AGGREGATION OF DIMYRISTOYLPHOSPHATIDYLGLYCEROL LIPOSOMES BY HUMAN PLASMA LOW-DENSITY-LIPOPROTEIN

Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physi ological pH and [NaCl], the aggregation of liquid crystalline large un ilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtai ned with fluorescent assays for these processes, in keeping with the l ack: of fusion of LUVs. Involvement of apoB is implicated by the findi ng that tryptic digestion of LDL abrogates its ability to cause aggreg ation. Aggregation is not caused by VLDL, HDL2, or HDL3. Interestingly , also oxidised LDL failed to aggregate DMPG vesicles. Aggregation of DMPG LUVs by LDL did depend on the ionic strength of the medium as wel l as on the phase state of the lipid. More specifically, below the mai n transition temperature T-m maximal aggregation was seen in the prese nce of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] w ere required when T> T-m. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylg lycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were nor aggregated. I nvolvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that i ncreasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at X-DMPC = 0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-ol eyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at X-POPG = 0.50 there was complete inhibition. The latter eff ect of POPG is likely to be due to augmented hydration of the unsatura ted lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strong ly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanc ed the aggregation and could partly reverse the inhibition by POPG. (C ) 1998 Elsevier Science B.V. All rights reserved.