CHOLESTEROL TRANSFER FROM LOW-DENSITY LIPOPROTEINS TO RECONSTITUTED HIGH-DENSITY-LIPOPROTEINS IS DETERMINED BY THE PROPERTIES AND CONCENTRATIONS OF BOTH PARTICLES

Cholesterol spontaneously transfers from low density lipoproteins (LDL ) to high density lipoproteins (HDL). This transfer is important physi ologically as it supplies the major portion of cholesterol for the lec ithin:cholesterol acyltransferase reaction and is one mechanism for th e reduction of atherogenic LDL cholesterol. The objective of this work was to examine the properties of both HDL and LDL which modulate chol esterol transfer, as well as to obtain the relevant kinetic constants for the transfer at concentrations of lipoproteins approaching those e xisting in vivo. To examine the effects of HDL structural parameters o n cholesterol transfer, we prepared reconstituted HDL particles with s aturated or unsaturated phospholipid, with apolipoprotein AI or apolip oprotein AII, with increasing size and phospholipid content, and with increasing initial contents of cholesterol. We also prepared five LDL subfractions of variable density and size. The kinetics of cholesterol mass transfer were measured by incubating LDL with rHDL at 37 degrees C, separating the Lipoproteins by dextran sulfate/Mg2+ precipitation of LDL at timed intervals, and analyzing rHDL cholesterol content. The cholesterol content of rHDL at equilibrium, C-eq, and the halftime fo r transfer, t(1/2), as well as the ratio of the lipid surface areas of LDL to rHDL were used in the analysis of the kinetic data by the aque ous diffusion model for lipid transfer developed by Nichols and Pagano [(1982) Biochemistry 21, 1720-1726]. The only variables that signific antly affect the C-eq and/or tin are the phospholipid content and comp osition of the rHDL and the size or density of the LDL particles. Thes e effects are mostly related to the differences in the ratio of donor to acceptor surface areas. Experiments as a function of LDL and rHDL c oncentrations gave the off-rate constants, as well as the ratio of the affinity constants and the ratio of the on-rate constants for cholest erol in equilibrium with LDL and rHDL. We found that rHDL has 0.7-fold the affinity constant of LDL, but a 3.7-fold higher off-rate constant , and 2.7-fold higher on-rate constant for cholesterol adsorption than does LDL. In addition, we determined that over 40% of the cholesterol in LDL is not available for transfer and confirmed that these rHDL ha ve a limited capacity for cholesterol compared to a phospholipid bilay er.