Origin of the alkaline phosphatases in amniotic fluid.

The nature and origins of amniotic fluid ALP activity were investigated early (14 to 22 weeks) and late (25 to 44 weeks) in pregnancy. The total enzyme activities for both stages were significantly different and considerable changes in the activities of individual enzyme components occurred. Early activity consists of intestinal (81%), bone/liver/kidney (15%), and placental (4%) ALP. In late fluids, the values are 5%, 69% and 27%, respectively. The intestinal enzyme is shown to be of fetal origin, presumably arising from the direct entry of desquamated intestinal mucosal cells into the amniotic fluid. The bone/liver/kidney enzyme is mostly of maternal serum origin early with an increased fetal contribution toward term. Early in pregnancy, the placental ALP activity is probably derived both directly from the placenta and from the placental activity in maternal serum. The latter source contributes more toward term.