A continuous fluorometric assay for cytochrome P-450-dependent mixed function oxidases using 3-cyano-7-ethoxycoumarin

Area of Science:

• Biochemistry
• Pharmacology
• Enzymology

Background:

• Cytochrome P-450 enzymes are crucial for drug metabolism and detoxification.
• Accurate measurement of mixed function oxidase activity is vital for pharmacological and toxicological studies.
• Existing assays may lack sensitivity or continuous monitoring capabilities.

Purpose of the Study:

• To develop a direct, continuous fluorometric assay for cytochrome P-450-dependent mixed function oxidases.
• To utilize 3-cyano-7-ethoxycoumarin as a sensitive substrate for this assay.
• To evaluate the assay's performance in hepatic microsomal preparations and isolated hepatocytes.

Main Methods:

• A direct fluorometric procedure using 3-cyano-7-ethoxycoumarin as a substrate.
• Measurement of fluorescence of the reaction product, 3-cyano-7-hydroxycoumarin, at specific excitation/emission wavelengths (408/450 nm).
• Enzyme kinetic analysis (Km, Vmax) using rat hepatic microsomes and isolated hepatocytes.

Main Results:

• The assay demonstrated high sensitivity, with Vmax values significantly induced by phenobarbitone pretreatment.
• The method was approximately 50- to 100-fold more sensitive than the ethoxyresorufin deethylase assay.
• While effective in hepatocytes, a minor fluorescence quenching of the product was observed, leading to a ~5% underestimate of reaction rates.

Conclusions:

• 3-Cyano-7-ethoxycoumarin provides a sensitive and direct substrate for fluorometric determination of mixed function oxidase activity.
• This assay offers a valuable tool for studying cytochrome P-450 enzyme induction and activity in various biological matrices.
• Potential limitations due to product fluorescence quenching in isolated cells should be considered for precise quantification.