The interaction of human plasma high density apolipoproteins (apoHDL) with lipids was examined by circular dichroism (CD) of the peptide and aromatic chromophores. By CD criteria native HDL contained about 70% α helical, 5 to 15% β, and 15 to 20% disordered structure. Delipidation decreased the helical content by about 20% with a corresponding increase in disordered structure. The two major apoproteins of HDL (apoA-I and apoA-II), isolated by chromatography in urea, refolded to different extents after removal of urea; apoA-I had more ordered structure than apoA-II (approximately 55% and 35% α helix, respectively). Reconstitution of apoA-I, apoA-II, and apoHDL with both phosphatidylcholine and cholesteryl ester restored, respectively, 119%, 87%, and 100% of the helical structure of the parent HDL. Phosphatidylcholine alone restored 50 to 70% of the increase produced by both phosphatidylcholine and cholesteryl oleate. With each substrate, the increase in helical content on lipid-protein recombination was accompanied by a corresponding decrease in disordered structure.

Spectra of HDL in the near-ultraviolet range showed peaks at 258, 264, 283.5, and 290.5 nm. Delipidation reduced and markedly altered the ellipticity bands of the near-ultraviolet CD spectrum. The bands at 283.5 and 290.5 nm, tentatively assigned to one or more of the tryptophan chromophores in apoA-I, were reversed in sign and shifted to 286 and 292 nm, respectively. The near-ultraviolet CD spectrum of the parent HDL was partially restored by recombination of apoHDL with phosphatidylcholine alone and almost completely restored by recombination with both phosphatidylcholine and cholesteryl oleate. Recombination with phosphatidylcholine alone was sufficient to restore the 286 and 292 nm peaks to their original sign and position.

These studies indicate that about 20 to 30% of the amino acid residues in the two major HDL apoproteins are involved in a helix-disordered transition on lipid removal or restoration. The near-ultraviolet CD spectrum is especially sensitive to these lipid-induced structural changes. Both phospholipid and cholesteryl esters are required for complete reorganization of the secondary and tertiary structure of HDL.