Cholesteryl ester exchange protein in human plasma isolation and characterization

Area of Science:

• Biochemistry
• Lipid Metabolism
• Protein Chemistry

Background:

• Lipoprotein metabolism involves the exchange of lipids, including cholesteryl esters, among different lipoprotein particles.
• Understanding the proteins that mediate these exchanges is crucial for comprehending lipid transport and cardiovascular health.

Purpose of the Study:

• To identify and characterize a protein in human plasma responsible for catalyzing the exchange of cholesteryl esters among lipoproteins.
• To develop a reliable assay for quantifying this cholesteryl ester transfer activity.

Main Methods:

• A rapid assay was developed using radioactive cholesteryl esters transferred from low-density lipoprotein (LDL) with MnCl2 and phosphate.
• Protein purification involved ammonium sulfate precipitation, ultracentrifugation, and chromatography (Phenyl-Sepharose, CM-cellulose, Concanavalin A-Sepharose).
• Characterization included determining isoelectric point, molecular weight, and immunological properties, and separation from other plasma enzymes.

Main Results:

• A 3500-fold purified exchange protein was obtained from human plasma.
• The protein is a glycoprotein with an isoelectric point of 5 and an apparent molecular weight of 80,000.
• Immunological and biochemical properties indicated it is distinct from known apolipoproteins and phosphatidylcholine-cholesterol acyltransferase (PC-CAT).

Conclusions:

• A novel glycoprotein functioning as a cholesteryl ester transfer protein (CETP) exists in human plasma.
• This protein is biochemically and immunologically distinct from other known lipoprotein-associated proteins and enzymes involved in lipid metabolism.
• The identified exchange protein does not directly participate in cholesterol esterification or hydrolysis mediated by PC-CAT.