Topological studies on rat liver microsomal cholesterol ester hydrolase

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Biology

Background:

• Cholesterol ester hydrolase (CEH) plays a role in cellular lipid metabolism.
• Understanding the localization of CEH within hepatic microsomal membranes is crucial for elucidating its function.
• Microsomal membranes are key sites for various metabolic processes, including lipid modification.

Purpose of the Study:

• To investigate the lateral and transversal distribution of cholesterol ester hydrolase activity in rat liver microsomal membranes.
• To determine the specific location of CEH within the rough and smooth microsomal fractions.
• To examine the enzyme's orientation within the microsomal vesicle.

Main Methods:

• Differential centrifugation to isolate rough and smooth microsomes.
• Assay of cholesterol ester hydrolase activity in different microsomal fractions.
• Protease sensitivity assays (trypsin) to assess enzyme accessibility.
• Latency measurements of mannose-6-phosphate phosphatase as a marker for microsomal integrity and lumenal orientation.

Main Results:

• Total CEH activity was primarily found in rough microsomes (75%).
• Specific CEH activities were comparable between rough and smooth microsomal fractions.
• CEH was significantly inhibited by trypsin only after detergent-induced disruption of microsomal integrity, indicating lumenal exposure.
• Mannose-6-phosphate phosphatase, a lumenal marker, showed differential sensitivity to proteolysis, confirming the assay's validity.

Conclusions:

• Cholesterol ester hydrolase activity is predominantly associated with rough hepatic microsomes.
• The enzyme's resistance to proteolysis under intact membrane conditions and sensitivity upon disruption strongly suggest its localization on the lumenal surface of hepatic microsomal vesicles.
• This lumenal localization implies specific roles for CEH in processes occurring within the endoplasmic reticulum lumen.