Infinite-cis kinetics support the carrier model for erythrocyte glucose transport

Structural comparison of GLUT1 to GLUT3 reveal transport regulation mechanism in sugar porter family

The human glucose transporters GLUT1 and GLUT3 have a central role in glucose uptake as canonical members of the Sugar Porter (SP) family. GLUT1 and GLUT3 share a fully conserved substrate-binding site with identical substrate coordination, but differ significantly in transport affinity in line with their physiological function. Here, we present a 2.4 Å crystal structure of GLUT1 in an inward open conformation and compare it with GLUT3 using both structural and functional data. Our work shows that interactions between a cytosolic "SP motif" and a conserved "A motif" stabilize the outward conformational state and increases substrate apparent affinity. Furthermore, we identify a previously undescribed Cl^- ion site in GLUT1 and an endofacial lipid/glucose binding site which modulate GLUT kinetics. The results provide a possible explanation for the difference between GLUT1 and GLUT3 glucose affinity, imply a general model for the kinetic regulation in GLUTs and suggest a physiological function for the defining SP sequence motif in the SP family.

Extracellular gating of glucose transport through GLUT 1

The ubiquitous glucose transporter 1 (GLUT1) is physiologically and pathologically relevant in energy metabolism of the CNS, skeletal muscles, cancer cells etc. Extensive experiments on GLUT1 produced thorough understandings of its expressions, functions, and structures which were recently resolved to atomic accuracy. However, theoretical understandings are still controversial about how GLUT1 facilitates glucose diffusion across the cell membrane. Molecular dynamics (MD) simulations of the current literature have GLUT1 embedded in a symmetric bilayer of a single lipid type. They provide atomistic illustrations of the alternating access theory (AAT), but the simulation results are inconsistent with the undisputed experimental data of kinetics showing rapid transport of glucose at near-physiological temperatures, high Arrhenius activation barrier in zero-trans uptake, and large trans-acceleration at sub-physiological temperatures. In this research, we embedded GLUT1 in an asymmetric bilayer of multiple lipids to better mimic the erythrocyte membrane. We ran unbiased MD simulations at 37 °C and at 5 °C and found a new mechanism of glucose transport via GLUT1: The extracellular (EC) gate opened wide for EC glucopyranose at 37 °C and, only in the presence of intracellular (IC) glucose, at 5 °C. In the absence of IC glucose at 5 °C, the EC gate opened narrowly for acyclic glucose, gating out glucopyranose. This EC-gating mechanism is simpler than AAT and yet it well explains for the rapid glucose transport at near-physiological temperatures and large trans-acceleration at sub-physiological temperatures. It also explains why zero-trans uptake (involving the pyranose-to-aldehyde transformation) has an Arrhenius barrier ∼20 kcal/mol higher than the equilibrium exchange transport.

The SLC2 (GLUT) family of membrane transporters

GLUT proteins are encoded by the SLC2 genes and are members of the major facilitator superfamily of membrane transporters. Fourteen GLUT proteins are expressed in the human and they are categorized into three classes based on sequence similarity. All GLUTs appear to transport hexoses or polyols when expressed ectopically, but the primary physiological substrates for several of the GLUTs remain uncertain. GLUTs 1-5 are the most thoroughly studied and all have well established roles as glucose and/or fructose transporters in various tissues and cell types. The GLUT proteins are comprised of ∼500 amino acid residues, possess a single N-linked oligosaccharide, and have 12 membrane-spanning domains. In this review we briefly describe the major characteristics of the 14 GLUT family members.

The SLC2 (GLUT) family of membrane transporters

GLUT proteins are encoded by the SLC2 genes and are members of the major facilitator superfamily of membrane transporters. Fourteen GLUT proteins are expressed in the human and they are categorized into three classes based on sequence similarity. All GLUTs appear to transport hexoses or polyols when expressed ectopically, but the primary physiological substrates for several of the GLUTs remain uncertain. GLUTs 1-5 are the most thoroughly studied and all have well established roles as glucose and/or fructose transporters in various tissues and cell types. The GLUT proteins are comprised of ∼500 amino acid residues, possess a single N-linked oligosaccharide, and have 12 membrane-spanning domains. In this review we briefly describe the major characteristics of the 14 GLUT family members.

The SLC2 (GLUT) family of membrane transporters

GLUT proteins are encoded by the SLC2 genes and are members of the major facilitator superfamily of membrane transporters. Fourteen GLUT proteins are expressed in the human and they are categorized into three classes based on sequence similarity. All GLUTs appear to transport hexoses or polyols when expressed ectopically, but the primary physiological substrates for several of the GLUTs remain uncertain. GLUTs 1-5 are the most thoroughly studied and all have well established roles as glucose and/or fructose transporters in various tissues and cell types. The GLUT proteins are comprised of ∼500 amino acid residues, possess a single N-linked oligosaccharide, and have 12 membrane-spanning domains. In this review we briefly describe the major characteristics of the 14 GLUT family members.

Will the original glucose transporter isoform please stand up!

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

Model of the exofacial substrate-binding site and helical folding of the human Glut1 glucose transporter based on scanning mutagenesis

Transmembrane helix 9 of the Glut1 glucose transporter was analyzed by cysteine-scanning mutagenesis and the substituted cysteine accessibility method (SCAM). A cysteine-less (C-less) template transporter containing amino acid substitutions for the six native cysteine residues present in human Glut1 was used to generate a series of 21 mutant transporters by substituting each successive residue in predicted transmembrane segment 9 with a cysteine residue. The mutant proteins were expressed in Xenopus oocytes, and their specific transport activities were directly compared to that of the parental C-less molecule whose function has been shown to be indistinguishable from that of native Glut1. Only a single mutant (G340C) had activity that was reduced (by 75%) relative to that of the C-less parent. These data suggest that none of the amino acid side chains in helix 9 is absolutely required for transport function and that this helix is not likely to be directly involved in substrate binding or translocation. Transport activity of the cysteine mutants was also tested after incubation of oocytes in the presence of the impermeant sulfhydryl-specific reagent, p-chloromercuribenzene sulfonate (pCMBS). Only a single mutant (T352C) exhibited transport inhibition in the presence of pCMBS, and the extent of inhibition was minimal (11%), indicating that only a very small portion of helix 9 is accessible to the external solvent. These results are consistent with the conclusion that helix 9 plays an outer stabilizing role for the inner helical bundle predicted to form the exofacial substrate-binding site. All 12 of the predicted transmembrane segments of Glut1 encompassing 252 amino acid residues and more than 50% of the complete polypeptide sequence have now been analyzed by scanning mutagenesis and SCAM. An updated model is presented for the outward-facing substrate-binding site and relative orientation of the 12 transmembrane helices of Glut1.

A General Channel Model Accounts for Channel, Carrier, Countertransport and Cotransport Kinetics

In this work we propose a unifying model of mediated membrane transport, based upon the idea that the integral membrane proteins involved in these processes operate via complex channel mechanisms. In the first part, we briefly review literature about the structural aspects of membrane transporters. We conclude that there is a substantial amount of evidence suggesting that most membrane proteins performing transport are embodied with channel-like structures that may constitute the translocation paths. This includes cases where the phenomenological transport kinetics do not correspond to the classical channel behavior. In the second part of this article we introduce the general channel model of mediated transport and employ it to derive specific examples, like simple one- or two-ligand channels, water-ligand channels, simple carriers, co- and counter-transport systems and more complex water-ligand carriers. We show that, for the most part, these particular cases can be obtained by the application of the techniques of diagram reduction to the full model. The necessary conditions for diagram reduction reflect physical properties of the protein and its surroundings.

Relationship Between Glucose Transport and Metabolism in Isolated Bovine Mammary Epithelial Cells

Glucose transport by isolated bovine mammary epithelial cells involves translocation across the cell membrane into a compartment that exchanges slowly with the bulk cytosol. The significance to glucose metabolism of this compartmentalization was examined by generation, modeling, and analysis of transport and metabolism data. Net uptake of 5 mM 3-O-methyl-d-glucose by isolated bovine mammary epithelial cells was measured at 37 degrees C. Time-course curves were better fitted by a double exponential equation than a single exponential equation and were subjected to compartmental analysis to obtain glucose transport model parameters. Lactose synthesis and glucose oxidation rates and cellular concentrations of intermediary metabolites, glucose-6-phosphate and glucose-1-phosphate, were measured at varied media glucose concentrations. A model that integrates both glucose transport and metabolism under-predicted the rates of lactose synthesis and glucose oxidation by a factor of 3. To account for the observed glucose use rates, glucose must be available for phosphorylation once translocated across the cell membrane (intermediate compartmentalization of translocated glucose does not exclude access to hexokinase). Metabolic control analysis indicated that, at physiological glucose concentrations, phosphorylation by hexokinase exerts 80% of the control of glucose metabolism to lactose and CO(2), and transport exerts the remaining 20%.

Conventional transport assays underestimate sugar transport rates in human red cells

The time course of protein-mediated 3-O-methylglucose uptake by human red cells and by red cell ghosts containing or lacking 4 mM MgATP was measured at ice temperature and sub-saturating sugar levels by conventional sampling procedures and at 20 degrees C by use of a quench-flow apparatus. The temporal resolution of the quench-flow apparatus (as fast as 5-ms sample times) was confirmed by analysis of alkaline hydrolysis of dinitrophenolacetate. Red cell sugar uptake at 4 degrees C is consistent with two processes [fast (tau = 120 s) and slow (tau = 1100 s)] that occur in series. Intracellular ATP increases the size and the rate of equilibration of the fast compartment and slows the rate of filling of the slow compartment. Red cell ghost volume and protein content are unaffected by lysis/resealing in the presence of ATP. Uptake at 20 degrees C is also consistent with two processes [fast (tau = 10 ms) and slow (tau = 15 s)] that occur in series. ATP increases the size of both compartments and the rate of filling of the small compartment at 20 degrees C. Preliminary estimates indicate that the sugar uptake capacity of human red cells at 20 degrees C is underestimated by as much as 8-fold by measuring sugar uptake over 2 s vs. 26 ms. We discuss the implications of multiphasic sugar uptake in the context of models for protein-mediated sugar transport.

Description of glucose transport in isolated bovine mammary epithelial cells by a three-compartment model

Initial rates of glucose entry into isolated bovine mammary epithelial cells display moderate degrees of asymmetry and cooperative interactions between export and import sites. The present study examined the hypothesis that these kinetic features are due to compartmentalization of intracellular glucose. Net uptake of 3- O-methyl-d-[1-3H]glucose (3-OMG) by isolated bovine mammary epithelial cells was measured at 37°C. The time course of 3-OMG net uptake was better fitted by a double-exponential equation than by a single- or triple-exponential equation. Compartmental analysis of the time course curve suggested that translocated 3-OMG is distributed into two compartments with fractional volumes of 32.6 ± 5.7% and 67.4 ± 5.7%, respectively. The results support the view that glucose transport in bovine mammary epithelial cells is a multistep process consisting of two serial steps: fast, carrier-mediated, symmetric translocation of sugar across the cell plasma membrane into a small compartment and subsequent slow exchange of posttranslocated sugar between two intracellular compartments. A three-compartment model of this system successfully simulated the observed time course of 3-OMG net uptake and the observed dependence of unidirectional entry rates on intra- and extracellular 3-OMG concentrations. Simulations indicated that backflux of radiolabeled sugar from the small compartment to extracellular space during 15 s of incubation gives rise to the apparent asymmetry, trans-stimulation, and cooperativity of mammary glucose transport kinetics. The fixed-site carrier model overestimated the rate of glucose accumulation in cells, and its features can be accounted for by the compartmentalization of intracellular sugar.

Glucose transporter in bovine mammary epithelial cells is an asymmetric carrier that exhibits cooperativity and trans-stimulation

Glucose transport kinetics were quantified in isolated bovine mammary epithelial cells using 3-O-methyl-D-glucose. Isolated cells retained satisfactory viability and glucose uptake activity, which was inhibited by cytochalasin B, phloretin, HgCl2, and low temperature. Initial rates of entry were measured over a 15-s interval at 37 degrees C under zero-trans, equilibrium-exchange, high-cis, and high-trans concentrations of 3-O-methyl-D-glucose between 0 and 20 mM. The combined set of rate measurements from all experimental conditions was fit to the fixed-site carrier model by nonlinear regression to estimate parameters of transport. For the regression between predicted and observed initial rates, r2 was 0.97. Forward Vmax was estimated at 18.2 nmol.min-1.mg protein-1, and the Michaelis constant was 8.29 mM. The cooperativity parameter was 1.63, trans-stimulation was 2.13-fold, and asymmetry was 2.06-fold. On the basis of the kinetic parameters, variations in intracellular glucose concentrations are not responsible for the range of glucose uptakes by bovine mammary glands observed in vivo.

Overview of membrane transport

Pharmaceutical scientists increasingly utilize transporters for drug delivery and targeting. The biological barriers to drug delivery can basically be divided into epithelial, endothelial, elimination, and target cell barriers. Membrane transporters play an important role in drug entrance and exit from the body. In addition, it is possible to utilize transporters for drug delivery, e.g., improving oral absorption via the peptide transporter. Identification, a better understanding of their transport characteristics, and the regulation of the membrane transporters will allow the development of better drug delivery strategies.

Synthesis of O-methylsulfonyl derivatives of D-glucose as potential alkylating agents for targeted drug delivery to the brain. Evaluation of their interaction with the human erythrocyte GLUT1 hexose transporter

In order to obtain hydrophilic analogues of 1,4-dimethylsulfonyloxybutane (busulfan) with enhanced selectivity and improved brain penetration, we have synthesized 6-O-methylsulfonyl-D-glucose, 3-O-methylsulfonyl-D-glucose, 3,6-di-O-methylsulfonyl-D-glucose, 4-O-methylsulfonyl-D-glucose, and 4,6-di-O-methylsulfonyl-D-glucose, and we have studied their interactions with the human erythrocyte GLUT1 hexose transport system. Mesylation of OH-4 and OH-6 of glucose resulted in a slightly diminished affinity for the GLUT1 glucose transporter, whereas mesylation of OH-3 led to complete loss of affinity.

Synthesis of glucose-chlorambucil derivatives and their recognition by the human GLUT1 glucose transporter

A limitation of the use of chemotherapeutic agents against intracerebral tumors lies on their poor uptake into the central nervous system. An approach to enhance brain delivery is to design agents that are transported into the brain by one of the saturable nutrient carriers of the blood-brain barrier, the highly efficient brain and erythrocyte glucose transporter isoform GLUT1. Since the GLUT1 hexose transporter of the blood-brain barrier is also present on erythrocytes, new compounds designed to be transported by the GLUT1 transporter were studied on human erythrocytes, which represent unique, easily accessible human GLUT1 expressing cells. In this paper we describe the synthesis of four glucose-chlorambucil derivatives, namely methyl 6-O-4[bis(2-chloroethyl)amino]benzenebut anoyl-beta-D-glucopyranosi de (3), 6-O-4-[bis(2-chloroethyl)amino]benzenebu tanoyl-D-glucopyranose (6), methyl 6-[4-[bis(2-chloroethyl)amino]benzenebut anoylamido]-6-deoxy-beta-D-glucopyranoside (9) and 6-[4-[bis(2-chloroethyl)amino]benzenebut anoyl amido]-6-deoxy-D-glucopyranose (10), and the study of their interactions with the GLUT1 transporter of the human erythrocytes. All four compounds were able to inhibit [14C]glucose uptake in a concentration-dependent manner. One of them, compound 6, exhibited an approximately 160-fold higher inhibition of [14C]glucose uptake by the GLUT1 transporter than glucose itself. Compound 6 was also able to inhibit [3H]cytochalasin B binding to erythrocytes with approximately 1000-fold higher efficacy than does glucose. The inhibition of glucose uptake was entirely reversible, indicating that it was not due to alkylation of a nucleophilic group of the hexose transporter. The above results suggested specific interactions of compound 6 with the hexose transporter protein. Uptake studies of [14C]compound 6 indicated, in addition, some non-specific interactions with intact and open erythrocyte membranes: only a small amount of the bound [14C]compound 6 can be displaced by cytochalasin B. Collectively, these findings led us to conclude that the interactions of compound 6 with GLUT1 are presumably that of a non-transported inhibitor.

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.

Accelerated net efflux of 3-O-methylglucose from rat adipocytes: a reevaluation

In a study of 3-O-methylglucose transport in insulin-stimulated rat adipocytes (catalyzed primarily by the GLUT4 isoform), it was reported that at 37 degrees C the Km and Vmax were 2.8-fold higher for net efflux than for equilibrium exchange (Vinten, J. (1984) Biochim. Biophys. Acta 772, 244-250). Because of its implications for the relative sizes of steps in the transport cycle, we reinvestigated this phenomenon. Accelerated net efflux was apparent when the extracellular methylglucose was diluted 26-fold but not when it was diluted 11-fold. When analyzed according to the one-site alternating conformation model, the data indicate about a 1.7-fold higher Vmax for efflux than for exchange, only about 40% of the difference reported previously. Together with other results in the literature, the accelerated net flux indicates that the conformational change of the loaded transporter from its outward-facing to its inward-facing form is likely the slowest step in the transport cycle, in contrast to the case for GLUT1. Experiments at 25 degrees C indicate a lower degree of accelerated net flux than at 37 degrees C. This is consistent with the above conformational change being the step with the lowest activation energy, as for GLUT1.

Facilitative glucose transporters

Facilitative glucose transport is mediated by members of the Glut protein family that belong to a much larger superfamily of 12 transmembrane segment transporters. Six members of the Glut family have been described thus far. These proteins are expressed in a tissue- and cell-specific manner and exhibit distinct kinetic and regulatory properties that reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement. It also plays a special role in transporting glucose across epithelial and endothelial barrier tissues. Glut2 is a high-Km isoform expressed in hepatocytes, pancreatic beta cells, and the basolateral membranes of intestinal and renal epithelial cells. It acts as a high-capacity transport system to allow the uninhibited (non-rate-limiting) flux of glucose into or out of these cell types. Glut3 is a low-Km isoform responsible for glucose uptake into neurons. Glut4 is expressed exclusively in the insulin-sensitive tissues, fat and muscle. It is responsible for increased glucose disposal in these tissues in the postprandial state and is important in whole-body glucose homeostasis. Glut5 is a fructose transporter that is abundant in spermatozoa and the apical membrane of intestinal cells. Glut7 is the transporter present in the endoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphatase on glucose 6-phosphate. This review summarizes recent advances concerning the structure, function, and regulation of the Glut proteins.

Determination of the intravesicular ionized sodium concentration in a cell-free brain membrane vesicle preparation using the fluorescent indicator, SBFI

The intravesicular ionized Na concentration (Nai) was measured using the fluorescent Na indicator, SBFI, in microsacs, a cell-free brain vesicle preparation. SBFI fluorescence was monitored by a dual excitation-wavelength method at the same wavelengths commonly employed for Fura-2 determination of intracellular ionized calcium concentrations (Cai). Calibration of SBFI fluorescence was reliably performed in brain microsacs in situ. Resting Nai was dependent on the extravesicular Na concentration (Nao) and was about 36 mM in the presence of 120 mM extracellular Nao. In the presence of ouabain, an inhibitor of the plasma membrane Na/K-ATPase, Nai increased by 27 mM over 60 s. Nai was also increased by resuspension of microsacs in buffers of low free Ca concentrations (0 to 0.8 mM), indicating that the extravesicular Ca concentration (Cao) is an important regulator of Nai. Alkaloids active at voltage-sensitive Na channels, veratridine and aconitine, also increased Nai. These results demonstrate the presence of homeostatic mechanisms for neuronal Nai regulation and show that Nai can be measured in a cell-free brain vesicle preparation using SBFI.

Mechanisms for the facilitated diffusion of substrates across cell membranes

Two classes of theoretical mechanisms for protein-mediated, passive, transmembrane substrate transport (facilitated diffusion) are compared. The simple carrier describes a carrier protein that exposes substrate influx and efflux sites alternately but never both sites simultaneously. Two-site models for substrate transport describe carrier proteins containing influx and efflux sites simultaneously. Velocity equations describing transport by these mechanisms are derived. These equations take the same general form, being characterized by five experimental constants. Simple carrier-mediated transport is restricted to hyperbolic kinetics under all conditions. Two-site carrier-mediated transport may deviate from hyperbolic kinetics only under equilibrium exchange conditions. When both simple- and two-site carriers display hyperbolic kinetics under equilibrium exchange conditions, these models are indistinguishable by using steady-state transport data alone. Seven sugar transport systems are analyzed. Five of these systems are consistent with both models for sugar transport. Uridine, leucine, and cAMP transport by human red cells are consistent with both simple- and two-site models for transport. Human erythrocyte sugar transport can be modeled by simple- and two-site carrier mechanisms, allowing for compartmentalization of intracellular sugars. In this instance, resolution of the intrinsic properties of the human red cell sugar carrier at 20 degrees C requires the use of submillisecond transport measurements.

Glucose transport kinetics in human red blood cells

D-[14C]Glucose self exchange and unidirectional efflux from human red blood cells were studied at 20 degrees C (pH 7.2) by means of the Millipore-Swinnex filtering technique whose time resolution is greater than 1 s and the continuous flow-tube method with a time resolution of greater than 2 ms. The unidirectional efflux data were analyzed using both the method of initial rates and the integrated rate equation. Simple Michaelis-Menten kinetics apply to the results obtained under both experimental conditions. In self-exchange mode, the half-saturation constant, K1/2ex, was 10 (S.E. +/- 1) mM. In unidirectional efflux mode K1/2ue was 6.6 (S.E. +/- 0.5) mM (initial rates) or by the method of integrated rates 7.7 mM, with a range of 2.7-12.1 mM, K1/2ue increasing with an increased initial intracellular glucose concentration. Our results of K1/2ex oppose previous published values of 32 mM for self exchange (Eilam and Stein (1972) Biochim. Biophys. Acta 266, 161-173) and 25 mM for unidirectional efflux (Karlish et al. (1972) Biochim. Biophys. Acta 255, 126-132) that have been used extensively in kinetic considerations of glucose transport models. Under self-exchange conditions Jmaxex was 1.8 x 10(-10) mol cm-2s-1, and in unidirectional efflux mode Jmaxue was 8.3 x 10(-11) mol cm-2s-1 (initial rates) and 8.6 x 10(-11) mol cm-2s-1 (integrated rates). We suggest that the previous high values of Jmax and in particular K1/2 are due to the use of methods with insufficient time resolution. Our results indicate that the transport system is less asymmetric than was generally accepted, and that complicated transport models developed to account for the great difference between the determined K1/2 and J max values are redundant.

Ligand-dependent quenching of tryptophan fluorescence in human erythrocyte hexose transport protein

D-Glucose transport by the 492-residue human erythrocyte hexose transport protein may involve ligand-mediated conformational/positional changes. To examine this possibility, hydrophilic quencher molecules [potassium iodide and acrylamide (ACR)] were used to monitor the quenching of the total protein intrinsic fluorescence exhibited by the six protein tryptophan (Trp) residues in the presence and absence of substrate D-glucose, and in the presence of the inhibitors maltose and cytochalasin B. Protein fluorescence was found to be quenched under various conditions, ca. 14-24% by KI and ca. 25-33% by ACR, indicating that the bulk of the Trp residue population occurs in normally inaccessible hydrophobic regions of the erythrocyte membrane. However, in the presence of D-glucose, quenching by KI and ACR decreased an average of -3.4% and -4.4%, respectively; Stern-Volmer plots displayed decreased slopes in the presence of D-glucose, confirming the relatively reduced quenching. In contrast, quenching efficiency increased in the presence of maltose (+5.9%, +3.3%), while addition of cytochalasin B had no effect on fluorescence quenching. The overall results are interpreted in terms of ligand-activated movement of an initially aqueous-located protein segment containing a Trp residue into, or toward, the cellular membrane. Relocation of this segment, in effect, opens the D-glucose channel; maltose and cytochalasin B would thus inhibit transport by mechanisms which block this positional change. Conformational and hydropathy analyses suggested that the region surrounding Trp-388 is an optimal "dynamic segment" which, in response to ligand activation, could undergo the experimentally deduced aqueous/membrane domain transfer.

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)

Proteolytic dissection as a probe of conformational changes in the human erythrocyte glucose transport protein

Tryptic digestion has been used to investigate the conformational changes associated with substrate translocation by the human erythrocyte glucose transporter. The effects of substrates and inhibitors of transport on the rates of tryptic cleavage at the cytoplasmic surface of the membrane have confirmed previous observations that this protein can adopt at least two conformations. In the presence of phloretin or 4,6-O-ethylidene-D-glucose, the rate of cleavage is slowed. Because these inhibitors bind preferentially at the extracellular surface of the transporter, their effects must result from a conformational change rather than from steric hindrance. A conformational change must also be responsible for the effect of the physiological substrate D-glucose, which is to increase the rate of cleavage. The regions of the protein involved in the conformational changes include both of the large cytoplasmic regions that are cleaved by trypsin: these are the central hydrophilic region of the sequence (residues 213-269) and the hydrophilic C-terminal region (residues 457-492).