Morgan E H, Smith G D, Peters T J
Biochem J. 1986 Jul 1;237(1):163-73. doi: 10.1042/bj2370163.
The uptake of transferrin and iron by the rat liver was studied after intravenous injection or perfusion in vitro with diferric rat transferrin labelled with 125I and 59Fe. It was shown by subcellular fractionation on sucrose density gradients that 125I-transferrin was predominantly associated with a low-density membrane fraction, of similar density to the Golgi-membrane marker galactosyltransferase. Electron-microscope autoradiography demonstrated that most of the 125I-transferrin was located in hepatocytes. The 59Fe had a bimodal distribution, with a larger peak at a similar low density to that of labelled transferrin and a smaller peak at higher density coincident with the mitochondrial enzyme succinate dehydrogenase. Approx. 50% of the 59Fe in the low-density peak was precipitated with anti-(rat ferritin) serum. Uptake of transferrin into the low-density fraction was rapid, reaching a maximal level after 5-10 min. When livers were perfused with various concentrations of transferrin the total uptakes of both iron and transferrin and incorporation into their subcellular fractions were curvilinear, increasing with transferrin concentrations up to at least 10 microM. Analysis of the transferrin-uptake data indicated the presence of specific transferrin receptors with an association constant of approx. 5 X 10(6) M-1, with some non-specific binding. Neither rat nor bovine serum albumin was taken up into the low-density fractions of the liver. Chase experiments with the perfused liver showed that most of the 125I-transferrin was rapidly released from the liver, predominantly in an undegraded form, as indicated by precipitation with trichloroacetic acid. Approx. 40% of the 59Fe was also released. It is concluded that the uptake of transferrin-bound iron by the liver of the rat results from endocytosis by hepatocytes of the iron-transferrin complex into low-density vesicles followed by release of iron from the transferrin and recycling of the transferrin to the extracellular medium. The iron is rapidly incorporated into mitochondria and cytosolic ferritin.
在用125I和59Fe标记的二价铁大鼠转铁蛋白进行静脉注射或体外灌注后，研究了大鼠肝脏对转铁蛋白和铁的摄取情况。通过蔗糖密度梯度亚细胞分级分离表明，125I-转铁蛋白主要与低密度膜部分相关，其密度与高尔基体膜标记物半乳糖基转移酶相似。电子显微镜放射自显影显示，大部分125I-转铁蛋白位于肝细胞中。59Fe具有双峰分布，一个较大的峰位于与标记转铁蛋白相似的低密度处，一个较小的峰位于与线粒体酶琥珀酸脱氢酶一致的较高密度处。低密度峰中约50%的59Fe可被抗（大鼠铁蛋白）血清沉淀。转铁蛋白摄取到低密度部分的过程很快，5-10分钟后达到最高水平。当用不同浓度的转铁蛋白灌注肝脏时，铁和转铁蛋白的总摄取量以及它们在亚细胞部分的掺入量呈曲线关系，随着转铁蛋白浓度增加至至少10 microM而增加。对转铁蛋白摄取数据的分析表明存在特异性转铁蛋白受体，其结合常数约为5×10(6) M-1，同时存在一些非特异性结合。大鼠和牛血清白蛋白均未被摄取到肝脏的低密度部分。对灌注肝脏进行追踪实验表明，大部分125I-转铁蛋白迅速从肝脏释放，主要以未降解的形式，这通过三氯乙酸沉淀得以表明。约40%的59Fe也被释放。结论是，大鼠肝脏对转铁蛋白结合铁的摄取是由于肝细胞将铁-转铁蛋白复合物通过胞吞作用摄入低密度囊泡，随后铁从转铁蛋白中释放，转铁蛋白再循环至细胞外介质。铁迅速掺入线粒体和胞质铁蛋白中。
