Two important pathogens of developing countries, Mycobacteium leprae, the etiologic agent of leprosy, and Leishmania donovani, the protozoal parasite that causes kalaazar, persist in the human host primarily in mononuclear phagocytes. The mechanims by which they survive in these otherwise highly cytocidal cells are presently unknown. Since the best understood cytocidal mechanism of these cells is the oxygen-dependent system that provides lethal oxidants including the superoxide anion (O-), hydrogen peroxide (H2O2

Like the rat peritoneal macrophage, the isolated Kupffer cell is capable of processing and releasing iron acquired by phagocytosis of immunosensitized homologous red blood cells. When erythrophagocytosis is restrained to levels which do not affect cell viability, about one red cell per macrophage, close to 50% of iron acquired from red cells is released within 24 hr in the form of ferritin. Immunoradiometric assay of the extracellular medium indicates that 160 ng ferritin are released by 10(6) Kupffer cells after 24-hr incubation at 37 degrees C. Iron release is temperature-dependent, the rate at 37 degrees C being nearly 5-fold greater than at 4 degrees C. As estimated by sucrose-gradient ultracentrifugation, ferritin released by the erythrophagocytosing Kupffer cell averages 2,400 iron atoms per molecule. When reincubated with isolated hepatocytes, this released ferritin is rapidly taken up by the cells. Via this process, hepatocytes may accumulate more than 160,000 iron atoms per cell per min. Such accumulation is not impeded by the presence of iron-loaded transferrin in the culture medium, but is markedly depressed by rat liver ferritin. In contrast to the conservation of transferrin during its interaction with hepatocytes, the protein shell of the ferritin molecule is rapidly degraded into trichloroacetic acid-soluble fragments. Ferritin-mediated transfer of iron from Kupffer cells to hepatocytes may help explain the resistance of the liver to iron deficiency as well as the liver's susceptibility to iron overload.

The subcellular localization of ferritin and its iron taken up by rat hepatocytes was investigated by sucrose-density-gradient ultracentrifugation of cell homogenates. After incubation of hepatocytes with 125I-labelled [59Fe]ferritin, cells incorporate most of the labels into structures equilibrating at densities where acid phosphatase and cytochrome c oxidase are found, suggesting association of ferritin and its iron with lysosomes or mitochondria. Specific solubilization of lysosomes by digitonin treatment indicates that, after 8 h incubation, most of the 125I is recovered in lysosomes, whereas 59Fe is found in mitochondria as well as in lysosomes. As evidenced by gel chromatography of supernatant fractions, 59Fe accumulates with time in cytosolic ferritin. To account for these results a model is proposed in which ferritin, after being endocytosed by hepatocytes, is degraded in lysosomes, and its iron is released and re-incorporated into cytosolic ferritin and, to a lesser extent, into mitochondria.

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The binding and uptake of 59Fe-loaded 'H-labelled rat transferrin by cultured rat hepatocytes was investigated. At 4"C, there is no evidence for a specific binding of transferrin which could be related to the association of neo-synthesized transferrin with plasma membrane receptors. At 37"C, iron uptake is much more important than transferrin uptake; it proceeds linearly over the time of incubation, is largely proportional to the extracellular transferrin concentration, and is compatible with uptake by fluid phase endocytosis. The difference observed between iron and transferrin uptake implies the existence of a mechanism allowing the reutilization of transferrin after iron delivery.Transferrin Iron Uptake