculated kinetic parameters of L-NNA initial uptake indicate that the former is a low-affinity high-capacity system and the latter is a high-affinity low-capacity one, similar to those in neutral amino acid transport. Neutral and basic amino acids showed cis-inhibitory and trans-stimulatory effects on L-NNA uptake in the presence or absence of Na+. N(G)-Nitro-L-arginine methyl ester, another potent NO-synthase inhibitor, also had both effects, which were smaller than with amino acids. CONCLUSIONS: The present study clearly indicates that transport of L-NNA across the intestinal BBM occurs in the same manner as neutral amino acid transport. However, it is affected by both neutral and basic amino acids in the presence or absence of Na+ differently from that across plasma membranes of nonepithelial cells, because B0,+ and b0,+ amino acid transporters function partly in L-NNA transport across intestinal BBM.

Amino Acid Transport Systems↗
PubMed2003

In vitro polarized transport of L-phenylalanine in human nasal epithelium and partial characterization of the amino acid transporters involved.

PURPOSE: The purpose of this study was to provide functional and molecular evidence to support the existence of large neutral amino acid transporters in human nasal epithelium using nasal primary cell culture model. METHODS: L-Phenylalanine was used as a model substrate to characterize carrier-mediated permeation of amino acids across human nasal epithelium. The influence of temperature, concentration, other amino acids, metabolic/transport inhibitors, and polarity/stereo-selectivity on transport of the model compound was investigated. Reverse transcriptase polymerase chain reaction was used for molecular characterization of the existence of the transporters. RESULTS: The transport of L-phenylalanine across the human nasal epithelium was polarized (apical --> basolateral >> basolateral --> apical), saturable (Km = 1.23 mM; Vmax = 805.1 nmol/mg protein/min) and stereo-selective (permeation of L-phenylalanine >> D-Phenylalanine). Its permeation was significantly (< 0.05) reduced by cationic, small and large neutral amino acids, oubain, amiloride, sodium-free medium, and temperature lowering. Reverse transcriptase polymerase chain reaction revealed the presence of the broad-scope cationic-dependent amino acid transporter gene (y+LAT-2) in the human nasal epithelium. CONCLUSIONS: Based on the results of this study, one may postulate that the human nasal epithelium expresses L-amino acid transporters. More studies are necessary for detailed characterization of the transporters.

Amino Acid Transport Systems↗
PubMed2003

Restoration of high-level transport activity by human reduced folate carrier/ThTr1 thiamine transporter chimaeras: role of the transmembrane domain 6/7 linker region in reduced folate carrier function.

The reduced folate carrier (RFC; SLC19A1) is closely related to the thiamine transporter, SLC19A2 (ThTr1). Hydropathy models for these homologous transporters predict up to 12 transmembrane domains (TMDs), with internally oriented N- and C-termini and a large central loop between TMDs 6 and 7. The homologies are localized mostly in the TMDs. However, there is little similarity in their N- and C-terminal domains and the central peptide linkers connecting putative TMDs 1-6 and TMDs 7-12. To explore the functional role of the 61-amino acid central linker in the human RFC (hRFC), we introduced deletions of 49 and 60 amino acids into this region, differing by the presence of a stretch of 11 highly conserved amino acids between the human and rodent RFCs (positions 204-214). An additional hRFC construct was prepared in which only the 11 conserved amino acids were deleted. The resulting hRFC(D215-R263 Delta), hRFC(K204-R263 Delta) and hRFC(K204-R214 Delta) proteins were transfected into transport-impaired K562 cells. The deletion constructs were all expressed in plasma membranes; however, they were completely inactive for methotrexate and (6 S )5-formyl tetrahydrofolate transport. Insertion of non-homologous 73- and 84-amino acid fragments from the structurally analogous ThTr1 linker region into position 204 of hRFC(K204-R263 Delta) restored low levels of transport (16-21% of the wild type). Insertion of the ThTr1 linkers into hRFC(D215-R263 Delta) at position 215 restored 60-80% of wild-type levels of transport. Collectively, our results suggest that the role of the hRFC linker peptide is to provide the proper spatial orientation between the two halves of the hRFC protein for optimal function, and that this is largely independent of amino acid sequence. Our results also demonstrate a critical transport role for the stretch of 11 conserved amino acids starting at position 204 of hRFC.

Amino Acid Sequence↗
PubMed1991

Phorbol ester only partially mimics the effects of insulin on glucose transport and glucose-transporter distribution in 3T3-L1 adipocytes.

We examined the effects of the phorbol ester phorbol 12-myristate 13-acetate (PMA) on the rate of hexose transport into 3T3-L1 adipocytes. Exposure of adipocytes to PMA (1 microM) for 60 min results in a 1.7-2.5-fold increase in the rate of hexose transport. This effect was mediated by translocation of two isoforms of glucose transporters to the plasma membrane, as determined by labelling in situ, photoaffinity labelling with a membrane-impermeant glucose analogue, and by immunoblotting of subcellular fractions. The PMA-induced stimulation of both transport and transporter translocation was substantially less than that induced by insulin in this cell line; the PMA-induced increase in plasma-membrane GLUT 1 and GLUT 4 transporter isoforms was only about 40% and 10% respectively of that induced by insulin. We suggest that the stimulation of transport by insulin and PMA occurs via different mechanisms, which is manifested by the ability of insulin to induce a much greater increase in the plasma-membrane content of GLUT 4 compared with the phorbol ester.

Adipose Tissue↗
PubMed2003

Organic anion-transporting polypeptide (OATP) transporter family and drug disposition.

Drug transporters are increasingly recognized as a key determinant of drug disposition. Recent studies have revealed that targeted expression of drug uptake and efflux transporters to specific cell membrane domains allows for the efficient directional movement of many drugs in clinical use. While the role of certain efflux transporters such as MDR1 (P-glycoprotein) in drug disposition has been extensively studied, emerging evidence suggests that uptake transporters may also be important to the intestinal absorption and renal or hepatic elimination of drugs. Members of the organic anion-transporting polypeptide (OATP) family of drug uptake transporters have been found capable of transporting a large array of structurally divergent drugs. Moreover, expression of OATP isoforms in the gastrointestinal tract, liver and kidney, as well as at the level of the blood-brain barrier, has important implications for our understanding of the factors governing drug absorption, elimination and tissue penetration.

Animals↗
PubMed1998

An ABC-transporter of Arabidopsis thaliana has both glutathione-conjugate and chlorophyll catabolite transport activity.

An ABC-transporter of Arabidopsis thaliana exhibiting high sequence similarity to the human (MRP1) and yeast (YCF1) glutathione-conjugate transporters has been analysed and used to complement a cadmium-sensitive yeast mutant (DTY168) that also lacks glutathione-conjugate transport activity. Comparison of the hydrophobicity plots of this A. thaliana MRP-like protein with MRP1 and YCF1 demonstrates that the transmembrane domains are conserved, even at the N-terminus where sequence identity is low. Cadmium resistance is partially restored in the complemented ycf1 mutant, and glutathione-conjugate transport activity can be observed as well. The kinetic properties of the A. thaliana MRP-like protein (AtMRP3) are very similar to those previously described for the vacuolar glutathione-conjugate transporter of barley and mung bean. Furthermore, a hitherto undescribed ATP-dependent transport activity could be correlated with the gene product, i.e. vesicles isolated from the complemented yeast, but not from DTY168 or the wild type, take up the chlorophyll catabolite Bn-NCC-1. The results indicate that the product of the MRP-like gene of A. thaliana is capable of mediating the transport of the two different classes of compounds.

ATP-Binding Cassette Transporters↗
PubMed2003

The soybean NRAMP homologue, GmDMT1, is a symbiotic divalent metal transporter capable of ferrous iron transport.

Iron is an important nutrient in N2-fixing legume root nodules. Iron supplied to the nodule is used by the plant for the synthesis of leghemoglobin, while in the bacteroid fraction, it is used as an essential cofactor for the bacterial N2-fixing enzyme, nitrogenase, and iron-containing proteins of the electron transport chain. The supply of iron to the bacteroids requires initial transport across the plant-derived peribacteroid membrane, which physically separates bacteroids from the infected plant cell cytosol. In this study, we have identified Glycine max divalent metal transporter 1 (GmDmt1), a soybean homologue of the NRAMP/Dmt1 family of divalent metal ion transporters. GmDmt1 shows enhanced expression in soybean root nodules and is most highly expressed at the onset of nitrogen fixation in developing nodules. Antibodies raised against a partial fragment of GmDmt1 confirmed its presence on the peribacteroid membrane (PBM) of soybean root nodules. GmDmt1 was able to both rescue growth and enhance 55Fe(II) uptake in the ferrous iron transport deficient yeast strain (fet3fet4). The results indicate that GmDmt1 is a nodule-enhanced transporter capable of ferrous iron transport across the PBM of soybean root nodules. Its role in nodule iron homeostasis to support bacterial nitrogen fixation is discussed.

Amino Acid Sequence↗
PubMed1999

GSH transport in immortalized mouse brain endothelial cells: evidence for apical localization of a sodium-dependent GSH transporter.

We have previously shown GSH transport across the blood-brain barrier in vivo and expression of transport in Xenopus laevis oocytes injected with bovine brain capillary mRNA. In the present study, we have used MBEC-4, an immortalized mouse brain endothelial cell line, to establish the presence of Na+-dependent and Na+-independent GSH transport and have localized the Na+-dependent transporter using domain-enriched plasma membrane vesicles. In cells depleted of GSH with buthionine sulfoximine, a significant increase of intracellular GSH could be demonstrated only in the presence of Na+. Partial but significant Na+ dependency of [35S]GSH uptake was observed for two GSH concentrations in MBEC-4 cells in which gamma-glutamyltranspeptidase and gamma-glutamylcysteine synthetase were inhibited to ensure absence of breakdown and resynthesis of GSH. Uniqueness of Na+-dependent uptake in MBEC-4 cells was confirmed with parallel uptake studies with Cos-7 cells that did not show this activity. Molecular form of uptake was verified as predominantly GSH, and very little conversion of [35S]cysteine to GSH occurred under the same incubation conditions. Poly(A)+ RNA from MBEC expressed GSH uptake with significant (approximately 40-70%) Na+ dependency, whereas uptake expressed by poly(A)+ RNA from HepG2 and Cos-1 cells was Na+ independent. Plasma membrane vesicles from MBEC were separated into three fractions (30, 34, and 38% sucrose, by wt) by density gradient centrifugation. Na+-dependent glucose transport, reported to be localized to the abluminal membrane, was found to be associated with the 38% fraction (abluminal). Na+-dependent GSH transport was present in the 30% fraction, which was identified as the apical (luminal) membrane by localization of P-glycoprotein 170 by western blot analysis. Localization of Na+-dependent GSH transport to the luminal membrane and its ability to drive up intracellular GSH may find application in the delivery of supplemented GSH to the brain in vivo.

Animals↗
PubMed1996

Hepatocellular sinusoidal membrane organic anion transport and transporters.

This review focuses on oatp, a rat liver protein that has been cloned on the basis of its ability to transport organic anions such as bilirubin and sulfobromophthalein (BSP). Although other proteins have been suggested as having bilirubin or BSP transport activity, these data have primarily been indirect, and their cloning and expression have not yet been accomplished. Although preliminary data suggest that organic anion transporting polypeptide (oatp) accounts for a significant amount of BSP transport into the hepatocyte, there is certainly the possibility that other transporters exist. In addition, oatp appears to be a member of a new family of integral membrane transport proteins that may have overlapping substrate specificities. Organic anions such as bilirubin and sulfobromophthalein (BSP) circulate bound tightly to albumin from which they are rapidly extracted by hepatocytes. This process of organic anion uptake has been the subject of extensive investigation over many years. Although details remain unresolved, much has been learned recently regarding the mechanisms of organic anion transport. This article will examine the history and current status of this field.

Animals↗
PubMed1995

Expression cloning of the cDNA for a polypeptide associated with rat hepatic sinusoidal reduced glutathione transport: characteristics and comparison with the canalicular transporter.

Using the Xenopus oocyte expression system, we previously identified an approximately 4-kb fraction of mRNA from rat liver that expresses sulfobromophthalein reduced glutathione S-conjugate (BSP-GSH)-insensitive and an approximately 2.5-kb fraction expressing BSP-GSH-sensitive reduced glutathione (GSH) transport. From the former, a 4.05-kb cDNA was cloned and characterized as the putative rat canalicular GSH transporter. Starting with a cDNA library constructed from the approximately 2.5-kb fraction, we have now isolated a single clone that leads to expression of a BSP-GSH- and cystathionine-inhibitable GSH transporter activity with Km approximately 3 mM characteristic of the sinusoidal GSH transporter. The cDNA for the rat sinusoidal GSH transporter-associated polypeptide (RsGshT) is 2733 bases with an open reading frame of 1059 nucleotides encoding a polypeptide of 353 amino acids (39,968 Da) with two putative membrane-spanning domains. No identifiable homologies were found in searching various data bases. An approximately 40-kDa protein is generated in in vitro translation of cRNA for RsGshT. Northern blot analysis revealed a single approximately 2.8-kb transcript in rat and human liver with negligible hybridization signal in other organs. The abundance of mRNA for RsGshT did not increase with phenobarbital treatment. Cis-inhibition by BSP-GSH and trans-inhibition by cystathionine and lack of induction by phenobarbital are characteristic of sinusoidal GSH secretion and thus indicate that RsGshT either encodes the sinusoidal GSH transporter itself or a regulatory subunit of the transporter that determines its liver-specific activity.

Amino Acid Sequence↗
PubMed1998

Cloning of Leishmania nucleoside transporter genes by rescue of a transport-deficient mutant.

All parasitic protozoa studied to date are incapable of purine biosynthesis and must therefore salvage purine nucleobases or nucleosides from their hosts. This salvage process is initiated by purine transporters on the parasite cell surface. We have used a mutant line (TUBA5) of Leishmania donovani that is deficient in adenosine/pyrimidine nucleoside transport activity (LdNT1) to clone genes encoding these nucleoside transporters by functional rescue. Two such genes, LdNT1.1 and LdNT1.2, have been sequenced and shown to encode deduced polypeptides with significant sequence identity to the human facilitative nucleoside transporter hENT1. Hydrophobicity analysis of the LdNT1.1 and LdNT1.2 proteins predicted 11 transmembrane domains. Transfection of the adenosine/pyrimidine nucleoside transport-deficient TUBA5 parasites with vectors containing the LdNT1.1 and LdNT1.2 genes confers sensitivity to the cytotoxic adenosine analog tubercidin and concurrently restores the ability of this mutant line to take up [3H]adenosine and [3H]uridine. Moreover, expression of the LdNT1.2 ORF in Xenopus oocytes significantly increases their ability to take up [3H]adenosine, confirming that this single protein is sufficient to mediate nucleoside transport. These results establish genetically and biochemically that both LdNT1 genes encode functional adenosine/pyrimidine nucleoside transporters.

Amino Acid Sequence↗
PubMed1997

Membrane potential-generating malate (MleP) and citrate (CitP) transporters of lactic acid bacteria are homologous proteins. Substrate specificity of the 2-hydroxycarboxylate transporter family.

Membrane potential generation via malate/lactate exchange catalyzed by the malate carrier (MleP) of Lactococcus lactis, together with the generation of a pH gradient via decarboxylation of malate to lactate in the cytoplasm, is a typical example of a secondary proton motive force-generating system. The mleP gene was cloned, sequenced, and expressed in a malolactic fermentation-deficient L. lactis strain. Functional analysis revealed the same properties as observed in membrane vesicles of a malolactic fermentation-positive strain. MleP belongs to a family of secondary transporters in which the citrate carriers from Leuconostoc mesenteroides (CitP) and Klebsiella pneumoniae (CitS) are found also. CitP, but not CitS, is also involved in membrane potential generation via electrogenic citrate/lactate exchange. MleP, CitP, and CitS were analyzed for their substrate specificity. The 2-hydroxycarboxylate motif R1R2COHCOOH, common to the physiological substrates, was found to be essential for transport although some 2-oxocarboxylates could be transported to a lesser extent. Clear differences in substrate specificity among the transporters were observed because of different tolerances toward the R substituents at the C2 atom. Both MleP and CitP transport a broad range of 2-hydroxycarboxylates with R substituents ranging in size from two hydrogen atoms (glycolate) to acetyl and methyl groups (citromalate) for MleP and two acetyl groups (citrate) for CitP. CitS was much less tolerant and transported only citrate and at a low rate citromalate. The substrate specificities are discussed in the context of the physiological function of the transporters.

Amino Acid Sequence↗
PubMed1998

Identification and characterization of a membrane protein (y+L amino acid transporter-1) that associates with 4F2hc to encode the amino acid transport activity y+L. A candidate gene for lysinuric protein intolerance.

We have identified a new human cDNA (y+L amino acid transporter-1 (y+LAT-1)) that induces system y+L transport activity with 4F2hc (the surface antigen 4F2 heavy chain) in oocytes. Human y+LAT-1 is a new member of a family of polytopic transmembrane proteins that are homologous to the yeast high affinity methionine permease MUP1. Other members of this family, the Xenopus laevis IU12 and the human KIAA0245 cDNAs, also co-express amino acid transport activity with 4F2hc in oocytes, with characteristics that are compatible with those of systems L and y+L, respectively. y+LAT-1 protein forms a approximately 135-kDa, disulfide bond-dependent heterodimer with 4F2hc in oocytes, which upon reduction results in two protein bands of approximately 85 kDa (i.e. 4F2hc) and approximately 40 kDa (y+LAT-1). Mutation of the human 4F2hc residue cysteine 109 (Cys-109) to serine abolishes the formation of this heterodimer and drastically reduces the co-expressed transport activity. These data suggest that y+LAT-1 and other members of this family are different 4F2 light chain subunits, which associated with 4F2hc, constitute different amino acid transporters. Human y+LAT-1 mRNA is expressed in kidney >> peripheral blood leukocytes >> lung > placenta = spleen > small intestine. The human y+LAT-1 gene localizes at chromosome 14q11.2 (17cR approximately 374 kb from D14S1350), within the lysinuric protein intolerance (LPI) locus (Lauteala, T., Sistonen, P. , Savontaus, M. L., Mykkanen, J., Simell, J., Lukkarinen, M., Simmell, O., and Aula, P. (1997) Am. J. Hum. Genet. 60, 1479-1486). LPI is an inherited autosomal disease characterized by a defective dibasic amino acid transport in kidney, intestine, and other tissues. The pattern of expression of human y+LAT-1, its co-expressed transport activity with 4F2hc, and its chromosomal location within the LPI locus, suggest y+LAT-1 as a candidate gene for LPI.

Amino Acid Metabolism, Inborn Errors↗