A search for trace expression of placental-like alkaline phosphatase in non-malignant human tissues: demonstration of its occurrence in lung, cervix, testis and thymus.

A search for placental or placental-like human alkaline phosphatase (ALP) was made in human tissues. The tissue extracts were assayed for ALP before and after heating at 65 degrees C for 1 h. Trace amounts of heat-stable ALP activity (greater than 0.01 IU/g) were found in lung, testis, cervix and thymus. The heat-stable ALP in these four tissues gave in Ouchterlony double diffusion plates lines of apparent identity with placental ALP when a rabbit anti-human placental antiserum was used. Inhibition studies with L-phenyl-alanine (Phe), L-homoarginine (Har), L-phenylalanylglycylglycine (Pgg), L-leucine (Leu) and levamisole (Leva), were carried out on the heat-stable ALP and on the total ALP. The heat-stable ALPs from cervix and lung gave [I]50 values with each inhibitor comparable to those of placental ALP. The heat-stable ALPs froM testis and thymus gave [I]50 values for Leu and Pgg which were significantly different from the placental isoenzyme. Electrophoresis of heat-stable lung ALP from different individuals showed polymorphic differences similar to those seen with placental ALPs. Such differences were not seen with heat-stable testis ALP. We conclude that human non-malignant testis, cervix, lung and thymus tissues contain small amounts of placental or placental-like ALPs. The heat-stable ALPs in cervix and lung appear to have the same characteristics as placental ALP and are probably encoded by the same gene locus. The heat stable ALPs in testis and thymus, though immunologically very similar to placental ALP differ from it in inhibition profile and electrophoretically. The significance of the results in relation to the "ectopic' expression of placental and placental-like ALPs in malignancy is discussed.