Lauraeus S, Holopainen J M, Taskinen M R, Kinnunen P K
Biomembrane Research Group, Department of Medical Chemistry, Institute of Biomedicine, P.O. Box 8, University of Helsinki, Siltavuorenpenger 10A, Helsinki, FIN-00014, Finland.
Biochim Biophys Acta. 1998 Aug 14;1373(1):147-62. doi: 10.1016/s0005-2736(98)00102-3.
Turbidity (absorbance at 470 nm) measurements revealed human serum low density lipoprotein (LDL) to cause, within a few minutes and at physiological pH and [NaCl], the aggregation of liquid crystalline large unilamellar liposomes (LUVs) of dimyristoylphosphatidylglycerol (DMPG). No evidence for concomitant lipid or aqueous contents mixing was obtained with fluorescent assays for these processes, in keeping with the lack of fusion of LUVs. Involvement of apoB is implicated by the finding that tryptic digestion of LDL abrogates its ability to cause aggregation. 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 well as on the phase state of the lipid. More specifically, below the main transition temperature Tm maximal aggregation was seen in the presence of 25-100 mM NaCl, whereas slightly higher (up to 150 mM) [NaCl] were required when T>Tm. Aggregation due to LDL was also observed for dimyristoylphosphatidylserine as well as for dipalmitoylphosphatidylglycerol LUVs, whereas liposomes composed of either unsaturated acidic phospholipids or different phosphatidylcholines were not aggregated. Involvement of electrostatic attraction between the acidic phosphate of DMPG and cationic residues in apoB is suggested by the finding that increasing the content of dimyristoylphosphatidylcholine (DMPC) in DMPG liposomes reduced their aggregation and at XDMPC=0.50 no response was evident. Notably, increasing the mole fraction of 1-palmitoyl-2-oleyl-PG (POPG) in DMPG LUVs progressively reduced their aggregation by LDL and at XPOPG=0.50 there was complete inhibition. The latter effect of POPG is likely to be due to augmented hydration of the unsaturated lipid constituting a barrier for the contact between apoB and the vesicle surface. In keeping with this view, the presence of the strongly hygroscopic polymer, poly(ethylene glycol) at 1% (by weight) enhanced the aggregation and could partly reverse the inhibition by POPG.
浊度（470nm处的吸光度）测量结果显示，在生理pH值和[NaCl]条件下，人血清低密度脂蛋白（LDL）在几分钟内就能导致二肉豆蔻酰磷脂酰甘油（DMPG）的液晶态大单层脂质体（LUVs）发生聚集。对于这些过程，荧光检测未发现脂质或水相内容物混合的证据，这与LUVs未发生融合一致。载脂蛋白B的参与可通过以下发现得到证明：LDL经胰蛋白酶消化后丧失了其引起聚集的能力。极低密度脂蛋白（VLDL）、高密度脂蛋白2（HDL2）或高密度脂蛋白3（HDL3）不会引起聚集。有趣的是，氧化型LDL也无法使DMPG囊泡聚集。LDL引起的DMPG LUVs聚集确实取决于介质的离子强度以及脂质的相态。更具体地说，在低于主要转变温度Tm时，在25 - 100 mM NaCl存在下可见最大聚集，而当T>Tm时则需要略高（高达150 mM）的[NaCl]。二肉豆蔻酰磷脂酰丝氨酸以及二棕榈酰磷脂酰甘油LUVs也观察到了由LDL引起的聚集，而由不饱和酸性磷脂或不同磷脂酰胆碱组成的脂质体未发生聚集。DMPG的酸性磷酸基团与载脂蛋白B中的阳离子残基之间存在静电吸引，这一观点可通过以下发现得到证明：增加DMPG脂质体中二肉豆蔻酰磷脂酰胆碱（DMPC）的含量会降低其聚集，当XDMPC = 0.50时无明显反应。值得注意的是，增加DMPG LUVs中1 - 棕榈酰 - 2 - 油酰 - PG（POPG）的摩尔分数会逐渐降低其被LDL聚集的程度，当XPOPG = 0.50时完全抑制。POPG的后一种作用可能是由于构成载脂蛋白B与囊泡表面接触屏障的不饱和脂质的水合作用增强。与此观点一致的是，1%（按重量计）的强吸湿性聚合物聚乙二醇的存在增强了聚集，并可部分逆转POPG的抑制作用。
