title: Combined interaction of phospholipase C and apolipoprotein A-I with small unilamellar lecithin-cholesterol vesicles: influence of apolipoprotein A-I concentration and vesicle composition.
abstract: We report the combined effects of phospholipase C (PLC), a pronucleating factor, and apolipoprotein A-I (apo A-I), an antinucleating factor, in solutions of model bile. Results indicate that apo A-I inhibits cholesterol nucleation from unilamellar lecithin vesicles by two mechanisms. Initially, inhibition is achieved by apo A-I shielding of hydrophobic diacylglycerol (DAG) moieties so as to prevent vesicle aggregation. Protection via shielding is temporary. It is lost when the DAG/apo A-I molar ratio exceeds a critical value. Subsequently, apo A-I forms small ( approximately 5-15 nm) complexes with lecithin and cholesterol that coexist with lipid-stabilized (400-800 nm) DAG oil droplets. This microstructural transition from vesicles to complexes avoids nucleation of cholesterol crystals and is a newly discovered mechanism by which apo A-I serves as an antinucleating agent in bile. The critical value at which a microstructural transition occurs depends on binding of apo A-I and so varies with the cholesterol mole fraction of vesicles. Aggregation of small, unilamellar, egg lecithin vesicles (SUVs) with varying cholesterol composition (0-60 mol %) was monitored for a range of apo A-I concentrations (2 to 89 microg/mL). Suppression of aggregation persists so long as the DAG-to-bound-apo A-I molar ratio is less than 100. A fluorescence assay involving dansylated lecithin shows that the suppression is an indirect effect of apo A-I rather than a direct inhibition of PLC enzyme activity. The DAG-to-total apo A-I molar ratio at which suppression is lost increases with cholesterol because of differences in apo A-I binding. Above this value, a microstructural transition to DAG droplets and lecithin/cholesterol A-I complexes occurs, as evidenced by sudden increases in turbidity and size and enhancement of Forster resonance energy transfer; structures are confirmed by cryo TEM.
authors.affiliation: Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Manasa V
authors.initials: MV
authors.lastname: Gudheti
authors.forename: Sum P
authors.initials: SP
authors.lastname: Lee
authors.forename: Dganit
authors.initials: D
authors.lastname: Danino
authors.forename: Steven P
authors.initials: SP
authors.lastname: Wrenn
mesh_terms: D016632:Apolipoprotein A-I / Q000737:chemistry / Q000378:metabolism*; D001646:Bile / Q000201:enzymology / Q000378:metabolism; D002784:Cholesterol / Q000737:chemistry / Q000378:metabolism*; D020285:Cryoelectron Microscopy; D004075:Diglycerides / Q000737:chemistry; D004875:Ergosterol / Q000031:analogs & derivatives* / Q000737:chemistry; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D007700:Kinetics; D008027:Light; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism*; D008956:Models, Chemical; D010316:Particle Size; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism*; D011237:Predictive Value of Tests; D012542:Scattering, Radiation; D010738:Type C Phospholipases / Q000737:chemistry / Q000378:metabolism*
chemical_list: C011439:1,2-diacylglycerol; D016632:Apolipoprotein A-I; D004075:Diglycerides; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; C010905:dehydroergosterol; D002784:Cholesterol; D010738:Type C Phospholipases; D004875:Ergosterol