Abstract
1. Phospholipid transfer protein (PLTP) mediates conversion of high-density lipoprotein (HDL3) to large particles, with concomitant release 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 (PysPC) in their surface lipid layer (PyrPC-rHDL) were prepared. Upon incubation with PLTP they behaved as native HDL3, in that their size increased considerably. 2. When PyrPC-rHDL was incubated with HDL3 in the presence of PLTP, a rapid decline of the pyrene excimer/monomer fluorescence ratio (E/M) occurred, demonstrating that PLTP induced mixing of the surface lipids of PyrPC-rHDL and HDL3. As this mixing was almost complete before any significant increase in HDL particle size was observed, it represents PLTP-mediated phospholipid transfer or exchange that is not directly coupled to the formation of large HDL particles. 3. When core-labelled PyrCE-rHDL was incubated in the presence of PLTP, a much slower, time-dependent decrease of E/M was observed, demonstrating that PLTP also promotes mixing of the core lipids. The rate and extent of mixing of core lipids correlated with the amount of PLTP added and with the increase in particle size. The enlarged particles formed could be visualized as discrete, non-aggregated particles by electron microscopy. Concomitantly with the appearance of enlarged particles, lipid-poor apoA-I molecules were released. These data, together with the fact that PLTP has been shown not to mediate transfer of cholesterol esters, strongly suggest that particle fusion rather than (net) lipid transfer or particle aggregation is responsible for the enlargement of HDL particles observed upon incubation with PLTP.4.ApoA-I rHDL, but not apoA-II rHDL, were converted into large particles, suggesting that the presence of apoA-I is required for PLTP-mediated HDL fusion. A model for PLTP-mediated enlargement of HDL particles is presented.