Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter

Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter The kinetics of a type IIb Na+-coupled inorganic phosphate (Pi) cotransporter (NaPi-IIb) cloned from mouse small intestine were studied using the two-electrode voltage clamp applied to Xenopus oocytes. In the steady state, mouse NaPi-IIb showed a curvilinear I-V relationship, with rate-limiting behavior only for depolarizing potentials. The Pi dose dependence was Michaelian, with an apparent affinity constant for Pi ( $ {K_{\rm m}}^{\rm P_i} $ ) of 10 ± 1 μM at −60 mV. Unlike for rat NaPi-IIa, $ {K_{\rm m}}^{\rm P_i} $ increased with membrane hyperpolarization, as reported for human NaPi-IIa, flounder NaPi-IIb and zebrafish NaPi-IIb2. The apparent affinity constant for Na+ ( $ {K_{\rm m}}^{\rm Na} $ ) was 23 ± 1 mM at −60 mV, and the Na+ activation was cooperative with a Hill coefficient of approximately 2. Pre-steady-state currents were documented in the absence of Pi and showed a strong dependence on external Na+. The hyperpolarizing shift of the charge distribution midpoint potential was 65 mV/log[Na]. Approximately half the moveable charge was attributable to the empty carrier. A comparison of the voltage dependence of steady-state Pi-induced current and pre-steady-state charge movement indicated that for −120 mV ≤ V ≤ 0 mV the voltage dependence of the empty carrier was the main determinant of the curvilinear steady-state cotransport characteristic. External protons partially inhibited NaPi-IIb steady-state activity, independent of the titration of mono- and divalent Pi, and immobilized pre-steady-state charge movements associated with the first Na+ binding step. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Membrane Biology Springer Journals

Electrogenic Kinetics of a Mammalian Intestinal Type IIb Na+/Pi Cotransporter

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Copyright © 2007 by Springer Science+Business Media, Inc.
Life Sciences; Biochemistry, general; Human Physiology
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