title: Cholesterol determines and limits rHDL formation from human plasma apolipoprotein A-II and phospholipid membranes.
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.
authors.affiliation: Department of Medicine, Baylor College of Medicine, Houston, TX 77030, USA.
authors.forename: G Randall
authors.initials: GR
authors.lastname: Bassett
authors.forename: Baiba K
authors.initials: BK
authors.lastname: Gillard
authors.forename: Henry J
authors.initials: HJ
authors.lastname: Pownall
mesh_terms: D016632:Apolipoprotein A-I / Q000378:metabolism*; D016633:Apolipoprotein A-II / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism*; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000378:metabolism*; D008081:Liposomes / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D008075:Lipoproteins, HDL; D008081:Liposomes; D002784:Cholesterol; D004134:Dimyristoylphosphatidylcholine