EVIDENCE THAT CHOLESTERYL ESTER TRANSFER PROTEIN-MEDIATED REDUCTIONS IN RECONSTITUTED HIGH-DENSITY-LIPOPROTEIN SIZE INVOLVE PARTICLE FUSION

It is well established that cholesteryl ester transfer protein (CETP) changes the size of high density lipoproteins (HDL) during incubation in vitro. It has been suggested that HDL-CETP-HDL ternary complex form ation is involved in these changes. The present results, which are con sistent with CETP changing the size of spherical reconstituted HDL (rH DL) by a mechanism involving fusion, support the ternary complex hypot hesis. When rHDL containing a core of cholesteryl esters and either th ree molecules of apolipoprotein (ape) A-I/particle, (A-I)rHDL, or six molecules of apoA-II/particle, (A-II)rHDL, were incubated individually with CETP, their respective diameters decreased from 9.4 to 7.8 nn an d from 9.8 to 8.8 nm. The small (A-I)rHDL and (A-II)rHDL contained, re spectively, two molecules of apoA-I/particle and four molecules of apo A-II/particle. As all of the rHDL lipids and apolipoproteins were quan titatively recovered at the end of the incubations, it was apparent th at there was a 50% increase in the number of particles. This increase in the number of particles can be explained as follows: (i) sequential binding of two rHDL to CETP to generate a ternary complex, (ii) fusio n of the rHDL in the ternary complex, and (iii) rearrangement of the f usion product into three small particles. Various spectroscopic techni ques were used to show that the small rHDL were structurally distinct from the original rHDL. These results provide the first evidence that CETP mediates the fusion of spherical rHDL.