lipoprotein

Lipoprotein refers to a biochemically organized particle that transports lipids through the watery environment of the bloodstream. These complexes consist of a hydrophobic core containing triglycerides and cholesteryl esters surrounded by a hydrophilic surface made of phospholipids, free cholesterol, and apolipoproteins. The apolipoproteins help maintain particle structure, serve as cofactors for enzymes, and act as ligands for hepatic receptors, enabling the distribution and clearance of lipids.

The major classes of lipoproteins are chylomicrons, very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Chylomicrons form in the intestinal mucosa and transport dietary triglycerides. VLDL is produced by the liver and carries endogenous triglycerides; as triglycerides are removed, VLDL becomes IDL and then LDL. HDL is involved in reverse cholesterol transport, gathering cholesterol from tissues for delivery to the liver or to steroidogenic tissues.

Density and size vary among classes: chylomicrons are the largest and least dense, followed by VLDL, IDL, LDL, and HDL, which is the most dense and smallest. Metabolic processing involves lipoprotein lipase, which hydrolyzes triglycerides in circulating lipoproteins, and hepatic receptors that clear apoB-100–containing particles via LDL receptors or related pathways. HDL interacts with scavenger receptor class B type I to mediate cholesterol uptake.

Clinical relevance includes dyslipidemias, where abnormal lipoprotein levels or function contribute to cardiovascular risk. LDL cholesterol often receives primary focus due to its atherogenic potential, while HDL is generally considered protective. Genetic conditions such as familial hypercholesterolemia affect LDL receptor function, altering clearance of LDL particles. Routine lipid panels measure total cholesterol, LDL-C, HDL-C, and triglycerides to assess risk and guide management.