Two populations of
high-density lipoprotein (HDL) particles exist in human plasma. Both contain
apolipoprotein (
apo) A-I, but only one contains
apo A-II: Lp(AI w AII) and Lp(AI w/o AII). To study the extent of interaction between these particles,
apo B-free plasma prepared by the selective removal of
apo B-containing
lipoproteins (LpB) from the plasma of three normolipidemic (NL) subjects and whole plasma from two patients with
abetalipoproteinemia (ABL) were incubated at 37 degrees C for 24 h.
Apo B-free plasma samples were used to avoid
lipid-exchange between HDL and LpB. Lp(AI w AII) and Lp(AI w/o AII) were isolated from each
apo B-free plasma sample before and after incubation and their
protein and
lipid contents quantified. Before incubation, ABL plasma had reduced levels of Lp(AI w AII) and Lp(AI w/o AII), (40% and 70% of normals, respectively). Compared to the HDL of
apo B-free NL plasma, ABL HDL had higher relative contents of free
cholesterol,
phospholipid and total
lipid, and contained more particles with apparent hydrated Stokes diameter in the 9.2-17.0 nm region. These differences were particularly pronounced in particles without
apo A-II. Despite their differences, the total
cholesterol contents of Lp(AI w AII) increased, while that of Lp(AI w/o AII) decreased in all five plasma samples and the amount of
apo A-I in Lp(AI w AII) increased by 6-8 mg/dl in four during the incubation. These compositional changes were accompanied by a relative reduction of particles in the 7.0-8.2 nm Stokes diameter size region and an increase of particles in the 9.2-11.2 nm region. These data are consistent with intravascular modulation between HDL particles with and without
apo A-II. The observed increase in
apo A-II-associated
cholesterol and
apo A-I, could involve either the transfer of
cholesterol and
apo A-I from particles without
apo A-II to those with A-II, or the transfer of
apo A-II from Lp(AI w AII) to Lp(AI w/o AII). The exact mechanism and direction of the transfer remain to be determined.