Formation of mixed micelles and vesicles of human apolipoproteins A-I and A-II with synthetic and natural lecithins and the bile salt sodium taurocholate: quasi-elastic light scattering studies.

We employed quasi-elastic light scattering to systematically study the interactions of human apolipoproteins A-I and A-II (apo A-I and apo A-II) with synthetic and natural lecithins, analogues of the major membrane lipids found in high-density lipoproteins (HDL). Equilibrium values of the mean hydrodynamic radius (Rh) of systems with varying concentrations of dimyristoylphosphatidylcholine (DMPC) to apolipoprotein showed that as the percentage of DMPC was increased, three distinct regions were observed. At low DMPC to apolipoprotein ratios, Rh values either increased, remained constant, or decreased, depending upon the total apolipoprotein concentration, which influenced the size of pure apolipoprotein micelles [Donovan, J. M., Benedek, G. B., & Carey, M. C. (1987) Biochemistry (preceding paper in this issue)]. When the percentage of DMPC approached the micellar phase boundary, Rh values uniformly diverged (50 to approximately 150 A); with the percentage of DMPC in excess of the micellar phase boundary, large Rh values (200-300 A) were observed that were consistent with unilamellar apo A-I/DMPC or apo A-II/DMPC vesicles. Decreases in total solute concentration (1.0-0.25 mg/mL) and/or elevations in temperature (25-37 degrees C) shifted micellar phase limits to lower percentages of DMPC in the case of both apolipoproteins. Although apo A-I interacted spontaneously with DMPC at 25 degrees C, it was necessary to dilute mixed micellar solutions of sodium taurocholate (TC) and egg yolk phosphatidylcholine (EYPC) with apo A-I solutions to form apo A-I/EYPC mixed micelles. Despite the presence of submicellar concentrations of TC (below 3 mM, the lower limit of its critical micellar concentration), Rh values of apo A-I/EYPC mixed micelles were similar to those observed for the apo A-I/DMPC system. Dilution of micellar TC/EYPC solutions with low concentrations of apo A-I (0.001-0.10 mg/mL) influenced the width of the mixed micellar zone, the kinetics of micelle-to-vesicle transitions, and the size of metastable vesicles. Dilution with higher apo A-I concentrations (0.05 and 0.1 mg/mL) resulted in the transformation of bile salt rich EYPC micelles into apo A-I rich EYPC micelles with an intervening zone of metastable vesicles. The micelle-to-vesicle transition was abolished by dilution with an apo A-I concentration of 0.5 mg/mL, suggesting that bile salts and apo A-I can directly interchange in micellar solubilization of EYPC.(ABSTRACT TRUNCATED AT 400 WORDS)