Cholesterol unesterified

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

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.)...

One role of high density lipoprotein (HDL) is to collect unesterified cholesterol from cells, including endothelial cells of the artery walls, and return it to the liver where it can not only inhibit cholesterol synthesis but also provide the precursor for bile acid formation. The process is known as reverse cholesterol transfer and its overall effect is to lower the amount of cholesterol in cells and in the blood. Even an excessive intracellular level of cholesterol can be lowered by this reverse transfer process (Figure 22.10). Unfortunately, the level of HDL in the subendothelial space of the arteries is very low, so that this safety valve is not available and all the cholesterol in this space is taken up by the macrophage to form cholesteryl ester. This is then locked within the macrophage (i.e. not available to HDL) and causes damage and then death of the cells, as described above. 

The naturally occurring fatty acids are carboxylic acids with unbranched hydrocarbon chains of 4-24 carbon atoms. They are present in all organisms as components of fats and membrane lipids, in these compounds, they are esterified with alcohols (glycerol, sphingosine, or cholesterol). However, fatty acids are also found in small amounts in unesterified form. In this case, they are known as free fatty adds (FFAs). As free fatty acids have strongly amphipathic properties (see p. 28), they are usually present in protein-bound forms. 

Unesterified cholesterol Cholesterol esters Phospholipids Glycerides... 

The mixed micelles contain predominantly FFAs, 2-monoacylglycerols, and unesterified cholesterol in addition to other fat-soluble compounds, such as the fat-soluble vitamins A, D, E, and K. 

Lipoproteins have hydrophobic core regions containing cholesteryl esters and triglycerides surrounded by unesterified cholesterol, phospholipids, and apoproteins. Certain lipoproteins contain very high-molecular-weight proteins that exist in two forms B-48, formed in the intestine and found in chylomicrons and their remnants and B-lOO, synthesized in liver and found in VLDL, VLDL remnants(IDL),LDL (formed from VLDL), and Lp(a) lipoproteins. HDL consist of at least 15 discrete molecular species. All species contain apolipoprotein A-I (apoA-I). Fifty-three other proteins are known to be distributed variously among the HDL species. 

Chylomicrons are formed in the intestine and carry triglycerides of dietary origin, unesterified cholesterol, and cholesteryl esters. They transit the thoracic duct to the bloodstream. 

LCAT catalyzes the transfer of a preferentially unesterified fatty acid from the sn-2 position of phosphatidylcholine to the 3/i-hydroxy group of cholesterol, and thereby produces lysophosphatidylcholine and a cholesteryl ester [50]. Depending on the mutation in the LCAT gene, homozygous or compound heterozygous patients present with one of two clinical phenotypes, classical LCAT deficiency or fish-eye disease [58, 85]. Classical LCAT deficiency is caused by a broad spectrum of missense and non-sense mutations that interfere with the synthesis or secretion or affect the catalytic activity of LCAT [10]. Fish-eye disease is caused by a limited number of missense point mutations that alter the surface polarity, and thereby interfere with the binding of the enzyme to apoA-I containing lipoproteins [77]). 

CER The fractional esterification rate (FER) is calculated as [cpm [3H] cholesteryl esters/(cpm [3H]cholesteryl esters + cpm unesterified [3H] cholesterol)]. 

Considerable amounts of LCAT are carried by HDL therefore a-LCAT activity is also secondarily reduced in other forms of familial HDL deficiency. Notably, this partial LCAT deficiency has been repeatedly documented in forms of apoA-I deficiency due to structural defects in apoA-I. However, despite secondary LCAT deficiency these patients have a normal unesterified cholesterolitotal cholesterol ratio [35]. 

Assays for the determination of cholesterol in routine clinical laboratories include cholesterol esterase and thus quantify total cholesterol (i.e., unesterified and es-terified cholesterol). However, specific assays are also available that lack cholesterol esterase and hence allow the determination of unesterified or free cholesterol. The difference between total and unesterified cholesterol gives the concentration of cholesterol esters. 

According to the manufacturers protocols for both, the total cholesterol assay and for the unesterified cholesterol assay. 

From the measurement of the total cholesterol and the unesterified cholesterol in the sample, the ratio of the unesterified to total cholesterol is calculated. 

In normolipidemic controls the unesterified cholesterol is <30% of the total cholesterol. 

Patients with classical LCAT deficiency show an increased proportion of unesterified cholesterol in plasma (80-100%). By contrast, the plasma from patients with fish-eye disease has a slightly elevated proportion of unesterified cholesterol (up to 70%). 

An adult ingests about 60 to 150 g of lipids per day, of which more than n nety percent is normally triacylglycerol (formerly called triglyceride). Uhe remainder of the dietary lipids consists primarily of cholesterol, cholesteryl esters, phospholipids, and unesterified ("free") fatty acids. "The digestion of dietary lipids is summarized in Figure 15.2. 

The receptors can be recycled, whereas the lipoprotein remnants in the vesicle are transferred to lysosomes and degraded by lysosomal (hydrolytic) enzymes, releasing free cholesterol, amro acids, fatty acids, and phospholipids. These compounds can be reutilized by the cell. [Note Rare autosomal recessive deficiencies in the ability to hydrolyze lysosomal cholesteryl esters (Wolman disease), or to transport unesterified cholesterol out of the lyso some (Niemann-Pick disease, type C) have been identified.]... 

HDL uptake of unesterified cholesterol Nascent HDL are diskshaped particles containing primarily phospholipid (largely phosphatidylcholine) and apolipoproteins A, C, and E. They are rapidly converted to spherical particles as they accumulate cholesterol (Figure 18.23). [Note HDL particles are excellent acceptors of unesterified cholesterol (both from other lipoproteins particles and from cell membranes) as a result of their high concentration of phospholipids, which are important solubilizers of cholesterol.]... 

The body contains sulfate esters of cholesterol and other sterols,245 sometimes in quite high concentrations relative to those of unesterified steroids. These esters are presumably soluble transport forms. They... 

Fig. (3). Mechanisms implicated in the protective effect of flavonoids in LDL oxidation. OX-LDL oxidized LDL CE cholesteryl ester UC unesterified cholesterol GSH glutathione SOD superoxide dismutase ROS radical oxygen species. Dashed lines represent inhibition.

Unesterified fatty acids are carried in plasma by albumin (chapter 18). The plasma also transports more complex lipids (cholesterol, triacylglycerols) among the various tissues as components of lipoproteins (spherical particles composed of lipids and proteins). Because cholesterol and triacylglyc-erol are insoluble in an aqueous medium such as the plasma, these lipoproteins (which are soluble in plasma) have evolved for the purpose of transporting complex lipids among tissues. In this section we are concerned with the structure and metabolism of these lipoproteins. 

Some cholesterol entering from the diet may be esterified to various fatty acids, although the extent of esterification is variable. For example, egg yolk cholesterol is about 10% esterified (Bitman and Wood, 1980 Tattrie, 1972) cholesterol in meat and poultry is at least 50% esterified (Kritchevsky and Tepper, 1961). Esterified cholesterol entering the intestinal tract is mostly hydrolyzed by pancreatic enzymes, yielding free cholesterol and fatty acids (Howies et al., 1996). Only unesterified cholesterol is available for absorption. 

Biliary cholesterol is entirely unesterified and flows into the small intestine as a component of bile. The other major components of bile are phosphatidylcholine (lecithin) and bile acids. Absorption of cholesterol and other lipids depends on their ability to form micelles within the intestinal lumen. 

Another possible dietary factor concerns the essential fatty acid content of human and artificial milk. It has been postulated by Sinclair that many modern dietaries are deficient in the essential polyethenoid fatty acids (EFA) and that in consequence there is a rise in unesterified (and more active) vitamin D and in unesterified cholesterol. He has suggested that a part of the etiology of infantile idiopathic hypercalcemia may be attributed to EFA deficiency (S5). He has pointed to the lower content of certain unsaturated fatty acids in cow s milk as compared with human milk as a factor in the development of idiopathic hypercalcemia in artificially fed infants. He considers that dried milk has an even lower content of essential fatty acids than liquid cow s milk and that the longer it is stored the lower does the essential fatty acid content become. On the basis of some observations on rats, he suggests that a dietary deficiency of the essential fatty acids increases susceptibility to the possible toxic effects of vitamin D. The age of the rats, the duration of the essential fatty acid deficient diet, or the dosage of vitamin D is not mentioned, and there would appear to be no other experimental data to support these views. 

Hydrophobic lipids (triacylglycerols and cholesteryl esters) are virtually completely insoluble in water they are solubilized for transport in plasma by incorporation into lipoproteins. Lipoproteins are spherical complexes containing triacylglycerol (triglyceride) and cholesteryl ester surrounded by a layer containing phospholipids, unesterified cholesterol, and specific apolipoproteins. 

There is abundant evidence to support the concept that the outer layer of plasma lipoproteins is a monolayer of polar lipids (phospholipids, mainly phosphatidylcholine, and cholesterol) and apolipoproteins with the hydrophilic aspect of the apolipoproteins and the polar head groups of phospholipids on the surface. The evidence has been reviewed by others [e.g., (S24)] and will not further be examined here. Nuclear magnetic resonance studies on HDL have shown that about 40% of unesterified cholesterol molecules are in the lipoprotein core, and 60% are associated with phospholipid molecules in the surface. Neither surface nor core is saturated with cholesterol (L20). Presumably, unesterified cholesterol is also found in the core of other lipoproteins. 

VLDL containing apoB-100, apoE, and apoC is secreted by the liver into the space of Disse. Metabolism of this triglyceride-rich particle by lipoprotein lipase leads to shrinkage of the core triglyceride component. As Eisen-berg and others have shown experimentally (E4), other VLDL components must be removed before the mature LDL particle is formed. These changes include removal of about 75% of the phospholipid, 85% of the unesterified cholesterol, and most of the apoC and apoE from the VLDL surface (E3). The mass of apoB per particle stays constant during metabolism of VLDL to LDL, but all other surface and core materials are diminished. 

Fig. 4. Potential sources of the immediate precursor pool of unesterified cholesterol available for mitochondrial steroidogenesis. ACAT, microsomal acyl coenzyme A cholesterol acyltransferase SEH, sterol ester hydrolase. From Ref. 14.

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