The major component of high density lipoproteins (HDL) is:

Question

BlackTom AI · Accepted Answer

HDL particles have the highest protein-to-lipid ratio among plasma lipoproteins, so their major component is protein.

1) Relevant knowledge points and concepts
- Lipoproteins are complexes of lipids (triacylglycerols, cholesterol, cholesterol esters, phospholipids) and proteins (apolipoproteins). 
- Density of a lipoprotein class is inversely related to its lipid content: chylomicrons and VLDL are lipid-rich and low density; HDL is the most dense because it contains relatively more protein. 
- HDL particles are enriched in apolipoproteins such as ApoA-I and ApoA-II, which constitute a large fraction of HDL mass and determine its structure and function.

2) Step-by-step reasoning
- “High density” means more mass contributed by protein per unit volume than by lipids.
- Compare typical compositions: HDL proteins often make up roughly half of HDL mass, whereas in lower-density classes (VLDL, chylomicrons) lipids dominate and protein is a small fraction.
- Therefore, the predominant component by mass in HDL is protein (apolipoproteins), not lipids.

3) Calculations / intermediate steps
- No numeric calculation needed; reasoning is qualitative based on composition/density: higher density ⇔ higher protein fraction.

4) Why the correct answer is correct
- Proteins (apolipoproteins) are the principal structural and functional constituents of HDL and account for the largest single component by mass. ApoA-I in particular is abundant and essential for HDL formation and reverse cholesterol transport.

5) Why other options are incorrect
- Phospholipids: present on the HDL surface but are not the largest component by mass. 
- Triacylglycerol: HDL contains only small amounts of TG compared with VLDL/chylomicrons. 
- Cholesterol and cholesterol esters: important components but collectively are less than the protein fraction in HDL.

6) Conclusion
- The major component of HDL is Protein. This matches the provided correct answer: Protein.

The major component of high density lipoproteins (HDL) is:单项选择题

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