Environmental Effects on the Structure of Human Serum High Density Lipoprotein and Its Apoprotein

Abstract

Previous studies from this laboratory using the technique of optical rotatory dispersion (ORD) * have indicated that the human serum density lipoprotein ciass floating between solvent densities of 1.063–1.21 has a high content in a-helïx, which is to a large extent retained after the lipoprotein is exhaustively extracted with a mixture of ethanol-ether (1,2). A relatively small influence on the optical rotatory dispersion parameters was noted after relipidation of the lipid-free apoprotein (apo HDL) with aqueous dispersions of phospholipids (3,4). Although the results gave sufficient evidence that the overall secondary structure of apo HDL is relatively independent of bound lipids, the possibility remained that conformational differences between lipid-rich and lipid-poor forms of apo HDL did exist, but could not be detected under the experimental conditions employed. The availability of the technique of circular dichroism (CD), which has the advantage over ORD of more clearly resolving bands relative to the optically active chromophores in the ultraviolet and far ultraviolet regions (7) offered the opportunity of reexamining this question. Under the chosen conditions, the CD spectrum of HDL was found to differ significantly from that of its lipid-poor product. Distinction between these two forms was also observed in terms of their sensitivity to chemical modification of the protein and changes of the nature and temperature of the solvent medium.

A portion of this work was presented at the 1968 Joint Meeting of the American Oil Chemists’ Society and the American Association of Cereal Chemists, March 31–April 3, 1968, and published in abstract form (Ref. ^#05).

Recipient of U. S. Public Health Service Career Development Award ^#HE-24,867.

Operated by the University of Chicago for the United States Atomic Energy Commission.