168 BIOLOGICAL CHEMISTRY 



-^ Alkaline methaemoglobin 



by alkali .s \ 



by reducing agents 



by acid ^ O 2 CO Carbon 



Methaemoglobin ^ Oxyhaemoglobin - -> Reduced > monoxide 



7 Q haemoglobin haemoglobin 



Globin 

 removed 



Globin 

 removed 



Globin 

 removed 

 by alkali 



^ acid N^ O 2 -^ 



Oxyhaematin in + oxyhaematiu > Reduced haematin 



acid in alkali TO 



alkali "" 2 Iron removed by 



acid 



* 



H aematoporphy rin 



Bile pigments 



DIAGRAM OF RELATION OF HAEMOGLOBIN TO ITS COMPOUNDS AND 

 DERIVATIVES. 



The decomposition of haemoglobin proceeds by the formation 

 of Haematin, which contains the iron, and a protein (globin) 

 belonging to ithe histone class. By removing iron from 

 haematin haematoporphyrin is formed. Both haematin and 

 haematoporphyrin can exist in acid and alkaline forms. 

 Haematoporphyrin is closely allied to such substances as the 

 bile pigments. 



Haematoporphyrin when decomposed is found to contain 

 pyrrole groups which are identical with the decomposition 

 products of chlorophyll, so that we see that there is a structural 

 relationship between the two important animal and vegetable 

 pigments. 



Haemoglobin and many of its derivatives can be easily 

 crystallised. This can be brought about by lowering the 

 solubility (by alcohol, salts, etc.) and at the same time cooling 

 the solution. 



One of the crystalline decomposition products is Haemin. 

 Blood pigment is heated with glacial acetic acid, and if it is 

 an old sample from which the chlorides have been removed 

 by washing a crystal of sodium chloride is added. On cooling 

 dark brown crystals appear. This is the well-known Teich- 

 mann's Test for blood* and the crystals are composed of the 

 chloride of haematin. Other solvents, such as acetone, can 

 be used instead of glacial acetic acid. 



* L. Teichmann, Zeit. /. rat. Med., 1853, vol. 3, p. 375. 



