Iron is not a trace element in mammalian physiology. Using textbook values for blood volume (5.5 L), red blood cell (RBC) count (5 million/muL), and a lifespan of 120 days for red blood cells, the equilibrium value for the erythrocyte generation/death rate in the average adult male human is over 2 million/sec. It follows that the amount of iron required for hemoglobin synthesis in one day amounts to about 25 mg. Virtually every atom of that 25 mg is recycled by macrophages of the reticuloendothelial system (RES) that provide iron to the plasma for its subsequent delivery back to the erythron (with a small fraction going to other tissues). In light of the certain toxicity of unprotected iron, both erythroid precursors and RES macrophages perform remarkable tasks in handling such copious amounts of the catalytic metal. In my studies, I have examined specific aspects of iron metabolism in these two tissues. / Iron is taken up by nearly every cell through a mechanism of receptor-mediated endocytosis, whereby the plasma iron binding protein transferrin (Tf) binds to its cognate receptor (TfR) on the cell surface, followed by internalization of the complex into a membrane bound organelle. Subsequent to endocytosis, the endosome is acidified by a v-ATPase proton pump, facilitating the release of iron from Tf. Through an unknown mechanism, iron is targeted to the inner membrane of the mitochondria, where the enzyme that inserts Fe2+ into protoporphyrin IX, ferrochelatase, resides. Although it has been demonstrated that the divalent metal transporter, DMT1, is responsible for the egress of reduced Fe from the vesicle, the immediate fate of the iron atoms after their transport across the vesicular membrane remains unknown. Therefore, we have investigated the uptake of iron in reticulocytes, cells that are taking up large amounts of iron for the synthesis of hemoglobin. Through both biochemical and imaging techniques, we have demonstrated that iron is transferred via a direct interorganellar relation between the endosome and mitochondria. / The "haemoglobin-deficit" (hbd) mouse has an erythroid-specific mutation which is responsible for its microcytic, hypochromic phenotype. Previous studies have shown that these mice have normal dietary iron acquisition and normal to elevated serum iron levels. We therefore investigated the handling of iron in reticulocytes from these animals to determine whether the mutated gene possibly plays a role in the trafficking of transferrin-iron-containing organelles. A systematic examination of the steps in the transferrin pathway revealed that the intracellular trafficking of the protein is compromised in the hbd mice. / The rapid turnover of iron by macrophages of the RES requires heme oxygenase-1 (HO-1), which catalyzes the rate-limiting step in heme degradation. This highly inducible enzyme, besides its major role in erythrocyte iron recycling, has been demonstrated to confer astonishing cytoprotectivity to cells and tissues in which its expression is elevated (either through chemical induction or genetic manipulation). In addition to, reportedly protective, carbon monoxide and biliverdin, the HO-1 catalyzed reaction releases ferrous iron, which itself is a potent pro-oxidant. Also, it is unlikely that there exists a significant amount of free heme in most tissues (i.e., non-erythroid, non-erythrophagocytic), to provide significant amounts of substrate to this enzyme. Hence, it is tempting to speculate that the mechanism of heme oxygenase cytoprotection is removed from its function of heme catabolism. Therefore, we investigated whether increased expression of heme oxygenase will, in and of itself, alter iron metabolism in cultured cells. My experiments show that in the absence of exogenous hemin, elevation of HO-1 protein levels does not have any effect on cellular iron metabolism in cultured cells.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.103181 |
| Date | January 2006 |
| Creators | Sheftel, Alexander D. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Physiology.) |
| Rights | © Alexander D. Sheftel, 2006 |
| Relation | alephsysno: 002585448, proquestno: AAINR32384, Theses scanned by UMI/ProQuest. |
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