Structural phase transitions involved in the interaction of phospholipid bilayers with octyl glucoside

Solubilization of biomimetic lipid mixtures by some commonly used non-ionic detergents

Detergents are amphiphilic molecules often used to solubilize biological membranes and separate their components. Here we investigate the solubilization of lipid vesicles by the commonly used non-ionic detergents polyoxyethylene (20) oleyl ether (Brij 98), n-octyl-β-D-glucoside (OG), and n-dodecyl β-D maltoside (DDM) and compare the results with the standard detergent Triton X-100 (TX-100). The vesicles were composed of palmitoyl oleoyl phosphatidylcholine (POPC) or of a biomimetic ternary mixture of POPC, egg sphingomyelin (SM) and cholesterol (2:1:2 molar ratio). To follow the solubilization profile of large unilamellar vesicles (LUVs), 90° light scattering measurements were done along the titration of LUVs with the detergents. Then, giant unilamellar vesicles (GUVs) were observed with optical microscopy during exposure to the detergents, to allow direct visualization of the solubilization process. Isothermal titration calorimetry (ITC) was used to assess the binding constant of the detergents in POPC bilayers. The results show that the incorporation of TX-100, Brij 98 and, to a lesser extent, OG in the pure POPC liposomes leads to an increase in the vesicle area, which indicates their ability to redistribute between the two leaflets of the membrane in a short scale of time. On the other hand, DDM incorporates mainly in the external leaflet causing an increase in vesicle curvature/tension leading ultimately to vesicle burst. Only TX-100 and OG were able to completely solubilize the POPC vesicles, whereas the biomimetic ternary mixture was partially insoluble in all detergents tested. TX-100 and OG were able to incorporate in the bilayer of the ternary mixture and induce macroscopic phase separation of liquid-ordered (Lo) and liquid-disordered (Ld) domains, with selective solubilization of the latter. Combination of ITC data with turbidity results showed that TX-100 and OG can be incorporated up to almost 0.3 detergent/lipid, significantly more than Brij 98 and DDM. This fact seems to be directly related to their higher capacity to solubilize POPC membranes and their ability to induce macroscopic phase separation in the biomimetic lipid mixture.

Simple Thermodynamic Description of the Micellar-Bilayer State Transition of Assemblies Composed of n-Octyl-β-D-glucopyranoside and 1,2-Dioleolyl-sn-glycero-3-phosphocholine Dispersed in Aqueous Media or Supported on Solid Substrates

In the preceding paper, we investigated a mixed assembly composed of a nonionic surfactant, n-octyl-β-D-glucopyranoside (OG), and an amphoteric lipid, 1,2-dioleolyl-sn-glycero-3-phosphocholine (DOPC), formed on hydrophilized solid substrates immersed in aqueous solutions containing OG and DOPC. The experimental data could be interpreted in terms of the phase equilibrium; thus, the partition equilibrium profile of OG between the bulk solution phase and the supported assembly phase was obtained, as well as that between the bulk solution and the dispersed assembly. The partition equilibrium profiles suggested that micellar-bilayer state transitions occur both in the supported assembly and in the dispersed one in a roughly synchronized manner, even though there are significant discrepancies between them. In this paper, we propose a simple thermodynamic model for the micellar-bilayer transition of the dispersed and supported assembly of OG and DOPC, assuming that the micellar and bilayer states are also pseudo-phases distinct from each other. Using this model, we analyzed these partition equilibrium profiles and concluded that the transition in the supported assembly should mainly be attributed to the transition in the dispersed assembly, which is partly modified by the interaction energy between the supported assembly and the substrate.

Lipid Scrambling Induced by Membrane-Active Substances

The functional roles of the lipid asymmetry of biomembranes are attracting increasing attention. This study characterizes the activity of surfactants to induce transmembrane flip-flop of lipids and thus "scramble" this asymmetry. Detergent-induced lipid scrambling of liposomes mimicking the charge asymmetry of bacterial membranes with 20 mol % of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol in the outer leaflet only was quantified by ζ-potential measurements for octaethylene glycol dodecyl ether (C₁₂EO₈), octyl glucoside (OG), and dodecyl maltoside. Membrane leakage was separately measured by the fluorescence lifetime-based calcein leakage assay and the onset of the membrane-to-micelle transition by isothermal titration calorimetry. Partition coefficients and partial molar areas were obtained as well. For the quickly membrane-permeant C₁₂EO₈ and OG, leakage proceeds at a rather sharp threshold content in the membrane, which is well below the onset of solubilization and little dependent on incubation time; it is accompanied by fast lipid scrambling. However, unlike leakage, flip-flop is a relaxation process that speeds up gradually from taking weeks in the detergent-free membrane to minutes or less in the leaking membrane. Hence, after 24 h of incubation, 10 mol % of C₁₂EO₈ or 50 mol % of OG in the membrane suffice for virtually complete lipid scrambling, whereas leakage remains below 10% for up to 14 mol % of C₁₂EO₈ and 88 mol % of OG. There is thus a concentration window in which lipid scrambling proceeds without leakage. This implies that lipid scrambling must be considered a possible mode of action of antimicrobial peptides and other membrane-active drugs or biomolecules. A related, detergent-based protocol for scrambling the lipid asymmetry of liposomes and maybe cells without compromising their overall integrity would be a very valuable tool to study functions of lipid asymmetry.

The mechanism of detergent solubilization of lipid bilayers

Multiple data are available on the self-assembly of mixtures of bilayer-forming amphiphiles, particularly phospholipids and micelle-forming amphiphiles, commonly denoted detergents. The structure of such mixed assemblies has been thoroughly investigated, described in phase diagrams, and theoretically rationalized in terms of the balance between the large spontaneous curvature of the curvophilic detergent and the curvophobic phospholipids. In this critical review, we discuss the mechanism of this process and try to explain the actual mechanism involved in solubilization. Interestingly, membrane solubilization by some detergents is relatively slow and the common attribute of these detergents is that their trans-bilayer movement, commonly denoted flip-flop, is very slow. Only detergents that can flip into the inner monolayer cause relatively rapid solubilization of detergent-saturated bilayers. This occurs via the following sequence of events: 1), relatively rapid penetration of detergent monomers into the outer monolayer; 2), trans-membrane equilibration of detergent monomers between the two monolayers; 3), saturation of the bilayer by detergents and consequent permeabilization of the membrane; and 4), transition of the whole bilayer to thread-like mixed micelles. When the detergent cannot flip to the inner monolayer, the outer monolayer becomes unstable due to mass imbalance between the monolayers and inclusion of the curvophilic detergent molecules in a flat surface. Consequently, the outer monolayer forms mixed micellar structures within the outer monolayer. Shedding of these micelles into the aqueous solution results in partial solubilization. The consequent leakage of detergent into the liposome results in trans-membrane equilibration of detergent and subsequent micellization through the rapid bilayer-saturation mechanism.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Protective effect caused by the exopolymer excreted by Pseudoalteromonas antarctica NF(3) on liposomes against the action of octyl glucoside

The capacity of the glycoprotein (GP) excreted by Pseudoalteromonas antarctica NF(3), to protect phosphatidylcholine (PC) liposomes against the action of octyl glucoside (OG) was studied in detail. Increasing amounts of GP assembled with liposomes resulted for the same interaction step in a linear increase in the effective surfactant to PC molar ratios (Re) and in a linear fall in the surfactant partitioning between bilayer and the aqueous phase (partition coefficients K). Thus, the higher the proportion of GP assembled with liposomes the lower the surfactant ability to alter the permeability of vesicles and the lower its affinity with these bilayer structures. In addition, increasing GP proportions resulted in a progressive increase of the free surfactant concentration (S(W)) needed to produce the same alterations in liposomes. The fact that S(W) was always lower than the surfactant critical micelle concentration indicates that the interaction was mainly ruled by the action of surfactant monomers, regardless of the amount of assembled GP.

Correlation of membrane/water partition coefficients of detergents with the critical micelle concentration

The membrane/water partition coefficients, K, of 15 electrically neutral (non-charged or zwitterionic) detergents were measured with phospholipid vesicles by using isothermal titration calorimetry, and were compared to the corresponding critical micellar concentrations, cmc. The detergents measured were oligo(ethylene oxide) alkyl ethers (C(m)EO(n) with m = 10/n = 3, 7 and m = 12/n = 3.8); alkylglucosides (octyl, decyl); alkylmaltosides (octyl, decyl, dodecyl); diheptanoylphosphatidylcholine; Tritons (X-100, X-114) and CHAPS. A linear relation between the free energies of partitioning into the membrane and micelle formation was found such that K. CMC approximately 1. The identity K. CMC = 1 was used to classify detergents with respect to their membrane disruption potency. "Strong" detergents are characterized by K. CMC < 1 and solubilize lipid membranes at detergent-to-lipid ratios X(b) < 1 (alkylmaltosides, tritons, heptaethylene glycol alkyl ethers). "Weak" detergents are characterized by K. CMC > 1 and accumulate in the membrane- to detergent-to-lipid ratios X(b) > 1 before the bilayer disintegrates (alkylglucosides, pentaethylene glycol dodecyl ether).

Ectoplasmic insertion of a glycosylphosphatidylinositol-anchored protein in glycosphingolipid- and cholesterol-containing phosphatidylcholine vesicles

Glycosylphosphatidylinositol (glycosyl-PtdIns)-anchored proteins are proposed to be clustered in membrane microdomains enriched in cholesterol and glycosphingolipids (GlySphs). We have prepared biomimetic membranes in order to study the possible phenomena of surface aggregation of these membrane components. Phosphatidylcholine liposomes were treated by octylglucoside to insert a glycosyl-PtdIns-protein, alkaline phosphatase (ALP), some cholesterol, and a GlySph, the lactocerebroside. The association of these compounds was shown by centrifugation on a density gradient. The presence of ALP on the surface of the vesicles was shown by the action of a phospholipase, and the presence of the lactocerebroside was shown by the use of a galactose-specific tetravalent lectin. Our data show that total alkaline phosphatase and half to total lactocerebroside were ectoplasmically inserted in the vesicles membrane. In addition, we observed that the presence of small amounts of ALP in the liposomes led to significant changes in membrane stability with regard to detergent, as shown by the changes in the solubilization process monitored by turbidimetry. Furthermore, we have built an original method to study the cohesion of the vesicles membrane, in which some magnesium ions were trapped in the luminal space of the liposomes during several days. The ALP is magnesium-dependent for its catalytic activity and was inhibited after incubation of ALP-containing liposomes in a magnesium-free buffer. The ALP activity was restored by the addition of detergent to the liposomes, due to the release of the luminal magnesium ions. Surface aggregation phenomena will now be investigated by atomic force microscopy.

Ability of the exopolymer excreted by Pseudoalteromonas antarctica NF3, to coat liposomes and to protect these structures against octyl glucoside

The ability of an exopolymer of glycoproteic character (GP) excreted by a new gram-negative specie Pseudoalteromonas antarctica NF3, to coat phosphatidylcholine (PC) liposomes and to protect these bilayers against the action of the nonionic surfactant octyl glucoside (OG) has been investigated. TEM micrographs of freeze-fractured liposome/GP aggregates reveal that the addition of GP to liposomes led to the formation of a covering structure (polymer adsorbed onto the bilayers) that tightly coated PC bilayers. The complete coating was already achieved when the proportion of GP assembled with liposomes was approximately 10% (wt% vs total PC). Higher GP amounts resulted in a growth of this coating structure which exhibited at the highest GP proportion in the system (31% of assembled GP) a multilayered structure. An increasing resistance of PC liposomes to be affected by OG both at sublytic and lytic levels occurred as the proportion of GP in the system rose; this protective effect being more effective when the proportion of assembled GP was 10-20% in weight. Thus, although a direct dependence was found between the growth of the enveloping structure and the resistance of the coated liposomes to be affected by OG, the best protection occurred when the proportion of assembled GP was about 10 wt%.

Solubilization of phosphatidylcholine liposomes by the amphoteric surfactant dodecyl betaine

The interaction of the amphoteric surfactant N-dodecyl-N,N-dimethylbetaine (C12-Bet) with phosphatidylcholine (PC) liposomes was investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattering (SLS) of the system. At sublytic level a initial maximum in the bilayer/water partitioning (K) followed by an abrupt decrease of this parameter occurred as the surfactant to lipid molar ratio (Re) rose. At lytic level a direct dependence was established between both parameters. The fact that the free surfactant concentration at sublytic and lytic levels showed values lower than and similar to its critical micelle concentration indicates that permeability alterations and solubilization were determined, respectively, by the action of surfactant monomer and by the formation of mixed micelles. A direct correlation occurred in the initial interaction steps (up to 50% CF release) between the growth of vesicles their fluidity and Re. A similar direct dependence was established during solubilization (up to 30% SLS) between the fall in both the surfactant-lipid aggregate size, the SLS of the system and Re. This surfactant showed higher capacity to solubilize PC liposomes than that reported by the commonly used non-ionic surfactants octyl glucoside and Triton X-100 and by the anionic one sodium dodecyl sulfate.

Direct formation of mixed micelles in the solubilization of phospholipid liposomes by Triton X-100

The vesicle to micelle transition which results in the interaction of the Triton X-100 surfactant with phosphatidylcholine vesicles was studied by means of dynamic light scattering (at different reading angles) and by freeze-fracture electron microscopy techniques. Vesicle solubilization was produced by the direct formation of mixed micelles without the formation of complex intermediate aggregates. Thus, vesicle to micelle transformation was mainly governed by the progressive formation of mixed micelles within the bilayer. A subsequent separation of these micelles from the liposome surface (vesicle perforation by the formation of surfactant-stabilized holes on the vesicle surface) led to a complete solubilization of liposomes.

Subsolubilizing alterations caused by alkyl glucosides in phosphatidylcholine liposomes

The subsolubilizing alterations caused by a series of alkyl glucosides (alkyl chain lengths ranging from C8 to C12) in unilamellar phosphatidylcholine (PC) liposomes were investigated. The surfactant to phospholipid molar ratios (RE) and the normalized bilayer/aqueous phase partition coefficients (K) were determined by monitoring the increase of the fluorescence intensity of liposome suspensions due to the 5(6)-carboxyfluorescein (CF) released from the interior of vesicles to the bulk aqueous phase. Given that the free surfactant concentrations was always lower than the critical micelle concentration (CMC) of the surfactant tested we may assume that the surfactant-liposome interactions were mainly ruled by the action of surfactant monomers. In general terms, the decrease in the surfactant alkyl chain length (or the rise in the surfactant CMC) resulted in an increase in the ability of these surfactants to alter the permeability of liposomes and, inversely, in an abrupt decrease in their affinity with these bilayers structures. The overall balance of these opposite tendencies shows that at the two interaction levels studied (50 and 100% of CF release) the nonyl and the octyl glucoside showed, respectively, the highest ability to alter the release of the CF trapped in bilayers (lowest RE values), whereas the dodecyl glucoside showed the highest degree of partitioning into liposomes or affinity with these bilayer structures (highest K values).

Interaction of octyl-beta-thioglucopyranoside with lipid membranes

Octyl-beta-thioglucopyranoside (octyl thioglucoside, OTG) is a nonionic surfactant used for the purification, reconstitution, and crystallization of membrane proteins. The thermodynamic properties of the OTG-membrane partition equilibrium are not known and have been investigated here with high-sensitivity titration calorimetry. The critical concentration for inducing the bilayer <==> micelle transition was determined as cD* = 7.3 mM by 90 degree light scattering. All thermodynamic studies were performed well below this limit. Sonified, unilamellar lipid vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with and without cholesterol were employed in the titration calorimetry experiments, and the temperature was varied between 28 degrees C and 45 degrees C. Depending on the surfactant concentration in the membrane, the partition enthalpy was found to be exothermic or endothermic, leading to unusual titration patterns. A quantitative interpretation of all titration curves was possible with the following model: 1) The partitioning of OTG into the membrane follows a simple partition law, i.e., Xb = Kc(D,f), where Xb denotes the molar amount of detergent bound per mole of lipid and c(D,f) is the detergent concentration in bulk solution. 2) The partition enthalpy for the transfer of OTG from the aqueous phase to the membrane depends linearly on the mole fraction, R, of detergent in the membrane. All calorimetric OTG titration curves can be characterized quantitatively by using a composition-dependent partition enthalpy of the form deltaHD(R) = -0.08 + 1.7 R (kcal/mol) (at 28 degrees C). At low OTG concentrations (R < or = 0.05) the reaction enthalpy is exothermic; it becomes distinctly endothermic as more and more surfactant is incorporated into the membrane. OTG has a partition constant of 240 M(-1) and is more hydrophobic than its oxygen-containing analog, octyl-beta-D-glucopyranoside (OG). Including a third nonionic amphiphile, octa(ethyleneoxide) dodecylether (C12EO8), an empirical relation can be established between the Gibbs energies of membrane partitioning, deltaGp, and micelle formation, deltaGmic, with deltaGp = 1.398 + 0.647 deltaGmic (kcal/mol). The partition constant of OTG is practically independent of temperature and of the cholesterol content of the membrane. In contrast, the partition enthalpy shows a strong temperature dependence. The molar specific heat capacity of the transfer of OTG from the aqueous phase to the membrane is deltaCp = -98 cal/(mol x K). The OTG partition enthalpy is also dependent on the cholesterol content of the membrane. It increases by approximately 1 kcal/mol at 50 mol% cholesterol. As the partition constant remains unchanged, the increase in enthalpy is compensated for by a corresponding increase in entropy, presumably caused by a restructuring of the membrane hydration layer.

Assembly Properties of a Glycoprotein Produced by Pseudoalteromonas antarctica, NF3

The self-assembly properties of an extracellular material of glycoprotein character produced by a new Gram-negative species, NF3, Pseudoalteromonas antarctica, isolated from muddy soil samples of Antarctica have been investigated. The aggregation behavior of this exopolymer was studied directly by transmission electron microscopy (TEM) and analysis of digitalized TEM images of its aqueous dispersions before and after sonication. Increasing amounts of glycoprotein (GP) in water led to an abrupt decrease in the dispersion surface tensions up to a GP concentration of about 0.20 mg/ml (from 72 to 47 mN m-1), followed by an almost constant surface tension value. The size distribution curves of the aggregates formed always showed a bimodal distribution. The mean size of these two aggregates increased as GP concentration increased (first peak from 120 to 140 nm and second peak from 500 to 700 nm), reaching in both cases almost a constant value also for 0.20 mg of GP/ml of water. TEM images of unsonicated aqueous GP dispersions at concentrations lower and higher than 0.20 mg/ml always showed the coexistence of concentric multilamellar and small unilamellar aggregates, the small particles being the dominant class in the first case. Sonication of these dispersions revealed that each lamella of the initial multilamellar structures was made up of various subunits of coiled coil, whereas the smaller particles were not composed of these subunits. Profiles from digitalized TEM images of unsonicated and sonicated dispersions confirm that each lamella of large aggregates was composed of three subunits. Copyright 1997Academic Press

Solubilizing effects caused by the nonionic surfactant dodecylmaltoside in phosphatidylcholine liposomes

The interaction of the nonionic surfactant dodecylmaltoside (DM) with phosphatidylcholine liposomes was investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein released from the interior of vesicles and bilayer solubilization as a decrease in the static light scattered by liposome suspensions. This surfactant showed higher capacity to saturate and solubilize PC liposomes and greater affinity with these structures than those reported for the octyl glucoside. At subsolubilizing level an initial maximum in the bilayer/water partitioning (K) followed by an abrupt decrease of this parameter occurred as the effective molar ratio of surfactant to phospholipid in bilayers (Re) rose. However, at solubilizing level a direct dependence was established between both parameters. A direct correlation took place in the initial interaction steps (Re up to 0.28) between the growth of vesicles, their fluidity, and Re. A similar direct dependence was established during solubilization (Re range from 0.9 to 1.7) between the decrease in both the surfactant-PC aggregate size, the light scattering of the system, and Re (composition of aggregates). The fact that the free DM concentration at subsolubilizing and solubilizing levels showed values lower than and similar to its critical micelle concentration indicates that permeability alterations and solubilization were determined, respectively, by the action of surfactant monomer and by the formation of mixed micelles.

Octyl-beta-D-glucopyranoside partitioning into lipid bilayers: thermodynamics of binding and structural changes of the bilayer

The interaction of the nonionic detergent octyl-beta-D-glucopyranoside (OG) with lipid bilayers was studied with high-sensitivity isothermal titration calorimetry (ITC) and solid-state 2H-NMR spectroscopy. The transfer of OG from the aqueous phase to lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can be investigated by employing detergent at concentrations below the critical micellar concentration; it can be defined by a surface partition equilibrium with a partition coefficient of K = 120 +/- 10 M-1, a molar binding enthalpy of delta H degrees D = 1.3 +/- 0.15 kcal/mol, and a free energy of binding of delta G degrees D = -5.2 kcal/mol. The heat of transfer is temperature dependent, with a molar heat capacity of delta CP = -75 cal K-1 mol-1. The large heat capacity and the near-zero delta H are typical for a hydrophobic binding equilibrium. The partition constant K decreased to approximately 100 M-1 for POPC membranes mixed with either negatively charged lipids or cholesterol, but was independent of membrane curvature. In contrast, a much larger variation was observed in the partition enthalpy. delta H degrees D increased by about 50% for large vesicles and by 75% for membranes containing 50 mol% cholesterol. Structural changes in the lipid bilayer were investigated with solid-state 2H-NMR. POPC was selectively deuterated at the headgroup segments and at different positions of the fatty acyl chains, and the measurement of the quadrupolar splittings provided information on the conformation and the order of the bilayer membrane. Addition of OG had almost no influence on the lipid headgroup region, even at concentrations close to bilayer disruption. In contrast, the fluctuations of fatty acyl chain segments located in the inner part of the bilayer increased strongly with increasing OG concentration. The 2H-NMR results demonstrate that the headgroup region is the most stable structural element of the lipid membrane, remaining intact until the disordering of the chains reaches a critical limit. The perturbing effect of OG is thus different from that of another nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), which produces a general disordering at all levels of the lipid bilayer. The OG-POPC interaction was also investigated with POPC monolayers, using a Langmuir trough. In the absence of lipid, the measurement of the Gibbs adsorption isotherm for pure OG solutions yielded an OG surface area of AS = 51 +/- 3 A2. On the other hand, the insertion area AI of OG in a POPC monolayer was determined by a monolayer expansion technique as AI = 58 +/- 10 A2. The similar area requirements with AS approximately AI indicate an almost complete insertion of OG into the lipid monolayer. The OG partition constant for a POPC monolayer at 32 mN/m was Kp approximately 320 M-1 and thus was larger than that for a POPC bilayer.

Reconstitution of the hexose phosphate translocator from the envelope membranes of wheat endosperm amyloplasts

Amyloplasts were isolated and purified from wheat endosperm and the envelope membranes reconstituted into liposomes. Envelope membranes were solubilized in n-octyl beta-D-glucopyranoside and mixed with liposomes supplemented with 5.6 mol% cholesterol to produce proteoliposomes of defined size, which showed negligible leakage of internal substrates. Transport experiments with proteoliposomes revealed a counter-exchange of glucose 1-phosphate (Glc1P), glucose 6-phosphate (Glc6P), inorganic phosphate (Pi), 3-phosphoglycerate and dihydroxyacetone phosphate. The Glc1P/Pi counter-exchange reaction exhibited an apparent K(m) for Glc1P of 0.4 mM. Glc6P was a competitive inhibitor of Glc1P transport (Ki 0.8 mM), and the two hexose phosphates could exchange with each other, indicating the operation of a single carrier protein. Glc1P/Pi antiport in proteoliposomes showed an exchange stoichiometry at pH 8.0 of 1 mol of phosphate transported per mol of sugar phosphate.