Apolipoprotein

Apolipoproteins are protein components of lipoproteins, the particles that transport lipids through the aqueous environment of the bloodstream. They provide structural stability, regulate the assembly and secretion of lipoproteins, act as enzyme cofactors, and serve as ligands for cellular receptors, thereby directing lipid delivery and clearance in tissues.

The apolipoprotein family includes several major classes such as ApoA, ApoB, ApoC, and ApoE, along with smaller sets like ApoD, ApoJ, and the apolipoprotein(a). ApoB exists in two forms: ApoB-100, produced by the liver and secreted in very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), and low-density lipoproteins (LDL); and ApoB-48, produced in the intestine and incorporated into chylomicrons. ApoA-I is the principal protein of high-density lipoprotein (HDL) and activates lecithin–cholesterol acyltransferase (LCAT), contributing to reverse cholesterol transport. ApoA-II is another HDL-associated protein with a less clearly defined metabolic role. The ApoC family (C-I, C-II, C-III) is transferred among lipoproteins; ApoC-II activates lipoprotein lipase to hydrolyze triglycerides, while ApoC-III can slow clearance of triglyceride-rich particles.

ApoE mediates receptor-dependent clearance of remnant lipoproteins via LDL receptors and related pathways; human isoforms E2, E3, and E4 have distinct effects on lipid metabolism and disease risk, with ApoE4 associated with increased cardiovascular risk and certain neurodegenerative conditions. Apo(a) forms lipoprotein(a) when covalently attached to ApoB-100, and elevated Lp(a) levels are an established risk factor for atherosclerotic disease.

Apolipoproteins are routinely measured in research and clinical contexts to assess lipid transport and cardiovascular risk. They can be affected by genetic variation and post-translational modifications, and serve as biomarkers and functional tools in lipoprotein biology.