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This Article Full Text Full Text (PDF) All Versions of this Article: 294/2/C451    most recent 00439.2007v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Mackenzie, B. Articles by Hediger, M. A. Search for Related Content PubMed PubMed Citation Articles by Mackenzie, B. Articles by Hediger, M. A.

MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Transport model of the human Na⁺-coupled L-ascorbic acid (vitamin C) transporter SVCT1

¹Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, Ohio; ²Membrane Biology Program and Renal Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts; and ³Institute of Biochemistry and Molecular Medicine, University of Berne, Bern, Switzerland

Submitted 21 September 2007 ; accepted in final form 18 December 2007

Vitamin C (L-ascorbic acid) is an essential micronutrient that serves as an antioxidant and as a cofactor in many enzymatic reactions. Intestinal absorption and renal reabsorption of the vitamin is mediated by the epithelial apical L-ascorbic acid cotransporter SVCT1 (SLC23A1). We explored the molecular mechanisms of SVCT1-mediated L-ascorbic acid transport using radiotracer and voltage-clamp techniques in RNA-injected Xenopus oocytes. L-Ascorbic acid transport was saturable (K_0.5 70 µM), temperature dependent (Q₁₀ 5), and energized by the Na⁺ electrochemical potential gradient. We obtained a Na⁺-L-ascorbic acid coupling ratio of 2:1 from simultaneous measurement of currents and fluxes. L-Ascorbic acid and Na⁺ saturation kinetics as a function of cosubstrate concentrations revealed a simultaneous transport mechanism in which binding is ordered Na⁺, L-ascorbic acid, Na⁺. In the absence of L-ascorbic acid, SVCT1 mediated pre-steady-state currents that decayed with time constants 3–15 ms. Transients were described by single Boltzmann distributions. At 100 mM Na⁺, maximal charge translocation (Q_max) was 25 nC, around a midpoint (V_0.5) at –9 mV, and with apparent valence –1. Q_max was conserved upon progressive removal of Na⁺, whereas V_0.5 shifted to more hyperpolarized potentials. Model simulation predicted that the pre-steady-state current predominantly results from an ion-well effect on binding of the first Na⁺ partway within the membrane electric field. We present a transport model for SVCT1 that will provide a framework for investigating the impact of specific mutations and polymorphisms in SLC23A1 and help us better understand the contribution of SVCT1 to vitamin C metabolism in health and disease.

cotransporters; sodium dependent; intestinal absorption; model simulation; renal reabsorption; Xenopus oocyte