Kinetic tests of models for sugar transport in human erythrocytes and a comparison of fresh and cold-stored cells

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Structural basis of GLUT1 inhibition by cytoplasmic ATP

Cytoplasmic ATP inhibits human erythrocyte glucose transport protein (GLUT1)–mediated glucose transport in human red blood cells by reducing net glucose transport but not exchange glucose transport (Cloherty, E.K., D.L. Diamond, K.S. Heard, and A. Carruthers. 1996. Biochemistry. 35:13231–13239). We investigated the mechanism of ATP regulation of GLUT1 by identifying GLUT1 domains that undergo significant conformational change upon GLUT1–ATP interaction. ATP (but not GTP) protects GLUT1 against tryptic digestion. Immunoblot analysis indicates that ATP protection extends across multiple GLUT1 domains. Peptide-directed antibody binding to full-length GLUT1 is reduced by ATP at two specific locations: exofacial loop 7–8 and the cytoplasmic C terminus. C-terminal antibody binding to wild-type GLUT1 expressed in HEK cells is inhibited by ATP but binding of the same antibody to a GLUT1–GLUT4 chimera in which loop 6–7 of GLUT1 is substituted with loop 6–7 of GLUT4 is unaffected. ATP reduces GLUT1 lysine covalent modification by sulfo-NHS-LC-biotin by 40%. AMP is without effect on lysine accessibility but antagonizes ATP inhibition of lysine modification. Tandem electrospray ionization mass spectrometry analysis indicates that ATP reduces covalent modification of lysine residues 245, 255, 256, and 477, whereas labeling at lysine residues 225, 229, and 230 is unchanged. Exogenous, intracellular GLUT1 C-terminal peptide mimics ATP modulation of transport whereas C-terminal peptide-directed IgGs inhibit ATP modulation of glucose transport. These findings suggest that transport regulation involves ATP-dependent conformational changes in (or interactions between) the GLUT1 C terminus and the C-terminal half of GLUT1 cytoplasmic loop 6–7.

ATP-dependent sugar transport complexity in human erythrocytes

Human erythrocyte glucose sugar transport was examined in resealed red cell ghosts under equilibrium exchange conditions ([sugar](intracellular) = [sugar](extracellular), where brackets indicate concentration). Exchange 3-O-methylglucose (3MG) import and export are monophasic in the absence of cytoplasmic ATP but are biphasic when ATP is present. Biphasic exchange is observed as the rapid filling of a large compartment (66% cell volume) followed by the slow filling of the remaining cytoplasmic space. Biphasic exchange at 20 mM 3MG eliminates the possibility that the rapid exchange phase represents ATP-dependent 3MG binding to the glucose transport protein (GLUT1; cellular [GLUT1] of </=20 microM). Immunofluorescence-activated cell sorting analysis shows that biphasic exchange does not result from heterogeneity in cell size or GLUT1 content. Nucleoside transporter-mediated uridine exchange proceeds as rapidly as 3MG exchange but is monoexponential regardless of cytoplasmic [ATP]. This eliminates cellular heterogeneity or an ATP-dependent, nonspecific intracellular diffusion barrier as causes of biphasic exchange. Red cell ghost 3MG and uridine equilibrium volumes (130 fl) are unaffected by ATP. GLUT1 intrinsic activity is unchanged during rapid and slow phases of 3MG exchange. Two models for biphasic sugar transport are presented in which 3MG must overcome a sugar-specific, physical (diffusional), or chemical (isomerization) barrier to equilibrate with cell water. Partial transport inhibition with the use of cytochalasin B or maltose depresses both rapid and slow phases of transport, thereby eliminating the physical barrier hypothesis. We propose that biphasic 3MG transport results from ATP-dependent, differential transport of 3MG anomers in which V(max)/apparent K(m) for beta-3MG exchange transport is 19-fold greater than V(max)/apparent K(m) for alpha-3MG transport.

The effect of ATP-depletion on the inhibition of glucose exits from human red cells

The effect of ATP-depletion or its consequence, by metabolic inhibition, on the inhibition of glucose transport by various inhibitors was studied in human red cells. In cells depleted of ATP, glucose exit times were longer than in normal cells and the times increased with the duration of depletion. The Km for external glucose was higher in ATP-depleted cells than in normal undepleted cells (3.0 mM c.f. 2.5 mM at 30 degrees C). In contrast, the apparent Ki for cytochalasin B decreased from 0.85 microM in the normal cells to 0.5 microM after ATP-depletion. Half-maximal rates of glucose exit in the absence, and in the presence of 2 microM cytochalasin B were found at ATP concentrations of 0.43 and 0.68 microM, respectively. Although glucose exits from ATP-depleted cells exposed to the irreversible inhibitor of glucose transport, 1-fluoro-2,4-dinitrobenzene (FDNB) were slower than in normal cells, the relative degrees of inhibition were not significantly different. However, normal and ATP-depleted cells responded differently to treatment with 1,2-cyclohexanedione, a modifier of arginine residues which inhibits glucose exit. While normal cells were markedly inhibited, depleted cells were much less affected and the inhibitory effect of cytochalasin B seen in normal cells was reduced. These findings demonstrate that the glucose transport system of human red cells is affected by intracellular ATP and that ATP alters the affinity of the transporter for certain inhibitors. The implications of these findings are discussed.

Re-examination of hexose exchanges using rat erythrocytes: evidence inconsistent with a one-site sequential exchange model, but consistent with a two-site simultaneous exchange model

(1). The kinetic parameters of zero-trans net uptake and infinite-trans uptake of 3-O-methyl-D-glucoside, 2-deoxy-D-glucose and D-mannose into rat red cells at 24 degrees C were measured after taking account of the linear diffusion components of flux. (2). Zero-trans exists of 3-O-methyl-D-glucoside and D-mannose from rat cells were also measured. (3). After correction for linear flux via non-specific routes, the Vmax of zero-trans uptake of 3-O-methyl-D-glucoside was significantly higher, (1.25 +/- 0.06 mumol (10 min)-1 (ml cell water)-1) than the corresponding parameters of mannose or 2-deoxy-D-glucose, (0.33 +/- 0.01 and 0.39 +/- 0.01 mumol(10 min)-1 (ml cell water)-1, respectively; P < 0.001). (4). After correction for linear flux via non-specific uptake routes, the Vmax of zero-trans exit of 3-O-methyl-D-glucoside is significantly higher (1.70 +/- 0.1 mumol (10 min)-1 (ml cell water)-1) than the corresponding value for mannose exit flux, (1.10 +/- 0.1 mumol (10 min)-1 (ml cell water)-1; P < 0.001). (5). The acceleration ratio, i.e., the ratio of infinite-trans influx Vmax/zero-trans influx Vmax of mannose by mannose (9.12 +/- 0.03) is significantly higher than that of 3-O-methyl-D-glucose by 3-O-methyl-D-glucose (2.77 +/- 0.14)(P < 0.001). (6). The one-site simple carrier model of glucose transport in which sugar exchange is viewed as a sequential process, predicts that the acceleration ratio of the more rapidly moving sugar 3-O-methyl-D-glucose by 3-O-methyl-D-glucose should be greater than that of the slower sugar, mannose by mannose. Hence, the observed findings are inconsistent with the one-site model, but confirm the earlier disputed studies of Miller, D.M. (1968; Biophys. J. 8, 1329-1338). (7). A two-site model, in which sugar exchange is considered as a simultaneous process, predicts that the acceleration ratio of mannose influx by mannose should be higher than for 3-O-methyl-D-glucose by 3-O-methyl-D-glucose. The data are, therefore, consistent with a two-site model.

Effects of low ionic strength media on passive human red cell monovalent cation transport

1. The effect of low ionic strength media on the residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, K+ influx was characterized in human red blood cells. 2. This K+ flux was enhanced significantly in isotonic solutions of low ionic strength using sucrose to maintain constant osmolarity. This effect was found for fresh red blood cells as well as for stored (bank) red blood cells. However, the absolute magnitude of K+ influx in solutions of low ionic strength was halved for stored red blood cells. 3. Anion replacement of Cl- by CH3SO4- did not affect residual K+ fluxes, showing that Cl- -dependent transport pathways (e.g. the KCl co-transporter) are not involved in the low ionic strength effect. 4. The enhanced K+ influx in low ionic strength media was reversible when the cells were resuspended in a solution of physiological ionic strength. 5. K+ influx measured in light and dense fractions of erythrocytes (separated by centrifugation and corresponding to samples enriched with either 'young' or 'mature' red cells) showed that the low ionic strength effect does not change markedly with cell age. 6. Low ionic strength media elevated residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, influx of both K+ and Na+ by about the same amount. In both cases the flux was linear with concentration in the range investigated (0.25-10 mM). No significant increase in the uptake of the cations Ca2+ and lysine in low ionic strength solutions could be found. 7. In CH3SO4- -containing solutions of physiological ionic strength the residual K+ influx was almost independent of cell volume, whereas this flux in CH3SO4- -containing solutions of low ionic strength declined as cell volume was increased. 8. K+ flux measurements in solutions of different external pH, where NaCl was replaced by sodium gluconate or sodium glucuronate, showed that the reduced ionic strength is of more importance for the enhanced residual K+ influx than the changed transmembrane potential or the changed intracellular pH. However, a small pH dependence could be found, the K+ flux passing through a minimum around pHi 7.3. 9. Hydrostatic pressure enhanced the residual K+ flux in media of low ionic strength synergistically, so that very large fluxes (greater than 10 mmol (1 cells)-1 h-1) were obtained at 40 MPa. The apparent activation volumes (delta V*) for the pressure-sensitive K+ flux were -108 and -69 ml mol-1 in low ionic strength or physiological ionic strength solutions respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Testing transport models and transport data by means of kinetic rejection criteria

In the case of a transport system obeying Michaelis-Menten kinetics, completely general relationships are shown to exist between the final ratio of internal and external substrate concentrations, alpha, and the V/Km ratios found in zero-trans-entry, zero-trans-exit and equilibrium-exchange experiments (where V is a maximum substrate flux and Km a substrate half-saturation constant). The proof depends on a new method of derivation proceeding from the form of the experimental data rather than, as has been the practice in kinetic analysis, from a hypothetical reaction scheme. These general relationships, which will be true of all mechanisms giving rise to a particular type of behaviour (here Michaelis-Menten kinetics), provide a test for internal consistency in a set of experimental data. Other relationships, which are specific, can be derived from individual reaction schemes, with the use of traditional procedures in kinetic analysis. The specific relationships include constants for infinite trans entry and exit in addition to constants involved in the general relationships. In conjunction, the general and specific relationships provide a stringent test of mechanism. A set of results that fails to satisfy the general relationships must be rejected; here systematic error or unexpected changes in the transport system in different experiments may have distorted the calculated constants, or the system may not actually obey Michaelis-Menten kinetics. Results in accord with the general relationships, on the other hand, can be applied in specific tests of mechanism. The usefulness of the theorem is illustrated in the cases of the glucose-transport and choline-transport systems of erythrocytes. Experimental results taken from several studies in the literature, which were in accord with hyperbolic substrate kinetics, had previously been shown to disagree with relationships derived for the carrier model, and the model was rejected. The new analysis shows that the data violated the general relationships and therefore cannot decide the issue. More recent results on the glucose-transport system satisfy the general relations and agree with the carrier model.

ATP does not regulate the reconstituted glucose transporter

ATP has been reported to affect glucose transport in human erythrocytes and resealed erythrocyte ghosts [Jacquez, J. A. (1983) Biochim. Biophys. Acta 727, 367-378; Jensen, M. R., & Brahm, J. (1987) Biochim. Biophys. Acta 900, 282-290]. In more detailed studies, effects of micromolar levels of ATP on transport in ghosts and inside-out vesicles, and on the fluorescence of ghosts and the purified glucose transporter [Carruthers, A. (1986) Biochemistry 25, 3592-3602; Hebert, D. N., & Carruthers, A. (1986) J. Biol. Chem. 261, 10093-10099; Carruthers, A. (1986) J. Biol. Chem. 261, 11028-11037], have been interpreted as supporting a model in which ATP regulates the catalytic properties of the transporter. Both allosteric and covalent effects of ATP were proposed; among the allosteric effects was a 60% reduction in the Km for zero-trans uptake. In order to test whether allosteric ATP regulation of the transporter occurs, we reconstituted glucose transport activity into liposomes using erythrocyte membranes without detergent treatment. The effects of ATP, present either outside, inside, or both inside and outside the liposomes, on the transport activity were examined. Effects of ATP on trypsin-treated liposomes, which have only a single orientation of active transporters, were also tested. While the model predicts activation by ATP, only inhibition was observed. This was significant only at millimolar concentrations of ATP, in contrast to the previously reported effects at micromolar levels, and was primarily on the extracellular surface of the transporter. In addition, the ATP effects on reconstituted transport were nonspecific, with similar effects produced by tripolyphosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ATP depletion on the mechanism of hexose transport in intact human erythrocytes

Depletion of ATP is known to inhibit glucose transport in human erythrocytes, but the kinetic mechanism of this effect is controversial. Selective ATP depletion of human erythrocytes by 10 micrograms/ml A23187 in the presence of extracellular calcium inhibited 3-O-methylglucose influx noncompetitively and efflux competitively. ATP depletion also decreased the ability of either equilibrated 3-O-methylglucose or extracellular maltose to inhibit cytochalasin B binding in intact cells, whereas neither total high-affinity cytochalasin B binding nor its Kd was affected. Under the one-site model of hexose transport these data indicate that ATP depletion decreases both the affinity of the inward-facing glucose carrier for substrate and its ability to reorient outwardly in intact cells.

Inhibition by nucleosides of glucose-transport activity in human erythrocytes

The interaction of nucleosides with the glucose carrier of human erythrocytes was examined by studying the effect of nucleosides on reversible cytochalasin B-binding activity and glucose transport. Adenosine, inosine and thymidine were more potent inhibitors of cytochalasin B binding to human erythrocyte membranes than was D-glucose [IC50 (concentration causing 50% inhibition) values of 10, 24, 28 and 38 mM respectively]. Moreover, low concentrations of thymidine and adenosine inhibited D-glucose-sensitive cytochalasin B binding in an apparently competitive manner. Thymidine, a nucleoside not metabolized by human erythrocytes, inhibited glucose influx by intact cells with an IC50 value of 9 mM when preincubated with the erythrocytes. In contrast, thymidine was an order of magnitude less potent as an inhibitor of glucose influx when added simultaneously with the radioactive glucose. Consistent with this finding was the demonstration that glucose influx by inside-out vesicles prepared from human erythrocytes was more susceptible to thymidine inhibition than glucose influx by right-side-out vesicles. These data, together with previous suggestions that cytochalasin B binds to the glucose carrier at the inner face of the membrane, indicate that nucleosides are capable of inhibiting glucose-transport activity by interacting at the cytoplasmic surface of the glucose transporter. Nucleosides may also exhibit a low-affinity interaction at the extracellular face of the glucose transporter.

The Wellcome Trust lecture. Mechanisms of molecular trafficking in malaria

The asexual stages of Plasmodium living within the erythrocyte result in growth-related changes in the permeability properties of the red cell for substances such as glucose, amino acids, purine nucleosides, sodium, potassium, calcium, zinc, iron and several antimalarial drugs such as chloroquine, amodiaquine and mefloquine. In most cases such changes do not appear to be due to a modification in the affinity or specificity of red cell transporters; indeed, for most substances the membrane-associated transporters are either unaffected or are partially inactivated. In malaria-infected erythrocytes, where a striking increase in influx has been observed, it has been attributed to the insertion of parasite-encoded transporters into the red cell membrane or the formation of aqueous leaks and/or pores. Leak formation, in the vast majority of cases, does not appear to be dependent on the insertion of plasmodial proteins into the red cell membrane. However, since the data presently available are less than satisfactory for discriminating amongst the various possible transport mechanisms future studies will require painstaking efforts and carefully controlled conditions to discriminate amongst the various transport systems which are operational in the malaria-infected red cell and the parasite.

Glucose transport in human erythrocytes measured using 13C NMR spin transfer

We present the results of a new NMR-based procedure for measuring the fast transmembrane exchange of D-[1-13C]glucose in human erythrocytes. The method relies on different rates of exchange between the alpha- and beta-anomers of glucose inside and outside the cells; the rate outside the cells is greatly increased by the addition of mutarotase to the suspension. Theory is developed to describe nuclear-spin transfer in the present system and is used to analyse the data to yield estimates of transmembrane-exchange rate constants and their statistical uncertainties. For a total glucose concentration of 25.5 mmol/l at 40 degrees C the first order efflux rate constants for the alpha- and beta-anomers were 1.20 +/- 0.40 s-1 and 0.71 +/- 0.30 s-1, respectively.

Glucose transport in human red cell membranes. Dependence of age, ATP, and insulin

Glucose self-exchange flux (Jex) and net efflux (Jnet) in human red cells and ghosts were studied at 25 degrees C and pH 7.2 by means of the combined use of the Millipore-Swinnex filtering method and the continuous flow tube method to show the dependence of time of storage after aspiration, ATP and insulin. In fresh cells (RBC), ghosts (G), ghosts with 2 mM ATP (G +), and cells stored at 4 degrees C greater than 60 days (OC) both Jex and Jnet follow simple Michaelis-Menten kinetics where J = Jmax X Ci X (K1/2 + Ci)-1. Jmaxex and Jmaxnet (nmol X cm-2 X s-1), respectively, was: (RBC) 0.27 and 0.19, (G) 0.24 and 0.27, (G +) 0.23 and 0.24, (OC) 0.23 and 0.20. K1/2,ex and K1/2,net (mM), respectively, was: (RBC) 7.5 and 1.3, (G) 4.8 and 14.2, (G +) 11.6 and 6.8, (OC) 3.8 and 9.0. In ghosts, the ATP-dependent fraction of the permeability shows a hyperbolic dependence on glucose concentrations lower than 80 mM. Insulin up to 1 microM had effect on neither Jex nor Jnet in RBC. Based on reported values of cytochalasin B binding sites the turnover rate per site in RBC appears to be as high as in maximally insulin-stimulated fat cells. Our results suggest that the number of transport sites remains constant, independent of age, ATP and insulin.

Glucose transporters in isolated chromaffin cells. Effects of insulin and secretagogues

1. Isolated chromaffin cells from bovine adrenal medulla were used to study glucose transport in a homogeneous neural tissue. 2. The affinity of glucose transporters was 1.20 +/- 0.52 mM by the infinite-cis technique and 1.02 +/- 0.09 mM by the direct transport experiments. 3. The affinity for 2-deoxyglucose of these transporters was 2.3 mM. 4. The glucose transporters, quantified by [3H]cytochalasin B binding, were 419,532 +/- 120,740 receptors/cell, which corresponds to about 7.2 +/- 2 pmol/mg of protein, with KD = 0.1 microM. 5. High-affinity insulin receptors with KD = 3.95 nM were present at a density of 68,400 +/- 7500 per cell. 6. Insulin and secretagogues increased glucose transport, raising the transporter number at the plasma membrane without changes in the affinity.

Kinetics of glucose transport in human erythrocytes: zero-trans efflux and infinite-trans efflux at 0 degree C

The kinetic features of glucose transport in human erythrocytes have been the subject of many studies, but no model is consistent with both the kinetic observations and the characteristics of the purified transporter. In order to reevaluate some of the kinetic features, initial rate measurements were performed at 0 degree C. The following kinetic parameters were obtained for fresh blood: zero-trans efflux Km = 3.4 mM, Vmax = 5.5 mM/min; infinite-trans efflux Km = 8.7 mM, Vmax = 28 mM/min. For outdated blood, somewhat different parameters were obtained: zero-trans efflux Km = 2.7 mM, Vmax = 2.4 mM/min; infinite-trans efflux Km = 19 mM, Vmax = 23 mM/min. The Km values for fresh blood differ from the previously reported values of 16 mM and 3.4 mM for zero-trans and infinite-trans efflux, respectively (Baker, G.F. and Naftalin, R.J. (1979) Biochim. Biophys. Acta 550, 474-484). The use of 50 mM galactose rather than 100 mM glucose as the infinite-trans sugar produced no change in the infinite-trans efflux Km values but somewhat lower Vmax values. Simulations indicate that initial rates were closely approximated by the experimental conditions. The observed time courses of efflux are inconsistent with a model involving rate-limiting dissociation of glucose from hemoglobin (Naftalin, R.J., Smith, P.M. and Roselaar, S.E. (1985) Biochim. Biophys. Acta 820, 235-249). The results presented here support the adequacy of the carrier model to account for the kinetics.

Exofacial photoaffinity labelling of the human erythrocyte sugar transporter

The 4-azidosalicylate derivative of 1,3-bis(D-mannos-4'-yloxy)-2-[2-3H]propylamine (ASA-[2-3H]BMPA) has been tested as a photoaffinity label for the sugar transporter in human erythrocytes. When photolysed in the presence of intact erythrocytes, ASA-[2-3H]BMPA covalently binds to the exofacial surface of the transporter. This labelled protein appears as a broad band in the 4.5 region in sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. The peak of radiolabel incorporation gives an apparent Mr of approx. 50 000 on 5-20% acrylamide gels. The binding is 80% inhibitable by 320 mM 4,6-O-ethylidene-D-glucose, by 320 mM D-glucose and by 50 microM cytochalasin B. Photoirradiation of a saturating concentration of ASA-BMPA in the presence of erythrocytes results in a 25-30% loss of D-galactose transport activity. From transport inactivation data and estimations of the amount of ASA-[2-3H]BMPA binding to the transporter it is calculated that there are approx. 220 000 exofacial hexose-transport binding sites per erythrocyte. The labelling of the transporter has been carried out using freshly drawn blood and 4-weeks-old transfusion blood. No change in the binding profile on SDS-polyacrylamide gel electrophoresis was observed. Proteolytic digestion of the ASA-[2-3H]BMPA-labelled transporter with either trypsin or alpha-chymotrypsin results in the appearance of a labelled 19 kDa fragment on SDS-polyacrylamide gel electrophoresis.

Evidence for non-uniform distribution of D-glucose within human red cells during net exit and counterflow

The kinetic parameters of net exit of D-glucose from human red blood cells have been measured after the cells were loaded to 18 mM, 75 mM and 120 mM at 2 degrees C and 75 mM and 120 mM at 20 degrees C. Reducing the temperature, or raising the loading concentration raises the apparent Km for net exit. Deoxygenation also reduces the Km for D-glucose exit from red blood cells loaded initially to 120 mM at 20 degrees C from 32.9 +/- 2.3 mM (13) with oxygenated blood to 20.5 +/- 1.3 mM (17) (P less than 0.01). Deoxygenation increases the ratio Vmax/Km from 5.29 +/- 0.26 min-1 (13) for oxygenated blood to 7.13 +/- 0.29 min-1 (17) for deoxygenated blood (P less than 0.001). The counterflow of D-glucose from solutions containing 1 mM 14C-labelled D-glucose was measured at 2 degrees C and 20 degrees C. Reduction in temperature, reduced the maximal level to which labelled D-glucose was accumulated and altered the course of equilibration of the specific activity of intracellular D-glucose from a single exponential to a more complex form. Raising the internal concentration from 18 mM to 90 mM at 2 degrees C also alters the course of equilibration of labelled D-glucose within the cell to a complex form. The apparent asymmetry of the transport system may be estimated from the intracellular concentrations of labelled and unlabelled sugar at the turning point of the counterflow transient. The estimates of asymmetry obtained from this approach indicate that there is no significant asymmetry at 20 degrees C and at 2 degrees C asymmetry is between 3 and 6. This is at least 20-fold less than predicted from the kinetic parameter asymmetries for net exit and entry. None of the above results fit a kinetic scheme in which the asymmetry of the transport system is controlled by intrinsic differences in the kinetic parameters at the inner and outer membrane surface. These results are consistent with a model for sugar transport in which movement between sugar within bound and free intracellular compartments can become the rate-limiting step in controlling net movement into, or out of the cell.

A kinetic analysis of hexose transport in cultured human lymphocytes (IM-9)

3-O-Methyl-D-glucose transport across the plasma membrane of cultured human lymphocytes of the IM-9 line was followed for net entry into sugar-free cells (zero trans entry), net exit into sugar-free medium (zero trans exit) and for equilibration of labelled sugar in cells with the same sugar concentration in the intracellular water as in the medium (equilibrium exchange). The measurements were performed at 37 degrees C (pH 7.4). Equilibrium exchange of 1 mM 3-O-methylglucose (t 1/2 about 7 S) was exponential, suggesting a homogeneous cell suspension. Initial rates of transport showed a Michaelis-Menten dependency on the sugar concentration. The transport system was found to be asymmetric with the following kinetic parameters. Zero trans entry: Km = 2.8 mM, Vmax = 10.7 mM X min-1. Zero trans exit: Km = 9.5 mM, Vmax = 37.9 mM X min-1. Equilibrium exchange: Km = 9.9 mM, Vmax = 44.0 mM X min-1. Finally, the affinity constant for the internal site was measured as approx. 1.2 mM using the infinite cis protocol.

A new class of sugar analogues for use in the investigation of sugar transport

D-Mannose derivatives have been synthesised which are crosslinked through their C-4 hydroxyls to propyl-2-amine. Coupling to the amino group gave a fluorodinitrobenzene derivative, a nitroazidophenyl derivative and an azidosalicylamide derivative. Each of these derivatives was shown to have high affinity for the human erythrocyte sugar transport system. The affinity constant for the nitroazidophenyl derivative was not altered by temperature changes. In rat adipocytes treated with insulin, the affinity constants for the derivatives were up to 1000-fold lower than for the parent sugar. In the absence of insulin the affinity constants for the derivatives, but not for D-mannose, were 3-times higher than in insulin-treated cells. By preparation of radiolabelled derivatives we have shown that the compounds are not transported either by erythrocytes or by adipocytes. Thus the crosslinked sugars are good outside-specific analogues.

Kinetics of nucleoside transport in human erythrocytes. Alterations during blood preservation

The transmembrane equilibration of radiolabeled uridine was measured by rapid kinetic techniques in human erythrocytes from freshly drawn blood and in the same cells during conventional storage of the blood as well as in cells from outdated blood. Our results confirm earlier reports that the maximum velocity of uridine equilibrium exchange (Vee) at 25 degrees C is about 30% lower in outdated than fresh red cells, whereas the opposite is the case for the Michaelis-Menten constant for equilibrium exchange (Kee), and that maximum zero-trans efflux (Vzt21) is about 4-times greater than maximum zero-trans influx (Vzt12) in outdated cells (directional asymmetry), whereas they are about the same in fresh red cells. At 25 degrees C, the nucleoside-loaded carrier of fresh cells moves on the average 6-times more rapidly than the empty carrier, whereas the differential mobility of loaded and empty carrier from outdated cells is about 15-fold. Our results also show that greater efflux than influx in outdated cells is not due to a general leakiness of outdated cells, that the differences in kinetic properties of the transporter developed during the first two weeks of blood storage and that the differences are greatly amplified when transport is measured at 5 degrees C rather than 25 degrees C. At 5 degrees C, the loaded carrier from outdated red cells moves about 325-times more rapidly than the empty carrier and maximum zero-trans efflux exceeds maximum zero-trans influx about 14-times, whereas the transport of fresh cells exhibits directional symmetry just as at 25 degrees C. The changes in kinetic properties of transport induced by temperature and storage are probably related to structural alterations in the plasma membrane and suggest that the operation of carrier is subject to modification by the membrane environment. Other results show that the kinetics of the sugar transport of human red cells is not affected in the same manner by blood storage as those of the nucleoside transporter.