URINE 183 



stance for which the biuret test was named, cyamiric acid and 

 other substances. 



NH 2 



NH 2 0=0 



/ \ 



2 C = -> NH 



\ / 



NH 2 C=0 



NH 2 



Biuret. 



Urea is decomposed by nitrous acid, and by hypobromite ; the 

 nitrogen is liberated as the gas, and may be measured. Various 

 methods for the quantitative estimation of urea depend upon 

 these reactions. If heated to 153 C. urea gives up its nitrogen 

 as ammonia. This method also has been used in some of the most 

 successful quantitative urea methods. An enzyme ureas'e which 

 is found in the soy bean, and elsewhere, has the property of de- 

 composing urea, but no other nitrogenous constituent of the 

 urine. The urea is converted into ammonium carbonate from 

 which the ammonia easily may be liberated. This is the basis for 

 the method of urea determination in general use at present. 



The original source of urinary urea is the proteins of food and 

 tissues. Proteins are made up of amino acids. In digestion they 

 are split up into these compounds. Where and how is the amino 

 acid nitrogen converted into urea? This is a question which 

 has taken much labor to solve. A portion of the amino acids is 

 deaminized by bacteria in the intestine, a portion possibly in 

 their passage through the cells of the intestinal wall. The amino 

 group is converted into ammonia which circulates in the blood as 

 ammonium carbonate. It has been shown that an excised liver 

 if perfused with blood containing ammonium carbonate will con- 

 vert this substance into urea, but most of the amino acids pass 

 into the blood after absorption. Amino acids undoubtedly are 

 deaminized in the liver itself, and also in various other tissues, so 

 that the supply of ammonium carbonate may come from very 



