Prostaglandin endoperoxide synthase kinetics in human amnion before and after labor at term and following preterm labor.

To determine whether the kinetics of prostaglandin endoperoxide synthase (PGHS, commonly known as cyclooxygenase) in human amnion change with labor onset or between preterm and term labor, a specific enzyme assay was developed and characterized. The assay was linear for time (0-8 min) and protein concentration (5-30 micrograms/250 microliters incubation volume). The optimum pH was 8.0-8.5, and the enzyme reaction reached saturation at 10-20 microM arachidonic acid. Flufenamic acid was more efficacious than ibuprofen in the presence of 1 mM tryptophan in inhibiting enzyme activity. The Km and Vmax of PGHS were determined in 10 amnions obtained at elective caesarean section before labor onset (CS) at 39.3 +/- 0.8 wk gestational age (mean +/- SD, range = 38.5-41 wk) and 9 amnions obtained following spontaneous labor and vaginal delivery (SL) at 39.6 +/- 0.8 wk (range = 38.5-41 wk). The Km values were 1.4 +/- 1.2 mumol/l (CS) and 2.2 +/- 1.5 mumol/l (SL) (not different). However, the Vmax increased significantly (p < 0.05) from 11 +/- 8 (CS) to 19 +/- 4 (SL) pg PGE2/micrograms protein/min. In eight preterm amnions obtained following spontaneous labor and delivery at 32.9 +/- 2.1 wk (range = 29-36 wk), the Km and Vmax were 2.0 +/- 1.2 mumol/l and 17 +/- 9 pg PGE2/micrograms protein/min, respectively. Neither of these values was different from those of CS or SL amnions. None of the preterm pregnancies displayed histological evidence of infection. These results suggest that an increase in the mean amnion PGHS maximum velocity occurs in association with the onset of labor at term. The mean Vmax of PGHS in amnions obtained from idiopathic preterm spontaneous deliveries is between the CS and SL term values, reflecting, perhaps, multiple etiologies for preterm delivery.