Biochemistry · Topic
HDL (high-density lipoprotein) !
HDL collects cholesterol from tissues and returns it to the liver (reverse transport); it also stores apolipoproteins for other particles.
Updated 2026-11-01
Key facts
- Its main protein is ApoA-I, made by the liver and gut.
- It is the smallest and densest lipoprotein, with the most protein.
- It takes up cholesterol via ABCA1 (and ABCG1); LCAT esterifies it and the particle matures from disc to sphere (HDL3 → HDL2).
- It delivers cholesterol to the liver via SR-BI or, through CETP, to ApoB lipoproteins.
- It holds ApoC-II and ApoE for chylomicrons and VLDL.
In detail
- ApoA-I leaves the liver or gut almost lipid-free.
- ABCA1 hands it cholesterol and phospholipid from cells, especially macrophages: discoidal HDL forms.
- LCAT esterifies the cholesterol, which moves to the core: HDL becomes spherical and grows.
- The liver takes up cholesteryl ester via SR-BI without degrading the particle, or CETP transfers it to VLDL and LDL.
Clinical notes
- Low HDL (below 40 mg/dL in men and 50 mg/dL in women) is a metabolic syndrome criterion.
- Raising HDL with drugs (niacin, CETP inhibitors) has not clearly reduced heart attacks.
- In Tangier disease, without ABCA1, HDL is almost zero.
Related conditions
Related
- Lipoprotein metabolism
- LCAT (lecithin–cholesterol acyltransferase)
- CETP (cholesteryl ester transfer protein)
- Apolipoprotein E (ApoE)
- Cholesterol
- Chylomicron
Test yourself
Think of the answer, then tap the question to see it.
What is HDL's main apolipoprotein?
ApoA-I.
Which transporter loads cell cholesterol onto nascent HDL?
ABCA1; its absence causes Tangier disease.
Why does raising HDL with drugs not always protect?
Because the HDL number does not reflect its function in reverse transport; that function is what protects.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: HDL: The "Good" Cholesterol: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); HDL assembly (R-HSA-8963896) (Reactome, 2026); ABCA1 gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.