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PubMed2005

OSTalpha-OSTbeta: a major basolateral bile acid and steroid transporter in human intestinal, renal, and biliary epithelia.

The cellular and subcellular localization and mechanism of transport of the heteromeric organic solute transporter (OST) OSTalpha-OSTbeta was examined in human and rodent epithelia. The two subunits of the transporter were expressed together in human small intestine, kidney, and liver, tissues that also express the apical sodium-dependent bile acid uptake transporter ASBT (SLC10A2). Indirect immunofluorescence microscopy localized OSTalpha and OSTbeta to the basolateral membrane of mouse, rat, and human ileal enterocytes, renal proximal tubular cells, and cholangiocytes. Transport in OSTalpha-OSTbeta-expressing Xenopus laevis oocytes was unaffected by depletion of intracellular adenosine triphosphate, or by changes in transmembrane Na(+), K(+), H(+), or Cl(-) concentration gradients. However, the oocytes demonstrated robust substrate efflux and trans-stimulation, indicating that transport occurs by facilitated diffusion. Madin Darby canine kidney cells coexpressing mouse Ostalpha and Ostbeta exhibited enhanced apical to basolateral transport of the major glycine and taurine conjugated bile acid species. In conclusion, the selective localization of OSTalpha and OSTbeta to the basolateral plasma membrane of epithelial cells responsible for bile acid and sterol reabsorption, the substrate selectivity of the transporter, and the facilitated diffusion transport mode collectively indicate that OSTalpha-OSTbeta is a key basolateral transporter for the reabsorption of these important steroid-derived molecules.

Amino Acid Sequence↗
PubMed2004

Differential regulation by glucose and fructose of a gene encoding a specific fructose/H+ symporter in Saccharomyces sensu stricto yeasts.

Saccharomyces cerevisiae transports fructose through a facilitated diffusion system common to other hexoses and mediated by the Hxt proteins. The related species S. pastorianus (carlsbergensis) and S. bayanus produce, in addition, a specific fructose/H(+) symporter. We have previously cloned a gene (FSY1) encoding the active fructose symporter from S. pastorianus PYCC 4457. Expression of Fsy1p in a S. cerevisiae mutant (hxt-null) devoid of the facilitated diffusion system allows growth on fructose but not on glucose. Here we present results concerning the regulation of Fsy1p expression, both in S. pastorianus and S. bayanus, where it occurs naturally, and in suitably engineered S. cerevisiae transformants. To that purpose, we made use of both Northern blot analysis and a Fsy1p-GFP fusion protein. The expression of Fsy1p is strongly regulated by both the carbon source and its concentration in the growth medium. In S. pastorianus, as well as in S. bayanus, very low concentrations of either fructose or glucose induced expression but higher sugar concentrations prevented transcription of the gene. Glucose was considerably more effective than fructose in repressing FSY1 expression. Proper regulation of the gene in S. cerevisiae seems to be exquisitely dependent on sugar transport. Analysis of Fsy1 expression in S. cerevisiae mutants shows that repression is mainly dependent on Mig1p, the final effector of the main glucose repression pathway. Interestingly, Mig1p also seems to mediate repression of FSY1 expression by high maltose concentrations.

Biological Transport↗
PubMed2001

Evidence that fungal MEP proteins mediate diffusion of the uncharged species NH(3) across the cytoplasmic membrane.

Methylammonium and ammonium (MEP) permeases of Saccharomyces cerevisiae belong to a ubiquitous family of cytoplasmic membrane proteins that transport only ammonium (NH(4)(+) + NH(3)). Transport and accumulation of the ammonium analog [(14)C]methylammonium, a weak base, led to the proposal that members of this family were capable of energy-dependent concentration of the ammonium ion, NH(4)(+). In bacteria, however, ATP-dependent conversion of methylammonium to gamma-N-methylglutamine by glutamine synthetase precludes its use in assessing concentrative transport across the cytoplasmic membrane. We have confirmed that methylammonium is not metabolized in the yeast S. cerevisiae and have shown that it is little metabolized in the filamentous fungus Neurospora crassa. However, its accumulation depends on the energy-dependent acidification of vacuoles. A Deltavph1 mutant of S. cerevisiae and a Deltavma1 mutant, which lack vacuolar H(+)-ATPase activity, had large (fivefold or greater) defects in the accumulation of methylammonium, with little accompanying defect in the initial rate of transport. A vma-1 mutant of N. crassa largely metabolized methylammonium to methylglutamine. Thus, in fungi as in bacteria, subsequent energy-dependent utilization of methylammonium precludes its use in assessing active transport across the cytoplasmic membrane. The requirement for a proton gradient to sequester the charged species CH(3)NH(3)(+) in acidic vacuoles provides evidence that the substrate for MEP proteins is the uncharged species CH(3)NH(2). By inference, their natural substrate is NH(3), a gas. We postulate that MEP proteins facilitate diffusion of NH(3) across the cytoplasmic membrane and speculate that human Rhesus proteins, which lie in the same domain family as MEP proteins, facilitate diffusion of CO(2).

Biological Transport↗
PubMed1996

Glucose transporters in the transepithelial transport of glucose.

Glucose transporters are integral membrane proteins that mediate the transport of glucose and structurally-related substances across the cellular membranes. Two families of glucose transporter have been identified: the facilitated-diffusion glucose transporter family (GLUT family), and the NA(+)-dependent glucose transporter one (SGLT family). These transporters play a pivotal role in the transfer of glucose across the epithelial cell layers that separate distinct compartments in the mammalian body. In the small intestine, a Na(+)-dependent glucose transporter, SGLT1, is localized at the apical plasma membrane of the absorptive epithelial cells, whereas a facilitated-diffusion glucose transporter, GLUT2, is at the basolateral membrane of the cells. Similar localization is seen in the kidney proximal tubules in the reabsorption of glucose. For the absorption of fructose in the small intestine, fructose transporter GLUT5 is localized at the apical membrane. The expressed GLUT5 in polarized cultured cells is targeted to the apical membrane, showing that the GLUT5 molecule itself has sufficient information to determine its cellular localization. In the blood-tissue barriers, such as the blood-brain barrier, blood-ocular barrier, and placental barrier, either endothelial or epithelial cell layers constitute the barrier. GLUT1 is abundant at the plasma membrane of these barrier cells, and plays a crucial role in the specific transfer of glucose across the barrier. When the barrier is composed of a two-cell layer, gap junctions connecting them could serve as intercellular channels for glucose transfer in addition to GLUT1. Proper localization of glucose transporters and gap junctions is a prerequisite for the successful transepithelial transport of sugars.

Animals↗
PubMed1995

Effect of inhibitors and substrates on methyl mercury uptake by rat erythrocytes.

Methyl mercury (MeHg) uptake by isolated erythrocytes from rats was studied at 20 degrees C. Inhibitors and substrates were used to test which transport system was involved in MeHg uptake. Ouabain and ATP were used to test the active transport system. Glycine was used to test system Gly. DL-Methionine was used to test system L. Cysteine was used to test the cysteine-facilitated transport system. The effects of Ca2+, Mg2+ and Na+ on MeHg uptake have been examined. MeHgCl and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) were used to test C1- ion transport system. D-Glucose and cytochalasin B were used to test the facilitated diffusive D-Glucose transport system. Colchicine and vinblastine were used to test the microtubule system. Probenecid was used to test the organic acid transport system. Valinomycin was used to test the effect of the membrane potential on MeHg uptake. The results showed that MeHg uptake at 20 degrees C might be involved in the following transport systems: 1) an active transport system; 2) a cysteine-facilitated transport system; 3) a C1- ion transport system; 4) a facilitated diffusive D-glucose transport system; 5) an organic acid transport system. The transport systems for MeHg uptake were sensitive to the membrane potential.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1989

Saturable pharmacokinetics in the renal excretion of drugs.

The renal excretion of drugs is the result of different mechanisms: glomerular filtration, passive back diffusion, tubular secretion and tubular reabsorption. Of these mechanisms the last 2 are saturable, as they involve carrier transport. This also implies that both tubular secretion and tubular reabsorption are susceptible to competition between similar substrates for a common carrier site. Furthermore, transport via these mechanisms is energy-dependent, so-called active transport, able to concentrate a drug. Tubular secretion takes place in the proximal tubule of the nephron. Many organic compounds are actively secreted, but there are separate carrier systems for anions and cations. Anions appear to be transported actively over the basolateral membrane and by a less efficient non-active carrier-mediated process (facilitated diffusion) over the brush border membrane. As a result of these mechanisms, anions tend to accumulate in proximal tubular cells. For cations, however, the active transport step operates over the brush border membrane, whereas the uptake of the cation in the cell occurs via facilitated diffusion over the basolateral membrane. Active reabsorption is most prominent for many nutrients and endogenous substrates (amino acids, glucose, vitamins), but various exogenous compounds also have a certain affinity for the reabsorptive carrier systems. Uricosuric drugs, for instance, interfere with carrier-mediated reabsorption of urate. The occurrence of saturable excretion routes causes dose-dependent, non-linear pharmacokinetics. In clinical pharmacokinetics, tubular secretion can adequately be described with the use of a Michaelis-Menten equation. This implies that a compound undergoing tubular secretion exhibits a concentration-dependent renal clearance. At low plasma concentrations the clearance will be maximal, and for several drugs may be as high as the effective renal plasma flow. Increasing concentrations cause decreasing renal clearance, until eventually the secretion mechanism becomes fully saturated. Then the excretion of the drug in urine will depend primarily on its net rate of filtration. It is important to realise that the non-linear kinetics will be evident from the plasma kinetics only when the saturable pathway contributes to at least some 20% of the total body clearance. Interactions with other substrates, however, are likely to occur even when only a very small amount of drug is transported by the carrier system. Non-linear kinetics inevitably lead to disproportionate accumulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Drug Interactions↗
PubMed2004

The amino acid transporter asc-1 is not involved in cystinuria.

BACKGROUND: The human amino acid transporter asc-1 (SLC7A10) exhibits substrate selectivity for small neutral amino acids, including cysteine, is expressed in kidney, is located close to the cystinuria B gene and presents sequence variants (e.g., E112D) in some cystinuria patients. We have cloned human asc-1, assessed its transport characteristics, localized its expression in kidney, searched for mutations in cystinuria patients, and tested the transport function of variant E112D. METHODS: We used an EST-based homology cloning strategy. Transport characteristics of asc-1 were assessed by coexpression with 4F2hc in Xenopus oocytes and HeLa cells. Localization of asc-1 mRNA in kidney was assessed by in situ hybridization. Exons and intron-exon boundaries were polymerase chain reaction (PCR)-amplified from blood cell DNA and mutational screening was performed by single-stranded conformational polymorphism (SSCP). RESULTS: Asc-1 reaches the plasma membrane in HeLa cells, unlike in oocytes, most probably by interaction with endogenous 4F2hc and presents similar transport characteristics to those in oocytes coexpressing asc-1/4F2hc. Asc-1 mediates a substantial efflux of alanine in a facilitated diffusion mode of transport. Expression of asc-1 mRNA localized to Henle's loop, distal tubules, and collecting ducts. Finally, SLC7A10 polymorphisms were identified in cystinuria probands and the SLC7A10 sequence variant E112D showed full transport activity. CONCLUSION: The lack of expression of asc-1 in the proximal tubule indicates that it plays no role in the bulk of renal reabsorption of amino acids. No mutations causing cystinuria have been found in SLC7A10. The facilitated diffusion mode of transport and the expression in distal nephron suggest a role for asc-1 in osmotic adaptation.

Amino Acid Transport System y+↗
PubMed2004

Caspofungin uptake is mediated by a high-affinity transporter in Candida albicans.

The uptake of the echinocandin drug caspofungin acetate in Candida albicans was evaluated at drug levels at or near the MIC for the organism. Maximal uptake was achieved in 10 min and was energy independent. A saturable transport system, consistent with a facilitated-diffusion carrier, was observed with the unlabeled drug competing with the labeled drug for uptake and efflux. More than 90% of the transported drug was observed in a single kinetic compartment that was available for efflux, indicating that the drug was free in the cytoplasm following uptake. Efflux was also energy independent but was sensitive to the presence of a fully loaded carrier on both faces of the bilayer. Overall, the data presented are consistent with the presence of a high-affinity facilitated-diffusion transporter that mediates caspofungin uptake and could be a potential source of transport-related reduced susceptibility.

Antifungal Agents↗
PubMed2006

Uptake and antiproliferative effect of molecular iodine in the MCF-7 breast cancer cell line.

This study analyzes the uptake and antiproliferative effect of two different chemical forms of iodine, iodide (I-) and molecular iodine (I2), in MCF-7 cells, which are inducible for the Na+/I- symporter (NIS) and positive for pendrin (PDS). The mouse fibroblast cell line NIH3T3 was used as control. Our results show that in MCF-7 cells, I- uptake is sustained and dependent on NIS, whereas I2 uptake is transient with a maximal peak at 10 min and a final retention of 10% of total uptake. In contrast, no I- was taken up by NIH3T3 cells, and although I2 was captured with the same time pattern as in MCF-7 cells, its uptake was significantly lower, and it was not retained within the cell. The uptake of I2 is independent of NIS, PDS, Na+, and energy, but it is saturable and dependent on protein synthesis, suggesting a facilitated diffusion system. Radioiodine was incorporated into protein and lipid fractions only with I2 treatment. The administration of non-radiolabeled I2 and 6-iodo-5-hydroxy-8,11,14-eicosatrienoic acid (6-iodolactone, an iodinated arachidonic acid), but not KI, significantly inhibited proliferation of MCF-7 cells. Proliferation of NIH3T3 cells was not inhibited by 20 microM I2. In conclusion, these results demonstrate that I2 uptake does not depend on NIS or PDS; they suggest that in mammary cancer cells, I2 is taken up by a facilitated diffusion system and then covalently bound to lipids or proteins that, in turn, inhibit proliferation.

Animals↗
PubMed1983

Monosaccharide transport and hexokinase activity in leg muscles from cardiomyopathic hamsters.

The facilitated diffusion system for monosaccharides was studied with nonmetabolized 3-O-methyl-D-glucose (3-O-MG) and hexokinase activity was determined with 2-deoxyglucose (2-DG) in extensor digitorum longus (EDL) and soleus muscles from BIO 14.6 (dystrophic) hamsters. Motor activity was recorded at 2 months of age. F1B (normal) hamsters were controls. In EDL (fast-twitch) muscle, sugar transport was unaltered at 6 weeks; at 2 to 5 months the normal decrease in sugar transport with time was reduced, decreasing stimulation of the facilitated diffusion system by anoxia. Phosphorylation in EDL muscles, measured at 2 months, probably was decreased. In soleus (slow-twitch) muscle, 3-O-MG transport was unaltered with age but hexokinase activity, determined at 2 months, was greatly increased in the quiescent state and during stimulation at 2 Hz. Motor activity was less, but not significantly, in the dystrophic hamsters. The results indicate that the alterations in glucose metabolism depend upon both muscle type and age. We interpret the changes in dystrophic fast- and slow-twitch muscles as reflecting compensatory mechanisms to increase the generation of energy.

3-O-Methylglucose↗
PubMed1989

Regulation of glucose transport in Candida utilis.

The transport systems for glucose present in Candida utilis cells, growing in batch and continuous cultures on several carbon sources, have been studied. Two different systems were found: a proton symport and a facilitated diffusion system. The high-affinity symport (Km for glucose about 15 microM) transported one proton per mole of glucose and was partially constitutive, appearing in cells grown on gluconeogenic substrates such as lactate, ethanol and glycerol. It was also induced by glucose concentrations up to 0.7 mM and repressed by higher ones. The level of repression depended on the external glucose concentration at which cells had grown in a way similar to that shown by the maltose-uptake system, so both systems seem to be under a common glucose control. Initial uptake by facilitated diffusion, the only transport system present in cells growing at glucose concentrations higher than 10 mM, showed a complex kinetic dependence on the extracellular glucose concentration. This could be explained either by the presence of at least two different systems simultaneously active, one with a Km around 2 mM and the other with a Km of about 1 M, or by the allosteric or hysteretic behaviour of a single carrier whose apparent Km would oscillate between 2 and 70 mM.

Biological Transport↗
PubMed1991

Induction of the differentiation of HL-60 cells by phorbol 12-myristate 13-acetate activates a Na(+)-dependent uridine-transport system. Involvement of protein kinase C.

The Na(+)-dependent transport and facilitated diffusion of uridine were measured after differentiation of HL-60 leukaemia cells along the monocytic pathway by phorbol 12-myristate 13-acetate (PMA). PMA (200 ng/ml) caused a marked increase in Na(+)-dependent uridine transport within 48 h of exposure that was attributable to an increase in transport affinity (apparent Km values of 1.15 +/- 0.22 and 44 +/- 4.4 microM for PMA-induced and uninduced cells respectively), with no change in Vmax. (0.15 +/- 0.02 and 0.13 +/- 0.01 pmol/s per microliter of cell water for PMA-induced and uninduced cells respectively). A corresponding rapid decrease in both the rate of facilitated diffusion and the formation of uracil nucleotides occurred in PMA-induced cells. As a consequence of these changes, intracellular pools of uridine 3-4-fold greater than those in the medium were generated. A similar increase in Na(+)-dependent transport of adenosine, inosine, guanosine, thymidine and cytidine (Km values of 1-4 microM) was observed. The effects of PMA on the activation of the Na(+)-dependent uridine transporter were inhibited by staurosporine, suggesting the involvement of protein kinase C. The findings indicate that a change in the balance of the cellular mechanisms employed for nucleoside transport occurs during the monocytic differentiation of HL-60 leukaemia cells.

Alkaloids↗
PubMed1984

Estimates of Michaelis-Menten constants for the two membranes of the brain endothelium.

Tracer studies on facilitated diffusion across the blood-brain barrier lead to the calculation of Michaelis-Menten constants that describe the rate of transport. However, the barrier consists of two endothelial cell membranes, and the relevance of single Michaelis-Menten constants in relation to the two cell membranes is unknown. We have formulated a model of two endothelial cell membranes and show that the measured Michaelis-Menten constants are simple functions of the properties of the individual membranes when transport across the endothelium is rapid (P1 greater than 10(-6) cm s-1). We also show that the Michaelis-Menten constants determined in tracer experiments describe facilitated diffusion in the steady state only if the two membranes have similar transport properties. As an application of this observation, we have examined three experimental studies that measure glucose transport in the steady state and show that the Michaelis-Menten constants for glucose transport calculated from the tracer experiments are equal to the constants calculated from the steady-state experiments. We conclude that the luminal and abluminal membranes of brain capillary endothelial cells have equal glucose transport properties.

Biological Transport, Active↗
PubMed2000

Separate sites and mechanisms for placental transport of calcium, iron and glucose in the equine placenta.

The placenta is the only channel for transport of nutrients to the conceptus and the fetal nutrient demands increase exponentially to term. The 9 kDa calcium binding protein (calbindin, 9CBP) and the iron binding protein uteroferrin (UF) are proving to be reliable markers for epithelia that mediate active transcellular calcium and iron transport and the glucose transporter proteins (GT1 and GT3) for glucose transport by facilitated diffusion. Light and electron microscope immunocytochemistry have been used on perfusion fixed resin embedded material to establish the distribution of 9CBP, UF, GT1 and GT3 in the equine placenta from 100 days of pregnancy to term (336 days). The equine placenta has two main structural components, flat areolae and microcotyledons. From 100 days of pregnancy to term immunoreactive 9CBP is found only in the cytoplasm of the maternal glands and the areolar trophoblast cells with none in the microcotyledons; whereas GT1 is present exclusively in the microcotyledons on the basolateral plasmalemma of both trophoblast and uterine epithelia with GT3 on the apical microvilli. The glands show neither GT1 nor GT3 expression. The areas of both areolae and microcotyledons increase enormously during gestation but there is no indication of increasing amounts of 9CBP, GT1 or GT3 protein per cell. Glucose transport through the placental cell cytoplasm is by diffusion of the free molecule, but calcium ions in transit must be sequestered in some way since the high calcium fluxes needed to support fetal bone growth in later pregnancy would be deleterious to calcium based homeostasis and cellular control systems. Electron microscope immunocytochemistry shows that 9CBP is uniformly distributed in the cytoplasm and nucleoplasm of the areolar trophoblast cells but excluded from all membrane bounded compartments such as mitochondria, Golgi saccules and pinocytotic transport vesicles. Such apical transport vesicles can be identified immunocytochemically by their content of uteroferrin, a component of the secretion from the uterine glands. It is suggested that transcellular calcium transport is therefore based on facilitated diffusion, not the vesicular method followed by the iron in the UF molecules, with 9CBP providing both transfer and sequestration functions for the transient calcium ions. These results show that the equine placenta has transport systems with restricted regional distribution similar to those recently shown for the ruminant placenta.

Acid Phosphatase↗
PubMedDate not supplied

Fingernail fluoride: a method for monitoring fluoride exposure.

This work was based on the hypothesis that fingernail clippings can be used as a biomarker for the subchronic exposure to fluoride. The results provide data on factors that may affect the concentration of fluoride in fingernail clippings as determined with the electrode following HMDS-facilitated diffusion. The following variables had only minor or no effects on the concentrations: (1) the surface area of the clippings (intact, minced or filed into powder) that were placed into the diffusion dishes; (2) soaking in deionized water for up to 6 h; (3) soaking in fluoridated water (1.0 ppm) for 2 h, and (4) removal of the organic material of nails by dry ashing. Fingernail fluoride concentrations were approximately 50% higher than those in toenails. A 1-month period of increased fluoride intake by one of the authors resulted in significant increases in fingernail fluoride concentrations after a lag time of approximately 3.5 months. The fluoride concentrations in fingernail clippings obtained from three groups of Brazilian children were directly related to the concentrations in the drinking water (0.1, 1.6 or 2.3 ppm). The results indicate that: (1) HMDS-facilitated diffusion completely separates fluoride from intact nail clippings, so the need for ashing or other preparative methods is obviated; (2) fingernail fluoride is derived mainly from the systemic circulation, and (3) fluoride intake is reflected by the concentrations in fingernails.

Analysis of Variance↗
PubMed1994

Alternative strategies of 2-deoxyglucose resistance and low affinity glucose transport in the ruminal bacteria, Streptococcus bovis and Selenomonas ruminantium.

Streptococcus bovis and Selenomonas ruminantium grew in the presence of the glucose analog, 2-deoxyglucose (2-DG), but the cells no longer had high affinity glucose transport. In S. bovis, 2-DG resistance was correlated with a decrease in phosphoenolpyruvate (PEP)-dependent glucose phosphotransferase (PTS) activity. The 2-DG-selected S. bovis cells relied solely upon a low affinity, facilitated diffusion mechanism of glucose transport and a 2-DG-resistant glucokinase (ATP-dependent). The glucokinase activity of S. ruminantium was competitively inhibited by 2-DG, and the 2-DG selected cells continued to use PEP-dependent PTS as a mechanism of glucose transport. In this latter case, the 2-DG selected cells switched from a mannosephosphotransferase (enzyme II) that phosphorylated glucose, mannose, and 2-DG, but not alpha-methylglucose to a glucosephosphotransferase (enzyme II) that phosphorylated glucose and alpha-methylglucoside but not 2-DG or mannose. The glucosephosphotransferase (enzyme II) had a very low affinity for glucose and the transport kinetics were similar to the facilitated diffusion system of S. bovis.

Animals↗
PubMed1991

Fine structure and sugar transport functions of the tegument in Clinostomum marginatum (Digenea: Clinostomatidae): environmental effects on the adult phenotype.

Digenean flukes can be classified into 3 groups according to their location in the host: the lumen of the alimentary canal or associated organ, body cavity or tissue, and external surfaces. We obtained adults of Clinostomum marginatum that had matured in these 3 habitats and compared the fine structure and glucose transporting capacity of their teguments. Adults from the esophagus of herons, Ardea herodias, had thick, smooth teguments and took up glucose by facilitated diffusion, the type of transport that is Na(+)-independent and insensitive to phlorizin. By contrast, the surfaces of adults cultured from metacercariae in body cavities of laboratory mice were amplified 3-5-fold due to numerous irregular projections of the tegument. Glucose transport by these worms was largely Na(+)-dependent and inhibited by phlorizin, indicating active transport. Ectoparasites from herons' mouths had relatively thick, smooth teguments, but these worms always were encrusted with bacteria and yeast that are known to absorb and metabolize glucose. Most of the attached bacteria, and the apparent glucose uptake associated with their presence, were removed by treating the worms with antibiotics prior to transport assays. As facilitated diffusion and active transport are operational simultaneously in metacercariae, the type of transport function, if any, expressed in the adult is determined by environmental conditions associated with the worm's habitat.

Animals↗
PubMed1993

A comparative study on the transport of L(-)malic acid and other short-chain carboxylic acids in the yeast Candida utilis: evidence for a general organic acid permease.

Cells of the yeast Candida utilis grown in medium with short-chain mono-, di- or tricarboxylic acids transported L(-)malic acid by two transport systems at pH 3.0. Results indicate that probably a proton symport for the ionized form of the acid and a facilitated diffusion for the undissociated form were present. Dicarboxylic acids such as succinic, fumaric, oxaloacetic and alpha-ketoglutaric acids were competitive inhibitors of the malic acid for the high-affinity system, suggesting that these acids used the same transport system. In turn, competitive inhibition uptake studies of labelled carboxylic acid in the low-affinity range indicated that this system was non-specific and able to accept not only carboxylic (mono-, di- or tri-) acids but also some amino acids. Additionally, under the same growth conditions, C. utilis produced two mediated transport systems for lactic acid: a proton symport for the anionic form which appeared to be a common monocarboxylate carrier and a facilitated diffusion system for the undissociated acid displaying a substrate specificity similar to that observed for the low-affinity dicarboxylic acid transport. The mediated carboxylic acid transport systems were inducible and subjected to repression by glucose. In glucose-grown cells the undissociated dicarboxylic acids entered the cells slowly by simple diffusion. Repressed glucose-grown cells were only able to produce both transport systems if an inducer, at low concentration (0.5%, w/v), was present during starvation in buffer. This process was inhibited by the presence of cycloheximide indicating that induction requires de novo protein synthesis. If a higher acid concentration was used, only the low-affinity transport system was detectable, showing that the high-affinity system was also repressed by high concentrations of the inducer.

Biological Transport↗