PROTEIN METABOLISM 873 



take an important part in the building up of the molecule of haematin, 

 the'essential prosthetic group of the haemoglobin molecule. 



Another ring grouping, iminazol, occurs in histidine, which is 

 iminazol rt-amino-propionic acid. This too undergoes complete oxida- 

 tion in the body. It is important to bear in mind that this ring may 

 be produced synthetically by very simple means, i.e. by the action of 

 zinc oxide*and ammonia on glucose, which results in a rich yield of 

 methyl iminazol (v. p. 130). The same grouping is found in creatinine, 

 as is seen by comparing the formulae : 



H,C N< HC -NH 



| > C=NH || >CH 



OC NH HC N 



Creatinine Iminazol 



and it is possible that this may furnish a clue to the mode of formation 

 of creatinine in muscle. Creatine has generally been regarded as 

 the primary product of muscular metabolism, but it is possible that 

 the ring- grouping is the original one and that creatine is produced 

 by hydrolysis occurring in this ring. 



The iminazol group is at present chiefly interesting in that it 

 contributes to the formation of the complex ring compounds known 

 as the purines. Since the purine metabolism is closely connected 

 with the question of the origin of uric acid we may consider these 

 questions together. 



