Inhibition of ( 3 H)phlorizin binding to isolated kidney brush border membranes by phlorizin-like compounds

Renal trehalase: two subsites at the substrate-binding site

Phlorizin, phloretin, Tris and beta-methylglucoside are competitive inhibitors, with respect to the substrate trehalose, of purified renal trehalase. Mercuric chloride is a noncompetitive inhibitor. The active site of trehalase was examined further by multi-inhibition kinetic studies involving combinations of inhibitors. Phlorizin vs. phloretin and phlorizin vs. Tris were mutually non-competitive. In contrast, phloretin vs. Tris was mutually competitive. These findings suggest that the binding site of phlorizin to the enzyme differed from that of phloretin or Tris, and that phloretin and Tris might bind at a common site. These findings suggest a model in which trehalase has two binding sites at the substrate-binding site, a phlorizin (glucosyl) and a phloretin (phenyl) binding site, analogous to the model proposed previously for the glucose carrier. In addition, mercuric chloride vs. beta-methylglucoside was mutually competitive, although mercuric chloride and beta-methylglucoside, respectively, were noncompetitive and competitive inhibitors with respect to the substrate. Thus, it is suggested that the substrate binding and the SH-inhibitor binding sites are located very close to each other.

Chromatofocussing and centrifugal reconstitution as tools for the separation and characterization of the Na+-cotransport systems of the brush-border membrane

Chromatofocussing was used for the separation of brush-border membrane proteins from calf kidney into 4 or 5 fractions over the pH range 4.0 to 7.4. These fractions were reconstituted into proteoliposomes by gradient centrifugation. Determination of the sodium-dependent solute uptake by proteoliposomes reconstituted from different chromatofocussed fractions showed that the sodium-D-glucose cotransport system was present in the fraction eluted between pH 5.3 and 5.8, and that the sodium-phosphate cotransport was present in fractions eluted between pH 4.6 and 5.3 and between pH 5.8 and 6.6, sodium-alanine cotransport could be detected in almost all fractions. Marker enzymes of the brush-border membrane, such as alkaline phosphatase, gamma-glutamyltransferase and aminopeptidase M etc. were also found to be eluted at pH 7.0-7.4, 4.0-4.1 and 5.6-5.8, respectively. These results suggest that chromatofocussing is a promising tool for the separation of membrane proteins and for pre-purification of the sodium-D-glucose cotransport system. It can be further concluded that the sodium-dependent phosphate transport across the brush-border membrane is not dependent upon alkaline phosphatase activity.

Effect of low-molecular-weight proteins on protein (lysozyme) binding to isolated brush-border membranes of rat kidney

Filtered proteins including the low-molecular-weight protein lysozyme are reabsorbed by the proximal tubule via adsorptive endocytosis. This process starts with binding of the protein to the brush-border membrane. The binding of 125I-labelled egg-white lysozyme (EC 3.2.1.17) to isolated brush-border membranes of rat kidney and the effect of several low-molecular weight proteins on that binding was determined. The Scatchard plot revealed a one-component binding type with a dissociation constant of 5.3 microM and 53.0 nmol/mg membrane protein for the number of binding sites. The binding of the cationic lysozyme was inhibited competitively by the addition of cationic cytochrome c to the incubation medium, while the neutral myoglobin had no effect. The anionic beta-lactoglobulin A inhibited the lysozyme binding in a noncompetitive manner. These data suggest that the binding takes place between positively charged groups of the protein molecule and negative sites on the brush-border membrane, and, the competition between the cationic cytochrome c and the cationic lysozyme for the binding sites may be responsible for the inhibitory effect of cytochrome c on renal lysozyme reabsorption. The binding step at the brush-border membrane appears to be cation-selective.

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.

Isolation of the sodium-dependent d-glucose transport protein from brush-border membranes

Rabbit kidney brush-border membrane vesicles were exposed to bacterial protease which cleaves off a large number of externally oriented proteins. Na+-dependent D-glucose transport is left intact in the protease-treated vesicles. The protease-treated membrane was solubilized with deoxycholate and the deoxycholate-extracted proteins were further resolved by passage through Con A-Sepharose columns. Sodium-dependent D-glucose activity was found to reside in a fraction containing a single protein band of Mr approximately equal to 165 000 which is apparently a dimer of Mr approximately or equal to 85 000. When reconstituted and tested for transport, this protein showed Na+-dependent, stereo-specific and phlorizin-inhibitable glucose transport. Transport activity is completely recovered and is 20-fold increased in specific activity. A similar isolate was obtained from rabbit small intestinal brush-border membranes and kidneys from several other species of animals.

Binding of lysozyme to brush border membranes of rat kidney

The binding of 125I-labelled egg-white lysozyme to isolated brush border membranes of rat kidney cortex was investigated. The lysozyme binding was reversible and saturable. The Scatchard plot revealed a one-component binding type with a dissociation constant of 7.8 microM and 15.6 nmol/mg membrane protein for the number of binding sites. The binding of the basic lysozyme could be reduced by basic amino acids such as L-lysine, L-ornithine or L-arginine, while neutral amino acids such as L-citrulline or L-alanine had no effect. The inhibitory effect of lysine was competitive.

Synthesis of [3H]phlorizin and its binding behavior to renal brush-border membranes

Tetra- and tribromophlorizin have been synthesized under mild brominating conditions. With catalytic debromination in the presence of hydrogen or tritium gas, bromine atoms in the derivatives were completely substituted by hydrogen or tritium. The product was identical to the native phlorizin and was chemically pure. Tritiated phlorizin with extremely high specific radioactivity (45 Ci/mmol) was obtained when hydrogen gas was replaced by tritium gas. While the brominated compounds showed little inhibition of sodium D-glucose co-transport by isolated renal brush-border membranes. [3H]phlorizin had the same binding affinity to the brush-border membranes as native phlorizin and a Ki value of 1.2 microM for the sodium-dependent D-glucose transport. Binding studies performed using a flow-dialysis method resulted in 150 pmol of phlorizin-binding sites per milligram of membrane protein. This radioactive phlorizin can be a useful tool for determining D-glucose-(phlorizin) binding sites at a low phlorizin concentration in membranes, in nonvesicle forms such as collapsed membrane vesicles, and in purified protein fractions.

Synthesis of phlorizin derivatives and their inhibitory effect on the renal sodium/D-glucose cotransport system

To characterize further the Na+/D-glucose cotransport system in renal brush border membranes, phlorizin - a potent inhibitor of D-glucose transport - has been chemically modified without affecting the D-glucose moiety or changing the side groups that are essential for the binding of phlorizin to the Na+/D-glucose cotransport system. One series of chemical modifications involved the preparation of 3-nitrophlorizin and the subsequent catalytic reduction of the nitro compound to 3-aminophlorizin. From 3-aminophlorizin, 3-bromoacetamido-, 3-dansyl- and 3-azidophlorizin have been synthesized. In another approach, 3'-mercuryphlorizin was obtained by reaction of phlorizin with Hg(II) acetate. The phlorizin derivatives inhibit sodium-dependent but not sodium-independent D-glucose uptake by hog renal brush border membrane vesicles in the following order of potency: 3'-mercuryphlorizin = phlorizin greater than 3-aminophlorizin greater than 3-bromoacetamidophlorizin greater than 3-azidophlorizin greater than 3-nitrophlorizin greater than 3-dansylphlorizin. 3-Bromoacetamidophlorizin - a potential affinity label - also inhibits sodium-dependent but not sodium-independent phlorizin binding to brush border membranes. In addition, sodium-dependent phosphate and sodium-dependent alanine uptake are not affected by 3-bromoacetamidophlorizin. The results described above indicate that specific modifications of the phlorizin molecule at the A-ring or B-ring are possible that yield phlorizin derivatives with a high affinity and high specificity for the renal Na+/D-glucose cotransport system. Such compounds should be useful in future studies using affinity labeling (3-bromoacetamido- and 3-azidophlorizin) or fluorescent probes (3-dansylphlorizin).

Transepithelial transport in cell culture: stoichiometry of Na/phlorizin binding and Na/D-glucose cotransport. A two-step, two sodium model of binding and translocation

The renal cell line LLC-PK1 cultured on a membrane filter forms a functional epithelial tissue. This homogeneous cell population exhibits rheogenic Na-dependent D-glucose coupled transport. The short-circuit current (Isc) was accounted for by net apical-to-basolateral D-glucose coupled Na flux, which was 0.53 +/- 0.09(8) mueq cm-2hr-1, and Isc, 0.50 +/- 0.50(8) mueq cm-2hr-1. A linear plot of concurrent net Na vs. net D-glucose apical-to-basolateral fluxes a gave a regression coefficient of 2.08. As support for a 2:1 transepithelial stoichiometry, sodium was added in the presence of D-glucose and the response of Isc analyzed by a Hill plot. A slope of 2.08 +/- 0.06(5) was obtained confirming a requirement of 2 Na for 1 D-glucose coupled transport. A Hill plot of Isc increase to added D-glucose in the presence of Na gave a slope of 1.02 +/- 0.02(5). A direct determination of the initial rates of Na and D-glucose translocation across the apical membrane using phlorizin, a nontransported glycoside competitive inhibitor to identify the specific coupled uptake, gave a stoichiometry of 2.2. A coupling ratio of 2 for Na, D-glucose uptake, doubles the potential energy available for Na-gradient coupled D-glucose transport. In contrast to coupled uptake, the stoichiometry for Na-dependent-phlorizin binding was 1.1 +/- 0.1(8) from Hill plot analyses of Na-dependent-phlorizin binding as a function of [Na].(ABSTRACT TRUNCATED AT 250 WORDS)

4-Azidophlorizin, a high affinity probe and photoaffinity label for the glucose transporter in brush border membranes

A new phlorizin derivative (2'-O-(beta-D-glucopyranosyl)-4-azidophloretin, 4-azidophlorizin) has been synthesized and its affinity for the D-glucose, Na+ co-transport system in brush border vesicles from intestinal and renal membranes has been compared with that of phlorizin. The extent of the reversible interaction of the ligand with the transporter in dim light has been evaluated from three separate measurements: (1) Ki', the constant for fully-competitive inhibition of (Na+, delta psi)-dependent D-glucose uptake, (2) Kd', the dissociation constant of 4-azido[3H]phlorizin binding in the presence of an NaSCN inward gradient, and (3) Ki", the constant for fully-competitive inhibition of the specific ((Na+, delta psi)-dependent, D-glucose protectable) high-affinity [3H]phlorizin binding. In experiments with vesicles derived from rat kidney, all three constants (Ki', Kd' and Ki") were essentially equal and ranged between 3.2 and 5.2 microM, that is, the azide derivative has almost the same affinity for this transporter as phlorizin itself. On the other hand, compared to phlorizin, the 4-azidophlorizin has a lower affinity for the transporter in vesicles prepared from rabbit; its Ki' values are some 15-20-times larger than those determined with rat membranes. However, the affinity of the azide for the sugar transporter in membranes from either the intestine or kidney of the same animal species (rabbit or rat) was essentially the same. In spite of the lower affinity for the transporter in either membrane system from the rabbit, results described elsewhere (Hosang, M., Gibbs, E.M., Diedrich, D.F. and Semenza, G. (1981) FEBS Lett., 130, 244-248) indicate that 4-azidophlorizin is an effective photoaffinity label in this species also. Photolysis of the azide yields a reactive intermediate which reacts with a 72 kDa protein in rabbit intestine brush borders. Covalent labeling of this protein occurred under conditions which suggests that it is (a component of) the glucose transporter.

Further studies of proximal tubular brush border membrane D-glucose transport heterogeneity

The properties of two sodium-dependent D-glucose transporters previously identified in renal proximal tubule brush border membrane (BBM) vesicles are studied. The low-affinity system, found in BBM vesicles from the outer cortex (early proximal tubule), is shown to be associated with the high-affinity phlorizin binding site typically found in renal BBM preparations. The high-affinity system, found in BBM vesicles from the outer medulla (late proximal tubule), is almost two orders of magnitude less sensitive to inhibition by phlorizin and is apparently not associated with high-affinity phlorizin binding. The sodium/glucose stoichiometry of the outer medullary transporter is found to be 2:1 by two independent methods. Previous measurements have established that the stoichiometry of the outer cortical system is 1:1. It is suggested that this arrangement of transporters in series along the proximal tubule enables the kidney to reabsorb glucose from the urine in an energy-efficient fashion. The bulk of the glucose load is reabsorbed early in the proximal tubule at an energetic cost of one Na+ per glucose molecule. Then in the late proximal tubule a larger coupling ratio and hence a larger driving force is employed to reabsorb the last traces of glucose from the urine.

High- and low-affinity transport of D-glucose from blood to brain

Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D- and L-glucose. The Tmax and Km for the high-affinity system were 0.16 mmol 100 g-1 min-1 and 1 mM; for the low-affinity system, approximately 5 mmol 100 g-1 min-1 and approximately 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235 X 10(-7) cm s-1, a specific but nonsaturable diffusion pathway of permeability 85 x 10(-7) cm s-1, and a nonspecific passive diffusion pathway of permeability 2 x 10(-7) cm s-1.

Interaction of phlorizin and sodium with the renal brush-border membrane D-glucose transporter: stoichiometry and order of binding

The order and stoichiometry of the binding of phlorizin and sodium to the renal brush-border membrane D-glucose transporter are studied. The experimental results are consistent with a random-binding scheme in which the ratio of phlorizin- to sodium-binding sites is one-to-one. When the kinetics of phlorizin binding are measured as a function of increasing sodium concentration no significant variation is found in the apparent number of binding sites; however, the apparent binding constant for phlorizin decreases rapidly from approximately 16 microM at [Na] = 0 to 0.1 microM at [Na] = 100 mM and approaches 0.05 microM as [Na] leads to infinity. The experimental data are fit to a random carrier-type model of the coupled transport of sodium and D-glucose. A complete parameterization of the phlorizin binding properties of this model under sodium equilibrium conditions is given.

Testing carrier models of cotransport using the binding kinetics of non-transported competitive inhibitors

The kinetic equations representing the binding of a non-transported competitive inhibitor are derived from three variations of the carrier model of cotransport. These are (a) the model in which the binding sequence of activator and substrate is random (random bi-bi); (b) the model in which activator must bind before substrate (ordered bi-bi, activator essential), and (c) the model in which substrate must bind before activator (ordered bi-bi, activator non-essential). In general it is found that the kinetic equations for inhibitor binding are considerably simpler and easier to test than the corresponding transport equations. The effect of trans-inhibitor, transported substrate, activator concentration and membrane potential on inhibitor binding are examined in some detail. The use of these results to test and characterize the three transport models is emphasized. Applications to transport mechanisms which are not of the mobile carrier type are also discussed. A summary of relevant experimental data interpreted in terms of the theoretical models concludes the paper.

Energy-dependence of phlorizin binding to isolated renal microvillus membranes. Evidence concerning the mechanism of coupling between the electrochemical Na+ gradient the sugar transport

In order to elucidate the mechanism by which the electrochemical Na+ gradient energizes glucose transport, the energy-dependence of high affinity phlorizin binding to isolated renal microvillus membrane vesicles was examined. Phlorizin is a competitive inhibitor of glucose transport but is not itself transported. Extravesicular Na+ accelerated the rate of phlorizin binding and inhibited the rate of dissociation of bound glycoside. Maneuvers to enhance intravesicular electronegativity stimulated phlorizin uptake and those to enhance intravesicular electronegativity stimulated phlorizin uptake and those to enhance intravesicular electropositivity inhibited. However, alterations in electrical potential were without effect on the rate release of bound phlorizin. Intravesicular Na+ inhibited the phlorizin uptake rate. The results are consistent with a model of the glucose transporter in which (i) Na+ increases the binding affinity of the carrier, (ii) the free carrier is negatively charged, and (iii) the translocation of the carrier is inhibited by the binding of Na+ in the absence of sugar. The electrochemical Na+ gradient thus energizes both glucose transport and phlorizin binding through its effect on the affinity and appearance of the free carrier at the membrane surface rather than through an effect on sugar translocation per se.

High-affinity phlorizin binding to brush border membranes from small intestine: identity with (a part of) the glucose transport system, dependence on Na +-gradient, partial purification

In the presence of an NaSCN gradient phlorizin binds with a high affinity (Kd similar or equal to 4.7 micron) to vesicles derived from brush border membranes of intestinal cells of rabbits. The value for Kd corresponds closely to that of Ki determined from phlorizin inhibition of sugar transport. The apparent affinity for phlorizin is decreased if NaCl is substituted for NaSCN and decreased substantially if the gradient of NaSCN is allowed to dissipate prior to the phlorizin binding. The number of high affinity binding sites is about 11 pmol/mg protein. Additional binding to low affinity sites can amount to as much as 600 pmol/mg protein after prolonged exposure to phlorizin (5 min.). The high affinity sites are related to glucose transport based on the similarity of the Kd and Ki values under a variety of conditions and on the inhibition of the binding by D-glucose but not by D-fructose. The transport system and the high affinity phlorizin binding sites can be enriched by a factor of 2-3 by treatment of vesicles with papain, which does not affect the transport system, but considerably hydrolyzes nonrelevant protein.

Carrier-mediated transfer of D-glucose in brush border vesicles derived from rabbit renal tubules. Na+-dependent versus Na+-independent transfer

A brush border preparation from rabbit renal tubules containing a high yield of vesicles has been used to study the transfer of D-glucose through the brush border membrane. In the presence of an Na+ gradient across the vesicular membrane, the vesicles could concentrate D-glucose to a factor of 1.5, whereas in the absence of an Na+ gradient, only equilibrium with the medium was achieved. Two types of transfer could be distinguished by their requirement of Na+, their sensitivity to phlorizin and their pH optimum. The Na+-independent transfer was about 100 times less sensitive to phlorizin than the Na+-dependent path and exhibited a pH optimum between 7 and 8, whereas the Na+-dependent transfer was highest at a pH between 8 and 9. The brush border preparation could be freed of most of the contaminating material derived from the basal and lateral tubular cell membrane by a discontinuous density gradient centrifugation. It still showed both forms of transfer to a similar extent, indicating that both are located in the brush border membrane. A study of the sensitivity of D-glucose transfer to phlorizin, in the presence and absence of Na+ at different temperature, suggests a single carrier species functioning in two interchangeable conformational states with different affinities for phlorizin rather than two transfer systems working independently.

High affinity phlorizin receptor sites and their relation to the glucose transport mechanism in the proximal tubule of dog kidney

A set of high affinity phlorizin receptors in a brush border membrane preparation from dog kidney cortex is described. The dissociation constant, Kd is approximately equal to 0.3 muM (20 mM Tris-HCl, 150 mM Na-+, 5mM EDTA pH 7.45, 37 degree C). The number of receptor sites is approximately equal to 12-10- minus 12 mol/mg membrane protein. Preincubation with sugar substrates shows that the high affinity phlorizin binding is completely abolished by D-glucose (100 mM), 3-deoxy-30fluoro-D-glucose (125 mM) and alpha-methyl-D-glucopyranoside (125 mM), while 40-50% inhibition is observed with glucose concentrations as low as 5 mM. D-Galactose adn beta-methyl-D-galactopyranoside inhibit 20-40% at 125 mM while 2-deoxy-D-glucose and 2-deoxy-D-galactose inhibit minimally (approximately equal to 25%) at the same concentration. L-glucose, D-mannose, D-xylose, myoinositol, D-fructose and 3-O-methyl-D-glucose do not inhibit significantly in concentrations up to 600 mM. Unlabelled phlorizin (1 muM) and D-glucose (125 mM) completely wash off bound [3-H] phlorizin from the high affinity site. In contrast, phloretin (100 muM) is only about 50% as effective in displacing bound [3-H] phlorizin. Binding decreases with decreasing sodium concentration and is abolished by N-ethylmaleimide (7 mM). No inhibition is observed with ouabain (0.125 mM), cytochalasin B (0.1- 42 muM) and concanavalin A (10-10 000 mug/ml). The specificity of inhibition of phlorizin binding in vivo to the luminal membrane of the proximal tubule in dog kidney has also been investigated. Alpha-methyl-D-glucopyranoside completely washes off bound [3-H] phlorizin. D-galactose is only about 10% as effective at equivalent doses. There is no observable wash off of bound [3-H] phlorizin with D-fructose, myoinositol, D-mannose or 2-deoxy-D-glucose. The relative affinity of monosaccharides for the glucose transport receptor at the brush border was investigated in vivo using the multiple indicator dilution technique to determine their fractional reabsorption under identical conditions of phlorizin blockade. The relative affinities are in the order D-glucose approximately equal to alpha-methyl-D-glucopyranoside greater than D-galactose greater than 2-deoxy-D-glucose greater than D-fructose approximately equal to myoinositol. It is concluded (i) that phlorizin receptors on the brush border of the proximal tubule in vivo are identical to the high affinity phlorizin binding sites in the brush border membrane fraction in vitro and (ii) that these phlorizin receptor sites are either in close proximity, or identical, to the glucose transport receptor.