Effect of polyenephosphatidylcholine on cholesterol uptake by human high density lipoprotein

The lipid and protein composition of human HDL was changed by incorporation of polyenephosphatidylcholine (PPC) into HDL in vitro. HDL with incorporated PPC (HDL-PPC) had a higher molar PC/apoprotein ratio than native HDL. PPC accounted for up to 50% of the PC fraction of HDL. The fluidity of HDL-PPC was higher than that of native HDL but lower than that of PPC liposomes. Zonal ultracentrifugation separated HDL-PPC into a major and a minor component. The AI/AII ratio of the major fraction was reduced compared with native HDL. The storage capacity of HDL-PPC and native HDL for cholesterol was studied by incubation of these fractions with [14]cholesterol-LDL. Significantly more cholesterol (55%) was taken up by HDL-PPC from LDL than by native HDL. The transfer of cholesterol from LDL to HDL in human serum was studied by an in vitro [14C]cholesterol distribution test. In this test the lipoproteins of serum were labelled with [14C]cholesterol. An analytical procedure was developed to quantify the transfer of cholesterol from LDL to HDL after addition of PC. The transfer depended on the fluidity and the dose of the PC fraction used as well as on the initial LDL + VLDL/HDL ratio and was independent of LCAT activity.