Serum alkaline phosphatase in pregnancy. I. A comparative study of total, L-phenylanine-sensitive and heat-stable alkaline phosphatase at 56 degrees C and 65 degrees C in normal pregnancy.

312 paired, serial estimations of total, L-phenylalanine-sensitive and heat-stable alkaline phosphatase (at two temperatures, 56 degrees C and 65 degrees C) were performed on sera from 31 pregnant healthy women from 18 weeks to term. Heat-stable alkaline phosphatase, determined at 65 degrees C, enriched the serum in a smooth exponential course throughout the second and third trimester, whereas the other enzymic activities investigated showed more irregular increments. There was a slight, but significant correlation between total alkaline phosphatase activity of the placenta and serum heat-stable alkaline phosphatase at term, whereas there was no correlation between the weight of the child at birth and heat-stable alkaline phosphatase in the serum of the mother. It is concluded that to test the feto-maternal unit, determination of serum alkaline phosphatase should be performed at 65 degrees C with due regard to appropriate sampling and experimental design.