Abstract

Alkaline phosphatase prepared from human placentae shows greater resistance to heat inactivation than any other known alkaline phosphatase of human origin. In the presence of magnesium this enzyme may be heated at 70°C. for 30 minutes without loss of activity whereas other human alkaline phosphatases lose most of their activity on being heated at 56°C. for this period of time. This heat stability is seen in freshly prepared enzyme, in alcohol-fractionated and freeze-dried material, and in the sera of individuals into whom placental alkaline phosphatase has been infused. The clinical implications of our observations are briefly indicated.