Effects of phospholipids on the conformation of chicken high density lipoprotein

Apolipoprotein A-I (apo-A-I), the major protein component of chicken high density lipoprotein (HDL), was isolated by hydrophobic interaction chromatography from the lipoprotein fraction of plasma. The apo-A-I was identified from its amino acid composition and molecular weight (by electrophoresis). The isolated protein contained 15% lipid, in the form of neutral lipids and free fatty acids, as compared with 50% lipid, including phospholipid, in intact HDL. The conformation of the isolated protein, as determined from the circular dichroism spectrum, was essentially that of the protein in intact chicken HDL, and similar to that of human HDL. The addition of phospholipid had little effect on the spectrum of this protein. Total delipidation of the protein by extraction with ethanol-diethyl ether mixtures removed the residual lipid, thereby producing an apoprotein with decreased helical structure. The secondary structure of this apoprotein could be restored by the addition of phospholipid.