title: 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.
abstract: 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.
mesh_terms: D000469:Alkaline Phosphatase / Q000037:antagonists & inhibitors; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000528:radiation effects; D002352:Carrier Proteins / Q000378:metabolism* / Q000528:radiation effects; D002417:Cattle; D005947:Glucose / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D008871:Microvilli / Q000378:metabolism; D008970:Molecular Weight; D009004:Monosaccharide Transport Proteins; D010695:Phlorhizin / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology*; D005723:gamma-Glutamyltransferase / Q000037:antagonists & inhibitors
chemical_list: D002352:Carrier Proteins; D009004:Monosaccharide Transport Proteins; D012964:Sodium; D010695:Phlorhizin; D005723:gamma-Glutamyltransferase; D000469:Alkaline Phosphatase; D005947:Glucose