## Apolipoprotein B-100: The LDL Receptor Ligand **Key Point:** Apolipoprotein B-100 (ApoB-100) is the structural protein of VLDL and LDL particles and serves as the primary ligand for the LDL receptor. It remains on the particle throughout its lifecycle from VLDL → IDL → LDL. ### Mechanism of LDL Uptake ApoB-100 binds to the LDL receptor via a specific domain, allowing receptor-mediated endocytosis of LDL particles into hepatocytes. This is the major pathway for LDL clearance from circulation. ### Comparison of Key Apolipoproteins | Apolipoprotein | Primary Lipoprotein | Function | Receptor | | --- | --- | --- | --- | | ApoB-100 | VLDL, IDL, LDL | Structural; LDL receptor ligand | LDL receptor | | ApoB-48 | Chylomicrons | Structural; intestinal origin | Not recognized by LDL receptor | | ApoA-I | HDL | Structural; LCAT activator | SR-B1 | | ApoC-II | VLDL, chylomicrons | Lipoprotein lipase cofactor | — | | ApoE | VLDL remnants, HDL | Ligand for LDL receptor & LRP | LDL receptor, LRP | **High-Yield:** ApoB-100 is present in ONE copy per VLDL/LDL particle and is NOT exchangeable between particles. This is why it is the defining structural protein. **Clinical Pearl:** Familial hypercholesterolemia (FH) results from mutations in the LDL receptor gene or less commonly in the ApoB-100 gene itself, preventing normal LDL clearance and causing severe elevation of LDL-C. **Mnemonic:** **BALE** — ApoB-100 for LDL, ApoA-I for HDL, ApoE for remnants, ApoC-II for Lipoprotein lipase. 
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