Baker E, van Bockxmeer F M, Morgan E H
Q J Exp Physiol. 1983 Jul;68(3):359-72. doi: 10.1113/expphysiol.1983.sp002731.
The quantity and distribution of transferrin and transferrin-binding sites in the placenta were investigated in rabbits on the 28th-29th days of pregnancy. The animals were injected intravenously with a mixture of 59Fe-125I-labelled rabbit diferric transferrin and 131I-labelled rabbit albumin. The binding of transferrin to placentas removed 3-75 min later was determined by using the 131I-labelled albumin values to correct for tissue content of plasma. Mean values for transferrin binding of 1460 and 560 micrograms/g tissue were obtained 3-15 and 45-75 min after injection, respectively. Gel filtration of placental extracts prepared with the non-ionic detergent, Teric 12A9, showed that the 125I-labelled transferrin bound to a large molecular weight component which had the properties of a specific receptor. The receptor had a higher affinity for diferric transferrin than for apotransferrin. The subcellular distribution of transferrin binding sites was determined by differential centrifugation of placental homogenates and by electron microscope autoradiography. The results with the former method indicated that the transferrin was bound to the microsomal fraction of the cells. Autoradiography showed that the majority of the transferrin molecules were at intracellular sites, mainly on the membrane of intracellular vesicles. It is concluded that iron-containing transferrin molecules enter the trophoblast cells by endocytosis or via a canalicular system after binding to cell membrane receptors. The higher affinity of the receptors for diferric transferrin than for apotransferrin explains the difference in amount of transferrin binding found within 15 min of injecting labelled diferric transferrin and that found 45-75 min later when much of the iron had been removed from the transferrin.
在妊娠第28 - 29天的兔子中，研究了胎盘中转铁蛋白及其结合位点的数量和分布。给动物静脉注射59Fe - 125I标记的兔双铁转铁蛋白和131I标记的兔白蛋白的混合物。3 - 75分钟后取出胎盘，通过用131I标记的白蛋白值校正血浆组织含量来测定转铁蛋白与胎盘的结合。注射后3 - 15分钟和45 - 75分钟，转铁蛋白结合的平均值分别为1460和560微克/克组织。用非离子去污剂Teric 12A9制备的胎盘提取物进行凝胶过滤，结果显示125I标记的转铁蛋白与一种具有特异性受体性质的大分子成分结合。该受体对双铁转铁蛋白的亲和力高于对脱铁转铁蛋白的亲和力。通过胎盘匀浆的差速离心和电子显微镜放射自显影来确定转铁蛋白结合位点的亚细胞分布。前一种方法的结果表明转铁蛋白与细胞的微粒体部分结合。放射自显影显示大多数转铁蛋白分子位于细胞内位点，主要在细胞内小泡的膜上。结论是含铁的转铁蛋白分子在与细胞膜受体结合后，通过内吞作用或经由小管系统进入滋养层细胞。受体对双铁转铁蛋白的亲和力高于对脱铁转铁蛋白的亲和力，这解释了注射标记的双铁转铁蛋白后15分钟内发现的转铁蛋白结合量与45 - 75分钟后发现的转铁蛋白结合量之间的差异，此时转铁蛋白中的大部分铁已被去除。
