Xem mã nguồn của Thành viên:Naazulene/Chylomicron

{{Short description|One of the five major groups of lipoprotein}}
{{More citations needed|date=December 2009}}
[[File:2512 Chylomicrons Contain Triglycerides Cholesterol Molecules and Other Lipids.jpg|Sơ đồ cấu trúc chylomicron|right|thumb|300px]]

'''Chylomicron''' (từ tiếng Hy Lạp, '''chylos''' là nước dịch, '''micron''' là hạt nhỏ), còn được gọi là lipoprotein mật độ siêu thấp), là những hạt lipoprotein chứa [[triglyceride]] (85–92%), [[phospholipid]] (6–12%), [[cholesterol]] (1–3%), và [[protein]] (1–2%). Nó có chức năng vận chuyển lipid đã được tiêu hóa từ ruột đến những cơ quan khác. ULDL là một trong năm nhóm lipoprotein chính cho phép cholesterol và chất béo di chuyển trong dung môi nước, cụ thể là máu.<ref>{{cite journal|last1=Gofman|first1=John W.|last2=Jones|first2=Hardin B.|last3=Lindgren|first3=Frank T.|last4=Lyon|first4=Thomas P.|last5=Elliott|first5=Harold A.|last6=Strisower|first6=Beverly|year=1950|title=Blood Lipids and Human Atherosclerosis|url=http://circ.ahajournals.org/content/2/2/161.full.pdf|journal=Circulation|volume=2|issue=2|pages=161–178|doi=10.1161/01.cir.2.2.161|pmid=15427204|doi-access=free|s2cid=402420}}</ref> Protein đặc hiệu của chylomicron là ApoB48. 

Với lipoprotein, mật độ tỉ lệ nghịch với thể tích hạt. Đó là vì chất béo có khối lượng riêng rất thấp, thấp hơn nước và protein (nên những hạt chứa nhiều chất béo thì sẽ có thể thích lớn nhưng khối lượng thấp, tức mật độ thấp). Chylomicron có mật độ thấp nhất, nên kích thước lớn nhất, khoảng 1,000 nm trở lên, nên có thể được quan sát bằng kính hiển vi quang học. 

==Chức năng==
[[Image:Chylomicron.svg|thumb|250px|'''Chylomicron structure'''<br/>'''ApoA, ApoB, ApoC, ApoE''' ([[apolipoprotein]]s); '''T''' ([[triacylglycerol]]); '''C''' ([[cholesterol]]); green ([[phospholipid]]s)]]

Chylomicron vận chuyển lipid từ ruột đến mô mỡ, mô tim và mô xương

Chylomicrons transport lipids absorbed from the intestine to [[adipose]], cardiac, and skeletal muscle tissue, where their triglyceride components are hydrolyzed by the activity of the [[lipoprotein lipase]], allowing the released [[free fatty acids]] to be absorbed by the tissues. When a large portion of the triglyceride core has been hydrolyzed, chylomicron remnants are formed and are taken up by the liver, thereby also transferring dietary fat to the liver.

==Origin==
[[Pancreatic lipase family|Pancreatic lipases]] digest dietary [[Triglyceride|triglycerides]] in the lumen of the [[small intestine]], forming [[Monoglyceride|monoglycerides]] and [[Fatty acid|fatty acids]]. These lipids are absorbed into [[Enterocyte|enterocytes]] via [[Passive transport|passive diffusion]]. Inside the cell, these lipids are transported to the [[smooth endoplasmic reticulum]], where they are re-esterified to form triglycerides.<ref>{{Cite journal |last=Schwenk |first=Robert W. |last2=Holloway |first2=Graham P. |last3=Luiken |first3=Joost J. F. P. |last4=Bonen |first4=Arend |last5=Glatz |first5=Jan F. C. |date=2010-04-01 |title=Fatty acid transport across the cell membrane: Regulation by fatty acid transporters |url=https://www.plefa.com/article/S0952-3278(10)00073-6/abstract |journal=Prostaglandins, Leukotrienes and Essential Fatty Acids |language=English |volume=82 |issue=4 |pages=149–154 |doi=10.1016/j.plefa.2010.02.029 |issn=0952-3278 |pmid=20206486}}</ref> These triglycerides, along with phospholipids and cholesterol, are added to [[Apolipoprotein B|apolipoprotein B48]] to form immature chylomicrons.<ref>{{Citation |last=Kiani |first=Reza |title=Chapter 18 - Dyslipidemia |date=2018-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780323511490000183 |work=Practical Cardiology |pages=303–309 |editor-last=Maleki |editor-first=Majid |publisher=Elsevier |language=en |doi=10.1016/b978-0-323-51149-0.00018-3 |isbn=978-0-323-51149-0 |access-date=2022-12-06 |editor2-last=Alizadehasl |editor2-first=Azin |editor3-last=Haghjoo |editor3-first=Majid}}</ref> Immature chylomicrons are transported from the [[smooth ER]] to the [[Golgi apparatus]] via [[SAR1B]] proteins, where they are processed, resulting in mature chylomicrons.<ref>{{Cite web |title=SAR1B gene: MedlinePlus Genetics |url=https://medlineplus.gov/genetics/gene/sar1b/ |access-date=2022-12-06 |website=medlineplus.gov |language=en}}</ref>

Mature chylomicrons are secreted through the basolateral membrane into the [[lacteal]]s, where they join [[lymph]] to become [[chyle]]. The [[lymphatic vessel]]s carry the chyle to the [[lymphatic ducts]] before they join the [[venous return]] of the systemic [[circulatory system|circulation]] via [[Subclavian vein|subclavian veins]]. From there, the chylomicrons supply the tissue with fat absorbed from the diet.<ref>{{cite book|last1=Smith|first1=Sareen S. Gropper, Jack L.|last2=Smith|first2=Jack S.|title=Advanced nutrition and human metabolism|date=2013|publisher=Wadsworth/Cengage Learning|location=Belmont, CA|isbn=978-1133104056|edition=6th}}</ref> It is important to note that, unlike digested [[carbohydrate]]s (in the form of [[Monosaccharide|monosaccharides]]) and [[protein]]s (in the form of [[Amino acid|amino acids]]), digested lipids (in the form of chylomicrons) bypass the [[hepatic portal system]], avoiding [[first pass metabolism]].

==Stages==
The three stages of the chylomicron are nascent, mature, and remnant.

===Nascent chylomicrons===
Triglycerides are emulsified by bile and hydrolyzed by the enzyme [[lipase]], resulting in a mixture of fatty acids and monoglycerides. These then pass from the intestinal lumen into the enterocyte, where they are re-esterified to form triglycerides. The triglycerides are then combined with phospholipids, cholesteryl esters, and [[Apolipoprotein B#Molecular biology|apolipoprotein B48]] (ApoB48) to form a nascent chylomicron. These are then released by [[exocytosis]] from the [[enterocyte]]s into the [[lacteal]]s, [[lymph vessels|lymphatic vessels]] originating in the villi of the small intestine, and are then secreted into the [[bloodstream]] at the [[thoracic duct|thoracic duct's]] connection with the left [[subclavian vein]].

Nascent chylomicrons are composed primarily of triglycerides (85%) and contain some cholesterol and [[cholesteryl ester]]s. The main [[apolipoprotein]] component is ApoB48.

===Mature chylomicron===
While circulating in blood, chylomicrons exchange components with [[high-density lipoprotein]]s (HDL). The HDL donates [[apolipoprotein C-II]] (APOC2) and [[apolipoprotein E]] (APOE) to the nascent chylomicron and, thus, converts it to a mature chylomicron (often referred to simply as "chylomicron"). APOC2 is the coenzyme for [[lipoprotein lipase]] (LPL) activity.

===Chylomicron remnant===
Once triglyceride stores are distributed, the chylomicron returns APOC2 to the [[High-density lipoprotein|HDL]] (but keeps APOE), and, thus, becomes a chylomicron remnant, now only 30–50&nbsp;nm. ApoB48 and APOE are important to identify the chylomicron remnant in the liver for endocytosis and breakdown.

== Pathology ==
There are some disorders in which chylomicrons are involved.<ref name=":0">{{Cite journal|title=Chylomicrons: Advances in biology, pathology, laboratory testing, and therapeutics|url=https://www.researchgate.net/publication/293638020|journal= Clinica Chimica Acta|year=2016|doi=10.1016/j.cca.2016.02.004|last1=Julve|first1=Josep|last2=Martín-Campos|first2=Jesús M.|last3=Escolà-Gil|first3=Joan Carles|last4=Blanco-Vaca|first4=Francisco|volume=455|pages=134–148|pmid=26868089}}</ref>

=== Hyperchylomicronemia ===
The hyperchylomicronemia syndrome is a disorder characterized by extreme hypertriglyceridemia, the presence of chylomicrons, and one or more of the following clinical manifestations: eruptive xanthomas, [[lipaemia retinalis]], hepatosplenomegaly, recurrent abdominal pain, and/ or acute pancreatitis.<ref name=":0" />

=== Hypochylomicronemia ===
Hypochylomicronemia is defined as the low level or absence of postprandial CMs, and it can result from genetic or acquired causes.<ref name=":0" />

=== Chylomicron remnants and cardiovascular disease ===
Postprandial hyperlipidemia is considered an important risk factor for cardiovascular disease.<ref name=":0" />

==References==
{{Reflist}}

{{Lipoproteins}}
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[[Category:Lipoproteins]]