Epoxide hydrolase in human fetal liver

Area of Science:

• Biochemistry
• Toxicology
• Developmental Biology

Background:

• Epoxide hydrolase (EH) enzymes are crucial for metabolizing epoxides, including environmental toxins like styrene oxide.
• Understanding fetal liver EH activity is vital for assessing developmental toxicity and xenobiotic metabolism.
• Previous studies have characterized adult liver EH, but fetal data is limited.

Purpose of the Study:

• To quantify epoxide hydrolase activity in human fetal liver microsomes.
• To investigate the kinetic properties and inhibition patterns of fetal liver EH towards styrene oxide.
• To compare fetal and adult liver EH activity and identify potential developmental differences.

Main Methods:

• Microsomal fractions were isolated from 20 human fetal livers.
• Epoxide hydrolase activity was measured using styrene oxide as a substrate.
• Enzyme kinetics were determined using Lineweaver-Burk plots, and inhibition studies were conducted with TCPO and BPO.

Main Results:

• Fetal liver epoxide hydrolase activity was 5.60 ± 0.52 nmol/min/mg, approximately 40% of adult levels.
• No significant correlation was found between enzymatic activity and fetal age.
• Michaelis-Menten kinetics were observed, with Km values ranging from 0.25–0.54 mmol/l and Vmax from 7.2–16.7 nmol/min/mg.
• 1,1,1-trichloropropene-2,3-oxide (TCPO) and benzo(a)pyrene-4,5-oxide (BPO) inhibited the enzyme; TCPO was a more potent inhibitor (61% inhibition vs. 14% for BPO at 1 mmol/l styrene oxide).
• TCPO demonstrated uncompetitive mixed-type inhibition.

Conclusions:

• Human fetal liver exhibits significant epoxide hydrolase activity towards styrene oxide, albeit lower than adult levels.
• Fetal liver EH activity is independent of gestational age within the studied range.
• The enzyme's kinetic properties and susceptibility to inhibition by TCPO suggest specific regulatory mechanisms and potential vulnerabilities during development.