Altered form of placental alkaline phosphatase produced by JAR choriocarcinoma cells in culture.

The alkaline phosphatase activity expressed by JAR choriocarcinoma cells was compared to the placental isoenzyme of human alkaline phosphatase by several criteria. JAR cell alkaline phosphatase was similar to the placental isoenzyme with respect to heat and urea stability and sensitivity to most inhibitors, but it differed significantly from placental alkaline phosphatase in its sensitivity to L-phenylalanylglycylglycine. In contrast to the serum form of placental alkaline phosphatase, the JAR cell enzyme was highly hydrophobic as determined by octyl agarose chromatography and appeared to be larger than the placental isoenzyme based on gel filtration and polyacrylamide gel electrophoresis. The slowly migrating hydrophobic JAR cell activity could be converted into a fast-migrating hydrophilic form similar to the serum placental isoenzyme by treatment with trypsin-like proteases. Conversion to the hydrophilic form was accompanied by a decrease in subunit molecular weight from 68,000 to 66,000, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 33P-labeled enzyme. Peptide mapping of the 33P-labeled molecules by limited proteolysis in the presence of sodium dodecyl sulfate showed JAR cell alkaline phosphatase to be closely related to the placental isoenzyme with respect to primary structure. However, placental alkaline phosphatase appeared to be richer in sialic acid residues than was the JAR cell enzyme. We conclude that JAR cells produce a form of placental alkaline phosphatase that contains a hydrophobic region not associated with the free serum enzyme and that the tumor cell enzyme is modified or processed differently than is the enzyme found in the term placenta.