Apolipoproteins: Functions of A-I, B-48, B-100, C-II & E

Five apolipoproteins answer almost every Step 1 lipid question: apoA-I (HDL; activates LCAT), apoB-48 (chylomicrons; made in the intestine), apoB-100 (VLDL, IDL and LDL; binds the LDL receptor), apoC-II (chylomicrons and VLDL; activates lipoprotein lipase) and apoE (remnants; the ligand the liver uses to clear chylomicron remnants and IDL). Each defect has a signature: apoE2/E2 → dysbetalipoproteinemia with palmar xanthomas, apoC-II deficiency → chylomicronemia and pancreatitis, defective apoB-100 → familial hypercholesterolemia and APOA1 variants → low HDL with early coronary disease. If all apoB particles are missing, think abetalipoproteinemia, which is an MTP defect, not an apolipoprotein gene defect.

The Q-bank vignette

A 52-year-old man is referred after a myocardial infarction at age 50. He has obesity and type 2 diabetes. Examination shows yellow-orange discoloration in the creases of both palms and firm yellow nodules over both elbows. Fasting total cholesterol is 410 mg/dL, triglycerides 430 mg/dL and HDL cholesterol 36 mg/dL. Lipoprotein analysis shows accumulation of cholesterol-enriched chylomicron remnants and intermediate-density lipoprotein (IDL). A defect in which apolipoprotein best explains these findings?

• A. ApoA-I
• B. ApoB-48
• C. ApoB-100
• D. ApoC-II
• E. ApoE

Answer: E. ApoE. This is familial dysbetalipoproteinemia (Fredrickson type III), most often caused by inheriting two copies of the E2 variant.

Why it's apoE

• ApoE is the exit pass for remnants. New chylomicrons and VLDL borrow apoE and apoC-II from HDL. After lipoprotein lipase strips their triglyceride, the liver takes up the leftover remnants (chylomicron remnants and IDL) through receptors that recognize apoE.
• ApoB-48 can't stand in. Chylomicron remnants carry apoB-48, which lacks the LDL receptor-binding domain, so their hepatic uptake depends on apoE.
• A poorly functioning apoE (usually E2/E2) traps remnants, so cholesterol and triglycerides rise together; Merck gives 250–500 mg/dL for each.
• Signature findings: palmar xanthomas (yellow palmar creases), tuberous xanthomas and premature coronary disease. Merck estimates the prevalence at about 1 in 5,000; inheritance is usually recessive (E2/E2) and less often dominant.
• Treatment: low-fat diet plus lipid-lowering drugs.

For the full pathway around this step, see lipoprotein metabolism.

The five apolipoproteins at a glance

Alphabet hook: A-I Activates LCAT; B-48 Builds chylomicrons in the Bowel; B-100 Binds the LDL receptor; C-II Cuts triglyceride by switching on LPL; E is the Exit pass for remnants.

Why every other answer choice is wrong

• A. ApoA-I: the main HDL protein. It accepts cholesterol and phospholipid from cells and activates LCAT, which esterifies cholesterol so it can move into the HDL core. APOA1 variants cause familial HDL deficiency: low HDL and cardiovascular disease that often starts before age 50. That doesn't make remnants pile up or cause palmar xanthomas, and his modestly low HDL doesn't explain the combined rise in cholesterol and triglycerides.
• B. ApoB-48: the structural protein of chylomicrons, made only in the intestine. Failure to build chylomicrons causes fat malabsorption and low lipids, the opposite of this picture. ApoB-48 is actually present on the remnants accumulating here; it just can't clear them.
• C. ApoB-100: the LDL receptor ligand on LDL. Familial defective apoB-100 (about 1 in 700 per Merck) mimics familial hypercholesterolemia: high LDL with normal triglycerides, Achilles tendon xanthomas, corneal arcus and premature coronary disease. Tendon xanthomas and normal triglycerides, not palmar xanthomas and high triglycerides.
• D. ApoC-II: the LPL activator. Its deficiency, like LPL deficiency, causes familial chylomicronemia: triglycerides above 1,000 mg/dL, pancreatitis, eruptive xanthomas and lipemia retinalis (in LPL deficiency, symptoms usually start before age 10). Here triglycerides are 430 mg/dL and cholesterol is just as high.

Change one clue, change the answer

All five Fredrickson phenotypes, with their mnemonic, are decoded in the familial dyslipidemias mnemonic.

Beyond the big five

• ApoC-III inhibits LPL. Olezarsen, an antisense drug that degrades apoC-III mRNA, is FDA-approved as an adjunct to diet to lower triglycerides in adults with familial chylomicronemia syndrome; its label, revised in June 2026, adds adults with severe hypertriglyceridemia (triglycerides of 500 mg/dL or higher), to reduce triglycerides and the risk of acute pancreatitis.
• ApoA-V is another LPL cofactor. APOA5 variants join LPL, APOC2, GPIHBP1 and LMF1 as causes of familial chylomicronemia syndrome.
• Apo(a) attaches to an LDL-like, apoB-100-containing particle to form Lp(a); Merck describes a causal relationship between high Lp(a) and cardiac risk.
• ApoE alleles: E3 is the most common. One E4 copy raises the risk of late-onset Alzheimer disease and two copies raise it further; E4 carriers are also more prone to atherosclerosis.
• Familial hypobetalipoproteinemia: APOB variants that make a shortened apoB. Heterozygotes have LDL cholesterol below 80 mg/dL and may develop fatty liver.

Exam traps

• Abetalipoproteinemia is not an APOB defect. MTP can't load lipid onto apoB-48 or apoB-100, so chylomicrons, VLDL and LDL never form and apoB is absent from plasma. The APOB gene disease is familial hypobetalipoproteinemia.
• B-48 and B-100 come from one gene. In the intestine, APOBEC-1 edits a CAA (glutamine) codon in apoB mRNA to a UAA stop codon, so apoB-48 contains 48% of the apoB-100 sequence and lacks the LDL receptor-binding domain. The human liver does not express APOBEC-1, so it makes full-length apoB-100.
• Two apolipoproteins bind the LDL receptor: apoB-100 and apoE. ApoB-48 does not.
• Pancreatitis tracks triglycerides. Merck lists triglycerides above 500 mg/dL as a cause of acute pancreatitis and above 2,000 mg/dL as the level that turns retinal vessels creamy (lipemia retinalis).
• Xanthoma location sorts the diagnosis: palmar creases and tuberous (apoE, type III); Achilles tendon and xanthelasma (high LDL); eruptive papules over the trunk, buttocks, elbows and knees (very high triglycerides).
• HDL is the apolipoprotein bank. It lends apoC-II and apoE to new chylomicrons and VLDL and takes them back once the triglyceride is gone. Statins lower LDL by making hepatocytes express more of the LDL receptors that bind apoB-100 (see the cholesterol synthesis pathway).

Worked example: the pancreatitis version

Vignette. A 12-year-old girl has had three episodes of severe epigastric pain with an elevated lipase. She has crops of small yellow papules on the buttocks and knees and an enlarged liver and spleen. Her fasting plasma looks milky, triglycerides are 2,600 mg/dL, and the retinal vessels appear pale and creamy. Sequencing of the LPL gene is normal. Choosing from the same five apolipoproteins, which is most likely deficient?

Answer: ApoC-II. Chylomicrons form normally but can't be unloaded, because LPL needs apoC-II to work. ApoE is wrong (type III raises cholesterol and triglycerides together, usually to 250–500 mg/dL, with palmar and tuberous xanthomas), apoB-48 would stop chylomicrons from forming at all, apoB-100 would raise LDL, and apoA-I would lower HDL. Variants in APOA5, which encodes another LPL cofactor, can cause the same syndrome.

Key takeaways

• ApoA-I: HDL; activates LCAT.
• ApoB-48: chylomicrons; intestinal mRNA editing; no LDL receptor-binding domain.
• ApoB-100: VLDL, IDL, LDL, Lp(a); LDL receptor ligand; defects mimic familial hypercholesterolemia.
• ApoC-II: activates LPL; deficiency causes chylomicronemia, pancreatitis and eruptive xanthomas. ApoC-III inhibits LPL.
• ApoE: remnant clearance; E2/E2 gives type III with palmar xanthomas; E4 raises late-onset Alzheimer risk.
• No apoB particles at all = MTP defect (abetalipoproteinemia), not an apolipoprotein gene defect.

Sources

• Overview of Lipid Metabolism — Merck Manual Professional Edition
• Dyslipidemia — Merck Manual Professional Edition
• Hypolipidemia — Merck Manual Professional Edition
• 6.2: Lipid Transport, in Cell Biology, Genetics, and Biochemistry for Pre-Clinical Students — Medicine LibreTexts
• Apolipoprotein B48 — ScienceDirect Topics (Elsevier)
• APOB gene — MedlinePlus Genetics
• APOE gene — MedlinePlus Genetics
• APOA1 gene — MedlinePlus Genetics
• Familial HDL deficiency — MedlinePlus Genetics
• Tangier disease — MedlinePlus Genetics
• Familial lipoprotein lipase deficiency — MedlinePlus Genetics
• Tryngolza (olezarsen) prescribing information — DailyMed, NLM