Antigenic determinants of human placental and testicular placental-like alkaline phosphatases as mapped by monoclonal antibodies.

Placental alkaline phosphatase (PLAP) in humans shows a high degree of genetic polymorphism as disclosed by electrophoretic analysis. Human testes contain trace amounts of a PLAP-like enzyme, that although immunologically cross-reactive with PLAP, shows unique catalytic properties. As an alternative approach to study enzyme polymorphism we have developed monoclonal antibodies to purified allelic variants of PLAP. Five different monoclonal antibodies are described in this report. The antibodies react with different epitopes on the PLAP molecule. Both conformational dependent and independent determinants are detected. Two epitopes are modified when comparing the S and F allelic variants of PLAP. One epitope is common to PLAP and the intestinal isoenzyme of alkaline phosphatase. The five epitopes appear to be mapped on two rather distant antigenic domains. Combinations of any two antibodies binding to different domains give immunoprecipitates with PLAP on Ouchterlony tests and give good response in sandwich enzyme-linked immunosorbent assays. A study of PLAP-like enzyme in 32 individual testis samples indicates differences in four of the epitopes when compared with PLAP. Four types of testicular enzymes can be distinguished based on their reactivities. These results indicate structural differences between the testicular PLAP-like enzyme and PLAP. These differences are compatible with an underlying genetic mechanism.