title: Studies of iron overload. Rat liver siderosome ferritin.
abstract: To investigate storage of ferritin and its transition to hemosiderin under conditions of iron overload, rats were either given multiple injections of iron dextran over 4 to 5 weeks or fed a diet containing 1.3% Fe as ferric ammonium citrate for 60 days. Then, preparations of liver siderosomes (heavily iron-laden lysosomes) were examined for content of buffer-soluble ferritin and buffer-insoluble, ferritin-related protein, total nonheme iron and protein, cathepsin D activity, and ability to incorporate 14C-leucine into ferritin. Total liver nonheme iron, ferritin protein and iron, and cathepsin D activity were also determined. Although parenteral iron loading produced higher total nonheme iron in livers than dietary loading, the iron content of ferritin was approximately 20% in both groups, reflecting saturation of ferritin with iron. Siderosome nonheme iron content was greater than 40% in relation to protein. The siderosomes contained little buffer-soluble ferritin; on isoelectric focusing this was composed of isoferritins present also in cytosol ferritin. Buffer-insoluble ferritin protein, identified in siderosomes by immunofluorescence, was solubilized and found to contain immunoreactive material corresponding to L and H subunits of buffer-soluble ferritin. Transmission electron microscopy indicated the presence of relatively large quantities of "ferritin" in siderosomes, and it is argued that this was mostly buffer insoluble (denatured) or represented ferritin [FeOOH]x cores divested of protein shells. Although siderosomes had substantial cathepsin D activity, the known resistance of ferritin to this and other proteases makes it unlikely that proteolysis is an early event in the decomposition of ferritin in siderosomes. Heavily iron-laden siderosomes did not take up newly labeled ferritin or ferritin protein or 14C-precursor within 24 hours of labeling, when 14C-labeled ferritin was abundant in cytosol. The author proposes a sequence of steps leading from sequestration of buffer-soluble cytosol ferritin to storage of insoluble "hemosiderin."
mesh_terms: D000818:Animals; D002402:Cathepsin D; D002403:Cathepsins / Q000378:metabolism; D005260:Female; D005293:Ferritins / Q000378:metabolism*; D006801:Humans; D007123:Immunoelectrophoresis, Two-Dimensional; D007501:Iron / Q000378:metabolism*; D007525:Isoelectric Focusing; D008099:Liver / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008854:Microscopy, Electron; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D005293:Ferritins; D007501:Iron; D002403:Cathepsins; D002402:Cathepsin D