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| __NOTOC__
| | #Redirect [[High density lipoprotein physiology]] |
| {{High density lipoprotein}}
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| {{CMG}}; {{AE}} {{AN}}; {{RT}}
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| ==Overview==
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| ==Biochemistry==
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| ===Structure===
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| {| class="wikitable" border="1" style="background:PaleGoldenrod"
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| |-
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| | '''Lipoprotein'''|| '''Density''' || '''Diameter'''|| '''% Protein'''|| '''% Cholesterol'''|| '''% Triglyceride'''|| '''Major Lipid'''|| '''Apolipoprotein'''
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| |- style="background:LemonChiffon"
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| | '''HDL''' ||1.063-1.210 ||5-12mm || || || || ||
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| |-
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| |}
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| ''For more information about the biochemistry of all lipoproteins, click '''[[lipoprotein|here]]'''.''
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| Shown below is a a schematic image depicting the structure of the HDL. Note that the inner core is made of triglyceride and cholesterol esters whereas the surface is made of amphiphilic phospholipids along with apolipoproteins.
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| [[Image:HDL-structure.gif|400px|The structure of the HDL: the inner core is made of triglyceride and cholesterol esters whereas the surface is made of amphiphilic phospholipids along with apolipoproteins.]]
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| * HDL are the smallest of the lipoproteins. They are the densest because they contain the highest proportion of [[protein]]. They contain the A class of [[apolipoprotein]]s.
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| * The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipids, which resemble cholesterol-free flattened spherical lipoprotein particles. They are capable of picking up cholesterol from cells they interact with.
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| * A [[Blood plasma|plasma]] enzyme called [[lecithin-cholesterol acyltransferase]] (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol) which is then sequestered into the core of the lipoprotein particle eventually making the newly synthesized HDL spherical. They increase in size as they circulate through the bloodstream and incorporate more cholesterol molecules into their structure.
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| * Thus it is the concentration of large HDL particles which more accurately reflects protective action, as opposed to the concentration of total HDL particles.<ref>Kwiterovich PO. The Metabolic Pathways of High-Density Lipoprotein, Low-Density Lipoprotein, and Triglycerides: A Current Review. Am J Cardiol 2000;86(suppl):5L.</ref> This ratio of large HDL to total HDL particles varies widely and is only measured by more sophisticated lipoprotein assays using either [[electrophoresis]], originally developed in the 1970s, or newer [[Nuclear magnetic resonance|Nuclear magnetic resonance]] (NMR) spectroscopy which was developed in the 1990s.
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| * HDL particles are not inherently protective. It is only the HDL particles which become the largest (actually picking up and carrying cholesterol) which are protective. There is no reliable relationship between total HDL and large HDL, and more sophisticated analyses which actually measure large HDL, not just total, correlate much better with clinical outcomes.<ref name="pmid23488589">{{cite journal |author=Tran-Dinh A, Diallo D, Delbosc S, ''et al.'' |title=HDL and endothelial protection |journal=[[British Journal of Pharmacology]] |volume= |issue= |pages= |year=2013 |month=March |pmid=23488589 |doi=10.1111/bph.12174 |url=}}</ref>
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| * Many studies have postulated an association between cholesterol efflux from peripheral tissue, and Apo A-I HDL particles, whereas the HDL3 containing both Apo AI and A-II are less effective. <ref name="pmid23564081">{{cite journal |author=Yin K, Tang SL, Yu XH, ''et al.'' |title=Apolipoprotein A-I inhibits LPS-induced atherosclerosis in ApoE-/- mice possibly via activated STAT3-mediated upregulation of tristetraprolin |journal=[[Acta Pharmacologica Sinica]] |volume= |issue= |pages= |year=2013 |month=April |pmid=23564081 |doi=10.1038/aps.2013.10 |url=}}</ref> <ref name="pmid23426429">{{cite journal |author=Mazer NA, Giulianini F, Paynter NP, Jordan P, Mora S |title=A Comparison of the Theoretical Relationship between HDL Size and the Ratio of HDL Cholesterol to Apolipoprotein A-I with Experimental Results from the Women's Health Study |journal=[[Clinical Chemistry]] |volume= |issue= |pages= |year=2013 |month=March |pmid=23426429 |doi=10.1373/clinchem.2012.196949 |url=}}</ref> <ref name="pmid23351584">{{cite journal |author=Kappelle PJ, Gansevoort RT, Hillege HJ, Wolffenbuttel BH, Dullaart RP |title=Common variation in cholesteryl ester transfer protein: relationship of first major adverse cardiovascular events with the apolipoprotein B/apolipoprotein A-I ratio and the total cholesterol/high-density lipoprotein cholesterol ratio |journal=[[Journal of Clinical Lipidology]] |volume=7 |issue=1 |pages=56–64 |year=2013 |month=January |pmid=23351584 |doi=10.1016/j.jacl.2012.05.003 |url=}}</ref>
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| ===HDL Receptors===
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| ===Enzymes Associated with HDL===
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| ====Cholesterol Ester Transfer Protein (CETP)====
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| * This protein mediates exchange of cholesterol between HDL particles, and [[triglyceride]] rich [[LDL]] and [[VLDL]] in both directions.
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| * It is normally present in both periphery and liver and functions to channel cholesterol to the liver for uptake and metabolism.
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| ==References==
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| {{Reflist|2}}
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| [[Category:Lipid disorders]]
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| [[Category:Cardiology]]
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| [[Category:Lipoproteins]]
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