Lithium interactions with Na+-coupled inorganic phosphate cotransporters: insights into the mechanism of sequential cation binding

Am J Physiol Cell Physiol. 2012 Feb 1;302(3):C539-54. doi: 10.1152/ajpcell.00364.2011. Epub 2011 Nov 9.


Type IIa/b Na(+)-coupled inorganic phosphate cotransporters (NaPi-IIa/b) are considered to be exclusively Na(+) dependent. Here we show that Li(+) can substitute for Na(+) as a driving cation. We expressed NaPi-IIa/b in Xenopus laevis oocytes and performed two-electrode voltage-clamp electrophysiology and uptake assays to investigate the effect of external Li(+) on their kinetics. Replacement of 50% external Na(+) with Li(+) reduced the maximum transport rate and the rate-limiting plateau of the P(i)-induced current began at less hyperpolarizing potentials. Simultaneous electrophysiology and (22)Na uptake on single oocytes revealed that Li(+) ions can substitute for at least one of the three Na(+) ions necessary for cotransport. Presteady-state assays indicated that Li(+) ions alone interact with the empty carrier; however, the total charge displaced was 70% of that with Na(+) alone, or when 50% of the Na(+) was replaced by Li(+). If Na(+) and Li(+) were both present, the midpoint potential of the steady-state charge distribution was shifted towards depolarizing potentials. The charge movement in the presence of Li(+) alone reflected the interaction of one Li(+) ion, in contrast to 2 Na(+) ions when only Na was present. We propose an ordered binding scheme for cotransport in which Li(+) competes with Na(+) to occupy the putative first cation interaction site, followed by the cooperative binding of one Na(+) ion, one divalent P(i) anion, and a third Na(+) ion to complete the carrier loading. With Li(+) bound, the kinetics of subsequent partial reactions were significantly altered. Kinetic simulations of this scheme support our experimental data.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Biological Transport
  • Lithium / metabolism*
  • Membrane Potentials
  • Oocytes / cytology
  • Oocytes / metabolism
  • Patch-Clamp Techniques
  • Phosphates / metabolism
  • Sodium / metabolism*
  • Sodium-Phosphate Cotransporter Proteins, Type II / metabolism*
  • Xenopus laevis


  • Phosphates
  • Sodium-Phosphate Cotransporter Proteins, Type II
  • Lithium
  • Sodium