Apolipoproteins – What is the principal protein component of high-density lipoprotein (HDL) particles?

Introduction / Context:
HDL mediates reverse cholesterol transport and exerts anti-atherogenic effects. Its functional properties are closely tied to its apolipoprotein composition, particularly ApoA-I, which activates lecithin:cholesterol acyltransferase (LCAT) and scaffolds HDL structure.

Given Data / Assumptions:

• HDL is protein-rich relative to other lipoproteins.
• ApoA-I is the dominant structural apolipoprotein in HDL.
• ApoC and ApoE are exchangeable apolipoproteins but not the principal scaffold of HDL.

Concept / Approach:
ApoA-I binds phospholipids to form nascent HDL, activates LCAT to esterify cholesterol, and promotes cholesterol efflux from cells via ABCA1/ABCG1 transporters. Therefore, ApoA-I is the defining major protein of HDL.

Step-by-Step Solution:

Verification / Alternative check:
Proteomic analyses show ApoA-I as the predominant HDL apolipoprotein; genetic deficiency of ApoA-I leads to markedly reduced HDL levels.

Why Other Options Are Wrong:

• ApoC-I and ApoE: present but not the principal HDL scaffold.
• ApoB-100: signature of VLDL/IDL/LDL, not HDL.
• “None of these” is incorrect because ApoA-I is correct.

Common Pitfalls:
Assuming ApoE is the main HDL apolipoprotein because of its role in receptor binding; it is important but not the major constituent.

Final Answer:
Apolipoprotein A-I