Enzymes cholesterol esterase, mucosal

Mucosal esterification of cholesterol is due to the activity of at least two enzymes Cholesterol Esterase and Acyl-CoA Cholesterol Acyltransferase (ACAT). 

This enzyme and its possible functions have been recently reviewed [78], Cholesterol esterase is secreted by the pancreas into the intestinal lumen where it hydrolyzes dietary cholesterol esters. It has been suggested that the intact enzyme is subsequently bound to the brush border membrane, possibly to high-affinity binding sites, and taken up by the mucosal cell. Once inside the cell, the enzyme is supposed to re-esterify absorbed cholesterol with various free fatty acids [78]. 

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. 

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. 

Some thirty years ago Swell et al. showed that the rat intestinal mucosa possesses an enzymatic activity which catalyzes the formation of cholesterol esters[46,47]. The enzyme was called Cholesterol Esterase and its properties were similar to those of the pancreatic cholesterol esterase. Over the past three decades Vahouny and colleagues extensively studied the properties of this enzyme, its subcellular localization, the possible sources and finally they proceeded to the purification of the enzyme[5,26,28,48]. The authors hypothesize that the pancreas secretes subunits of cholesterol esterase. In the intestinal lumen the subunits, in presence of trihydroxy bile acids, aggregate to the active enz3me which catalyzes the hydrolysis of cholesterol esters from a micellar substrate and at the optimal pH of 6.6. The enz3one is then absorbed and localizes mostly in the supernatant of the mucosal cells here, at the optimal pH of 6.2, cholesterol esterase operates in reverse inducing the esterification of cholesterol and speeding up its... 

Apart from the fact that mucosal esterase activity can be reduced by diverting pancreatic secretions from the intestinal lumen, no studies are available on how dietary manipulations alter the activity of this enzyme. Thus, for example, there is no indication that esterase activity can be increased by increasing the load of dietary cholesterol [98], as is the case with ACAT activity [41,85]. 

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