Clustered distribution of human placental alkaline phosphatase on the surface of both placental and cancer cells. Electron microscopic observations using gold-labeled antibodies.

The cell-surface distribution of human placental alkaline phosphatase (PLAP) on cultured cancer cells, A431 and HeLa TCRC-1, and on normal syncytial cells of placental tissue was examined in immunoelectron transmission microscopy using the gold-labeling technique. Chemically fixed cells were reacted with affinity-purified rabbit polyclonal antibodies to PLAP, and the antibodies were visualized using gold particles tagged with goat antirabbit IgG. On all cells PLAP was observed in clusters distributed throughout the membrane surface, including microvilli, but it was not expressed in desmosomes or along other dense regions on the membrane. Previous histochemical and immunochemical techniques failed to demonstrate clusters. The results show that (1) the gold-labeling technique allows a more precise localization of PLAP on the cell surface than previously employed methods, and (2) the distribution of the enzyme is the same on cultured cancer cells and on normal placental syncytial cells. The clustered distribution of PLAP is thus a general phenomenon and is probably influenced by the physiological function of the enzyme, which has yet to be defined.