BIOLOGICAL CHEMISTRY 



individual amino acids, and a certain amount of these are 

 deamidised with the formation of ammonia. Ammonia is 

 formed into urea mainly by the cells of the liver. If the liver 

 is removed from the circulation ammonium salts accumulate 

 in the blood and increase in the urine.* 



Urea is formed from ammonium carbonate by a process of 

 dehydration, as shown by the following formulae : 

 ^X)NH 4 -H 2 ^ONH 4 -H 2 ^NH 2 

 C=O -~* C=O -- C=O 



^ONH 4 ^NH 2 >NH 2 



Ammonium Carbonate. Ammonium Carbamate. Urea. 



These changes can be shown by perfusing the excised liver 

 with solutions of either ammonium carbonate or ammonium 

 carbamate, urea being formed in each instance. | The same 

 synthesis can be accomplished but in lesser degree by other 

 tissue cells. 



By the formation of urea the alkaline salt ammonium car- 

 bonate is turned into the neutral substance urea, thus there is a 

 decrease in alkalinity, or in other words an increase in acidity. 

 According to previous discussions (pp. 59, 112) an increase in 

 acidity will therefore interfere with the formation of urea. 

 It is found that administration of inorganic acids causes a 

 decrease in urea and an increase of ammonium salts in the 

 urine. J In conditions when large quantities of organic acids 

 are formed owing to incomplete combustion of fatty acids, 

 there is an increase in the ammonia of the urine. 



Urea is often estimated by measuring the amount of nitrogen 

 liberated when it is decomposed by sodium hypobromite. 

 This method is not very accurate, as other substances are also 

 decomposed. By hydrolysis urea can be converted into 

 ammonia and by estimating the increase in the ammonia of 

 the urine the amount of urea can be calculated. The best 

 method of hydrolysis is by the enzyme, urease, of the soy 

 bean. 



Ammonia. There is always a small amount of ammonia 

 in the blood, therefore, also in the urine. Increase in acids 

 causes an increase in ammonia, as mentioned above. In 



* H. Hahn, A. Massen, M. Nencki and J. Pavlov, Arch. f. exper. 

 Path. u. Pharm., 1893, vol. 32, p. 161. 



f W. v. Schroder, Arch. f. exper. Path. u. Pharm., 1882, vol. 15, 

 p. 364. 



% F. Walter, Arch. f. exper. Path. u. Pharm., 1877, vol. 7, p. 148. 



E. K. Marshall, Jr., Journ. Biol. Chem., 1913, vol. 14, p. 283 ; 

 R. H, A. Plimmer and R. F. Skelton, Biochem. Journ., 1914, vol. 8, 

 p. 70. 



