The influence of cholesteryl ester transfer protein on the composition, size, and structure of spherical, reconstituted high density lipoproteins

The effect of cholesteryl ester transfer protein (CETP) on the size, composition, and structure of spherical, reconstituted HDL (rHDL) which contain apolipoprotein (apo) A-I as their sole apolipoprotein has been studied. Spherical rHDL were incubated with CETP and Intralipid for up to 24 h. During this time CETP promoted transfers of cholesteryl esters (CE) and triglyceride (TG) between rHDL and Intralipid. As a result, the rHDL became depleted of CE and enriched in TG. However, as the loss of CE from the rHDL was greater than the gain of TG, the concentration of core lipids in the rHDL decreased. The decrease in the concentration of rHDL core lipids, which was evident throughout the incubation, was accompanied by a reduction in rHDL diameter from 9.2 to 8.0 nm, the dissociation of apoA-I from rHDL and a decrease in the number of apoA-I molecules, from three/particle in the 9.2-nm rHDL, to two/particle in the 8.0-nm rHDL. Spectroscopic studies showed that the lipid-water interface and phospholipid packing of the 8.0-nm rHDL were, respectively, more polar and less ordered than those of the 9.2-nm rHDL. Quenching studies with KI revealed that the number of exposed apoA-I Trp residues in the 9.2- and 8.0-nm rHDL was two and three, respectively. Circular dichroism established that the 9.2- and 8.0-nm rHDL had identical apoA-I alpha-helical contents. The 9.2- and 8.0 nm rHDL also had identical surface charges as determined by agarose gel electrophoresis. Denaturation studies with guanidine hydrochloride demonstrated that apoA-I is more stable in 8.0-nm rHDL than in 9.2-nm rHDL. It is concluded that CETP converts rHDL to small, TG-enriched, apoA-I-depleted particles with increased lipid-water interfacial hydration and less ordered phospholipid packing. These changes are associated with enhanced stability and minor changes to the conformation of the apoA-I which remains associated with the rHDL.