Human placental sodium-dependent vitamin C transporter (SVCT2): molecular cloning and transport function

We report here on the cloning and functional characterization of human SVCT2, a sodium-dependent vitamin C (ascorbate) transporter. The hSVCT2 cDNA obtained from a human placental choriocarcinoma cell cDNA library, codes for a protein of 650 amino acids with a predicted molecular mass of 70 kDa. At the level of amino acid sequence, the human SVCT2 exhibits 95% identity to its rat homolog. When functionally expressed in mammalian cells, hSVCT2 induces the transport of ascorbic acid. The transport process induced by hSVCT2 is Na(+)-dependent and is specific for ascorbate. The Michaelis-Menton constant (K(t)) for the transport of ascorbate in cDNA-transfected cells is 69 +/- 5 microM. The relationship between the cDNA-specific uptake rate of ascorbate and Na(+) concentration is sigmoidal with a Na(+):ascorbate stoichiometry of 2:1. Northern blot analysis shows that SVCT2-specific transcripts are present in heart, brain, placenta, and liver and is absent in lung and skeletal muscle. The size of the principal transcript is approximately 7.5 kb.