Displacement of apo A-I by A-II in lipid-apoprotein complexes and human HDL.

The aim of this study was to define the specific affinity of human apo A-I and apo A-II for HDL lipids and to investigate the possible transfer of apoproteins from the HDL molecule. For this purpose we incubated apo A-I -- lipid complexes prepared "in vitro", as well as human HDL with increasing amounts of isolated apo A-II. After incubation the reaction products were separated by gradient ultracentrifugation and gel chromatography. The apoproteins were quantitated separately by immunonephelometry and the apo A-I content was monitored by measuring the intrinsic tryptophan fluorescence. These results suggest that apo A-II has a higher affinity than apo A-I for the lecithin-cholesterol vesicle and that 2 mol apo A-II are able to displace 1 mol apo A-I from the apo A-I lipid complexes. Analogous results were obtained with HDL where two mol apo A-II substitute to 1 mol apo A-I to yield en apo A-II - rich HDL with identical lipid composition, hydrodynamic properties and fluidity. Such a mechanism might contribute to the regulation of the HDL2 in equilibrium with HDL3 distribution in plasma.