Morrison J, Fidge N H, Tozuka M
Protein Chemistry and Molecular Biology Unit, Baker Medical Research Institute, Prahran, Victoria, Australia.
J Biol Chem. 1991 Oct 5;266(28):18780-5.
There is good evidence that high density lipoprotein (HDL) interacts with high affinity sites present on hepatocytes. The precise nature of the ligand recognized by putative HDL receptors remains controversial, although there is a consensus that apolipoprotein AI (apoAI) is involved. This suggestion would be strengthened if a biologically active site demonstrating a high affinity for the receptor could be isolated. Cyanogen bromide fragments (CF) of apoAI (CF1-CF4) were complexed with phospholipid, and their ability to associate with the receptor was compared in various binding studies. Careful analysis of the concentration-dependent association of 125I-labeled dimyristoyl phosphatidylcholine (DMPC) recombinants to rat liver plasma membranes revealed high and low affinity binding components. As all DMPC recombinants displayed the low affinity binding component, it was postulated that this interaction was independent of the protein present in the particle and may well represent a lipid-lipid or lipid-protein association with the membranes. Only 125I-labeled CF4.DMPC displayed a high affinity binding component with similar Kd and Bmax (8 x 10(-9) M, 1.6 x 10(-12) mol/mg plasma membrane protein) to that of 125I-labeled AI.DMPC (7 x 10(-9), 1.4 x 10(-12) mol/mg plasma membrane protein). Similarly, egg yolk phosphatidylcholine complexes containing CF4 (CF4.egg PC) showed higher affinity binding than CF1-egg yolk phosphatidylcholine complexes confirming the results obtained with DMPC complexes. Furthermore, ligand blotting studies showed that only 125I-labeled CF4.DMPC associated specifically with HB1 and HB2, two HDL binding proteins recently identified in rat liver plasma membranes. We conclude that a region within the carboxyl-terminus of apoAI is responsible for the interaction with putative HDL receptors present in rat liver plasma membranes.
有充分证据表明高密度脂蛋白（HDL）与肝细胞上存在的高亲和力位点相互作用。尽管人们普遍认为载脂蛋白AI（apoAI）参与其中，但假定的HDL受体识别的配体的确切性质仍存在争议。如果能够分离出对该受体具有高亲和力的生物活性位点，这一观点将得到加强。将apoAI的溴化氰片段（CF）（CF1 - CF4）与磷脂复合，并在各种结合研究中比较它们与受体结合的能力。对125I标记的二肉豆蔻酰磷脂酰胆碱（DMPC）重组体与大鼠肝细胞膜的浓度依赖性结合进行仔细分析，发现了高亲和力和低亲和力结合成分。由于所有DMPC重组体都显示出低亲和力结合成分，因此推测这种相互作用与颗粒中存在的蛋白质无关，很可能代表脂质 - 脂质或脂质 - 蛋白质与膜的结合。只有125I标记的CF4.DMPC显示出高亲和力结合成分，其Kd和Bmax（8×10^(-9) M，1.6×10^(-12) mol/mg质膜蛋白）与125I标记的AI.DMPC（7×10^(-9)，1.4×10^(-12) mol/mg质膜蛋白）相似。同样，含有CF4的蛋黄磷脂酰胆碱复合物（CF4.egg PC）显示出比CF1 - 蛋黄磷脂酰胆碱复合物更高的亲和力结合，证实了用DMPC复合物获得的结果。此外，配体印迹研究表明，只有125I标记的CF4.DMPC与HB1和HB2特异性结合，HB1和HB2是最近在大鼠肝细胞膜中鉴定出的两种HDL结合蛋白。我们得出结论，apoAI羧基末端的一个区域负责与大鼠肝细胞膜中假定的HDL受体相互作用。
