Effects of pH on the activity of the human red cell glucose transporter Glut 1: transport retention chromatography of D-glucose and L-glucose on immobilized Glut 1 liposomes

Immobilized-biomembrane affinity chromatography for binding studies of membrane proteins

Analyses of specific interactions between solutes and a membrane protein can serve to characterize the protein. Frontal affinity chromatography of an interactant on a column containing the membrane protein immobilized in a lipid environment is a simple and robust approach for series of experiments with particular protein molecules. Regression analysis of the retention volumes at a series of interactant concentrations shows the affinity of the protein for the interactant and the amount of active binding sites. The higher the affinity, the fewer sites are required to give sufficient retention. Competition experiments provide the affinities of even weakly binding solutes and the non-specific retention of the primary interactant. Hummel and Dreyer size-exclusion chromatography allows complementary analyses of non-immobilized membrane materials. Analyses of the human facilitative glucose transporter GLUT1 by use of the inhibitor cytochalasin B (radioactively labeled) and the competitive substrate D-glucose (non-labeled) showed that GLUT1 interconverted between two states, exhibiting one or two cytochalasin B-binding sites per two GLUTI monomers, dependent on the membrane composition and environment. Similar analyses of a nucleoside transporter, a photosynthetic reaction center, nicotinic acetylcholine receptors and a P-glycoprotein, alternative techniques, and immobilized-liposome chromatographic approaches are presented briefly.

Effects of cholesterol and model transmembrane proteins on drug partitioning into lipid bilayers as analysed by immobilized-liposome chromatography

We have analysed how cholesterol and transmembrane proteins in phospholipid bilayers modulate drug partitioning into the bilayers. For this purpose we determined the chromatographic retention of drugs on liposomes or proteoliposomes entrapped in gel beads. The drug retention per phospholipid amount (the capacity factor Ks) reflects the drug partitioning. Cholesterol in the bilayers decreased the Ks value and hence the partitioning into the membrane in proportion to the cholesterol fraction. On average this cholesterol effect decreased with increasing temperature. Model transmembrane proteins, the glucose transporter GLUT1 and bacteriorhodopsin, interacted electrostatically with charged drugs to increase or decrease the drug partitioning into the bilayers. Bacteriorhodopsin proteoliposomes containing cholesterol combined the effects of the protein and the cholesterol and approached the partitioning properties of red blood cell membranes. For positively charged drugs the correlation between calculated intestinal permeability and log Ks was fair for both liposomes and bacteriorhodopsin-cholesterol proteoliposomes. Detailed modeling of solute partitioning into biological membranes may require an extensive knowledge of their structures.

Advantages of quantitative affinity chromatography for the analysis of solute interaction with membrane proteins

The use of membrane proteins as chromatographic stationary phases for the quantitation of biospecific interaction between the proteins and solutes is reviewed. This method is one among the few where a membrane protein is immobilized for repeated analyses of solute binding. To our knowledge, five transmembrane proteins have been immobilized in chromatographic matrices: the glucose and nucleoside transporters from human red blood cells, the photosynthetic reaction center from Rhodobacter sphaeroides, the nicotinic acetylcholine receptor from rat brain and a recombinant P-glycoprotein. Proteoliposomes and membrane vesicles have thereby been entrapped in size-exclusion beads, such as Superdex 200, and membrane proteins have been adsorbed on 'immobilized artificial membrane' monolayers of lipid analogs grafted to silica beads. Encouragingly, immobilized glucose transporter and P-glycoprotein showed constant interactant affinities for months. Analysis is done in the frontal mode at equilibrium because there is no separation between bound and free ligand. Both the affinity constant, which generally coincides with the corresponding constant determined by use of nonchromatographic methods, and the amount of active binding sites are obtained. The method has been successfully applied to functional analysis of membrane proteins in cells or reconstituted in lipid mono- or bilayers, screening of low-molecular interactants, investigation of protein-protein interaction and studies of effects of physico-chemical parameters on solute-protein interaction. The analyses require sensitive detection of the analyte and matching between amount of binding sites and affinity.

Characterization of dynamically prepared phospholipid-modified reversed-phase columns

We have modified a reversed-phase (RP8) column by passing through it an aqueous solution of phosphatidylcholine-based liposomes. The phospholipids from the liposomes adsorb onto the octyl chain of the stationary phase, thus altering the nature of the stationary phase and of the chromatographic interactions. The properties of the phospholipid-modified column were investigated using solutes belonging to several chemical classes. We found that the retention factors of negatively and positively charged solutes decreased as the amount of phospholipid in the column was increased. For the solutes studied here the extent of the decrease was smaller for the positive solutes. With neutral solutes, the retention factors of some (benzenediols) increased markedly while with others (ketones) the retention factors decreased. The selectivities between the various solutes on the phospholipid-modified column were different than on the original reversed-phase column. The retention behavior of the solutes can be explained in terms of (1) effective shielding of the hydrophobic part of the stationary phase by the polar head groups of the phospholipids and (2) hydrogen bond formation between the solutes and the carbonyl oxygens as well as the non-ester phosphate oxygens in the polar head groups of the phospholipids.

Freeze-thaw immobilization of liposomes in chromatographic gel beads: evaluation by confocal microscopy and effects of freezing rate

Biological membranes immobilized in chromatographic gel beads constitute a multifunctional affinity matrix. Membrane protein-solute interactions and drug partitioning into the lipid bilayers can conveniently be studied. By the use of confocal laser-scanning microscopy (CLSM) the distribution of immobilized model membranes in the beads has been visualized for the first time. Freeze-thaw-immobilized liposomes in Superdex 200 gel beads were situated in a thick shell surrounding a liposome-free core. The amount of phospholipids immobilized by freeze-thawing was dependent on the temperature in the cooling bath and the type of test tube used. A bath temperature of -25 degrees C gave higher immobilization yield than freezing at -75 or -8 degrees C did. Freeze-thawing in the presence of liposomes did not affect the gel bead shape or the refractive index homogeneity of the agarose network of the beads, as shown by confocal microscopy.

Biomembrane-affinity centrifugal analyses of solute interactions with membrane proteins

We have developed a rapid centrifugal method for analyzing solute interactions with membrane proteins in cytoskeleton-depleted membrane vesicles or proteoliposomes sterically immobilized in Superdex 200 gel beads. The size and density of the gel beads allow fast sedimentation in a bench-top centrifuge. Biospecific interactions of cytochalasin B and D-glucose with the human red cell glucose transporter, Glut1, were analyzed. The binding constants and the molar ratio of inhibitor sites per protein monomer agreed well with recent results obtained by frontal affinity chromatography.

Liquid chromatographic studies with immobilized neuronal nicotinic acetylcholine receptor stationary phases: effects of receptor subtypes, pH and ionic strength on drug-receptor interactions

Nicotinic acetylcholine receptor (nAChR) alpha3-subunits, beta4-subunits, alpha3/beta4-subunit combination and alpha4/beta2-subunit combination were immobilized on chromatographic stationary phases and the binding affinities of the different nAChR subtypes were chromatographically evaluated. The observed relative binding affinities of epibatidine were alpha4/beta2>alpha3/beta4 and epibatidine did not bind at alpha3-subunits and beta4-subunits. No significant difference in binding affinities was observed on the alpha4/beta2 nAChRs immobilized in immobilized artificial membrane (IAM) particles and those sterically immobilized on Superdex 200 beads. The effects of mobile phase pH and ionic strength on the binding affinities of the alpha3/beta4 nAChRs support were also investigated. The results are consistent with the proposed ligand-nAChR binding model in which a cationic center exists at the binding site.

Chromatographic approaches to liposomes, proteoliposomes and biomembrane vesicles

Size-exclusion chromatography has been used for fractionation of liposomes, proteoliposomes and biomembrane vesicles of up to approximately 500 nm in size and for separation of these entities from smaller components. Liposome sizes, encapsulation stability, and solute affinities for membrane proteins have been determined. Counter-current distribution in aqueous two-phase systems has widened the range of applications to larger structures. Immobilized biomembrane vesicles and (proteo)liposomes provide stationary phases for chromatographic analysis of specific or nonspecific membrane-solute interactions.

Chromatography on cells and biomolecular assemblies

Red cells, biomembrane vesicles, proteoliposomes and liposomes non-covalently immobilized in gel particles or beads have been used as stationary phases for biomembrane affinity analyses and ion-exchange chromatographic separation. Lipid monolayers coupled to silica beads have been utilized for membrane protein purification in detergent solution and plant cell walls for group separation of macromolecules according to size and charge. Further methodological studies are essential to implement general practical application.

Glucose affinity for the glucose transporter Glut1 in native or reconstituted lipid bilayers. Temperature-dependence study by biomembrane affinity chromatography

The affinity of D-glucose and the transport inhibitor cytochalasin B (CB) for the glucose transporter Glut1 was studied at 5-42 degrees C by quantitative frontal affinity chromatography on sterically immobilized human red cell membrane vesicles, and on proteoliposomes containing reconstituted red cell membrane proteins. Glut1 in the vesicles showed the highest glucose affinity; the dissociation constant Kd(glc) was nearly constant (16 +/- 3 mM) from 15 degrees C to 37 degrees C. For Glut1 in proteoliposomes Kd(glc) decreased from 56 mM at 5 degrees C to 26 mM at 42 degrees C. The CB-Glut1 affinity was strongest around 20 degrees C and was mostly higher with the vesicles, Kd (CB) being 49 nM at 19 degrees C. The entropy and entropy and enthalpy changes for the interactions were calculated.

D-Glucose, forskolin and cytochalasin B affinities for the glucose transporter Glut1. Study of pH and reconstitution effects by biomembrane affinity chromatography

The affinities of D-glucose and the transport inhibitors, forskolin and cytochalasin B (CB), for Glut1 were studied by frontal affinity chromatography at pH 5-10 on sterically immobilized proteoliposomes with reconstituted human red cell glucose transporter Glut1. The affinity of D-glucose for Glut1 became slightly weaker as the pH was increased. The inhibitor affinities decreased and became immeasurably weak above pH 9. At pH 7.4, the dissociation constants were 44 mM for glucose, 1.8 microM for forskolin and 72 nM for CB. The affinities of these solutes for Glut1 in red cell membrane vesicles and particularly for Glut1 in red cells were higher, as shown by chromatographic analyses.

Immobilization of human red cells in gel particles for chromatographic activity studies of the glucose transporter Glut1

Chromatography on a novel stationary phase, human red cells immobilized in a gel bed, was introduced for analysis of activities of the glucose transporter Glut1 in the cell membrane. A gel containing positively charged ligands was synthesized from derivatized acrylamide monomers. Red cells were immobilized in gel particles which were packed into a column tube for chromatographic analyses over periods of 10-15 days. D-Glucose was separated from L-glucose on a 1.1-ml bed with a retention volume difference of 0.23 ml, approximately equal to the total inner volume of immobilized intact cells and of ghosts probably formed from lysed cells during the immobilization. The separation was suppressed by the glucose-transport inhibitor cytochalasin B. The interactions between D-glucose, the transport inhibitor forskolin and Glut1 were analyzed by quantitative frontal affinity chromatography. The dissociation constants at room temperature were 6.8 mM for D-glucose binding and 1.8 microM for glucose-displaceable binding of forskolin, in good agreement with published values. The results suggest that chromatography on immobilized cells is a potentially useful tool for studies on cellular membrane functions.

Immobilized liposome chromatography of drugs on capillary continuous beds for model analysis of drug-membrane interactions

Liposomes were immobilized in capillary continuous beds with covalently linked C4 or C8 alkyl ligands for chromatographic analysis of drug interaction with phospholipid bilayers, as reflected by drug retention volumes and calculated differences in interaction free energies. This procedure is a high-resolution micro-scale version of immobilized liposome chromatography for prediction of diffusion of drugs across biological membranes. The logarithm of the specific capacity factors of several structurally unrelated drugs showed a linear correlation with the logarithm of known apparent drug permeabilities through Caco-2 epithelial cell monolayers. The latter values are used for prediction of absorption of orally administered drug doses.

Quantitative micellar chromatographic analysis of interaction between peptides and sodium dodecyl sulfate micelles

Interaction between short, water-soluble peptides and sodium dodecyl sulfate (SDS) micelles was analyzed by chromatography of the peptides on a Sephadex G-50 M el bed in an eluent containing the micelles. Peptides that interacted with the micelles showed intermediate elution volumes. In the absence of micelles in the eluent, the peptides (7-16 amino acid residues) were eluted near the total volume, whereas micelles applied as a sample appeared at the void volume. The association constants, KA, were calculated from the capacity factors k' obtained at different SDS micelle concentrations. The range of the KA values was (0.5-11) x 10(4) M-1. As a rule, the longest peptides and the positively charged ones interacted most strongly with the micelles, but the amino acid sequences also affected the interaction. Partially hydrophobic peptides that may correspond to interfacial segments of a transmembrane protein, the glucose transporter Glutl, showed relatively weak interactions with SDS micelles. The peptide interaction with sodium 1-decane sulfonate micelles supplemented with SDS was similar to that with SDS micelles, whereas none of several peptides tested interacted significantly with micelles of the non-ionic detergent n-dodecyl octaoxyethylene. The peptide-micelle interaction was proposed to occur mostly at the micelle surface as in the protein-decorated micelle structure for SDS-protein complexes.

Monomeric human red cell glucose transporter (Glut1) in non-ionic detergent solution and a semi-elliptical torus model for detergent binding to membrane proteins

The self-association state of the human red cell glucose transporter (Glut1) in octaethylene glycol n-dodecyl ether (C12E8) and n-octyl beta-D-glucopyranoside (OG) solution was analyzed in the presence of reductant by gel filtration with light-scattering, refractivity and absorbance detection, and by ultracentrifugation. The C12E8-Glut1 complex was essentially monomeric, whereas OG-Glut1 also formed dimers and larger oligomers. C12E8-Glut1 retained substantial glucose transport activity even after depletion of endogenous lipids by gel filtration, as shown by reconstitution and transport measurements. Removal of endogenous lipids from OG-Glut1 abolished the activity unless phosphatidylcholine was included in the eluent. The binding of C12E8 and OG to Glut1 was determined by gel filtration with refractivity and absorbance detection or with radioactive tracer to be 1.86 +/- 0.07 and 1.84 +/- 0.09 g/g polypeptide, respectively. A structural model was proposed in which non-ionic detergent forms a semi-elliptical torus (SET) surrounding the transmembrane protein. The torus thickness was assumed to be equal to the radius (short half-axis) of a spherical (oblate ellipsoidal) free detergent micelle and the polar head groups of the detergent molecules were predicted to be situated just outside the hydrophobic surface of the protein. The experimental detergent binding values and those obtained from the SET model together confirmed that Glut1 was monomeric in C12E8 solution and provided constraints on the shape and size of the hydrophobic transmembrane region of Glut1 in alpha-helical and beta-barrel topology models.

Liposome capillary electrophoresis for analysis of interactions between lipid bilayers and solutes

Liposomes, which mimic biomembranes, were used as a pseudostationary phase in capillary zone electrophoresis. The decrease in the mobility of an analyte owing to the presence of liposomes reflected interaction between the analyte and the liposomes. Equations were derived to calculate the specific capacity factor Ks (the capacity factor, k', normalized to the liposome concentration 1 M) from the migration times and to estimate the difference in free energy, delta(delta G0), of the weak analyte/liposome interactions. The order of Ks values for the drugs tested was aspirin < salicylic acid < warfarin << sulfasalazine. The peptide TyrGlySerThrProGlyCysCys interacted more strongly with the liposomes (Ks = 10.1 M-1) than did TyrGlySerThrProGlySerSer (Ks = 9.1 M-1). These results were similar to those obtained earlier by immobilized liposome chromatography.

Immobilized-liposome chromatographic analysis of drug partitioning into lipid bilayers

The chromatographic retardation of drugs on a gel bed with immobilized liposomes was shown to correlate with the absorption of the drugs through epithelial cell layers, which is related to drug partitioning into the lipid bilayers of cell membranes. The capacity factors were divided by the lipid concentration (mM) in the gel bed to obtain specific capacity factors, Ks. The logarithm of the octanol-water distribution ratios showed a linear correlation with log Ks, whereas the logarithm of the apparent permeability coefficients in epithelial cell monolayers and the absorption of drugs orally administered in humans (data from other laboratories) increased over the interval 0 < log Ks < 1 and attained a saturation level in the interval 1 < log Ks < 3. Immobilized-liposome chromatography may be applicable for prediction of drug uptake through epithelial cell membranes.