BIOTIN TRANSPORT IN MICROVILLOUS MEMBRANE-VESICLES, CULTURED TROPHOBLASTS, AND ISOLATED PERFUSED HUMAN PLACENTA

Biotin, essential for normal fetal growth and development, must be transported across the placenta to reach the fetus. This study evaluated placental transport of biotin using microvillous membrane vesicles (MMV), cultured trophoblasts, and isolated perfused cotyledon. Biotin uptake in MMV was stimulated by an inward Na+ gradient. In the presence of Na+, maximal stimulation was observed with Cl-, among various anions. Biotin uptake required 1 Na+ per biotin molecule. Kinetic analysis in MMV showed saturable transport with a Michaelis constant (K(m)) of 26.1 +/- 2.9-mu-M. Increases in membrane potential did not alter biotin uptake. Biotin uptake by cultured trophoblasts was also stimulated in the presence of Na+ and was saturable (K(m) = 7.0 +/- 1.5-mu-M). In the perfused placental cotyledon, maternal-to-fetal (M-to-F) biotin transfer was not saturable. However, biotin transfer in the M-to-F direction was significantly greater than the reverse. When the fetal circulation was closed to allow accumulation, an F/M ratio of only 1.056:1 was achieved. Tissue analysis of biotin contents suggested an active accumulation within the placental compartment. This study demonstrates that biotin is actively transported into the placenta, across the microvillous membrane, and released into the fetal compartment at a slower rate.