126 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



carriers. This use is characteristic of a group of animals, the sipunculids. 

 The absorption spectra of the haemerythrins are analogous to those of the 

 haemocyanins. Like the latter, they do not show the absorption character- 

 istics of haem derivatives that we have noted in the case of the haemoglobins, 

 cytochromes and hydroperoxidases. Oxyhaemerythrin is wine-red whilst 

 haemerythrin is colourless. The spectrum of oxyhaemerythrin is like that 

 of ferriproteins. The iron of haemerythrin, like the copper of haemocyanin, 

 appears to be attached to an -SH group. 



2. Ferritin 



Ferritin is a compound of a protein with an iron hydroxide whose formula 

 is [(FeOOH)8.(FeOP03H2)]. In certain organisms, and particularly in 

 mammals, it serves as a means of storing iron in organs such as the liver 

 and spleen. 



2 



5000 



4000 



3000 



2000 



.& 



Fig. 26 (Florkin) — Absorption spectrum of the haemerythrin from Sipunculus nudus. 



3. Transferrin (siderophilin) 



In the blood plasma of mammals there is present a ^-pseudo-globulin 

 called transferrin or siderophilin, and which at a definite point in its mole- 

 cule forms an iron complex. This protein, whose molecular weight is in 

 the region of 90,000, makes up about 3% by weight of the plasma proteins. 

 The iron is complexed by it in the ferric form and only in the presence of 

 CO2. When the level of iron in the plasma is at its normal value, which in 

 man is 129y/100 ml, the transferrin is saturated to the extent of 30% of 

 the maximum amount of iron which it can carr}\ The same protein can 

 also fix copper. 



