Placental alkaline phosphatase as the placental IgG receptor

We review data from our studies of the physiological role of placental alkaline phosphatase (PLAP) and report that, in addition to functioning in catalysis, PLAP has the capacity to bind the Fc portion of human IgG. The dissociation constant for the interaction (3.86 mumol/L) indicates that the PLAP-IgG complex probably occurs in vivo. Furthermore, the electrophoretic and immunochemical properties of PLAP are identical to those of the purified placental Fc receptor. This receptor is believed to participate in the transfer of IgG molecules from the maternal circulation to the fetus during pregnancy. Studies with HEp2 cells show that PLAP is necessary for the internalization of IgG molecules. PLAP behaves, at least in this cell line, as an Fc receptor. The presence of large amounts of PLAP in clathrin-coated vesicles prepared from placenta strongly indicates that PLAP is involved in the endocytic machinery in this organ. We conclude that these results, taken together, suggest a novel biological role for PLAP.