Phlorizin Binding Sites Research Articles

Inhibitor Binding in the Human Renal Low- and High-Affinity Na+/Glucose Cotransporters

The kidney contains two Na(+)/glucose cotransporters, called SGLT2 and SGLT1, arranged in series along the length of the proximal tubule. The low-affinity transporter, SGLT2, is responsible for the reabsorption of most of the glucose in the kidney. There is recent interest in SGLT2 as a target for the treatment of type II diabetes using selective inhibitors based on the structure of the phenylglucoside, phlorizin (phloretin-2'-beta-glucoside). In this study, we examined the inhibition of alpha-methyl-d-glucopyranose transport by phlorizin and a new candidate drug, sergliflozin-A [(2-[4-methoxyphenyl]methyl)phenyl beta-d-glucopyranoside], in COS-7 cells expressing hSGLT1 and hSGLT2. Inhibition by phlorizin was competitive, with K(i) values of 0.3 muM in hSGLT1 and 39 nM in hSGLT2. Inhibition by sergliflozin-A was also competitive, with K(i) values of 1 muM in hSGLT1 and 20 nM in hSGLT2. Phloretin [3-(4-hydroxyphenyl)-1-(2,4,6-trihydroxyphenyl)-1-propanone; the aglucone of phlorizin] was a less potent inhibitor, with IC(50) values of 142 muM in hSGLT1 and 25 muM in hSGLT2. Site-directed mutagenesis of residues believed to be in the phlorizin binding site showed that only Cys610 is involved in inhibitor binding in the human transporters. Mutation of Cys610 in hSGLT1 to lysine resulted in an increased IC(50) for all inhibitors. In contrast, mutagenesis of the analogous Cys615 in hSGLT2 produced the opposite effect, a decrease in IC(50) for phlorizin and sergliflozin-A. The differences in the effects of the mutations between hSGLT1 and hSGLT2 suggest that this cysteine holds key residues in place rather than participating directly in inhibitor binding.

Membrane topology of loop 13–14 of the Na +/glucose cotransporter (SGLT1): A SCAM and fluorescent labelling study

The accessibility of the hydrophilic loop between putative transmembrane segments XIII and XIV of the Na +/glucose cotransporter (SGLT1) was studied in Xenopus oocytes, using the substituted cysteine accessibility method (SCAM) and fluorescent labelling. Fifteen cysteine mutants between positions 565 and 664 yielded cotransport currents of similar amplitude than the wild-type SGLT1 (wtSGLT1). Extracellular, membrane-impermeant MTSES (−) and MTSET (+) had no effect on either cotransport or Na + leak currents of wtSGLT1 but 9 mutants were affected by MTSES and/or MTSET. We also performed fluorescent labelling on SGLT1 mutants, using tetramethylrhodamine-5-maleimide and showed that positions 586, 588 and 624 were accessible. As amino acids 604 to 610 in SGLT1 have been proposed to form part of a phlorizin (Pz) binding site, we measured the K i Pz and K m αMG for wtSGLT1 and for cysteine mutants at positions 588, 605–608 and 625. Although mutants A605C, Y606C and D607C had slightly higher K i Pz values than wtSGLT1 with minimal changes in K m αMG, the effects were modest and do not support the original hypothesis. We conclude that the large, hydrophilic loop near the carboxyl terminus of SGLT1 is thus accessible to the external solution but does not appear to play a major part in the binding of phlorizin.

Synthesis of photoaffinity probes [2 ′-iodo-4 ′-(3 ′′-trifluoromethyldiazirinyl)phenoxy]- d-glucopyranoside and [(4 ′-benzoyl)phenoxy]- d-glucopyranoside for the identification of sugar-binding and phlorizin-binding sites in the sodium/ d-glucose cotransporter protein

In this paper we describe the synthesis and photochemical and biochemical properties of two new photoaffinity probes designed for studies on the structure–function relationship of the sodium d-glucose cotransporter (SGLT1). The two probes are [2 ′-iodo-4 ′-(3 ′′-trifluoromethyldiazirinyl)phenoxy]- d-glucopyranoside (TIPDG), a mimic for the phenyl glucopyranoside arbutin which is transported by SGLT1 with a very high affinity, and [(4 ′-benzoyl)phenoxy]- d-glucopyranoside (BzG), a model compound for phlorizin, the most potent competitive inhibitor of sugar translocation by SGLT1. Both photoaffinity probes TIPDG ( λ max=358 nm) and BzG ( λ max=293 nm) can be activated at 350–360 nm, avoiding protein-damaging wavelengths. In inhibitor studies on sodium-dependent d-glucose uptake into rabbit intestinal brush border membrane vesicles TIPDG and BzG showed a fully competitive inhibition with regard to the sugar with respective K i values of 22 ± 5 μM for TIPDG and 12 ± 2 μM for BzG. These K i values are comparable to those of their parent compounds arbutin (25 ± 6 μM) and phlorizin (8 ± 1 μM). To further test the potential of TIPDG and BzG as photoaffinity probes, truncated loop 13 protein, supposed to be part of the substrate recognition site of SGLT1, was exposed to TIPDG and BzG in solution. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis demonstrated that TIPDG and BzG successfully labeled the protein. These preliminary results suggest that both photoaffinity probes are promising tools for the study of the structure–function relationship of SGLT1 and other SGLT1 family transporter proteins.

Rotavirus infection impairs intestinal brush-border membrane Na(+)-solute cotransport activities in young rabbits.

The mechanism of rotavirus diarrhea was investigated by infecting young, specific pathogen-free, New Zealand rabbits with a lapine rotavirus, strain La/RR510. With 4-wk-old animals, virus shedding into the intestinal lumen peaked at 72 h postinfection (hpi), and a mild, watery diarrhea appeared at 124 hpi. No intestinal lesions were seen up to 144 hpi, indicating that diarrhea does not follow mucosal damage but can precede it, as if cell dysfunction were the cause, not the consequence, of the histological lesions. Kinetic analyses with brush-border membrane vesicles isolated from infected rabbits revealed strong inhibition of both Na(+)-D-glucose (SGLT1) and Na(+)-L-leucine symport activities. For both symporters, only maximum velocity decreased with time. The density of phlorizin-binding sites and SGLT1 protein antigen in the membrane remained unaffected, indicating that the virus effect on this symporter is direct. Because SGLT1 supports water reabsorption under physiological conditions, the mechanism of rotavirus diarrhea may involve a generalized inhibition of Na(+)-solute symport systems, hence, of water reabsorption. Massive water loss through the intestine may eventually overwhelm the capacity of the organ for water reabsorption, thereby helping the diarrhea to get established.

Probing the conformation of the sugar transport inhibitor phlorizin by 2D-NMR, molecular dynamics studies, and pharmacophore analysis.

Sodium/D-glucose cotransport, one of the prototypes for sodium gradient-driven symport systems in kidney and intestine, is known to be inhibited by aromatic and aliphatic glucosides (Diedrich, D. F. Biochim. Biophys. Acta 1963, 71, 688-700; Diedrich, D. F. Arch. Biochem. Biophys. 1966, 117, 248-256; Kipp, H.; et al. Biochim. Biophys. Acta 1996, 1282, 124-130; Ramaswamy, K.; et al. Biochim. Biophys. Acta 1976, 433, 32-38). The conformation in which the most potent inhibitor, phlorizin, interacts with the transport protein was investigated with different approaches. Phlorizin consists of the glucose moiety and two aromatic rings (A and B) joined by an alkyl spacer. First the interaction of these various parts of the molecule was determined by two-dimensional (2D) solution NMR. From the 2D-NOESY (nuclear Overhauser effect) measurements spatial distances (up to 5 A) between various interacting H atoms could be detected. Using these values as distance constraints, conformations of phlorizin were calculated and analyzed by the valence force-field method. As a result, a set of conformations could be obtained. The most probable phlorizin conformation shows a nearly perpendicular arrangement of the two aromatic rings (A and B) with the ring B situated above the sugar ring. A very similar conformation could be found by using molecular dynamics simulations when water was chosen as the solvent. This phlorizin conformation in aqueous solution then served as a template for conformational analysis of various phlorizin derivatives. The resulting conformations of derivatives were taken as input to establish a pharmacophore model using the DISCO calculation. As a result, the essential elements of phlorizin for interaction with its binding pocket could be deduced: namely hydrogen bonding via hydroxyl groups of the pyranoside at C(2), C(3), C(4), and C(6) and at C(4) and C(6) of aromatic ring A and hydrophobic interactions via the pyranoside ring and aromatic ring A. Finally, from these conformational features of the pharmacophore the dimension of the phlorizin binding site on the sodium/D-glucose cotransporter was estimated to be 17 x 10 x 7 A(3).

Effect of cross-linkers on the structure and function of pig-renal sodium-glucose cotransporters after papain treatment.

Kidney brush-border membranes contain two sodium-dependent glucose transporters, one with low and one with high affinity for phlorizin, the specific inhibitor of these transporters. Using Scatchard analysis of phlorizin binding and Western blotting with specific antibodies against these transporters, we demonstrate in this study that although both transporters were proteolysed by papain treatment, only the high-affinity phlorizin-binding sites were decreased. Papain treatment followed by cross-linking with homobifunctional disuccinimidyl tartarate restored only the structure of the low-affinity phlorizin-binding protein (approx. molecular mass 70 kDa) without modifying the phlorizin-binding sites. When disuccinimidyl tartarate was replaced with dithiobis(succinimidyl acetate), another homobifunctional cross-linker with a higher spacer arm, the low- and high-affinity sites were both restored, with reappearance of two phlorizin-binding proteins with approx. molecular masses of 70 and 120 kDa. We conclude that high-affinity phlorizin-binding sites depend on the presence of the heterodimeric 120 kDa protein.

Cystine loading induces Fanconi's syndrome in rats: in vivo and vesicle studies

To better understand the link between lysosomal cystine accumulation and the renal impairment seen in cystinosis, we have studied the effect of cystine loading in vivo, on renal function of rats, and in brush-border membrane vesicles (BBMV) prepared from the kidney cortex of the treated rats. Intraperitoneal injection of cystine dimethyl ester (CDME) (400 mumol, twice a day, for 5 days) led to an increased urine volume and excretion of glucose, phosphate, and protein. Kinetic analysis of alpha-methylglucoside initial flux in BBMV showed reduction in maximal transport capacity (Vmax, from 10.1 +/- 1.3 to 8.5 +/- 0.7 nmol.min-1.mg protein-1; P < 0.01) with no change in Michaelis constant (Km, 4.80 +/- 0.08 and 4.90 +/- 0.05 mM). The number of phlorizin binding sites declined (from 6.5 +/- 0.7 to 4.1 +/- 0.4 pmol/mg protein; P < 0.01) with no significant change in the affinity for phlorizin (0.64 +/- 0.08 and 0.59 +/- 0.06 microM). In the cortex homogenate, cystine concentration, which was undetectable in controls, increased to 0.97 +/- 0.09 nmol 1/2 cystine/mg protein. Two hours after CDME administration, ATP content declined to approximately 50% of control values. This decline was transient, and ATP content was recovered to control values 5 h after CDME administration. The treatment did not affect ouabain-sensitive adenosinetriphosphatase activity (40.0 +/- 3.9 and 38.6 +/- 4.7 nmol Pi.mg protein-1.min-1) or the number and affinity of ouabain binding sites (Bmax = 1.48 +/- 0.25 and 1.44 +/- 0.18 pmol/mg, and Kd = 0.68 +/- 0.09 and 0.72 +/- 0.12 microM, respectively). (ABSTRACT TRUNCATED AT 250 WORDS)

Oligomeric structure of the sodium-dependent phlorizin binding protein from kidney brush-border membranes

Immunodetection of solubilized kidney brush-border proteins on Western blots using antibodies against the 70 kDa phlorizin binding component of sodium-glucose cotransporter allows to identify an additional protein band with apparent molecular mass of 120 kDa in the presence of reducing agent dithiothreitol. Antibodies specifically eluted from the 70 kDa protein still recognize the 120 kDa protein on Western blot. The lack of dissociation of the 120 kDa protein from native brush borders or Triton X-100 extract in the presence of dithiothreitol can be improved by an extended incubation at 25°C; this protein is fully dissociated when purified by electroelution from polyacrylamide gel and gives two subunits with apparent molecular masses of 70 and 60 kDa by Coomassie staining and Western blot analysis. The effect of dithiothreitol on the renal brush-border membrane phlorizin binding is studied; a decrease in the number of high-affinity phlorizin binding sites without modification of the affinity to the binding molecule is observed. These data suggest that the high-affinity phlorizin binding moiety of sodium-glucose cotransporter exists in the kidney as a dimeric structure.

Crypt-villus site of glucose transporter induction by dietary carbohydrate in mouse intestine.

Intestinal brush-border glucose uptake rate is regulated by dietary carbohydrate level. However, this uptake response takes a day or more after a change in dietary carbohydrate level. Is this dietary signal perceived in the crypts, and is the glucose transporter activity of enterocytes irreversibly programmed there? If so, this time lag could reflect cell migration times along the crypt-villus axis, since glucose transport is not fully expressed until cells reach the midvillus. Alternatively, however, the time lag could arise from the induction process itself, if glucose transporter activity in mature villus enterocytes can be reversibly reprogrammed by dietary carbohydrate levels. Hence, we measured glucose-protectable phlorizin binding (as a measure of glucose transporter site density) in mouse enterocytes fractionated along the crypt-villus axis by the Weiser method, as a function of time after an abrupt switch in dietary carbohydrate level. For an increase or decrease in dietary carbohydrate, an increase or decrease in phlorizin binding site density first appeared in the crypts and marched over the course of several days to the villus tips. Hence, the signal for glucose transporter regulation is perceived in the crypts, and the observed lag in uptake is due largely to cell migration times.

Effect of diet on glucose transporter site density along the intestinal crypt-villus axis.

High-carbohydrate diets stimulate intestinal brush-border glucose uptake and increase the number of glucose-protectable phlorizin binding sites, but it has been unknown where along the crypt-villus axis these effects are expressed. We attacked this problem by three methods. First, by measuring phlorizin binding to isolated mouse enterocytes fractionated along the crypt-villus axis by the Weiser method, we identified a high-affinity binding site predominating from villus tip to midvillus and a site of possibly lower affinity predominating in the crypts. A high-carbohydrate diet increased by severalfold the density of the villus sites and probably also of the crypt sites, without changing their binding constants. Second, autoradiography revealed increased glucose-protectable phlorizin binding along the whole crypt-villus axis on a high-carbohydrate diet. Finally, a polyclonal antibody against the Na(+)-glucose cotransporter recognized a protein in the brush-border membrane of villus cells. Hence, substrate-dependent upregulation of intestinal glucose transport involves increased numbers of transporters along the crypt-villus axis.

Increased Na(+)-dependent D-glucose transport and altered lipid composition in renal cortical brush-border membrane vesicles from bile duct-ligated rats.

Erythrocyte membranes of patients with liver disease are characteristically enriched in cholesterol, a change known to impair several carrier-mediated membrane transport functions. In the present study we have assessed whether experimental liver disease can affect the membrane lipid composition and transport function of kidney epithelial cells. Small (about 5%) but significant (P less than 0.01) increases were found in the cholesterol-to-phospholipid molar ratio (C/PL) of rat renal cortical brush-border membrane (BBM) vesicles 3, 8, and 15 days after bile duct ligation which correlated closely with increased fluorescence polarization, i.e., decreased membrane fluidity (r = 0.75, P less than 0.001; n = 27). A lipoprotein-mediated pathogenesis was suggested by the close relationship between BBM C/PL and plasma C/PL (r = 0.69, P less than 0.001). The mean high-affinity Na(+)-coupled D-glucose uptake by BBM vesicles was higher 1, 3, 8, and 15 days after ligation than in non-operated rats, significantly so at 3 and 8 days (611 +/- 37 and 593 +/- 22 vs. 507 +/- 21 pmol/mg protein per 4 sec; P less than 0.05), and was positively correlated with BBM C/PL (r = 0.58, P less than 0.01) and fluorescence polarization (r = 0.41, P less than 0.05). Brief incubation of BBM vesicles from normal rats with cholesterol-rich phospholipid liposomes simultaneously increased BBM C/PL and Na(+)-dependent D-glucose uptake. Stimulation of BBM Na(+)-glucose cotransport in ligated rats was not due to delayed dissipation of the Na+ gradient or to a more rapid development of membrane potential. High-affinity Na(+)-dependent D-glucose uptake kinetics in 3-day bile duct-ligated rats showed a lower Kt, without an alteration in maximum velocity, Vmax, compared to sham-operated animals (0.298 +/- 0.015 vs. 0.382 +/- 0.029 mM; P less than 0.05), whilst the binding dissociation constant, Kd of high-affinity phlorizin binding sites was reduced by ligation (0.453 +/- 0.013 vs. 0.560 +/- 0.015 microM; P less than 0.001). We conclude that an early effect of bile duct ligation is to enrich renal cortical brush-border membranes in cholesterol, thereby decreasing membrane fluidity and stimulating Na(+)-dependent D-glucose uptake by increasing the affinity of the carrier.

Altered glucose carrier expression: mechanism of intestinal adaptation during streptozocin-induced diabetes in rats

Intestinal adaptation of glucose transport during streptozocin-induced diabetes in rats was examined using microdensitometric analysis of [3H]phlorizin binding. Results of specific phlorizin binding were correlated with measurements of maximal transport capacity, carrier affinity, villus height, and enterocyte birth rate determined by the metaphase arrest technique. Animals diabetic for 14 days (acute) and 60 days (chronic) were compared with age-matched controls. In the jejunum, adaptation occurred only in chronically diabetic rats and consisted of a 10-fold increase in the density of phlorizin binding sites in the upper villus region (i.e., that portion normally transporting glucose), while in the ileum, adaptation occurred both in acute and chronically diabetic rats and consisted of 1) a 3-fold increase in density of phlorizin binding sites in the upper villus region of acutely diabetic rats and 2) an increased density in the upper villus region as well as the recruitment of phlorizin binding sites in the mid to lower villus region (i.e., that portion not normally transporting glucose) of chronically diabetic rats. Enhancement of glucose Vmax and villus length accompanied changes in binding, whereas enterocyte birth rates were similar in each group.

Two substrate sites in the renal Na+-d-glucose cotransporter studied by model analysis of phlorizin binding and-d-glucose transport measurements

Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na+ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na+) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na+) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na+ on both membrane sides and with a clamped membrane potential, KD values of 0.4 and 7.9 microM were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence of D-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na+ (out greater than in). Low and high affinity transport could be fitted with identical Km values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparent Km of high affinity transport whereas the apparent Km of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pig high and low affinity Na(+)-D-glucose cotransporters are present which contain low and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose 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+.

Protein kinase C activation has dissimilar effects on sodium-coupled uptakes in renal proximal tubular cells in primary culture

Protein kinase C (Ca2+/phospholipid-dependent enzyme) was shown to be present in renal brush border membranes. To evaluate the influence of protein kinase C activation on three apical transport systems, we studied the effect of phorbol myristate acetate (PMA) and of two diacylglycerol analogs, oleoylacetylglycerol and dioctanoylglycerol, on sodium-dependent uptakes of phosphate (Pi), L-alanine, and alpha-methyl-D-glucopyranoside (MGP), as well as on specific phlorizin binding, in cultured rabbit proximal tubular cells. PMA, at 100 ng/ml, decreased the Vmax of Pi and MGP uptake by 30 and 17%, respectively, but not that of alanine uptake. None of the Km values were affected. PMA also decreased the number of phlorizin binding sites by 40%. PMA-induced inhibition of Pi and MGP uptake was time- and concentration-dependent, was mimicked by oleoylacetylglycerol, dioctanoylglycerol, and the diacylglycerol kinase inhibitor R59022, and was reversed by the protein kinase C antagonist 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). The effects of PMA persisted in the presence of amiloride and dimethyl amiloride, and were potentiated by Ca2+ ionophore A23187. Opening of tight junctions blunted subsequent PMA-induced decrease of MGP uptake, but not of Pi uptake. It is concluded that: (i) activation of protein kinase C does not affect similarly Na-Pi, Na-hexose, and Na-alanine cotransport; and (ii) different pathways are likely to be involved in the observed effects.

Ischemia induces surface membrane dysfunction. Mechanism of altered Na+-dependent glucose transport.

Reversible ischemia reduced renal cortical brush border membrane (BBM) Na+-dependent D-glucose uptake (336 +/- 31 vs. 138 +/- 30 pmol/mg per 2 s, P less than 0.01) but had no effect on Na+-independent glucose or Na+-dependent L-alanine uptake. The effect on D-glucose uptake was present after only 15 min of ischemia and was due to a reduction in maximum velocity (1913 +/- 251 vs. 999 +/- 130 pmol/mg per 2 s; P less than 0.01). This reduction was not due to more rapid dissipation of the Na+ gradient, altered sidedness of the vesicles, or an alteration in membrane potential. Ischemia did, however, reduce the BBM sphingomyelin-to-phosphatidylcholine (SPH/PC) and cholesterol-to-phospholipid ratios and the number of specific high-affinity Na+-dependent phlorizin binding sites (390 +/- 43 vs. 146 +/- 24 pmol/mg; P less than 0.01) without altering the binding dissociation constant (Kd). 20 mM benzyl alcohol also reduced the number of Na+-dependent phlorizin binding sites (418 +/- 65 vs. 117 +/- 46; P less than 0.01) without altering Kd. The reduction in Na+-dependent D-glucose transport correlated with ischemic-induced changes in the BBM SPH/PC and cholesterol-to-phospholipid ratios and membrane fluidity. Taken together these data indicate the cellular site responsible for ischemic-induced reduction in renal cortical transcellular glucose transport is the BBM. We propose the mechanism involves marked alterations in BBM lipids leading to large increases in BBM fluidity which reduces the binding capacity of Na+-dependent glucose carriers. These data indicate that reversible ischemia has profound effects on the surface membrane function of epithelial cells.

Intestinal adaptation to diabetes. Altered Na-dependent nutrient absorption in streptozocin-treated chronically diabetic rats.

To examine the pattern and mechanisms of enhanced intestinal nutrient absorption in diabetes, we measured intestinal transport of 3-O-methylglucose (3OMG), l-alanine (ALA), and SO4 in male Lewis rats made diabetic with streptozocin. Diabetes enhanced 3OMG absorption fivefold in ileum and threefold in jejunum; ALA absorption increased twofold in ileum but not at all in jejunum; ileal SO4 transport was unaffected. Increases in 3OMG and ALA transport were due solely to increases in maximum velocity. The enhancement of ileal glucose absorption was half-maximal in 40-45 d, could be reversed by 10 d of treatment with insulin and did not result from adrenergic denervation. The density of glucose carriers per milligram brush border protein (measured as [3H]phlorizin binding sites) was not altered but there was a sixfold increase in the number of glucose-inhibitable [3H]phlorizin-binding sites in the intact epithelium. Generalized mucosal hypertrophy accounted for less than 30% of this increase. We conclude that the intestine adapts to streptozocin-induced diabetes through recruitment of additional brush border carriers for sugar, probably in the midvillus-to-crypt region.

Identification of the D-glucose binding polypeptide of the renal Na +-D-glucose cotransporter with a covalently binding D-glucose analog

The covalently binding D-glucose analog 10- N-(bromoacetyl)amino-1-decyl-β-D-glucopyranoside (BADG) was synthesised and shown to be a high-affinity inhibitor of the renal Na +-D-glucose contransporter. From renal brush-border membranes a protein fraction was isolated, in which the concentration of Na +-dependent phlorizin binding sites per mg protein was enriched 7-fold. In labeling experiments with this protein fraction a polypeptide of M r ~ 79000 was identified as containing the D-glucose binding site of the renal Na +-D-glucose cotransporter. Affinity labeling D-Glucose analog D-Glucose binding site Na±D-glucose cotransporter Brush-border membrane Kidney

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°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 γ-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.