Interaction of apolipoprotein A-I with dimyristoylphosphatidylcholine particles of various sizes

Dimyristoylphosphatidylcholine (DMPC) particles of different sizes were prepared by a short sonication of the lipid followed by fractionation on a Sepharose CL-4B column. Column fractions were pooled to give DMPC multilamellar liposomes, and vesicles ranging in average diameters from 300 to 220 A. These particles were used to prepare complexes with bovine apolipoprotein A-I (apo A-I). Reaction mixtures covering molar ratios from 2000:1 to 100:1 DMPC/apo A-I were equilibrated at 25 degrees C for over 15 h and were fractionated by gel filtration in order to separate vesicular and micellar protein.lipid complexes. The results indicate that under identical reaction conditions, larger particles with a smaller total surface area per mol of lipid give proportionately more micellar complexes. In fact, the multilamellar liposomes give only micellar complexes at all the initial molar ratios. For the smaller DMPC vesicles, the vesicular complexes are saturated when 6 or 7 apo A-I molecules are bound per particle, which corresponds to 2,83 X 10(4) A2 of vesicle surface per apo A-I and an approximate 16% coverage of the surface by apo A-I.