THE MECHANISM OF HUMAN PLASMA PHOSPHOLIPID TRANSFER PROTEIN-INDUCED ENLARGEMENT OF HIGH-DENSITY-LIPOPROTEIN PARTICLES - EVIDENCE FOR PARTICLE FUSION

1. Phospholipid transfer protein (PLTP) mediates conversion of high-de nsity lipoprotein (HDL(3)) to large particles, with concomitant releas e of apolipoprotein A-I (apoA-I). To study the mechanisms involved in this conversion, reconstituted HDL (rHDL) particles containing either fluorescent pyrenylacyl cholesterol ester (PyrCE) in their core (PyrCE -rHDL) or pyrenylacyl phosphatidylcholine (PyrPC) in their surface lip id layer (PyrPC-rHDL) were prepared. Upon incubation with PLTP they be haved as native HDL,, in that their size increased considerably. 2. Wh en PyrPC-rHDL was incubated with HDL(3) in the presence of PLTP, a rap id decline of the pyrene excimer/monomer fluorescence ratio (E/M) occu rred, demonstrating that PLTP induced mixing of the surface lipids of PyrPC-rHDL and HDL(3). As this mixing was almost complete before any s ignificant increase in HDL particle size was observed, it represents P LTP-mediated phospholipid transfer or exchange that is not directly co upled to the formation of large HDL particles. 3. When core-labelled P yrCE-rHDL was incubated in the presence of PLTP, a much slower, time-d ependent decrease of E/M was observed, demonstrating that PLTP also pr omotes mixing of the core lipids. The rate and extent of mixing of cor e lipids correlated with the amount of PLTP added and with the increas e in particle size. The enlarged particles formed could be visualized as discrete, nonaggregated particles by electron microscopy. Concomita ntly with the appearance of enlarged particles, lipid-poor apoA-I mole cules were released. These data, together with the fact that PLTP has been shown not to mediate transfer of cholesterol esters, strongly sug gest that particle fusion rather than (net) lipid transfer or particle aggregation is responsible for the enlargement of HDL particles obser ved upon incubation with PLTP. 4. ApoA-I rHDL, but not apoA-II rHDL, w ere converted into large particles, suggesting that the presence of ap oA-I is required for PLTP-mediated HDL fusion. A model for PLTP-mediat ed enlargement of HDL particles is presented.