The Biochemistry of Ferritin 207 



after treatment of ferritin with iodoacetamide, o-iodosobenzoate, or 

 p-chloromercuribenzoate, a decrease in ferrous iron content was noted. 

 Since iodoacetamide and p-chloromercuribenzoate are known to react 

 only with sulfhydryl groups and not with iron, the result may be 

 explained if we assume that the function of the sulfhydryl groups in 

 active ferritin is to stabilize the ferrous iron against autoxidation, a 

 reaction which occurs spontaneously when ferrous iron is added to 

 water or to most proteins at neutral pH. Alteration of the sulfhydryl 

 groups would thus lead to autoxidation of ferrous iron and inactivation 

 of ferritin. It should be noted that chelation of metals like ferrous 

 iron or cuprous copper tends to stabilize the lower valence state of 

 these metals. 



The finding, mentioned previously, of equal vasodepressor activity 

 of ferritin fractions with varying total iron content was now reinvesti- 

 gated. Fractions were prepared from ferritin by serial precipitation 

 using increasing concentrations of ammonium sulfate. In this way a 

 number of fractions were obtained with decreasing ratios of total iron : 

 total nitrogen as the concentration of ammonium sulfate needed to 

 precipitate these fractions increased (Table 9). The sulfhydryl con- 

 Table 9. Relationship of Total Iron, Ferrous Iron, and SH groups in 



Ferritin Fraetions 



A. Mazur, S. Baez, and E. Shorr, ./. Biol. Chem., March, 1955. 



tent among these fractions was fairly constant with the exception of 

 that fraction containing the least total iron; it had a higher sulfhydryl 

 content and would correspond to a mixture of molecules relatively rich 

 in apoferritin. In contrast, the ratio of ferrous iron: total nitrogen 

 was constant for all fractions, a result to be expected if ferrous iron 

 were more specifically associated with ferritin activity. 



The other difficulty with the argument for iron participation in 

 ferritin activity is the equivalent activity of iron-free apoferritin. 

 There is no direct evidence for the reaction of apoferritin with plasma 



