Evidence of active transfer of certain non-electrolytes across the human red cell membrane

Establishing mammalian GLUT kinetics and lipid composition influences in a reconstituted-liposome system

Glucose transporters (GLUTs) are essential for organism-wide glucose homeostasis in mammals, and their dysfunction is associated with numerous diseases, such as diabetes and cancer. Despite structural advances, transport assays using purified GLUTs have proven to be difficult to implement, hampering deeper mechanistic insights. Here, we have optimized a transport assay in liposomes for the fructose-specific isoform GLUT5. By combining lipidomic analysis with native MS and thermal-shift assays, we replicate the GLUT5 transport activities seen in crude lipids using a small number of synthetic lipids. We conclude that GLUT5 is only active under a specific range of membrane fluidity, and that human GLUT1-4 prefers a similar lipid composition to GLUT5. Although GLUT3 is designated as the high-affinity glucose transporter, in vitro D-glucose kinetics demonstrates that GLUT1 and GLUT3 actually have a similar K_M, but GLUT3 has a higher turnover. Interestingly, GLUT4 has a high K_M for D-glucose and yet a very slow turnover, which may have evolved to ensure uptake regulation by insulin-dependent trafficking. Overall, we outline a much-needed transport assay for measuring GLUT kinetics and our analysis implies that high-levels of free fatty acid in membranes, as found in those suffering from metabolic disorders, could directly impair glucose uptake.

An appraisal of the current status of inhibition of glucose transporters as an emerging antineoplastic approach: Promising potential of new pan-GLUT inhibitors

Neoplastic cells displayed altered metabolism with accelerated glycolysis. Therefore, these cells need a mammoth supply of glucose for which they display an upregulated expression of various glucose transporters (GLUT). Thus, novel antineoplastic strategies focus on inhibiting GLUT to intersect the glycolytic lifeline of cancer cells. This review focuses on the current status of various GLUT inhibition scenarios. The GLUT inhibitors belong to both natural and synthetic small inhibitory molecules category. As neoplastic cells express multiple GLUT isoforms, it is necessary to use pan-GLUT inhibitors. Nevertheless, it is also necessary that such pan-GLUT inhibitors exert their action at a low concentration so that normal healthy cells are left unharmed and minimal injury is caused to the other vital organs and systems of the body. Moreover, approaches are also emerging from combining GLUT inhibitors with other chemotherapeutic agents to potentiate the antineoplastic action. A new pan-GLUT inhibitor named glutor, a piperazine-one derivative, has shown a potent antineoplastic action owing to its inhibitory action exerted at nanomolar concentrations. The review discusses the merits and limitations of the existing GLUT inhibitory approach with possible future outcomes.

Quantitative study of unsaturated transport of glycerol through aquaglyceroporin that has high affinity for glycerol

The structures of several aquaglyceroporins have been resolved to atomic resolution showing two or more glycerols bound inside a channel and confirming a glycerol-facilitator's affinity for its substrate glycerol. However, the kinetics data of glycerol transport experiments all point to unsaturated transport that is characteristic of low substrate affinity in terms of the Michaelis-Menten kinetics. In this article, we present an in silico-in vitro research focused on AQP3, one of the human aquaglyceroporins that is natively expressed in the abundantly available erythrocytes. We conducted 2.1 μs in silico simulations of AQP3 embedded in a model erythrocyte membrane with intracellular-extracellular asymmetries in leaflet lipid compositions and compartment salt ions. From the equilibrium molecular dynamics (MD) simulations, we elucidated the mechanism of glycerol transport at high substrate concentrations. From the steered MD simulations, we computed the Gibbs free-energy profile throughout the AQP3 channel. From the free-energy profile, we quantified the kinetics of glycerol transport that is unsaturated due to glycerol-glycerol interactions mediated by AQP3 resulting in the concerted movement of two glycerol molecules for the transport of one glycerol molecule across the cell membrane. We conducted in vitro experiments on glycerol uptake into human erythrocytes for a wide range of substrate concentrations and various temperatures. The experimental data quantitatively validated our theoretical-computational conclusions on the unsaturated glycerol transport through AQP3 that has high affinity for glycerol.

Erythroid glucose transport in health and disease

Glucose transport is intimately linked to red blood cell physiology. Glucose is the unique energy source for these cells, and defects in glucose metabolism or transport activity are associated with impaired red blood cell morphology and deformability leading to reduced lifespan. In vertebrate erythrocytes, glucose transport is mediated by GLUT1 (in humans) or GLUT4 transporters. These proteins also account for dehydroascorbic acid (DHA) transport through erythrocyte membrane. The peculiarities of glucose transporters and the red blood cell pathologies involving GLUT1 are summarized in the present review.

Single-channel permeability and glycerol affinity of human aquaglyceroporin AQP3

For its fundamental relevance, transport of water and glycerol across the erythrocyte membrane has long been investigated before and after the discovery of aquaporins (AQPs), the membrane proteins responsible for water and glycerol transport. AQP1 is abundantly expressed in the human erythrocyte for maintaining its hydrohomeostasis where AQP3 is also expressed (at a level ~30-folds lower than AQP1) facilitating glycerol transport. This research is focused on two of the remaining questions: How permeable is AQP3 to water? What is the glycerol-AQP3 affinity under near-physiological conditions? Through atomistic modelling and large-scale simulations, we found that AQP3 is two to three times more permeable to water than AQP1 and that the glycerol-AQP3 affinity is approximately 500/M. Using these computed values along with the data from the latest literature on AQP1 and on erythrocyte proteomics, we estimated the water and glycerol transport rates across the membrane of an entire erythrocyte. We used these rates to predict the time courses of erythrocyte swelling-shrinking in response to inward and outward osmotic gradients. Experimentally, we monitored the time course of human erythrocytes when subject to an osmotic or glycerol gradient with light scattering in a stopped-flow spectrometer. We observed close agreement between the experimentally measured and the computationally predicted time courses of erythrocytes, which corroborated our computational conclusions on the AQP3 water-permeability and the glycerol-AQP3 affinity.

Carriers, exchangers, and cotransporters in the first 100 years of the Journal of General Physiology

Jennings reviews the many contributions of JGP articles to our current understanding of solute transporter mechanisms.

Transporters, pumps, and channels are proteins that catalyze the movement of solutes across membranes. The single-solute carriers, coupled exchangers, and coupled cotransporters that are collectively known as transporters are distinct from conductive ion channels, water channels, and ATP-hydrolyzing pumps. The main conceptual framework for studying transporter mechanisms is the alternating access model, which comprises substrate binding and release events on each side of the permeability barrier and translocation events involving conformational changes between inward-facing and outward-facing conformational states. In 1948, the Journal of General Physiology began to publish work that focused on the erythrocyte glucose transporter—the first transporter to be characterized kinetically—followed by articles on the rates, stoichiometries, asymmetries, voltage dependences, and regulation of coupled exchangers and cotransporters beginning in the 1960s. After the dawn of cDNA cloning and sequencing in the 1980s, heterologous expression systems and site-directed mutagenesis allowed identification of the functional roles of specific amino acid residues. In the past two decades, structures of transport proteins have made it possible to propose specific models for transporter function at the molecular level. Here, we review the contribution of JGP articles to our current understanding of solute transporter mechanisms. Whether the topic has been kinetics, energetics, regulation, mutagenesis, or structure-based modeling, a common feature of these articles has been a quantitative, mechanistic approach, leading to lasting insights into the functions of transporters.

Current technical approaches to brain energy metabolism

Neuroscience is a technology-driven discipline and brain energy metabolism is no exception. Once satisfied with mapping metabolic pathways at organ level, we are now looking to learn what it is exactly that metabolic enzymes and transporters do and when, where do they reside, how are they regulated, and how do they relate to the specific functions of neurons, glial cells, and their subcellular domains and organelles, in different areas of the brain. Moreover, we aim to quantify the fluxes of metabolites within and between cells. Energy metabolism is not just a necessity for proper cell function and viability but plays specific roles in higher brain functions such as memory processing and behavior, whose mechanisms need to be understood at all hierarchical levels, from isolated proteins to whole subjects, in both health and disease. To this aim, the field takes advantage of diverse disciplines including anatomy, histology, physiology, biochemistry, bioenergetics, cellular biology, molecular biology, developmental biology, neurology, and mathematical modeling. This article presents a well-referenced synopsis of the technical side of brain energy metabolism research. Detail and jargon are avoided whenever possible and emphasis is given to comparative strengths, limitations, and weaknesses, information that is often not available in regular articles.

Concentration dependence of the cell membrane permeability to cryoprotectant and water and implications for design of methods for post-thaw washing of human erythrocytes

For more than fifty years the human red blood cell (RBC) has been a widely studied model for transmembrane mass transport. Existing literature spans myriad experimental designs with varying results and physiologic interpretations. In this review, we examine the kinetics and mechanisms of membrane transport in the context of RBC cryopreservation. We include a discussion of the pathways for water and glycerol permeation through the cell membrane and the implications for mathematical modeling of the membrane transport process. In particular, we examine the concentration dependence of water and glycerol transport and provide equations for estimating permeability parameters as a function of concentration based on a synthesis of literature data. This concentration-dependent transport model may allow for design of improved methods for post-thaw removal of glycerol from cryopreserved blood. More broadly, the consideration of the concentration dependence of membrane permeability parameters may be important for other cell types as well, especially for design of methods for equilibration with the highly concentrated solutions used for vitrification.

Glucose transport: meeting the metabolic demands of cancer, and applications in glioblastoma treatment

GLUT1, and to a lesser extent, GLUT3, appear to be interesting targets in the treatment of glioblastoma multiforme. The current review aims to give a brief history of the scientific community's understanding of these glucose transporters and to relate their importance to the metabolic changes that occur as a result of cancer. One of the primary changes that occurs in cancer, the Warburg Effect, is characterized by an extreme shift toward glycolysis from the usual reliance on oxidative phosphorylation and is currently being investigated to target the upstream and downstream factors responsible for Warburg-induced changes. Further, it aims to explain the differential expression of GLUT1 and GLUT3 in glioblastoma tissue, and how these modulations in expression can serve as targets to restore a more normal metabolism. Additionally, hypoxia-induced factor-1α's (HIF1α) role in a number of transcriptional changes typical to GBM will be discussed, including its role in GLUT upregulation. Finally, the four known subtypes of GBM [proneural, neural, mesenchymal, and classical] will be characterized in order to discuss how metabolic changes differ in each subtype. These changes have the potential to be selectively targeted in order to provide specificity to the clinical treatment options in GBM.

Structural Biology of the Major Facilitator Superfamily Transporters

The ancient and ubiquitous major facilitator superfamily (MFS) represents the largest secondary transporter family and plays a crucial role in a multitude of physiological processes. MFS proteins transport a broad spectrum of ions and solutes across membranes via facilitated diffusion, symport, or antiport. In recent years, remarkable advances in understanding the structural biology of the MFS transporters have been made. This article reviews the history, classification, and general features of the MFS proteins; summarizes recent structural progress with a focus on the sugar porter family transporters exemplified by GLUT1; and discusses the molecular mechanisms of substrate binding, alternating access, and cotransport coupling.

Transport of sugars

Soluble sugars serve five main purposes in multicellular organisms: as sources of carbon skeletons, osmolytes, signals, and transient energy storage and as transport molecules. Most sugars are derived from photosynthetic organisms, particularly plants. In multicellular organisms, some cells specialize in providing sugars to other cells (e.g., intestinal and liver cells in animals, photosynthetic cells in plants), whereas others depend completely on an external supply (e.g., brain cells, roots and seeds). This cellular exchange of sugars requires transport proteins to mediate uptake or release from cells or subcellular compartments. Thus, not surprisingly, sugar transport is critical for plants, animals, and humans. At present, three classes of eukaryotic sugar transporters have been characterized, namely the glucose transporters (GLUTs), sodium-glucose symporters (SGLTs), and SWEETs. This review presents the history and state of the art of sugar transporter research, covering genetics, biochemistry, and physiology-from their identification and characterization to their structure, function, and physiology. In humans, understanding sugar transport has therapeutic importance (e.g., addressing diabetes or limiting access of cancer cells to sugars), and in plants, these transporters are critical for crop yield and pathogen susceptibility.

Permeability of the human red blood cell tomeso-erythritol

Using(14)C-erythritol, we measured net as well as unidirectional erythritol fluxes. Up to near saturation, net and unidirectional fluxes were virtually identical and linearly related to the erythritol concentration in the medium (isotonic saline). No saturation of the transfer system was observed. At 20°C, a maximum of 60 to 70% of the erythritol flux could be inhibited by glucose, phlorizin, or a combination of both substances. Dinitrofluorobenzene and HgCl2 also reduce erythritol permeability. These findings confirm the earlier conclusion of F. Bowyer and W. F. Widdas that the glucose transport system is involved in erythritol permeation. Glycerol partially inhibits the glucose-phlorizin-sensitive component of erythritol flux, but not the glucose-phlorizin-insensitive component. Apparently glycerol has a slight affinity to that portion of the glucose transport system which is involved in erythritol transfer, whereas the glucosephlorizin-insensitive fraction of erythritol movements is not identical with the glycerol system. This latter inference is supported by the observation that, in contrast to glycerol permeability, erythritol permeability is insensitive to variations of pH or to the addition of copper. The apparent activation energy of the glucose-phlorizin-sensitive and-insensitive fractions of erythritol permeation are 22.2 and 20.7 kcal/mole, respectively. These values are not significantly different from one another.

Encapsulation of FITC to monitor extracellular pH: a step towards the development of red blood cells as circulating blood analyte biosensors

A need exists for a long-term, minimally-invasive system to monitor blood analytes. For certain analytes, such as glucose in the case of diabetics, a continuous system would help reduce complications. Current methods suffer significant drawbacks, such as low patient compliance for the finger stick test or short lifetime (i.e., 3-7 days) and required calibrations for continuous glucose monitors. Red blood cells (RBCs) are potential biocompatible carriers of sensing assays for long-term monitoring. We demonstrate that RBCs can be loaded with an analyte-sensitive fluorescent dye. In the current study, FITC, a pH-sensitive fluorescent dye, is encapsulated within resealed red cell ghosts. Intracellular FITC reports on extracellular pH: fluorescence intensity increases as extracellular pH increases because the RBC rapidly equilibrates to the pH of the external environment through the chloride-bicarbonate exchanger. The resealed ghost sensors exhibit an excellent ability to reversibly track pH over the physiological pH range with a resolution down to 0.014 pH unit. Dye loading efficiency varies from 30% to 80%. Although complete loading is ideal, it is not necessary, as the fluorescence signal is an integration of all resealed ghosts within the excitation volume. The resealed ghosts could serve as a long-term (>1 to 2 months), continuous, circulating biosensor for the management of diseases, such as diabetes.

Changes in abundance of aquaporin-like proteins occurs concomitantly with seasonal acquisition of freeze tolerance in the goldenrod gall fly, Eurosta solidaginis

The accumulation of cryoprotectants and the redistribution of water between body compartments play central roles in the capacity of insects to survive freezing. Aquaporins (AQPs) allow for rapid redistribution of water and small solutes (e.g. glycerol) across the cell membrane and were recently implicated in promoting freeze tolerance. Here, we examined whether aquaporin-like protein abundance correlated with the seasonal acquisition of freezing tolerance in the goldenrod gall fly, Eurosta solidaginis (Diptera: Tephritidae). Through the autumn, larvae became tolerant of freezing at progressively lower temperatures and accumulated the cryoprotectant glycerol. Furthermore, larvae significantly increased the abundance of membrane-bound aquaporin and aquaglyceroporin-like proteins from July through January. Acute exposure of larvae to cold and desiccation resulted in upregulation of the AQP3-like proteins in October, suggesting that their abundance is regulated by environmental cues. The seasonal increase in abundance of both putative aquaporins and aquaglyceroporins supports the hypothesis that these proteins are closely tied to the seasonal acquisition of freeze tolerance, functioning to permit cells to quickly lose water and take-up glycerol during extracellular ice formation, as well as reestablish water and glycerol concentrations upon thawing.

Glucose transporters in the 21st Century

The ability to take up and metabolize glucose at the cellular level is a property shared by the vast majority of existing organisms. Most mammalian cells import glucose by a process of facilitative diffusion mediated by members of the Glut (SLC2A) family of membrane transport proteins. Fourteen Glut proteins are expressed in the human and they include transporters for substrates other than glucose, including fructose, myoinositol, and urate. The primary physiological substrates for at least half of the 14 Glut proteins are either uncertain or unknown. The well-established glucose transporter isoforms, Gluts 1-4, are known to have distinct regulatory and/or kinetic properties that reflect their specific roles in cellular and whole body glucose homeostasis. Separate review articles on many of the Glut proteins have recently appeared in this journal. Here, we provide a very brief summary of the known properties of the 14 Glut proteins and suggest some avenues of future investigation in this area.

Probing bioinorganic chemistry processes in the bloodstream to gain new insights into the origin of human diseases

In the context of elucidating the origin of human diseases, past poisoning epidemics have revealed that exceedingly small doses of inorganic environmental pollutants can result in dramatic effects on human health. Today, numerous organic and inorganic pollutants have been quantified in human blood, but the interpretation of these concentrations remains--from a public health point of view--problematic. Conversely, the biomolecular origin for several grievous human diseases is essentially unknown. Taken together and viewed in the context of recent bioinorganic research findings, the established human blood concentrations of toxic metals and metalloids may be functionally connected with the etiology of specific human diseases. To unravel the underlying biomolecular mechanisms, and taking into account the basic flow of dietary matter through mammalian organisms, a better understanding of the bioinorganic chemistry of toxic metals and metalloid compounds in the bloodstream is emerging as a promising avenue for future research. To this end, the concerted application of modern proteomic methodologies, synchrotron-based X-ray absorption spectroscopy and established spectroscopic techniques will contribute to better define the role that blood-based bioinorganic chemistry-related processes play in the origin of human diseases. The application of this and other modern proteomic methodologies could contribute to a better understanding of the role that blood-based bioinorganic chemistry-related processes play in the origin and etiology of human diseases.

Drug transporters in pharmacokinetics

This review deals with the drug transporters allowing drugs to enter and leave cells by carrier-mediated pathways. Emphasis is put on liver transporters but systems in gut, kidney, and blood-brain barrier are mentioned as well. Drug-drug interactions on carriers may provoke significant modification in pharmacokinetics as do carrier gene polymorphisms yielding functional carrier protein mutations. An integrated phase concept should reflect the interplay between drug metabolism and drug transport.

Experimental models, protocols, and reference values for evaluation of iodinated analogues of glucose

For an iodinated analogue of glucose to be useful for evaluating glucose uptake using single-photon emission computed tomography (SPECT), it must enter the cell via the same transporter as glucose and accumulate within the cell without being degraded. The biological behavior of the iodinated tracer must therefore be similar to that of 2-deoxy-D(-)[1-14C]-glucose (2-DG). In the present study, four experimental models (biodistribution in mouse, isolated rat heart, human erythrocytes in suspension and cultured neonatal rat cardiomyocytes) have been chosen and protocols have been set up which allow for the examination of small quantities of iodinated analogues of glucose. The uptakes of 2-DG and of L(-)[1-14C]-glucose have been measured in these models to establish reference values which will be compared with uptake values for iodinated analogues of glucose.

Insulin-stimulated glucose transport inhibits Ca2+ influx and contraction in vascular smooth muscle

BACKGROUND

Insulin attenuates serotonin-induced Ca2+ influx, the intracellular Ca2+ transient, and contraction of cultured vascular smooth muscle cells from dog femoral artery. These studies were designed to test whether insulin-induced glucose transport was an early event leading to the inhibitory effects of insulin on Ca2+ influx, intracellular Ca2+ concentration, and contraction in these cells.

METHODS AND RESULTS

Insulin 1 nmol/L stimulated the 30-minute uptake of [3H]2-deoxyglucose in these cells via a phloridzin-inhibitable mechanism. Contraction of individual cells was measured by photomicroscopy, intracellular Ca2+ concentration was monitored by measuring fura 2 fluorescence by use of Ca(2+)-sensitive excitation wavelengths, and Ca2+ influx was estimated by the rate of Mn2+ quenching of intracellular fura 2 fluorescence when excited at a Ca(2+)-insensitive wave-length. In the presence of 5 mmol/L glucose, preincubation of cells for 30 minutes with 1 nmol/L insulin inhibited 10(-5) mol/L serotonin-induced contraction of individual cells by 62% (P < .01) and decreased the serotonin-stimulated component of Mn2+ influx by 78% (P < .05). Removing glucose from the preincubation medium or adding 1 mmol/L phloridzin completely eliminated these effects of insulin. Insulin lowered the serotonin-induced intracellular Ca2+ peak by 37% (P < .05), and phloridzin blocked this effect of insulin. When glucose uptake was increased to the insulin-stimulated level by preincubation of the cells for 30 minutes with 25 mmol/L glucose in the absence of insulin, serotonin failed to stimulate Mn2+ influx, the serotonin-induced Ca2+ peak was decreased by 46% (P < .05), serotonin-induced contraction was inhibited by 60% (P < .01), and addition of insulin did not further inhibit contraction.

CONCLUSIONS

Since the effects of insulin on serotonin-stimulated Ca2+ transport, intracellular Ca2+ concentration, and contraction were dependent on glucose transport and were duplicated when glucose transport was stimulated by high extracellular glucose concentration rather than insulin per se, it is concluded that insulin-stimulated glucose transport is an early event that leads to decreased Ca2+ influx and contraction in vascular smooth muscle.

Conformational changes in human red cell membrane proteins induced by sugar binding

We have previously shown that the human red cell glucose transport protein and the anion exchange protein, band 3, are in close enough contact that information can be transmitted from the glucose transport protein to band 3. The present experiments were designed to show whether information could be transferred in the reverse direction, using changes in tryptophan fluorescence to report on the conformation of the glucose transport protein. To see whether tryptophan fluorescence changes could be attributed to the glucose transport protein, we based our experiments on procedures used by Helgerson and Carruthers [Helgerson, A. L., Carruthers, A., (1987) J. Biol. Chem. 262:5464-5475] to displace cytochalasin B (CB), the specific D-glucose transport inhibitor, from its binding site on the inside face of the glucose transport protein, and we showed that these procedures modified tryptophan fluorescence. Addition of 75 mM maltose, a nontransportable disaccharide which also displaces CB, caused a time-dependent biphasic enhancement of tryptophan fluorescence in fresh red cells, which was modulated by the specific anion exchange inhibitor, DBDS (4,4'-dibenzamido-2,2'-stilbene disulfonate). In a study of nine additional disaccharides, we found that both biphasic kinetics and DBDS effects depended upon specific disaccharide conformation, indicating that these two effects could be attributed to a site sensitive to sugar conformation. Long term (800 sec) experiments revealed that maltose binding (+/- DBDS) caused a sustained damped anharmonic oscillation extending over the entire 800 sec observation period. Mathematical analysis of the temperature dependence of these oscillations showed that 2 microM DBDS increased the damping term activation energy, 9.5 +/- 2.8 kcal mol-1 deg-1, by a factor of four to 39.7 +/- 5.1 kcal mol-1 deg-1, providing strong support for the view that signalling between the glucose transport protein and band 3 goes in both directions.

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.

New permeability pathways induced by the malarial parasite in the membrane of its host erythrocyte: potential routes for targeting of drugs into infected cells

Malarial parasites propagate asexually inside the erythrocytes of their vertebrate host. Six hours after invasion, the permeability of the host cell membrane to anions and small nonelectrolytes starts to increase and reaches its peak as the parasite matures. This increased permeability differs from the native transport systems of the normal erythrocyte in its solute selectivity pattern, its enthalpy of activation and its susceptibility to inhibitors, suggesting the appearance of new transport pathways. A biophysical analysis of the permeability data indicates that the selectivity barrier discriminates between permeants according to their hydrogen bonding capacity and has solubilization properties compared to those of iso-butanol. The new permeability pathways could result from structural defects caused in the host cell membrane by the insertion of parasite-derived polypeptides. It is suggested that the unique transport properties of the new pathways be used to target drugs into infected cells, to affect the parasite either directly or through the modulation of the intraerythrocytic environment. The feasibility of drug targeting is demonstrated in in vitro cultures of the human malarial parasite Plasmodium falciparum.

Glucose transport by uterine plasma membranes

Uterine plasma membrane preparations were obtained by centrifugation on discontinuous sucrose gradients. The specific activity of the plasma membrane marker 5'-nucleotidase was increased 10-fold while the specific activity of glucose-6-phosphatase was increased 3-fold. Electron microscopy showed mainly closed vesicles having diameters mainly in the range of 0.1 to 0.4 micron and an absence of other recognizable organelles such as mitochondria. D-Glucose transport was inhibited by sulfhydryl reagents, phloretin, and cytochalasin B. Uptake was prevented at high osmotic pressures. The Km of glucose transport was 12.2 +/- 1.1 mM. Studies of the inhibition of [3H]cytochalasin B binding by D-glucose indicated that the value of the Kd of the cytochalasin B-transporter complex was larger than 1 microM. These data demonstrate the potential usefulness of these preparations in the study of glucose transport in rat uterus and its control by steroid hormones.

Kinetics of glucose transport in human erythrocytes

The rate of unidirectional D-[14C]glucose efflux from human red blood cells was determined at self-exchange and net-efflux conditions by means of the Millipore-Swinnex filtering technique and the rapid continuous flow tube technique with which initial rates can be measured within fractions of a second. Determinations at 38, 25 and 10 degrees C of the concentration dependence of glucose self-exchange flux and net efflux showed that both self exchange and net efflux followed simple Michaelis-Menten kinetics at all temperatures. At 38 degrees C the maximal self-exchange flux and the maximal net efflux were identical (6 X 10(-10) mol/cm2.sec). The cellular glucose concentration for half-maximal flux (K1/2) was 6.7 mM for self exchange and 8.2 mM for net efflux. By lowering temperature the maximal glucose self-exchange flux progressively exceeded the maximal net efflux, and was about three times larger at 10 degrees C. K1/2 for self exchange increased to 12.6 mM at 10 degrees C, while K1/2 for net efflux decreased to 4.4 mM. At 38 degrees C the glucose permeability at self exchange at a constant extracellular glucose concentration of 40 mM showed a bell-shaped pH dependence between pH 6 and pH 9. A maximum was found at pH 7.2, whereas the apparent permeability coefficient was halved both at pH 6 and pH 9. The temperature dependence of glucose transport was determined between 47 and 0 degrees C at a cellular glucose concentration of 100 mM which ensured greater than 85% saturation of the glucose transport system within the temperature range. The Arrhenius activation energy of glucose transport was not constant. By lowering the temperature, the activation energy increased gradually for net efflux from 55 kJ/mole between 38 and 47 degrees C to 151 kJ/mole between 0 and 10 degrees C. The temperature dependence of self-exchange flux showed a more pronounced change around 10 degrees C. The Arrhenius activation energy was found to be 61 kJ/mole above and 120 kJ/mole below 10 degrees C.

New permeability pathways induced in membranes of Plasmodium falciparum infected erythrocytes

The permeability properties of the membrane of human erythrocytes infected with malaria parasites (Plasmodium falciparum) were studied by the method of osmotic hemolysis. At the trophozoite stage, the host membrane becomes permeable to substrates such as sorbitol and glucose. The new permeability pathway is insensitive to most inhibitors of the glucose carrier, but is highly susceptible to the membrane dipole modifier phloretin. It is blocked by disaccharides and oligosaccharides, both of which are impermeant to non-infected and infected cells. It has an enthalpy of activation of solute penetration of 10 +/- 1 kcal mol-1 (range of 5-37 degrees C). It appears that new permeability pathways with pore-like properties are induced in parasitized cells. The pore(s) admit(s) neutral and anionic substances of a discrete molecular volume, but exclude(s) cations. Apparently they play an essential role in parasite development.

Uptake of L-tryptophan by erythrocytes infected with malaria parasites (Plasmodium falciparum)

The initial rates of uptake of L-tryptophan into normal human red blood cells and into cells infected by the malarial parasite Plasmodium falciparum in vitro, were investigated. We find that transport in non-infected cells, which is mediated by the specific saturable T system and the apparently non-saturable L system (Rosenberg, Young and Ellory (1980) Biochim. Biophys. Acta 598, 375-384) is considerably enhanced by blood preservation and culture conditions. This increase is mostly due to an increase in the maximal velocity of the saturable component and of the rate constant of the linear component. Uptake is further enhanced in non-infected cells by factors released from infected cells into the culture medium and, even more so, in infected cells at the advanced stage of intraerythrocytic parasite development. At these stages the susceptibility of the transport system to the non-specific inhibitor phloretin and to the competitive inhibitor phenylalanine, is virtually lost. The effect of the parasite on L-tryptophan uptake by the host cell membrane is exerted only on the maximal velocity of the T system, which is carrying most of the substrate under physiological conditions. The possible implications of these findings to the life of the intraerythrocytic parasite are briefly discussed.

Similar activities of nerve growth factor and its homologue proinsulin in intracellular hydrogen peroxide production and metabolism in adipocytes. Transmembrane signalling relative to insulin-mimicking cellular effects

Generation of hydrogen peroxide in adipocyte plasma membrane and its intracellular metabolism and regulatory role have been shown by Mukherjee and co-workers to be a major effector system for insulin [Fedn Proc. 35, 1694 (1976); Archs Biochem. Biophys. 184, 69 (1977); Biochem. Pharmac. 27, 2589 (1978); Fedn Proc. 37, 1689 (1978); and Biochem. Pharmac. 29, 1239 (1980)]. The possible involvement of this mechanism in the action of structurally similar polypeptides having some insulin-like metabolic effects was investigated. The beta-subunit of nerve growth factor (2.5 S NGF, mol. wt 13,500) which has a striking structural homology with proinsulin and has been reported to exert certain insulin-like metabolic effects in its own target tissues (e.g. growing neurites and sympathetic ganglia), and the insulin-derived polypeptides, desalanine-insulin and desoctapeptide-insulin, as well as proinsulin, were examined for their effects on rat adipocytes, employing the technique of formate oxidation. Both NGF and proinsulin caused increased [14C]formate oxidation, showing similar intrinsic activities, up to a maximum of 140-160% of the basal rate; insulin increased the rate to 190-210% of the basal rate. The relative potencies of the hormones toward H2O2 formation and stimulation of the pentose phosphate pathway activity were: insulin (EC50: 2.5 x 10(-11) M), desalanine-insulin (EC50: 2.5 x 10(-10) M), proinsulin (EC50: 8 x 10(-9) M), and NGF (EC50: 10(-9) M). The biologically inactive derivative, desoctapeptide-insulin, did not stimulate glucose oxidation, although it caused a small increase in formate oxidation, with an EC50 of 5 x 10(-7) M, indicating a suboptimal level of H2O2 formation in the elevation of the hexose monophosphate shunt activity. 3-Amino-1,2,4-triazole (50 mM), which irreversibly decomposes the peroxidatic compound II of the catalase: H2O2 complex, inhibited formate oxidation to a greater extent in the hormone-treated cells than in the control cells, whereas sodium azide, an inhibitor of the hemoprotein, catalase, completely inhibited it. The abilities of the polypeptides to stimulate H2O2 formation correlated with their abilities to promote lipogenesis from [U-14C]-D-glucose, as expected of insulin. The cellular GSH/GSSG ratio increased concomitantly with the stimulation of glucose oxidation via the shunt, indicating a tight coupling between these processes. The results confirm that the hydrogen peroxide production is a common basis of the metabolic actions of growth-promoting polypeptide hormones or mitogens beyond their respective receptors.