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Title:Effect of Lecithin on the Esterification of Lipoprotein Cholesterol by Lecithin-Cholesterol Acyltransferase
Author(s):Wiessner, John Henry
Department / Program:Food Science
Discipline:Food Science
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Biochemistry
Abstract:The initial rate of the lecithin-cholesterol acyltransferase reaction in human plasma was enhanced by the addition of lecithin dispersions, especially in the presence of apo A-I. To clarify the nature of this enhancement, purified enzyme and isolated human plasma lipoproteins were used. Without added lecithin the enzyme did not efficiently utilize the cholesterol of all lipoproteins; very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL). However, the preincubation of lipoproteins with egg lecithin and apo A-I enhanced the cholesterol esterification. At a constant level of lipoprotein cholesterol, the esterification of LDL and VLDL cholesterol was enhanced to a greater extent at lower lecithin levels than HDL. Although the esterification of HDL cholesterol was enhanced by the addition of lecithin alone, the presence of both lecithin and apo A-I further increased the cholesterol esterification rate at higher lecithin levels. When a constant level of apo A-I was included in the plasma and the model systems, a decrease in cholesterol esterification was observed at high lecithin levels. This decrease in esterification rate was overcome by adding apo A-I in increasing amounts. The level of apo A-I required for the maximum esterification of lipoprotein cholesterol was dependent on the amount of lecithin added.
The efficient esterification of cholesterol in fasting plasma and in model systems in the presence of exogenous lecithin and apo A-I appears to reflect the formation of substrate particles readily utilizable by lecithin-cholesterol acyltransferase. These substrate particles were produced by the efficient transfer of free cholesterol from all lipoproteins to the lecithin vesicles and by the association of apo A-I with the vesicles. This cholesterol transfer was not appreciably influenced by the presence or absence of apo A-I. However, the presence of apo A-I reduced the amount of lecithin incorporated into VLDL and possibly LDL and stabilized these apo B containing lipoproteins. No appreciable difference in the incorporation of lecithin into HDL was observed in the presence or absence of apo A-I, possibly reflecting the capacity of HDL to accomodate a large amount of exogenous phospholipids. Nevertheless neither intact HDL nor the HDL enriched with lecithin served as effectively as a substrate as the vesicles. The slower response of HDL cholesterol esterification to increasing concentrations of lecithin, as compared to VLDL or LDL, may largely be a reflection of the competition between HDL and the vesicles for the binding of the enzyme. It was speculated that the enhancement of cholesterol esterification in fasting plasma by the addition of lecithin vesicles could parallel the situation in vivo when nascent HDL is secreted by the liver or when phospholipid rich particles are formed by the action of lipoprotein lipase on chylomicrons or VLDL.
Issue Date:1980
Description:92 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
Other Identifier(s):(UMI)AAI8026620
Date Available in IDEALS:2014-12-13
Date Deposited:1980

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