title: Simultaneous occlusion of Na+ and phosphate by the intestinal brush border membrane Na+/phosphate cotransporter.
abstract: The molecular mechanism of Na+ and phosphate transport by the intestinal brush border membrane Na+/phosphate cotransporter was examined using anion and cation exchange columns interfaced to a light-activated microsecond timer. The nature of ion:cotransporter interaction was examined for sensitivity to itself versus non-substrates, and sensitivity to amino acid specific reagents. Two possible ion:cotransporter interactions were examined. (a) ion bound, where ion exchanges rapidly with medium ion but not non-substrates, and (b) ion occluded, where ion exchanges slowly with medium ion. The Na+/phosphate cotransporter was found to occlude Na+ or Na+ and H2PO4, but not H2PO4 alone or HPO4. Phosphate occlusion had an absolute requirement for Na+ with K+ or Cs+ unable to substitute. Phosphate occlusion distinguished between phosphate and sulfate. Deocclusion from the fully loaded cotransporter was consistent with the release of 2 Na+'s prior to phosphate. These results predict two ternary cotransporter conformations differing in the Na+ sensitivity of their phosphate affinity.
authors.affiliation: Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, USA.
authors.forename: B E
authors.initials: BE
authors.lastname: Peerce
mesh_terms: D001667:Binding, Competitive / Q000502:physiology; D001692:Biological Transport / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism*; D007422:Intestines / Q000166:cytology; D015640:Ion Channel Gating / Q000502:physiology; D008871:Microvilli / Q000737:chemistry / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D010761:Phosphorus Radioisotopes / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D012979:Sodium Radioisotopes / Q000378:metabolism; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters
chemical_list: D002352:Carrier Proteins; D010710:Phosphates; D010761:Phosphorus Radioisotopes; D012979:Sodium Radioisotopes; D050608:Sodium-Phosphate Cotransporter Proteins; D027981:Symporters; D012964:Sodium