Adsorption of apolipoprotein A-IV to phospholipid monolayers spread at the air/water interface. A model for its labile binding to high density lipoproteins.
The mechanisms that mediate the labile binding of apolipoprotein A-IV (apoA-IV) to high density lipoproteins (HDL) are not known. We therefore used a surface balance and surface radioactivity detector to investigate the adsorption of apoA-IV to egg phosphatidylcholine monolayers spread at the air/water interface. ApoA-IV bound rapidly and reversibly to phospholipid monolayers and generated a maximum increase in surface pressure of 19 millinewtons (mN)/m at a subphase concentration of 2 x 10(-5) g/dl. Binding decreased linearly with increasing initial surface pressure; at pressures greater than 28-29 mN/m, apoA-IV could no longer penetrate the lipid monolayer. The area occupied by the amino acid residues in apoA-IV reached an unusually low limiting molecular area of 10-12 A2/residue at surface saturation. The surface pressure of native HDL3 was calculated to be 33 mN/m, and it rapidly decreased with the action of lecithin:cholesterol acyltransferase on the particle surface. We conclude that the surface activity of apoA-IV is lower than that of any other human apolipoprotein; its binding and surface conformation are particularly sensitive to pressure; and at saturation, a significant portion of the molecule is excluded from the interface. The exclusion pressure of apoA-IV may be only slightly lower than the surface pressure of HDL; in vivo, the action of lecithin:cholesterol acyltransferase and lipid transfer proteins may cause the HDL3 surface pressure to oscillate about a narrow range that spans the exclusion pressure of apoA-IV. The resultant labile association of apoA-IV and HDL may be of central importance to its role in lipoprotein metabolism.
Weinberg
R B
RB
Department of Medicine, Bowman Gray School of Medicine, Winston-Salem, North Carolina 27157.
Ibdah
J A
JA
Phillips
M C
MC
D000327:Adsorption; D000388:Air; D001054:Apolipoproteins A / Q000378:metabolism*; D001665:Binding Sites; D006801:Humans; D008075:Lipoproteins, HDL / Q000378:metabolism*; D008956:Models, Chemical; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000378:metabolism; D010713:Phosphatidylcholines / Q000378:metabolism*; D014867:Water
D001054:Apolipoproteins A; D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; C022205:apolipoprotein A-IV; D014867:Water; D007862:Phosphatidylcholine-Sterol O-Acyltransferase