[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Lipoprotein metabolism [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # Lipoprotein metabolism Lipoproteins carry fat in the blood: the exogenous pathway delivers dietary fat, the endogenous pathway liver fat, and HDL returns cholesterol to the liver. Updated 2026-11-01 ## Key facts - Exogenous: the gut builds chylomicrons (ApoB-48); LPL removes their triglyceride and the liver takes up remnants via ApoE. - Endogenous: the liver secretes VLDL (ApoB-100); LPL turns it into IDL and then LDL, which the LDL receptor clears. - Reverse transport: HDL picks up tissue cholesterol via ABCA1, LCAT esterifies it, and it reaches the liver via SR-BI or, through CETP, in ApoB particles. - Key apos: B-48 (chylomicron), B-100 (VLDL/LDL, binds the receptor), C-II (activates LPL), E (remnant uptake), A-I (HDL, activates LCAT). ## In detail - In the enterocyte, MTP loads triglyceride, cholesterol and phospholipid onto ApoB-48: the chylomicron is born and leaves through the lymph (thoracic duct), bypassing the liver. - In blood, HDL gives it ApoC-II and ApoE. In muscle and fat capillaries, LPL (activated by ApoC-II) releases the fatty acids. - The shrunken chylomicron returns ApoC-II to HDL and becomes a cholesterol-rich remnant that the liver takes up via ApoE. - The liver packs triglyceride made from surplus carbohydrate into VLDL with ApoB-100; LPL empties it, leaving IDL. - Some IDL returns to the liver via ApoE; the rest loses more triglyceride to hepatic lipase and becomes cholesteryl-ester-rich LDL. - LDL delivers cholesterol to tissues and liver via the LDL receptor; if it lingers in blood, it is oxidised in the artery wall and taken up by macrophages (foam cells). - Nascent HDL (lipid-poor ApoA-I) takes cholesterol and phospholipid from cells via ABCA1; LCAT esterifies it and the particle becomes spherical. - HDL hands cholesteryl ester to the liver via SR-BI, or CETP passes it to VLDL and LDL for triglyceride; the liver excretes it in bile. ## Clinical notes - A lipid panel measures total cholesterol, HDL and triglycerides; LDL is usually calculated (Friedewald), unreliable when triglycerides are very high. - Fredrickson classes: I (chylomicrons), IIa (LDL), IIb (LDL and VLDL), III (remnants), IV (VLDL), V (VLDL and chylomicrons). - Statins, ezetimibe and PCSK9 inhibitors lower LDL; fibrates activate PPAR-α and raise LPL activity. ## Related conditions - [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/) - [Familial hypercholesterolemia](https://durgical.com/en/conditions/familial-hypercholesterolemia/) - [Familial chylomicronemia syndrome (lipoprotein lipase deficiency)](https://durgical.com/en/conditions/familial-chylomicronemia-syndrome/) - [Abetalipoproteinemia (Bassen–Kornzweig syndrome)](https://durgical.com/en/conditions/abetalipoproteinemia/) - [Familial dysbetalipoproteinemia (type III hyperlipoproteinemia)](https://durgical.com/en/conditions/familial-dysbetalipoproteinemia/) - [Tangier disease](https://durgical.com/en/conditions/tangier-disease/) - [Acute pancreatitis](https://durgical.com/en/conditions/acute-pancreatitis/) ## Related - [Chylomicron](https://durgical.com/en/biochemistry/topics/chylomicron/) - [VLDL (very-low-density lipoprotein)](https://durgical.com/en/biochemistry/topics/vldl/) - [LDL (low-density lipoprotein)](https://durgical.com/en/biochemistry/topics/ldl/) - [HDL (high-density lipoprotein)](https://durgical.com/en/biochemistry/topics/hdl/) - [Lipoprotein lipase (LPL)](https://durgical.com/en/biochemistry/enzymes/lipoprotein-lipase/) - [LCAT (lecithin–cholesterol acyltransferase)](https://durgical.com/en/biochemistry/enzymes/lcat/) - [CETP (cholesteryl ester transfer protein)](https://durgical.com/en/biochemistry/enzymes/cetp/) - [LDL receptor](https://durgical.com/en/biochemistry/enzymes/ldl-receptor/) - [PCSK9](https://durgical.com/en/biochemistry/enzymes/pcsk9/) - [Apolipoprotein B (ApoB-100 and ApoB-48)](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-b/) - [Apolipoprotein E (ApoE)](https://durgical.com/en/biochemistry/enzymes/apolipoprotein-e/) - [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/) - [Bile acid synthesis](https://durgical.com/en/biochemistry/topics/bile-acid-synthesis/) - [Cholesterol synthesis (mevalonate pathway)](https://durgical.com/en/biochemistry/pathways/cholesterol-synthesis/) ## Test yourself Think of the answer, then tap the question to see it. ## Which apolipoprotein defines the chylomicron, and which LDL? ApoB-48 on chylomicrons; ApoB-100 on VLDL, IDL and LDL. ## Why do chylomicrons reach the blood without passing through the liver? They are too large for gut capillaries: they travel through lymphatics and the thoracic duct. ## Which steps make up reverse cholesterol transport? Efflux through ABCA1 to ApoA-I, esterification by LCAT and delivery to the liver (SR-BI or via CETP), which sends it to bile. ## Which Fredrickson type is remnant accumulation? Type III (dysbetalipoproteinaemia). [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/lipoprotein-metabolism/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Plasma lipoprotein assembly, remodeling, and clearance (R-HSA-174824)](https://reactome.org/PathwayBrowser/#/R-HSA-174824) (Reactome, 2026); [Cholesterol | High Cholesterol | MedlinePlus](https://medlineplus.gov/cholesterol.html) (MedlinePlus, US National Library of Medicine, 2026); [Triglycerides | MedlinePlus](https://medlineplus.gov/triglycerides.html) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/biochemistry/topics/lipoprotein-metabolism/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). 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