Which apolipoprotein is predominantly distributed in HDL, chylomicrons, and acts as a LCAT activator?

• A. ApoA1
• B. ApoA2
• C. ApoB100
• D. ApoB48

Answer: (A)

Apolipoprotein A-I is the main protein component of HDL and serves as the key activator of LCAT (lecithin-cholesterol acyltransferase). LCAT sits on the surface of HDL and catalyzes the esterification of free cholesterol to cholesteryl ester, a step essential for HDL maturation from discoidal to spherical particles and for efficient reverse cholesterol transport. ApoA-I binds and activates LCAT, enabling this esterification process that allows HDL to carry more cholesterol as esters.

While ApoA-I is highly enriched in HDL, it can also be present on chylomicrons, though these particles contain other apolipoproteins as their primary structural components. The other listed apolipoproteins have different primary roles: ApoB-100 is the main protein of VLDL and LDL, ApoB-48 is the intestinal form on chylomicrons, and ApoA-II is another HDL-associated apolipoprotein but does not stimulate LCAT activity to the same extent as ApoA-I.