High-density lipoproteins discoidal

Narayanaswami, V., Maiorano, J. N., Dhanasekaran, P. et al. Helix orientation of the functional domains in apolipoprotein e in discoidal high density lipoprotein particles. /. Biol. Chem. 279 14273-14279, 2004. 

Fig. 5.2.1 The major metabolic pathways of the lipoprotein metabolism are shown. Chylomicrons (Chylo) are secreted from the intestine and are metabolized by lipoprotein lipase (LPL) before the remnants are taken up by the liver. The liver secretes very-low-density lipoproteins (VLDL) to distribute lipids to the periphery. These VLDL are hydrolyzed by LPL and hepatic lipase (HL) to result in intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), respectively, which then is cleared from the blood by the LDL receptor (LDLR). The liver and the intestine secrete apolipoprotein AI, which forms pre-jS-high-density lipoproteins (pre-jl-HDL) in blood. These pre-/ -HDL accept phospholipids and cholesterol from hepatic and peripheral cells through the activity of the ATP binding cassette transporter Al. Subsequent cholesterol esterification by lecithinxholesterol acyltransferase (LCAT) and transfer of phospholipids by phospholipid transfer protein (PLTP) transform the nascent discoidal high-density lipoproteins (HDL disc) into a spherical particle and increase the size to HDL2. For the elimination of cholesterol from HDL, two possible pathways exist (1) direct hepatic uptake of lipids through scavenger receptor B1 (SR-BI) and HL, and (2) cholesteryl ester transfer protein (CfiTP)-mediated transfer of cholesterol-esters from HDL2 to chylomicrons, and VLDL and hepatic uptake of the lipids via the LDLR pathway...

H5. Hamilton, R. L., Williams, M. C., Fielding, C. J., and Havel, R. J., Discoidal bilayer structure of nascent high density lipoproteins from perfused rat liver J Clin. Invest. 58, 667-680 (1976). 

Lipoproteins are often called pseudomicellar because their outer shell is in part composed of amphipathic phospholipid molecules. Unlike simple micelles, lipoproteins contain apolipoproteins, or apoproteins, in their outer shell and a hydrophobic core of triacylglycerol and cholesteryl esters. Unesterified, or free, cholesterol, which contains a polar group, can be found as a surface component and in the region between the core and surface (Figure 20-1). Most lipoproteins are spherical. However, newly secreted high-density lipoproteins (HDLs) from the liver or intestine are discoidal and require the action of lecithin-cholesterol acyltransferase (LCAT) in plasma to expand their core of neutral lipid and become spherical. The hydrophobic core of the low-density lipoprotein (LDL) molecule may contain two concentric layers one of triacylglycerol and another of cholesteryl ester. 

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