Complement fixation for study of placental-type alkaline phosphatase

Preparations of human placental alkaline phosphatase differing in specific enzyme activities were compared by microcomplement fixation assays using monospecific antisera. While both specific enzyme activity and complement fixation units increased 15,000-fold upon purification, the ratio between these units remained constant. Separation of an alkaline phosphatase preparation into 'A' and 'B' forms by ampholine isoelectric focusing indicated that these forms also possessed the same ratio of immunoreactive enzyme protein to enzyme activity. The correspondence of complement fixation units with specific enzyme activity indicates that complement fixation with monospecific antisera can be used to analyze structural differences among alkaline phosphatase isoenzymes.