title: Lithium interactions with Na+-coupled inorganic phosphate cotransporters: insights into the mechanism of sequential cation binding.
abstract: 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.
authors.affiliation: Institute of Physiology, Univ. of Zurich, Zurich, Switzerland.
authors.forename: Olga
authors.initials: O
authors.lastname: Andrini
authors.forename: Anne-Kristine
authors.initials: AK
authors.lastname: Meinild
authors.forename: Chiara
authors.initials: C
authors.lastname: Ghezzi
authors.forename: Heini
authors.initials: H
authors.lastname: Murer
authors.forename: Ian C
authors.initials: IC
authors.lastname: Forster
mesh_terms: D000818:Animals; D001692:Biological Transport; D008094:Lithium / Q000378:metabolism*; D008564:Membrane Potentials; D009865:Oocytes / Q000166:cytology / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010710:Phosphates / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D050610:Sodium-Phosphate Cotransporter Proteins, Type II / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D010710:Phosphates; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D008094:Lithium; D012964:Sodium