CE transfer protein

Enzymes and proteins that synthesize, transport, and hydrolyze CE are found both inside and outside of cells. In most cases, the intracellular and extracellular enzymes use entirely different cofactors, and have different pH optima. The enzymes found within cells typically include both a CE-synthesizing enzyme, acyl-CoA cholesterol acyltransferase (ACAT), and 2 or 3 CE-degrading enzymes acid CE hydrolase (CEH), neutral CEH, and possibly a mitochondrial CEH. Blood plasma and the extracellular fluid, on the other hand, contain only a CE-synthesizing enzyme, lecithin cholesterol acyltransferase (LCAT), and a CE transfer protein (CETP). Finally, pancreatic juice contains a CE-degrading enzyme, pancreatic CEH. Each of these very different proteins is discussed below, with the exception of pancreatic CEH, which is discussed in Chapter 5. 

CETP is a plasma protein of unknown origin that transfers CE from one lipoprotein or artificially prepared bilayer to another (Fig. 2A). In addition to CE, the protein may also transfer other lipids such as TG or PC. It has been referred to as lipid transfer complex (ETC) [68,69], esterified cholesterol transfer/exchange protein (ECTEP) [70], CE transfer protein (CETP) [71], and lipid transfer fraction (LTP-1) [72]. The original observations regarding plasma CE transfer activity were made nearly 20 years ago [73], but even today the literature is confusing and incomplete and no generally accepted recent review is available. 

The lipid content and composition of the major lipoprotein classes are listed in Table 1 [6], which shows that the total lipid content is inversely correlated with the density of the lipoproteins. Glycerolipids, mainly triacylglycerols, are the major lipid components of CM and VLDL, but constitute less than 11% of the lipids of LDL and HDL, which are enriched in CE (24-51%). Unesterified cholesterol is found in all the lipoprotein classes in relatively low proportions because it is actively esterified by LCAT on HDL and then redistributed to LDL and VLDL by CE transfer protein. The total PL content of lipoproteins increases with increasing density and is directly related to the surface area of the... 

In general, in the fasting state, the fatty acid compositions across the lipoprotein classes for specific types of lipids are fairly similar due to the transfers of lipids among all the lipoproteins by CE transfer protein and the PL transfer protein. Postprandially, the fatty acid composition of VLDL glycerolipids reflects to some extent the fatty acid composition of dietary fat, but the fatty acid compositions of LDL and HDL are hardly affected. In CM, the fatty acid compositions reflect the fatty acid composition of the meal, especially... 

HDL particle. Apo A1 is a cofactor for LCAT (A. Jonas, 2000) and the conversion of cholesterol to CE (Section 2.2) induces the formation of a neutral lipid core and the concomitant change in particle shape. Triacylglycerol molecules are introduced into the core of spherical HDL as a consequence of CE transfer protein activity. The apo A1 amphipathic a-helices are embedded among the PL molecules on the surface of spherical HDL (Fig. 7) but the detailed conformations of the apo A1 molecules are not known. Spherical HDL in plasma can be remodeled via a fusion event into large and small particles by PL transfer protein (K.A. Rye, 2001). Some HDL particles contain both apo A1 and apo A2 molecules and the interactions between these two proteins need to be understood. The presence of apo A2 inhibits HDL remodeling by CE transfer protein and the dissociation of apo A1 molecules to create pre-p-HDL. It seems that apo A2 interacts with apo A1 and reduces the ability of the latter to desorb from the HDL particle surface (K.A. Rye, 2003). 

Part of the CE formed in HDL is transferred to apo B lipoproteins, particularly VLDL and LDL, by a CE transfer protein (CETP), prior to removal of these lipoproteins by the liver. Another part of the CE in HDL is selectively internalized by the liver and by tissues synthesizing steroid hormones, by a scavenger receptor protein (SR-BI) (Chapter 20). Finally some CE is internalized as part of intact HDL by hepatic HDL receptors/binding proteins. 

Figure 26-6. Transport of cholesterol between the tissues in humans. (C, unesterified choiesterol CE, cho-iesteryi ester TG, triacyigiyceroi VLDL, very iow density iipoprotein iDL, intermediate-density iipoprotein LDL, iow-density iipoprotein HDL, high-density iipoprotein ACAT, acyi-CoA choiesteroi acyitransferase LCAT, iecithinxhoiesteroi acyitransferase A-i, apoiipoprotein A-i CETP, choiesteryi ester transfer protein LPL, lipoprotein iipase HL, hepatic iipase LRP, LDL receptor-reiated protein.)...

This enzyme [EC 2.4.1.22] is a protein complex of two proteins (designated A and B) and catalyzes the reaction of UDP-galactose with D-glucose to generate UDP and lactose. In the absence of the ce-lactalbumin (protein B), the enzyme catalyzes the transfer of galactose from UDP-galactose to A-acetylglucosamine (Le., the activity of A-acetyllactosamine synthase, EC 2.4.1.90). 

The transfer of CE from HDL to TRL and LDL and the transfer of TG back to HDL is facilitated by cholesteryl ester transfer protein (CETP) or lipid transfer protein 1 (LTP 1). CETP also catalyzes the transfer of phospholipids... 

Cholesteryl ester transfer protein (CETP) promotes exchange and transfer of neutral lipids such as cholesteryl ester (CE) and TG between plasma lipoproteins [63-65], The function of CETP is illustrated in Fig. 3. CETP is a very hydrophobic and heat-stable glycoprotein with an apparent molecular weight of 74 kDa as determined by SDS-PAGE analysis [66,67], The cDNA from human liver was cloned and sequenced [68], It encodes for a 476-amino acid protein (53 kDa), suggesting that the apparent higher molecular weight is due to the addition of carbohydrate residues by posttranslational modification. 

H Tomoda, N Tabata, M Shinose, Y Takahashi, B Woodruff, S Omura. Ferrover-dins, potent inhibitors of cholesteryl ester transfer protein, produced by Streptomy-ces sp. WK-5344. I. J Antibiot 52 1101-1107, 1999. 

The species differences observed in lipid transfer protein activity may, in part, be due to the presence of an inhibitor which markedly reduces CE and TG transfer and can be separated from LTP-I in human plasma (M42). Inhibitory activity has also been demonstrated in lipoprotein-free plasma from rat, pig, goat, chicken, and cow, but not in rabbit lipoprotein-free plasma. The levels of inhibitor in the species studied were not quantitated, but it seems possible that the level of inhibitor in the plasma of different species may be an important factor in determining LTP-I activity (M42). 

Finally, many fundamental questions related to the metabolism and fimction of CE remain to be answered. Virtually nothing is known regarding the molecular biology of any of the enzymes or transfer proteins involved in the metabolism of CE. Indeed, many of these enzymes have not even been purified to homogeneity. Furthermore, the close association between disordered CE metabolism and atherosclerosis suggests that the accumulation of CE may affect cellular function in ways that have yet to be explored. 

Though chylomicrons and VLDLs are both substrates for LPL, the processing of the end-products of their metabolism is quite different. Chylomicron remnants recirculate until about 80% of their original TG content has been removed. These remnants retain almost the whole of their content of CE and retinyl ester. Excess surface molecules (mainly apo C proteins, cholesterol, and phospholipids) are transferred from the remnants, either spontaneously or by the activity of phospholipid transfer protein, mainly to HDLs. The chylomicron remnants, with apo E as the major ligand, are cleared quantitatively by hepatic receptors of the LDL receptor family. 

There is strong evidence that the recycling of apo A1 out of large spheroidal HDLs is linked to the selective removal or hydrolysis of its lipids. A number of lipases, lipid transfer proteins, cell-surface receptors, and apolipoproteins have been indicated as key determinants of the rate of recycling based on the hypothesis that loss of core lipids (CE and TG) from large HDL would generate excess surface components (cholesterol, apo A1 and particularly, phospholipids). What is not yet clear is why apo A1 is initially released from cells as a lipid-poor pre-beta,-HDL, instead of as a surface remnant rich in polar lipids and protein reflecting the surface composition of the donor HDLs. 

Fig. 34.16. Functions and fate of FiDL. Nascent FiDL is synthesized in liver and intestinal cells. It exchanges proteins with chylomicrons and VLDL. HDL picks up cholesterol (C) from cell membranes. This cholesterol is converted to cholesterol ester (CE) by the LCAT reaction. HDL transfers CE to VLDL in exchange for triacylglycerol (TG). The cholesterol ester transfer protein (CETP) mediates this exchange. PL = phospholipids.

Fig. 34.17. Function of cholesterol ester transfer protein (CETP). CETP transfers cholesterol esters (CE) from HDL to VLDL in exchange for triacylglycerol (TG).

Electrospray (ESI) is an atmospheric pressure ionization source in which the sample is ionized at an ambient pressure and then transferred into the MS. It was first developed by John Fenn in the late 1980s [1] and rapidly became one of the most widely used ionization techniques in mass spectrometry due to its high sensitivity and versatility. It is a soft ionization technique for analytes present in solution therefore, it can easily be coupled with separation methods such as LC and capillary electrophoresis (CE). The development of ESI has a wide field of applications, from small polar molecules to high molecular weight compounds such as protein and nucleotides. In 2002, the Nobel Prize was awarded to John Fenn following his studies on electrospray, for the development of soft desorption ionization methods for mass spectrometric analyses of biological macromolecules. ... 

LTP-I was originally referred to as cholesteryl ester exchange protein, as the transfer of cholesteryl ester (CE) tracer between plasma HDL and LDL by this protein did not result in mass changes to either lipoprotein (Pll). However, it has since been shown not only to facilitate the transfer of triglyceride (TG) and phospholipid in addition to cholesteryl ester (A33), but also to facilitate net mass changes in lipoprotein fractions under appropriate condi-... 

Ortiz de Montellano PR, Catalano CE (1985) Epoxidation of styrene by haemoglobin and myoglobin. Transfer of oxygen equivalents to the protein surface. J Biol Chem 260 9265-9271... 

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