Foam cells absorption

Third, acyl-CoA cholesterol acyltransferase (ACAT) [EC 2.3.1.26], an enzyme that works after the formation of cholesterol, was considered a unique target of inhibition [32], ACAT catalyzes the synthesis of cholesteiyl esters from cholesterol and long-chain fatty acyl-CoA. ACAT plays important roles in the body, for example, in the absorption of dietary cholesterol from the intestines, production of lipoprotein in liver and formation of foam cells from macrophages in arterial walls. Therefore, ACAT inhibition is expected not only to lower plasma cholesterol levels but also to have a direct effect at the arterial wall. A number of synthetic ACAT inhibitors such as ureas, imidazoles, and acyl amides have been developed [33], Several groups have searched for novel ACAT inhibitors... 

The foam scrubbing technique is effective because it brings the hazardous material into close contact with the foam by getting it into the bubbles. This is different from using a foam blanket as a cover for spills (see Chapter 3). With the large internal surface area of the foam available for absorption or mass transfer, an equilibrium concentration between the contaminated air inside the bubble and the foam cell wall liquid can be developed rapidly. Unabsorbed gas that is still in the foam bubbles when they collapse is released. This results in the slower release of a smaller quantity of hazardous material, which should result in a reduced hazard zone downfield. 

Excess cholesterol can also be metabolized to CE. ACAT is the ER enzyme that catalyzes the esterification of cellular sterols with fatty acids. In vivo, ACAT plays an important physiological role in intestinal absorption of dietary cholesterol, in intestinal and hepatic lipoprotein assembly, in transformation of macrophages into CE laden foam cells, and in control of the cellular free cholesterol pool that serves as substrate for bile acid and steroid hormone formation. ACAT is an allosteric enzyme, thought to be regulated by an ER cholesterol pool that is in equilibrium with the pool that regulates cholesterol biosynthesis. ACAT is activated more effectively by oxysterols than by cholesterol itself, likely due to differences in their solubility. As the fatty acyl donor, ACAT prefers endogenously synthesized, monounsaturated fatty acyl-CoA. 

The cloning of ACAT-2 (R.A. Anderson, 1998 S. Cases, 1998 R Oelkers, 1998) revealed a protein of simitar size to ACAT-1, with a novel N-terminus but a C-terminus highly similar to ACAT-1. In adult humans, ACAT-2 is expressed in the apical region of intestinal enterocytes and in hepatocytes. Disruption of the ACAT-2 gene in mice led to dramatic reduction in cholesterol absorption and prevention of hypercholesterolemia (A.K.K. Buhman, 2000). The data suggest that, in humans, ACAT-1 plays a critical role in foam-cell formation and cholesterol homeostasis in extrahepatic tissues, whereas ACAT-2 has an important role in absorption of dietary cholesterol [29]. Interest in ACAT inhibitors as a therapeutic strategy has been revived by liver-specific knockdown of ACAT-2 in atherosclerotic-prone mice, which significantly reduced aortic atherosclerotic lesions (T.A. Bell, 2006). 

Inhibitors of acyl CoA-cholesterol acyltransferase (ACAT) are currently being Investigated as cholesterol-lowering or antiatherosclerotic agents. In addition to its role in foam cell formation, ACAT also is required for esterification of cholesterol in intestinal mucosal cells and for synthesis of cholesterol esters in hepatic VLDL formation. Thus, ACAT inhibitors have the potential of providing three beneficial effects in patients with hypercholesterolemia decreased cholesterol absorption, decreased hepatic VLDL synthesis, and decreased foam cell formation. Initial successes at inhibiting ACAT were dampened by the discovery of accompanying adrenal toxicity. Subsequent structural modifications have lead to the development of... 

Lee et al. (99) investigated the sound absorption characteristics of PE qCjqT foams, particularly, the effect of both cell size and foam dimensions on the absorption coefficients. It was observed that larger foam cell sizes exhibited a significant increase in absorption, which was dependent on the frequency of the incoming acoustic energy. 

Figure 13.1 Mid-infrared reflectivity of an aortic atheroma. Depending on the refractive index, absorption coefficients and the surface roughness of components in an atheroma, incident IR light may encounter a refractive index boundary and undergo (a) diffuse reflection, (b) quasi-specular reflection, or (c) their combination. CD, calcium deposits EC, endothelial cells ECF, extracellular collagen fiber FC, foam cells MC, macrophages LC, lipid particles and lipid core SMC, smooth muscle cells TC, T-cells Arrows and lines, IR light paths.

Acyl-CoA cholesterol acyl transferase (ACAT) catalyzes the intracellular formation of cholesteryl esters (CE) in all mammalian cells. It has been implicated as a key enzyme involved in cholesterol absorption, very low density lipoprotein secretion, and the formation of lipid-laden macrophages. The accumulation of CE in macrophage-derived foam cells is characteristic of the early step in the development of atherosclerosis. ACAT inhibitors reduced TC levels without affecting HDL-C. This can be attributed to decreased intestinal cholesterol absorption based on binding to bile acid (Turley SD. and Herndon MW. 1994)... 

Foam, Sound Absorption, Emulsion Polymerization, Open Cell... 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

The absorption and the permeability to water or moisture are low for those foams with closed cells and their hydrolysis behaviour is generally fair. 

Sekisni Kaseihin Kogy Co. has started production of closed cell type PE foam board. The PE board has a low density cnshion effect and high heat insulation. Boards will be prodnced with thicknesses of 40,50 and 60mm in 1 metre widths for packaging, shock absorption and moistnre barrier uses. The company is already producing PS and PU foamed materials. This abstract includes all the information contained in the original article. 

Procedure Assemble the aerating apparatus as shown in Fig. 16, with bottles c and d empty and stopcock b in the bypass position. Connect the apparatus to the absorption cell (f) in the instrument, and adjust the air or nitrogen flow rate so that in the following procedure, maximum absorption and reproducibility are obtained without excessive foaming in the test solution. Obtain a baseline reading at 253.6 nm, following the manufacturer s instructions for operating the instrument. 

As the cell walls are squeezed into polyhedra, a wall-thinning effect takes place, and liquid is drained from cell-wall faces into the lines of cell intersections to form ribs or struts, which are typically triangular in cross-section. This cell wall membrane thinning can continue to the point where the cell walls collapse and the cells open. This becomes a very important characteristic of most plastic foams, and affects properties such as thermal conductivity, moisture absorption, breathability, and load bearing. 

The acoustic properties of polymers are altered in a cellular structure. Sound transmission changes only slightly, because it depends predominantly upon the barrier density, in this case the polymer phase. Therefore, closed-cell cellular polymers by themselves are poor materials for reducing sound transmission. They are, however, effective in absorbing sound waves of certain frequencies [64]. Materials with open cells on the surface are particularly effective in this respect. The combination of other advantageous physical properties with fair acoustic properties has led to the use of plastic foams in soundproofing [65,66]. The sound absorption of a number of cellular polymers has been reported [7,64,65,67]. 

Rodents chew through cellular polymers but do not ingest the foam as a foodstuff. The resistance to rot, mildew, and fungi is related to moisture absorption [64]. Therefore, open-cell foams support such growth better than closed-cell foams. High humidity and temperature are necessary for the growth of microbes on any plastic foam. 

In Eq. (14) the index is in the preexponential factor and con quently affects t>w less than the volume fraction dp which is in the exponent. This leads to a very important conclusion it is not the volume fraction of open cells, as usually considered, but the volume fraction of polymer and the type of packing of cells (through the value 7) which primarUy determine the extent of equilibrium moisture absorption of plastic foams. 

This idea has been confirmed by Lowe et aL l describing the dependence of water absorption of phenolic foams (7 = 35 kg/m ) on the fraction of closed cells this dependence recalculated for the fraction of open cells is shown in Fig. 9. It is noteworthy that an increase of d< from 10 to 98% corresponds to an increase of the maximum water absorption only from 6 to 8 g/lOOmL On the basis of these data, Lowe et al. concluded that surface chemistry is more important for foamed polymers than the closed cell content They explain the obtained data by the presence of microcells and interstices between macrocells (see Chap. 5.2). 

In order to elucidate the mechanism of moisture and water absorption by phenolic foams, Lowe et al." calculated the wall thickness of cells using three simplified models of cell packing 1) spheres with point contacts 2) den packing of cubes without distortion of faces 3) fused cubes with distorted faces. 

These differences in physical structure are responsible for the different properties of foamed polymers containing varying proportions of open cells. In contrast to closed-cell foamed polymers, open-cell foamed plastics have a higher water and moisture absorptive capacity, a higher gas and vapor permeability, less pronounced electric and heat insulation characteristics but a stronger sound absorptive and damping power... 

The structure of syntactic foams is composed of closed-cell microspheres and matrix resins, and the resultant foam has the advantages of (a) isotropic properties not available in other types of foams (b) very low water absorption due to highly closed-cell structures and (c) very high compressive strengthAveight ratio. Table 40 shows properties of syntactic foams composed of epoxy resin and glass microspheres. 

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