Which apolipoprotein is the major component of HDL?

• A. Apo A-I
• B. Apo B-100
• C. Apo C-II
• D. Apo E

Answer: (A)

HDL particles are defined by their apolipoprotein content, and the principal protein in HDL is Apo A-I. This apolipoprotein forms the main scaffold of HDL and acts as a cofactor for lecithin–cholesterol acyltransferase, promoting cholesterol esterification and the maturation of HDL from discoidal to spherical forms. Apo A-I also drives cholesterol efflux from peripheral cells via the ABCA1 transporter, which is a key step in reverse cholesterol transport. Other apolipoproteins have different primary associations: Apo B-100 is the major protein of VLDL/LDL, Apo C-II activates lipoprotein lipase on triglyceride-rich particles, and Apo E serves as a receptor ligand for remnant clearance. Therefore, Apo A-I is the major apolipoprotein of HDL.