Iacopetta B J, Morgan E H
J Biol Chem. 1983 Aug 10;258(15):9108-15.
The endocytosis of diferric transferrin and accumulation of its iron by freshly isolated rabbit reticulocytes was studied using 59Fe-125I-transferrin. Internalized transferrin was distinguished from surface-bound transferrin by its resistance to release during treatment with Pronase at 4 degrees C. Endocytosis of diferric transferrin occurs at the same rate as exocytosis of apotransferrin, the rate constants being 0.08 min-1 at 22 degrees C, 0.19 min-1 at 30 degrees C, and 0.45 min-1 at 37 degrees C. At 37 degrees C, the maximum rate of transferrin endocytosis by reticulocytes is approximately 500 molecules/cell/s. The recycling time for transferrin bound to its receptor is about 3 min at this temperature. Neither transferrin nor its receptor is degraded during the intracellular passage. When a steady state has been reached between endocytosis and exocytosis of the ligand, about 90% of the total cell-bound transferrin is internal. Endocytosis of transferrin was found to be negligible below 10 degrees C. From 10 to 39 degrees C, the effect of temperature on the rate of endocytosis is biphasic, the rate increasing sharply above 26 degrees C. Over the temperature range 12-26 degrees C, the apparent activation energy for transferrin endocytosis is 33.0 +/- 2.7 kcal/mol, whereas from 26-39 degrees C the activation energy is considerably lower, at 12.3 +/- 1.6 kcal/mol. Reticulocytes accumulate iron atoms from diferric transferrin at twice the rate at which transferrin molecules are internalized, implying that iron enters the cell while still bound to transferrin. The activation energies for iron accumulation from transferrin are similar to those of endocytosis of transferrin. This study provides further evidence that transferrin-iron enters the cell by receptor-mediated endocytosis and that iron release occurs within the cell.
使用59Fe-125I-转铁蛋白研究了新鲜分离的兔网织红细胞对双铁转铁蛋白的内吞作用及其铁的积累。内化的转铁蛋白通过其在4℃用链霉蛋白酶处理期间对释放的抗性与表面结合的转铁蛋白区分开来。双铁转铁蛋白的内吞作用与脱铁转铁蛋白的胞吐作用速率相同，在22℃时速率常数为0.08 min-1，在30℃时为0.19 min-1，在37℃时为0.45 min-1。在37℃时，网织红细胞对转铁蛋白的最大内吞速率约为500分子/细胞/秒。在此温度下，与受体结合的转铁蛋白的循环时间约为3分钟。在细胞内转运过程中，转铁蛋白及其受体均未降解。当配体的内吞作用和胞吐作用达到稳态时，约90%的总细胞结合转铁蛋白被内化。发现在10℃以下转铁蛋白的内吞作用可忽略不计。在10至39℃之间，温度对内吞速率的影响是双相的，在26℃以上速率急剧增加。在12-26℃的温度范围内，转铁蛋白内吞作用的表观活化能为33.0±2.7千卡/摩尔，而在26-39℃时活化能则低得多，为12.3±1.6千卡/摩尔。网织红细胞从双铁转铁蛋白中积累铁原子的速率是转铁蛋白分子内化速率的两倍，这意味着铁在仍与转铁蛋白结合时进入细胞。转铁蛋白中铁积累的活化能与转铁蛋白内吞作用的活化能相似。这项研究提供了进一步的证据，表明转铁蛋白-铁通过受体介导的内吞作用进入细胞，并且铁在细胞内释放。
