Mechanistic insights and functional determinants of the transport cycle of the ascorbic acid transporter SVCT2: Activation by sodium and absolute dependence on bivalent cations

Alejandro Godoy, Valeska Ormazabal, Gustavo Moraga-Cid, Felipe A. Zúñiga, Paula Sotomayor, Valeria Barra, Osmán Vasquez, Viviana Montecinos, Lorena Mardones, Catherine Guzmán, Marcelo Villagrán, Luis G. Aguayo, Sergio A. Oñate, Alejandro M. Reyes, Juan G. Cárcamo, Coralia I. Rivas, Juan Carlos Vera

Research output: Contribution to journalArticlepeer-review

88 Citations (Scopus)

Abstract

We characterized the human Na+-ascorbic acid transporter SVCT2 and developed a basic model for the transport cycle that challenges the current view that it functions as aNa+-dependent transporter. The properties of SVCT2 are modulated by Ca2+/Mg2+ and a reciprocal functional interaction between Na+ and ascorbic acid that defines the substrate binding order and the transport stoichiometry. Na+ increased the ascorbic acid transport rate in a cooperative manner, decreasing the transport Km without affecting the Vmax, thus converting a low affinity form of the transporter into a high affinity transporter. Inversely, ascorbic acid affected in a bimodal and concentration-dependent manner the Na+ cooperativity, with absence of cooperativity at low and high ascorbic acid concentrations. Our data are consistent with a transport cycle characterized by a Na+:ascorbic acid stoichiometry of 2:1 and a substrate binding order of the type Na +:ascorbic acid:Na+. However, SVCT2 is not electrogenic. SVCT2 showed an absolute requirement for Ca2+/Mg2+ for function, with both cations switching the transporter from an inactive into an active conformation by increasing the transport Vmax without affecting the transport Km or the Na+ cooperativity. Our data indicate that SVCT2 may switch between a number of states with characteristic properties, including an inactive conformation in the absence of Ca2+/Mg2+. At least three active states can be envisioned, including a low affinity conformation at Na+ concentrations below 20 mM and two high affinity conformations at elevated Na+ concentrations whose Na+ cooperativity is modulated by ascorbic acid. Thus, SVCT2 is a Ca2+/Mg2+-dependent transporter.

Original languageEnglish
Pages (from-to)615-624
Number of pages10
JournalJournal of Biological Chemistry
Volume282
Issue number1
DOIs
Publication statusPublished - 5 Jan 2007

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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