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-II and (dimyristoylglycerophosphocoline)-cholesterol vesicles were performed with apoprotein mixtures at apoprotein A-I/A-II molar ratios varying between 0 and 3. The apoproteins were incubated at 24.degree. C, 28.degree. C and 32.degree. C with either pure dimyristoylglycerophosphocholine vesicles or with dimyristoylglycerophosphocholine cholesterol vesicles containing 0, 5, 10, 15 mol/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-II were quantified separately by immunonephelometry. The content of apoprotein A-I was also monitored by measuring the intrinsic tryptophan fluorescence. Apoprotein A-II apparently has a greater affinity than apoprotein A-I for the phospholipidcholesterol vesicles, and apoprotein A-II is able to quantitatively displace apoprotein A-I from the lipid-protein complexes. The content of apoprotein A-II in the complexes increases proportionally to the concentration of apoprotein A-II in the incubation mixture until saturation is reached. At saturation, the dimyristoylglycerophosphocholine/apoprotein A-II ratio in the complex is dependent upon the cholesterol content of the orignial vesicles and increases from 60 to 275 mol/mol between 0 and 15 mol/100 mol cholesterol. One mole human apoprotein A-I is displaced by 2 mol human apoprotein A-II.