CONFORMATION OF APOLIPOPROTEIN AI IN RECONSTITUTED LIPOPROTEIN PARTICLES AND PARTICLE-MEMBRANE INTERACTION - EFFECT OF CHOLESTEROL

Discoidal recombinant high density lipoproteins (rHDL) of apolipoprote in AT (apoAI) and palmitoyloleoylphosphatidylcholine (POPC), with or w ithout cholesterol, were prepared by cholate dialysis, By gel filtrati on, rHDL containing 2-4 (Lp2, Lp3 and Lp4) apoAI molecules/particle we re obtained. The ApoAI conformation in these rHDL was investigated by tryptophan fluorescence, denaturation with guanidine HCl, and immunore activity with two monoclonal antibodies recognizing epitopes in the N- terminal and central domains. Data show that apoAI conformation is hig hly dependent on particle size as well as on cholesterol. The ability of rHDL to interact with lipid bilayer was studied by measuring leakag e induction on POPC and POPC/cholesterol vesicles loaded with terbium/ dipicolinic acid. Among the cholesterol-free rHDL, the most efficient ones were the smallest Lp2. Leakage induction on POPC vesicles is dram atically decreased by the presence of cholesterol in Lp2 and Lp3. All the rHDL, but specially those containing cholesterol, induced more lea kage on the POPC/cholesterol than on the POPC vesicles. These results suggest that in small cholesterol-poor particles, apoAI could have a c onformation determining a high affinity for membranes, which could fac ilitate cholesterol efflux. After cholesterol enrichment, a conformati onal change in apoAI could decrease the affinity for membranes allowin g the lipoprotein release. (C) 1998 Elsevier Science B.V.