Serum heat-stable alkaline phosphatase in pregnancy.

It is generally recognized that a dependable test of placental function is one of the most urgent needs in obstetrics. The advantages of using various placental enz,ymes as an index of placental function were found in the fact that their measurements are simple procedures. Coryn (1934) showed for the first time that serum alkaline phosphatase is elevated in late pregnancy. Serum alkaline phosphatase is composed of different iso-enzymes. Unless the placental fraction is well differentiated from non-placental iso-enzymes, estimates of alkaline phosphatase could not be a reliable test for assessing placental function. Various methods like taurocholate inhibition, electrophoresis, histochemistry, electron microscopy, KD.T.A. inactivation and substrate specificity have been employed to study the behaviour of alkaline phosphatase in pregnancy. Neale et al (1965) first showed that the placental fraction of alkaline phosphatase is heatstable at 56°C. Later, Hunter (1969) demonstrated that the critical temperature is 65°C, not 56°C.