Interactions between solubilized cytochrome P-450 and hepatic microsomes

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• Cytochromes P-450 (CYP) and P-448 are hemoproteins found in liver microsomes.
• These enzymes play crucial roles in xenobiotic metabolism and detoxification.
• Microsomal enzyme activities can be modulated by the presence and organization of these CYPs.

Purpose of the Study:

• To investigate the binding and catalytic activity of solubilized cytochromes P-450 and P-448 when added to rat liver microsomes.
• To determine the conditions and mechanisms by which these exogenous hemoproteins become active.
• To explore the relationship between CYP incorporation into the microsomal membrane and enhanced enzyme function.

Main Methods:

• Preparation of solubilized cytochromes P-450 and P-448 from rat liver microsomes.
• Incubation of solubilized CYPs with native or hydroperoxide-treated microsomes.
• Assay of microsomal enzyme activities such as benzphetamine demethylase, ethoxycoumarin dealkylase, and benzo[a]pyrene hydroxylase.
• Separation of incorporated CYPs from free hemoproteins using gel filtration.
• Measurement of enzyme kinetics and temperature dependence.

Main Results:

• Solubilized cytochromes P-450 and P-448 bind to microsomes, increasing specific monoxygenase activities.
• Cytochrome P-450 enhances benzphetamine demethylase activity; cytochrome P-448 boosts ethoxycoumarin dealkylase and benzo[a]pyrene hydroxylase activities.
• Exogenous CYPs are incorporated into the microsomal membrane to become catalytically active.
• Hydroperoxide-treated microsomes, depleted of native CYPs but retaining reductase activity, show restored enzyme function upon addition of exogenous CYPs.
• Incorporated CYPs directly interact with NADPH-cytochrome P-450 reductase, receiving electrons for catalytic activity.

Conclusions:

• Exogenous cytochromes P-450 and P-448 can be functionally integrated into the microsomal membrane.
• Membrane incorporation is essential for the catalytic activity of added hemoproteins.
• The results support a dynamic, non-rigid model for the organization of CYPs and NADPH-cytochrome P-450 reductase in the endoplasmic reticulum membrane.