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.

The aryl hydrocarbon hydroxylase (AHH) and epoxide hydrolase (EH) activities with styrene oxide and benzo[a]pyrene-4,5-oxide as substrates were investigated and compared in the nuclear and microsomal fractions isolated from the human fetal liver, adrenals, kidneys and lungs. The purity of the fractions was estimated by electron microscopy and found to be around 85% for the nuclear and 90% for the microsomal fractions. All tissues catalyzed the hydration of the two epoxides at significant rates. The EH followed Michaelis-Menten kinetics in all fractions. The highest activities were seen in the liver and the adrenals. The nuclear/microsomal ratios of the EH activity was tissue dependent, being highest in the kidneys and lungs.

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