Lipoproteiine

Lipoproteins are complexes that transport hydrophobic lipids through the aqueous environment of the bloodstream. Each particle consists of a hydrophobic core of triglycerides and cholesteryl esters surrounded by a monolayer of phospholipids, free cholesterol, and specific apolipoproteins. The apolipoproteins provide structural stability, enzyme cofactors, and receptor-binding interactions, enabling lipid transport, metabolism, and tissue delivery. Lipoproteins are traditionally classified by density into chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).

Chylomicrons form in enterocytes after dietary fat is absorbed. They are rich in triglycerides and contain ApoB-48 along with various apolipoproteins acquired from HDL, such as ApoC and ApoE. VLDL is produced by the liver and contains ApoB-100; it delivers hepatic triglycerides to peripheral tissues. Lipoprotein lipase (LPL) in capillaries hydrolyzes triglycerides from chylomicrons and VLDL, producing remnant particles that are cleared by the liver via receptor pathways.

As triglycerides are removed, VLDL is transformed into IDL and eventually LDL, which delivers cholesterol to cells via LDL receptor–mediated endocytosis. HDL participates in reverse cholesterol transport, collecting cholesterol from peripheral cells and transferring cholesteryl esters to other lipoproteins (via CETP) or directly to the liver through receptors such as SR-B1.

Clinical significance: Abnormal lipoprotein levels contribute to atherosclerosis and cardiovascular disease. LDL cholesterol is a major risk factor; HDL cholesterol is generally protective, though the biology is complex. Genetic disorders, such as familial hypercholesterolemia, involve defective LDL receptor function. Lipid panels measuring LDL-C, HDL-C, and triglycerides are commonly used in clinical assessment.