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.

Size-selective uptake of colloidal low density lipoprotein aggregates by cultured white blood cells

“…Colloid science can be used to study and explain some of the pathophysiological mechanisms in which LDL is involved. For example, a different set of investigations showed that apolipoprotein A-1 from high density lipoprotein inhibits LDL aggregation through a well-studied hydrophobic shielding mechanism [27,28]. Using colloid science techniques to study Smase-mediated LDL aggregation, we have found that LDL aggregate size is a sole function of ceramide concentration; that is, at any point in the hydrolysis reaction, sizes can be described by ceramide concentration, independent of Smase concentration.…”

Triglycerides Stabilize Water/Organic Interfaces of Changing Area via Conformational Flexibility

“…Their two-chain analogue, diacylglycerides (DGs), are implicated in a variety of biological processes involving membrane fusion and budding. ,− DGs are considered amphiphiles and are able to self-assemble in water due to their polar free hydroxyl group, while TGs are not. The two do, however, share a unique combination of biophysical properties, including inducement of negative curvature, transbilayer activity, and the ability to aggregate in a distinct, isotropic phase. ,,− …”