Enzymes involved in prostaglandin I2 (PGI2) and thromboxane A2 (TXA2) synthesis were studied in maternal and fetal platelets and venous endothelium from normotensive pregnant controls (n = 70), women with mild preeclampsia (MP, n = 45), and severe preeclampsia (SP, n = 34). Activities of phospholipase A2 (PHA2), cyclooxygenase (PGHS), and PGI2 synthetase (PGIS) or TXA2 synthetase (TXAS) were determined in platelets and in endothelial cells. The PGHS enzyme was studied further by immunoblot methodology. In maternal platelets: Vmax (per 10(-10) mol/mg protein) and Michaelis-Menten constant (Km) (10(-7) mol, mean +/- SEM) of PHA2 were 3.0 +/- 0.8, 3.0 +/- 0.7, and 31.7 +/- 10.9* maximum velocity (Vmax) and 1.8 +/- 0.3, 2.0 +/- 0.8, and 0.8 +/- 0.2 (Km) in normal control (NC), mild preeclampsia (MP), and severe preeclampsia (SP), respectively (*P less than 0.05 against NC). The apparent overall PGHS plus TXAS activity was 10.2 +/- 1.8, 23.8 +/- 7.1, and 68.8 +/- 18.8* (Vmax) and 3.2 +/- 1.3, 5.4 +/- 1.4, and 6.9 +/- 1.2* (Km, *P less than 0.05 against NC). TXA synthesis in fetal platelets demonstrated PHA2 activity of 6.4 +/- 1.4, 12.0 +/- 1.3, and 17.2 +/- 3.2* (Vmax) and 3.5 +/- 0.9, 2.2 +/- 1.5, and 0.7 +/- 0.3* (Km, *P less than 0.05 against NC), respectively, whereas an apparent overall PGHS plus TXAS activity was 18.5 +/- 2.8, 87.5 +/- 12.5*, and 3.6 +/- 0.1* (Vmax) and 4.8 +/- 1.0, 8.8 +/- 1.2, and 0.8 +/- 0.3* (Km, *P less than 0.05 against NC).(ABSTRACT TRUNCATED AT 250 WORDS)