Interaction of the high density lipoprotein conversion factor with recombinant discoidal complexes of egg phosphatidylcholine, free cholesterol, and apolipoprotein A-I.

An HDL conversion factor which promotes the conversion of HDL3 to populations of larger and smaller particles has recently been identified in human plasma. In the present report a partially purified preparation of this factor has been used to examine the conversion of 79:0:1, 77:5:1, and 74:10:1 (mol:mol:mol) egg phosphatidylcholine-free cholesterol-apolipoprotein A-I (apoA-I) recombinant discoidal complexes. The study was carried out in order to ascertain whether the conversion process is regulated by the concentration of free cholesterol in the complexes. The complexes comprised one major and two minor populations of particles with respective Stokes' diameters of 96 A, 84 A, and 78 A. The 74:10:1 complexes also contained a population of particles 112 A in diameter. The 79:0:1 and 77:5:1 complexes contained two molecules of apoA-I per particle. The 74:10:1 complexes comprised two classes of particles with two or three molecules of apoA-I. When the 74:10:1 complexes were incubated with the conversion factor, the 96 A and 84 A particles were converted to a population of particles 78 A in diameter that contained two apoA-I molecules. In the case of the 79:0:1 and 77:5:1 complexes, the 96 A particles were converted to 78 A particles but the concentration of 84 A particles did not change. The rate of conversion of 96 A particles to 78 A particles was dependent on the concentration of free cholesterol in the complexes. When the 74:10:1 complexes were incubated for 24 hr with the conversion factor, the 96 A particles were completely converted to particles 78 A in diameter. In the case of the 77:5:1 complexes, complete conversion was achieved by 48 hr. Conversion of the 79:0:1 complexes did not proceed to completion, even when the incubation was extended beyond 48 hr. The rate of conversion of 96 A particles to 78 A particles was also dependent on the concentration of the conversion factor in the incubation mixtures. The previous incubations contained equivalent concentrations of apoA-I and conversion factor. When the concentration of the conversion factor relative to apoA-I was reduced, there was a concomitant decrease in the rate of conversion of 96 A particles to 78 A particles. Conversion was not evident when the concentration of the conversion factor was reduced to one-tenth that of apoA-I.(ABSTRACT TRUNCATED AT 400 WORDS)