Apolipoprotein A-I structure and lipid properties in homogeneous, reconstituted spherical and discoidal high density lipoproteins.

We prepared a spherical reconstituted high density lipoprotein (rHDL) particle in pure form and compared it with its homogeneous discoidal rHDL precursors, in terms of the structure and stability of the apolipoprotein A-I (apoA-I) component, the dynamics of the surface lipids, and the relative reactivity with lecithin-cholesterol acyltransferase. The apoA-I-structure was examined in the rHDL particles by circular dichroism and fluorescence spectroscopic methods, and the binding of monoclonal antibodies specific for apoA-I epitopes. The stability of apoA-I on the rHDL particles was assessed by the effects of guanidine hydrochloride on the wavelength of maximum intrinsic fluorescence of the apolipoprotein. Lipid dynamics in the acyl chain region and the polarity of the lipid-water interface were investigated by means of fluorescence probes. The conformation of apoA-I in the spherical 93-A rHDL particles was found to be very similar to that in the 96-A rHDL discs but distinct from the apoA-I structure in the 78-A rHDL discs. The stability of apoA-I to denaturation by guanidine hydrochloride was highest in the 93-A rHDL spheres. The experiments on the lipids indicate somewhat more ordered and motionally restricted acyl chains in the spheres, relative to the discs, but a similar surface polarity. These results suggest that the folding and organization of apoA-I on the three particles include protein domains consisting of interacting alpha-helical segments in the carboxyl-terminal region and a globular domain in the amino-terminal region of each apoA-I molecule. The reactivity with lecithin-cholesterol acyltransferase was highest for the 96-A rHDL disc, and 16- and 34-fold lower for the 78-A rHDL disc and the 93-A rHDL sphere, respectively, possibly as a result of differences in apoA-I structure and product inhibition in these particles.