CHAPTER V 



UEEA. HIPPURIC ACID. EXCRETION OF AMINOACIDS 



UREA 



IT has been the earnest wish of the author to present in 

 logical sequence as fully as possible the world of biochemical 

 fact and error; but, as has been said before, and that not 

 without feelings of decided rebellion, at a single stride the 

 darkly yawning chasm of intermediate metabolism must be 

 passed, with the vast number of unsolved enigmas it harbors, 

 and attention next directed to the end-products of protein 

 metabolism. 



The present chapter has first to deal with urea, the most 

 important of the nitrogenous end-products of mammalian 

 metabolism. What do we know of the method and manner 

 of urea-formation? 



Theories of the Formation of Urea in the Living Body. 

 Of the many theories which have been proposed to ex- 

 plain the origin of urea there are today practically, in the 

 author's opinion, only two worthy of consideration, that of 

 Schmiedeberg and that of Hofmeister. The former, the 

 "anhydride theory/' regards urea as originating from 

 ammonium carbonate by withdrawal of water : 



AMMONIUM CARBONATE AMMONIUM CARJBAMATE UEEA 



/Q(NH 4 ) /NH 2 /NH 2 



CO >- CO * CO 



\C(NH 4 ) H 2 o \O(NH4> H 2 o \NH 2 . 



In this schema the carbamate of ammonium may be regarded 

 as an intermediate product. Hofmeister, on the other hand, 

 in his theory holds that urea is formed by an oxydative 

 synthesis, in which one of the components is supposed to be 

 an NH 2 "rest" arising from oxidation of ammonia, and the 

 other an NH 2 .CO rest. The hydrocyanic acid theory of 

 Hoppe-Seyler, which for decades has trailed through the 

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