Insights into the mechanism for transporting copper, iron, and zinc across the placenta have tended to lag behind nutritional understandings. Cell cultures are a powerful tool for elucidating the fine details of the transport mechanism. Here, we analyze placental Cu transport and compare the mechanism with Zn transport and the well-characterized receptor- mediated mechanism for Fe transport (as transferrin). We hypothesize that maternal ceruloplasmin (Cp) is a major plasma source of Cu for the developing fetus; that transport occurs through specific receptors for Cp on the placental surface. We further postulate that placental Cu transport is mediated by specific membrane proteins acting as receptors, that these proteins contain specific binding sites for Cu as well as recognition signals that guide translocation to other areas of the cell. An immediate objective will be to identify the internal Cu-binding proteins; to learn if Cp enters or Cu passes the membrane as a Cp-free ion. Subsequent work will attempt to isolate Cu-binding ligands in the membrane. BeWo cells grown on membrane filters to obtain in situ polarized apical and basolateral surfaces will be used to assay intracellular movement. Apical and basolateral surfaces will be analyzed for Cp binding sites; a Scatchard analysis will characterize binding strength and receptor number. Subsequently, we will isolate the membrane proteins that receive Cu from Cp. The proteins will be sequenced to determine whether targeting residues that route the protein through the cell reside in their structure. Another objective is the learn whether BeWo cells release Cu as Cp, i.e., placental Cp translocates Cu between maternal and fetal circulations. Carriers for Zn will also be studied to expand the understanding to this important metal. We will examine Cu, Fe, and Zn as individual ions and as antagonists or facilitators of one another. The latter pertains to potential interactions between metals. Ultimately, we hope to learn the molecular events in the movement of metals through the placental cells by identifying the key components that take part in the transition.
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