Human placental alkaline phosphatase (AP) exhibits a genetic polymorphism that is determined by the genotype of the fetus. A similar enzyme is produced by certain neoplasms and by some cells in culture. The relations between these enzymes were investigated by immunologic methods comparing human placental AP to the AP of HeLa71 cells. Antiserum prepared against placental AP precipitated 90% of the enzyme and the catalytic activity was quantitatively recovered in the antigen antibody precipitate. Enzyme antibody complexes failed to migrate on starch gel electrophoresis. Antiserum against placental AP cross reacted with HeLa71 AP and HeLa71 AP antiserum reacted with the placental enzyme. Immunologic analysis by double diffusion in agar showed that the 3 common genetic variants of placental AP - F, FS and S - and HeLa71 AP were closely related when studied by antisera against both placental or HeLa71 AP. When studied by immunodiffusion the genetic variants of human placental AP and the HeLa enzyme reacted with identity at the points of contact of the precipitation lines when precipitated by antisera against either enzyme. These findings support the view that the human placental and HeLa71 APs are products of the same genetic locus. Derepression of a portion of the genome in association with malignant transformation might be responsible for ectopic production of this enzyme in HeLa71 cells.
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