670 THE PRECURSORS OF UREA 



produced artificially from proteids, and that when heated to 140 C. 

 it loses two molecules of water and is converted into urea. 



H0\ NH 4 0\ 



CO + 2NH 3 = C0-H 9 



HO/ NH 4 0/ 



(Ammonium 

 carbonate) 



NH 4 0\ NH 9 \ 



C0-H.,0 = CO. 



NH 2 / NH 2 / 



(Ammonium ( Urea) 



carbamate) 



According to this view the N in such a group as CH 2 NH 2 in 

 the amino-acid is converted into ammonia, combines with car- 

 bonate acid to form ammonium carbonate, and is dehydrated into 

 urea. A similar terminal synthesis with loss of water is known 

 to take place in the production of hippuric acid from benzoic acid 

 and glycin. There is no necessary antagonism between the views 

 of Schmiedeberg and Drechsel, for ammonium carbamate could be 

 looked upon as an intermediate product in the dehydration of 

 ammonium carbonate to urea. 



We have abundant evidence that ammonium salts are con- 

 stantly formed in the body, and, being highly poisonous, are con- 

 verted into a relatively innocuous body, urea. Ammonia is formed 

 during the autolysis of tissues, arterial blood contains 0'4 mgr. 

 per cent., the urine contains only small quantities, and not more 

 than about 5-10 per cent, of its total nitrogen is present as 

 ammonia. When the liver is put out of circulation or extensively 

 destroyed, the ammonia content of the blood and urine is increased 

 and the area decreased. Schroder has proved that ammonium salts 

 circulated through the surviving liver of a dog are converted into 

 urea. He found that not only the carbonate but any ammonium 

 salt whose acid radicle could be oxidised in the body, such as the 

 formate, lactate, citrate, and acetate, was converted into urea. 

 We have, therefore, conclusive evidence that ammonia is one of 

 the immediate precursors of urea, but we have no evidence which 

 shows how this conversion is carried out, and whether it takes 

 place by an oxidative synthesis or by a process of dehydration. 



(5) Origin of Urea from Ammonium Cyanate. This salt has 

 been produced artificially from proteid, and Wohler made the 



