Inactivation of the renal outer cortical brush-border membrane D-glucose transporter by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline

The inactivation of the renal outer cortical brush-border membrane D-glucose transporter by the covalent carboxyl reagent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) is studied by monitoring its effects on sodium-dependent phlorizin binding to the active site of the carrier. In the presence of EEDQ, this component of phlorizin binding decreases exponentially and irreversibly with time. The order of this inactivation reaction is very close to 1, indicating that EEDQ modifies the transporter at a single essential site. This site can be partially protected by glucose and by other substrates of the transporter and completely protected by phlorizin, a nontransported competitive inhibitor. By contrast, sodium, a co-transported activator, has no protective effect. The concentration dependence of the protection provided by glucose and phlorizin indicates that the site of action of EEDQ is at or closely related to the substrate binding site on the carrier. The effects of EEDQ on the transporter are mimicked by another carboxyl specific reagent, 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide metho-p-toluenesulfonate. The rate of inactivation of the transporter by EEDQ increases dramatically with decreasing pH, consistent with the hypothesis that the rate-limiting step in the inactivation process is a reaction with an essential carboxyl group. The properties of this group indicate, however, that it is distinct from the carboxyl group proposed by others as forming (a part of) the sodium binding site of sodium-coupled sugar carriers.     

High affinity phlorizin binding to the LLC-PK1 cells exhibits a sodium:phlorizin stoichiometry of 2:1

The phlorizin binding properties of luminal membrane vesicles isolated from the LLC-PK1 cells, a continuous epithelial cell line derived from pig kidney, are studied. Scatchard analysis of this binding indicates the existence of a single high affinity sodium-dependent site with KD = 0.4 microM at 266 mM sodium. The specificity properties of this site indicate that it represents the binding of phlorizin to the hexose binding site of the sodium-dependent D-glucose transporter previously identified in this cell line. Both phlorizin equilibrium binding and the rate of phlorizin binding were found to be sigmoidal functions of sodium concentration. A Hill analysis of these data was consistent with a sodium:phlorizin stoichiometry of 2:1 in good agreement with the sodium:glucose stoichiometry already established in these cells. Phlorizin dissociation was also found to be sodium-dependent. On the basis of the phlorizin binding data presented here, a number of models of the binding of phlorizin and sodium to the transporter can be excluded. An analysis of a random binding model consistent with the data is presented. The significance of the LLC-PK1 sodium-dependent D-glucose transporter as a model system for related renal and intestinal transporters is discussed.     

Single molecule recognition of protein binding epitopes in brush border membranes by force microscopy

Sidedness and accessibility of protein epitopes in intact brush border membrane vesicles were analyzed by detecting single molecule interaction forces using molecular recognition force microscopy in aqueous physiological solutions. Frequent antibody-antigen recognition events were observed with a force microscopy tip carrying an antibody directed against the periplasmically located gamma-glutamyltrans- peptidase, suggesting a right side out orientation of the vesicles. Phlorizin attached to the tips bound to NA+/D-glucose cotransporter molecules present in the vesicles. The recognition was sodium dependent and inhibited by free phlorizin and D-glucose, and revealed an apparent K(D) of 0.2 microM. Binding events were also observed with an antibody directed against the epitope aa603-aa630 close to the C terminus of the transporter. In the presence of phlorizin the probability of antibody binding was reduced but the most probable unbinding force f(u) = 100 pN remained unchanged. In the presence of D-glucose and sodium, however, both the binding probability and the most probable binding force (f(u) = 50 pN) were lower than in its absence. These studies demonstrate that molecular recognition force microscopy is a versatile tool to probe orientation and conformational changes of epitopes of membrane components during binding and trans-membrane transport.     

Characterization of an essential disulfide bond associated with the active site of the renal brush-border membrane d-glucose transporter

In a previous report (J. Biol. Chem. 258 (1983) 3565–3570) we have demonstrated that the disulfide-reducing agent dithiothreitol has two effects on the sodium-dependent outer cortical brush border membrane d-glucose transporter; the first results in a reversible increase in the affinity of the transporter for the non-transported competitive inhibitor phlorizin, while the second results in a partially reversible loss of phlorizin binding and glucose-transport activity. Evidence was presented that both of these effects are the result of the reduction of disulfide bonds on the transport molecule. In the present paper we extend our observations on the inactivation of the transporter by dithiothreitol. We provide evidence here (i) that the inactivation of the transporter by dithiothreitol is independent of the effect of the reducing agent on the affinity of the transporter, (ii) that this inactivation process is first-order in dithiothreitol and thus presumably due to the reduction of a single disulfide bond essential to the functioning of the transporter. (iii) that it is the reduction of this disulfide bond and not some subsequent conformational or other change in the transporter which results in its inactivation, (iv) that phlorizin and substrates of the transporter provide protection against inactivation by dithiothreitol and that the degree of protection provided correlates well with the known specificity and phlorizin-binding properties of the transporter, and (iv) that the reactivity of the transporter with dithiothreitol is pH-dependent, decreasing with increasing pH over the pH range 6.5–8.5. We conclude that this site of action of dithiothreitol is a single essential disulfide bond intimately associated with the glucose-binding site on the transport molecule.     

Cyclosporin binding to a protein component of the renal Na(+)-D-glucose cotransporter

The immunosuppressive and nephrotoxic agent cyclosporin binds to a renal polypeptide with an apparent molecular weight of 75,000 which has been identified as a component of the renal Na(+)-D-glucose cotransporter (Neeb, M., Kunz, U., and Koepsell, H. (1987) J. Biol. Chem. 262, 10718-10729). The same Mr 75,000 polypeptide was covalently labeled with the D-glucose analog 10-N-(bromoacetyl)amino-1-decyl-beta-D-glucopyranoside and with the cyclosporin analog N epsilon-(diazotrifluoroethyl)benzyl-D-Lys8- cyclosporin (CSDZ). CSDZ labeling was decreased when the brush-border membrane proteins were incubated with monoclonal antibodies against the Na(+)-D-glucose cotransporter. In the presence of 145 mM Na+, CSDZ labeling was decreased by D-glucose (1 microM, 1 mM, or 100 mM) and by phlorizin (100 or 500 microM). In the absence of Na+, CSDZ labeling was distinctly increased by 50 microM phlorizin and was slightly increased by 1 mM D-glucose, whereas CSDZ labeling was decreased by 50 microM phloretin and by 500 microM phlorizin. Furthermore, Na(+)-dependent high affinity phlorizin binding to the Na(+)-D-glucose cotransporter was competitively inhibited by cyclosporin A (Ki = 0.04 microM) while Na(+)-D-glucose cotransport was not influenced. The data suggest that a part of the cyclosporin binding domain on the Na(+)-D-glucose cotransporter is identical to the phloretin binding domain of the high affinity phlorizin binding site. While phloretin or the phloretin moiety of phlorizin may directly displace cyclosporin, interaction of D-glucose or of the D-glucose moiety of phlorizin with the transporter may alter the conformation of the cyclosporin binding site and this conformational change may be modulated by Na+.     

Radiation inactivation studies of the renal brush-border membrane phlorizin-binding protein

Renal brush-border membrane vesicles were irradiated in the frozen state with a high energy electron beam. The integral membrane proteins, alkaline phosphatase and 5'-nucleotidase, each showed a single exponential loss of activity with radiation dose, indicating target sizes of 67,000 and 58,000 daltons, respectively. Inactivation of sodium-dependent phlorizin binding to the brush-border membrane D-glucose transporter was more complex. One-half of the phlorizin binding sites were lost after even the smallest doses of radiation suggestive of large functional units (greater than 4 X 10(6) daltons) for a subpopulation of phlorizin binding proteins. The remaining sites behaved as a single radiation target of 110,000 +/- 8,000 daltons and retained the kinetic characteristics commonly associated with phlorizin binding to the glucose transporter. Thus, the data are consistent with the assignment of a molecular weight of 110,000 to the phlorizin binding moiety of the brush-border membrane D-glucose transport protein.     

High-affinity phlorizin binding in Mytilus gill.

The gill of the marine mussel, Mytilus, contains a high affinity, Na-dependent D-glucose transporter capable of accumulating glucose directly from sea water. We examined the ability of the beta-glucoside, phlorizin, to act as a high-affinity ligand of this process in intact gills and isolated brush border membrane vesicles (BBMV). The time course of association of nanomolar [3H]phlorizin to gills and BBMV was slow, with t50 values between 10 and 30 min, and a half-time for dissociation of approx. 30 min. 1 mM D-glucose reduced equilibrium binding of 1 nM phlorizin by 90-95%, indicating that there was little non-specific binding of this ligand to the gill. In addition, there was little, if any, hydrolysis by the gill of phlorizin to its constituents, glucose and phloretin. Phlorizin binding to gills and BBMV was significantly inhibited by the addition of 50 microM concentrations of D-glucose and alpha-methyl-D-glucose, and unaffected by the addition of L-glucose and fructose. Binding to gills and BBMV was reduced by greater than 90% when Na+ was replaced by K+. Replacement of Na+ by Li+ effectively blocked binding to the intact gill, although Li+ did support a limited amount of glucose-specific phlorizin binding in BBMV. The Kd values for glucose-specific phlorizin binding in intact gills and BBMV were 0.5 nM and 6 nM, respectively. We conclude that phlorizin binds with extremely high affinity to the Na-dependent glucose transporter of Mytilus gill, which may be useful in future efforts to isolate and purify the protein(s) involved in integumental glucose transport.     

Histidyl residues at the active site of the Na/succinate cotransporter in rabbit renal brush borders

Summary Mono-, dicarboxylic acid-, andd-glucose transport were measured in brush border vesicles from renal cortex after treatment with reagents known to modify terminal amino, lysyl, -amino, guanidino, serine/threonine, histidyl, tyrosyl, tryptophanyl and carboxylic residues. All three sodium-coupled cotransport systems proved to possess sulfhydryl (and maybe tryptophanyl sulfhydryl, disulfide, thioether and tyrosyl) residues but not at the substrate site or at the allosteric cavity for the Na coion. Histidyl groups seem to be located in the active site of the dicarboxylic transporter in that the simultaneous presence of Na and succinate protects the transporter against the histidyl specific reagent diethylpyrocarbonate. Lithium, which specifically competes for sodium sites in the dicarboxylic acid transporter, substantially blocked the protective effect of Na and succinate. Hydroxylamine specifically reversed the covalent binding of diethylpyrocarbonate to the succinate binding site. The pH dependence of the Na/succinate cotransport is consistent with an involvement of histidyl and sulfhydryl residues. We conclude that a histidyl residue is at, or is close to, the active site of the dicarboxylate transporter in renal brush border membranes.     

Similarity in effects of Na+ gradients and membrane potentials ond-glucose transport by,and phlorizin binding to,vesicles derived from brush borders of rabbit intestinal mucosal cells

Both the presence of sodium and of an electrical potential difference across the membrane have been found to be necessary in order to achieve optimal D-glucose-protectable phlorizin binding to brush border membranes from rabbit small intestine. The effect of delta approximately muNa on phlorizin binding shows a close similarity to that on D-glucose transport, confirming that phlorizin is indeed bound to the D-glucose transporting protein. Possible modulations of binding by a transmembrane potential are discussed on the basis of some models.     

Structural state of the Na+/D-glucose cotransporter in calf kidney brush-border membranes. Target size analysis of Na+-dependent phlorizin binding and Na+-dependent D-glucose transport

Target sizes of the renal sodium-D-glucose cotransport system in brush-border membranes of calf kidney cortex were estimated by radiation inactivation. In brush-border vesicles irradiated at -50 degrees C with 1.5 MeV electron beams, sodium-dependent phlorizin binding, and Na+-dependent D-glucose tracer exchange decreased exponentially with increasing doses of radiation (0.4-4.4 Mrad). Inactivation of phlorizin binding was due to a reduction in the number of high-affinity phlorizin binding sites but not in their affinity. The molecular weight of the Na+-dependent phlorizin binding unit was estimated to be 230 000 +/- 38 000. From the tracer exchange experiments a molecular weight of 345 000 +/- 24 500 was calculated for the D-glucose transport unit. The validity of these target size measurements was established by concomitant measurements of two brush-border enzymes, alkaline phosphatase and gamma-glutamyltransferase, whose target sizes were found to be 68 570 +/- 2670 and 73 500 +/- 2270, respectively. These findings provide further evidence for the assumption that the sodium-D-glucose cotransport system is a multimeric structure, in which distinct complexes are responsible for phlorizin binding and D-glucose translocation.     

A two sodium ion/D-glucose symport mechanism: membrane potential effects on phlorizin binding

Apical membrane vesicles isolated from a continuous renal cell line, LLC-PK1, catalyze electrogenic Na+-stimulated hexose transport and Na+-dependent binding of 3H-labeled 1-[2-(beta-D-glucopyranosyloxy)-4, 6-dihydroxyphenyl]-3-(4-hydroxyphenyl)-1-propanone [( 3H]phlorizin), a competitive ligand of this transport system. Phlorizin was not itself transported across the membrane and thus can serve as a probe of the binding step. The stoichiometry of Na+-dependent phlorizin binding in vesicles was 1:1, whereas Na+/hexose cotransport in vesicles exhibited a 2:1 stoichiometry. Na+ increased the affinity of phlorizin binding without affecting the total number of binding sites. An increased number of Na+-dependent phlorizin binding sites was observed under conditions of interior-negative membrane potential. These results are consistent with a model of the Na+/glucose cotransport cycle in which the unloaded transporter is negatively charged and its orientation influenced by membrane potential. Glucose and one sodium ion interact with the transporter, resulting in an uncharged complex. Binding of a second sodium ion triggers translocation of glucose and both sodium ions via formation of a loaded carrier complex bearing a single positive charge.     

Isolation and partial purification of a Na+-dependent phlorizin receptor from dog kidney proximal tubule

This paper describes a new method for solubilization and partial purification of a Na+-dependent phlorizin receptor from dog kidney proximal convoluted tubule. Selective solubilization is carried out with 0.1% Na+-deoxycholate followed by complete solubilization with 0.5% deoxycholate. The 100,000 X g supernatant of the deoxycholate extract is then subjected to a combination of chromatofocusing and gel exclusion chromatography. Purification is monitored by a new column assay which permits detection of the Na+-dependent high affinity phlorizin receptor in solubilized preparations. Na+-dependent phlorizin binding exhibits the same characteristics on the column assay as in intact brush border vesicles. Binding is temperature-dependent, inhibited by proteolytic agents, Na+-dependent, and inhibited by excess cold phlorizin and D-glucose but not L-glucose. Quantitation of specific binding at different stages of the isolation procedure indicates a final purification of approximately 80-140-fold compared to intact brush border membrane fragments. Enrichment of specific phlorizin binding is paralleled by enrichment of a 61-66-kDa polypeptide on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. It is postulated that this polypeptide contains both the Na and the sugar specific binding site and represents a subunit of the intact Na+-dependent glucose transporter from dog kidney proximal tubule brush border membrane.     

Inactivation of leucine aminotransferase with diethylpyrocarbonate and rose bengal: evidence for an active site histidine residue

Modification of leucine aminotransferase by diethylpyrocarbonate or rose bengal-sensitized photo-oxidation caused rapid inactivation of the enzyme. The inactivation of leucine aminotransferase depended on the concentration of the reagent, the time of incubation and exhibited pseudo-first order kinetics. Rose bengal-sensitized photo-oxidation was maximum at pH 6.5 and 9. Substrates leucine and alpha-ketoglutarate protected the enzyme against inactivation by these reagents, thus suggesting participation of histidine residue at the substrate binding site.     

Cytochalasin B interferes with conformational changes of the human erythrocyte glucose transporter induced by internal and external sugar binding.

To gain insight into the mechanism of facilitated sugar transport and possible mechanisms by which glucose transporter intrinsic activity might be altered, we have investigated conformational changes of the human erythrocyte glucose transporter induced by internal and external sugar binding and by the transporter inhibitor, cytochalasin B. Changes in the ability of thermolysin to digest glucose transporters present in erythrocyte ghosts were used to monitor conformational changes of the glucose transporter. The degree of protease digestion was determined by the amount of undigested glucose transporter remaining after the protease treatment, as assessed in Western blots using the glucose transporter specific monoclonal antibody 7F7.5. D-Glucose, the physiological substrate of the transporter, increased the transporter's susceptibility to cleavage by thermolysin. Nontransportable glucose analogues which bind specifically to either an internal or external glucose transporter sugar binding site also altered susceptibility of the transporter to thermolysin. Both methyl and propyl glucoside, which preferentially bind the internal sugar site, increased thermolysin susceptibility of the glucose transporter in a manner similar to that of D-glucose. In contrast, 4,6-O-ethylideneglucose, which preferentially binds the external sugar site, protected the transporter from thermolysin digestion. These results suggest that sugar binding to internal and external sugar sites induces distinct conformational changes and that the observed D-glucose effect on the susceptibility of the glucose transporter to thermolysin is due to D-glucose at equilibrium predominantly forming a complex with the internal sugar site. The protection from cleavage by thermolysin caused by external sugar binding is attenuated by the addition of an internally binding sugar.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sugar uptake into brush border vesicles from dog kidney. II. Kinetics

The kinetics of D-glucose transport over the concentration range 0.07--20 mM have been investigated in a vesiculated membrane preparation from dog kidney cortex. 1. A sodium-dependent and a sodium-independent component of D-glucose uptake are observed. The sodium-dependent component is phlorizin sensitive (KI approximately 0.6 micron) and electrogenic. 2. The sodium-dependent component of D-glucose uptake yields non-linear Eadie-Hofstee plots consistent with the presence of high (GH) and low (GL) affinity sites (KH approximately 0.2 mM, KL approximately 4.5 mM, VL/VH approximately 7 at pH 7.4, 25 degrees C, 100 mM NaC1 gradient). Alternative explanations are cooperative effects of non-Michaelis-Menten kinetics. 3. The initial uptake of D-glucose increases as the intravesicular membrane potential become more negative but the numerical values of KH and KL show little, if any, change. 4. alpha-Methyl-D-glucoside transport is also sodium dependent and phlorizin sensitive (KI approximately 1.9 micron). 5. In contrast to the results for D-glucose, the sodium-dependent component of alpha-methyl-D-glucoside uptake exhibits a nearly linear Eadie-Hofstee plot consistent with a single carrier site with Km approximately 1.9 mM and Vmax approximately 27 nmol/min per mg protein at pH 7.4, 25 degrees C, 100 mM NaCl gradient. 6. The kinetics of D-glucose transport in newborn dog kidney are similar to those in the adult except that the low affinity (GL) system appears to be less well developed.     

Characterization of an electrogenic sodium/glucose cotransporter in a human colon epithelial cell line

In this study, we have characterized the Na/glucose transporter in polarized monolayers formed by the clonal human colon carcinoma cell line HT-29-D4. Isotopic tracer flux measurements show that differentiated HT-29-D4 cells possess a sodium-dependent α-methyl-D-glucopyranoside (AMG) uptake that is competed for by increasing concentrations of D-glucose, D-galactose, and phlorizin. This transport is exclusively localized on the apical side of the epithelium. Kinetic data demonstrate the existence of a single Michaelian sodium-dependent AMG transporter with a Km of 1.2 ± 0.12 mM and a Vmax of 3.24 ± 0.25 nmol/mg of protein per min. Hill analysis reveals a coefficient of 1.9 ± 0.03, consistent with at least two sodium ions involved in AMG transport. Interestingly, the cotransporter function is not modulated by glucose in the culture medium. Transepithelial electrical parameter measurements show that the transepithelial potential difference (Vt) is glucose dependent and phlorizin sensitive. Antibodies directed against a peptide of the rabbit intestinal glucose cotransporter (Ser402-Lys420) recognize, in western blot experiments, the characteristic bands of the cotransporter on a crude membrane preparation of differentiated HT-29-D4 cells and react strongly with the apical domain of the monolayer in immunofluorescence experiments. We conclude that HT-29-D4 cells express the sodium/glucose cotransporter SGLT1 at their apical membrane and that this transporter generates the basal transepithelial potential difference. © 1995 Wiley-Liss, Inc.     

Evidence for histidine residues in the immunoglobulin-binding site of human C1q

The immunoglobulin-binding activity of subcomponent Clq of human complement is lost following treatment with diethylpyrocarbonate; the inactivation showed first-order kinetics with respect to time and modifier concentration. Soluble IgG oligomers protected Clq against diethylpyrocarbonate modification. Treatment of modified Clq with hydroxylamine resulted in an 85% recovery of its ability to bind to aggregated immunoglobulin. The inactivation process was associated with modification of 12.1 +/- 0.7 histidine residues per Clq molecule. These data are consistent with the presence of histidine residues in the immunoglobulin-binding sites of Clq; these residues may participate in ionic interactions with the carboxyl groups known to be in the Clq binding site of IgG.     

Chemical modification of the small intestinal Na+/d-glucose cotransporter by amino group reagents: Evidence for a role of amino group(s) in the binding of the sugar

Some amino group reagents inactivate the small-intestinal Na⁺/d-glucose cotransporter, as measured either as a catalyst of Na⁺-dependent d-glucose transport or as a Na⁺-dependent phlorizin ligand. The amino group(s) studied in this paper are not identical with those investigated previously (Biber, J., Weber, J. and Semenza, G. (1983) Biochim. Biochim. Acta 728, 429–437): these are protected from inactivation by the simultaneous presence of Na⁺ plus sugar substrates. They are thus likely to be located within the substrate-binding site.     

An essential histidine residue for the activity of UDPglucose 4-epimerase from Kluyveromyces fragilis.

UDPglucose 4-epimerase from Kluyveromyces fragilis was completely inactivated by diethylpyrocarbonate following pseudo-first order reaction kinetics. The pH profile of diethylpyrocarbonate inhibition and reversal of inhibition by hydroxylamine suggested specific modification of histidyl residues. Statistical analysis of the residual enzyme activity and the extent of modification indicated modification of 1 essential histidine residue to be responsible for loss in catalytic activity of yeast epimerase. No major structural change in the quarternary structure was observed in the modified enzyme as shown by the identical elution pattern on a calibrated Sephacryl 200 column and association of coenzyme NAD to the apoenzyme. Failure of the substrates to afford any protection against diethylpyrocarbonate inactivation indicated the absence of the essential histidyl residue at the substrate binding region of the active site. Unlike the case of native enzyme, sodium borohydride failed to reduce the pyridine moiety of the coenzyme in the diethylpyrocarbonate-modified enzyme. This indicated the presence of the essential histidyl residue in close proximity to the coenzyme binding region of the active site. The abolition of energy transfer phenomenon between the tryptophan and coenzyme fluorophore on complete inactivation by diethylpyrocarbonate without any loss of protein or coenzyme fluorescence are also added evidences in this direction.     

Purification of a putative Na+/D-glucose cotransporter from pig kidney brush border membranes on a phlorizin affinity column

Phlorizin, a potent inhibitor of the Na+/D-glucose cotransporter, was derivatised to 3-aminophlorizin and subsequently coupled to Affi-Gel 15. Affinity chromatography of pig kidney brush border membranes solubilised in Triton X-100 allowed the purification of a 60 kDa protein on this resin. We consider this protein to be the Na+/D-glucose cotransporter, or part of it, for the following reasons: (i) binding of this protein to Affi-Gel 15 specifically requires phlorizin covalently attached to the resin and is lowered when phlorizin is replaced by phloretin; (ii) binding of the 60 kDa protein to a phlorizin affinity column requires the presence of Na+; (iii) polyclonal as well as monoclonal antibodies against the 60 kDa protein inhibit binding of phlorizin to brush border membranes from rabbit and pig kidney.