LDL is not cholesterol itself but a tiny shuttle that carries fats through blood
Fat does not dissolve in water, so the body ships it in particles. A low-density lipoprotein is a speck about 22 to 27.5 nanometres across, wrapped around thousands of fat molecules and held together by one giant protein. The familiar LDL number on a blood test is only an estimate of what these particles carry.
LDL is one of five lipoprotein families, ranked by density from chylomicrons through VLDL and IDL to HDL, and its job is to deliver fats to cells. Each particle carries 3,000 to 6,000 fat molecules around a water-repelling core, organised by a single apolipoprotein B-100, a protein of 4,536 amino acids, plus 80 to 100 helper proteins. LDL forms when an enzyme strips triglycerides from VLDL, leaving a smaller, denser particle richer in cholesterol esters.
Cells collect it on demand. A cell that needs more cholesterol than it can make builds LDL receptors, which drift in the membrane until they gather in coated pits that swallow the bound particle. Inside, acidity makes the receptor let go, the LDL is broken down in lysosomes, and the receptor usually returns to the surface to go again, unless a protein called PCSK9 has tagged it for destruction. LDL even has a defensive side: its apolipoprotein B can jam the chemical signalling Staphylococcus aureus uses to turn invasive.
Elevated LDL is an established cause of atherosclerotic heart disease, and the details matter. Small, dense particles, called pattern B, may slip more easily through gaps in artery linings. When LDL is oxidised, scavenger receptors on immune cells gulp it far faster than normal receptors would, stuffing macrophages until they become the foam cells found in artery plaques.
Measurement is its own puzzle. Standard lipid tests do not count particles; they calculate LDL cholesterol with the Friedewald equation, which assumes a 12 to 14 hour fast and fails when triglycerides exceed 400 mg/dL. Since cholesterol's share inside particles can vary eightfold, direct particle counts by NMR, recognised as superior in 2008, can tell a different story.
Source: Low-density lipoprotein
