Cholesterol esterase, pancreatic

The overall metabolism of vitamin A in the body is regulated by esterases. Dietary retinyl esters are hydrolyzed enzymatically in the intestinal lumen, and free retinol enters the enterocyte, where it is re-esterified. The resulting esters are then packed into chylomicrons delivered via the lymphatic system to the liver, where they are again hydrolyzed and re-esterified for storage. Prior to mobilization from the liver, the retinyl esters are hydrolyzed, and free retinol is complexed with the retinol-binding protein for secretion from the liver [101]. Different esterases are involved in this sequence. Hydrolysis of dietary retinyl esters in the lumen is catalyzed by pancreatic sterol esterase (steryl-ester acylhydrolase, cholesterol esterase, EC 3.1.1.13) [102], A bile salt independent retinyl-palmitate esterase (EC 3.1.1.21) located in the liver cell plasma hydrolyzes retinyl esters delivered to the liver by chylomicrons. Another neutral retinyl ester hydrolase has been found in the nuclear and cytosolic fractions of liver homogenates. This enzyme is stimulated by bile salts and has properties nearly identical to those observed for... 

J. Hyun, C. R. Treadwell, G. V. Vahouny, Pancreatic Juice Cholesterol Esterase. Studies on Molecular Weigth and Bile Salt-Induced Polymerization , Arch. Biochem. Bio-phys. 1972, 752, 233-242. 

R. M. Stroud, Structure of Bovine Pancreatic Cholesterol Esterase at 1.6 A Novel Structural Features Involved in Lipase Activation , Biochemistry 1998, 37, 5107-5117. 

E. M. Kyger, D. J. S. Riley, C. A. Spilburg, L. G. Lange, Pancreatic Cholesterol Esterases. 3. Kinetic Characterization of Cholesterol Ester Resynthesis by Pancreatic Cholesterol Esterases , Biochemistry 1990, 29, 3853-3858. 

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... 

Cholesterol assay solution. The stock reagent contains pancreatic cholesterol esterase, microbial cholesterol oxidase, horseradish peroxidase, 4-aminoantipyrine, and phenol. Your instructor will reconstitute the stock reagent by addition of water. 

Pancreatic cholesterol esterase (3.1.1.3.) aids in transporting cholesterol to the enterocyte. By utilizing a selective and potent cholesterol esterase inhibitor 6-chloro-3-(l-ethyl-2-cyclohexyl)-2-pyrone, the absorption of cholesterol in hamsters could be reduced [71]. Wadkins et al. [72] synthesized novel sulfonamide derivatives, which demonstrated greater than 200-fold selectivity for human intestinal carboxylesterase compared with the human liver carboxylesterase hCEl, and none of them was an inhibitor of human acetylcholinesterase or butyrylcholinester-ase. Maybe these agents can serve as lead compounds for the development of effective, selective carboxylesterase inhibitors for clinical applications. Also the potent P-gp inhibitor verapamil [73] as well as S,S,S-tributylphosphortrithionate (DEF) [74] may exhibit carboxylesterase inhibitory properties. Various other inhibitors of human esterases are listed in Table 5.6. 

Spilburg, C.A., et al. 1995. Identification of a species specific regulatory site in human pancreatic cholesterol esterase. Biochemistry 34 15532. 

Heidrich, J.E., et al. 2004. Inhibition of pancreatic cholesterol esterase reduces cholesterol absorption in the hamster. BMC Pharmacol 4 5. 

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). 

While it was previously thought that the lipase of pancreatic origin, the classical lipase, was responsible for the digestion of most dietary fat, it has become clear in recent years that lipolysis in the gastrointestinal rat is a result of the conceited action of four different types of lipases gastric Kpase, classical pancreatic lipase, pancreatic carboxyl ester lipase (identical to cholesterol esterase), and phos-... 

E. Kyger, D. Riley, C. Spilburg, and U Lange, Pancreatic cholesterol esterases. 3. Kinetic characterisation of cholesterol ester resynthesls by the pancreatic cholesterol esterases. Biochemistry 29 3853 (1990). 

B. J. Hiiyn, C. Treadwell, and G. Vahouny. Pancreatic J uice cholesterol esterase. Studies on molecular weight and bile salt-induced polymerisation. Arch. Biochem. Biophys. 752533(1972). 

C Spilburg, M. Lowe and L. Lange. Structure of the human pancreatic cholesterol esterase gene. Biochemistry 37 6077 (1992). 

Orally administered fluorescein is readily absorbed in the small intestine. By contrast fluorescein coupled to dilaurate cannot be absorbed unless the composite molecule is cleaved intra-duodenally by the pancreatic cholesterol esterase to form lauric acid and (absorbable) fluorescein. After its absorption, fluorescein is partly glucuronidated in the liver and then excreted in urine, predominantly as fluorescein diglucuronide. Thus, in pancreatic-insufficient... 

Figure 26-4. Principle of the Pancreolauryl and the NBT-PABA (V-benzoyl-i.-tyrosyl-P-aminobenzoic acid, bentiromide) test. The composite molecule consisting of a substrate and a marker molecule cannot be absorbed but can be cleaved intraduodenally by pancreatic enzymes (cholesterol esterase and chymotrypsin, respectively), leading to release of an absorbable marker substance. Following absorption and hepatic conjugation, the marker is excreted in urine. In pancreatic-insufficient patients, decreased secretion of pancreatic enzymes results in incomplete cleavage of the composite molecule. This results in decreased absorption and subsequent excretion of the marker, which can be measured photometrically.

The best-known cholesterol esterase (Table I) is also a pancreatic enzyme. Cholesterol esterases not only hydrolyze cholesterol esters but also synthesize them from cholesterol and fatty acid in a reversible reaction. This is important in the digestion of cholesterol itself because... 

Lipolysis starts at the water/emulsion interphase. The major enzymes involved are pancreatic lipase, pancreatic phospholipase A2, and pancreatic cholesterol esterase, the latter also named bile-salt-dependent lipase. 

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),... 

The pancreolauryl test (fluorescein dilaurate test, Temmler Pharma, Germany) is similar in its principle to the NBT-PABA test. Fluorescein dilaurate (FDL) is poorly water-soluble. It is given orally in the middle of a standard breakfast (50 g white bread, 20 g butter, and one cup of tea) and is specifically hydrolyzed by pancreatic cholesterol esterase, which requires bile salts for its activity. The released... 

Riley D.IS. Kyger EM, Spilburg CA, Lange LG. Pancreatic cholesterol esterases. 2. Purification and characterization of human pancreatic fatty acid ethyl ester synthase. Biochemistry 1990 29 3848-3852. 

Pancreatic carboxylester lipase, secreted by the pancreas as an active enzyme without proteolytic activation, displays broad substrate specificity and has therefore received many names in the literature carboxylesterase, bile salt-stimulated (or activated or dependent) lipase (due to its absolute requirement for bile salts to hydrolyze insoluble substrates), carboxylester lipase or hydrolase, cholesterol esterase, lysophospholipase, nonspecific lipase, and monoglyceride lipase. The IUPAC classification of the enzyme has been either EC.3.1.1.1 (carboxylester hydrolase) or EC.3.1.1.13 (cholesterolester hydrolase) (Table 2). 

Figure 9 The pancreolauryl test allows an indirect assessing of exocrine pancreatic function. Orally administered fluorescein dilaurate is hydrolyzed by carboxylester lipase (identical to cholesterol esterase) liberating lauric acid and free, water-soluble fluorescein. The latter is readily absorbed in the small intestine, partly conjugated in the liver, and excreted in urine mainly as fluorescein glucuronide. By measuring the concentration of fluorescein in the urine over a period of, for instance, 10 hours, the total quantity of this dye is determined. (From Ref. 52.)...

D. Hui and Y. Kissel. Sequence identity between human pancreatic cholesterol esterase and bile salt-stimulated milk lipase. FEBS Lett. 276 131 (1990). 

First attempts to predict the selectivity of enzymes are dated back to 1964 when Prelog described an empirically determined rule for the addition of hydrogen to ketones by the yeast Culvaria lunata [13]. In 1991 Kazlauskas published the hydrolysis of acetates of secondary alcohols by Pancreatic cholesterol esterase, Pseudomonas cepacia and Candida rugosa and formulated the widely applicable Kazlauskas rule according to which esters of secondary alcohols with a specific substitution pattern of large (L) and medium (M) substituents are cleaved faster than the corresponding enantiomer [14]. 

Most of the evidence implicating the pancreatic cholesterol esterase as playing an important role physiologically in the intestinal mucosa during cholesterol absorption is based on the observation that the lymphatic secretion of cholesterol esters is... 

The bile-salt-dependent lipase of pancreatic juice has many names such as cholesterol esterase, nonspecific lipase, the most rational being carboxyl ester lipase [27], In the case of water-insoluble substrates this enzyme has an absolute requirement for bile salts specifically having hydroxyl groups in the 3a and la positions [28.29]. The best documented role for this enzyme is to allow the absorption of dietary cholesterol, through hydrolysis of cholesterol esters in the lumen. The enzyme also catalyzes the esterification of cholesterol and a role for it has been proposed in cholesterol absorption [30]. In addition, a wide range of primary and secondary fatty acyl esters including glycerides, vitamin A and E esters are hydrolyzed by this enzyme. 

Enzymic processes in the enterocyte which handle a number of lipid species other than triglycerides are also known to be influenced by bile salts. For example, it has been postulated that pancreatic cholesterol esterase is absorbed by the mucosal cells and catalyzes the esterification of cholesterol in the cell [87]. This postulate has been challenged by Watt and Simmonds [88] although Bhat and Brockman [30] recently produced evidence for the importance of pancreatic cholesterol esterase for transport across the enterocyte membrane. Another enzyme, acyl-CoA cholesterol acyltransfera.se (ACAT) of microsomal origin, has been ascribed a major role in mucosal esterification of cholesterol [89], Unlike pancreatic cholesterol esterase, this enzyme is inhibited by taurocholate in vitro. 

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