Identification of trophoblast membrane-originated proteins in human placental blood

The human placenta secretes various fetoplacental proteins, including a trophoblast membrane (TM)-bound alkaline phosphatase. Five TM-originated proteins (TOPs), molecular weights 100, 39, 37, 35, and 26 kilodaltons (kDa), were isolated from placental blood by using an anti-TM Sepharose immunoaffinity column. Apparently, these five TOPs are secreted from TM into the maternal circulation during pregnancy. One of these five TOP bands (26 kDa) was identified by Western blot analysis to be human placental lactogen. The other four bands are newly identified as trophoblast-specific proteins and, therefore, could have potential diagnostic applications as serum markers for monitoring tumor development.