Clinical Pathology: Clinical Chemistry

• Chylomicrons (density, <0.95 g/mL; size, 80 to 1200 nm) are formed in the intestine during absorption of exogenous fat. Chylomicrons are the largest and lightest lipoproteins. Chylomicrons contain mainly triglycerides (TGs) (90% to 96%), phospholipids (2% to 8%), free cholesterol (1%), cholesteryl esters (1% to 3%), and proteins (1% to 2%). After a 12-hour fast, chylomicrons are absent from the plasma. During fasting, the intestine releases small amounts of small chylomicrons similar in size, density, and lipid composition to very-low-density lipoprotein (VLDL) particles. Chylomicrons also carry fat-soluble vitamins absorbed from the diet.

• VLDL is synthesized in the liver (density, 0.95 to 1.006 g/mL;, size, 30 to 80 nm). Nascent VLDL contains cholesterol, cholesteryl esters, and TGs. Mature VLDL contains predominantly TGs (50% to 65%), phospholipids (12% to 18%), cholesteryl esters (8% to 14%), free cholesterol (5% to 8%), and proteins (6% to 102%).

• LDL (density, 1.019 to 1.063 g/mL; size, 19 to 25 nm) originates from VLDL lipolysis. LDL particles carry 60% to 70% of total plasma cholesterol in healthy human subjects. The main components of LDL are cholesteryl esters (35% to 45%), phospholipids (20% to 25%), proteins (20% to 25%), TGs (6% to 12%), and free cholesterol (6% to 10%). Apoprotein B (ApoB) is the only apoprotein found in LDL particles.

• High-density lipoprotein (HDL) (density, 1.063 to 1.210 g/mL; size, 6 to 11 nm) is the smallest lipoprotein. The liver and the intestine produce nascent lipid-poor HDL particles. Free cholesterol is acquired from macrophages and other cells and esterified by lecithin-cholesterol acyltransferase (LCAT) to generate mature HDL.

• HDL particles can be separated further into HDL2 and HDL3, which differ by size. HDL3 is the smallest fraction (density, >1.125 g/mL, size, 6 to 9 nm). The components are proteins (35% to 55%), phospholipids (25% to 40%), cholesteryl esters (8% to 20%), free cholesterol (1% to 6%), and TGs (3% 6%). Apoprotein A-1 is the main apoprotein in HDL.