Flavin-containing monooxygenase activity in human liver microsomes.

Area of Science:

• Biochemistry
• Pharmacology
• Drug Metabolism

Background:

• Human liver microsomes contain enzymes crucial for drug metabolism.
• Flavin-containing monooxygenases (FMOs) and cytochrome P-450s (CYPs) are key enzyme families involved in xenobiotic oxidation.
• Understanding the specific roles of these enzymes under different conditions is vital for predicting drug interactions and toxicity.

Purpose of the Study:

• To investigate the activity of human liver microsomal flavin-containing monooxygenase (FMO) using dimethylaniline N-oxidation.
• To compare the contribution of FMOs and CYPs to thiobenzamide S-oxidation at different pH levels.
• To elucidate the impact of pH on enzyme kinetics and substrate specificity in human liver microsomes.

Main Methods:

• Enzyme activity assays were performed using dimethylaniline N-oxidation and thiobenzamide S-oxidation in human liver microsomes.
• Reactions were studied at pH 7.4 and pH 8.4 to assess the effect of pH.
• Specific inhibitors and antibodies targeting NADPH-cytochrome P-450 reductase and cytochrome P-450 enzymes were used to differentiate enzyme contributions.
• Thermal inactivation studies were conducted to assess enzyme stability.

Main Results:

• Dimethylaniline N-oxidation activity was significantly higher at pH 8.4 compared to pH 7.4, suggesting FMO dominance.
• An antibody against NADPH-cytochrome P-450 reductase inhibited dimethylaniline N-oxidation at pH 7.4 but not at pH 8.4.
• Cytochrome P-450 inhibitors showed minimal effect on dimethylaniline N-oxidation.
• Thiobenzamide S-oxidation was significantly inhibited by the anti-reductase antibody at both pH values, indicating a substantial role for CYPs.

Conclusions:

• Flavin-containing monooxygenase is the primary enzyme responsible for dimethylaniline N-oxidation in human liver microsomes, particularly at alkaline pH.
• Cytochromes P-450 play a significant role in thiobenzamide S-oxidation in human liver microsomes.
• Enzyme activity and substrate preference in human liver microsomes are influenced by pH, highlighting the complexity of drug metabolism.