EFFECTS OF ACCEPTOR PARTICLE-SIZE ON THE EFFLUX OF CELLULAR FREE-CHOLESTEROL

Several subspecies of human high density lipoprotein (HDL) have been s hown to exist, and particle size is one parameter that can be used to distinguish them, Recently, a small HDL subspecies has been described that may be a particularly efficient acceptor of peripheral cell unest erified (free) cholesterol (FC), To address the effects of particle si ze on the ability of HDL to remove FC from cells, homogeneous, well de fined HDL particles were reconstituted (rHDL) that varied in particle diameter within the size range of human HDL particles (7-13 nm), The a bilities of each of these particles to remove cellular FC from mouse L -cells and rat Fu5AH hepatoma cells were compared on the basis of thei r phospholipid (PL) content as well as on a per particle basis, The ef fect of particle size was also examined using small unilamellar vesicl es (SUV) of 25 nm in diameter and large unilamellar vesicles (LUVs) of 70-180 nn in diameter, The SUV were prepared by sonication, and the L UVs were prepared by extrusion techniques, The FC efflux efficiency of these particles (in order of decreasing efficiency) was: rHDL > SUV > LUV when compared on the basis of acceptor PL content across a range of concentrations (i.e. at a given PL concentration for these three ac ceptor classes, smaller particles were more efficient), The FC efflux differences between the rHDL and the vesicles were not due to the abse nce of apolipoprotein in the vesicles, No difference was detected amon g the rHDL of varying size, nor was a difference detected among the LU Vs of varying size when compared on the basis of PL content, When the FC efflux data for rHDL and LUVs were normalized on the basis of the n umber of acceptor particles present at a given PL concentration, a cor relation was found between acceptor particle radius and the ability to accept cellular FC with larger particles being the most efficient, Ho wever, the dependence of the rate of FC efflux on acceptor particle si ze was not quantitatively the same within the rHDL and LUV classes of acceptor particles, The dependence of FC efflux on acceptor particle s ize may reflect differing abilities of the variously sized acceptor pa rticles to access the region very close to the cell plasma membrane wh ere most of the FC removal is expected to occur.