Cholesterol 7 -hydroxylase in rat liver microsomal preparations

Area of Science:

• Biochemistry
• Cell Biology
• Metabolic Pathways

Background:

• Cholesterol metabolism is crucial for various physiological processes.
• Liver microsomes play a significant role in xenobiotic and endogenous compound metabolism.
• Understanding cholesterol oxidation pathways is key to comprehending lipid metabolism disorders.

Purpose of the Study:

• To investigate the oxidation products of cholesterol by rat liver microsomes.
• To characterize the enzymatic requirements and conditions for cholesterol oxidation.
• To develop a method for assaying cholesterol 7alpha-hydroxylase activity.

Main Methods:

• Preparation of subcellular fractions (microsomes) from rat livers.
• Incubation of cholesterol with microsomal preparations under varying conditions (with/without EDTA, presence of oxygen, NADPH, CO).
• Quantification of cholesterol oxidation products using a double-isotope-derivative dilution procedure.

Main Results:

• Microsomes catalyzed cholesterol oxidation to 7alpha-hydroxycholesterol, 7-oxocholesterol, 7beta-hydroxycholesterol, and 5alpha-cholestane-3beta,5,6beta-triol in the absence of EDTA.
• 7alpha-hydroxycholesterol was the main product when microsomes were prepared in the presence of EDTA.
• The reactions required native protein, molecular oxygen, and NADPH, and were inhibited by CO, suggesting a cytochrome P450-mediated process.

Conclusions:

• Rat liver microsomes catalyze cholesterol oxidation, producing several hydroxylated metabolites.
• The formation of these compounds may involve a peroxidation mechanism analogous to fatty acid peroxidation.
• A sensitive assay for cholesterol 7alpha-hydroxylase activity was established using a double-isotope-derivative dilution method.