Mao S J, Downing M R, Kottke B A
Biochim Biophys Acta. 1980 Oct 6;620(1):100-10. doi: 10.1016/0005-2760(80)90189-7.
Pigeon plasma high density lipoproteins (HDL) were isolated by ultracentrifugation between the densities of 1.063 and 1.21 g/ml. Gel filtration of delipidated HDL in 5 M guanidine-HCl on Sephadex G-150 yielded a major fraction which eluted at the same position as human apolipoprotein A-I isolated from HDL. In SDS-gel electrophoresis, the isolated apolipoprotein co-migrated with human apolipoprotein A-I with a molecular weight of approx. 28 000. The amino acid composition was similar to the apolipoprotein A-I isolated from human and hen plasma. The isolated apolipoprotein from pigeon plasma had therefore been designated as apolipoprotein A-I. As judged by circular dichroism (CD), the apolipoprotein A-I displayed a maximum mean residue ellipticity of approx. -3 000 at 222 nm while at concentrations greater than 0.2 mg/ml. Calculations of alpha-helicial content gave values of 85%. Lowering the concentration of apolipoprotein A-I was found to concomitantly decrease the ellipticity (absolute value) suggesting that there was some conformational change when the apolipoprotein A-I concentration varied. The isolated pigeon apolipoprotein A-I was found bound to the phospholipid (dimyristoyl phosphatidylcholine) and there was no significant conformational change upon lipid binding as judged by CD. Under the same experimental conditions, human apolipoprotein A-I exhibited a drastic conformational change by increasing its helicity in the presence of phospholipid. The helical content of human apolipoprotein A-I was increased from 48 to 85%. This finding suggests that the apolipoprotein may not necessarily increase its helical content during lipid binding. Moreover, immunochemical studies showed that rabbit antiserum prepared against pigeon apolipoprotein A-I could partially react with human apolipoprotein A-I determined by quantitative radioimmunoassay.
通过在密度为1.063至1.21 g/ml之间进行超速离心分离鸽血浆高密度脂蛋白（HDL）。在5 M盐酸胍中对脱脂HDL进行Sephadex G - 150凝胶过滤，得到一个主要组分，其洗脱位置与从HDL中分离的人载脂蛋白A - I相同。在SDS - 凝胶电泳中，分离的载脂蛋白与人载脂蛋白A - I共迁移，分子量约为28000。氨基酸组成与从人及鸡血浆中分离的载脂蛋白A - I相似。因此，从鸽血浆中分离的载脂蛋白被命名为载脂蛋白A - I。通过圆二色性（CD）判断，载脂蛋白A - I在222 nm处显示出约 - 3000的最大平均残基椭圆率，而浓度大于0.2 mg/ml时。α - 螺旋含量计算值为85%。发现降低载脂蛋白A - I的浓度会相应降低椭圆率（绝对值），这表明当载脂蛋白A - I浓度变化时存在一些构象变化。发现分离的鸽载脂蛋白A - I与磷脂（二肉豆蔻酰磷脂酰胆碱）结合，通过CD判断，脂质结合后没有明显的构象变化。在相同实验条件下，人载脂蛋白A - I在磷脂存在下通过增加其螺旋度表现出剧烈的构象变化。人载脂蛋白A - I的螺旋含量从48%增加到85%。这一发现表明载脂蛋白在脂质结合过程中不一定会增加其螺旋含量。此外，免疫化学研究表明，通过定量放射免疫测定，用鸽载脂蛋白A - I制备的兔抗血清可与人载脂蛋白A - I部分反应。
