Heterogeneity in the effects of membrane potentials on pantothenate and glucose uptakes by rabbit renal apical membranes

Structure and function of mammalian sodium-dependent multivitamin transporter

This review focuses on the advances made in recent years in the understanding of the multivitamin transporter, a unique and important transporter that transports not one but three different unrelated water-soluble vitamins namely, pantothenate, biotin and lipoate. The transport process mediated by the transporter is active and is energized by the transmembrane sodium ion gradient as well as the membrane potential. The transporter belongs to the sodium-coupled glucose transporter family. The ubiquitous expression of this transporter in mammalian tissues and the fact that it is highly conserved across the species indicate the nutritional relevance and importance of this transporter.

Human placental Na+-dependent multivitamin transporter. Cloning, functional expression, gene structure, and chromosomal localization

We have cloned the human Na+-dependent multivitamin transporter (SMVT), which transports the water-soluble vitamins pantothenate, biotin, and lipoate, from a placental choriocarcinoma cell line (JAR). The cDNA codes for a protein of 635 amino acids with 12 transmembrane domains and 4 putative sites for N-linked glycosylation. The human SMVT exhibits a high degree of homology (84% identity and 89% similarity) to the rat counterpart. When expressed in HRPE cells, the cDNA-induced transport process is obligatorily dependent on Na+ and accepts pantothenate, biotin, and lipoate as substrates. The relationship between the cDNA-specific uptake rate of pantothenate or biotin and Na+ concentration is sigmoidal with a Na+:vitamin stoichiometry of 2:1. The human SMVT, when expressed in Xenopus laevis oocytes, induces inward currents in the presence of pantothenate, biotin, and lipoate in a Na+-, concentration-, and potential-dependent manner. We also report here on the structural organization and chromosomal localization of the human SMVT gene. The SMVT gene is approximately 14 kilobase pairs in length and consists of 17 exons. The SMVT gene is located on chromosome 2p23 as evidenced by somatic cell hybrid analysis and fluorescence in situ hybridization.

Characterization of a sodium-dependent vitamin transporter mediating the uptake of pantothenate, biotin and lipoate in human placental choriocarcinoma cells

The characteristics of the uptake of the vitamin pantothenate into JAr human placental choriocarcinoma cells were investigated and these cells were found to accumulate the vitamin against a concentration gradient by a Na(+)-dependent process. Substitution of Na+ with over other monovalent cations abolished the uptake completely. The transport process showed no preference for any particular anion. Kinetic analysis indicated the presence of a single saturable transport system with a Michaelis-Menten constant of 2.1 +/- 0.2 microM and a maximal velocity of 341 +/- 12 pmol/mg of protein per 10 min. The dependence of the uptake rate of pantothenate on Na+ concentration exhibited sigmoidal kinetics, indicating interaction of more than one Na+ ion with the transporter. The Hill coefficient for this process was calculated to be 1.6. The Na+/pantothenate coupling ratio being greater than unity suggests that the transport process is electrogenic, resulting in net transfer of positive charge across the membrane. This was confirmed in plasma membrane vesicles prepared from JAr cells where the uptake of pantothenate was found to be significantly stimulated by valinomycin-induced inside-negative K(+)-diffusion potential. Substrate specificity studies showed that, in addition to pantothenate, the transporter interacts with two other vitamins, namely biotin and lipoate. The characteristics of pantothenate uptake in the placental cell line BeWo was also investigated. These cells were also found to express a pantothenate transport system similar to that expressed in the JAr cells.