362 PHYSIOLOGICAL CHEMISTRY. 



UREA. 



The relation of this substance to ammonium carbonate has 

 been referred to many times, but especially in discussing the 

 enzymic processes of the liver. The nutritive proteins contain 

 many amino groups which seem to be split off in the general 

 combustion processes going on in the body; also a great ex- 

 cess of groups which oxidize more completely and yield car- 

 bon dioxide. The large part of this escapes by way of the 

 lungs, while another part is evidently taken care of in the 

 liver through combination with the amino groups to form 

 urea. It has just been explained that normally some of this 

 amino nitrogen fails to take this simple course because of the 

 presence of strong acid radicles, which have great tenacity in 

 their combining relations. The ammonium salts so formed 

 are stable and cannot be worked over into urea. 



It appears also that the nitrogen of some other groups in 

 addition fails to reach the urea stage. Creatinine and uric 

 acid nitrogen are not included here, as these substances seem 

 to have an independent origin which will be discussed below. 

 But there are obscure compounds in the urine in small amount 

 of which we know but little, and some of these contain nitro- 

 gen. The oxyproteic acid referred to above is an illustration. 

 What the relation of this is to urea we cannot say, but an 

 idea of this kind suggests itself : the original protein complex 

 may contain certain groups which do not fall an easy prey to 

 the work of the oxidation enzymes in the body; they do not 

 break down to amino compounds and carbon dioxide, but re- 

 main intact as very resistant residues, and hence when the 

 liver is reached they are not in condition to pass into the urea 

 stage. In the katabolic changes of protein it is possible that 

 a number of such resistant groups may be produced, and it is 

 likely that the amount of nitrogen or other element which so 

 escapes the normal end reaction depends largely on the 

 strength of the enzymic functions. These must vary in dif- 

 ferent individuals, and hence sometimes more and sometimes 

 less of these resistant, or left over, residues will find their way 

 into the urine. 



