ABSTRACT
Amniotic fluid acid protease and acid protease inhibitory activities were examined
in normal pregnancies as a function of gestational age. The acid proteolytic activity
of the amniotic fluid is almost constant during gestational weeks 16-29 (26 ± 13 μg
globin/ml/2 hrs, mean ± SD, n = 64). The activity sharply increases after 29 weeks
in a time-dependent fashion and reaches a value of 302 ± 89 (mean ± SD, n = 13) at
39-40 weeks gestation. Under standard conditions, the ability of amniotic fluid to
inhibit bovine pepsin declined during gestation in a Iinearfashion from 44 ± 13% (mean
± SD, n = 36) at 16-18 weeks to 9 ± 10% (mean ± SD, n = 41) at 36-40 weeks. A correlation
coefficient of r = 0.72 was found between pepsin inhibitory activity and gestational
age. No consistent change was noted in the extent of inhibition of the endogenous
acid protease throughout pregnancy. In 61 amniotic fluid samples, a correlation coefficient
of r = 0.70 was found between acid protease activity and the lecithin/sphingomyelin
(L/S) ratio.
During the course of this study, five cases of respiratory distress syndrome (RDS)
were diagnosed clinically. All five infants had a low protease activity (55 ± 22 μg
globin/ml/2 hr, mean ± SD) as well as a low L/S ratio (0.68 ± 0.20, mean ± SD). In
contrast, no case of RDS of the newborn was observed among 29 pregnancies with high
protease activity and a high L/S ratio. The present observations may suggest a predictive
value of amniotic fluid acid protease activity in assessment of fetal lung maturity.
