¹³ C Nuclear Magnetic Resonance Spectroscopic Evidence for Hydrophobic Lipid-Protein Interactions in Human High Density Lipoproteins

Abstract

Phosphatidylcholines, sphingomyelins, cholesterol, and cholesterol esters were enriched with 3IC by chemical synthesis in specific positions of their hydrophilic groups and aliphatic chains. Their spin-lattice relaxation times were determined in organic solvents. The substances were organized as liposomes and recombined with total human high density apolipoproteins and the two separated main components, apolipoprotein A-I (apoLp-Gln-I) and apolipoprotein A-II (apoLp-Gln-II). The enrichment of '3C in the hydrophilic and hydrophobic moiety of these lipids increases the sensitivity of the method and allows the determination of T, times of special regions of these molecules in reasonable periods of time. It should give insight into hydrophilic and hydrophobic interactions of lipidprotein complexes.A readily accessible system for the study of lipid-lipid and lipid-protein interactions in their natural molecular organization is the high density lipoprotein (HDL) fraction of human serum. Detailed information is available on the lipid and protein composition of these particles. Two apoproteins, apoA-I and apoA-II, comprise almost 90% of the molecule, with a high degree of a-helical structure (61-70%) (3)(4)(5)(6)(7)(8)(9). The aminoacid sequence of the latter has been elucidated (10). The apoproteins are assembled with an approximately equal amount of lipids. The lipid components are mainly phosphatidylcholine, sphingomyelin, cholesterol esters, and cholesterol. The acyl residues of the ester lipids are predominantly unsaturated fatty acids (oleic and linoleic acid).The molecular organization of the molecules in the HDL particle relates closely to other important questions concerning the structure and interactions of lipids and proteins in more complex lipoprotein aggregates, such as biological membranes.Several models of the HDL structure have been proposed (7,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). There is no general agreement on any one of these structures, most likely because of insufficient knowledge of the lipid-protein interactions and the tertiary structure of the proteins.

Ionization Behaviour of Native Apolipoproteins and of Their Complexes with Lecithin

“…The interpretation proposed is consistent with the experimental titration data and supports the hypothesis that the amphipathic helices are involved in phospholipid-apoprotein binding. These data suggest that the interaction of the phospholipid polar head groups with the acidic and basic side chains of amino acids may be more important than suggested by earlier work [19].…”

Isolation and Characterization of an Abnormal High Density Lipoprotein in Tangier Disease

“…Cholesteryl esters were the major neutral lipids and the quantity of free cholesterol was only slightly higher than normal (Table I). The (16,(28)(29)(30). When Tangier Apo A-TI was recombined with [3H]SPM, complexes with a SPM:protein molar ratio of 37:1 were isolated after sequential flotation in the ultracentrifuge (Table III).…”

High Density Lipoprotein Metabolism in Man

“…This suggests a smaller mean HDL particle size because available data indicate that HDL particles are spherical and are composed of an amiiphiphilic surface of protein, phospholipid, an(l uniesterified cholesterol, and a hydrophobic core of cholesteryl ester andl triglyceride (42)(43)(44)(45)(46)(47). Mlost of the cholesterol in HDL is esterified (48).…”