Triacylglycerols high-density lipoproteins

High density lipoprotein (HDL) (a-lipoprotein) Triacylglycerols, phospholipids, cholesterol 75... 

Figure 25-4. Metabolic fate of very low density lipoproteins (VLDL) and production of low-density lipoproteins (LDL). (A, apolipoprotein A B-100, apolipoprotein B-100 , apolipoprotein C E, apolipoprotein E HDL, high-density lipoprotein TG, triacylglycerol IDL, intermediate-density lipoprotein C, cholesterol and cholesteryl ester P, phospholipid.) Only the predominant lipids are shown. It is possible that some IDL is also metabolized via the LRP.

Four major groups of lipoproteins are recognized Chylomicrons transport lipids resulting from digestion and absorption. Very low density lipoproteins (VLDL) transport triacylglycerol from the liver. Low-density lipoproteins (LDL) deliver cholesterol to the tissues, and high-density lipoproteins (HDL) remove cholesterol from the tissues in the process known as reverse cholesterol transport. 

The plasma lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They function to keep lipids (primarily triacylglyc-erol and cholesteryl esters) soluble as they transport them between tissues. Lipoproteins are composed of a neutral lipid core (containing triacylglycerol, cholesteryl esters, or both) surrounded by a shell of amphipathic apolipoproteins, phospholipid, and nonesterified cholesterol. Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily, triacylglycerol) plus additional lipids synthesized in these cells. Each nascent chylomicron particle has one molecule of apolipoprotein B-48 (apo B-48). They are released from the cells into the lymphatic system and travel to the blood, where they receive apo C-ll and apo E from HDLs, thus making the chylomicrons functional. Apo C-ll activates lipoprotein lipase, which degrades the... 

The plasma lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They keep lipids (primarily, triacylglycerol and cholesteryl esters) soluble as they transport them in the plasma, and provide an efficient mechanism for transporting their lipid contents between tissues. 

The small particles of plasma lipoprotein, which carry triacylglycerols, can be separated according to their buoyant densities by centrifugation. They have been classified into five groups of increasing density but smaller size as chylomicrons, very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low density (LDL), and high density lipoproteins (HDL) (Table 21-1 and Fig. 21-2). Each lipoprotein particle contains one or more apolipoproteins (Table 21-2), whose sizes vary from the enormous 4536-residue apoB-100 to apoC-II and apoC-III, each of which contains just 79 residues73 and the 57-residue apoC-I.7b... 

Lipoproteins are globular, micelle-like particles consisting of a hydrophobic core of triacylglycerols and cholesterol esters surrounded by an amphipathic coat of protein, phospholipid and cholesterol. The apolipoproteins (apoproteins) on the surface of the lipoproteins help to solubilize the lipids and target the lipoproteins to the correct tissues. There are five different types of lipoprotein, classified according to their functional and physical properties chylomicrons, very low density lipoproteins (VLDLs), intermediate density lipoproteins (IDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). The major function of lipoproteins is to transport triacylglycerols, cholesterol and phospholipids around the body. 

R2. Rajaram, O. V., and Barter, P. J., Influence of lipoprotein concentration on the exchanges of triacylglycerol between rabbit plasma low density and high density lipoproteins. Biochim. Biophys. Acta 620, 438-448 (1980). 

Alterations in serum lipid patterns The p-blockers may disturb lipid metabolism, decreasing high-density lipoproteins (HDL) and increasing plasma triacylglycerol. 

Cholesterol and triacylglycerols are transported in body fluids in the form of lipoprotein particles. Each particle consists of a core of hydrophobic lipids surrounded by a shell of more polar lipids and apoproteins. The protein components of these macromolecular aggregates have two roles they solubilize hydrophobic lipids and contain cell-targeting signals. Lipoprotein particles are classified according to increasing density (Table 26.1) chylomicrons, chylomicron remnants, very low density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Ten principal apoproteins have been isolated and characterized. They are synthesized and secreted by the liver and the intestine. 

Protein molecules associated with triacylglycerol, cholesterol or phospholipids are called lipoproteins. Triacyl glycerols derived from intestinal absorption or from the liver are not transported in the free form in circulating blood plasma, but move as chylomicrons, as very low density lipoproteins (VLDL) or as free fatty acids (FFA) - albumin complexes. Besides, two more physiologically important groups of lipoproteins are low density lipoprotein (LDL) and high density lipoprotein (FIDL). 

Figure 6-1. Overview of lipid metabolism in the fed state. TG = triacylglycerol 2-MG = 2-monoacylglyceroi FA = fatty acid LPL = lipoprotein lipase VLDL = very-low-density lipoprotein HDL = high-density lipoprotein circled TG = triacylglycerols of VLDL and chylomicrons.

N. Hilaire, A. Negre-Salvayre, and R. Salvayre, Cellular uptake and catabolism of high-density-lipoprotein triacylglycerols in human cultured fibroblasts degradation block in neutral lipid storage disease, Biochem. J., 1994, 297, 467-473. 

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. 

More recently the term hyperlipoproteinemia has become useful. Saturated fatty acids, usually esterified to glycerol as triacylglycerides (i.e., fats), can be related to cardiovascular disease since they increase plasma cholesterol levels. This cholesterol is now known to be primarily associated with a low-density lipoprotein (LDL) fraction. Lipoproteins are complex particles consisting of proteins, triacylglycerol (fat), phospholipids, cholesterol, and cholesterol esters. LDL has density of 1.00-1.06 and contains at least 45% cholesterol. It is the cholesterol found in the LDL fraction that is the harbinger of human arterial plaque manifested as atheroma. An inverse relationship has been established between cholesterol in high-density lipoprotein (HDL) (d = 1.20, 18% cholesterol) and CHD. HDL transports cholesterol from accumulation in arterial walls to the liver for biodegradation. 

Fig. 1. Simplified schematic summary of the essential pathways for receptor-mediated human lipoprotein metabolism. The liver is the crossing point between the exogenous pathway (left-hand side), which deals with dietary lipids, and the endogenous pathway (right-hand side) that starts with the hepatic synthesis of VLDL. The endogenous metabolic branch starts with the production of chylomicrons (CM) in the intestine, which are converted to chylomicron remnants (CMR). Very-low-density lipoprotein particles (VLDL) are lipolyzed to LDL particles, which bind to the LDL receptor. IDL, intermediate-density lipoproteins LDL, low-density lipoproteins HDL, high-density lipoproteins LCAT, lecithinxholesterol acyltransferase CETP, cholesteryl ester transfer protein A, LDL receptor-related protein (LRPl) and W, LDL receptor. Lipolysis denotes lipoprotein lipase-catalyzed triacylglycerol lipolysis in the capillary bed.

Abbreviations LPL, lipoprotein lipase LDL, low-density lipoprotein HDL, high-density lipoprotein triacylglycerols, triglyoerides PPAR, peroxisome proliferators-aoti-vated reoeptor (the Table is adapted from Ciroulation 2002 106 3145-3457). 

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