A simple and efficient method for reconstitution of amino acid and glucose transport systems from Ehrlich ascites cells

Construction of an alkaline phosphatase-liposome system: a tool for biomineralization study

Alkaline phosphatase is required for the mineralization of bone and cartilage. This enzyme is localized in the matrix vesicle, which plays a role key in calcifying cartilage. In this paper, we standardize a method for construction an alkaline phosphatase liposome system to mimic matrix vesicles and examine a some kinetic behavior of the incorporated enzyme. Polidocanol-solubilized alkaline phosphatase, free of detergent, was incorporated into liposomes constituted from dimyristoylphosphatidylcholine (DMPC), dilaurilphosphatidylcholine (DLPC) or dipalmitoylphosphatidylcholine (DPPC). This process was time-dependent and >95% of the enzyme was incorporated into the liposome after 4h of incubation at 25 degrees C. Although, incorporation was more rapid when vesicles constituted from DPPC were used, the incorporation was more efficient using vesicles constituted from DMPC. The 395nm diameter of the alkaline phosphatase-liposome system was relatively homogeneous and more stable when stored at 4 degrees C. Alkaline phosphatase was completely released from liposome system only using purified phosphatidylinositol-specific phospholipase C (PIPLC). These experiments confirm that the interaction between alkaline phosphatase and lipid bilayer of liposome is via GPI anchor of the enzyme, alone. An important point shown is that an enzyme bound to liposome does not lose the ability to hydrolyze ATP, pyrophosphate and p-nitrophenyl phosphate (PNPP), but a liposome environment affects its kinetic properties, specifically for pyrophosphate. The standardization of such system allows the study of the effect of phospholipids and the enzyme in in vitro and in vivo mineralization, since it reproduces many essential features of the matrix vesicle.

Association of hepatic system A amino acid transporter with the membrane-cytoskeletal proteins ankyrin and fodrin

System A activity is a highly regulated mechanism for the active transport of zwitterionic amino acids into mammalian cells. Monoclonal antibodies generated against a previously unidentified rat liver plasma membrane-associated protein were shown to immunoprecipitate solubilized System A transport activity. The immunoreactive protein was later determined by immunoblotting and peptide microsequencing to be rat liver alpha-fodrin (non-erythroid spectrin). Antibody against ankyrin, a protein that often serves as a bridge between integral membrane proteins and fodrin, also immunoprecipitated System A transport activity. Fractionation of solubilized plasma membrane proteins on sucrose gradients revealed that the System A transporter co-migrated as a complex with fodrin and ankyrin, even in the presence of detergent and urea. In contrast, the System N amino acid transporter does not co-migrate with ankyrin and fodrin, nor does the anti-fodrin antibody immunoprecipitate System N activity. The present data are the first to demonstrate an association between an organic solute transporter and the membranocytoskeletal proteins ankyrin and fodrin.

Identification of the integrin alpha 3 beta 1 as a component of a partially purified A-system amino acid transporter from Ehrlich cell plasma membranes

We have previously reported [McCormick and Johnstone (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7877-7881] the partial purification of the Na(+)-dependent A-system amino acid transporter from Ehrlich cell plasma membranes and have suggested that a 120-130 kDa peptide, a major component of the purified fraction [octyl glucoside (OG) extract], is involved in Na(+)-dependent amino acid transport. In the present study, N-terminal sequence analysis of the 120-130 kDa peptide revealed a sequence similar to that of the alpha 3 subunit of the integrin alpha 3 beta 1. The presence of alpha 3 beta 1 was confirmed by Western blots of the OG extract probed with anti-alpha 3 or -beta 1 antibodies. Western blots also showed that an antibody originally raised against the 120-130 kDa peptide crossreacts with both the alpha 3 and beta 1 integrin subunits. Co-purification of alpha 3 beta 1 and Na(+)-dependent transport activity suggested that the two activities might be associated. Evidence that alpha 3 plays a role in transport is shown by the fact that an antibody against human alpha 3, but not beta 1, removed transport activity (approximately 25% loss) from cholate-solubilized Ehrlich membranes. Further purification of OG extracts using concanavalin A and wheat-germ lectin columns resulted in the separation of transport activity from the bulk (but not all) of alpha 3 beta 1 integrin without loss of the transport activity. These results indicate that the integrin itself is not essential for amino acid transport. Reconstitution of a purified alpha 3 beta 1-depleted protein fraction showed high levels of Na(+)-dependent, alpha-methylaminoisobutyric-acid-inhibitable amino acid transport in proteoliposomes, whereas reconstituted integrin alone showed little transport activity. However, in the integrin-depleted fractions, high amino acid uptake occurred in K+ which compromised the accurate measurement of the Na(+)-dependent component of uptake. The data suggest that alpha 3 may be associated with the A-system transporter and may modulate the activity of this carrier. Moreover, transfection of K562 and RD cells with human alpha 3 and alpha 2 cDNA showed that the former but not the latter increased A-system transport, thus providing more direct evidence that alpha 3 may modulate A-system transport activity.

Solubilization and functional reconstitution of the human placental taurine transporter

The taurine transporter from purified human placental brush-border membranes was solubilized and reconstituted into proteoliposomes in a functional form. Solubilization was done with 2.5% cholate in the presence of 4 M urea. The proteins in the solubilizate were precipitated with 6% poly(ethylene glycol) and the precipitated proteins were reconstituted into proteoliposomes with an asolectin/protein ratio of 10:1. Under these experimental conditions, the taurine transport activity in the proteoliposomes was maximal. SDS-PAGE analysis of proteins, however, revealed that the proteoliposomes still contained a majority of the proteins originally present in the brush-border membranes. Uptake of taurine in the reconstituted proteoliposomes was obligatorily dependent on the presence of Na+ as well as Cl-. Substitution of Na+ with other monovalent cations such as K+ and Li+ reduced the taurine transport activity drastically. Similarly, substitution of Cl- with other monovalent anions such as SCN-, F-, I- and NO3- could support the transport activity only to a maximum of 30% of the control activity. In the presence of Cl-, the uptake rate was sigmoidally related to Na+ concentration, resulting in a Na+/taurine coupling ratio of 2:1. The apparent dissociation constant for Na+ was about 195 mM. In the presence of Na+, the uptake rate was hyperbolically related to Cl- concentration, indicating a Cl-/taurine coupling ratio of 1:1. The apparent dissociation constant for Cl- was about 205 mM. The NaCl-dependent taurine uptake was stimulated by an inside-negative membrane potential, showing that the uptake process was electrogenic. The uptake system was specific for beta-amino acids. The affinity of the system for taurine was high with an apparent dissociation constant of 2.7 +/- 0.1 microM. It is concluded that the taurine transporter can be dislodged from the placental brush-border membranes and reconstituted in a catalytically active form in proteoliposomes with no significant change in its characteristics.

Reconstitution of the human placental 5-hydroxytryptamine transporter in a catalytically active form after detergent solubilization

The 5-hydroxytryptamine (5-HT; serotonin) transporter was solubilized from purified human placental brush border membranes by cholate in the presence of urea, and the solubilized transporter was reconstituted into proteoliposomes in a functionally active form. Solubilization of the membranes with cholate in the absence of urea inactivated the transporter. The reconstitution procedure involved precipitation of the solubilized proteins and simultaneous removal of cholate and urea by poly(ethylene glycol), and incorporation of the precipitated proteins into proteoliposomes in the presence of asolectin by a freeze-thaw/sonication technique. Optimal conditions included the use of 6% poly(ethylene glycol) during the precipitation step and an asolectin/protein ratio of 10:1 during the reconstitution step. K+ was present in the reconstitution medium. The reconstituted proteoliposomes showed the ability to transport 5-HT against a concentration gradient when an inwardly directed NaCl gradient was imposed. The 5-HT transport system in the proteoliposomes had an absolute requirement for Na+ and Cl-. The system was specific for 5-HT and was inhibited by imipramine, paroxetine and fluoxetine. The Na+/Cl-/5-HT stoichiometry was found to be 1:1:1. The transport process was electrically silent, indicating that one K+ ion was countertransported for each 5-HT molecule. The reconstituted 5-HT transporter showed high affinity for 5-HT, with an apparent Michaelis-Menten constant of 0.34 +/- 0.01 microM. It is concluded that the human placental 5-HT transporter can be solubilized and reconstituted into proteoliposomes in a transport-competent form and that the characteristics of the reconstituted transporter are similar to those of the native transporter.

Amino acid deprivation leads to the emergence of System A activity and the synthesis of a specific membrane glycoprotein in the bovine renal epithelial cell line NBL-1

1. Amino acid deprivation of confluent monolayers of the bovine renal epithelial cell line NBL-1 causes a stimulation of Na(+)-dependent alanine transport. 2. This stimulation is mediated by a protein-synthesis-dependent induction of 2-(methylamino)isobutyric acid (methyl-AIB)-sensitive alanine transport activity (System A), which was not previously present in these cells. 3. Induction was prevented by the addition of methyl-AIB, alanine or glutamine. 4. Tunicamycin prevented the induction of alanine transport activity. 5. Induction of System A activity was accompanied by incorporation of [3H]mannose into a single membrane protein band of molecular mass 113-140 kDa. 6. These results are consistent with the possibility that induced System A activity in confluent NBL-1 cells is mediated by the synthesis of a 113-140 kDa membrane glycoprotein.

Reconstitution and identification of the major Na(+)-dependent neutral amino acid-transport protein from bovine renal brush-border membrane vesicles

Amino acid transport activity from bovine renal brush-border membrane vesicles (BBMV) was reconstituted into phospholipid vesicles composed of phosphatidylcholine/5% stearylamine. Reconstitutable transport activity was enhanced in protein fractions binding to various lectins. When solubilized BBMV were fractionated on peanut lectin, a single protein band of average molecular mass 132 kDa was obtained. When this protein fraction was reconstituted into phospholipid membrane vesicles, amino acid transport activity was obtained with properties similar to those in native BBMV with regard to amino acid specificity, although the cation specificity was different. A monoclonal antibody which reacted with the same protein removed reconstitutable amino acid transport activity from solubilized BBMV. These findings may provide the first identification of a renal amino acid-transporting protein, although confirmation of this identification by other approaches will be required.

Diamines interfere with the transport of L-ornithine in Ehrlich-cell plasma-membrane vesicles

1. L-Ornithine transport by plasma-membrane vesicles isolated from Ehrlich cells is Na(+)-independent and shows a saturable and a diffusional component. 2. Putrescine, histamine, 5-hydroxytryptamine and 2,3-diaminopropane at 55 microM concentration significantly inhibit 0.5 mM-L-ornithine transport at least for the first 10 min of incubation. 3. There is a trans-stimulatory effect of putrescine on L-ornithine transport.

Sodium-dependent L-serine transport in plasma membrane vesicles isolated from Ehrlich cells by two-phase compartmentation

Plasma membrane vesicles were prepared from Ehrlich cells using two-phase system compartmentation. The highly pure plasma membrane vesicles obtained presented a negligible mitochondrial contamination and were suitable for studies of amino acid transport. L-Serine transport showed a clear ionic specificity, maximum incorporation being observed when an inwardly directed NaSCN gradient was used. Na(+)-dependent L-serine transport was dependent on assay temperature and membrane potential, and it seemed to be carried out by two different transport systems. An essential sulfhydryl group seemed to be involved in the transport process.

L-glutamine transport in native vesicles isolated from Ehrlich ascites tumor cell membranes

Native vesicles isolated from Ehrlich ascites tumor cells accumulate glutamine by means of Na(+)-dependent transport systems; thiocyanate seems to be the more effective anion. The apparent affinity constant for the process was 0.38 mM. The Arrhenius plot gave an apparent activation energy of 12.3 kJ/mol. The structural analogs of glutamine, acivicin (2.5 mM) and azaserine (2.5 mM), inhibited the net uptake by 67 and 70%, respectively. The sulfhydryl reagents mersalyl, PCMBS, NEM, and DTNB also inhibited net uptake, suggesting that sulfhydryl groups may be involved in the activity of the carrier protein. A strong inhibition was detected when the vesicles were incubated in the presence of alanine, cysteine, or serine; in addition, histidine, but not glutamate or leucine, had a negative effect on glutamine transport.

Functional reconstitution of the hepatic system N amino acid transport activity

Hepatic System N is responsible for the active plasma-membrane transport of glutamine, histidine and asparagine. This report describes the solubilization and reconstitution of System N activity. Differential solubility resulted in an approximate enrichment of almost 600-fold compared with total cell homogenate. The results indicate that reconstitution can be utilized as a functional assay during purification of the hepatic System N carrier protein.

Reconstitution of the L-leucine-H+ cotransporter of the plasma membrane from Chang liver cells into proteoliposomes

L-Leucine is cotransported with H+ in the plasma membrane of Chang liver cells (Mitsumoto, Y. et al. (1986) J. Biol. Chem. 261, 4549). The leucine transport system was solubilized from the plasma membrane of the cells with ocytl glucoside and reconstituted in proteoliposomes prepared by a rapid dilution of a mixture of the solubilized proteins, octyl glucoside and liposomes. The proteoliposomes exhibited H(+)-gradient and electrical potential-stimulated leucine uptake. The H(+)-gradient-stimulated leucine uptake could be completely inhibited by carbonyl cyanide p-trifluoro-methoxyphenylhydrazone (FCCP) and 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH). The stimulatory effect of H+ gradient on leucine uptake was shown to be mainly due to decrease of the Km, but not to change of the Vmax, of the transport kinetics. These results suggest that the leucine-H+ cotransporter is solubilized and reconstituted into proteoliposomes.

Substrate-dependent adaptive regulation and trans-inhibition of System A-mediated amino acid transport. Studies using rat hepatoma plasma membrane vesicles

Substrate-dependent regulation of amino acid transport by System A occurs by both direct action at the carrier (trans-inhibition) and transcriptional control (adaptive regulation). While experiments with intact cells have led to working models that describe these regulatory phenomena, the use of subcellular approaches will serve to refine the present hypotheses. Adaptive induction of System A transport following amino acid starvation of cells was shown to be dependent on de novo RNA and protein synthesis, and the stimulated activity was shown to be retained in isolated plasma membrane vesicles. This stimulated transport activity was tightly associated with the plasma membrane, but could be solubilized by 4 M urea and 2.5% cholate, and recovered following reconstitution of the protein into artificial proteoliposomes. These data support the working hypothesis that adaptive induction of transport is the result of de novo synthesis and insertion into the plasma membrane of System A carrier protein. In contrast, the activity of System ASC in the vesicles from the amino acid starved cells was actually reduced by 2-5-fold when compared to amino acid-fed cells. A more rapid form of regulation of System A activity is trans-inhibition. The use of isolated plasma membrane vesicles demonstrated that trans-inhibition in whole cells did not survive membrane isolation. However, substrate loading of isolated membrane vesicles containing high levels of System A activity, produced trans-inhibition in a very specific manner in that System A substrates resulted in decreased transport activity, while those amino acids which are poor substrates for the System A carrier did not. Thus, trans-inhibition is not the result of a recycling process involving an intracellular pool of carriers, but rather can be accounted for by differences in the kinetics for amino acid binding and/or translocation on the two sides of the membrane.

Solubilization and reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells

Uptake of [3H]uridine by Ehrlich cells was mediated by both nitrobenzylthioinosine (NBMPR)-sensitive (75%) and NBMPR-insensitive (25%) mechanisms. Each cell contained approx. 26,000 high-affinity (KD = 0.19 nM) recognition sites for [3H]NBMPR, and binding was inhibited by dipyridamole and adenosine at concentrations similar to those required for inhibition of [3H]uridine uptake. Calculations show that each cell contains a total of about 35,000 nucleoside transporters. Photoaffinity labelling of a partially purified preparation of plasma membranes with [3H]NBMPR resulted in a single broad 3H-labelled band on SDS/polyacrylamide gels, with an apparent molecular-mass peak of 42 kDa. This is in contrast with human erythrocyte membranes, where [3H]NBMPR photolabelled two broad bands with peaks at 55 and 80 kDa. Treatment of photoaffinity-labelled membranes with endoglycosidase F decreased the apparent molecular masses of both the Ehrlich-cell and erythrocyte [3H]NBMPR-labelled proteins to approx. 40 kDa. These results suggest that the human erythrocyte [3H]NBMPR-binding polypeptides are more extensively glycosylated than the corresponding Ehrlich-cell polypeptides. Octyl beta-D-glucopyranoside [1.0% (w/v) + asolectin] solubilized over 90% of the [3H]NBMPR-binding sites, with near-complete retention of [3H]NBMPR-binding characteristics. The only major change was a 65-fold decrease in affinity for dipyridamole, which was partly reversed upon incorporation of the solubilized proteins into asolectin membranes. Proteoliposomes, prepared by using asolectin and the octyl glucoside-solubilized plasma membranes, were capable of accumulating [3H]uridine via a protein-dependent dipyridamole/nitrobenzylthioguanosine/dilazep-sensitive mechanism. We have thus demonstrated the efficient solubilization and functional reconstitution of a nucleoside-transport system from Ehrlich ascites-tumour cells.

Solubilization and reconstitution of the sodium-and-chloride-coupled glycine transporter from rat spinal cord

Synaptic membranes from rat spinal cord were solubilized in the presence of 2% sodium cholate, phospholipids and 15% ammonium sulphate. The soluble extract was incorporated into liposomes consisting of asolectin and crude rat brain lipids. Reconstitution of the functional transporter protein was achieved by removal of detergent by gel filtration. Several parameters proved to be important for optimal reconstitution efficiency: (a) the lipid composition of the liposomes, (b) the type of detergent, and (c) the phospholipid/protein and detergent/protein ratio during reconstitution. In the reconstituted system, the transport of glycine showed a specific activity about twice that of native vesicles. The ionic dependence of the transport, the inhibitory effect of nigericin in the presence of external sodium and the stimulatory effect of valinomycin in the presence of internal potassium on glycine transport were preserved and more clearly observed in the reconstituted system. These results indicate that, in this preparation, the glycine transporter protein retains the same features displayed in the synaptic plasma membrane vesicles, namely dependence on sodium and chloride, electrogenicity and inhibitor sensitivity.

A rapid method for the functional reconstitution of amino acid transport systems from rat liver plasma membranes. Partial purification of System A

A rapid method for the functional reconstruction of amino acid transport from liver plasma-membrane vesicles using the neutral detergent decanoyl-N-glucamide ('MEGA-10') is described. The method is a modification of that previously employed in this laboratory for reconstitution of amino acid transport systems from kidney brush-border membranes [Lynch & McGivan (1987) Biochem. J. 244, 503-508]. The transport activities termed 'System A', 'System N', and 'System L' are all reconstituted. The reconstitution procedure is rapid and efficient and is suitable as an assay for transport activity in studies involving membrane fractionation. By using this reconstitution procedure, System A transport activity was partially purified by lectin-affinity chromatography.

Simple and effective purification of a Na+-dependent amino acid transport system from Ehrlich ascites cell plasma membrane

A reconstitution assay was used to measure transport activity during purification of a Na+-dependent amino acid transporter from Ehrlich cell plasma membrane. Cholate/urea-solubilized membranes were fractionated on a Sepharose 6B column and transport activity was recovered in the column void volume. Centrifugation of the void volume fraction at 105,000 X g and reextraction of the pellet with 1% octyl glucoside led to recovery of an extract whose specific transport activity was nearly 30-fold higher than that of the original solubilized extract with a recovery of 38% of the original activity. The properties of amino acid uptake in the purified reconstituted transporter were identical to those in native plasma membrane vesicles. The major component present in the purified fraction had a molecular mass of 120-130 kDa. Strong evidence that this 120- to 130-kDa peptide contains a component of the amino acid transporter was obtained by immunoprecipitation of transport activity from solubilized membranes with an antibody against the 120- to 130-kDa peptide. This study tentatively identifies a component of the Na+-dependent amino acid transporter as a peptide with an apparent molecular mass of 120-130 kDa.

Kinetic characterization and radiation-target sizing of the glucose transporter in cardiac sarcolemmal vesicles

Stereospecific glucose transport was assayed and characterized in bovine cardiac sarcolemmal vesicles. Sarcolemmal vesicles were incubated with D-[3H]glucose or L-[3H]glucose at 25 degrees C. The reaction was terminated by rapid addition of 4 mM HgCl2 and vesicles were immediately collected on glass fiber filters for quantification of accumulated [3H]glucose. Non-specific diffusion of L-[3H]glucose was never more than 11% of total D-[3H]glucose transport into the vesicles. Stereospecific uptake of D-[3H]glucose reached a maximum level by 20 s. Cytochalasin B (50 microM) inhibited specific transport of D-[3H]glucose to the level of that for non-specific diffusion. The vesicles exhibited saturable transport (Km = 9.3 mM; Vmax = 2.6 nmol/mg per s) and the transporter turnover number was 197 glucose molecules per transporter per s. The molecular sizes of the cytochalasin B binding protein and the D-glucose transport protein in sarcolemmal vesicles were estimated by radiation inactivation. These values were 77 and 101 kDa, respectively, and by the Wilcoxen Rank Sum Test were not significantly different from each other.

A rapid method for the reconstitution of Na+-dependent neutral amino acid transport from bovine renal brush-border membranes

1. A simple and rapid method for the reconstitution of Na+-dependent neutral amino acid transport activity from bovine renal brush border membranes is described. 2. The neutral detergent decanoyl-N-methylglucamide ('MEGA-10') was employed to solubilize the membrane protein. This obviated the necessity for a prolonged dialysis step. 3. The properties of amino acid transport in these vesicles were similar to those observed in native membranes. 4. This should be a useful procedure in the eventual identification and isolation of amino acid transport proteins.

Asymmetric reconstitution of the glucose transporter from Ehrlich ascites cell plasma membrane: role of alkali cations

Gel chromatography of solubilized Ehrlich cell plasma membranes and preformed asolectin vesicles coupled to a freeze-thaw cycle results in the reconstitution of 3-O-methyl-D-glucose transport. The transport activity of the liposomes formed is critically dependent on the cation present during reconstitution. Liposomes formed in K+ show high levels of carrier-mediated 3-O-methyl-D-glucose uptake (495 pmol/min/mg protein) while those formed in Na+ do not (33 pmol/min/mg protein). The inactivity in Na+ is not due to a diminished incorporation of glucose transporter nor is it due to carrier molecules reconstituted with a different orientation from those in K+ liposomes. Instead, the low glucose transport level in Na+ liposomes is related to the small size of vesicles formed with Na+. A second freeze-thaw cycle in K+ causes a two- to threefold increase in the available intravesicular volume of Na+ liposomes and results in an eightfold increase in carrier-mediated 3-O-methyl-D-glucose uptake. K+ liposomes, treated in an identical manner, show only a twofold increase in uptake. The glucose transporter was identified as a protein with a molecular mass range of 44.7 to 66.8 kDa, by the D-glucose-inhibitable photoincorporation of [3H]cytochalasin B. The carrier protein is inserted in reconstituted vesicles in a nonrandom manner with at least 80% of the molecules oriented with the cytoplasmic domain accessible to the external medium. In contrast, the neutral Na+-dependent amino acid transport system appears to be randomly reconstituted.

Methionine-sensitive glycolysis in transformed cells

Glycolysis in several tumor cell lines grown in tissue culture was inhibited by methionine. Kirsten murine sarcoma virus-transformed rat kidney cells (K-NRK) were inhibited 60-75% by 10 mM methionine, whereas normal rat kidney (NRK-49F) cells showed little or no inhibition. Inhibition of glycolysis in K-NRK cells was manifest 2-4 hr after exposure to the amino acid. Glycolysis in a chemically transformed cell line of Madin-Darby canine kidney cells was also sensitive to methionine, but maximal inhibition (75%) required 18-24 hr of incubation with the amino acid. Under the same conditions glycolysis in the nontransformed canine cells was less than 20% inhibited by methionine. In Ehrlich ascites tumor cells grown in tissue culture, 10 mM methionine inhibited glycolysis by about 50%. Inhibition of glycolysis, even by 50 mM methionine, was rapidly reversible. Within 2 hr after removal of methionine the rate of glycolytic activity was restored to that observed in control cells. Furthermore, inhibition by methionine required a minimum level (7%) of serum in the growth medium and inhibition was not sensitive to cycloheximide. Only amino acids that are transported by system A (including the nonmetabolized analogue methylaminoisobutyric acid) specifically inhibited glycolysis in tumor cells. The only exception was phenylalanine, which was toxic to both transformed and normal cell lines.

Reconstitution of the glucose transporter from bovine heart

Reconstitution of the glucose transporter from heart should be useful as an assay in its purification and in the study of its regulation. We have prepared plasma membranes from bovine heart which display D-glucose reversible binding of cytochalasin B (33 pmol sites/mg protein; Kd = 0.2 muM). The membrane proteins were reconstituted into liposomes by the freeze-thaw procedure. Reconstituted liposomes showed D-glucose transport activity which was stereospecific, saturable and inhibited by cytochalasin B, phloretin, and mercuric chloride. Compared to membrane proteins reconstituted directly, proteins obtained by dispersal of the membranes with low concentrations of cholate or by cholate solubilization showed 1.2- or 2.3-fold higher specific activities for reconstituted transport, respectively. SDS-polyacrylamide gel electrophoresis followed by electrophoretic protein transfer and labeling with antisera prepared against the human erythrocyte transporter identified a single band of about 45 kDa in membranes from both dog and bovine hearts, a size similar to that reported for a number of other glucose transporters in various animals and tissues.