Human apolipoprotein A-IV: displacement from the surface of triglyceride-rich particles by HDL2-associated C-apoproteins

Abstract

Human apolipoprotein A-IV rapidly dissociates from the surface of lymph chylomicrons following their entry into circulation by an unknown mechanism. We have therefore investigated the binding of human apoA-IV to triglyceride-rich particles and the interaction of these apoA-IV/lipid complexes with human HDL2. Human apoA-IV was purified from lipoprotein depleted serum (J. Lipid Res. 1983. 24:52-59). Triglyceride-rich particles of well-defined properties were isolated from Intralipid, a commercially available phospholipid-triglyceride emulsion. Various concentrations of radiolabeled human apoA-IV were incubated at 24 degrees C with a fixed quantity of lipid particles; the particles were reisolated by centrifugation, and bound and free apoA-IV were quantitated. In 50 mM Tris, pH 7.4, apoA-IV bound to the triglyceride-rich particles in a non-cooperative manner, with a Kd of 2.0 microM. The calculated maximal binding was 4.96 X 10(-4) mol of apoA-IV bound per mol of phospholipid. The addition of increasing amounts of human HDL2 to the incubations caused the progressive dissociation of apoA-IV from the triglyceride-rich particles. Analysis of the reisolated particles by isoelectric focusing demonstrated the presence of C-apoproteins, suggesting their transfer from HDL2. Addition of purified apoC-III-1 to the incubations at concentrations equivalent to those present in HDL2 caused a similar dissociation of apoA-IV. HDL2 was modified to selectively remove C-apoproteins, without alteration of other physical characteristics. This modified HDL2 was four times less effective in causing apoA-IV dissociation. These results demonstrate that the lipid binding properties of human apoA-IV may be quantitatively examined using triglyceride-rich particles as model chylomicrons. This approach reproduces in vitro the dissociation of apoA-IV that occurs in vivo when mesenteric lymph chylomicrons enter the circulation, and suggests that the primary mechanism for this phenomenon is the transfer of C-apoproteins from high density lipoproteins to the triglyceride-rich particle surface. We hypothesize that this mechanism may play an important role in the modulation of chylomicron apoA-IV content in man.