The Biochemistry of Ferritin 201 



also associated with phosphate which is removed together with the iron 

 during this procedure (Table 2). Granick has assigned (FeOOH) 8 - 



Table 2. Elementary Analysis of Ferritin and Apoferritin 



A. Mazur, I. Litt, and E. Shorr, J. Biol. Client., 187, 473, 1950; analytical 

 data reported on the basis of dry weight. Various preparations of ferritin vary 

 in their ratio of N:Fe:P. 



(FeOOP0 3 H 2 ) as the formula for the colloidal micelles in ferritin. 



As a result of measurements of the paramagnetic susceptibility of 

 ferritin as well as of the ferric hydroxide-ferric phosphate prepared 

 from ferritin by alkali treatment, Michaelis 5 reported that the iron 

 atoms in this protein have an orbital arrangement corresponding to 3 

 unpaired electrons. Ferric iron may also exist with 1 or 5 unpaired 

 electrons. This type of orbital arrangement in ferritin iron makes it 

 unique among biological iron compounds and stresses the highly spe- 

 cific nature of the iron-incorporation reaction which takes place during 

 ferritin biosynthesis, since the iron compounds ingested with food rep- 

 resent all types of iron with respect to paramagnetic susceptibility. It 

 also points to a specific type of iron binding to the protein, the nature 

 of which is still unknown. 



The ultracentrifugal pattern obtained with ferritin solutions 6 indi- 

 cates that it is a mixture of molecules consisting of approximately 20 

 to 25% iron-free apoferritin together with a series of ferritin molecules 

 of varying total iron content and presumably in varying states of 

 aggregation. Apoferritin, however, behaves as a single component dur- 

 ing ultracentrifugation with a sedimentation constant corresponding 

 to a molecular weight of 465,000 (horse apoferritin). If considered as 

 an ellipsoid, apoferritin has an axial ratio of 3:1. 



In contrast to its behavior in the ultracentrifuge, ferritin behaves as 

 a single component on electrophoresis. Apoferritin has mobilities 

 identical with those of ferritin over a range of pH from 4 to 8.6, 2 indi- 

 cating that the large quantity of iron in ferritin does not affect the 

 surface charge density of the protein in solution at these pH's. Other 

 evidences of similarities of these two proteins from the point of view 

 of surface properties are: identical viscosities 7 calculated on the basis 

 of nitrogen content, and identical quantitative immunochemical re- 



