PRODUCTION OF PLACENTAL ALKALINE-PHOSPHATASE (PLAP) AND PLAP-LIKE MATERIAL BY EPITHELIAL GERM-CELL AND NON-GERM CELL TUMORS IN-VITRO

Placental and placental-like alkaline phosphatase (FLAP) levels in the culture media of 87 cell lines of neoplastic and 'normal' origin were measured by a conventional immunosorbent enzymatic assay (IAEA) and b y a new immunoradiometric assay (IRMA). The IRMA detected immunoreacti ve FLAP in 37 of 80 (46%) human epithelial and germ cell cultures, whi le the IAEA detected FLAP in only 25 (33%). Of the 52 non-germ cell tu mour cultures, the IRMA detected expression in 24 (46%) and the IAEA i n only 16 (31%). In 17 cases (21%) the IRMA recorded levels double tha t of the IAEA, while in five cultures (6%) the reverse was true. The I RMA was much more robust than the IAEA and had considerably lower inte r- and intra-assay coefficients of variation (3.75-8.5% vs 5.2-46%). D etection of PLAP(-like) expression by IAEA is dependent on neoplastic expression of enzymically functional molecules and quantification assu mes constant enzyme kinetics. FLAP-like material has a higher catalyti c rate constant than FLAP and thus will give higher values on a stoich iometric basis in an IAEA. The higher detection rate and levels of FLA P-like material in neoplastic cultures when measured by the IRMA clear ly demonstrate ectopic expression of non-enzymatic FLAP and FLAP-like genes. The incidence of PLAP(-like) expression by non-germ cell and po ssible germ cell tumours has been underestimated and its utility as a tumour marker should be re-examined using assays which measure antigen mass rather than phosphatase activity.