Bjornson L K, Gniewkowski C, Kayden H J
J Lipid Res. 1975 Jan;16(1):39-53.
The simultaneous exchange of (3h)tocopherol and (14C)cholesterol between rat plasma, rat plasma lipoproteins, and RBC was studied in vitro to compare quantitavely (a) the fractional exchange rates and (b) the half-times for isotope equilibration. In all incubations of RBC with plasma or with plasma lipoprotein fractions, (14C)cholesterol approached equilibrium more rapidly than (3H)tocopherol. When the RBC contained the initial radioactivity, the half-times for equilibration with plasma of cholesterol and of tocopherol were 1.0 and 2.2 hr, respectively. However, the fractional exchange rates (KRBC leads to plasma) were 0.097/hr for cholesterol and 0.188/hr for tocopherol, indicating that the RBC tocopherol pool is turning over almost twice as rapidly as the RBC cholesterol pool. The rat plasma lipoproteins were separated into five fractions by successive ultracentrifugation. Only two fractions, the high density lipoproteins (d 1.063-1.21) and the very low density lipoproteins (d is less than 1.006), participated to a significant extent in the exchange of either tocopherol or cholesterol with RBC. Cholesterol exchange between individual rat plasma lipoproteins and RBC had the same half-times for isotope equilibrium for the very low and high density lipoproteins, and the RBC fractional exchange rates were proportional to the amount of cholesterol in the lipoproteins. In tocopherol exchange between individual rat plasma lipoproteins and RBC, the very low density lipoprotein tocopherol did not equilibrate completely with the RBC. However, the initial rate of tocopherol exchange appeared to be the same for very low and high density lipoproteins. The very low density lipoproteins were disrupted by repeated freezing and thawing or by dehydrating and rehydrating, and analysis of the resulting lipoproteins indicated that free cholesterol was associated more closely than tocopherol with the phospholipid-protein portion of the molecule, which is thought to be on the surface. This difference in distribution of tocopherol and free cholesterol within very low density lipoproteins could account for their different rates of exchange and for the nonequilibrium of tocopherol between RBC and very low density lipoproteins.
在体外研究了大鼠血浆、大鼠血浆脂蛋白和红细胞之间（3H）生育酚与（14C）胆固醇的同时交换，以定量比较（a）分数交换率和（b）同位素平衡的半衰期。在红细胞与血浆或血浆脂蛋白组分的所有孵育中，（14C）胆固醇比（3H）生育酚更快地达到平衡。当红细胞含有初始放射性时，胆固醇和生育酚与血浆平衡的半衰期分别为1.0小时和2.2小时。然而，分数交换率（红细胞→血浆）胆固醇为0.097/小时，生育酚为0.188/小时，这表明红细胞生育酚池的周转速度几乎是红细胞胆固醇池的两倍。通过连续超速离心将大鼠血浆脂蛋白分离成五个组分。只有两个组分，即高密度脂蛋白（d 1.063 - 1.21）和极低密度脂蛋白（d小于1.006），在很大程度上参与了生育酚或胆固醇与红细胞的交换。单个大鼠血浆脂蛋白与红细胞之间的胆固醇交换，极低密度脂蛋白和高密度脂蛋白的同位素平衡半衰期相同，并且红细胞分数交换率与脂蛋白中的胆固醇含量成正比。在单个大鼠血浆脂蛋白与红细胞之间的生育酚交换中，极低密度脂蛋白生育酚与红细胞未完全平衡。然而，极低密度脂蛋白和高密度脂蛋白的生育酚交换初始速率似乎相同。极低密度脂蛋白通过反复冻融或脱水再水化而被破坏，对所得脂蛋白的分析表明，游离胆固醇比生育酚更紧密地与分子的磷脂 - 蛋白质部分相关联，而磷脂 - 蛋白质部分被认为在表面。生育酚和游离胆固醇在极低密度脂蛋白内的这种分布差异可以解释它们不同的交换速率以及红细胞与极低密度脂蛋白之间生育酚的非平衡状态。
