Electron microscopic studies of human haemosiderin and ferritin

Ferritin and haemosiderin were isolated from fresh frozen human spleens that had been removed from patients with secondary iron overload due to multiple transfusions. Haemosiderin was solubilised by a novel technique that maintains its integrity. Unstained preparations of haemosiderin and ferritin were visualised and quantitative measurements made of the volumes of iron core. The mean diameter of the ferritin core (6.4 nm) was larger than that of haemosiderin (5.7 nm). In addition, haemosiderin, in contrast to ferritin, showed a large number of cores of less than 5 nm in diameter. Negatively stained preparations of haemosiderin and ferritin were visualised, confirming the small core size of the haemosiderin. The protein shell of haemosiderin, unlike that of ferritin, was thinner and irregular. These findings are consistent with the suggestion that haemosiderin is derived from ferritin by partial proteolysis and partial solubilisation of the iron core, presumably by lysosomal action.