apolipoprotein B

ApoB-100 is synthesized in the liver and is a required structural component of VLDL, IDL, and LDL particles—each particle contains exactly one ApoB-100 molecule, making ApoB level a direct measure of the total number of atherogenic lipoprotein particles. ApoB-48 is synthesized in the intestine via mRNA editing of the same APOB gene (a premature stop codon truncates the protein to 48% of full length) and is incorporated into chylomicrons for dietary lipid transport, lacking the LDL-receptor binding domain present in ApoB-100. Clinically, ApoB-100 mediates LDL particle uptake by binding the LDL receptor on hepatocytes; mutations in the ApoB gene (familial hypobetalipoproteinemia) or LDL receptor defects (familial hypercholesterolemia) disrupt this clearance pathway, leading to abnormal lipid profiles. Because each atherogenic particle carries one ApoB, plasma ApoB concentration reflects particle number rather than cholesterol content, and is considered a superior predictor of cardiovascular risk compared to LDL-C, especially in patients with discordant LDL-C and triglyceride levels (e.g., diabetes, metabolic syndrome, small dense LDL phenotype). ApoB is also central to the pathogenesis of atherosclerosis: retention of ApoB-containing particles in the arterial subendothelial space triggers oxidation, macrophage uptake (foam cell formation), and plaque development—the "response-to-retention" hypothesis. Measurement of ApoB is increasingly used clinically alongside or in place of LDL-C for cardiovascular risk stratification, particularly per updated lipid guidelines (ACC/AHA, ESC). Abetalipoproteinemia (MTP gene mutation) results in absence of ApoB-containing lipoproteins, causing fat malabsorption, acanthocytosis, and neurologic deficits due to vitamin E deficiency.