Enzymes cholesterol esters hydrolase

A few enzymes, such as the previously mentioned CNP, are believed to be fairly specific for myelin/oligodendro-cytes. There is much more in the CNS than in peripheral nerve, suggesting some function more specialized to the CNS. In addition, a unique pH 7.2 cholesterol ester hydrolase is also enriched in myelin. On the other hand, there are many enzymes that are not myelin-specific but appear to be intrinsic to myelin and not contaminants. These include cAMP-stimulated kinase, calcium/calmodulin-dependent kinase, protein kinase C, a neutral protease activity and phosphoprotein phosphatases. The protein kinase C and phosphatase activities are presumed to be responsible for the rapid turnover of MBP phosphate groups, and the PLP acylation enzyme activity is also intrinsic to myelin. 

The chemical methods for the quantitation of cholesterol measure total cholesterol, i.e. free and esterified, and so a digitonin precipitate must be prepared if free cholesterol is to be measured. Enzymic methods do not measure the esters and a hydrolysis stage, either chemical or enzymic (using cholesterol ester hydrolase, EC 3.1.1.13), is necessary for the measurement of total cholesterol. 

The release of cholesterol from cholesterol esters occurs extra-mitochondrially by means of a cholesterol ester hydrolase in adrenals, ovaries and testicular Ley dig cells (see Ref. 6 for review). This enzyme has been studied mostly in adrenal preparations, and is known to be activated and de-activated by reversible phosphorylation [14] and that the phosphorylation was brought about by a c-AMP-dependent protein kinase [15]. Hence, ACTH stimulation of cholesterol ester activity in the adrenal occurs via the kinase and, in a similar way, trophic hormone stimulation of ovarian and testicular cholesterol ester hydrolases may occur and provide a large pool of cholesterol for steroidogenesis [16-18],... 

Enzymes. Many enzyme activities have been found in myelin neuraminidase, cholesterol ester hydrolase, lipid synthesizing and catabolizing enzymes, proteases, protein kinases, and phosphatases. Two of them have been especially characterized. 

Pancreatic lipase, in the presence of bile salts and coUpase, acts at the oil-water interface of the triglyceride emulsion to produce fatty acids and 2-monoacylglycerols. Cohpase is secreted in pancreatic juice as an inactive proenzyme, which is converted to the active form by trypsin. Other significant enzymes involved in the breakdown of fats within the intestinal lumen are cholesterol ester hydrolase, phospholipase A, and a nonspecific bile salt-activated lipase. 

ADP AFP ab as ALAT AP ASAT ATP BQ BSA CEH CK CME COD con A CV d D E E EC ECME EDTA EIA /e FAD FET FIA G GOD G6P-DH HBg HCG adenosine diphosphate a-fetoprotein antibody antigen alanine aminotranferase alkaline phosphatase aspartate aminotransferase adenosine triphosphate benzoquinone bovine serum albumin cholesterol ester hydrolase creatine kinase chemically modified electrode cholesterol oxidase concanavalin A coefficient of variation (relative standard deviation) layer thickness diffusion coefficient enzyme potential Enzyme Classification enzyme-chemically modified electrode ethylene diamine tetraacetic acid enzyme immunoassay enzyme loading factor flavin adenine dinucleotide field effect transistor flow injection analysis amplification factor glucose oxidase glucose-6-phosphate dehydrogenase hepatitis B surface antigen human chorionic gonadotropin... 

This enzyme (triacylglycerol acyl-hydrolase) has a molecular mass of approximately 42 kDa (Hide, Chan, and Li 1992) and a short half-life of about 1-3 h in dogs. Pancreatic lipase is secreted in its active form, and this activity is enhanced by colipase and bile salts the enzyme hydrolyzes triglycerides to monoglycerides. Other lipases— phospholipase a, phospholipase b, and cholesterol ester hydrolase—are also secreted by the pancreas. 

It appears there are two main reasons for the accumulation of cholesterol esters in rats that are fed HEAR oil or erucic acid containing diets. First, the cholesterol ester hydrolase fails to increase in activity when these rats are stressed, while in control rats, when stressed, the enzyme doubles its activity (Beckett and Boyd, 1975). Second, cholesteryl erucate, which accumulates in the adrenals of rats fed HEAR oil, is only slowly hydrolyzed by the enzyme, i.e., at 25-30% of the rate of cholesteryl oleate. This may be very significant, since there is considerable cholesteryl erucate accumulation in the adrenal glands of rats fed diets high in erucic acid, i.e., this ester may constitute 29-35% of the total (Carroll, 1962 Walker and Carney, 1971). In addition, in these rats there was an accumulation of 8% cholesteryl eicose-noate. In agreement with this evidence of impaired adrenal function, the results indicate that plasma levels of one of the adrenal hormones, corticosterone, are lower in these rats than in control rats when exposed to an environmental stress (Walker and Carney, 1971 Budzynska-Topolowska eta/., 1975). 

The luminal requirements for bile salts are general, and do not adequately describe the known specifity for cholate and its conjugates which has been demonstrated in several species including man. This specifity appears to be a function, at least in part, of a cholic acid - dependent enzyme, sterol ester hydrolase (cholesterol esterase). 

The cholesterol esters present in the intestinal lumen during digestion are rapidly hydrolyzed to the free alcohol and fatty acids[5]. Vahouny et al. showed that a pancreatic enzyme - cholesterol esterase or cholesterol esters hydrolase - is responsible for both the hydrolysis of cholesterol esters in the intestinal lumen and the esterification of the absorbed cholesterol in the intestinal mucosa[6,26] Because of the pH, the nature of the substrate and other undefined factors the reaction is directed towards hydrolysis in the lumen and towards esterification in the mucosa[5]. Although the ideal substrate for hydrolysis seems to be micellar cholesterol [27], an action of the enzyme at the oil-water interface of the lipid droplets cannot be excluded. The pancreatic origin of the rat cholesterol esterase has been repeatedly suggested, since in absence of pancreatic juice both hydrolysis of cholesterol esters and cholesterol absorption are significantly lower[28]. 

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... 

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

Cholesteryl esters that are internalized via the LDL receptor are hydrolyzed to produce cholesterol and an acyl chain. Cholesterol, in (urn, activates the enzyme acyl-CoA cholesterol acyl-transferase (ACAT) which re-esterifies cholesterol. In an apparently futile cycle, the cholesteryl esters are hydrolyzed by cholesteryl ester hydrolase. The cholesterol moiety has several fates it may leave the cell and bind to an acceptor such as high-density lipoprotein (HDL), it may be converted to steroid hormones, or it may be reesterified by ACAT. When the cellular cholesterol concentration falls, the activity of HMG-CoA reductase is increased, as is the number of LDL receptors, which results in an increase of cellular cholesterol, due both to de novo synthesis and to the uptake of cholesterol-rich lipoproteins in the circulation. An increase in cellular cholesterol results in the rapid decline in the mRNA levels for both HMG-CoA reductase and the LDL receptor. This coordinated regulation is brought about by the presence of an eight nucleotide sequence on the genes which code for both proteins this is termed the sterol regulatory element-1. 

Enzymes that hydrolyze lysophospholipids are found in nearly all tissues and organisms. They seem to be non-specific esterases of the serine-histidine type (25) and hardly deserve the name lysophospholipase because they also hydrolyze esters other than phospholipids. They should probably be considered together with such enzymes as cholesterol esterases and monoglyceride lipases as amphiphilic carboxyl ester hydrolases. These non-specific esterases have a preference for amphiphilic (hydrophilic-lipophilic) substrates. Such an enzyme may perhaps hydrolyze lysophospholipis, monoglycerides, diglycerides, and cholesterol esters. 

Modified LDL taken up by the cell are delivered to the endolysosome pathway, where enzymes hydrolyze cholesteryl esters to free cholesterol and fatty acids. The levels of free cholesterol and cholesteryl esters are regulated with the help of neutral cholesteryl ester hydrolase, which converts cholesteryl ester to free cholesterol. After cholesterol leaves the lysosome, it is transported to the ER and to the plasma membrane by means of an intermediate step through the Golgi apparatus. Recycling compartments, especially multivesicular endosomes, harbor most of the cholesterol in the endocytic pathway. The intra-endosomal membranes of multivesicular late endosomes that are enriched in the phospholipids lyso-bisphosphatidic acid/bismonoacylglycerophosphate serve as important regulators of cholesterol transport (Kobayashi et al. 1999 Ikonen 2008). Lysobisphosphatidic acid is structurally... 

Fig. 8. A schematic diagram showing cellular processes known to require SCP2. The reactions in cholesterol biosynthesis and esterification have been shown for liver. The reactions involving cholesterol transport from cytoplasmic lipid inclusion droplets to mitochondria have been demonstrated in endocrine tissues. Choi and C. cholesterol ACAT, acyl-CoA cholesterol acyl transferase C.E., cholesterol ester SEH, sterol ester hydrolase (hormone-dependent) P-450s,, cytochrome P-450 cholesterol side-chain cleavage enzyme PREG, pregnenolone.

The pancreatic enzyme mainly responsible for retinyl ester hydrolysis appears to be the same enzyme that catalyzes intraluminal cholesteryl ester hydrolysis (Erlanson and Boigstibm, 1968 Lombardo and Guy, 1980). This enzyme has been purified from rat (Calame et al.. 1975) and from porcine (Momsen and Brockman, 1977) pancreas, and from human pancreatic juice (Lombardo et al.. 1979). The enzyme appears to be a relatively nonspecific carboxylic ester hydrolase that can act on a wide variety of esters as substrates. Thus the purified human enzyme hydrolyzes triacetin, tributyrin, p-nitrophenylacetate, and lyso-phosphatidylcholine, as well as esters of cholesterol and of vitamins A, Dj, and E and glycerides solubilized by bile salts. Its molecular weight (approximately 100,000) is greater than that of the rat or pig enzyme, and it can hydrolyze... 

Finally it is clear that, unlike long-chain fatty acids which can appear in portal blood in limited amounts[32], cholesterol is absorbed exclusively into the lymphatic circulation[32,33]. A small amount of unesterified cholesterol can appear in lymph and is largely associated with chylomicron and lipoprotein "coat , or limiting membrane. However, this coat has a limited capability for significant transport, and the mass transport cholesterol (like that of triglycerides) requires esterification and incorporation into the lipoprotein core. Thus, the transfer of significant amounts of cholesterol from intestinal lumen to l3onph is associated with extensive esterification (70-90%) with fatty acids, and this occurs in the mucosal epithelial cells[11]. There are two intestinal enzymes potentially important in the esterification of unesterified cholesterol cholesterol esterase, or sterol ester hydrolase (EC. 

Of the two potentially esterifying enzymes associated with intestinal mucosa, sterol ester hydrolase, also referred to as cholesterol esterase (CE ase) has been extensively investigated in... 

Inside the cell, cholesteryl esters are hydrolysed by cholesteryl ester hydrolase. Incorporation of cholesterol into the endoplasmic reticulum membranes serves to inhibit hydroxymethylglutaryl-CoA reductase, the rate limiting enzyme in cholesterol biosynthesis (section 7.5). Thereby, an abundant supply of cholesterol in the plasma is able to suppress its own endogenous biosynthesis and ensure that excessive amounts do not accumulate. The importance of this pathway for cholesterol homeostasis... 

During a study on the resolution of the sterically demanding bicyclic acetate shown in Scheme 2.56 [397], which represents an important chiral building block for the synthesis of leukotrienes [398], it was found that crude steapsin is a highly selective catalyst for its resolution. On the other hand, pure PPL and a-chymotryp-sin were unable to hydrolyze the substrate. Cholesterol esterase, another known hydrolase impurity in crude steapsin capable of accepting bulky substrates, was able to hydrolyze the ester but with low selectivity. Finally, a novel hydrolase which was isolated from crude PPL proved to be the enzyme responsible for the highly selective transformation. 

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