THE EFFECT OF HIGH-DENSITY-LIPOPROTEIN PHOSPHOLIPID ACYL-CHAIN COMPOSITION ON THE EFFLUX OF CELLULAR FREE-CHOLESTEROL

High density lipoprotein (HDL) phospholipid (PL) fatty acyl chain comp osition has been proposed to affect the ability of HDL to participate in the first step of reverse cholesterol transport. To examine the eff ects of PL fatty acid chain length and degree of unsaturation in this process, reconstituted HDL (rHDL) particles were made with human apoli poprotein (apo) A-I and PL containing fatty acid chains from 14 to 18 carbons in length, which were either fully saturated or unsaturated in one or both chains. These particles were characterized structurally a nd for their ability to promote free (unesterified) cholesterol (FC) e fflux from cells growing in culture, The discoidal rHDL particles were homogeneous and exhibited similar hydrodynamic diameters (10.4 +/- 1. 0 nm) indicating that apoA-I forms similarly sized discs with a variet y of PL. Measurements of particle surface charge, apoA-I alpha-helix c ontent, and conformational stability indicated that the conformation o f apoA-I varies among the particles. These conformational effects on a poA-I are consistent with the PL fluidity influencing the interaction between the amphipathic Lu-helical segments and FL acyl chains. Differ ential scanning calorimetry demonstrated that the physical state of th e rHDL PL at 37 degrees C varied according to acyl chain length and de gree of unsaturation; the FC efflux efficiencies for particles with PL in either the gel or liquid crystal states were determined. The abili ty of the rHDL to accept cellular FC depended on the physical state of the PL in the rHDL. Liquid crystal PL formed the most efficient FC ac ceptor particles exhibiting a maximal efflux velocity (V-max) of 12-14 % release of total cellular FC per h. Gel-phase PL formed inefficient rHDL acceptors with a V-max of about 3%/h. A similar hierarchy of FC e fflux efficiency was noted when either mouse L-cells or rat Fu5AH hepa toma cells were used as the FC donors. Furthermore, this hierarchy was found to be due to the characteristics of the PL and not due to varia ble apoA-I conformation because protein-free, small unilamellar vesicl es made with the same PL exhibited similar relative efflux capabilitie s. Generally, the ability of a given rHDL particle to accept cellular FC was related to rHDL PL acyl chain length and degree of unsaturation ; decreases in FL acyl chain length and increases in chain unsaturatio n tended to result in more efficient FC acceptor particles. These resu lts suggest that rHDL acceptor particles that contain highly fluid sur faces sequester FC molecules that have diffused from the cell plasma m embrane at a significantly faster rate than those containing highly or ganized lipid surfaces with restricted FL acyl chain mobility, This in formation forms a basis for understanding the role of lipid content in the structural and functional diversity of HDL.