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 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 μM 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.