666 THE PRECURSORS OF UREA 



and the rest of the combination holds together, producing (la) 

 and (2a). 



(la) -COOH + NH .C- 



(2a) -COOH + NH".C.NH.C- 



NH 



(2'6) NH 2 . CO + NH 2 . C - 

 NH 2 



As examples of (la) we may take the hydrolysis of hippuric acid 

 and of glycyl glycin by an acid. Hippuric acid is hydrolysed into 

 benzoic acid and glycin. 



C 6 H 5 . CO . NH . CH, . COOH + H 9 O 

 = C 6 H 5 . COOH + NH 2 . CH 2 . COOH. 



In the same way glycyl glycin, the simplest polypeptide, splits 

 up into glycin. 



NH 2 . CH 2 . CO . NH . CFf 9 . COOH + H 2 O 

 = NH 2 . CH 2 . COOH + NH 2 . CH 2 . COOH. 



In the case of (2 a) it is clear that hydrolysis by acid will have 

 produced a free amino-acid and some combination with the 

 guanidin group. If, however, the substance containing the 

 guanidin group is heated with baryta, a more profound change 

 is produced. The guanidin group itself is broken up and urea 

 formed, (2') becoming (2'b). For example, guanidin acetic acid 

 treated with an acid yields guanidin and'oxalic acid. 



NH 2 . C . NH . CH 2 . COOH + H 2 O = NH 2 . C . NH 2 + COOH . COOH. 

 NH NH 



But when the same substance is heated with baryta, it splits up 

 into urea and glycin. 



NH 2 . C . NH . CH 2 . COOH + H 2 O = NH 2 . CO + NH 2 . CH 2 . COOH. 

 NH NH 2 



Corresponding to this difference in the hydrolytic action of 

 acids and baryta, the imidolytic ferments of the body may be 

 divided into two groups. (1) Those which act like acids, 

 namely, trpysin, erepsin, and some of the proteolytic and other 

 autolytic tissue ferments. (2) Those which can split up the 



