Isolation and characterization of a human serum cholesteryl ester transfer protein.

Abstract

Human plasma has been shown to contain an apolipoprotein that mediates the transport of cholesteryl ester from high density lipoprotein (HDL) to very low density lipoprotein (VLDL) or low density lipoprotein (LDL) This activity, confined to the density >1.063 g/ml interval, has been isolated from HDL and appears as a single migrating species by anionic or sodium dodecyl sulfate/polyacrylamide gel electrophoresis.It is unreactive to antibodies to the major HDL apolipoproteins. Antibodies prepared against this factor and immobilized on Sepharose remove the capacity of HDL and density > 1.21 g/ml fractions as well as whole plasma to transport cholesteryl ester from HDL. The system shows saturation kinetics with respect to plasma LDL and VLDL concentrations, and transport of cholesteryl ester was associated with reciprocal and equimolar back-transport of triglyceride from VLDL and LDL to the HDL fraction. The possible relationship of this apoprotein to apoprotein D is discussed.In human plasma, cholesterol esterification occurs predominately via the lecithin:cholesterol acyltransferase (LCATase; EC 2.3.1.43) reaction. Indeed an acyl-CoA:cholesterol 0-acyltransferase could not be detected in human liver (1), and familial LCATase deficiency is associated with the almost complete absence of cholesteryl ester from plasma liporoteins (2). After incubation of lipoproteins from LCAT-deficient subjects with this enzyme, all lipoprotein fractions were found to contain significant levels of cholesteryl ester, even though low density lipoproteins (LDL) and very low density lipoproteins (VLDL) were not significant substrates for the enzyme (3). It therefore has seemed likely that cholesteryl ester, formed by the LCATase reaction from high density lipoprotein (HDL) substrates, is transferred to nonsubstrate lipoproteins by a transport factor whose nature has not been ascertained. It was earlier shown that, when human plasma was incubated at 370, there was an increase in VLDL cholesteryl ester that was associated with an increased triglyceride content in HDL (4, 5). A cholesteryl ester exchange protein has been identified in the plasma of cholesterol-fed rabbits and is in the density > 1.21 g/ml density fraction (6) although net transport of cholesteryl ester in this case was not shown. Finally, increased cholesteryl ester synthesis by LCATase in human plasma has been shown during alimentary lipemia (7), possibly due to the release of a cholesteryl ester-mediated inhibition of LCATase activity (8).The identity of such a cholesteryl ester transport protein is of considerable potential interest because of its central role in lipoprotein lipid homeostasis. The present research describes the isolation and partial characterization of such a transport protein from human serum. METHODSPreparation of Plasma Lipoproteins. Plasma was taken from blood withdrawn into 0.38% (wt/vol) citrate from healthy volunteers. The major lipoprotein classes (VLDL, p < 1.006 g/ml; LDL, 1.019 < p < 1.063 g/ml; HDL, 1.063 < p < 1.21 g/ml) were prepar...

Factors Affecting the Lipid and Apolipoprotein Levels of Cord Sera

“…An unexpected relationship also existed between the levels of cord sera triglyceride and ApoD values. ApoD had been reported to be involved in the lecithin cholesterol acyl transferase reaction (9). The close correlation between ApoD and triglyceride levels suggests that ApoD may play a specific role in triglyceride metabolism as discussed by Chajek and Fielding (9), perhaps as an acyl carrier protein.…”

Transfers and exchanges of esterified cholesterol between plasma lipoproteins

“…This has been interpreted as a molecular exchange of one moiety for the other, a conclusion which has been reinforced recently with a report that there is a mole-for-mole exchange of esterified cholesterol and triacylglycerol between HDL and VLDL (Chajek & Fielding, 1978). It has been shown also that net mass transfers of esterified cholesterol from LDL to VLDL are accompanied by a reciprocal transfer of triacylglycerol from VLDL to LDL Hopkins & Barter, 1982a).…”

“…It has been reported also that apolipoprotein D, a minor HDL apoprotein, promotes an irreversible transfer of esterifed cholesterol from human HDL to both VLDL and LDL (Chajek & Fielding, 1978).…”

Inhibition of cholesteryl ester transfer protein activity by monoclonal antibody. Effects on cholesteryl ester formation and neutral lipid mass transfer in human plasma.

“…Nichols et al (3,4) first reported the net transfer of CE and triglycerides between lipoproteins in incubated plasma. Using reconstituted lipopro-teins, proteins purified from plasma were subsequently shown to promote the reciprocal transfer of triglycerides and CE between triglyceride-rich lipoproteins and HDL particles (5)(6)(7)(8).…”