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At least 199 records · Page 11Linked to original sources
PubMed1996

Rapid appearance and asymmetric distribution of glucose transporter SGTP4 at the apical surface of intramammalian-stage Schistosoma mansoni.

Adult Schistosoma mansoni blood flukes reside in the mesenteric veins of their vertebrate hosts, where they absorb immense quantities of glucose through their tegument by facilitated diffusion. Previously, we obtained S. mansoni cDNAs encoding facilitated-diffusion schistosome glucose transporter proteins 1 and 4 (SGTP1 and SGTP4) and localized SGTP1 to the basal membranes of the tegument and the underlying muscle. In this study, we characterize the expression and localization of SGTP4 during the schistosome life cycle. Antibodies specific to SGTP4 appear to stain only the double-bilayer, apical membranes of the adult parasite tegument, revealing an asymmetric distribution relative to the basal transporter SGTP1. On living worms, SGTP4 is available to surface biotinylation, suggesting that it is exposed at the hose-parasite interface. SGTP4 is detected shortly after the transformation of free-living, infectious cercariae into schistosomula and coincides with the appearance of the double membrane. Within 15 min after transformation, anti-SGTP4 staining produces a bright, patchy distribution at the surface of schistosomula, which becomes contiguous over the entire surface of the schistosomula by 24 hr after transformation. SGTP4 is not detected in earlier developmental stages (eggs, sporocysts, and cercariae) that do not possess the specialized double membrane. Thus, SGTP4 appears to be expressed only in the mammalian stages of the parasite's life cycle and specifically localized within the host-interactive, apical membranes of the tegument.

Amino Acid Sequence↗
PubMed1988

Inorganic carbon transport in biological systems.

1. The flux of inorganic carbon (Ci) is an important biological process. 2. CO2 crosses membranes through passive diffusion and, perhaps active transport while HCO3- crosses membranes via facilitated diffusion and active transport mechanisms. 3. Carbonic anhydrase is ubiquitous and enhances the flux of Ci. 4. Generally, Ci crosses membranes through passive and facilitated diffusion when the flux of Ci, per se, is important and crosses membranes via active transport when cells are regulating their intracellular pH and/or ion levels.

Animals↗
PubMed1999

Extending the limits to enzymatic catalysis: diffusion of ribonuclease A in one dimension.

Bovine pancreatic ribonuclease A (RNase A) is a distributive endoribonuclease that catalyzes the cleavage of the P-O5' bond of RNA on the 3' side of pyrimidine residues. Here, RNase A is shown to cleave the P-O5' bond of a pyrimidine ribonucleotide faster when the substrate is embedded within a longer tract of poly(adenylic acid) [poly(A)] or poly(deoxyadenylic acid) [poly(dA)]. These data indicate that a ribonuclease can diffuse in one dimension along a single-stranded nucleic acid. This facilitated diffusion is mediated by Coulombic interactions, as the extent is diminished by the addition of NaCl. RNase A is more effective at cleaving a pyrimidine ribonucleotide embedded within a poly(dA) tract than within a poly(deoxycytidylic acid) [poly(dC)] tract. T45G RNase A, which catalyzes the processive cleavage of poly(A) but the distributive cleavage of poly(cytidylic acid) [poly(C)], has the same preference. Apparently, processive catalysis by the T45G enzyme arises from the expanded substrate specificity of the variant superimposed upon an intrinsic ability to diffuse along poly(A). Homologous ribonucleases with cytotoxic activity may rely on facilitated diffusion along poly(A) tails for efficient degradation of the essential information encoded by cellular mRNA.

Animals↗
PubMedDate not supplied

Modeling of palmitate transport in the heart.

Transport of palmitate from the albumin-palmitate complex in the plasma to inside mitochondria where it undergoes beta-oxidation is a multistep process. Albumin's large size prevents permeation via interendothelial clefts. Palmitate dissociation from albumin in solution is too slow to provide an adequate supply of the unbound palmitate. The discovery that the dissociation occurs upon albumin binding to an endothelial surface receptor resolves the conundrum. Palmitate transport across the luminal surface membrane may be either carrier-mediated or passive. Fatty-acid binding protein inside endothelial and cardiac muscle cells facilitates diffusion through cytosol while maintaining the unbound palmitate concentration at a very low level. Within the interstitium, albumin is again the palmitate carrier. Still controversial is whether or not there is a saturable sarcolemmal transporter or simply passive exchange. Inside the myocyte palmitate is again bound to the fatty acid binding protein which buffers the free palmitate concentration, facilitates diffusion, and may facilitate further intracellular reactions.

Animals↗
PubMed1980

A theoretical model for studying the rate of oxygenation of blood in pulmonary capillaries.

A mathematical analysis of the process of gas exchange in the lung is presented taking into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion of the species due to haemoglobin. Since the rate at which blood gets oxygenated in the pulmonary capillaries is very fast, it is difficult to set up an experimental study to determine the effects of various parameters on equilibration rate. The proposed study is aimed at determining the effects of various physiological parameters on equilibration rate in pathological conditions. Among the significant results are that 1. dissolved oxygen takes longer to achieve equilibration across the pulmonary membrane and carbon dioxide attains equilibration faster, 2. the equilibration length increases with increase in blood velocity, haemoglobin concentration, calibre of pulmonary capillaries and fall in alveolar PO2, 3. the alveolar PCO2 and forward and backward reaction rates of haemoglobin with CO2 do not materially affect the equilibration rate or length. 4. At complete equilibration, by the end of the pulmonary capillary 92% of the total haemoglobin has combined with oxygen and 8% free pigment is left which is present as carbamino haemoglobin, met haemoglobin, carboxy haemoglobin etc. These results are of some importance for anaemic conditions, muscular exercise, meditation, altitude physiology, hypo-ventilation, hyperventilation, etc.

Capillaries↗
PubMed1987

A numerical study of the nonsteady transport of gases in the pulmonary capillaries.

A mathematical model is formulated for simulating the unsteady transport of gases in the blood flowing through the pulmonary capillaries. The formulation takes into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion of the species due to haemoglobin. A time dependent situation is created by allowing to vary suddenly the partial pressures of the gases either in the venous blood or in the alveolar air. A numerical technique is described to solve the resulting time-dependent system of nonlinear coupled partial differential equations with the physiologically relevant boundary, entrance and initial conditions. The time required by the gases to achieve equilibrium is computed. It is shown that the dissolved oxygen takes longest in reaching equilibration whereas the carbon dioxide is the fastest. The various physiologically relevant unsteady situations have been examined.

Capillaries↗
PubMed1987

The process of gas exchange in the pulmonary circulation incorporating the contribution of axial diffusion.

A mathematical model is made to describe the process of gas exchange in the pulmonary circulation incorporating the contribution of axial diffusion. The model takes into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion due to the presence of haemoglobin as a carrier of the gases. The mathematical formulation leads to a coupled system of non-linear elliptic partial differential equations. A numerical scheme is described to solve such a system. It is found that the axial diffusion does not have an appreciable effect on the transport of the species in the blood.

Diffusion↗
PubMed1990

A numerical model for studying the effect of plasma layer on the process of blood oxygenation in the pulmonary capillaries.

A two layer model for the blood oxygenation in pulmonary capillaries is proposed. The model consists of a core of erythrocytes surrounded by a symmetrically placed plasma layer. The governing equations in the core describe the free molecular diffusion, convection, and facilitated diffusion due to the presence of haemoglobin. The corresponding equations in the plasma layer are based on the free molecular diffusion and the convective effect of the blood. According to the axial train model for the blood flow proposed by Whitmore (1967), the core will move with a uniform velocity whereas flow in the plasma layer will be fully developed. The resulting system of nonlinear partial differential equations is solved numerically. A fixed point iterative technique is used to deal with the nonlinearities. The distance traversed by the blood before getting fully oxygenated is computed. It is shown that the concentration of O2 increases continuously along the length of the capillary for a given ratio of core radius to capillary radius. It is found that the rate of oxygenation increases as the core to capillary ratio decreases. The equilibration length increases with a heterogeneous model in comparison to that in a homogeneous model. The effect of capillary diameters and core radii on the rate of oxygenation has also been examined.

Capillaries↗
PubMed1971

Promoter-like mutant with increased expression of the glycerol kinase operon of Escherichia coli.

A glycerol-specific phenotypic revertant isolated from a mutant of Escherichia coli missing enzyme I of the phosphoenolpyruvate phosphotransferase system was studied. This revertant is capable of producing higher levels of glycerol kinase and the protein mediating the facilitated diffusion of glycerol (facilitator) than wild-type cells. The kinase of the revertant is indistinguishable from the wild-type enzyme with respect to its sensitivity to feedback inhibition by fructose-1,6-diphosphate, its pH optimum, and its turnover number. The synthesis of glycerol kinase in strains bearing the suppressor locus is resistant to catabolite repression. The suppressor mutation mapped at the known glpK locus. Thus, it is suggested that the mutation occurred in the promoter of the operon specifying the kinase and the facilitator.

Arabinose↗
PubMed1990

Characterization of transmembrane movement of glucose and glucose analogs in Streptococcus mutants Ingbritt.

The transmembrane movement of radiolabeled, nonmetabolizable glucose analogs in Streptococcus mutants Ingbritt was studied under conditions of differing transmembrane electrochemical potentials (delta psi) and pH gradients (delta pH). The delta pH and delta psi were determined from the transmembrane equilibration of radiolabeled benzoate and tetraphenylphosphonium ions, respectively. Growth conditions of S. mutants Ingbritt were chosen so that the cells had a low apparent phosphoenolpyruvate (PEP)-dependent glucose:phosphotransferase activity. Cells energized under different conditions produced transmembrane proton potentials ranging from -49 to -103 mV but did not accumulate 6-deoxyglucose intracellularly. An artificial transmembrane proton potential was generated in deenergized cells by creating a delta psi with a valinomycin-induced K+ diffusion potential and a delta pH by rapid acidification of the medium. Artificial transmembrane proton potentials up to -83 mV, although producing proton influx, could not accumulate 6-deoxyglucose in deenergized cells or 2-deoxyglucose or thiomethylgalactoside in deenergized, PEP-depleted cells. The transmembrane diffusion of glucose in PEP-depleted, KF-treated cells did not exhibit saturation kinetics or competitive inhibition by 6-deoxyglucose or 2-deoxyglucose, indicating that diffusion was not facilitated by a membrane carrier. As proton-linked membrane carriers have been shown to facilitate diffusion in the absence of a transmembrane proton potential, the results therefore are not consistent with a proton-linked glucose carrier in S. mutans Ingbritt. This together with the lack of proton-linked transport of the glucose analogs suggests that glucose transmembrane movement in S. mutans Ingbritt is not linked to the transmembrane proton potential.

Acetone↗
PubMed1999

Glucose transporter gene expression in bovine mammary gland.

Transport of glucose across the plasma membrane of mammary epithelial cells is believed to be a passive process of facilitated diffusion mediated by facilitative glucose transporter(s). This article presents three lines of evidence that indicate the expression of sodium/glucose cotransporter (SGLT1) in the mammary gland of lactating and nonlactating cows. First, transcripts of SGLT1 mRNA ranging in size from 1.5 to 5.2 kb were detected in polyadenylated RNA preparations of mammary glands of lactating and nonlactating cows. Second, SGLT1 cotransporter protein was also detected in plasma membrane preparations of mammary glands of lactating cows. Third, partial amino acid sequence deduced from the reverse transcriptase-PCR fragment of SGLT1 from bovine mammary glands was similar to the sequence reported for ovine SGLT1. We conclude that mammary gland expression of SGLT1 mRNA and protein suggests that an active glucose transport system may be involved in glucose transport and metabolism in the mammary gland of dairy cows. However, the physiological significance of the expression of SGLT1 in mammary gland remains unknown.

Amino Acid Sequence↗
PubMed1996

Adenosine transporters.

1. In mammals, nucleoside transport is an important determinant of the pharmacokinetics, plasma and tissue concentration, disposition and in vivo biological activity of adenosine as well as nucleoside analogues used in antiviral and anticancer therapies. 2. Two broad types of adenosine transporter exist, facilitated-diffusion carriers and active processes driven by the transmembrane sodium gradient. 3. Facilitated-diffusion adenosine carriers may be sensitive (es) or insensitive (ei) to nanomolar concentrations of the transport inhibitor nitrobenzylthioinosine (NBMPR). Dipyridamole, dilazep and lidoflazine analogues are also more potent inhibitors of the es carrier than the ei transporter in cells other than those derived from rat tissues. 4. The es transporter has a broad substrate specificity (apparent Km for adenosine approximately 25 microM in many cells at 25 degrees C), is a glycoprotein with an average apparent Mr of 57,000 in human erythrocytes that has been purified to near homogeneity and may exist in situ as a dimer. However, there is increasing evidence to suggest the presence of isoforms of the es transporter in different cells and species, based on kinetic and molecular properties. 5. The ei transporter also has a broad substrate specificity with a lower affinity for some nucleoside permeants than the es carrier, is genetically distinct from es but little information exists as to the molecular properties of the protein. 6. Sodium-dependent adenosine transport is present in many cell types and catalysed by four distinct systems, N1-N4, distinguished by substrate specificity, sodium coupling and tissue distribution. 7. Two genes have been identified which encode sodium-dependent adenosine transport proteins, SNST1 from the sodium/glucose cotransporter (SGLT1) gene family and the rat intestinal N2 transporter (cNT1) from a novel gene family including a bacterial nucleoside carrier (NupC). Transcripts of cNT1, which encodes a 648-residue protein, are found in intestine and kidney only. 8. Success in cloning the remaining adenosine transporter genes will improve our understanding of the diversity of nucleoside transport processes, with a view to better targeting of therapeutic nucleoside analogues and protective use of transport inhibitors.

Adenosine↗
PubMed1978

Accumulation of amines by rabbit erythrocytes in vitro.

1. Accumulation of noradrenaline (NA), 5-hydroxytryptamine (5HT) and tyramine by rabbit erythrocytes was measured at 37 degrees C in vitro. 2. Of the amines used only NA was broken down during incubation. This was a result of intracellular catechol-O-methyl transferase activity. 3. NA and 5HT entered the red cells by similar processes which were temperature-sensitive (cooling to 0 degrees C inhibited accumulation) and had saturation kinetics. The entry of NA was partially stereospecific; the (-)-isomer accumulated twice as fast as did (+)-NA. 5HT and NA competed for entry. Tyramine entry was unaffected by cooling, was not saturable and did not affect the entry of either NA or 5HT. NA and 5HT entered the erythrocytes at rates which were proportional to their lipid solubilities. 4. Metabolic inhibitors had no effect on amine transport. Inhibitors of amine transport in other tissues produced only small non-specific reductions of NA accumulation in the red cells. 5. Amine accumulation was a symmetrical process (no amine was retained by the red cells if the concentration gradient was reversed). It is concluded that NA and 5HT enter the cells by facilitated diffusion. The entry of NA and 5HT displayed countertransport, an additional feature of facilitated diffusion. 6. The relationship between the physical properties of the amines and the routes by which they entered the erythrocytes is discussed.

Animals↗
PubMed1977

Effect of methylprednisolone on the nucleoside metabolism of a human lymphoblastoid cell line.

Concentrations of methylprednisolone which have lethal effects on human lymphoblastoid cell lines in vitro cause a reduction both in the uptake of uridine and thymidine into acid-soluble material and their incorporation into acid-insoluble material. These effects are virtually instantaneous, which indicates that they do not depend on alterations in gene activity. Normal uptake of nucleosides into cells is by both simple and facilitated diffusion, and methylprednisolone appears to act directly on the cell surface to inhibit only facilitated diffusion uptake.

Cell Line↗
PubMed1998

Thiamine transport in human placental brush border membrane vesicles.

Pathways for transport of thiamine by the human placental epithelium were investigated using brush border membrane vesicles isolated by divalent cation precipitation. The presence of thiamine transport mechanisms mediating Na+-thiamine cotransport, proton/thiamine exchange and facilitated diffusion was assessed from [3H]-thiamine tracer flux measurements. The magnitude of intravesicular thiamine accumulation was unaffected by the imposition of an inwardly directed sodium gradient suggesting an absence of a mechanism mediating brush border membrane Na+-thiamine cotransport. Intravesicular thiamine accumulation was indistinguishable when measured in the presence and absence of conditions favoring the development of an inside-negative, potassium diffusion potential. The observed absence of conductive thiamine uptake suggests the absence of a mechanism mediating facilitated diffusion of thiamine in placental brush border membrane. The imposition of an inside-acid pH gradient was observed to induce concentrative accumulation of thiamine to levels exceeding equilibrium, suggesting the presence of a placental brush border membrane proton/thiamine exchange mechanism. Protonophore- induced dissipation of an imposed inside-acid pH gradient in the absence of membrane potential was observed to abolish concentrative accumulation of thiamine, suggesting a direct chemical coupling of protons and thiamine via a mediated exchange mechanism. Consistent with the functional properties expected for a mechanism mediating thiamine transport by organic cation exchange, the rate and magnitude of intravesicular [3H]-thiamine accumulation was increased when measured in the presence compared to the absence of an outwardly directed thiamine concentration gradient. Substrate specificity studies of the proton/thiamine exchange mechanism suggest that the amine at position four of the pyrimidine ring, but not the hydroxyethyl side chain or an unmodified thiazolium ring, is an important chemical determinant for interaction with the transporter substrate binding site(s). Substrate specificity studies further suggest the possible presence of three separate organic cation exchange mechanisms mediating transport of thiamine, guanidine and MIA across placental brush border membrane.

Biological Transport↗
PubMedDate not supplied

[Erythrocyte membrane transport and plasma levels of tyrosine and tryptophan in depression].

L-Tyrosine and L-Tryptophan were studied in 80 depressed patients, hospitalised in Genova and Marseille. After a washout period of ten days, the erythrocyte membrane transports (MT) of L-Tyrosine and L-Tryptophan were measured, as well as their plasma levels; plasma phenylalanine was moreover measured. 33 normal subjects served as control group. In the whole population of patients, the mean of MT's was different compared to controls (MT tyrosine low and MT tryptophan high) and the plasma tyrosine was low. The perturbations of these variables were different according to diagnostic groups (DSM III): MT tyrosine, plasma tyrosine and tryptophan low in bipolar disorders depressed; MT tyrosine low, MT tryptophan high, plasma tryptophan low in major depressions; MT tryptophan high in dysthymic disorders. Phenylalanine was in the normal range compared to controls. The perturbations of MT's involved the part incubated at 37 degrees C for tyrosine, e.g. the facilitated diffusion, and the part incubated at 0 degrees C for tryptophan, e.g. probably the passive and facilitated diffusion. These results were in agreement with the monoaminergic hypothesis of affective disorders and might provide a useful peripheral model.

Adult↗
PubMed1984

A simple analysis of the "phosphocreatine shuttle".

The diffusive mobility of solutes chemically connected by reversible reactions in cells is analyzed as a problem of facilitated diffusion. By this term we mean that the diffusive flux of any substance, X, which is in one metabolic pathway, is effectively increased when it participates in a second and equilibrium reaction with another substance Y because the total flux of X in the pathway is the sum of the fluxes of X and Y. This notion is generalized and is seen to include the familiar enhanced intracellular diffusion of oxygen by oxymyoglobin. In this framework the function of creatine kinase (CK) is seen to have two aspects: 1) phosphocreatine (PCr) via the CK reaction buffers the cellular ATP and ADP concentrations and 2) transport of high-energy phosphates is predominantly in the chemical form of PCr. This predominance of PCr is a consequence of the maintained ATP, ADP, and total creatine levels and of the apparent equilibrium constant of the reaction. Thus experimental results demonstrating the transport aspects of the CK reaction emphasize only one feature of a more general notion of facilitated diffusion by near-equilibrium metabolic reactions and do not per se establish the existence of any physical or functional compartmentation of ATP, ADP, PCr, or creatine. PCr can be a large source for increasing inorganic phosphate levels during contractile activity, possibly as a metabolic regulator. Neither the transport nor buffer aspects can be quantitatively important in cells with small distances between ATP-utilizing and ATP-generating sites, such as is the case with cardiac myofibrils and mitochondria.

Adenosine Diphosphate↗
PubMed1986

Thiol-dependent regulation of glycerate metabolism in leaf extracts : the role of glycerate kinase in c(4) plants.

We have recently reported that the activity of maize leaf glycerate kinase [EC 2.7.1.31] is regulated in vivo by the light/dark transition, possibly involving the ferredoxin/thioredoxin mechanism, and that the stimulating effect of light can be mimicked in vitro by incubation of crude leaf extract with reducing compounds (LA Kleczkowski, DD Randall 1985 Plant Physiol 79: 274-277). In the present study it was found that the time course of thiol activation of the enzyme was substantially dependent on the presence of some low molecular weight inhibitor(s) of activation found both in leaf extracts and mesophyll chloroplasts. Activity of glycerate kinase from maize as well as wheat leaves increased upon greening of etiolated plants and was correlated with the development of photosynthetic apparatus in these species. The maize enzyme was strongly activated by thiols at all stages of development from etiolated to green seedlings. Thiol activation of glycerate kinase was observed for a number of C(4) plants, notably of the nicotinamide adenine dinucleotide phosphate-malic enzyme type, with the strongest effect found for the enzyme from leaf extracts of maize and sorghum (10- and 8-fold activation, respectively). Among the C(3) species tested, only the enzyme from soybean leaves was affected under the same conditions (1.6-fold activation). This finding was reflected by an apparent lack of cross-reactivity between the enzyme from maize leaves and antibodies raised against purified spinach leaf glycerate kinase. We suggest that, in addition to its role as a final step of photorespiration in leaves, glycerate kinase from C(4) species may serve as a part of the facilitative diffusion system for the intercellular transport of 3-phosphoglycerate. Simultaneous operation of both the passive and the facilitative diffusion mechanisms of 3-phosphoglycerate transport in C(4) plants is postulated.

Journal Article↗