2l8 DYEING 



ribonuclease, the mitochondria are still colourable by iron 

 haematein; ^'^^ this has been attributed to their lipoprotein con- 

 tent. ^^^ Other acidic lipids besides phospholipids may perhaps 

 also bind iron, but the idea that iron haematein attaches itself to 

 nothing in cells except acidic lipids ^^^ is untenable. 



Presumably the cationic iron complex first makes a salt-linkage 

 with the available acidic groups, but the characteristic reactions of 

 ferric iron are not given and it is thought that a non-ionizing 

 complex is formed with adjacent carboxyl and hydroxyl groups. ^^^ 



O 

 O 



NH 



I 

 HC(CH2)2C 



c=o 



I 

 NH 



"? 

 f 



Fe 



CH2( >0 



\— /H 



Diagrammatic representation of a possible boyiding of iron with glutamic acid 

 atid tyrosine residues in a protein chain. {The zchole of the iron complex is not 

 shown.) 



A theoretically possible arrangement in conformity with this 

 view is shown here. Two amino-acids of a protein are seen to be 

 holding the iron in their grip. Similar bonding could take place 

 with nucleic acids, through a sugar hydroxyl and a phosphoric 

 group. 



When iron has been taken up by the tissues, the compound into 

 which it has entered is readily broken down by ammonium sul- 

 phide and it appears once more in an ionic form. Everything is at 

 first coloured blue or blue-grey; but when the section is washed 

 in water and thus exposed to atmospheric oxygen, an interesting 

 differentiation occurs. Certain parts, such as the chromatin, 

 remain blue or bluish, while others, such as collagen and the 

 contractile substance of muscle, become brown. It is thought that 

 the iron is ferrous in the one case, ferric in the other. At first 

 the iron appears everywhere as ferrous sulphide, and remains 

 as such wherever there are reducers (such as sulphydryl groups) 

 in the proteins or other cellular constituents; but where there 

 are not, oxidation can occur, with development of a brown 

 colour. ^'^^ 



