ApoB

Apolipoprotein B (ApoB) is a large, amphipathic protein that serves as the main structural component of several plasma lipoproteins and acts as a ligand for lipoprotein receptors. There are two major isoforms in humans: ApoB-100 and ApoB-48. ApoB-100 is synthesized in the liver and incorporated into very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL). ApoB-48 is produced in enterocytes of the small intestine via RNA editing and is essential for the assembly and secretion of chylomicrons that transport dietary lipids.

The APOB gene on chromosome 2 encodes ApoB-100, whereas ApoB-48 arises from post-transcriptional RNA editing that creates a truncated protein corresponding to roughly the first 48% of ApoB-100. This distinction underlies the different roles of ApoB isoforms in hepatic versus intestinal lipoprotein production. Functionally, ApoB provides the scaffold for lipoprotein particles and serves as a ligand for receptors such as the LDL receptor. ApoB-48-containing chylomicrons carry dietary lipids from the intestine, while ApoB-100–containing VLDL/LDL transport hepatic lipids.

Clinically, ApoB concentration reflects the number of atherogenic lipoprotein particles and is used with, or as an alternative to, LDL-C in cardiovascular risk assessment and in guiding lipid-lowering therapy. Genetic disorders include familial hypobetalipoproteinemia (FHBL), caused by APOB mutations that reduce ApoB production or yield truncated ApoB, often with low LDL-C and fat-soluble vitamin deficiencies. A related disorder, abetalipoproteinemia, results from mutations in MTP and leads to absence of ApoB-containing lipoproteins.