Lecithin:cholesterol acyltransferase, reaction catalyzed

Reaction catalyzed by lecithin cholesterol acyltransferase (LCAT). The resulting cholesteryl ester is transferred to VLDL and LDL particles by a lipid transfer protein. 

Do you expect the equilibrium position of the reaction catalyzed by CoA cholesterol acyltransferase (fig. 20.13) to be any different from that of lecithin cholesterol acyltransferase (fig. 20.17) ... 

Reaction Catalyzed by Lecithin Cholesterol Acyltransferase (LCAT). 

There are two pieces of evidence in the literature that support the prediction that the cholesterol content of tissue membranes of children with SCD is increased relative to children without this hematological disease, and both are related to the phenomenon of reverse cholesterol transport, which allows the liver to eliminate excess cholesterol in peripheral tissues. Central to the reverse cholesterol transport is the efflux of cholesterol from the membranes followed by the lecithin-cholesterol acyltransferase-catalyzed acylation of that cholesterol with a fatty acid from phosphatidylcholine. This reaction is activated by high density lipoprotein (HDL) and is favored by n-3 PUFA in the HDL particles. The cholesterol esters are finally delivered to the liver bound to low density lipoprotein or by very low density Hpoproteins. 

Fatty acids are also found esterified to the 3-hydroxyl group of cholesterol (cholesterol esters ChE). ChE, which are more hydrophobic than free cholesterol, are a transport and storage form of cholesterol. ChE are found in high concentrations in low-density lipoproteins. Intracellular lipid droplets containing ChE are found in steroidogenic tissues and are thought to be a reservoir of cholesterol for steroid-hormone synthesis. The fatty acid most commonly found in ChE is 18 1. It must be activated to its CoA derivative before transfer to cholesterol in a reaction catalyzed by acyl-CoA cholesterol acyltransferase. ChE are also formed within lipoproteins by the transfer of one fatty acyl chain from phosphatidyl choline to cholesterol, a reaction catalyzed by circulating lecithin cholesterol acyltransferase. 

Finally, the enzymatic nature of CPIA-cholesterol ester formation will be briefly mentioned. None of the enzyme preparations of three known biosynthetic pathways for cholesterol esters, namely, acyl-CoA cholesterol Q-acyltransferase (ACAT), lecithin cholesterol 0-acyltransferase (LCAT), nor cholesterol esterase, was effective in producing CPIA-cholesterol ester from the Ba isomer or CPIA. In contrast, the 9,000 g supernatant or microsomal fractions from liver or kidney homogenate were found to be capable of producing CPIA-cholesterol ester without the addition of any cofactors. As substrate, only the Ba isomer was effective, and none of the 3 other fenvalerate isomers nor free CPIA was effective. The hepatic enzyme preparation also catalyzed hydrolysis of fenvalerate, and in this case all the 4 isomers were utilized as substrates. These facts imply that CPIA-cholesterol ester is formed from the Ba isomer through a transesterification reaction via intermediary acyl-enzyme complex. 

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