Displacement of the Human Apoprotein A‐I by the Human Apoprotein A‐II from Complexes of (Apoprotein A‐I)‐Phosphatidylcholine‐Cholesterol

Abstract

Reassembly experiments, involving isolated human apoproteins A-I and A-I1 and (dimyristoylglycerophosphocholine)-cholesterol vesicles were performed with apoprotein mixtures at apoprotein A-I/A-I1 molar ratios varying between 0 and 3. The apoproteins were incubated at 24 "C, 28 "C and 32 "C with either pure dimyristoylglycerophosphocholine vesicles or with dimyristoylglycerophosphocholine cholesterol vesicles containing 0, 5, 10, 15 mo1/100 mol cholesterol. The kinetics of association were followed by measuring the increase of the fluorescence polarization ratio after labeling the lipids with diphenyl hexatriene. The complexes were separated from the free protein by gradient ultracentrifugation. Total protein was assayed and the apoproteins A-I and A-I1 were quantified separately by immunonephelometry . The content of apoprotein A-I was also monitored by measuring the intrinsic tryptophan fluorescence.The results suggest that apoprotein A-I1 has a greater affinity than apoprotein A-I for the phospholipidcholesterol vesicles and that apoprotein A-I1 is able to quantitatively displace apoprotein A-I from the lipidprotein complexes. The content of apoprotein A-I1 in the complexes increases proportionally to the concentration of apoprotein A-11 in the incubation mixture until saturation is reached. At saturation the dimyristoylglycerophosphocholine/apoprotein A-I1 ratio in the complex is dependent upon the cholesterol content of the original vesicles and increases from 60 to 275 mol/mol between 0 and 15 mo1/100 mol cholesterol. From these experiments one can calculate that 1 mol human apoprotein A-I is displaced by 2 mol human apoprotein A-11.The physiological importance of apoprotein-phospholipid complexes has been emphasized by the observations of Hamilton et al. [I], indicating that discoidal particles consisting of phospholipids and apoproteins are generated as 'nascent' high-density lipoproteins (HDL) and by those of Forte et al.[2], who isolated discoidal HDL deprived of cholesterolesters in the plasma of patients deficient in phosphatidylcholine-cholesterol acyl transferase. During lypolysis and transfer of surface components from very low density lipoproteins (VLDL) and chylomicra to HDL, such intermediary structures are probably exchanged between lipoprotein classes [3].In view of the important role of these transient complexes in the synthesis and metabolism of the HDL particles, we have previously studied the composition, the structural properties and the thermodynamics of complexes generated between phosphatidylcholine-cholesterol mixtures and either apoprotein A-I or apoprotein A-I1 [4,5]. In this paper, we extend these studies to the mixed complexes containing phosphatidylcholine, cholesterol, apoproteins A-I and A-11, in an attempt to define the interrelationship between the apoproteins in the interaction with phospholipid and phospholipidcholesterol mixtures. These data should contribute to the understanding of the formation and the stabilization of circulating apoprotein-lipid complexes.Abbreviation...

Potential involvement of dissociated apoA-I in the ABCA1-dependent cellular lipid release by HDL

“…To confirm this view by expanding apoA-I dissociation from HDL, displacement of apoA-I by apoA-II was attempted. When apoA-II was incubated with HDL, apoA-I was displaced and released from HDL by apoA-II as previously observed (36)(37)(38)(39) ( Fig. 4A , B).…”

Dissociation of Apolipoprotein AI from Apoprotein‐Lipid Complexes and from High‐Density Lipoproteins

“…The structure of denatured apoAI can also be partially restored by the addition of phospholipids as indicated by the fluorescence measurements. We have shown that the incubation of either apoAI-lipid complex, HDL or plasma with human apoAII displaces the apoAI from its lipidassociation and induces the formation of apoAII-rich HDL [6,7]. Treatment with 3 -4 M GdmCl induces a similar effect, whereas higher GdmCl concentrations are needed for the rupture of the apoAII-lipid bond [9].…”

“…The protein conformation is also sensitive to temperature [2] and to denaturating agcnts such as guanidinium hydrochloride [3 -51. The lipid-apoAl association can be disrupted by the competition with another apoprotein of higher affinity [6,7], by heat treatment [8] or by exposure to guanidinium hydrochloride [9 -1 I].We have previously studied the kinetics if apoAI-lipid association and characterized the isolated complexes [12 -151.In this paper we report on the dissociation of apoAI from these complexes and from native HDL in an attempt to evaluate the stability of the lipid-apoAI association in relation to the functional metabolic transfer of apoAI and apoAI-lipid complexes between lipoprotein fractions. …”

“…The protein conformation is also sensitive to temperature [2] and to denaturating agcnts such as guanidinium hydrochloride [3 -51. The lipid-apoAl association can be disrupted by the competition with another apoprotein of higher affinity [6,7], by heat treatment [8] or by exposure to guanidinium hydrochloride [9 -1 I].…”

Lipid‐Binding Properties of Synthetic Peptide Fragments of Human Apolipoprotein A‐II

“…Among the shorter apolipoproteins, human apo A-I1 appears as an homodinier of 77 residues with a cysteine at position 6 linking two identical peptide chains (Brewer et al, 1972). Apo A-11, and especially its C-terminal domain , has a high affinity for lipids and is able to displace apo A-I from high-density lipoproteins (HDL) and from recombinant HDL consisting of apo A-Uphospholipidkholesterol (Lagocki and Scanu, 1980;Rosseneu et al, 1981). The precise function of apo A-I1 has not been elucidated, although it was proposed that it could act as a modulator of the activity of the lecithin-cholesterol acyltransferase (LCAT), by displacing the LCAT activator, apo A-I, from the substrate (Fielding et al, 1972).…”