Cholesteryl esterase

E. Neither hormone sensitive lipase nor lipoprotein lipase is a digestive enzyme. The patient s symptoms are consistent with an inability to absorb triglycerides, which wonld eliminate cholesteryl esterase from consideration. Since the patient did not have any problems while being breast-fed, then the most likely enzyme to be deficient is pancreatic lipase, since gastric lipase is most active on short chain triglycerides, such as those that are found in breast milk. 

Figure 18-16 depicts a model for the selective uptake of cholesteryl esters by a cell-surface receptor called SR-BI (scavenger receptor, class B, type I). SR-BI binds HDL, LDL, and VLDL and can mediate selective uptake from all of these lipoproteins. The detailed mechanism of selective llpid uptake has not yet been elucidated, but It may entail hemifuslon of the outer phospholipid monolayer of the lipoprotein and the exoplasmic leaflet of the plasma membrane. The cholesteryl esters Initially enter the hydrophobic center of the plasma membrane, are subsequently transferred across the Inner leaflet, and are eventually hydrolyzed by cytosolic, not lysosomal, cholesteryl esterases. The llpid-depleted particles remaining after llpid transfer dissociate from SR-BI and return to the circulation they can then extract more phospholipid and cholesterol from other cells by means of the ABCAl protein or other cell-surface transport proteins (see Figure 18-13c). Eventually, small llpid-depleted HDL particles circulating In the bloodstream are filtered out by the kidney and bind to a different receptor on renal epithelial cells. After these particles have been Internalized by receptor-mediated endocytosis, they are degraded by lysosomes.

Biological/Medical Applications Analyzing lipid aggregates diagnosis of Niemaim-Pick disease in cultured cells,Wolman disease as substrates for ceramidase activity,cholesteryl esterases activity, " lipase activity,phospholipases A2 activity, sphingomyelinase activity, sphingosine kinases 1 and 2 activity, synthetase activity ... 

There are two forms of the hydrolytic enzyme, one of which is specific for short chain esters like retinyl acetate, even though this ester does not occur naturally The other has maximum activity with retinyl palmitate as substrate but also hydrolyses other long chain esters. As in the hydrolysis of cholesteryl esters, the enzyme is not just a non-specific esterase, but has quite definite specificity for retinyl esters. In vitamin A deficiency, the activity of the enzyme increases one hundred fold. The esterification enzyme resembles the low energy cholesteryl esterase in that neither ATP nor coenzyme A appear to take part in the reaction nor are free fatty acids or acyl-CoA thiolesters incorporated into retinyl esters. One of the major problems in this area of research is to identify the acyl donor, which may be, as in plasma cholesteryl ester biosynthesis, a phospholipid. 

This is a broad class of enzymes that catalyze the hydrolysis of esters some of these enzymes are quite specific. See also Cholesteryl Ester Hydrolase specific esterase... 

Cholesteryl ester degradation Most dietary cholesterol is present in the free (nonesterified) form, with ten to fifteen percent present in the esterified form. Cholesteryl esters are hydrolyzed by pancreatic cholesterol ester hydrolase (cholesterol esterase), which produces cholesterol plus free fatty acids (see Figure 15.2). Cholesteryl esteh hydrolase activity is greatly increased in the presence of bile salts. ... 

In the duodenum, dietary lipids are degraded by pancreatic enzymes triacylglycerol by pancreatic lipase, phospholipids by phospholipase A2 and lysophospholipase, and cholesteryl esters by cholesterol esterase. Enzyme release from the pancreas is controlled by cholecystokinin, produced by cells in the intestinal mucosa. 

Serum cholesterol. Most cholesterol is carried in the blood by low density lipoprotein (LDL, Tables 21-1,21-2), which delivers the cholesteryl esters directly to cells that need cholesterol. Both a 74-kDa cholesteryl ester transfer protein193-1953 and a phospholipid transfer protein196 1963 are also involved in this process. Cholesterol esterases, which release free cholesterol, may act both on lipoproteins and on pancreatic secretions.197-199... 

The search for intestinal cholesterol transporters extended for many years, beginning with a debate about whether or not it was even a protein-facilitated process (4, 5). The pancreatic enzyme carboxyl ester lipase (CEL, also called cholesterol esterase) was believed to be important to this process (6,7) and several companies devoted considerable resources to the development and testing of compounds to inhibit CEL, with mixed results (8-10). These efforts were abandoned in the mid-1990s, however, after studies with gene-knockout mice demonstrated that the enzyme was important only for absorption of cholesteryl ester (11, 12), which is a minor component of dietary cholesterol and is present at very low levels in bile. Interestingly, CEL is also found in liver where it has been shown to affect HDL metabolism (13). Thus, it may ultimately play an important role in cholesterol metabolism and may yet prove to be a useful drug target for CVD treatment (Camarota and Howies, unpublished). 

I he average daily intake of total dietary cholesterol is 400-500 mg. Cholesterol also enters the gastrointestinal tract via the bile. Between fiOO and 1200 mg of free cholesterol is secreted in the bile per day. By weight, bile consists of 92% water, 6% bile salts, 0,3% cholesterol, and small amounts of bilirubin, fatty acids, phosphatidylcholine, and sails. The cholesteryl esters of the diet are hydmlyzed to free cholesterol and a fatty add by pancreatic cholesterol esterase. After entry into the enterocyte, the free cholesterol is nmverted back to cholesteryl esters by acyl CoA cholesterol acyl transferase. Some evidence suggests that the absorption of dietary cholesterol (from the bile salt micelles) is mediated by a membrane-bound transport protein of the brush border (1 humhofer and Hauser, 1990),... 

Enzymatic methods are commonly used for this measurement, and many of the current methods use a cholesterol esterase to hydrolyze lipoprotein-cholesteryl esters, followed by further reactions where cholesterol oxidase is linked to a peroxidase-chromogen system (Richmond 1992). Eree cholesterol can be measured by omitting cholesterol esterase of the first reaction step, although many reagent formulations prevent this elimination of the esterase. 

Some of the cholesterol esterases used in these analytical systems show varying degrees of specificity toward the different cholesterol esters, and problems have occurred when these reagents are used for several species. For example, in rats, where there is a fivefold-higher concentration of cholesteryl arachidonate ester compared to human plasma, plasma cholesterol may be underestimated with some reagents (Demacker et al. 1983 Noel, Dupras, and Pillion 1983 Wiebe and Bernert 1984 Evans 1986). 

Another pancreatic enzyme, cholesterol esterase, is responsible for intraluminal cholesteryl ester hydrolysis. Complete hydrolysis of dietary cholesteryl esters occurs in the lumen before absorption of cholesterol can occur (Shiratori and Goodman, 1965). This enzyme is discussed further below. 

Human serum paraoxonase (PON 1) is an esterase that is physically associated with high-density lipoprotein (HDL) and is also distributed in tissues such as liver, kidney, and intestine [38,39]. Activities of PON 1, which are routinely measured, include hydrolysis of organophosphates, such as paraoxon (the active metabolite of the insecticide parathion) hydrolysis of arylesters, such as phenyl acetate and lactonase activities. Human serum paraoxonase activity has been shown to be inversely related to the risk of cardiovascular disease [40,41], as shown in atherosclerotic, hypercholester-olemic, and diabetic patients [42-44]. In 1998 HDL-associated PON 1 was shown to protect LDL, as well as the HDL particle itself, against oxidation induced by either copper ions or free radical generators [45,46], and this effect could be related to the hydrolysis of the specific lipoproteins oxidized lipids such as cholesteryl linoleate hydroperoxides and oxidized phospholipids. Protection of HDL from oxidation by PON 1 was shown to preserve... 

VLDL are assembled in the liver, and contain newly synthesized triacylglycerol, cholesterol and cholesteryl esters and phospholipids as well as lipids from chylomicron remnants. These lipids are taken up by peripheral tissues which have cell-surface lipoprotein lipase, phospholipase and cholesterol esterase. 

Several systems for formation and hydrolysis of cholesteryl esters in rat liver are known. Microsomes contain an acyl-CoA cholesterol acyltransferase, which requires coenzyme A and ATP for fatty add activation, and operates at neutral pH. Enzymic transfer of fatty adds from lecithin to cholesterol occurs in the soluble fraction of rat liver. A third enzyme, cholesterol esterase, occurs in rat liver and its main function is probably hydrolytic. Although human liver apparently does not have acyl-CoA cholesterol acyltransferase activity, it does have a reversible cholesterol esterase (E.C.3.1.1.13) with optimal... 

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