NONHEMATIN IRON IN TISSUES 563 



of hemoglobin iron in the erythrocyte made it difficult to detect the 

 presence of other forms of iron, and the chemical pitfalls in the inter- 

 pretation of "easily detachable iron" have been discussed in Chap- 

 ter X. The normal level of bile pigment iron in the erythrocytes is 

 probably about one per cent of the total iron, and may be increased 

 under pathological conditions. 



It may be considered almost certain that this iron fraction is 

 identical with that found histologically in the erythrocyte by the 

 appropriate methods. Iron granules were first reported in erythro- 

 cytes at the end of last century (cf. Jt.16), but their significance has 

 recently been established by the work of Griineberg (1065) and Case 

 {Jf.16). The former of these two workers found that their occurrence 

 was under genetic control in mice (cf. Chapter XIII), and concluded 

 that the cells containing iron were young cells. Case has provided 

 good evidence, however, that they are aged cells (cf. Sections 5.2. 

 and 5.3.) although there is evidence that an easily detachable iron 

 fraction may exist in immature avian erythrocytes (cf. Chapter XIII, 

 Section 4.). 



The significance of bile pigment iron in tissues other than the cir- 

 culating blood must be related to the role these tissues play in the 

 catabolism of hemoglobin as well as to their content of other heme 

 compounds. Thus, the catalase content of the liver would make some 

 contribution to the bile pigment iron content of this organ although 

 this may be negligible. 



In view of the fact that analytical methods which are able to distinguish 

 between bile pigment iron and the various forms of storage iron are lacking, 

 the most accurate value for this fraction can best be obtained by calculation 

 from the content of bile pigment hematin determined spectrophotometrically 

 or perhaps by estimation of pigments such as biliverdin. In the presence of 

 complex mixtures, such methods are only able to give very approximate 

 results. At present the histochemical methods for iron (cf. J^lGJOSJ^j suffer 

 from similar defects although they enable histological differentiation of cells 

 of importance for iron metabolism. 



10.3.2. Transport Iron. Normal human plasma contains 50-250 

 Mg. iron per 100 ml., the average value for men (120-140 Mg-) being 

 significantly higher than for women (90-115 Mg) (H9,151,928,1221, 

 2^09,28U,28Ur29S8). In the plasma of various animals quite 

 similar values have been found (152,915,1242,2180). ' 



The importance of the plasma iron as transport iron has been cor- 

 rectly stressed by Moore and co-workers (1979,1982); the assumption 



