lipoproteins

Lipoproteins are soluble macromolecular particles that transport hydrophobic lipids in the aqueous bloodstream. They consist of a hydrophobic core of triglycerides and cholesteryl esters surrounded by a surface layer of phospholipids, free cholesterol, and apolipoproteins. The apolipoproteins stabilize the particle, serve as enzyme cofactors, and act as ligands for cellular receptors. Lipoproteins are classified by density and size into chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL).

Chylomicrons form in the intestinal mucosa after fat ingestion and contain apoB-48; they deliver dietary triglycerides to peripheral tissues and leave behind chylomicron remnants taken up by the liver. Hepatic VLDL, containing apoB-100, transports endogenous triglycerides to tissues. Lipoprotein lipase hydrolyzes these triglycerides, converting VLDL to IDL and then to LDL, which carries cholesterol to cells via LDL receptors. HDL, rich in apoA-I, participates in reverse cholesterol transport, picking up cholesterol from cells and transferring it to the liver, a process supported by LCAT and CETP and involving interactions with SR-BI receptors.

Clinical relevance: Aberrations of lipoprotein metabolism contribute to cardiovascular disease. Lipid panels measure total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides. Genetic defects affecting lipoprotein assembly, remodeling, or receptor uptake lead to familial hyperlipoproteinemias, such as defects in the LDL receptor, apoB, or lipoprotein lipase. Treatments often target LDL reduction or triglyceride clearance and HDL function.