Differential effects of lecithin and cholesterol on the immunoreactivity and conformation of apolipoprotein A-I in high density lipoproteins.
Recently identified epitopes in apoA-I define a distinct N-terminal region with a complex tertiary structure, characterized by multiple discontinuous epitopes. Other epitopes are constituted of short domains centered either on beta-turns or random coils or on the 22-mer amphipathic alpha-helices (Marcel, Y. L., Provost, P. R., Koa, H., Raffaï, E., Vu Dac, N., Fruchart, J.-C., and Rassart, E. (1991) J. Biol. Chem. 266, 3644-3653). The compared immunoreactivity of seven epitopes studies here in response first to delipidation of high density lipoprotein (HDL) apoA-I by detergents, and second to modifications of HDL lipid composition by phospholipase A2 or by enrichment in surface lipids demonstrates that apoA-I has a flexible conformation which is readily responsive to the nature and concentration of bound lipids and that the structure of lipid-free apoA-I is significantly different from that of HDL-bound apoA-I, possibly representing a condensed molecule with several masked domains. In HDL apoA-I, these epitopes define five distinct domains which are characterized by particular responses to lipid modifications. However, two domains, each starting at the N-terminal beta-turn of an amphipathic alpha-helical repeat (residues 99-121 and 186-209, respectively) have almost identical immunoreactivity whether after detergent treatment or after changes in cholesterol and phospholipid levels, a property which probably reflects the known periodicity of apoA-I structural 22-mers. The immunoreactivity of a discontinuous epitope, representative of the N-terminal domain, is inversely related to the concentration of phospholipids, a unique characteristic among the epitopes tested here which indicates that the complex N-terminal region interacts with phospholipids, either directly or indirectly. These studies demonstrate that the conformation of multiple domains of HDL apoA-I is dependent on lipid phase composition and differentially affected by cholesterol and phospholipids.
Collet
X
X
Laboratory of Lipoprotein Metabolism, Institut de Recherches Cliniques de Montreal, Québec, Canada.
Perret
B
B
Simard
G
G
Raffaï
E
E
Marcel
Y L
YL
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000032:analysis / Q000276:immunology* / Q000648:ultrastructure; D002784:Cholesterol / Q000494:pharmacology*; D003902:Detergents; D000939:Epitopes; D006801:Humans; D066298:In Vitro Techniques; D008075:Lipoproteins, HDL / Q000032:analysis / Q000737:chemistry / Q000276:immunology; D010713:Phosphatidylcholines / Q000494:pharmacology*; D010741:Phospholipases A / Q000494:pharmacology; D054467:Phospholipases A2; D011136:Polysorbates; D011487:Protein Conformation / Q000187:drug effects; D011863:Radioimmunoassay
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D003902:Detergents; D000939:Epitopes; D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; D011136:Polysorbates; D002784:Cholesterol; D010741:Phospholipases A; D054467:Phospholipases A2