Biosynthesis and processing of placental alkaline phosphatase.

Polypeptides of 61,500 and 64,500 apparent molecular weights were the precursor and fully processed forms of placental alkaline phosphatase monomer synthesized by choriocarcinoma cells in vivo. [3H] Mannose was incorporated into both polypeptides whereas [3H] glucosamine was incorporated mainly into the 64,500-dalton polypeptide, suggesting processing by the addition of glucosamine moieties. In the absence of microsomal membranes, choriocarcinoma mRNA directed the cell-free synthesis of the preprotein form of alkaline phosphatase monomer of apparent Mr = 60,000. The unglycosylated monomer had an apparent Mr = 58,000. In the presence of microsomal membranes, the 60,000-dalton polypeptide was processed to a polypeptide of apparent Mr = 61,500, comigrating with the precursor form of alkaline phosphatase monomer.