Cholesterol Determines
and Limits rHDL Formation from
Human Plasma Apolipoprotein A‑II and Phospholipid Membranes
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Abstract
Apolipoprotein (apo) A-II, the second most abundant protein
after
apo A-I of human plasma high-density lipoproteins (HDL), is the most
lipophilic of the exchangeable apolipoproteins. The rate of microsolubilization
of dimyristoylphosphatidylcholine (DMPC) membranes by apo A-I to give
rHDL increases as the level of membrane free cholesterol (FC) increases
up to 20 mol % when the level of reaction decreases to nil. Given
its greater lipophilicity, we tested the hypothesis that apo A-II
and its reduced and carboxymethylated monomer (rcm apo A-II) would
form rHDL at a membrane FC content of >20 mol %. According to turbidimetric
titrations, the DMPC/apo A-II stoichiometry is 65/1 (moles to moles).
At this stoichiometry, apo A-II forms rHDL from DMPC and FC. Contrary
to our hypothesis, apo A-II, like apo A-I, reacts poorly with DMPC
containing ≥20 mol % FC. The rate of formation of rHDL from
rcm apo A-II and DMPC at all FC mole percentages is faster than that
of apo A-II but nil at 20 mol % FC. In parallel reactions, monomeric
and dimeric apo A-II form large FC-rich rHDL coexisting with smaller
FC-poor rHDL; increasing the FC mole percentage increases the number
and size of FC-rich rHDL. On the basis of the compositions of coexisting
large and small rHDL, the free energy of transfer of FC from the smallest
to the largest particle is approximately −1.2 kJ. On the basis
of our data, we propose a model in which apo A-I and apo A-II bind
to DMPC via surface defects that disappear at 20 mol % FC. These data
suggest apo A-II-containing HDL formed intrahepatically are likely
cholesterol-rich compared to the smaller intracellular lipid-poor
apo A-I HDL