THE PRECURSORS OF UREA 671 



classical discovery that it became converted into urea gradually, 

 at ordinary temperatures, and at once at 100 C. Dry ammonia 

 and cyanic acid (CN . OH) together produce ammonium cyanate, 

 and the view supposes that during proteid katabolism in the 

 body these two substances are formed. Cyanates are produced 

 by the oxidation of cyanides, and several observers have obtained 

 prussic acid during the oxidation of proteid. Plimmer has re- 

 cently shown that many proteids when oxidised with equal parts 

 of strong nitric and sulphuric acids yield constantly more than 0*5 

 per cent, by weight of prussic acid. He further showed that 

 the products of hydrolysis of the proteid gave the same yield. 

 He found that the prussic acid came neither from the mono-amino 

 acids nor the bulk of it from the hexone bases, and he was unable 

 to determine its origin farther than by showing that tyrosin 

 yielded appreciable quantities. Oxidation of the same proteids 

 with chromic acid gave constantly a greater yield than oxidation 

 with the nitric acid mixture, and in this case he was able to 

 show that none of the prussic acid arose from tyrosin but 

 chiefly from the amino-acids, glycin and aspartic acid. 



Although this view is chemically possible, it lacks the necessary 

 physiological evidence that cyanates do occur in the body. 



Besides arginin and the various ammonium compounds already 

 considered, there are other intermediate products of nitrogenous 

 metabolism which can be converted into urea by chemical means, 

 and which are either possible sources of urea in the body or 

 are already known to be converted into urea or some similar 

 substance. 



(a) The Mono-amino Acids. There are two places in the body 

 where mono-amino acids might be formed from proteid, the 

 alimentary canal and the tissues. In the alimentary canal we 

 know that these bodies are produced, and we have already seen 

 that erepsin can outside the body convert more than 60 per cent, 

 of the nitrogen of peptone into the form of mono-amino acid ; 

 but we have no information to what extent this change goes 

 on during normal digestion. Since normal urine contains at most 

 traces of these bodies, it seems likely that any amino acid pro- 

 duced in the alimentary canal must be either synthesised during 

 absorption and made use of by the body or else excreted as 

 some other substance. There are experiments to show that 

 glycin, leucin, or aspartic acid given to an animal by the mouth 



