Roboflavin Uptake by Microvillous Membrane Vesicles from Human Placental Syncytiotrophoblast
Powell, Kristen A.
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A layer of syncytiotrophoblast cells separates the maternal and fetal circulations in the human placenta. The microvillous membrane of the syncytiotrophoblast faces the mother and the basal membrane is exposed to the fetus. In order for transport from mother to fetus to occur, nutrients must first cross the microvillous membrane. A variety of transport mechanisms have been studied: simple diffusion, facilitated diffusion, carrier-mediated transport, and receptor-mediated endocytosis. The present study was undertaken to determine the mechanism of transport of the water-soluble vitamin, riboflavin (vitamin B2). Isolated microvillous membrane vesicles were chosen for the study so that transport across this initial membrane could be examined exclusively. It was determined that a significant component of riboflavin uptake is binding, rather than transport. Via further characterization of this binding component, the Kd (binding constant), Bmax (maximum amount of substrate that binds to a saturable site), and non-specific binding were elucidated. Riboflavin could be involved in receptor-mediated endocytosis, however further investigation is required to substantiate this hypothesis. Discovery of the mechanism of riboflavin transport may aid our understanding of how riboflavin deficiency occurs.