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PubMed1976

Studies on the lithium transport across the red cell membrane. I. Li+ uphill transport by the Na+-dependent Li+ counter-transport system of human erythrocytes.

Li+ net-transfer across cell membranes was studied on human erythrocytes and ghosts preloaded with 1-2 mM Li+ and incubated in saline media of varying composition at initial thermodynamic equilibrium for Li+. The following results were obtained: 1. Li+ is extruded from glycolyzing erythrocytes against an electrochemical gradient until a steady-state Li+ distribution is established after 24-28 h. 2. The initial rate of Li+ extrusion is not altered by ouabain or by reduction of ATP levels to less than 25% of the normal value. 3. Replacement of external Na+ by K+ or choline+ abolishes the establishment of an electrochemical Li+ gradient. 4. The Li+ distribution ratio Lie+/Lii+ increases proportional to the ratio Nae+/Nai+ at constant extravellular K+ concentrations. 5. In ghost suspension an uphill Li+ transport is driven by an oppositely directed Na+ gradient. The direction of the Li+ uphill transport can be reversed by reversing the Na+ gradient. From the results it is concluded that the Li+ uphill transport across human red cell membranes is mediated by a Na+-dependent Li+ counter-transport system. This system is not inhibited by ouabain and does not appear to be identical to the Na+-Na+ exchange system described by Garrahan and Glynn.

Adenosine Triphosphate↗
PubMed1991

The axonal transport motor 'kinesin' is bound to anterogradely transported organelles: quantitative cytofluorimetric studies of fast axonal transport in the rat.

Monoclonal antibodies to the axonal transport ATPase kinesin were used in an immunofluorescent study on mammalian nerves. Following crushing of the sciatic nerve and the ventral roots of adult rats, immunoreactive material was found to accumulate rapidly, mainly proximal to a crush but also, to some degree, distal to a crush. The strongest immunofluorescence was observed after incubation with the H2 antibody against the heavy subunit of kinesin. Using the cytofluorimetric scanning (CFS) procedure, the accumulated amounts were quantified and it was found that the retrogradely accumulating kinesin-like immunoreactivity (IR) was about 4-12% of the anterogradely transported kinesin-IR. The results were compared to the vesicle marker p38 (synaptophysin), which was found to accumulate to a significant extent on both sides of the crush. Cytofluorimetric scanning measurements indicated that nearly 50% of the anterogradely accumulated p38-IR was recycling to the cell body. The results demonstrate that kinesin in the living axon is affiliated with anterogradely transported organelles. Retrogradely transported organelles appeared to carry very little kinesin-IR, suggesting that kinesin may be subject to turnover, distinct from that of p38, in the distal regions of the axon.

Adenosine Triphosphatases↗
PubMed2000

Gene expression of taurine transporter and taurine biosynthetic enzymes in brain of rats with acute or chronic hyperosmotic plasma. A comparative study with gene expression of myo-inositol transporter, betaine transporter and sorbitol biosynthetic enzyme.

Cells exposed to hyperosmotic conditions maintain their volume by accumulating organic osmolytes. Taurine is considered as an osmolyte in brain cells. Accumulation of other osmolytes (sorbitol, myo-inositol and betaine), was shown in renal cells to result from an upregulation of the expression of the genes regulating osmolyte cell content. We have investigated the gene expression of the taurine transporter (TauT) and of the taurine biosynthetic enzymes, cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) by measuring their mRNA levels in brain of salt-loaded rats. mRNA levels of genes previously identified as osmosensitive, namely aldose reductase (AR), myo-inositol transporter (SMIT) and betaine transporter (BGT1) were also determined. In whole brain, TauT-, SMIT- and BGT1-mRNA levels were significantly increased following acute salt-loading but SMIT-mRNA levels only remained elevated following chronic salt-loading while CDO-, CSD- and AR-mRNA levels remained unchanged in both conditions. Following acute salt-loading, mRNA levels of TauT, CDO, CSD, SMIT, BGT1 and AR were increased in cerebral cortex while SMIT- and BGT1-mRNA levels only were increased in striatum and habenula.TauT, CDO and CSD genes may be upregulated in brain of salt-loaded rats but the upregulation of the TauT gene appears more widespread. TauT, CDO and CSD are thus putative osmosensitive genes. However the actual pattern (amplitude, time course and regional occurrence) of the upregulation of each of the putative (TauT, CDO and CSD) and established (AR, SMIT and BGT1) osmosensitive genes differs markedly. This indicates that there exist other factors in brain cells which can selectively prevent the upregulation of these genes by hyperosmolarity.

Acute Disease↗
PubMed1997

Substrate transport and cocaine binding of human dopamine transporter is reduced by substitution of carboxyl tail with that of bovine dopamine transporter.

A chimeric dopamine transporter (DAT) cDNA encoding mutant human DAT (hDAT) protein in which the intracellular carboxyl-terminal tail is replaced by that of the bovine dopamine transporter (bDAT) was constructed. The chimeric hDAT cDNA was expressed in COS-7 cells, and [3H]dopamine and [3H]MPP+ uptake and [3H]CFT binding capacities were assessed. Substrate transport and ligand binding of bDAT were reduced by 32-43% as a result of substitution of the carboxyl tail in hDAT, suggesting that the functional characteristics of bDAT arise from differences in the carboxyl tail between human and bovine DAT. Thus, it appears that the sequences encoded within the carboxyl terminal of DAT would be one of the important determinants for its functions.

1-Methyl-4-phenylpyridinium↗
PubMed1995

Ligand-directed immunoaffinity purification and properties of the one-carbon, reduced folate transporter. Interspecies immuno-cross-reactivity and expression of the native transporter in murine and human tumor cells and their transport-altered variants.

Almost complete purification (> 95%) of the 46-kDa murine, one-carbon, reduced folate transporter (RFT) at a recovery of 20% was obtained by ligand-directed immunoaffinity fractionation from transporter overproducing L1210/R83 cells. These cells were labeled with the N-hydroxysuccinimide ester of [3H]aminopterin (AMT), the isolated plasma membrane alkaline washed to remove nonintegral membrane proteins, detergent-solubilized, and RFT-separated on an anti-AMT antibody-protein G-Sepharose column followed by preparative SDS-polyacrylamide gel electrophoresis. Anti-RFT antibody, subsequently derived, differentially blotted (L1210/R83 >> L1210/0) a 46-kDa protein during SDS-polyacrylamide gel electrophoresis of plasma membrane from L1210/R83 and L1210 cells and in L1210/R83 cells after trichloroacetic acid precipitation. In contrast to that reported for human tumor cells, glycosidase treatment of RFT revealed no common N- or O-linked core oligosaccharides associated with this protein. The same 46-kDa protein at different relative levels was revealed in a Western blot of plasma membrane from other murine tumors. Blotting of plasma membrane from methotrexate resistant, transport defective L1210 cell variants exhibited wild-type levels of a less electrophoretically mobile RFT or greater levels of the same 46-kDa RFT which could not be affinity labeled with N-hydroxysuccinimide-[3H]AMT. The same antibody differentially blotted a 83-kDa plasma membrane protein from human HL-60 and CCRF-CEM cells with different levels of reduced folate transport and affinity labeling of RFT, verifying the conserved nature of this protein consistent with earlier functional studies.

Aminopterin↗
PubMed1998

Identity of the organic cation transporter OCT3 as the extraneuronal monoamine transporter (uptake2) and evidence for the expression of the transporter in the brain.

We investigated the transport of cationic neurotoxins and neurotransmitters by the potential-sensitive organic transporter OCT3 and its steroid sensitivity using heterologous expression systems and also analyzed the expression of OCT3 in the brain. When expressed in mammalian cells, OCT3 mediates the uptake of the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) and the neurotransmitter dopamine. Competition experiments show that several cationic neuroactive agents including amphetamines interact with OCT3. When expressed in Xenopus laevis oocytes, OCT3-mediated MPP+ uptake is associated with inward currents under voltage-clamp conditions. The MPP+-induced currents are saturable with respect to MPP+ concentration, and half-maximal saturation (K0.5) occurs at about 25 microM MPP+ with membrane potential clamped at -50 mV. The K0.5 for MPP+ is markedly influenced by membrane potential. OCT3 is inhibited by several steroids, and beta-estradiol is the most potent inhibitor (Ki approximately 1 microM). The pattern of steroid sensitivity of OCT3 is different from that of OCT1 and OCT2 but correlates significantly with that of the extraneuronal monoamine transporter (uptake2). The transport characteristics and steroid sensitivity provide strong evidence for the molecular identity of OCT3 as uptake2. OCT3 is expressed in the brain as evidenced from Northern blot analysis, reverse transcription-polymerase chain reaction, and in situ hybridization using OCT3-specific probes. The molecular identity of the transcript hybridizing to the probe has been established by sequencing the reverse transcription-polymerase chain reaction product and also by the isolation of the OCT3 cDNA from a brain cDNA library. Regional distribution studies with in situ hybridization show that OCT3 is expressed widely in different brain regions, especially in the hippocampus, cerebellum, and cerebral cortex. OCT3 is likely to play a significant role in the disposition of cationic neurotoxins and neurotransmitters in the brain.

1-Methyl-4-phenylpyridinium↗
PubMed1994

Vibrio cholerae iron transport systems: roles of heme and siderophore iron transport in virulence and identification of a gene associated with multiple iron transport systems.

Vibrio cholerae iron transport mutants were tested for their ability to cause disease in an infant mouse model. The mice were challenged with either the wild-type strain, a vibriobactin synthesis mutant, a heme utilization mutant, or double mutants containing both the vibriobactin synthesis defect and the heme utilization defect. When mice were challenged with 10(7) bacteria, the ability of the double mutant to survive in the intestines was greatly reduced and that of the heme utilization mutant was slightly reduced compared with that of the wild type or the vibriobactin synthesis mutant. When the inoculum size was reduced 10-fold, all of the iron transport mutants failed to colonize the intestines and failed to cause diarrhea in the mice, whereas the wild-type strain was not cleared and elicited a diarrheal response. These data indicate that disruption of either the heme utilization or the vibriobactin uptake system reduces the ability of V. cholerae to cause disease. One of the heme utilization mutants, DHH1, was found to be defective also in utilization of vibriobactin and ferrichrome, mimicking the Escherichia coli TonB- phenotype. This mutant was the least virulent of the iron transport mutants tested. Transformation of DHH1 with the recombinant plasmid pHUT4 restored the abilities to use hemin, vibriobactin, and ferrichrome as iron sources, suggesting that pHUT4 encodes a gene(s) involved globally in the iron transport systems. Hybridization of Vibrio DNA with the V. cholerae heme utilization genes demonstrated the presence of DNA homologous to the genes encoding the outer membrane protein HutA and the inner membrane protein HutB in all the V. cholerae strains tested. The probe containing hutA, but not that containing hutB, also hybridized to DNA from Vibrio parahaemolyticus.

Animals↗
PubMed1988

Further studies on a novel class of genetic variants of the L1210 cell with increased folate analogue transport inward. Transport properties of a new variant, evidence for increased levels of a specific transport protein, and its partial characterization following affinity labeling.

Studies are reported on the characterization of a new isolate within a novel class of variants of the L1210 cell exhibiting markedly increased transport inward of folate analogues. This variant (L1210/R83), which was selected in the presence of the antifolate metoprine, exhibited a 40-fold increase in [3H]aminopterin influx compared to parental cells and a modest (4-5-fold) increase in [3H]aminopterin efflux. The increase in influx was associated with a comparable increase in influx Vmax for the one-carbon, reduced folate transport system and the same increase in the amount of specific binding of [3H]aminopterin on the cell surface. Values for influx Km for [3H]aminopterin and specificity for various folate structures were unchanged. The alteration in influx Vmax and more rapid efflux accounted for the different level of intracellular exchangeable level of drug at steady state in this variant compared with parental L1210 cells. Otherwise, membrane potential was unchanged. The N-hydroxysuccinimide ester of [3H]aminopterin was used to covalently label the specific binding protein for folate compounds in the plasma membrane of variant and parental L1210 cells. Incorporation of label into this protein was stable under a variety of conditions and accounted for 97 and 52% of total cellular labeling, respectively, for membrane derived from R83 and parental L1210 cells at a reagent concentration of 20 nM. Specific affinity labeling on the surface of parental and variant cells was decreased in the presence of aminopterin, methotrexate, or 5-formyltetrahydrofolate, but not in the presence of folic acid. Also, [3H]aminopterin influx in these cells was inhibited by the N-hydroxysuccinimide ester of aminopterin or methotrexate, but not the N-hydroxysuccinimide ester of folic acid. These findings, in addition to the increased affinity labeling of this variant, which corresponds to the increase in influx of [3H] aminopterin also seen, appears to identify the affinity labeled protein as a component of the "classical" one-carbon, reduced folate transport system in these cells. The affinity labeled protein from each cell type was solubilized in sodium dodecyl sulfate or extracted in detergent in the presence of proteinase inhibitors and was found to elute from Sephacryl S-300 and migrate during sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a single peak of Mr = 45,000-48,000. Recovery of labeled binding protein in these fractions from R83 variant cells was approximately 40 times greater than that from parental cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Affinity Labels↗
PubMed2001

Mutagenesis and derivatization of human vesicle monoamine transporter 2 (VMAT2) cysteines identifies transporter domains involved in tetrabenazine binding and substrate transport.

The vesicle monoamine transporter (VMAT2) concentrates monoamine neurotransmitter into synaptic vesicles. Photoaffinity labeling, chimera analysis, and mutagenesis have identified functionally important amino acids and provided some information regarding structure and ligand binding sites. To extend these studies, we engineered functional human VMAT2 constructs with reduced numbers of cysteines. Subsets of cysteines were discovered, which restore function to an inactive cysteine-less human VMAT2. Replacement of three transmembrane (TM) cysteines together (net removal/replacement of three atoms) significantly enhanced monoamine transport. Cysteine modification studies involving single and combination cysteine mutants with methanethiosulfonate ethylamine revealed that [(3)H]dihydrotetrabenazine binding is > 90% inhibited by modification of two sets of cysteines. The primary target (responsible for approximately 80% of inhibition) is Cys(439) in TM 11. The secondary target (responsible for approximately 20% of inhibition) is one or more of the four non-TM cysteines. [(3)H]Dihydrotetrabenazine protects against modification of Cys(439) by a 10,000-fold molar excess of methanethiosulfonate ethylamine, demonstrating that Cys(439) is either at the tetrabenazine binding site, or conformationally linked to tetrabenazine binding. Supporting a direct effect, the position of tetrabenazine-protectable Cys 439 is consistent with previous mutagenesis, chimera, and photoaffinity labeling data, demonstrating involvement of TM 10-12 in a tetrabenazine binding domain.

Animals↗
PubMed1998

Conversion of the coprogen transport protein FhuE and the ferrioxamine B transport protein FoxA into ferrichrome transport proteins.

The FhuA protein of Escherichia coli K-12 transports ferrichrome and the structurally related antibiotic albomycin across the outer membrane and serves as a receptor for the phages T1, T5, and phi 80 and for colicin M. In this paper, we show that chimeric proteins consisting of the central part of FhuA and the N- and C-terminal parts of FhuE (coprogen receptor) or the N- and/or C-terminal parts of FoxA (ferrioxamine B receptor), function as ferrichrome transport proteins. Although the hybrid proteins contained the previously identified gating loop of FhuA, which is the principal binding site of the phages T5, T1, and phi 80, only the hybrid protein consisting of the N-terminal third of FoxA and the C-terminal two thirds of FhuA conferred weak phage sensitivity to cells. Apparently, the gating loop is essential, but not sufficient for wild-type levels of ferrichrome transport and for phage sensitivity. The properties of FhuA-FoxA hybrids suggest different regions of the two receptors for ferric siderophore uptake.

Bacterial Outer Membrane Proteins↗
PubMed1984

Identification of the nucleoside transporter in cultured mouse lymphoma cells. Photoaffinity labeling of plasma membrane-enriched fractions from nucleoside transport-competent (S49) and nucleoside transport-deficient (AE1) cells with [3H]nitrobenzylthioinosine.

Plasma membrane-enriched fractions from disrupted S49 lymphoma cells contained high affinity sites for [3H]nitrobenzylthioinosine, a potent and specific inhibitor of nucleoside transport. These sites were absent from similar preparations from AE1 cells, a nucleoside-transport deficient clone derived from the S49 cell line. Reversible binding of [3H]nitrobenzylthioinosine to the S49 membrane preparations was inhibited by adenosine, nitrobenzylthioguanosine, and dipyridamole. Exposure of S49 membrane preparations to UV light in the presence of [3H]nitrobenzylthioinosine resulted in the covalent radiolabeling of a membrane protein(s) which migrated on sodium dodecyl sulfate-polyacrylamide gels with an apparent Mr of 45,000 to 66,000. Labeling of this protein was abolished in the presence of nitrobenzylthioguanosine and markedly reduced in the presence of adenosine and dipyridamole. AE1 membrane proteins were not covalently labeled under these conditions.

Adenosine↗
PubMed2005

Changes in vesicular glutamate transporter 2, vesicular GABA transporter and vesicular acetylcholine transporter labeling of sacrocaudal motoneurons in the spastic rat.

Spasticity of the midline musculature can significantly hinder performing transfers and lead to development of pressure sores. Currently, significant gaps exist in our knowledge of the pathophysiology involved in spasticity development following SCI, especially regarding the axial musculature. The goals of this study were: (1) to determine the effects of S(2) transection on the number and distribution of glutamatergic, GABAergic and cholinergic inputs on more caudal motoneurons, (2) to correlate these changes with the development of spasticity within the tail musculature, which are the caudal counterparts to the axial musculature. Animals with S(2) spinal transection were tested behaviorally for the progression of spasticity within the tail musculature. At 1, 2, 4, or 12 weeks post-injury, the animals were sacrificed and temporal changes in glutamatergic, GABAergic, and cholinergic inputs to sacrocaudal motoneurons were assessed using antibodies for the specific vesicular transporter of each neurotransmitter and confocal microscopy. At 1 week post-injury, when the tail musculature demonstrated decreased responsiveness, an overall increase in the ratio of excitatory to inhibitory input to sacrocaudal motoneurons was observed. From 2 to 12 weeks post-injury, when the tail musculature demonstrated increased reflex behavior, an overall decrease in the ratio of excitatory to inhibitory inputs was observed. Additionally, from 2 to 12 weeks following spinal transection, a progressive loss of cholinergic labeling of sacrocaudal motoneurons was observed. The increase in the overall level of excitation with a concomitant loss of cholinergic influence following spinal transection could, in part, explain the development of spasticity within the tail musculature.

Animals↗
PubMed2002

Effects of streptozotocin-induced diabetes on markers of skeletal muscle metabolism and monocarboxylate transporter 1 to monocarboxylate transporter 4 transporters.

Diabetes is known to alter both oxidative and glycolytic pathways in a fiber type-dependent manner. In various skeletal muscles of normal rats, monocarboxylate transporter 1 (MCT1) has been found to be highly correlated to lactate uptake, as well as to oxidative capacity, whereas the distribution and characteristics of MCT4 make it a good candidate for the extrusion of lactic acid from glycolytic muscle cells. Since a previous study found decreased sarcolemmal lactate uptake in streptozotocin (STZ)-diabetic rats, we investigated the presence of MCT1 in relation to enzymatic markers of both oxidative and glycolytic pathways, as well as MCT4 content, in STZ-diabetic rats. Soleus (SOL), red tibialis anterior (RTA), extensor digitorus longus (EDL), heart, and preparations of purified sarcolemmal vesicles (SV) from control and STZ-diabetic rats were harvested for MCT1 and MCT4 content, citrate synthase activity (CS), and lactate dehydrogenase (LDH) isozymes. Basal blood lactate concentration was increased by 38% in the diabetic rats (close to 1.91 mmol/L). However, no change was found in either MCT1 or MCT4 content in these rats. The diabetic rats presented fiber type-specific decrease in CS activity. We noted a redistribution in LDH isozymes in diabetic muscles with a general increase in type H-LDH. Regression analyses indicated (1) a strong relationship between LDH-4 and LDH-5 and (2) MCT1 was still correlated with CS activity in diabetic muscles. These results suggest that diabetes-induced hyperlactatemia is not associated with changes in MCT1 or MCT4 expression, but with alterations of oxidative and glycolytic enzymes.

Animals↗
PubMed1995

The mechanism of nuclear transport of natural or artificial transport substrates in digitonin-permeabilized cells.

Characterization of nuclear protein transport in digitonin-permeabilized cells revealed that the number of the nuclear localization signal sequences (NLS) within the transport substrate basically influences the mechanism of the transport reaction. Phycoerythrine-NLS transport substrate carrying a maximum of 4-5 conjugated NLSs/subunit, or Bsp methyltransferase-NLS fusion protein were efficiently transported into the nuclei of digitonin-permeabilized cultured cells without any exogenously added cytosolic protein. All the characteristic properties of in vivo nuclear transport are faithfully reproduced with these transport substrates: (i) the transport requires a functional NLS in the transported protein, a transport-incompetent mutant NLS being ineffective; (ii) the transport is energy dependent; (iii) the wild type nuclear localization peptide efficiently competes for transport, while the transport-incompetent mutant peptide does not; and (iv) wheat germ agglutinin inhibits this transport reaction. Nuclear transport observed with these substrates was not due to any damage of the nuclear membrane or inefficient extraction of the cytosolic proteins during the permeabilization of the cells. The nuclear transport was proportional to the number of conjugated NLSs. Nuclear transport of phycoerythrine carrying 7-8 conjugated NLSs/subunit required the addition of exogenous cytosolic proteins. This transport also fulfilled all the characteristic properties of an authentic nuclear transport. Nuclear transport with different combinations of transport substrates further supported the assumption that distinct transport mechanisms operate for different substrates. From a mixture of PE-NLS7-8 and Bsp methyltransferase-NLS, the highly conjugated substrate was completely retained in the cytoplasm in the absence of exogenous cytosol, while Bsp methyltransferase-NLS was efficiently transported. Exogenous cytosol promoted the nuclear transport of the highly conjugated substrate.

Amino Acid Sequence↗
PubMedDate not supplied

Peritoneal fluid transport in CAPD patients with different transport rates of small solutes.

BACKGROUND: Continuous ambulatory peritoneal dialysis (CAPD) patients with high peritoneal solute transport rate often have inadequate peritoneal fluid transport. It is not known whether this inadequate fluid transport is due solely to a too rapid fall of osmotic pressure, or if the decreased effectiveness of fluid transport is also a contributing factor. OBJECTIVE: To analyze fluid transport parameters and the effectiveness of dialysis fluid osmotic pressure in the induction of fluid flow in CAPD patients with different small solute transport rates. PATIENTS: 44 CAPD patients were placed in low (n = 6), low-average (n = 13), high-average (n = 19), and high (n = 6) transport groups according to a modified peritoneal equilibration test (PET). METHODS: The study involved a 6-hour peritoneal dialysis dwell with 2 L 3.86% glucose dialysis fluid for each patient. Radioisotopically labeled serum albumin was added as a volume marker.The fluid transport parameters (osmotic conductance and fluid absorption rate) were estimated using three mathematical models of fluid transport: (1) Pyle model (model P), which describes ultrafiltration rate as an exponential function of time; (2) model OS, which is based on the linear relationship of ultrafiltration rate and overall osmolality gradient between dialysis fluid and blood; and (3) model G, which is based on the linear relationship between ultrafiltration rate and glucose concentration gradient between dialysis fluid and blood. Diffusive mass transport coefficients (K(BD)) for glucose, urea, creatinine, potassium, and sodium were estimated using the modified Babb-Randerson-Farrell model. RESULTS: The high transport group had significantly lower dialysate volume and glucose and osmolality gradients between dialysate and blood, but significantly higher K(BD) for small solutes compared with the other transport groups. Osmotic conductance, fluid absorption rate, and initial ultrafiltration rate did not differ among the transport groups for model OS and model P. Model G yielded unrealistic values of fluid transport parameters that differed from those estimated by models OS and P. The K(BD) values for small solutes were significantly different among the groups, and did not correlate with fluid transport parameters for model OS. CONCLUSION: The difference in fluid transport between the different transport groups was due only to the differences in the rate of disappearance of the overall osmotic pressure of the dialysate, which was a combined result of the transport rate of glucose and other small solutes. Although the glucose gradient is the major factor influencing ultrafiltration rate, other solutes, such as urea, are also of importance. The counteractive effect of plasma small solutes on transcapillary ultrafiltration was found to be especially notable in low transport patients. Thus, glucose gradient alone should not be considered the only force that shapes the ultrafiltration profile during peritoneal dialysis. We did not find any correlations between diffusive mass transport coefficients for small solutes and fluid transport parameters such as osmotic conductance or fluid and volume marker absorption. We may thus conclude that the pathway(s) for fluid transport appears to be partly independent from the pathway(s) for small solute transport, which supports the hypothesis of different pore types for fluid and solute transport.

Algorithms↗
PubMed1993

Endogenous amino acid transport systems and expression of mammalian amino acid transport proteins in Xenopus oocytes.

Oocyte amino acid transport has physiological significance to oocytes and practical importance to molecular biologists and transport physiologists. Expression of heterologous mRNA in Xenopus oocytes is currently being used to help clone cDNAs for amino acid transporters and their effectors. A major question to be resolved in many of these studies is whether the injected mRNA codes for a transporter or an activator of an endogenous system. Nevertheless, the cDNAs of several families of amino acid transporters or their activators appear already to have been cloned. One such transporter is the anion exchanger, band 3, which may also transport glycine and taurine under some important physiological conditions such as hypoosmotic stress. Site-directed mutagenesis of band 3 has already shown that an amino acid residue believed to be at or near the active site nevertheless does not appear to influence Cl- transport in Xenopus oocytes expressing the modified band 3 protein. Continuation of such studies along with examination of transport of all possible substrates of band 3 should yield insight into the relationship between the structure and function of this transporter. Each of three other families not only contains amino acid transporters, but also appears to contain members that serve as transporters of neurotransmitters or their metabolites. Because of the distinct structural differences in the preferred substrates of different transporters within some of these families, elucidation of the tertiary and possibly quaternary structural relationships among the members of such families may reveal transport mechanisms. In addition, the grouping of neurotransmitters or their metabolites according to the family to which their transport systems and transporters belong could yield insight into mechanisms of brain development, function and evolution. Another family of transporters for cationic amino acids also serves, at least in one case, as a viral receptor. Hence, these or other transporters also could conceivably function in eggs as receptors for sperm and, more broadly, in cell-cell interactions as well as in amino acid transport. Moreover, a family of apparent amino acid transport activators are homologous to a family of glycosidases, so these activators could also serve to recognize carbohydrate structures on other cells or the extracellular matrix. Some of these activators appear to increase more than one amino acid transport activity in Xenopus oocytes. In other studies, expression of heterologous mRNA in oocytes has led apparently to detection of inhibitors as well as activators of amino acid transport. Some amino acid transport systems also could conceivably contain nucleic acid as well as glycoprotein components.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Transport Systems↗
PubMed1981

Uridine and uracil transport in Escherichia coli and transport-deficient mutants.

Mutants of Escherichia coli K-12 which are defective in components of transport systems for uracil and uridine were isolated and utilized to characterize the transport mechanism of uracil and uridine. Mutant U-, isolated from a culture of the parent strain, is resistant to 5-fluorouracil and is deficient in the uracil transport system. Mutant UR-, isolated from a culture of the parent strain, is resistant to a low concentration of showdomycin and lacks the capacity to transport intact uridine. Mutant U-UR- isolated from a culture of mutant U-, is resistant to a low concentration of showdomycin and is defective in both uracil and intact uridine transport processes. Mutant UR-R- was isolated from a culture of mutant UR-, and is resistant to a high concentration of showdomycin. This mutant is defective for transport of intact uridine and addition lacks the transport system for the ribose moiety of uridine. Characteristics of uracil and uridine transport in parent and mutant cells demonstrate the existence of specific transport processes for uracil, intact uridine and the uracil and ribose moieties of uridine. Mutants U- and UR-, which are defective for uracil transport, lack uracil phosphoribosyltransferase activity and retain a small but significant capacity to transport uracil. The data support the conclusion that uracil is transported by two mechanisms, the major one of which requires uracil phosphoribosyltransferase activity, while the other process involves the transport of uracil as such. The characteristics of uridine transport in parent and mutant strains show that, in addition to transport as the intact nucleoside, uridine is rapidly cleaved to the uracil and ribose moieties. The latter is transported into the cell by a process which, in contrast to transport of intact uridine, does not require an energy source. The uracil moiety is released into the medium and is transported by the uracil transport system. Whole cells of the parent and mutant strains differ in their ability to cleave uridine even though cell-free extracts of all the strains have similar uridine phosphorylase activity. The data implicate a uridine cleavage enzyme in a group transport of the ribose moiety of uridine, a process which is nonfunctional in mutants which lack the capacity to transport the ribose moiety of uridine. A common transport component for this process and the transport of intact uridine is indicated by similarities in the inhibitory effects of heterologous nucleosides on these processes.

Adenine↗