Stoichiometry and Na+ binding cooperativity of rat and flounder renal type II Na+-Pi cotransporters.
The stoichiometry of the rat and flounder isoforms of the renal type II sodium-phosphate (Na+-Pi) cotransporter was determined directly by simultaneous measurements of phosphate (Pi)-induced inward current and uptake of radiolabeled Pi and Na+ in Xenopus laevis oocytes expressing the cotransporters. There was a direct correlation between the Pi-induced inward charge and Pi uptake into the oocytes; the slope indicated that one net inward charge was transported per Pi. There was also a direct correlation between the Pi-induced inward charge and Na+ influx; the slope indicated that the influx of three Na+ ions resulted in one net inward charge. This behavior was similar for both isoforms. We conclude that for both Na+-Pi cotransporter isoforms the Na+:Pi stoichiometry is 3:1 and that divalent Pi is the transported substrate. Steady-state activation of the currents showed that the Hill coefficients for Pi were unity for both isoforms, whereas for Na+, they were 1.8 (flounder) and 2.5 (rat). Therefore, despite significant differences in the apparent Na+ binding cooperativity, the estimated Na+:Pi stoichiometry was the same for both isoforms.
Forster
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Physiologisches Institut, Universität Zürich, CH-8057 Zurich, Switzerland. forster@physiol.unizh.ch
Loo
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Eskandari
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D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D055598:Chemical Phenomena; D002621:Chemistry; D004563:Electrochemistry; D005260:Female; D005432:Flounder; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D009865:Oocytes / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters; D014982:Xenopus laevis
D002352:Carrier Proteins; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters; D012964:Sodium