200 



Essays in Biochemistry 



The Bulk of Ferritin Iron 



Ferritin is found mostly in the liver and spleen and to a lesser extent 

 in bone marrow, kidneys, and placenta, and in much smaller quantities 

 in skeletal muscle, testes, and pancreas. It has also been reported to 

 be present in the intestinal mucosa of the anemic guinea pig in re- 

 sponse to iron feeding. 1 Table 1 lists quantitative data for the ferritin 



Table 1. Ferritin Content of Various Tissues 



A. Mazur and E. Shorr, J. Biol. Chem., 182, 607 (1950); determined by the quantitative immuno- 

 chemical method. 



content of dog tissues 2 and human placenta, obtained by the quantita- 

 tive immunochemical method of Heidelberger. The best source for 

 the isolation of crystalline ferritin is horse spleen, from which it is 

 prepared by the method of Granick 3 using CdS0 4 for crystallization 

 of the protein as first recommended by Laufberger. 4 Concentrated 

 ferritin solutions free of inorganic ions are obtained by dialysis and 

 can be stored in sterile bottles in the refrigerator after filtration through 

 a Seitz filter. Such ferritin preparations have a low cadmium content 

 and are quite stable. 



The bulk of iron in ferritin appears to exist in the form of colloidal 

 ferric hydroxide, since the visible absorption spectrum is identical at 

 equivalent concentrations of iron with that of solutions of colloidal 

 ferric hydroxide. The iron is tightly held by the protein but can be 

 removed by prolonged dialysis of ferritin solutions in the presence of 

 sodium hydrosulfite at pH. 4.6 in concentrated acetate buffer and 

 a,a'-dipyridyl. This treatment results in the reduction of ferric to 

 ferrous iron and the removal of the latter by complexing with di- 

 pyridyl. After several such treatments and dialysis against water, 

 the protein is undenatured and essentially colorless. Addition of 

 CdS0 4 produces crystals of apoferritin, almost entirely free of iron, 

 and identical in form with those obtained from ferritin. Ferritin is 



