Studies of iron overload. Rat liver siderosome ferritin

G W Richter
Laboratory Investigation; a Journal of Technical Methods and Pathology 1984, 50 (1): 26-35
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."

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