Examination of the substrate stoichiometry of the intestinal Na+/phosphate cotransporter

Phosphophloretin sensitivity of rabbit renal NaPi-IIa and NaPi-Ia

The effect of phosphorylated phloretins on Na+-dependent phosphate uptake into rabbit renal brush-border membrane vesicles (BBMV) was examined. Na+-dependent phosphate uptake into isolated rabbit cortex BBMV was sensitive to 2′-phosphophloretin (2′-PP) and 2′-phospho-4′,4,6′-trimethoxy phloretin (PTMP) in a dose-dependent and pH-dependent manner. PTMP inhibition of Na+-dependent phosphate uptake was maximum at alkali pH, and 2′-PP inhibition of Na+-dependent phosphate uptake was maximum at acidic pH. Increasing Na+concentrations did not increase PTMP inhibition of renal cortex BBMV Na+-dependent phosphate uptake at pH 6. The effect of phosphophloretins on Na+-dependent phosphate uptake was examined in BBMV isolated from purified proximal tubules and distal tubules. 2′-PP and PTMP inhibition of Na+-dependent phosphate uptake into BBMV isolated from purified proximal tubules was similar to the inhibition seen with BBMV from renal cortex. 2′-PP, but not PTMP, inhibited Na+-dependent phosphate uptake into BBMV isolated from purified distal tubules. The pH dependence of inhibition, the absence of PTMP inhibition of Na+-dependent phosphate uptake into distal tubule BBMV, and the inhibition of Na+-dependent phosphate uptake into distal tubule BBMV suggest that NaPi-Ia is 2′-PP sensitive and NaPi-IIa is PTMP sensitive.

Inhibition of human intestinal brush border membrane vesicle Na+-dependent phosphate uptake by phosphophloretin derivatives

Hyperphosphatemia and II(o) hyperparathyroidism are common and severe complications of chronic renal failure. Reduced dietary phosphorus has been shown to be an effective treatment in reducing serum phosphate and serum PTH. 2(')-Phosphophloretin inhibited small intestine apical membrane Na(+)/phosphate cotransport and reduced serum phosphate in adult rats. 2(')-PP and phosphoesters of phloretin were tested for inhibition of human small intestine brush border membrane alkaline phosphatase activity and for inhibition of Na(+)-dependent phosphate uptake. The IC(50)'s for inhibition of alkaline phosphatase suggested an order of inhibitory potency of 4-PP > phloretin > 4(')-PP > 2(')-PP. Inhibition of Na(+)-dependent phosphate uptake followed the sequence 2(')-PPz.Gt;4(')-PP > 4-PP > phloretin. These results are consistent with 2(')-PP being a specific inhibitor of human intestinal brush border membrane Na(+)/phosphate cotransport.

A phosphorylated phloretin derivative. Synthesis and effect on intestinal Na(+)-dependent phosphate absorption

2'-Phosphophloretin (2'-PP), a phosphorylated derivative of the plant chalcone, was synthesized. The effect of 2'-PP, on Na(+)-dependent phosphate uptake into intestinal brush-border membrane vesicles (BBMV) isolated from rabbit and rat duodenum and jejunum was examined. 2'-PP decreased Na(+)-dependent phosphate uptake into rabbit BBMV with an IC(50) of 55 nM and into rat BBMV with an IC(50) of 58 nM. 2'-PP did not affect Na(+)-dependent glucose, Na(+)-dependent sulfate, or Na(+)-dependent alanine uptake by rabbit intestinal BBMVs. 2'-PP inhibition of rabbit intestinal BBMV Na(+)-dependent phosphate uptake was sensitive to external phosphate concentration, suggesting that 2'-PP inhibition of Na(+)-dependent phosphate uptake was competitive with respect to phosphate. Binding of [(3)H]2'-PP to rabbit intestinal BBMV was examined. Binding of [(3)H]2'-PP was Na(+)-dependent with a K(0.5) for Na(+)(Na(+) concentration for 50% 2'-PP binding) of 30 mM. The apparent K(s) for Na(+)-dependent [(3)H]2'-PP binding to rabbit BBMVs was 58 nM in agreement with the IC(50) for 2'-PP inhibition of Na(+)-dependent phosphate uptake. These results indicate that 2'-PP bound to rabbit or rat intestinal BBMV Na(+)-phosphate cotransporter and inhibited Na(+)-dependent phosphate uptake. In rats treated with 2'-PP by daily gavage, the effect of 2'-PP on serum phosphate, serum glucose, and serum calcium was examined. In a concentration-dependent manner, 2'-PP reduced serum phosphate by 45% 1 wk after starting treatment. 2'-PP did not alter serum calcium or serum glucose. The apparent IC(50) for 2'-PP in vivo was 3 microM.

A 40-kDa polypeptide from papain digestion of the rabbit intestinal Na+/phosphate cotransporter retains Na+ and phosphate cotransport

The rabbit intestinal brush border membrane Na+/phosphate cotransporter was digested with a variety of proteolytic enzymes. Limited papain digestion generated a 40-kDa polypeptide (P40) which retained putative substrate site markers, fluorescein isothiocyanatophenyl glyoxal and eosin n-acetyl imidazole. P40 retained Na+- and phosphate-selective tryptophan fluorescence quenching, pH sensitivity of ion-induced conformational changes, and tight Na+ and H(2)PO(4)(-) binding. Reconstituted into proteoliposomes, P40 catalyzed Na+-dependent phosphate uptake. The N-terminus of P40 was blocked. An internal sequence of P40 demonstrated that it was derived from NaPi II b. These results suggest that P40 may be a useful model system for studies of the molecular mechanism of Na+-dependent phosphate cotransport and a starting point for structural studies.

Sodium-dependent transport of phosphate in neuronal and related cells

Sodium-dependent phosphate entry into neuronal cells was demonstrated in synaptic plasma membrane vesicles and synaptosomes prepared from rat brains, in PC12 cells and in primary culture of pituitary cells. The extent of the sodium-dependent phosphate transport in the synaptic plasma membrane preparation, at [Na]out = 110 mM and [P(i)]out = 0.1 mM, varied between 0.28 to 1.02 nmol phosphate/mg membrane protein/min. In pituitary cells the value was only about 0.05 nmol P(i)/mg protein/min. In PC12 cells the activity increased from 0.0085 to 0.26 nmol P(i)/mg protein/min in the transit from undifferentiated to differentiated cells. The dependence of phosphate on sodium concentrations fits a model in which two sodium ions are required to transfer the phosphate into the cells with a K[Na]0.5 of 43 mM. The K(m) for the phosphate transport in the synaptic plasma membrane preparations was between 0.1 and 0.45 mM. It is concluded that sodium-driven active transport of phosphate is a ubiquitous activity in various types of neuronal cells.

Interaction of substrates with the intestinal brush border membrane Na/phosphate cotransporter

The interaction of Na+ and phosphate with the intestinal brush border membrane Na+/phosphate cotransporter was examined using stopped-flow tryptophan fluorescence and ion-exchange Dowex columns coupled to a light-activated microsecond timer (LAM timer) which measures exchange kinetics between protein-bound ions and the external medium Na+ or Na+ + H2PO4- induced tryptophan fluorescence quenching with apparent rate constants of 35 s-1 and 13 s-1, respectively. Dilution of substrate-bound cotransporter resulted in tryptophan fluorescence recovery consistent with cotransporter return to the substrate-free conformation. Recovery of the substrate-free conformation was slow (1.6 s-1) in the absence of phosphate, was accelerated by H2PO4 (7 s-1) and was inhibited by HPO4(2) (1.1 s-1). The effects of substrates on tryptophan fluorescence were sensitive to substrate site blockers consistent with tryptophan fluorescence monitoring cotransporter conformations and substrate-induced changes in conformation. Equivalent experiments using the LAM timer and either (22Na+) or Na+ + (32P) phosphate verified the rate constants for the substrate-induced quenching of tryptophan fluorescence, suggested that 2 Na+ 's were occluded by the cotransporter as part of the Na(+)-induced conformational change and that H2PO4 accelerated deocclusion of Na+. The association of phosphate with the cotransporter was also examined. Although cotransporter-bound phosphate was medium anion-insensitive, a cotransporter conformational change preceding the release of phosphate from the cotransporter was not observed. However, three lines of evidence suggest that release of phosphate from the cotransporter involved a unique cotransporter conformation which may suggest that phosphate was also occluded by the intestinal brush border Na+/phosphate cotransporter.

Simultaneous occlusion of Na+ and phosphate by the intestinal brush border membrane Na+/phosphate cotransporter

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.

Examination of the molecular mechanism of SH reagent-induced inhibition of the intestinal brush-border membrane Na+/phosphate cotransporter

SH residues on the rabbit intestinal brush-border membrane Na+/phosphate cotransporter were examined using a variety of SH specific reagents, proteolytic digestion and HPLC separation of SH-labeled cotransporter, and partial reaction assays. Of the seven SH-containing peptide fragments on the non-denatured non-reduced cotransporter six peptides were labeled: five SH-containing peptides were labeled with acrylodan or IAF (iodoacetamidofluorescein) and three peptides were labeled with IAEDANS. One SH-containing peptide was labeled with IAEDANS or fluorescein maleimide only. Selective SH labeling conditions employing acrylodan and IAEDANS were used to identify the environments of these SH-containing peptides in the native cotransporter. The nature of SH reagent-induced inhibition of Na(+)-dependent phosphate uptake was examined using substrate-induced conformational changes, and substrate-induced changes in IAEDANS and acrylodan fluorescence of the SH-labeled Na+/phosphate cotransporter. The results indicate that five of the SH-labeled peptides sense the Na(+)-induced conformational change, three peptides sense the Na++ difluorophosphate-induced conformational change, and one peptide senses only the Na++ monofluorophosphate-induced conformational change. Five of the SH-labeled peptides are passive participants in the substrate-induced conformational changes with only SH 51 involved in cotransporter function. Alkylation of SH 51 resulted in a cotransporter conformation which differed from the substrate-mediated conformations and was characterized by increased monofluorophosphate sensitivity.

Effect of substrates and pH on the intestinal Na+/phosphate cotransporter: evidence for an intervesicular divalent phosphate allosteric regulatory site

Intervesicular divalent phosphate-induced inhibition of the intestinal brush-border membrane Na+/phosphate cotransporter was examined using Na(+)-dependent phosphate uptake, substrate-induced tryptophan fluorescence quenching, and the apparent pKa values for substrate-induced conformational changes. In right-side-out (RSO) reconstituted proteoliposomes, only monovalent phosphate inhibited Na(+)-dependent phosphate uptake in the absence of pre-equilibration. Addition of divalent phosphate to inside-out (ISO) proteoliposomes resulted in 80 +/- 5% inhibition of Na(+)-dependent phosphate uptake in the absence of pre-equilibration. The nature of divalent phosphate-induced inhibition of cotransporter function was examined using cotransporter partial reaction assays based on substrate-induced conformational changes reported as changes in tryptophan fluorescence. Na+ but not K+ induced a quenching of tryptophan fluorescence with a K0.5 of 25 mM and an apparent Hill coefficient of 1.8. Monovalent phosphate (difluorophosphate) induced a further quenching of tryptophan fluorescence with a K0.5 of 53 microM. Divalent phosphate (monofluorophosphate) had no effect on tryptophan fluorescence, but inhibited the difluorophosphate-induced quenching of tryptophan fluorescence. The Na+ to Na++ divalent phosphate (monofluorophosphate) conformation and the Na+ to Na++ monovalent phosphate (difluorophosphate) conformations were compared using tryptophan quench reagents. These transitions had different apparent pKa values and different phenylglyoxal sensitivities consistent with monovalent phosphate and divalent phosphate interacting with the cotransporter at separate sites.