The change of alkaline phosphatase binding conditions with trophoblast membrane at different stages of human placenta.

Alkaline phosphatase was brought into solution from microsomal fractions of placentas of varying gestational age by using gradually increasing concentrations of proteinase papain. When the activity of the soluble alkaline phosphatase (S) was compared with that of the non-soluble residue (R), the S/R ratio rose as pregnancy progressed. The electrophoretic pattern showed that in serum from pregnant women the papain-soluble alkaline phosphatase corresponded to the heat-stable one. These results indicate that the cytoplasmic membrane of the trophoblast changes with the growth of the placenta so that this enzyme is easily dissolved by papain. It is probable that alkaline phosphatase molecules easily enter the maternal blood stream in late pregnancy.