Brainard J R, Knapp R D, Morrisett J D, Pownall H J
J Biol Chem. 1984 Aug 25;259(16):10340-7.
The most abundant lipid and protein components of human plasma high density lipoproteins are phosphatidylcholine and apolipoprotein A-I (A-I). Under appropriate conditions, A-I spontaneously associates with dimyristoylphosphatidylcholine (DMPC) to quantitatively form a lipid-protein complex with a DMPC/A-I molar ratio of 100:1. Differential scanning calorimetry of this complex reveals two broad thermal transitions centered at approximately 27 and 72 degrees C. 13C NMR spectra of the complex have been obtained above, at, and below the lower transition temperature. The 13C resonance arising from the 3' carbon of the fatty acyl chains is a doublet, split by approximately 0.2 ppm, suggesting that the 3' carbon nuclei occupy two magnetically inequivalent sites. By replacing the sn-2 fatty acyl chain with myristate selectively 13C-enriched at carbon 3', we have shown that the splitting is, in fact, a result of magnetic inequivalence of the two sites and have assigned the lower field resonance to the 3' carbon nucleus of the sn-2 chain. The temperature dependence of the NMR relaxation rates indicates that the endothermic transition at 27 degrees C is associated with increased motional freedom for the phospholipids within this complex. The temperature dependence of the fatty acyl chain methylene 13C chemical shifts suggests that the population of gauche conformers increases above the transition temperature. These dynamic and conformational changes are characteristic of gel----liquid crystalline phase transitions observed in pure phospholipid systems. For the DMPC-A-I complex at 37 degrees C, the chemical shifts of the fatty acyl C 4'- 11' methylene envelope and of the C 7' and C 13' resonances occur significantly downfield from the corresponding chemical shifts for the DMPC vesicle. These results suggest that the apoprotein rigidifies the acyl chains by increasing their number of trans conformers.
人血浆高密度脂蛋白中含量最丰富的脂质和蛋白质成分是磷脂酰胆碱和载脂蛋白A-I（A-I）。在适当条件下，A-I可自发地与二肉豆蔻酰磷脂酰胆碱（DMPC）缔合，以定量形成DMPC/A-I摩尔比为100:1的脂蛋白复合物。对该复合物进行差示扫描量热法分析，可发现两个宽的热转变峰，中心温度分别约为27℃和72℃。已在较低转变温度之上、该温度以及该温度之下获得了该复合物的13C NMR谱。由脂肪酰链3'位碳产生的13C共振为双峰，分裂约0.2 ppm，这表明3'位碳核占据两个磁不等价位点。通过用在碳3'位选择性富集13C的肉豆蔻酸取代sn-2脂肪酰链，我们已证明这种分裂实际上是两个位点磁不等价的结果，并将低场共振归属于sn-2链的3'位碳核。NMR弛豫速率的温度依赖性表明，27℃的吸热转变与该复合物中磷脂运动自由度的增加有关。脂肪酰链亚甲基13C化学位移的温度依赖性表明，高于转变温度时，gauche构象体的数量增加。这些动态和构象变化是在纯磷脂体系中观察到的凝胶-液晶相转变的特征。对于37℃下的DMPC-A-I复合物，脂肪酰C 4'-11'亚甲基包络以及C 7'和C 13'共振的化学位移相对于DMPC囊泡的相应化学位移显著向低场移动。这些结果表明，载脂蛋白通过增加反式构象体的数量使酰链刚性化。
