Uptake and subcellular processing of 59Fe-125I-labelled transferrin by rat liver

Abstract

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.

ZRT/IRT-like Protein 14 (ZIP14) Promotes the Cellular Assimilation of Iron from Transferrin

“…In addition to clearing NTBI rapidly, the liver readily takes up TBI and assimilates the iron (8). Studies in isolated primary mouse and rat hepatocytes have shown that TBI can inhibit the uptake of NTBI (and vice versa), suggesting that NTBI and TBI share a common membrane iron transporter (9,23,24).…”

“…Studies in perfused rat liver document that the liver takes up TBI, almost exclusively into hepatocytes (8). In iron overload conditions, such as hereditary hemochromatosis, the liver can also take up non-TF-bound iron (NTBI), a form of iron that appears in the plasma when the iron-carrying capacity of TF becomes exceeded (9).…”

Iron transport proteins: Gateways of cellular and systemic iron homeostasis

“…2D). TBI is taken up by receptor-mediated endocytosis after it binds to transferrin-receptor 1 (TFR1) located at the sinusoidal membrane (67). Acidification of endosomes causes transferrin to release its Fe 3ϩ , which is subsequently reduced to Fe 2ϩ and then transported into the cytosol via DMT1.…”

Iron transfer from ferritin to transferrin

“…The apotransferrin remains firmly bound to its receptor and is recycled back to the plasma membrane where it dissociates from the receptor at the neutral pH of the extracellular fluid. The iron released within the cell is rapidly incorporated into cytosol ferritin and into mitochondria [5,6].…”