Voltage and substrate dependence of the inverse transport mode of the rabbit Na(+)/glucose cotransporter (SGLT1)

SSM-based electrophysiology, a label-free real-time method reveals sugar binding & transport events in SGLT1

Here, we present a solid-supported membrane (SSM)-based electrophysiological approach to study sugar binding and Na⁺/glucose cotransport by SGLT1 in membrane vesicles. SSM-based electrophysiology delivers a cumulative real-time current readout from numerous SGLT1 proteins simultaneously using a gold-coated sensor chip. In contrast to conventional techniques, which mainly operate with voltage steps, currents are triggered by sugar or sodium addition. Sugar concentration jumps in the presence of sodium lead to transport currents between 5 and 10 nA. Remarkably, in the absence of sodium (i.e. no transport), we observed fast pre-steady-state (PSS) currents with time constants between 3 and 10 ms. These PSS currents mainly originate from sugar binding. Sodium binding does not induce PSS currents. Due to high time resolution, PSS currents were distinguished from transport and eventually correlated with conformational transitions within the sugar translocation pathway. In addition, we analyzed the impact of driving forces on transport and binding currents, showing that membrane voltage and sodium concentration gradients lead to an increased transport rate without affecting sugar binding kinetics. We also compared Na⁺/sugar efflux with physiologically relevant influx and found similar transport rates, but lower affinity in efflux mode. SSM-based electrophysiology is a powerful technique, which overcomes bottlenecks for transport measurements observed in other techniques such as the requirement of labels or the lack of real-time data. Rapid solution exchange enables the observation of substrate-induced electrogenic events like conformational transitions, opening novel perspectives for in-depth functional studies of SGLT1 and other transporters.

Stochastic steps in secondary active sugar transport

Secondary active transporters, such as those that adopt the leucine-transporter fold, are found in all domains of life, and they have the unique capability of harnessing the energy stored in ion gradients to accumulate small molecules essential for life as well as expel toxic and harmful compounds. How these proteins couple ion binding and transport to the concomitant flow of substrates is a fundamental structural and biophysical question that is beginning to be answered at the atomistic level with the advent of high-resolution structures of transporters in different structural states. Nonetheless, the dynamic character of the transporters, such as ion/substrate binding order and how binding triggers conformational change, is not revealed from static structures, yet it is critical to understanding their function. Here, we report a series of molecular simulations carried out on the sugar transporter vSGLT that lend insight into how substrate and ions are released from the inward-facing state of the transporter. Our simulations reveal that the order of release is stochastic. Functional experiments were designed to test this prediction on the human homolog, hSGLT1, and we also found that cytoplasmic release is not ordered, but we confirmed that substrate and ion binding from the extracellular space is ordered. Our findings unify conflicting published results concerning cytoplasmic release of ions and substrate and hint at the possibility that other transporters in the superfamily may lack coordination between ions and substrate in the inward-facing state.

Electrophysiological characterization of membrane transport proteins

Active transport in biological membranes has been traditionally studied using a variety of biochemical and biophysical techniques, including electrophysiology. This review focuses on aspects of electrophysiological methods that make them particularly suited for the investigation of transporter function. Two major approaches to electrical recording of transporter activity are discussed: (a) artificial planar lipid membranes, such as the black lipid membrane and solid supported membrane, which are useful for studies on bacterial transporters and transporters of intracellular compartments, and (b) patch clamp and voltage clamp techniques, which investigate transporters in native cellular membranes. The analytical power of these methods is highlighted by several examples of mechanistic studies of specific membrane proteins, including cytochrome c oxidase, NhaA Na(+)/H(+) exchanger, ClC-7 H(+)/Cl(-) exchanger, glutamate transporters, and neutral amino acid transporters. These examples reveal the wealth of mechanistic information that can be obtained when electrophysiological methods are used in combination with rapid perturbation approaches.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Effect of substrate on the pre-steady-state kinetics of the Na(+)/glucose cotransporter

When measuring Na(+)/glucose cotransporter (SGLT1) activity in Xenopus oocytes with the two-electrode voltage-clamp technique, pre-steady-state currents dissipate completely in the presence of saturating alpha-methyl-glucose (alphaMG, a nonhydrolyzable glucose analog) concentrations. In sharp contrast, two SGLT1 mutants (C255A and C511A) that lack a recently identified disulfide bridge express the pre-steady-state currents in the presence of alphaMG. The dose-dependent effects of alphaMG on pre-steady-state currents were studied for wild-type (wt) SGLT1 and for the two mutants. Increases in alphaMG concentration reduced the total transferred charge (partially for the mutants, totally for wt SGLT1), shifted the transferred charge versus membrane potential (Q-V) curve toward positive potentials, and significantly modified the time constants of the pre-steady-state currents. A five-state kinetic model is proposed to quantitatively explain the effect of alphaMG on pre-steady-state currents. This analysis reveals that the reorientation of free transporter is the slowest step for wt SGLT1 either in the presence or in the absence of alphaMG. In contrast, the conformational change of the fully loaded mutant transporters constitutes their rate-limiting step in the presence of substrate and explains the persistence of pre-steady-state currents in this situation.

Mechanisms of net chloride secretion during rotavirus diarrhea in young rabbits: do intestinal villi secrete chloride?

Rotaviral diarrheal illness is one of the most common infectious diseases in children worldwide, but our understanding of its pathophysiology is limited. This study examines whether the enhanced net chloride secretion during rotavirus infection in young rabbits may occur as a result of hypersecretion in crypt cells that would exceed the substantial Cl(-) reabsorption observed in villi. By using a rapid filtration technique, we evaluated transport of (36)Cl and D-(14)C glucose across brush border membrane (BBM) vesicles purified from villus tip and crypt cells isolated in parallel from the entire small intestine. Rotavirus infection impaired SGLT1-mediated Na(+)-D-glucose symport activity in both villus and crypt cell BBM, hence contributing to the massive water loss along the cryptvillus axis. In the same BBM preparations, rotavirus failed to stimulate the Cl(-) transport activities (Cl(-)/H(+) symport, Cl(-)/anion exchange and voltage-activated Cl(-) conductance) at the crypt level, but not at the villus level, questioning, therefore, the origin of net chloride secretion. We propose that the chloride carrier might function in both normal (absorption) and reversed (secretion) modes in villi, depending on the direction of the chloride electrochemical gradient resulting from rotavirus infection, agreeing with our results that rotavirus accelerated both Cl(-) influx and Cl(-) efflux rates across villi BBM.

Intracellular hypertonicity is responsible for water flux associated with Na+/glucose cotransport

Detection of a significant transmembrane water flux immediately after cotransporter stimulation is the experimental basis for the controversial hypothesis of secondary active water transport involving a proposed stoichiometry for the human Na(+)/glucose cotransporter (SGLT1) of two Na(+), one glucose, and 264 water molecules. Volumetric measurements of Xenopus laevis oocytes coexpressing human SGLT1 and aquaporin can be used to detect osmotic gradients with high sensitivity. Adding 2 mM of the substrate alpha-methyl-glucose (alphaMG) created mild extracellular hypertonicity and generated a large cotransport current with minimal cell volume changes. After 20, 40, and 60 s of cotransport, the return to sugar-free, isotonic conditions was accompanied by measurable cell swelling averaging 0.051, 0.061, and 0.077 nl/s, respectively. These water fluxes are consistent with internal hypertonicities of 1.5, 1.7, and 2.2 mOsm for these cotransport periods. In the absence of aquaporin, the measured hypertonicites were 4.6, 5.0, and 5.3 mOsm for the same cotransport periods Cotransport-dependent water fluxes, previously assumed to be water cotransport, could be largely explained by hypertonicities of such amplitudes. Using intracellular Na(+) injection and Na(+)-selective electrode, the intracellular diffusion coefficient for Na(+) was estimated at 0.29 +/- 0.03 x 10(-5) cm(2) s(-1). Using the effect of intracellular alphaMG injection on the SGLT1-mediated outward current, the intracellular diffusion coefficient of alphaMG was estimated at 0.15 +/- 0.01 x 10(-5) cm(2) s(-1). Although these intracellular diffusion coefficients are much lower than in free aqueous solution, a diffusion model for a single solute in an oocyte would require a diffusion coefficient three times lower than estimated to explain the local osmolyte accumulation that was experimentally detected. This suggests that either the diffusion coefficients were overestimated, possibly due to the presence of convection, or the diffusion in cytosol of an oocyte is more complex than depicted by a simple model.

Kinetics of the reverse mode of the Na+/glucose cotransporter

This study investigates the reverse mode of the Na(+)/glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of alpha-methyl-D: -glucopyranoside (alphaMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na(+)- and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The current-voltage relationship saturated at positive membrane voltages (30-50 mV), and approached zero at -150 mV. The half-maximal concentration for alphaMDG-evoked outward current (K(0.5) (alphaMDG)) was 35 mM (at 0 mV). In comparison, K(0.5) (alphaMDG) for forward sugar transport was 0.15 mM (at 0 mV). K(0.5) (Na) was similar for forward and reverse transport ( approximately 35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: alphaMDG (1.0) > D: -galactose (0.84) > 3-O-methyl-glucose (0.55) > D: -glucose (0.38), whereas for forward transport, specificity was: alphaMDG approximately D: -glucose approximately D: -galactose > 3-O-methyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na(+)/sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

Phloem-localized, Proton-coupled Sucrose Carrier ZmSUT1 Mediates Sucrose Efflux under the Control of the Sucrose Gradient and the Proton Motive Force

The phloem network is as essential for plants as the vascular system is for humans. This network, assembled by nucleus- and vacuole-free interconnected living cells, represents a long distance transport pathway for nutrients and information. According to the Münch hypothesis, osmolytes such as sucrose generate the hydrostatic pressure that drives nutrient and water flow between the source and the sink phloem (Münch, E. (1930) Die Stoffbewegungen in der Pflanze, Gustav Fischer, Jena, Germany). Although proton-coupled sucrose carriers have been localized to the sieve tube and the companion cell plasma membrane of both source and sink tissues, knowledge of the molecular representatives and the mechanism of the sucrose phloem efflux is still scant. We expressed ZmSUT1, a maize sucrose/proton symporter, in Xenopus oocytes and studied the transport characteristics of the carrier by electrophysiological methods. Using the patch clamp techniques in the giant inside-out patch mode, we altered the chemical and electrochemical gradient across the sucrose carrier and analyzed the currents generated by the proton flux. Thereby we could show that ZmSUT1 is capable of mediating both the sucrose uptake into the phloem in mature leaves (source) as well as the desorption of sugar from the phloem vessels into heterotrophic tissues (sink). As predicted from a perfect molecular machine, the ZmSUT1-mediated sucrose-coupled proton current was reversible and depended on the direction of the sucrose and pH gradient as well as the membrane potential across the transporter.

Perturbation analysis of the voltage-sensitive conformational changes of the Na+/glucose cotransporter

Conformational changes of the human Na(+)/glucose cotransporter (hSGLT1) were studied using voltage-jump methods. The cotransporter was expressed in Xenopus laevis oocytes, and SGLT1 charge movements were measured in the micro- to millisecond time scale using the cut-open oocyte preparation and in the millisecond to second time scale using the two-electrode voltage clamp method. Simultaneous charge and fluorescence changes were studied using tetramethylrhodamine-6-maleimide-labeled hSGLT1 Q457C. In 100 mM external [Na(+)], depolarizing voltage steps evoked a charge movement that rose initially to a peak (with time constant tau = 0.17 ms) before decaying to steady state with two time constants (tau = 2-30 and 25-150 ms). The time to peak (0.9 ms) decreased with [Na(+)], and was not observed in 0 mM [Na(+)]. In absence of Na(+), charge movement decayed monotonically to steady state with three time constants (0.2, 2, and 150 ms). Charge movement was accompanied by fluorescence changes with similar time courses, indicating that global conformational changes monitored by charge movement are reflected by local environmental changes at or near Q457C. Our results indicate that the major voltage-dependent step of the Na(+)/glucose transport cycle is the return of the empty carrier from inward to outward facing conformations. Finally, we observed subtle differences between time constants for charge movement and for optical changes, suggesting that optical recordings can be used to monitor local conformational changes that underlie the global conformational changes of cotransporters.

Charge translocation during cosubstrate binding in the Na+/proline transporter of E.coli

Charge translocation associated with the activity of the Na(+)/proline cotransporter PutP of Escherichia coli was analyzed for the first time. Using a rapid solution exchange technique combined with a solid-supported membrane (SSM), it was demonstrated that Na(+)and/or proline individually or together induce a displacement of charge. This was assigned to an electrogenic Na(+)and/or proline binding process at the cytoplasmic face of the enzyme with a rate constant of k>50s(-1) which preceeds the rate-limiting step. Based on the kinetic analysis of our electrical signals, the following characteristics are proposed for substrate binding in PutP. (1) Substrate binding is electrogenic not only for Na(+), but also for the uncharged cosubstrate proline. The charge displacement associated with the binding of both substrates is of comparable size and independent of the presence of the respective cosubstrate. (2) Both substrates can bind individually to the transporter. Under physiological conditions, an ordered binding mechanism prevails, while at sufficiently high concentrations, each substrate can bind in the absence of the other. (3) Both substrate binding sites interact cooperatively with each other by increasing the affinity and/or the speed of binding of the respective cosubstrate. (4) Proline binding proceeds in a two-step process: low affinity (approximately 1mM) electroneutral substrate binding followed by a nearly irreversible electrogenic conformational transition.

Efflux properties of basolateral peptide transporter in human intestinal cell line Caco-2

Small peptides and some pharmacologically active compounds are absorbed from the small intestine by the apical H(+)-coupled peptide transporter 1 (PEPT1) and the basolateral peptide transporter. Here we investigated the efflux properties of the basolateral peptide transporter in Caco-2 cells using two strategies, efflux measurements and a kinetic analysis of transepithelial transport of glycylsarcosine (Gly-Sar). [(14)C]Gly-Sar efflux through the basolateral membrane was not affected significantly by the external pH. Both approaches revealed that the basolateral peptide transporter was saturable in the efflux direction, and that the affinity was lower than that in the influx direction. For two peptide-like drugs, there was no difference in substrate recognition by the basolateral peptide transporter between the two sides of the membrane. Using the kinetic parameters of PEPT1 and the basolateral peptide transporter, a computational model of Gly-Sar transport in Caco-2 cells was constructed. The simulation fitted the experimental data well. Our findings suggested that substrate affinity of the basolateral peptide transporter is apparently asymmetric, but pH-dependence and substrate specificity are symmetric for the two directions of transport. The behaviour of Gly-Sar in Caco-2 cells could be predicted by a mathematical model describing the peptide transporters.

Localization of the Na+-D-glucose cotransporter SGLT1 in the blood-brain barrier

Immunoreactivity of the Na+-D-glucose cotransporter SGLT1 was demonstrated in intracerebral capillaries of rat and pig. Immunostaining suggested that SGLT1 is located in the luminal membrane of the endothelial cells and in intracellular vesicles. Using in situ hybridization, SGLT1 mRNA was not detectable in intracerebral capillaries of non-treated or sham-operated Wistar rats. However, 1 day after a transient occlusion of the right middle cerebral artery, SGLT1 mRNA was detected in capillaries of both brain hemispheres. Expression of SGLT1 was also demonstrated in primary cultures of capillary endothelial cells from pig using polymerase chain reaction after reverse transcription and western blotting. The data suggest that SGLT1 participates in transport of D-glucose across the blood-brain barrier and is upregulated after brain ischemia and reperfusion.

Functional asymmetry of the human Na+/glucose transporter (hSGLT1) in bacterial membrane vesicles

The functional characteristics of the forward and reverse transport modes of the human Na(+)/glucose transporter (hSGLT1) were investigated using plasma membrane vesicles of E. coli expressing the recombinant transporter. Correctly and inverse-oriented vesicles were employed to measure the initial rates of methyl-alpha-D-glucose uptake, under zero-trans conditions, as a function of Na(+), sugar, and phlorizin concentrations and membrane potential. This approach enabled the analysis of the two faces of hSGLT1 in parallel, revealing the reversibility of Na(+)/sugar cotransport. While the key characteristics of secondary active sugar transport were maintained in both modes, namely, Na(+) and voltage dependence, the kinetic properties of the two sides indicated a functional asymmetry of the transporter. That is, the apparent affinity for sugar and driver cation Na(+) exhibited a difference of more than 1 order of magnitude between the two modes. Furthermore, the selectivity pattern of ligands and the interaction of the transporter with the competitive inhibitor phlorizin were different. Whereas the high-affinity substrates, D-glucose and D-galactose, inhibited uptake of radioactive sugar tracer at their physiological concentrations (10 mM) in the forward reaction, they were poor inhibitors even at high concentrations in the reverse transport mode. Taken together, these results confirm the successful employment of E. coli to express and characterize a human membrane protein (hSGLT1), elucidating the functional asymmetry of this cotransporter.

Forging the link between structure and function of electrogenic cotransporters: the renal type IIa Na+/Pi cotransporter as a case study

Electrogenic cotransporters are membrane proteins that use the electrochemical gradient across the cell membrane of a cosubstrate ion, for example Na(+) or H(+), to mediate uphill cotransport of a substrate specific to the transport protein. The cotransport process involves recognition of both cosubstrate and substrate and translocation of each species according to a defined stoichiometry. Electrogenicity implies net movement of charges across the membrane in response to the transmembrane voltage and therefore, in addition to isotope flux assays, the cotransport kinetics can be studied in real-time using electrophysiological methods. As well as the cotransport mode, many cotransporters also display a uniport or slippage mode, whereby the cosubstrate ions translocate in the absence of substrate. The current challenge is to define structure-function relationships by identifying functionally important elements in the protein that confer the transport properties and thus contribute to the ultimate goal of having a 3-D model of the protein that conveys both structural and functional information. In this review we focus on a functional approach to meet this challenge, based on a combination of real-time electrophysiological assays, together with molecular biological and biochemical methods. This is illustrated, by way of example, using data obtained by heterologous expression of the renal Na(+)-coupled inorganic phosphate cotransporter (NaP(i)-IIa) for which structure-function relationships are beginning to emerge.

Employing Escherichia coli to functionally express, purify, and characterize a human transporter

Large-scale purification of recombinant human membrane proteins represents a rate-limiting step toward the understanding of their role in health and disease. There are only four mammalian membrane proteins of known structure, and these were isolated from natural sources (see http://www.mpibp-frankfurt.mpg.de/michel/public/memprotstruct.html). In addition, genetic diseases of membrane proteins are frequently caused by trafficking defects, and it is enigmatic whether these mutants are functional. Here, we report the employment of Escherichia coli for the functional expression, purification, and reconstitution of a human membrane protein, the human Na+/glucose cotransporter (hSGLT1). The use of an E. coli mutant defective in the outer membrane protease OmpT, incubation temperatures below 20 degrees C, and transcriptional regulation from the lac promoter/operator are crucial to reduce proteolytic degradation. Purification of a recombinant hSGLT1 through affinity chromatography yields about 1 mg of purified recombinant hSGLT1 per 3 liters of cultured bacterial cells. Kinetic analysis of hSGLT1 in proteoliposomes reveals that a purified recombinant transporter, which is missorted in eukaryotic cells, retains full catalytic activity. These results indicate the power of bacteria to manufacture and isolate human membrane proteins implicated in genetic diseases.

Single molecule recognition of protein binding epitopes in brush border membranes by force microscopy

Sidedness and accessibility of protein epitopes in intact brush border membrane vesicles were analyzed by detecting single molecule interaction forces using molecular recognition force microscopy in aqueous physiological solutions. Frequent antibody-antigen recognition events were observed with a force microscopy tip carrying an antibody directed against the periplasmically located gamma-glutamyltrans- peptidase, suggesting a right side out orientation of the vesicles. Phlorizin attached to the tips bound to NA+/D-glucose cotransporter molecules present in the vesicles. The recognition was sodium dependent and inhibited by free phlorizin and D-glucose, and revealed an apparent K(D) of 0.2 microM. Binding events were also observed with an antibody directed against the epitope aa603-aa630 close to the C terminus of the transporter. In the presence of phlorizin the probability of antibody binding was reduced but the most probable unbinding force f(u) = 100 pN remained unchanged. In the presence of D-glucose and sodium, however, both the binding probability and the most probable binding force (f(u) = 50 pN) were lower than in its absence. These studies demonstrate that molecular recognition force microscopy is a versatile tool to probe orientation and conformational changes of epitopes of membrane components during binding and trans-membrane transport.

Fluorescence studies of ligand-induced conformational changes of the Na(+)/glucose cotransporter

Conformational changes in the human Na(+)/glucose cotransporter (hSGLT1) were examined using hSGLT1 Q457C expressed in Xenopus laevis oocytes and tagged with tetramethylrhodamine-6-maleimide (TMR6M). Na(+)/glucose cotransport is abolished in the TMR6M-labeled mutant, but the protein binds Na(+) and sugar [Loo et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 7789-7794]. Under voltage clamp the fluorescence of labeled Q457C was dependent on external cations. Increasing [Na(+)] increased fluorescence with a Hill coefficient of 2 and half-maximal concentration (K(Na)(0.5)) of 49 mM at -90 mV. Li(+) also increased fluorescence, whereas choline, tetraethylammonium, and N-methyl-D-glucamine did not. Fluorescence was increased by sugars with specificity: methyl alpha-D-glucopyranoside > D-glucose > D-galactose >> D-mannitol. Voltage-jump experiments (in 100 mM NaCl buffer in absence of sugar) elicited parallel changes in pre-steady-state charge movement and fluorescence. Charge vs voltage and fluorescence vs voltage curves followed Boltzmann relations with the same median voltage (V(0.5) = -50 mV), but the apparent valence was 1 for charge movement and 0.4 for fluorescence. V(0.5) for fluorescence and charge movement was shifted by -100 mV per 10-fold decrease in [Na(+)]. Under Na(+)-free conditions, there was a voltage-dependent change in fluorescence. Voltage-jump experiments showed that the maximal change in fluorescence increased 20% with sugar. These results indicate that Na(+), sugar, and membrane voltage change the local environment of the fluorophore at Q457C. Our interpretation of these results is (1) the conformational change of the empty transporter is voltage dependent, (2) two Na(+) ions can bind cooperatively to the protein before sugar, and (3) sugar binding induces a conformational change.

Bidirectional electrogenic transport of peptides by the proton-coupled carrier PEPT1 in Xenopus laevis oocytes: its asymmetry and symmetry

1. The giant patch clamp technique in the inside-out configuration and the two-electrode voltage clamp technique were used to characterize the bidirectional transport properties of the proton-coupled peptide carrier PEPT1 expressed in Xenopus laevis oocytes. 2. The addition of the neutral dipeptide Gly-L-Gln to the cytoplasmic solution induced a net outward transport current in a membrane potential range between -80 and +60 mV, even in the absence of a pH gradient. 3. The concentration dependency of the outwardly directed transport currents followed Michaelis-Menten-type kinetics, with an apparent K0.5 of 3.28 mM (at pH 7.5 and +60 mV membrane potential). This apparent affinity is around fivefold lower than the apparent affinity measured for the inward transport mode (K0.5 of 0.70 mM (at pH 7.5 and -60 mV) under identical experimental conditions). 4. Apparent K0.5 values were strongly pH and potential dependent only on the external face for inward transport. The transport currents were potential dependent, but essentially pH independent for inward transport and only modestly altered by pH in the reverse direction. In addition to the membrane potential, the transmembrane substrate gradient acts as a driving force and contributes significantly to total transport currents. 5. The differences in apparent substrate affinity under identical experimental conditions suggest major differences in the conformation of the substrate binding pocket of PEPT1 when exposed to the external versus the internal face of the membrane. The lower affinity on the internal face allows the substrate to be released into the cytosolic compartment even in the absence of a proton-motive force. 6. Our study demonstrates for the first time that PEPT1 can transport dipeptides bidirectionally in an electrogenic and proton-coupled symport mode. When substrates are present on both sides of the membrane in sufficiently high concentrations, the direction and rate of transport are solely dependent on the membrane potential, and transport occurs symmetrically.

Renal Na(+)-glucose cotransporters

In humans, the kidneys filter approximately 180 g of D-glucose from plasma each day, and this is normally reabsorbed in the proximal tubules. Although the mechanism of reabsorption is well understood, Na(+)-glucose cotransport across the brush-border membrane and facilitated diffusion across the basolateral membrane, questions remain about the identity of the genes responsible for cotransport across the brush border. Genetic studies suggest that two different genes regulate Na(+)-glucose cotransport, and there is evidence from animal studies to suggest that the major bulk of sugar is reabsorbed in the convoluted proximal tubule by a low-affinity, high-capacity transporter and that the remainder is absorbed in the straight proximal tubule by a high-affinity, low-capacity transporter. There are at least three different candidates for these human renal Na(+)-glucose cotransporters. This review will focus on the structure-function relationships of these three transporters, SGLT1, SGLT2, and SGLT3.