scholarly journals Evidence for a proton/sugar symport in the yeast Rhodotorula gracilis (glutinis)

1978 ◽  
Vol 172 (1) ◽  
pp. 15-22 ◽  
Author(s):  
M Höfer ◽  
P C Misra
Keyword(s):  
Sugar Uptake ◽  
Sugar Accumulation ◽  
Electrochemical Gradient ◽  
Apparent Dissociation Constant ◽  
Physiological Conditions ◽  
Half Saturation Constant ◽  
Monosaccharide Transport ◽  
Rhodotorula Gracilis ◽  
Proton Uptake ◽  
Effective Carrier

1. The uptake of monosaccharides and polyols in the obligatory aerobic yeast Rhodotorula gracilis (glutinis) was accompanied by proton uptake. 2. The half-saturation constant of transport, KT, depended on pH, changing from about 2mM at pH 4.5 to 80mM at pH8.5 for D-xylose; this change of the effective carrier affinity was reversible. 3. The apparent dissociation constant of the monosaccharide carrier was estimated at pKa 6.75. 4. At pH8.5, when the pH gradient across the cell membrane vanished, no sugar accumulation was demonstrable. 5. The half-saturation constants of sugar uptake and H+ co-transport were very similar to each other, the latter obviously being controlled by the former. 6. The H+/sugar stoicheiometry remained constant under various physiological conditions; it amounted to one H+ ion per sugar molecule taken up. 7. The data are interpreted as a strong piece of evidence in favour of the active monosaccharide transport in R. gracilis (glutinis) being an H+-symport energized by the electrochemical gradient of H+ across the plasma membrane of the yeast.

scholarly journals Active transport of charged substrates by a proton/sugar co-transport system. Amino-sugar uptake in the yeast Rhodotorula gracilis

1981 ◽  
Vol 194 (2) ◽  
pp. 433-441 ◽  
Author(s):  
C Niemietz ◽  
R Hauer ◽  
M Höfer
Keyword(s):  
Membrane Potential ◽  
Amino Sugar ◽  
Protonated Form ◽  
Sugar Uptake ◽  
Amino Sugars ◽  
Maximal Velocity ◽  
Deprotonated Form ◽  
Half Saturation Constant ◽  
Carbonyl Cyanide ◽  
Rhodotorula Gracilis

1. In the yeast Rhodotorula gracilis several amino sugars were actively transported. Glucosamine, which is largely protonated at physiological pH (pK 7.75) was used as a model substrate. At pH 6.75 its half-saturation constant was 1 mM and the maximal velocity was 50 nmol/min per mg dry wt. 2. Amino sugars were taken up via the monosaccharide carrier. The transport of glucosamine was strongly restricted by monosaccharides. D-Xylose inhibited competitively the uptake of glucosamine. The inhibition constant was 1 mM. Cells preloaded with D-xylose showed exchange transport on subsequent addition of glucosamine. 3. Transport of glucosamine was energized by the membrane potential. Uncoupling agents such as carbonyl cyanide m-chlorophenyl-hydrazone and the lipophilic cation TPP+ (tetraphenylphosphonium ion) at concentrations that depolarized the membrane potential inhibited the uptake of glucosamine. Conversely the transport of glucosamine partly dissipated the membrane potential, which was monitored by radioactively labelled lipophilic cations. 4. The translocated charges were electrically compensated by the extrusion of protons and K+ (1 glucosamine molecule/0.85 H+ + 0.15 K+). 5. An increase of the pH in the range 4.75-8.75 lead to a decrease of the half-saturation constant from 5 mM to 1 mM and to an optimum of the maximal velocity at pH 6.75. We suggest that this fair constancy is due to the carrier not distinguishing between the protonated form of glucosamine (pH less than 7.75) and the deprotonated form (pH greater than 7.75). The increase of V(T) (maximal transport velocity) between pH 4.75 and 6.75 is due to the increase of the membrane potential: the decrease between pH 6.75 and 8.75 is due to the deprotonization of the carrier.

scholarly journals The Hexose-Proton Cotransport System of Chlorella

1974 ◽  
Vol 64 (5) ◽  
pp. 568-581 ◽  
Author(s):  
Ewald Komor ◽  
Widmar Tanner
Keyword(s):  
Sugar Transport ◽  
Protonated Form ◽  
Weak Acid ◽  
Sugar Uptake ◽  
Sugar Accumulation ◽  
Maximal Velocity ◽  
Uptake System ◽  
High Affinity ◽  
Ph Values ◽  
Proton Concentration

The proton concentration in the medium affects the maximal velocity of sugar uptake with a Km of 0.3 mM (high affinity uptake). By decreasing the proton concentration a decrease in high affinity sugar uptake is observed, in parallel the activity of a low affinity uptake system (Km of 50 mM) rises. Both systems add up to 100%. The existence of the carrier in two conformational states (protonated and unprotonated) has been proposed therefore, the protonated form with high affinity to 6-deoxyglucose, the unprotonated form with low affinity. A plot of extrapolated Vmax values at low substrate concentration versus proton concentration results in a Km for protons of 0.14 µM, i.e. half-maximal protonation of the carrier is achieved at pH 6.85. The stoichiometry of protons cotransported per 6-deoxyglucose is close to 1 at pH 6.0–6.5. At higher pH values the stoichiometry continuously decreases; at pH 8.0 only one proton is cotransported per four molecules of sugar. Whereas the translocation of the protonated carrier is strictly dependent on sugar this coupling is less strict for the unprotonated form. Therefore at alkaline pH a considerable net efflux of accumulated sugar can occur. The dependence of sugar accumulation on pH has been measured. The decrease in accumulation with higher pH values can quantitatively be explained by the decrease in the amount of protonated carrier. The properties of the unprotonated carrier resemble strikingly the properties of carrier at the inner side of the membrane. The inside pH of Chlorella was measured with the weak acid 5,5-dimethyl-2, 4-oxazolidinedion (DMO). At an outside pH of 6.5 the internal pH was found to be 7.2. To explain the extent of sugar accumulation it has to be assumed that the membrane potential also contributes to active sugar transport in this alga.

scholarly journals Dexamethasone inhibits the hexose monophosphate shunt in activated rat peritoneal macrophages by reducing hexokinase-dependent sugar uptake

1991 ◽  
Vol 278 (1) ◽  
pp. 129-135 ◽  
Author(s):  
R J Rist ◽  
R J Naftalin
Keyword(s):  
Sugar Transport ◽  
Peritoneal Macrophages ◽  
Free Sugar ◽  
Sugar Uptake ◽  
Sugar Accumulation ◽  
Dependent Manner ◽  
Hexose Monophosphate ◽  
Simultaneous Exposure ◽  
Hexose Monophosphate Shunt

Dexamethasone decreases 2-D-deoxyglucose (2-dGlc) uptake and accumulation into rat peritoneal macrophages in vitro in a concentration- and time-dependent manner (Ki for 1 microM-dexamethasone after a 2 h exposure = 0.71 +/- 0.21 microM; Ki for 0.1 microM-dexamethasone after exposure for 4 h = 0.10 +/- 0.06 microM). The inhibition of 2-dGlc uptake is consistent with a decrease in the coupling between endofacial hexokinase activity and the sugar transporter. The evidence for this is: (1) the Km for zero-trans 2-dGlc uptake in quiescent macrophages was increased by dexamethasone, but there was no significant effect on the Vmax.; (2) dexamethasone increased the rate of exit of sugar from cells preloaded with 2-dGlc; (3). the free sugar accumulation within the cytosol of the cells above the external solution concentration was significantly decreased by dexamethasone. These effects of dexamethasone on 2-dGlc transport were antagonized by simultaneous exposure to the steroid RU 38486 (Ki = 0.04 +/- 0.01 microM; 4 h incubation). Although dexamethasone inhibited zero-trans uptake, the maximum rate of infinite-trans exchange uptake of 2-dGlc into cells preloaded with 3-O-methyl-D-glucose (40 mM) was unaltered by dexamethasone or RU 38486, indicating that the dexamethasone-dependent decrease in zero-trans uptake was not due to a change in the number of transporters in the plasma membrane. Dexamethasone also inhibited the phorbol myristate acetate-induced stimulation of hexose monophosphate shunt (HMPS) activity, and this was reversed by RU 38486. Cytochalasin B, the potent sugar-transport inhibitor, inhibited HMPS activity and 2-d[2,6-3H]Glc uptake equally, indicating a single site of action. By contrast, dexamethasone showed differential inhibition of HMPS activity and 2-d[2,6-3H]Glc uptake, suggesting that it not only acts by decreasing the coupling between hexokinase and sugar transport, but also at one or more additional points.

scholarly journals Abnormal Accumulation of Sugars and Phenolics in Tobacco Roots Expressing the Agrobacterium T-6b Oncogene and the Role of These Compounds in 6b-Induced Growth

2007 ◽  
Vol 20 (1) ◽  
pp. 53-62 ◽  
Author(s):  
Bernadette Clément ◽  
Jonathan Perot ◽  
Pierrette Geoffroy ◽  
Michel Legrand ◽  
Jerzy Zon ◽  
...  
Keyword(s):  
Competitive Inhibitor ◽  
Sugar Uptake ◽  
Sugar Accumulation ◽  
Induction Medium ◽  
Abnormal Accumulation ◽  
Free Induction ◽  
Tobacco Seedlings ◽  
Growth Changes ◽  
Sugar Levels ◽  
Tobacco Roots

The Agrobacterium T-DNA oncogene 6b induces tumors and modifies the growth of transgenic plants by an unknown mechanism. We have investigated changes in roots of tobacco seedlings that express a dexamethasone-inducible T-6b (dex-T-6b) gene. On induction medium with sucrose, intact or isolated dex-T-6b roots accumulated sucrose, glucose, and fructose and changed their growth, contrary to noninduced roots. Root fragments bridging agar blocks with or without sucrose accumulated sugars at the site of sucrose uptake, resulting in local growth. Induced root fragments showed enhanced uptake of 14C-labeled sucrose, glucose, and fructose. When seedlings were placed on sucrose-free induction medium, sugar levels strongly decreased in roots and increased in cotyledons. Collectively, these results demonstrate that 6b stimulates sugar uptake and retention with drastic effects on growth. Apart from sugars, phenolic compounds also have been found to accumulate in 6b tissues and have been proposed earlier to play a role in 6b-induced growth. Induced dex-T-6b roots accumulated high levels of 5-caffeoylquinic acid (or chlorogenic acid [CGA]), but only under conditions where endogenous sugars increased. Inhibition of phenyla-lanine ammonia-lyase with the competitive inhibitor 2-ami-noindan-2-phosphonic acid (AIP) abolished CGA accumulation without modifying sugar accumulation or affecting the 6b phenotype. We conclude that the absorption, retention, and abnormal accumulation of sugars are essential factors in 6b-induced growth changes, whereas phenylpropanoids only marginally contribute to the 6b seedling phenotype.

scholarly journals α- and β-Monosaccharide transport in human erythrocytes

2009 ◽  
Vol 296 (1) ◽  
pp. C151-C161 ◽  
Author(s):  
Jeffry M. Leitch ◽  
Anthony Carruthers
Keyword(s):  
Sugar Transport ◽  
Human Erythrocytes ◽  
Slow Phase ◽  
Sugar Uptake ◽  
Cell Water ◽  
Rapid Phase ◽  
Monosaccharide Transport ◽  
Equilibrium Exchange ◽  
Transport Inhibitors ◽  
Slow Transport

Equilibrative sugar uptake in human erythrocytes is characterized by a rapid phase, which equilibrates 66% of the cell water, and by a slow phase, which equilibrates 33% of the cell water. This behavior has been attributed to the preferential transport of β-sugars by erythrocytes (Leitch JM, Carruthers A. Am J Physiol Cell Physiol 292: C974–C986, 2007). The present study tests this hypothesis. The anomer theory requires that the relative compartment sizes of rapid and slow transport phases are determined by the proportions of β- and α-sugar in aqueous solution. This is observed with d-glucose and 3- O-methylglucose but not with 2-deoxy-d-glucose and d-mannose. The anomer hypothesis predicts that the slow transport phase, which represents α-sugar transport, is eliminated when anomerization is accelerated to generate the more rapidly transported β-sugar. Exogenous, intracellular mutarotase accelerates anomerization but has no effect on transport. The anomer hypothesis requires that transport inhibitors inhibit rapid and slow transport phases equally. This is observed with the endofacial site inhibitor cytochalasin B but not with the exofacial site inhibitors maltose or phloretin, which inhibit only the rapid phase. Direct measurement of α- and β-sugar uptake demonstrates that erythrocytes transport α- and β-sugars with equal avidity. These findings refute the hypothesis that erythrocytes preferentially transport β-sugars. We demonstrate that biphasic 3- O-methylglucose equilibrium exchange kinetics refute the simple carrier hypothesis for protein-mediated sugar transport but are compatible with a fixed-site transport mechanism regulated by intracellular ATP and cell shape.

Monosaccharide transport into hemocytes of a sipunculan worm Themiste dyscrita

1985 ◽  
Vol 249 (1) ◽  
pp. R139-R144
Author(s):  
R. L. Ingermann ◽  
R. E. Hall ◽  
J. M. Bissonnette ◽  
R. C. Terwilliger
Keyword(s):  
Transport System ◽  
Concentration Gradient ◽  
Blood Cells ◽  
Cytochalasin B ◽  
Potent Inhibitor ◽  
Simple Diffusion ◽  
Initial Concentration ◽  
Half Saturation Constant ◽  
Monosaccharide Transport ◽  
Saturation Kinetics

The hemerythrin-containing blood cells, or hemocytes, of the sipunculan worm Themiste dyscrita were found to have a stereospecific and nonconcentrative monosaccharide transport system. The transport system transferred both D-glucose and 3-O-methyl-D-glucose (3-OMG), and transport into cells by this system was rapid, reaching 50% equilibrium in approximately 20 s at 10 degrees C with an initial concentration gradient of 0.1 mM; the contribution to total uptake by simple diffusion was very small. 3-OMG uptake showed saturation kinetics with a low half-saturation constant (Km less than or equal to 0.1 mM). The uptake of labeled 3-OMG by the hemocytes was strongly inhibited by unlabeled 3-OMG, 2-deoxy-D-glucose, alpha- and beta-D-glucose, D-galactose, and D-mannose. It was moderately inhibited by D-xylose, only slightly by alpha-methyl-D-glucoside and D-fructose, and uninhibited by sucrose, L-glucose, or D-sorbitol. Phloretin was more potent than phloridzin in blocking entry of 3-OMG. Cytochalasin B did not bind tightly to the T. dyscrita transporter and was not a potent inhibitor of transport; it half-maximally inhibited 3-OMG transport at 0.1 mM. Therefore, despite some differences the data suggest functional similarities in the mechanism of monosaccharide transport into blood cells of mammals and this invertebrate.

Porters and neurotransmitter transporters.

1994 ◽  
Vol 196 (1) ◽  
pp. 213-228 ◽  
Author(s):  
N Nelson ◽  
H Lill
Keyword(s):  
Structure And Function ◽  
Membrane Transporters ◽  
Aminobutyric Acid ◽  
Gamma Aminobutyric Acid ◽  
Electrochemical Gradient ◽  
Physiological Conditions ◽  
Neurotransmitter Transporters ◽  
Vesicular Transporters ◽  
And Function ◽  
Potassium Gradient

Uptake of neurotransmitters involves multiple transporters acting in different brain locations under different physiological conditions. The vesicular transporters are driven by a proton-motive force generated by a V-ATPase and their substrates are taken up via proton/substrate exchange. The plasma membrane transporters are driven by an electrochemical gradient of sodium generated by a Na+/K(+)-ATPase. Two distinct families of transporters were identified in this group. One cotransports sodium with glutamate and other amino acids and requires additionally an outwardly directed potassium gradient. The second cotransports sodium, chloride and a variety of neurotransmitters, including gamma-aminobutyric acid (GABA), glycine and monoamines. Genes and cDNA encoding several members of the latter family have been cloned and studied in detail. The structure and function as well as the evolutionary relationships among these neurotransmitter transporters are discussed.

Ultrastructure of Giant Mitochondria and Their Inclusions in Schwann Cells of Bovine Splenic Nerve

1974 ◽  
Vol 32 ◽  
pp. 194-195
Author(s):  
Å. Thureson-Klein
Keyword(s):  
Schwann Cells ◽  
Cell Organelles ◽  
Giant Mitochondria ◽  
Matrix Density ◽  
Physiological Conditions ◽  
Unmyelinated Axons ◽  
Internal Structures ◽  
Structural Abnormalities ◽  
Cytotoxic Compounds ◽  
Cell Components

Giant mitochondria of various shapes and with different internal structures and matrix density have been observed in a great number of tissues including nerves. In most instances, the presence of giant mitochondria has been associated with a known disease or with abnormal physiological conditions such as anoxia or exposure to cytotoxic compounds. In these cases degenerative changes occurred in other cell organelles and, therefore the giant mitochondria also were believed to be induced structural abnormalities.Schwann cells ensheating unmyelinated axons of bovine splenic nerve regularly contain giant mitochondria in addition to the conventional smaller type (Fig. 1). These nerves come from healthy inspected animals presumed not to have been exposed to noxious agents. As there are no drastic changes in the small mitochondria and because other cell components also appear reasonably well preserved, it is believed that the giant mitochondria are normally present jin vivo and have not formed as a post-mortem artifact.

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