Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 1. Inhibition of glucose transport

Studies on the Uptake of Glucose Derivatives by Red Blood Cells

Erythrocytes express the same glucose transporter (GLUT-1) as is present in the blood-brain barrier. With the aim of testing the viability of using this transport system to deliver glucosyl drug derivatives to the brain, the uptake of several dopamine-glucose conjugates and a few structurally related analogues by erythrocytes was studied with HPLC and (1)H MAS NMR spectroscopy. The results showed that slight structural changes determine the uptake of glycoconjugates by red blood cells. However, experiments in the presence of glucose transport inhibitors showed that none of the conjugates that efficiently crossed the cell membrane were transported by GLUT-1.

Synthesis of 6,7-dideoxy-7-isothiocyanatoheptoses: stable fully unprotected monosaccharide isothiocyanates

Methyl 6,7-dideoxy-7-isothiocyanato-alpha-D-gluco (manno)(galacto)-heptopyranosides have been synthesized in four steps by homologation of the respective methyl hexopyranosides via the corresponding heptopyranosydurononitriles. Neither intra- nor intermolecular thiocarbamate formation was observed, even under rather strenuous acidic or basic conditions. The reducing derivative 6,7-dideoxy-7-isothiocyanato-alpha-D-gluco-heptopyranose was also a stable compound in aqueous solution in the absence of base. Formation of a six-membered intramolecular cyclic thiocarbamate was achieved in DMF solution in the presence of triethylamine. The title compounds are the first examples of stable fully unprotected monosaccharide isothiocyanates.

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.

Selective labeling of the erythrocyte hexose carrier with a maleimide derivative of glucosamine: relationship of an exofacial sulfhydryl to carrier conformation and structure

Sulfhydryl-reactive derivatives of glucosamine were synthesized as potentially transportable affinity labels of the human erythrocyte hexose carrier. N-Maleoylglycyl derivatives of either 6- or 2-amino-2-deoxy-D-glucopyranose were the most potent inhibitors of 3-O-methylglucose uptake, with concentrations of half-maximal irreversible inhibition of about 1 mM. Surprisingly, these derivatives were very poorly transported into erythrocytes. They reacted rather with an exofacial sulfhydryl on the carrier following a reversible binding step, the latter possibly to the exofacial substrate binding site. However, their reactivity was determined primarily by access to the exofacial sulfhydryl, which, as predicted by the one-site model of transport, required a carrier conformation with the exofacial substrate binding site exposed. Once reacted, the carrier was "locked" in a conformation unable to reorient inwardly and bind cytochalasin B. In intact erythrocytes the N-maleoylglycyl derivative of 2-[3H]glucosamine labeled predominantly an Mr 45,000-66,000 protein on gel electrophoresis in a quantitative and cytochalasin B inhibitable fashion. By use of changes in carrier conformation induced by competitive transport inhibitors in a "double" differential labeling method, virtually complete selectivity of labeling of the carrier protein was achieved, the latter permitting localization of the reactive exofacial sulfhydryl to an Mr 18,000-20,000 tryptic fragment of the carrier.

Immunological evidence that band 3 is the major glucose transporter of the human erythrocyte membrane

We have previously reported that human erythrocyte band 3 contains 90-95% of the reconstitutable glucose transport activity of the erythrocyte membrane (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33). We have now found that monoclonal and polyclonal antibodies to epitopes on band 3 specifically removed band 3 and more than 90% of the reconstitutable glucose transport activity from unfractionated octylglucoside extracts of erythrocyte membranes; nonimmune serum removed neither. Western blots of whole membrane extracts revealed that the polyclonal antibody to band 4.5 used to isolate cDNA clones presumed to code for the transporter (Mueckler, M., Caruso, C., Baldwin, C.A., Pancio, M., Blench, J., Morris, H.B., Allard, W.J., Lienhard, G.E. and Lodish, H.F. (1985) Science 229, 941-945) reacts strongly with six discrete bands in the 4.5 region. A monoclonal antibody to band 3 also reacts with a Mr 55,000 component of band 4.5. We conclude that band 3 contains the major glucose transporter of human erythrocytes, and that the transport activity in band 4.5 might be attributable to a band 3 fragment. Band 3 is probably a multifunctional transport protein responsible for transport of glucose, anions, and water.

Inhibition of hexose transport and labelling of the hexose carrier in human erythrocytes by an impermeant maleimide derivative of maltose

Maltose-maleimide was synthesized as a potential affinity label for the facilitative hexose carrier with selectivity for exofacial sulphydryl groups. This reagent, although probably a mixture of isomers, did not significantly penetrate the plasma membrane of human erythrocytes at concentrations below 5 mM at 37 degrees C. When allowed to react to completion, it irreversibly inhibited the uptake of 3-O-methylglucose, with a half-maximal response at about 1.5-2.0 mM-reagent. The rate of transport inactivation was a saturable function of the maltose-maleimide concentration. Studies of reaction kinetics and effects of known transport inhibitors demonstrated that irreversible reaction occurred on the exofacial outward-facing carrier, although not at a site involved in substrate binding. Reaction of intact erythrocytes with [14C]maltose-maleimide resulted in labelling of a broad band 4.5 protein of Mr (average) 45,000-66,000 in electrophoretic gels. This protein was very likely the hexose carrier, since its labelling was inhibited by cytochalasin B. Exofacial band 4.5 labelling was stoichiometric with respect to transport inhibition, yielding an estimated 300,000 carriers/cell. These results suggest that the exofacial sulphydryl which reacts with maltose-maleimide is distinct from the substrate binding site on the hexose carrier, but that it confers substantial labelling selectivity to impermeant maleimides. Additionally, the high efficiency of carrier labelling obtained with maltose-maleimide is useful in quantifying numbers of carriers in whole cells.

Binding of cytochalasin B to trypsin and thermolysin fragments of the human erythrocyte hexose transporter

The cleavage of the human erythrocyte hexose transporter by the proteinases trypsin and thermolysin has been studied. When red cell membranes are treated with trypsin, washed and then photolabelled with cytochalasin B, a labelled peak at 18 kDa is obtained. This labelling of the cleaved transporter is D-glucose inhibitable. This probably indicates that the residual 36 kDa portion of the transporter is not required for binding of ligands. Extensive cleavage of the transporter with low concentrations of thermolysin only occurs when transporter is prelabelled with cytochalasin B. This indicates that covalently bound cytochalasin B can cause a conformational change which exposes the thermolysin cleavage site.

Photolabeling of the adipocyte hexose carrier with an aryl azide derivative of maltose

A nitrophenyl azide derivative of maltose, N-(4-azido-2-nitrophenyl)-maltosylamine (NAP-maltosylamine), was synthesized as a potential photoaffinity label for the hexose carrier of the rat adipocyte. This derivative inhibited 3-O-methylglucose uptake with a Ki of 1.3 mM in the dark, while that of maltose was 10.0 mM. Carbon-14-labeled maltose and NAP-maltosylamine entered adipocytes via the hexose carrier, the latter in a concentrative fashion. Photolysis of NAP-[14C]maltosylamine in the presence of an adipocyte low density microsomal membrane fraction labeled several electrophoretic bands. Among these are a 45 kDa band which showed features expected of the hexose carrier: its labeling was decreased 40% by D- but not L-glucose and pretreatment of intact adipocytes with insulin decreased labeling of the 45 kDa band by 10-40%, as predicted by the translocation theory of insulin-stimulated transport activation. These studies show the suitability of using carbon-1-modified sugar photoaffinity labels as probes for the hexose carrier and possibly of its regulation in rat adipocytes.

Re-examination of hexose-transporter inhibition and labelling by hexose isothiocyanates

We have re-examined hexose-transport inhibition by hexose isothiocyanates and find that the inhibition is incomplete, probably because of decomposition of the reagent. The inhibition type is 'mixed', because hexose-transporter ligands such as maltose and cytochalasin B only give partial protection from inhibition. This suggests that a liganded-transporter-hexose isothiocyanate ternary complex is formed. We have compared the labelling of red-blood-cell membranes by [14C]MITC (D-maltose isothiocyanate) with the labelling obtained using a photoaffinity probe (ASA-BMPA [2-N-(4-azidosalicyloyl)-1,3-bis-(D-mannos-4'-yloxy)-2 -propylamine]) which gives specific labelling of the hexose transporter in band 4.5. [14C]MITC gives a partially D-glucose-displaceable labelling of a band 3 component in the same cell preparations which show ASA-BMPA labelling of band 4.5. This eliminates the possibility that band 4.5 labelling can only occur when the HITC (hexose isothiocyanate) binding protein in band 3 is proteolysed. HITC pretreatment does not decrease ASA-BMPA labelling of the exofacial site of band 4.5. This is also consistent with the formation of ternary complex. However, HITC pretreatment inhibits both reversible and photoactivated covalent [3H]cytochalasin B binding to band 4.5. These results suggest that, in the intact cell, interactions between a band 3 HITC-binding component and the inside cytochalasin B-binding site on the hexose transporter in band 4.5 may occur.

Exofacial photoaffinity labelling of the human erythrocyte sugar transporter

The 4-azidosalicylate derivative of 1,3-bis(D-mannos-4'-yloxy)-2-[2-3H]propylamine (ASA-[2-3H]BMPA) has been tested as a photoaffinity label for the sugar transporter in human erythrocytes. When photolysed in the presence of intact erythrocytes, ASA-[2-3H]BMPA covalently binds to the exofacial surface of the transporter. This labelled protein appears as a broad band in the 4.5 region in sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. The peak of radiolabel incorporation gives an apparent Mr of approx. 50 000 on 5-20% acrylamide gels. The binding is 80% inhibitable by 320 mM 4,6-O-ethylidene-D-glucose, by 320 mM D-glucose and by 50 microM cytochalasin B. Photoirradiation of a saturating concentration of ASA-BMPA in the presence of erythrocytes results in a 25-30% loss of D-galactose transport activity. From transport inactivation data and estimations of the amount of ASA-[2-3H]BMPA binding to the transporter it is calculated that there are approx. 220 000 exofacial hexose-transport binding sites per erythrocyte. The labelling of the transporter has been carried out using freshly drawn blood and 4-weeks-old transfusion blood. No change in the binding profile on SDS-polyacrylamide gel electrophoresis was observed. Proteolytic digestion of the ASA-[2-3H]BMPA-labelled transporter with either trypsin or alpha-chymotrypsin results in the appearance of a labelled 19 kDa fragment on SDS-polyacrylamide gel electrophoresis.

Reconstitution of the glucose transport activity of rat adipocytes

Rat epididymal fat cell membrane proteins were extracted from adipocyte ghosts with octylglucoside and incorporated by detergent dialysis into unilamellar phosphatidylcholine vesicles approx. 200 nm in diameter. The rate of glucose transport into the vesicles under zero-trans conditions was substrate dependent, saturable and inhibited by phloretin and cytochalasin B. Their maximum specific transport activity was 35.6 mumol/min per mg protein, and their half saturation constant for glucose was 15 mM. Glucose transport into the reconstituted vesicles was inhibited by only those sugars which competitively inhibited glucose transport into intact adipocytes. A major protein component of the vesicles was a 100 kDa protein which we had previously found to react with the affinity label maltosyl isothiocyanate (Malchoff, D.M., Olansky, L., Pohl, S. and Langdon, R.G. (1981) Fed. Proc. 40, 1893). Removal of adipocyte ghost membrane extrinsic proteins with dimethylmaleic anhydride followed by extraction of the resulting membrane pellet with octylglucoside yielded a solution which contained two major proteins, of Mr 100 000 and 85 000, with very small quantities of lower Mr proteins. Vesicles into which these proteins were incorporated had average specific transport activities of 624 mumol/min per mg protein and half saturation constants of 22 mM. Our results strongly indicate that the native glucose transporter of the rat adipocyte, like that of the human erythrocyte (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33), is a 100 kDa protein.

Possible mechanism of uptake for several compounds in ionized form through human erythrocyte membrane

The mechanism underlying uptake of certain compounds in ionized form across human red blood cell membrane was examined. The ionized forms of salicylate, 5-methoxy-salicylate and phenylalanylphenylalanine were significantly taken up into the interior space of human red blood cells instead of remaining in the membrane. Inhibition of the uptake of these compounds by 4,4'-diisothiocyano-2,2'-disulfonate stilbene and phlorizin indicates that their permeation of the erythrocyte membrane may involve the membrane protein fraction. Chelation at the protein site does not appear to occur. Instead, an amino group in the protein structure may mediate the transport of these ionized compounds.

Promoting effect of concanavalin A on transport of sodium cefoxitin and phenol red from rat rectal compartment

Concanavalin A enhanced the rat rectal absorption of phenol red and cefoxitin at pH 7.4 and the uptake of cefoxitin into brush border membrane vesicles prepared from rat rectal membrane. The enhancing action of concanavalin A demonstrated a sodium ion dependency and was inhibited by the presence of 4,4'-diisothiocyano-2,2'-disulfonate stilbene and phlorizin. This inhibition suggests the involvement of the membrane protein fraction.

Reconstitution of glucose transport using human erythrocyte band 3

Band 3 and the diffuse component of zone 4.5, designated band 4.5.B, have been separately prepared from human erythrocyte membranes and incorporated into the membranes of 150 nm 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles. The rates of glucose influx into these vesicles were measured under zero-trans conditions. Both sets of vesicles exhibited substrate-saturable transport which was inhibited by phloretin. However, the specific activity of the band 3 vesicles, 292 mumol X min-1 X (mg protein)-1, was more than twice that of the band 4.5.B vesicles, and the turnover number of transporters in the band 3 vesicles was at least 4-fold greater than those in the 4.5.B vesicles. Very little background density was visible in the band 4.5 region of erythrocyte membranes protected from degradation. In unprotected membranes, band 4.5.B was abundantly present, could be purified, and had glucose transport activity. Previously we have shown (Biochemistry 19, 1205 (1980] that maltosyl isothiocyanate, an affinity label for the glucose transporter, labelled a single 100 000 Mr protein of the intact erythrocyte membrane. Based upon the results of both affinity labelling and reconstitution we suggest that the native glucose transporter is a component of band 3, and that band 4.5.B contains a partially active fragment of the native transporter.

D-Glucose-sensitive and -insensitive cytochalasin B binding proteins from microvillous plasma membranes of human placenta. Identification of the D-glucose transporter

Cytochalasin B was found to bind to at least two distinct sites in human placental microvillous plasma membrane vesicles, one of which is likely to be intimately associated with the glucose transporter. These sites were distinguished by the specificity of agents able to displace bound cytochalasin B. [3H]Cytochalasin B was displaceable at one site by D-glucose but not by dihydrocytochalasin B; it was displaceable from the other by dihydrocytochalasin B but not by D-glucose. Some binding which could not be displaced by D-glucose + cytochalasin B binding site. Cytochalasin B can be photoincorporated into specific binding proteins by ultraviolet irradiation. D-Glucose specifically prevented such photoaffinity labeling of a microvillous protein component(s) of Mr = 60,000 +/- 2000 as determined by urea-sodium dodecyl sulfate acrylamide gel electrophoresis. This D-glucose-sensitive cytochalasin B binding site of the placenta is likely to be either the glucose transporter or be intimately associated with it. The molecular weight of the placental glucose transporter agrees well with the most widely accepted molecular weight for the human erythrocyte glucose transporter. Dihydrocytochalasin B prevented the photoincorporation of [3H]cytochalasin B into a polypeptide(s) of Mr = 53,000 +/- 2000. This component is probably not associated with placental glucose transport. This report presents the first identification of a sodium-independent glucose transporter from a normal human tissue other than the erythrocyte. It also presents the first molecular weight identification of a human glucose-insensitive high-affinity cytochalasin B binding protein.

An affinity label for alpha 2-adrenergic receptors in rat brain

Clonidine, a potent and highly selective alpha 2-adrenergic agonist of the central nervous system, was modified. Insertion of the strong alkylating isothiocyanate group (NCS) group, at its aromatic residue, makes clonidine a potential affinity label of the alpha 2-adrenergic receptors. In displacement of [3H]clonidine and p-[3H]aminoclonidine from rat brain membrane preparations, clonidine-NCS demonstrates high affinity for the alpha 2-adrenergic receptors (Kd = 50 mM). The covalent labelling of the central alpha 2-receptors requires higher concentrations of the irreversible ligand (1-70 microM), thus indicating possible non-productive interactions at the environment of the receptor site. Only partial protection of the receptors is observed with a reversible alpha 2-agonist. The new clonidine analog appears to be a general ligand for the alpha 2-adrenergic receptors and might serve as a potential affinity probe for these receptors.

The reconstitution of the human erythrocyte sugar transporter in planar bilayer membranes

The degradation of human erythrocyte membrane proteins in relation to the identification of the monosaccharide transporter has been investigated in whole membrane preparations and membrane protein extracts by polyacrylamide gel electrophoresis in sodium n-dodecyl sulphate and iodine-125 labelling. Evidence is presented for the degradation of band 3 polypeptide to lower molecular weight material some of which appears in region 4.5 of the polyacrylamide gel electrophoresis profile. It is found that the degradation process is inhibited by phenylmethylsulphonyl fluoride and is only significant in membrane extracts in the absence of detergent (Triton X-100) and on prolonged incubation at 37 degrees C, conditions which do not prevail during the isolation of membrane protein extracts for reconstitution studies. Extracts of band 3 and band 4.5 have been prepared and reconstituted in bilayer lipid membranes. The permeabilities of the reconstituted systems to D-glucose have been investigated and it is found that only bilayers incorporating band 4.5 exhibited enhanced monosaccharide transport. A linear relationship between D-glucose transport and the concentration of protein in the aqueous phase bathing the bilayers suggests a partitioning of the protein into the bilayer. Reconstitution is stereospecific and inhibited by cytochalasin B.

Hexose transport in L6 muscle cells. Kinetic properties and the number of [3H]cytochalasin B binding sites

(1) Myoblasts in culture (L6 cell line) were used as an in vitro model system, to study the kinetic and pharmacological properties of hexose transport in skeletal muscle tissue. (2) Uptake of 2-deoxy-D-[3H]glucose into L6 cells grown in monolayer culture was judged rate limiting since: (2) The time course of sugar uptake extrapolated to zero, (b) a parallel inhibition of hexose uptake and phosphorylation was caused by cytochalasin B, and (c) very little backflow of the hexose was detected. (3) Uptake of 2-deoxy-D-[3H]glucose by cells in monolayers was linear for at least 20 min and it was stimulated by countertransport. The Kt value was 0.83 mM. Cytochalasin B inhibited uptake non-competitively, and half maximal inhibition was achieved at 0.3 microM. Cytochalasin E (up to 5 microM) did not affect 2-deoxy-D-[3H]glucose uptake. (4) L6 myoblasts, detached by trypsinization, retained the hexose transport activity. Kt in detached cells was 0.96 mM. V was 3.2 nmol/min per mg protein, and half maximal inhibition was observed with 0.25 microM cytochalasin B. (5) [3H]Cytochalasin B binding to detached cells showed saturable and non-saturable components. The former could be further separated into cytochalasin E-sensitive binding (probably associated to cytoskeletal proteins) and cytochalasin E-insensitive binding, a fraction of which was inhibited by D-glucose. The D-glucose sensitive sites amount to 16.3 pmol/mg protein, and showed a Kd of 0.49 microM, which is in close agreement with the Ki of cytochalasin B inhibition of hexose uptake. These sites probably are equivalent to the hexose carrier molecules, and are present at a density of 6.8 . 10(6) sites/cell.

Clonidine p-isothiocyanate, an affinity label for alpha 2-adrenergic receptors on human platelets

Exposure of intact human platelets or platelet membranes to the clonidine analog clonidine p-isothiocyanate (clonidine-NCS), followed by extensive washing, results in the loss of [3H]yohimbine binding to platelet alpha 2-receptors. In addition, exposure of intact platelets to clonidine-NCS, followed by extensive washing, results in the loss of of epinephrine-induced inhibition of adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in frozen--thawed platelets and in purified platelet membranes. This effect is dependent on time and concentration (t 1/2 at 30 degrees C is less than 15 min; half-maximal effect occurs with clonidine-NCS at less than 10 microM). Clonidine-NCS appears to interact by irreversibly blocking the platelet alpha 2-receptors because (i) it abolishes alpha 2-receptor effects of adenylate cyclase activity (i.e., epinephrine-induced inhibition of basal and prostaglandin E1-stimulated activity) while not altering other cyclase activity (basal, prostaglandin E1-stimulated, and NaF-stimulated) and (ii) its effect on both [3H]yohimbine binding and epinephrine-induced inhibition of adenylate cyclase can be specifically prevented by alpha-agonists [(-)-epinephrine and clonidine] and alpha-antagonists (yohimbine and phentolamine). These observations indicate that clonidine-NCS is an effective affinity label for platelet alpha 2-receptors.

Phloretinyl-3'-benzylazide: a high affinity probe for the sugar transporter in human erythrocytes. II. Irreversible transport inhibition is induced by photolysis

In the dark, phloretinyl-3'-benzylazide (PBAz), at a nominal concentration of 10 microM, will inhibit the transport of D-glucose in human erythrocytes by more than 90%. This inhibition can be completely reversed by percolating the cell suspension through a small column of Sephadex G-10; cells recovered after this treatment, and then loaded with 100 mM D-glucose, possess a transport capacity (glucose efflux) equal to untreated cells. The Sephadex matrix completely removes non-covalently bound inhibitor even though, under these conditions (subdued light, 0.2% hematocrit, 0 degrees C, pH 6.2 or 7.8), from 70 to 80% of the PBAz added is bound to the cells (mostly non-specifically to hemoglobin). However, when erythrocytes exposed to 10 microM inhibitor are irradiated with long wavelength ultraviolet light, the glucose transporter is irreversibly inhibited; after 1 min irradiation, about 50% of transporter activity cannot be restored by Sephadex treatment. Under identical conditions, control cells (no PBAz, but irradiated and treated with Sephadex) retain over 90% of carrier activity. The photolytic conversion of the inhibition to an irreversible form is directly dependent on PBAz concentration. The results reaffirm our earlier conclusions that PBAz is a potentially useful photoaffinity labeling agent for the glucose transporter in erythrocyte membranes.

Phloretinyl-3'-benzylazide: a high affinity probe for the sugar transporter in human erythrocytes. I. Hexose transport inhibition and photolabeling of mutarotase

A new phloretin derivative, phloretinyl-3'-benzylazide (PBAz), has been synthesized and compared with phloretin for its ability to inhibit the hexose transporter in human erythrocyte membranes in subdued light. Transport measurements were made using the light scattering (Orskov optical) method and a Millipore filtration technique with isotopically labeled sugars. Initial rates of sugar flux were measured under four different conditions to test for inhibition asymmetry. In each experimental condition, PBAz is from 6-20-times more potent than phloretin, making it one of the most effective reversible inhibitors known. Although both agents penetrate the cell membrane, they apparently fail to reach inhibitory levels at the inner surface over the time course of our nonequilibrated experiments, because of extensive binding to hemoglobin. The mechanism by which PBAz and its parent phloretin inhibit transport is pure competition with hexose for the carrier which faces the exterior of the membrane. If given time to equilibrate with the cells, the inhibition by both agents converts to a mixed type, i.e., both competitive and noncompetitive. The noncompetitive component could be due to inhibition of those transporter units oriented internally. Alternatively pre-equilibration with the inhibitors may cause them to attain high levels in the lipid membrane and produce nonspecific effects. PBAz and its precursor amine, phloretinyl-3'-benzylamine (PBA), compete with glucose for the sugar binding site on mutarotase at least as well as phloretin. When exposed to long wavelength ultraviolet radiation, PBAz is converted to a reactive intermediate which becomes covalently bound to the enzyme. Both irreversible ligand attachment and mutarotase inhibition are related to dose of the azide and irradiation time, but inactivation is from 5 to 6-times greater than label incorporation. We conclude that PBAz is a potentially useful photoaffinity labeling agent capable of covalently interacting with the transporter site facing the exterior of the red cell.

The stereospecific D-glucose transport activity of cholate extracts from human erythrocyte membranes

The glucose transport protein of human erythrocyte membranes was solubilized with cholate to facilitate rapid reconstitution and direct glucose transport measurements. This may simplify the isolation of the native glucose transporter. In most experiments the membranes were prepared from fresh blood within 8 h, frozen in liquid nitrogen and stored at -70 degrees C to minimize proteolytic degradation. Solubilization with 25 mM cholate in the presence of 200 mM NaCl at pH 8.4 for 12 min at room temperature gave a high D-glucose transport activity. The solubilized mixture contained 20% of the total membrane protein, only 6% of the polypeptides of molecular weight around 90000, 23% of the polypeptides of molecular weight around 55000, 30% of the phospholipids and at least 6% of the stereospecific D-glucose transport activity. At cholate concentrations up to 22 mM the ratio of solubilized phospholipids to cholate increased steeply, concomitant with an increase in solubilized activity. Above 30 mM cholate the activity diminished. At 4 degrees C the activity of the extract decreased rapidly within the first day and slowly during the next few days. The initial changes seem to have produced a fairly stable, but not native form or fragment of the transporter. When 20 mM EDTA and 5 mM dithioerythritol were included in the solubilization mixture a high activity was preserved for about one day.

Preparation, properties, and applications of carbohydrate conjugates of proteins and lipids

As a result of the growing awareness of the involvement of the oligosaccharide moieties of glycoproteins and glycolipids in cell surface recognition and binding phenomena, a wide variety of methods have been developed, many quite recently, for preparing glycoconjugates. The chemical methods used for the attachment of sugars and certain hydrophilic polymers (e.g., polyethylene glycol) are discussed, as are the effects of such modifications on various properties of the protein (immune response, thermal stability and resistance to proteolysis, clearance, and specific binding to cell surface receptors). Enzymatic approaches to glycoconjugate preparation are also considered, and several examples are given of the preparation of model glycolipids, useful in studying cell surface phenomena. In a final section, three areas are considered in which rapid advances seem likely to occur: improved methods for the preparation of glycoconjugates; direct modification of cell surface glycoconjugates; and modification for the purpose of studying location and environment of membrane glycoconjugates.

Specific labeling of the lac carrier protein in membrane vesicles of Escherichia coli by a photoaffinity reagent

4-Nitrophenyl-alpha-D-galactopyranoside (NPG) was used as a photoaffinity reagent to specifically inactivate the beta-galactoside transport system in Escherichia coli ML 308-225 membrane vesicles. Photolysis of NPG produced time-dependent, irreversible loss of transport activity with corresponding incorporation of [3H]NPG into the membrane. Both processes were blocked by beta-D-galactopyranosyl 1-thio-beta-D-galactopyranoside, a high-affinity substrate of the lac carrier protein and inactivation of lactose transport was specific because NPG photolysis did not affect proline uptake or the ability of the vesicles to generate an electrochemical proton gradient. Arylation of the lac carrier protein was stoichiometric and resulted in the formation of 0.25 nmol of NPG adduct per mg of membrane protein. All attempts to regenerate transport activity by reillumination in the presence of externally added nucleophiles failed, indicating that arylation is functionally irreversible. When vesicles labeled with [3H]NPG under defined experimental conditions were solubilized and analyzed by gel electrophoresis, only one radioactive peak with an apparent molecular weight of 30,000 was observed, confirming that the reaction is highly specific. The results demonstrate that NPG is an active-site-directed photoaffinity label for the lac carrier protein.