Partial purification of the Na+-dependent D-glucose transport system from renal brush border membranes

Membrane fractions display different lipid and enzyme content in three cell types in 16-cell stage embryos of sea urchins

Three cell types were isolated from dissociated 16-cell sea urchin embryos. Four membrane density fractions from discontinuous gradients have different proportions of lipids, surfacer markers and enzymes for the three cell types. Assays of lipid content, CH/PLIPID and SPH/PC ratios, acyl chain length, level of unsaturation by proton NMR and assays of enzyme activity revealed variation at the same density between the three cell types and among different densities from one cell type. There were also differences between whole embryos and dissociated embryo cells. There was no typical membrane domain at a particular density common to the cell types. Cell surface characteristics and polarity of adult cells rely on which lipid domains and enzymes are present, their association with cytoskeleton and how they are localized. At the 16-cell stage these characteristics are still very dynamic as revealed by cytochemical localization of Na+/K(+)-ATPase which varied with cell type and suggests endocytosis at set times in the division cycle. Polarity has not been permanently set for Na+/K(+)-ATPase yet. Membrane enzyme and lipid distributions unique to the three cell types seen in this study suggest parcelling out or insertion of new membrane domains occurs during early sea urchin cleavage. Perturbation of membrane density distribution and lipid content occurs after treatment of embryos with animalizing and vegetalizing teratogens which alter development.

Monoclonal antibodies that bind the renal Na+/glucose symport system. 1. Identification

Phlorizin is a specific, high-affinity ligand that binds the active site of the Na+/glucose symporter by a Na+-dependent mechanism but is not itself transported across the membrane. We have isolated a panel of monoclonal antibodies that influence high-affinity, Na+-dependent phlorizin binding to pig renal brush border membranes. Antibodies were derived after immunization of mice either with highly purified renal brush border membranes or with apical membranes purified from LLC-PK1, a cell line of pig renal proximal tubule origin. Antibody 11A3D6, an IgG2b, reproducibly stimulated Na+-dependent phlorizin binding whereas antibody 18H10B12, an IgM, strongly inhibited specific binding. These effects were maximal after 30-min incubation and exhibited saturation at increased antibody concentrations. Antibodies did not affect Na+-dependent sugar uptake in vesicles but significantly prevented transport inhibition by bound phlorizin. Antibodies recognized a 75-kDa antigen identified by Western blot analysis of brush border membranes, and a 75-kDa membrane protein could be immunoprecipitated by 18H10B12. These properties, taken together with results in the following paper [Wu, J.-S.R., & Lever, J.E. (1987) Biochemistry (following paper in this issue)], provide compelling evidence that the 75-kDa antigen recognized by these antibodies is a component of the renal Na+/glucose symporter.

Solubilization and reconstitution of the renal phosphate transporter

Proteins from brush-border membrane vesicles of rabbit kidney cortex were solubilized with 1% octylglucoside (protein to detergent ratio, 1:4 (w/w). The solubilized proteins (80.2 +/- 2.3% of the original brush-border proteins, n = 10, mean +/- S.E.) were reconstituted into artificial lipid vesicles or liposomes prepared from purified egg yolk phosphatidylcholine (80%) and cholesterol (20%). Transport of Pi into the proteoliposomes was measured by rapid filtration in the presence of a Na+ or a K+ gradient (out greater than in). In the presence of a Na+ gradient, the uptake of Pi was significantly faster than in the presence of a K+ gradient. Na+ dependency of Pi uptake was not observed when the liposomes were reconstituted with proteins extracted from brush-border membrane vesicles which had been previously treated with papain, a procedure that destroys Pi transport activity. Measurement of Pi uptake in media containing increasing amounts of sucrose indicated that Pi was transported into an intravesicular (osmotically sensitive) space, although about 70% of the Pi uptake appeared to be the result of adsorption or binding of Pi. However, this binding of Pi was not dependent upon the presence of Na+. Both Na+-dependent transport and the Na+-independent binding of Pi were inhibited by arsenate. The initial Na+-dependent Pi transport rate in control liposomes of 0.354 nmol Pi/mg protein per min was reduced to 0.108 and 0 nmol Pi/mg protein per min in the presence of 1 and 10 mM arsenate, respectively. Future studies on reconstitution of Pi transport systems must analyze and correct for the binding of Pi by the lipids used in the formation of the proteoliposomes.

Reconstitution of a partially purified Na+-dependent D-glucose transport system from rat jejunal brush border membranes

The Na+-dependent D-glucose transport system of rat jejunal brush border membranes was partially purified and reconstituted into functional proteoliposomes. Brush border membrane vesciles isolated from villous cells were first extracted with 0.3% cholate to remove extrinsic proteins and the insoluble residual pellet was reextracted with 1.2% cholate. The 1.2% cholate-extracted soluble fraction was then further purified by hydroxylapatite and Concanavalin A affinity chromatography in tandem. When the HLP-unadsorbed-ConA-unadsorbed fraction was reconstituted into proteoliposomes, it showed a characteristic Na+-coupled, phlorizin inhibitable, D-glucose transport activity that was 3 fold higher than that of the reconstituted proteoliposomes of the 1.2% cholate-extracted fraction. This partially purified fraction also displayed the simplest polypeptide composition pattern among all the membrane fractions analysed in SDS-polyacrylamide gels.

Reconstitution of [3H]3-cyanoimipramine ([3H]Ro 11-2465) sites solubilised from human platelets

[3H]3-Cyanoimipramine has been characterised as a pseudoirreversible ligand at imipramine binding sites making it useful for molecular characterisation studies. Using this ligand, I now report that it is possible to solubilise the binding site molecule with cholate. Gel permeation chromatography on a Sepharose 4B column equilibrated in 0.1% cholate revealed a micellar size of 4.1 nm. Subsequent removal of cholate (from an initial concentration of 2% to less than 0.02%) in the presence of Azolectin (soybean lipid extract) led to reconstituted material that could be retained on glass fibre filters. Electron microscopy revealed apparent micelles of approximately 1 micron diameter. Over 16% of the original, membrane-bound, binding sites were recovered in the reconstituted material along with a similar percentage of the protein. This reconstituted material was moderately stable for up to 3 days after its preparation.

Isolation and partial purification of dicarboxylic acid binding protein from luminal-membrane vesicles of rabbit kidney cortex

A specific dicarboxylic acid binding protein was isolated by solubilizing highly purified renal luminal-membrane vesicles with the non-ionic detergent C12E8 , followed by affinity chromatographic procedures. SDS-polyacrylamide gel electrophoresis of the samples containing dicarboxylic acid binding protein showed a single sharp band of an apparent molecular weight of 50 000. After treatment with mercaptoethanol the protein was split in two subunits of apparent molecular weights of 35 000 and 15 000. By analytical ultracentrifugation the minimal molecular weight of the dicarboxylic acid binding protein preparation was calculated to be 54 000. Binding of the radioactive succinate and L-malate to the dicarboxylic acid binding protein preparation as studied by equilibrium dialysis showed saturation phenomenon and was specifically inhibited by addition of D-malate. The dissociation constants for succinate (0.18 mM) and L-malate (0.33 mM) calculated from the binding data agree extremely well with the apparent Km values for these organic acids found in transport studies utilizing intact luminal-membrane vesicles.

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

Solubilized Ehrlich cell plasma membrane proteins were incorporated into lipid vesicles in the presence of added phospholipid, using Sephadex G-50 chromatography combined with a freeze-thaw step. Liposomes formed in K+ exhibited high levels of Na+-dependent, alpha-aminoisobutyric acid uptake which was electrogenic and inhibited by other amino acids. The transport activity reconstituted was similar to that observed in native plasma membrane vesicles. In addition to transport by system A, leucine exchange activity (system L), Na+-dependent serine exchange activity (system ASC), and stereospecific glucose transport activity were also reconstituted. The latter was inhibited by D-glucose, D-galactose, cytochalasin B, and mercuric chloride. The medium used for reconstitution was critical for the recovery of Na+-dependent amino acid transport. The use of Na+ in the reconstitution procedure led to formation of liposomes which displayed little Na+-dependent and gradient-stimulated amino acid uptake. In contrast, all transport activities studied were efficiently reconstituted in K+ medium.

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.

A simple liposomal system to reconstitute and assay highly efficient Na+/D-glucose cotransport from kidney brush-border membranes

A simple procedure to reconstitute highly efficient Na+/D-glucose cotransport from solubilized brush-border membranes of proximal kidney tubules is described. Reconstitution of transport activity was possible with various phospholipid and cholesterol combinations; the presence, however, of cholesterol and at least one phospholipid was essential. When liposomes were synthesized from only one phospholipid and cholesterol, the highest uptake rats were observed with phosphatidylserine; phosphatidylcholine was less effective and phosphatidylethanolamine showed insignificant uptake of D-glucose in the presence of Na+. The rate at which an inward-directed Na+ gradient dissipated across the liposomal membranes was reduced if the cholesterol concentration of liposomes was increased. In the optimized system, proteoliposomes were formed from cholesterol and phosphatidylserine by a heat-sonication-freeze-thaw procedure. A Na+-gradient persisted for hours across these proteoliposomal membranes and a Na+/D-glucose cotransport with the following characteristics could be demonstrated: (1) dependency on the Na+ gradient; (2) a transient (3) rheogenicity; (4) stereospecificity; and (5) high-affinity phlorizin inhibition. Since the Na+-gradient-stimulated D-glucose uptake is linear for minutes, the initial uptake rates can be measured and the Na+/D-glucose cotransport activity of different protein fractions can be compared.

Reconstitution of a partially purified Na+-independent D-glucose transport system from rat jejunal basolateral membranes

1. Basolateral membranes of rat small intestine were first solubilized in a 0.6% cholate buffer and then the insoluble fraction was reextracted with a 1.2 or 1.6% cholate buffer. 2. Proteoliposomes reconstituted from the 1.2 or 1.6% cholate-extracted membrane fraction demonstrated characteristic Na+-independent D-glucose transport of the native basolateral membrane vesicles: inhibitable by mercuric chloride and D-galactose. 3. To further purify this D-glucose transport system, the 1.6% cholate-extracted membrane fraction was chromatographed on either hydroxylapatite, concanavalin A, wheat-germ lectin or castor bean lectin-120 affinity gels. 4. Proteoliposomes reconstituted from the membrane proteins adsorbed on hydroxylapatite and subsequently passed through agarose-castor bean lectin-120 showed a 12-fold enrichment of Na+-independent D-glucose transport activity over that of the native membrane vesicles. 5. SDS-electrophoretic analysis showed that the protein composition of the hydroxylapatite-castor bean lectin-120 treated fraction was much simpler than that of both 1.6% cholate-extracted fraction and the native membrane vesicles.

Membrane transport in the proximal tubule and thick ascending limb of Henle's loop: mechanisms and their alterations

Over the past few years, our knowledge on renal tubular transport mechanisms has increased considerably. Due to new technical developments, it is now possible to understand in part transepithelial transport and its pathological and pharmacological alterations at the level of the cell membranes. Different membrane transport mechanisms are discussed in this article, whereby sodium coupled solute transport in the proximal tubule and sodium chloride transport in the thick ascending limb of Henle's loop are taken as examples. It is indicated that an altered function of the kidney can often be equated with an alteration of the membrane transport.