Epoxide hydrolase in human placenta at different stages of pregnancy

The epoxide hydrolase activity towards styrene-7,8-oxide was investigated in the microsomal fraction of 18 human placentas from the midtrimester and the term of pregnancy. The enzymatic activity was 0.40 +/- 0.03 nmol/min/mg microsomal protein. No relation was found between gestational age and the rate of styrene oxide hydration. The kinetics of the enzyme were studied in 3 placentas. Biphasic kinetics were observed in each of the tissue specimens. The enzyme was inhibited both by 1,1,1-trichloropropene-2,3-oxide (0.25 mM) and benzo(a)pyrene-4,5-oxide (0.2 mM) which had similar inhibiting potency.