Specific photoaffinity inactivation of the D-glucose transporter in small intestinal brush border membrane using new phlorizin analogues

Cotransport systems in the brush border membrane of the human placenta

The cotransport systems present in the brush border membrane of the human placental syncytiotrophoblast are reviewed. Attention is focused on the systems that are powered by the electrochemical gradient of Na+ (for example, for neutral amino acids but not for glucose), and on recently described systems in which K+ flux is coupled to C1- flux or to that for Na+ and C1-. These systems are similar to those found in other tissues and may be significant for net trans-placental transport and its control, as well as for the regulation of placental trophoblast volume.

Glycosylation of the rabbit intestinal brush border Na+/glucose cotransporter

The glycosylation of the mature form of the rabbit intestinal Na+/glucose cotransporter was investigated by using both glycosidases and chemical treatment. The protein was identified on Western blots using polyclonal antibodies directed against peptide sequences from the cloned transporter as a Mr 68,000 polypeptide. The effect of these treatments on the size of the transporter is consistent with the major post-translational processing being a single N-linked glycosylation of either the tri- or tetra-antennary complex type. Either method of deglycosylation reduced the SDS-PAGE size by 11,000 to Mr 57,000. These results also suggest that O-linked glycosylation, if present, contributes little to the apparent size of the transporter. The relative size of the deglycosylated mature protein appears to be greater than that of the in vitro primary transcript (Mr 45,000), suggesting either a difference in a stable conformational state insensitive to reduction and denaturation by SDS or an additional post-translational modification. In addition, deglycosylation of the native transporter does not affect transport activity in brush border membrane vesicles. The transporter, an integral membrane protein having several membrane-spanning regions, has an anomalous mobility in SDS-PAGE as shown by Ferguson analysis. We estimate that the actual size of the mature Na+/glucose cotransporter is 86,000, and that N-linked glycosylation contributes about 15,000 to the mass.

Intestinal and renal Na+/glucose cotransporters share common structures

Polyclonal antibodies were raised to peptides selected from three different regions of the cloned rabbit intestinal Na+/glucose cotransporter. Western blot analysis was used to identify the fully mature protein in intestinal and renal brush borders. Two of the antibodies specifically identified a approximately 70-kDa protein band in rabbit intestinal brush borders but did not specifically immunoreact with membranes that do not have Na(+)-dependent glucose transport activity. The immunoreactive proteins had an apparent isoelectric point between pH 4.7 and 6.8. The antibodies also specifically recognized a similar-sized protein in human and seven other mammalian intestinal brush borders. Similar protein bands were identified in four mammalian renal brush-border membranes, indicating shared epitopes between intestinal and renal cotransport proteins. In some species, e.g., lamb and pig, the epitope for one antibody was missing in both intestinal and renal brush borders, suggesting that this epitope is not essential for function. These results suggest that 1) the cloned intestinal Na+/glucose cotransporter is that identified in earlier biochemical studies, 2) there is close structural similarity between intestinal and renal cotransporters, and 3) the structure of these proteins has been conserved during evolution.

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.

Use of mild detergent gel electrophoresis for isolation and characterization of the kidney brush border D-glucose transporter

Gradient gel electrophoresis was performed under mild detergent conditions to separate pig kidney brush border membrane proteins and to identify the smallest functional molecular protein entity of the D-glucose transporter. The various protein bands obtained from the nondenaturing gel system in a semipreparative scale were eluted by electrodialysis. These proteins were then reintegrated into proteoliposomes and tested for D-glucose-inhibitable [3H]phlorizin binding. The D-glucose transporter had a molecular mass of 70 kDa in mild detergent electrophoresis conditions and in subsequent SDS analysis.

Leakiness of brush-border vesicles

From the water content of pelleted brush-border vesicles and from a comparison of the aqueous volume within the pellet that is available to [3H]inulin (58%), inulin [14C]carboxylic acid (34%, both approx. 5000 daltons), [3H]raffinose (97%, 540 daltons) and [3H]glucose (94%, 180 daltons) it is concluded that only 1 in 4 to 6 of the brush-border vesicles is sealed. The implication of this finding for labelling and transport studies and for vesicle formation is discussed.

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.

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.

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.

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.