Chylomicrons structure

HDL and VLDL are assembled primarily in the endoplasmic reticulum of the liver (with smaller amounts produced in the intestine), whereas chylomicrons form in the intestine. LDL is not synthesized directly, but is made from VLDL. LDL appears to be the major circulatory complex for cholesterol and cholesterol esters. The primary task of chylomicrons is to transport triacylglycerols. Despite all this, it is extremely important to note that each of these lipoprotein classes contains some of each type of lipid. The relative amounts of HDL and LDL are important in the disposition of cholesterol in the body and in the development of arterial plaques (Figure 25.36). The structures of the various... 

Figure 4.4 Diagram of the structure of a villus. Most of the absorbed materials enter the blood vessel, but chylomicrons enter the lymph in the lacteals.

Figure 4.11 Detaib of the formation of a chylomicron and its structure. Triacylglycerol is synthesised upon the smooth endoplasmic reticulum, chylomicrons are synthesised in the cytosol and then secreted into the lacteal via the Golgi.

Apo B is structural apolipoprotein for chylomicrons and for VLDL and LDL particles. It is synthesized in enteric and hepatic cells. It is important for cholesterol transport to cells via interaction with LDL receptors. 

Apolipoprotein B. Apo B is a structural apolipoprotein for chylomicrons, VLDL, and LDL particles. It is synthesized in enteric and hepatic cells. It is important for cholesterol transport to cells via interaction with LDL receptors. Nowadays, it seems its clinical relevance in CSF investigation is near Apo A-I and Apo A-II, but in current studies some varieties can be found (A23, T3). 

RGURE 17-2 Molecular structure of a chylomicron. The surface is a layer of phospholipids, with head groups facing the aqueous phase. Triacylglycerolssequestered in theinterior (yellow) make up more than 80% of the mass. Several apolipoproteins that protrude from the surface (B-48, C-lll, C-ll) act as signals in the uptake and metabolism of chylomicron contents. The diameter of chylomicrons ranges from about 100 to 500 nm. 

As mentioned in Chapter 21, there are several related receptors with similar structures. Two of them have a specificity for apolipoprotein E and can accept remnants of VLDL particles and chylomicrons.216 220 The LDL receptor-related protein is a longer-chain receptor.216 221 LDL particles, especially when present in excess or when they contain oxidized lipoproteins, may be taken up by endocytosis into macrophages with the aid of the quite different scavenger receptors.221 225 The uptake of oxidized lipoproteins by these receptors may be a major factor in promoting development of atherosclerosis (Box 22-B). On the other hand, scavenger receptor SR-B1, which is also present in liver cells, was recently identified as the receptor for HDL and essential to the "reverse cholesterol transport" that removes excess cholesterol for excretion in the bile.213/213a... 

The plasma lipoproteins contain eight major apoproteins, the structure and function of which have recently been reviewed (5). Briefly, the primary amino acid sequence is known for five of these apoproteins. ApoB, a highly hydrophobic protein, is found in chylomicrons, VLDL and LDL. It is the major polypeptide in LDL and has been shown to be responsible, in part, for the recognition of LDL by its receptor in cultured human fibroblasts (7,10). The major polypeptides of HDL are apoA-I and apoA-II apoA-l activates lecithin cholesterol acyl transferase. In addition, studies on the cellular level suggest that apoA-I may regulate the content of the lipids in the cell membrane (8). 

With one exception, all apolipoproteins appear to exchange between plasma lipoproteins. The exception is apoB, an apolipoprotein that is present in all chylomicrons and VLDL, LDL, and Lp(a) particles, and which seems to be structurally essential to the integrity of these particles. It appears likely... 

Apolipoprotein C-II can also be isolated from VLDL or HDL (H20, L5, N3). It contains 78 residues (J3) and has been shown by Chou-Fasman analysis to bind phospholipids (M26, M40), with three predicted helical sequences (M26). ApoC-II has attracted a great deal of attention because it activates one of the most important enzymes in plasma lipid metabolism, lipoprotein lipase, responsible for the hydrolysis of triglyceride in chylomicrons and VLDL. Sparrow and Gotto have summarized a number of studies on structure-function relationships (S52). These, taken together, indicate that there are separate functional domains in apoC-II, in that lipoprotein lipase activation is mediated by residues 55-78 and phospholipid binding by... 

See Fig. 6-4. The polar surface of the spherical particle renders the assembly soluble in water. This structure can be considered to be a tentative one only. The amount of polar material in chylomicrons and VLDL is astonishingly small. Moreover, when lipoproteins come into contact with the membranes of the cells of target tissue, the proteins remain soluble and do not become incorporated into the membrane. This suggests that the proteins of lipoproteins have unusual properties. It is known that several species of proteins (apoproteins AI, All. B4K, B1(K), Cl, CII, CIII, D, and E) occur. The amino acid sequences of some of them have been determined, and they possess hydrophobic regions i.e., they have properties suggesting that parts of their structure are compatible with hydrocarbons (e.g., TAGs and the tails of phospholipids). 

ACAT transfers amino-acyl groups from one molecule to another. ACAT is an important enzyme in bile acid synthesis, and catalyses the intracellular esterification of cholesterol and formation of cholesteryl esters. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane and thus promotes accumulation of cholesterol ester in the fat droplets within the cytoplasm this process is important in preventing the toxic accumulation of free cholesterol that would otherwise damage ceU-membrane structure and function. Most of the cholesterol absorbed during intestinal transport undergoes ACAT-mediated esterification before incorporation into chylomicrons. In the liver, ACAT-mediated esterification of cholesterol is involved in the production and release of apo-B-containing lipoproteins. 

The effects of Caprenin, another structured lipid, on chylomicron fatty acid composition and postprandial semm lipid concentrations have also been studied (178). It was found that there is a very low uptake of C8 0, C10 0, and C22 0 into chylomicrons. Moreover, a postprandial lipemia after caprenin is comparable with that produced by other dietary fats as opposed to a fat-free meal. There is considerable... 

EFFECTS OF STEREOSPECIFIC STRUCTURE OF DIETARY ACYLGLYCEROLS ON CHYLOMICRON CLEARING AND TISSUE TARGETING... 

Special proteins, called apoLipoproteins, are required for handling and traruv port of lipid droplets. These proteins are synthesized on the ER and enter the lumen of the ER, where they are assembled into large macromolecular structures. The relevant proteins include apolipoprotein A apo A) and apo lipoprotein B (apo B), Apo A and apo B combine with lipid droplets to form structures called chylomicrons, microscopic particles with large cores of lipid coated with a thin shell of protein. The chylomicrons are transferred to secretory vesicles, which migrate through the cytoplasm to the basal membrane of the cell. Here the vesicles fuse with the membrane, resulhng in the expulsion of chylomicrons from the cell. (If the vesicles fused with the apical membrane of the enterocyte, the effect would be a futile transfer of the dietary lipids back to the lumen of the small intestine.)... 

ApolipoprcHein C ll serves as a ccifactor for lipoprotein lipase. This situation resernbSes that of colipase, which is required for the activity of pancieatic lipase. When chylomicrons or VLDLs pass through the capillaries of an organ, they encounter lipoprotein lipase. About half the fatty acids liberated by the action of this enzyme are taken up by the tissue, whereas the rest remain in the circulation and return bound to albumin) to the liver. Apo C-II is part of the structure of chylomicrons and VLDI,. ... 

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