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PubMed1982

Renal handling of 2'-deoxyadenosine and adenosine in humans and mice.

In a child lacking adenosine deaminase and in patients treated with deoxycoformycin (a potent inhibitor of the enzyme), apparent renal secretion of 2'-deoxyadenosine (dAdo) and reabsorption of adenosine (Ado) were observed. The renal clearance of dAdo in humans was approximately five-fold that of creatinine, whereas the renal clearance of Ado was only one-fifth that of creatinine. In mice treated with deoxycoformycin, a similar paradigm was observed. Specifically, plasma levels of Ado and dAdo were elevated to detectable levels and apparent renal secretion and reabsorption of these purine nucleosides became manifest. Thus, the mouse may serve as a suitable model to study the renal handling of these two compounds. The active renal secretion of dAdo may occur because the compound has not been appreciably synthesized by mouse kidney in situ, and 'ion-trapping' of dAdo in acid urine could not explain the net secretion. The differential transport of these similar purine nucleosides suggests a very selective transport system in mammalian kidney. Although carrier-mediated, facilitated diffusion of purine nucleosides across cell membranes is a well-known ph enomenon, the present data indicate the existence of (an) active transport system(s) for the transepithelial secretion of dAdo, and possibly for the reabsorption of Ado.

Adenine↗
PubMed1978

Analysis of regulatory mechanisms controlling the activity of the hexitol transport systems in Escherichia coli K12.

The transport systems (enzymeII-complexes of the PEP-dependent sugar:phosphotransferase system) coded for in the mtl and in the gut (srl) operon of E. coli K12 have been shown to be the pacemaker enzymes in the catabolism of the two hexitols D-mannitol and D-glucitol, respectively. As for other pacemaker enzymes their activity is regulated in a complex way: (i) via competitive inhibition by analogues. (ii) via non-competitive (feedback) inhibition by the simultaneous, rapid uptake of a number of structurally related or non-related carbohydrates, regardless if these are transported by group translocation, active transport or facilitated diffusion. This type of inhibition is strongly reinforced, if the inhibitory carbohydrates are converted efficiently into hexose-phosphates at the same time. Among these, predominantly D-fructose-6-P seems to act as a feedback inhibitor for the hexitol specific enzymeII-complexes: (iii) inhibition of hixitol-phosphate accumulation by D-glucose-6-P. The influence of additional parameters (PEP level, P approximately HPr level) and indications for the existence of further mechanisms controlling the activity of hexitol and other carbohydrate transport systems will be discussed, as will be the part the inhibitory mechanisms described above play in the phenomena of transient repression and inducer exclusion.

Biological Transport↗
PubMed1993

Transport and metabolism of glucose and arabinose in Bifidobacterium breve.

Glucose was required for the transport of arabinose into Bifidobacterium breve. The non-metabolisable glucose analogue 2-deoxy-D-glucose (2-DG) did not facilitate assimilation of arabinose. Studies using D-[U-14C]-labelled arabinose showed that it was fermented to pyruvate, formate, lactate and acetate, whereas the principal metabolic products of D-[U-14C]-labelled glucose were acetate and formate. In contrast to glucose, arabinose was not incorporated into cellular macromolecules. A variety of metabolic inhibitors and inhibitors of sugar transport (proton ionophores, metal ionophores, compounds associated with electron transport) were used to investigate the mechanisms of sugar uptake. Only NaF, an inhibitor of substrate level phosphorylation, and 2-DG inhibited glucose assimilation. 2-DC had no effect on arabinose uptake, but NaF was stimulatory. High levels of phosphorylation of glucose and 2-DC by PEP and to a lesser degree, ATP were seen in phosphoenolpyruvate: phosphotransferase (PEP:PTS) assays. These data together with strong inhibition of glucose uptake by NaF suggest a role for phosphorylation in the transport process. Arabinose uptake in B. breve was not directly dependent on phosphorylation or any other energy-linked form of transport but may be assimilated by glucose-dependent facilitated diffusion.

Arabinose↗
PubMed1994

Immunolocalization of glucose transporter GLUT1 in the rat placental barrier: possible role of GLUT1 and the gap junction in the transport of glucose across the placental barrier.

GLUT1 is an isoform of facilitated-diffusion glucose transporters and has been shown to be abundant in cells of blood-tissue barriers. Using antibodies against GLUT1, we investigated the immunohistochemical localization of GLUT1 in the rat placenta. Rat placenta is of the hemotrichorial type. Three cell layers (from the maternal blood side inward) cytotrophoblast and syncytiotrophoblasts I and II, lie between the maternal and fetal bloodstreams. GLUT1 was abundant along the invaginating plasma membrane facing the cytotrophoblast and the syncytiotrophoblast I. Also, the infolded basal plasma membrane of the syncytiotrophoblast II was rich in GLUT1. Apposing plasma membranes of syncytiotrophoblasts I and II, however, had only a small amount of GLUT1. Numerous gap junctions were seen between syncytiotrophoblasts I and II. Taking into account the localization of GLUT1 and the gap junctions, we suggest a possible major transport route of glucose across the placental barrier, as follows: glucose in the maternal blood passes freely through pores of the cytotrophoblast. Glucose is then transported into the cytoplasm of the syncytiotrophoblast I via GLUT1. Glucose enters the syncytiotrophoblast II through the gap junctions. Finally glucose leaves the syncytiotrophoblast II via GLUT1 and enters the fetal blood through pores of the endothelial cells.

Animals↗
PubMed1993

Urea derivatives as tools for studying the urea-facilitated transport system.

The effects of urea structural analogues on the urea-facilitated diffusion system were examined in human red cell membranes (pink ghosts) and in antidiuretic hormone(ADH)-stimulated frog urinary bladder epithelia. In both tissues, urea permeability (P(urea)) was dramatically but reversibly inhibited by a number of urea analogues, such as 1-(3,4-dichlorophenyl)-2-thiourea (DCPTU). This urea derivative reduced the urea flux in a dose-dependent manner (90% inhibition of P(urea) at 0.5 mM concentration of DCPTU). With the aim of obtaining irreversible markers of red cell and urinary bladder urea transport systems, urea derivatives were modified by addition of an azido residue (N3) and preliminary experiments of photoaffinity labelling were carried out. Two synthetic urea derivatives: 1-(3-azido-4-chlorophenyl)-2-thiourea (ACPTU) and 1-(3-azido-4-chlorophenyl)-3-methyl-2-thiourea (Me-ACPTU) were shown to be very potent inhibitors of P(urea) when used in the absence of light, with IC50 values 60.3 microM and 31.6 microM respectively, as measured in frog urinary bladder. Both these molecules appeared to bind covalently to the urea carrier in both frog urinary bladder and human pink red cell ghosts, when illuminated in the presence of the tissue: the urea flux, which fell to 30-70% of the value obtained in the presence of ADH after inhibitor addition, remained low after the preparation had been illuminated for 30 min and the inhibitor removed. These results provide an interesting approach to the urea carrier analysis, particularly to the urea or urea analogue binding site on the transport protein.

Affinity Labels↗
PubMed1984

The glucose-dependent transport of L-malate in Zygosaccharomyces bailii.

Zygosaccharomyces bailii possesses a constitutive malic enzyme, but only small amounts of malate are decomposed when the cells ferment fructose. Cells growing anaerobically on glucose (glucose cells) decompose malate, whereas fructose cells do not. Only glucose cells show an increase in the intracellular concentration of malate when suspended in a malate-containing solution. The transport system for malate is induced by glucose, but it is repressed by fructose. The synthesis of this transport system is inhibited by cycloheximide. Of the two enantiomers L-malate is transported preferentially. The transport of malate by induced cells is not only inhibited by addition of fructose but also inactivated. This inactivation is independent of the presence of cycloheximide. The transport of malate is inhibited by uranyl ions; various other inhibitors of transport and phosphorylation were of little influence. It is assumed that the inducible protein carrier for malate operates by facilitated diffusion. Fructose cells of Z. bailii and cells of Saccharomyces cerevisiae do not contain a transport system for malate.

Biological Transport↗
PubMed1990

Modes of lactose uptake in the yeast species Kluyveromyces marxianus.

Twelve lactose-assimilating strains of the yeast species Kluyveromyces marxianus and its varieties marxianus, lactis and bulgaricus were studied with respect to transport mechanisms for lactose, glucose and galactose, fermentation of these sugars and the occurrence of extracellular lactose hydrolysis. The strains fell into three groups. Group I (two strains): Fermentation of lactose, glucose and galactose, extracellular lactose hydrolysis, apparent facilitated diffusion of glucose and galactose; Group II (two strains): Lactose not fermented, glucose and galactose fermented and transported by an apparent proton symport, extracellular hydrolysis of lactose present (one strain) or questionable; Group III (eight strains): Lactose, glucose and galactose fermented, lactose transported by an apparent proton symport mechanism, extracellular hydrolysis of lactose and transport modes for glucose and galactose variable.

Biological Transport↗
PubMed1980

Analysis of the physiological effects of the antibiotic streptozotocin on Escherichia coli K 12 and other sensitive bacteria.

The antibiotic streptozotocin under a variety of growth conditions rapidly and irreversibly inactivates the capacity to divide or to form colonies of a series of sensitive bacteria, containing the phosphoenolpyruvate-dependent sugar-phosphotransferase system. Cells can be sensitized towards the drug by pregrowth in N-acetyl-glucosamine and can be protected by adding this amino-glucoside to the medium. Starvation for energy, especially for phosphoenolpyruvate, or prevention of the induction of a transport system involved in streptozotocin uptake will protect the cells, while a block in protein synthesis does not. The killed cells neither lyse, nor are they transformed into spheroplasts. At first, the capacity of such "dead" cells to respire, to swim actively or to keep the cytoplasmic membrane impermeable for small molecules remains intact. Their capacity for over-all RNA and protein synthesis, and for carbohydrate and amino acid uptake by facilitated diffusion or active transport is not affected. However, they loose rapidly their ability to take up carbohydrates by the phosphoenolpyruvate dependent process of group translocation or to synthesize inducible enzymes, e.g. the enzyme beta-galactosidase. These inhibitory effects apparently are caused by the accumulation of phosphorylated, toxic derivatives of the antibiotic and eventually lead to a pronounced bacteriostasis. Killing of the cells seems to be caused by a direct effect of the strongly mutagenic drug on replicating DNA.

Acetylglucosamine↗
PubMed1978

Role of adenine phosphoribosyltransferase in adenine uptake in wild-type and APRT- mutants of CHO.

Adenine uptake in cultured Chinese hamster fibroblasts showed biphasic saturation kinetics. The transport system was highly specific for adenine and was competitively inhibited by adenosine. Utilizing mutant clones of Chinese hamster fibroblasts that have either reduced or negligible adenine phosphoribosyltransferase (APRT) activity, we found that (1) adenine was not accumulated against a concentration gradient in the absence of APRT activity and (2) after rapid initial uptake equal to that of the parent the rates of adenine accumulation found for the mutants correlated strongly with their residual APRT activities. Furthermore, using either artificially depressed phosphoribosylpyrophosphate pool size and APRT activities or the mutants with decreased APRT activity, we found that adenine transport was independent of phosphorylation by APRT. These studies suggest that adenine is transported as the free base by facilitated diffusion and is subsequently phosphorylated by APRT.

Adenine↗
PubMed1991

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucleotides, eicosanoids.

Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent Ki values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. Ki,PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. Ki,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: Km = 1.5 mmol/l, Jmax = 0.34 pmol S-1 cm-1, r (extracellular/intracellular amount at steady state) = 0.91; for cGMP: Km = 0.29 mmol/l, Jmax = 0.31 pmol S-1 cm-1, r = 0.55. Comparison of app. Ki,cGMP with app. Ki,PAH of ten substrates gave a linear relation with a ratio of 1.83 +/- 0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1 alpha, A1, B1, E2, F2 alpha, D2, A2 and B2 with an app. Ki,PAH between 0.08 and 0.18 mmol/l. The app. Ki of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1 alpha (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. Ki,PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1 alpha (0.77 mmol/l) and 2,3-dinor-6-keto-PGF1 alpha (0.57 mmol/l) as well as that of thromboxane B2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. Ki,SO4(2-) 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed Km = 0.61 mmol/l and Jmax of 4.26 pmol S-1 cm-1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

A stopped flow capillary perfusion method to evaluate contraluminal transport parameters of methylsuccinate from interstitium into renal proximal tubular cells.

In order to study the transport of dicarboxylic acids through the contraluminal cell membrane of proximal tubular cells, 3H- methylsuccinate has been synthetized by catalytic hydration of methylfumarate . As the chromatography of radioactive material excreted in the urine after i.v. injection of 3H- methylsuccinate shows, no metabolite is detectable during the first 3 min. After 10 min, less than 10% of the excreted radiolabel is metabolized. To measure the contraluminal influx of 3H- methylsuccinate from the interstitium into cortical tubular cells, the renal vessels were clamped so that the proximal tubular lumina collapsed. Then Ringer solution was injected into the blood capillaries. It contained different concentrations of 3H- methylsuccinate and 14C-inulin as extracellular space marker. After contact times between 1 and 10 s, this fluid was withdrawn from the capillaries and the disappearance of 3H- methylsuccinate relative to 14C-inulin was measured. The morphological compartments in the outer cortex of the clamped glutaraldehyde-fixed kidney were evaluated by a stereological method. For proximal tubular cells a ratio of extracellular water space to intracellular space of 1:3.1 and a ratio extracellular water space to free cell water space of 1:2 was found. It was tested whether the experimental disappearance curves with 4 different starting concentrations of 3H- methylsuccinate fit with the data from four model calculations. It was found that the data and the conditions of transport are consistent with the predictions of a facilitated diffusion model.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1982

Asymmetry in the transport of lactate by basolateral and brush border membranes of rat kidney cortex.

The uptake of L(+)lactate into rat renal cortical brush border (BBV) and basolateral (BLV) membrane vesicles, isolated through differential centrifugation and free flow electrophoresis, were studied using a rapid filtration technique. In contrast to the lactate transport into the BBV, that into the BLV: 1) was found to proceed only towards equilibrium, 2) showed Na+ -independent coupling of the influx of L(+)lactate and the efflux of L(+) but not to the efflux of D(-)lactate, 3) was not inhibited by D(-)lactate, 2-thiolactate or 3-phenyl-lactate, but 4) was inhibited by 3-thiolactate and alpha-hydroxybutyrate and 5) was accelerated by changes in inwardly directed ionic gradients or by increases in cation conductance both of which led to increased intravesicular positivity. The latter changes had the opposite effect on the uptake of L(+)lactate by BBV. Thus, while the L(+)lactate transport system present in BBV showed the characteristics of Na-dependent electrogenic cotransport system, that in the BLV was consistent with a carrier mediated Na-dependent, facilitated diffusion system.

Animals↗
PubMed1991

A theoretical evaluation of growth yields of yeasts.

Growth yields of Saccharomyces cerevisiae and Candida utilis in carbon-limited chemostat cultures were evaluated. The yields on ethanol and acetate were much lower in S. cerevisiae, in line with earlier reports that site I phosphorylation is absent in this yeast. However, during aerobic growth on glucose both organisms had the same cell yield. This can be attributed to two factors: --S. cerevisiae had a lower protein content than C. utilis; --uptake of glucose by C. utilis requires energy whereas in S. cerevisiae it occurs via facilitated diffusion. Theoretical calculations showed that, as a result of these two factors, the ATP requirement for biomass formation in C. utilis is 35% higher than in S. cerevisiae (theoretical YATP values of 20.8 and 28.1, respectively). The experimental YATP for anaerobic growth of S. cerevisiae on glucose was 16 g biomass.mol ATP-1. In vivo P/O-ratios can be calculated for aerobic growth on ethanol and acetate, provided that the gap between the theoretical and experimental ATP requirements as observed for growth on glucose is taken into account. This was done in two ways: --via the assumption that the gap is independent of the growth substrate (i.e. a fixed amount of ATP bridges the difference between the theoretical and experimental values). --alternatively, on the assumption that the difference is a fraction of the total ATP expenditure, that is dependent on the substrate. Calculations of P/O-ratios for growth of both yeasts on glucose, ethanol, and acetate made clear that only by assuming a fixed difference between theoretical and experimental ATP requirements, the P/O-ratios are more or less independent of the growth substrate. These P/O-ratios are approximately 30% lower than the calculated mechanistic values.

Acetates↗
PubMed1980

Polar distribution of sodium-dependent and sodium-independent transport system for L-lactate in the plasma membrane of rat enterocytes.

The uptake of L-lactate by rat small intestinal brush-border and basal-lateral plasma membrane vesicles has been studied. L-Lactate uptake by the isolated membrane vesicles is osmotically sensitive and represents predominantly transport into an intravesicular space and not binding to the membranes. The transport of L-lactate across the brush-border membrane is stimulated by sodium, whereas the transport across the basal-lateral plasma membrane is sodium-independent. In both types of membrane vesicles L-lactate is transported faster than D-lactate and L-lactate transport is inhibited by alpha-cyano-cinnamic acid. L-Lactate transport across basal-lateral membranes is inhibited by D-lactate and pyruvate and transstimulated by L-lactate and pyruvate. The polar distribution of transport system for L-lactate in the plasma membrane of rat enterocytes--a Na+/L-lactate cotransport system in the brush-border membrane and a facilitated diffusion system in the basal-lateral membrane--can explain the fact that in the intact epithelium L-lactate produced by cell metabolism is preferentially released on the serosal side and could enable the cell to perform vectorial, secondary active transport of L-lactate from the intestinal lumen to the serosal compartment.

Animals↗
PubMed1980

Evidence for carrier-mediated uptake of sugars at the serosal side of lamb colon mucosa.

Sugar uptake through the basolateral membrane into epithelial cells was investigated in lamb colon stripped of serosa and muscle layers. Only the antiluminal surface of the mucosa was exposed to the incubtion medium. 2-Deoxy-D-glucose (2-DG) and 2-O-methyl-D-glucose (3-MG) were used as model substrates. Both sugars were taken up by a saturable process. Transport apparently occurred by facilitated diffusion. 2-DG uptake was inhibited by D-glucose and 3-MG, but not by D-galactose and alpha-methyl-D-glucoside and 3-MG uptake was inhibited by 2-DG and D-glucose but not by alpha-methyl-D-glucoside. Thus 2-DG, 2-MG and glucose appear to compete for a common transport mechanism. Carrier-mediated uptake of glucose through the basolateral membranes is probably important for the energy supply of colon epithelium.

Animals↗
PubMed1981

Active transport of urea across the skin of the euryhaline toad, Bufo viridis.

Urea is accumulated in considerable amounts (greater than 100 mM) in the blood of the euryhaline toad Bufo viridis, under conditions of adaptation to high salinities. Salt adaptation increases active transport of urea (inward direction) in the skin, which was measured in vitro. The active transport of urea is insensitive to ADH, and was inhibited nearly 50% by 0.5 mM phloretin. This transport system is different from the facilitated diffusion of urea which has been studied extensively in the toad urinary bladder, and may offer a simple model system for the study of active urea transport.

Animals↗
PubMed1981

Uptake and release of choline in cultures of human glioma cells.

Human glioma cells (138MG) have a low-affinity uptake system for choline (Km = 20 microM; Vmax = 56 pmol/min/10(6) cells). The uptake is reduced by acetylcholine, hemicholinium-3, HgCl2, and phosphodiesterase inhibitors. Release of [3H]choline from preloaded cultures showed two pools with half-lives of 1.3 and 160 min. Choline release was stimulated by 8-bromo-cAMP or isobutylmethylxanthine. The results suggest that release of choline occurs by a facilitated diffusion transport system and is increased by elevations of intracellular cAMP.

Cell Line↗
PubMed1993

Mathematical models for multidrug resistance and its reversal.

Mathematical models describing drug resistance are briefly reviewed. One model which describes the molecular function of the P-glycoprotein pump in multidrug resistant (MDR) cell lines has been developed and is presented in detail. The pump is modeled as an energy dependent facilitated diffusion process. A partial differential equation linked to a pair of ordinary differential equations forms the core of the model. To describe MDR reversal, the model is extended to add an inhibitor. Equations for competitive, one-site noncompetitive, and two-site noncompetitive inhibition are derived. Numerical simulations have been run to describe P-glycoprotein dynamics both in the presence and absence of these kinds of inhibition. These results are briefly reviewed. The character of the pump and its response to inhibition are discussed within the context of the models. All discussions, descriptions, and conclusions are presented in nonmathematical terms. The paper is aimed at a scientifically sophisticated but mathematically innocent audience.

ATP Binding Cassette Transporter, Subfamily B, Mem↗