Human Na⁺-dependent vitamin C transporter 1 (hSVCT1): primary structure, functional characteristics and evidence for a non-functional splice variant

Abstract

We report here on the cloning and functional characterization of human Na⁺-dependent vitamin C transporter 1 (SVCT1). The human SVCT1 cDNA, obtained from a Caco2 cell cDNA library, encodes a protein of 598 amino acids with 12 putative transmembrane domains. The SVCT1-specific transcript, 2.4 kb in size, is expressed in kidney, liver, small intestine, thymus and prostate. When expressed heterologously in HRPE cells, SVCT1 mediates the transport of ascorbate, the reduced form of vitamin C, in a Na⁺-dependent manner. The transporter is specific for ascorbate with a K_t of ∼75 μM. The relationship between the cDNA-specific uptake rate of ascorbate and Na⁺ concentration is sigmoidal with a Na⁺:ascorbate stoichiometry of 2:1, indicating that the transport process is electrogenic. In Caco2 cells and in normal human intestine, SVCT1 also exists as a non-functional splice variant with a four amino acid sequence inserted between E-155 and V-156. The splice variant results from the use of a donor site 12 bp downstream of the normal donor site.