Human oxysterol-binding protein. I. Identification and characterization in liver

Area of Science:

• Biochemistry
• Molecular Biology
• Human Physiology

Background:

• The precise function of cellular oxysterol-binding proteins remains largely unknown.
• Oxysterol-binding proteins are hypothesized to have a receptor-like role in regulating cholesterol synthesis.
• The liver is crucial for systemic cholesterol metabolism, making it a key site for investigation.

Purpose of the Study:

• To identify and characterize oxysterol-binding proteins in human liver specimens.
• To investigate the potential role of these proteins in cholesterol metabolism regulation.

Main Methods:

• Utilized 25-hydroxy-[3H]cholesterol as a ligand to detect specific binding components in human liver cytosolic extracts.
• Fractionated extracts using sucrose density gradients to identify binding components at 4S and 7S.
• Characterized binding kinetics, determined the protein's isoelectric point (pI), and used antiserum for immunoprecipitation and photoaffinity labeling.
• Analyzed immunoprecipitates via electrophoresis to determine the molecular weight of the labeled protein.

Main Results:

• Identified a protein in human liver cytosol that binds specific oxygenated cholesterol derivatives (oxysterols) with high affinity.
• Binding components were found in 4S and 7S fractions, with cholesterol and steroid hormones showing no competition, while other oxysterols did.
• The binding exhibited a single kinetic class with an apparent dissociation constant (Kd) of 28 x 10(-9) mol/L.
• The purified protein has a pI of approximately 4.8, and immunoprecipitation revealed a specifically labeled protein band of about 57,000 mol wt.

Conclusions:

• A novel oxysterol-binding protein has been identified and characterized in human liver cytosol.
• This protein exhibits specific high-affinity binding to oxysterols, suggesting a role beyond general cholesterol binding.
• The findings indicate that this 57 kDa protein may function as a regulatory component in human cholesterol synthesis.